xref: /petsc/src/snes/interface/snes.c (revision 8ca48ce972ee21275074d66b56733913e788a0a1)

#include <petsc/private/snesimpl.h> /*I "petscsnes.h"  I*/
#include <petscdmshell.h>
#include <petscdraw.h>
#include <petscds.h>
#include <petscdmadaptor.h>
#include <petscconvest.h>

PetscBool         SNESRegisterAllCalled = PETSC_FALSE;
PetscFunctionList SNESList              = NULL;

/* Logging support */
PetscClassId  SNES_CLASSID, DMSNES_CLASSID;
PetscLogEvent SNES_Solve, SNES_SetUp, SNES_FunctionEval, SNES_JacobianEval, SNES_NGSEval, SNES_NGSFuncEval, SNES_NPCSolve, SNES_ObjectiveEval;

/*@
   SNESSetErrorIfNotConverged - Causes `SNESSolve()` to generate an error immediately if the solver has not converged.

   Logically Collective

   Input Parameters:
+  snes - iterative context obtained from `SNESCreate()`
-  flg - `PETSC_TRUE` indicates you want the error generated

   Options Database Key:
.  -snes_error_if_not_converged <true,false> - cause an immediate error condition and stop the program if the solver does not converge

   Level: intermediate

   Note:
   Normally PETSc continues if a solver fails to converge, you can call `SNESGetConvergedReason()` after a `SNESSolve()`
   to determine if it has converged. Otherwise the solution may be inaccurate or wrong

.seealso: [](chapter_snes), `SNES`, `SNESGetErrorIfNotConverged()`, `KSPGetErrorIfNotConverged()`, `KSPSetErrorIfNotConverged()`
@*/
PetscErrorCode SNESSetErrorIfNotConverged(SNES snes, PetscBool flg)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidLogicalCollectiveBool(snes, flg, 2);
  snes->errorifnotconverged = flg;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESGetErrorIfNotConverged - Indicates if `SNESSolve()` will generate an error if the solver does not converge?

   Not Collective

   Input Parameter:
.  snes - iterative context obtained from `SNESCreate()`

   Output Parameter:
.  flag - `PETSC_TRUE` if it will generate an error, else `PETSC_FALSE`

   Level: intermediate

.seealso: [](chapter_snes), `SNES`, `SNESSolve()`, `SNESSetErrorIfNotConverged()`, `KSPGetErrorIfNotConverged()`, `KSPSetErrorIfNotConverged()`
@*/
PetscErrorCode SNESGetErrorIfNotConverged(SNES snes, PetscBool *flag)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidBoolPointer(flag, 2);
  *flag = snes->errorifnotconverged;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
    SNESSetAlwaysComputesFinalResidual - tells the `SNES` to always compute the residual (nonlinear function value) at the final solution

   Logically Collective

    Input Parameters:
+   snes - the shell `SNES`
-   flg - `PETSC_TRUE` to always compute the residual

   Level: advanced

   Note:
   Some solvers (such as smoothers in a `SNESFAS`) do not need the residual computed at the final solution so skip computing it
   to save time.

.seealso: [](chapter_snes), `SNES`, `SNESSolve()`, `SNESGetAlwaysComputesFinalResidual()`
@*/
PetscErrorCode SNESSetAlwaysComputesFinalResidual(SNES snes, PetscBool flg)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  snes->alwayscomputesfinalresidual = flg;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
    SNESGetAlwaysComputesFinalResidual - checks if the `SNES` always computes the residual at the final solution

   Logically Collective

    Input Parameter:
.   snes - the `SNES` context

    Output Parameter:
.   flg - `PETSC_TRUE` if the residual is computed

   Level: advanced

.seealso: [](chapter_snes), `SNES`, `SNESSolve()`, `SNESSetAlwaysComputesFinalResidual()`
@*/
PetscErrorCode SNESGetAlwaysComputesFinalResidual(SNES snes, PetscBool *flg)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  *flg = snes->alwayscomputesfinalresidual;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESSetFunctionDomainError - tells `SNES` that the input vector, a proposed new solution, to your function you provided to `SNESSetFunction()` is not
     in the functions domain. For example, a step with negative pressure.

   Logically Collective

   Input Parameters:
.  snes - the `SNES` context

   Level: advanced

   Note:
   You can direct `SNES` to avoid certain steps by using `SNESVISetVariableBounds()`, `SNESVISetComputeVariableBounds()` or
   `SNESLineSearchSetPreCheck()`, `SNESLineSearchSetPostCheck()`

.seealso: [](chapter_snes), `SNESCreate()`, `SNESSetFunction()`, `SNESFunction`, `SNESSetJacobianDomainError()`, `SNESVISetVariableBounds()`,
          `SNESVISetComputeVariableBounds()`, `SNESLineSearchSetPreCheck()`, `SNESLineSearchSetPostCheck()`
@*/
PetscErrorCode SNESSetFunctionDomainError(SNES snes)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscCheck(!snes->errorifnotconverged, PETSC_COMM_SELF, PETSC_ERR_ARG_OUTOFRANGE, "User code indicates input vector is not in the function domain");
  snes->domainerror = PETSC_TRUE;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESSetJacobianDomainError - tells `SNES` that the function you provided to `SNESSetJacobian()` at the proposed step. For example there is a negative element transformation.

   Logically Collective

   Input Parameters:
.  snes - the `SNES` context

   Level: advanced

   Note:
   You can direct `SNES` to avoid certain steps by using `SNESVISetVariableBounds()`, `SNESVISetComputeVariableBounds()` or
   `SNESLineSearchSetPreCheck()`, `SNESLineSearchSetPostCheck()`

.seealso: [](chapter_snes), `SNESCreate()`, `SNESSetFunction()`, `SNESFunction()`, `SNESSetFunctionDomainError()`, `SNESVISetVariableBounds()`,
          `SNESVISetComputeVariableBounds()`, `SNESLineSearchSetPreCheck()`, `SNESLineSearchSetPostCheck()`
@*/
PetscErrorCode SNESSetJacobianDomainError(SNES snes)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscCheck(!snes->errorifnotconverged, PETSC_COMM_SELF, PETSC_ERR_ARG_OUTOFRANGE, "User code indicates computeJacobian does not make sense");
  snes->jacobiandomainerror = PETSC_TRUE;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESSetCheckJacobianDomainError - tells `SNESSolve()` whether to check if the user called `SNESSetJacobianDomainError()` Jacobian domain error after
   each Jacobian evaluation. By default, we check Jacobian domain error in the debug mode, and do not check it in the optimized mode.

   Logically Collective

   Input Parameters:
+  snes - the `SNES` context
-  flg  - indicates if or not to check Jacobian domain error after each Jacobian evaluation

   Level: advanced

   Note:
   Checks require one extra parallel synchronization for each Jacobian evaluation

.seealso: [](chapter_snes), `SNES`, `SNESConvergedReason`, `SNESCreate()`, `SNESSetFunction()`, `SNESFunction()`, `SNESSetFunctionDomainError()`, `SNESGetCheckJacobianDomainError()`
@*/
PetscErrorCode SNESSetCheckJacobianDomainError(SNES snes, PetscBool flg)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  snes->checkjacdomainerror = flg;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESGetCheckJacobianDomainError - Get an indicator whether or not we are checking Jacobian domain errors after each Jacobian evaluation.

   Logically Collective

   Input Parameters:
.  snes - the `SNES` context

   Output Parameters:
.  flg  - `PETSC_FALSE` indicates that we don't check Jacobian domain errors after each Jacobian evaluation

   Level: advanced

.seealso: [](chapter_snes), `SNES`, `SNESCreate()`, `SNESSetFunction()`, `SNESFunction()`, `SNESSetFunctionDomainError()`, `SNESSetCheckJacobianDomainError()`
@*/
PetscErrorCode SNESGetCheckJacobianDomainError(SNES snes, PetscBool *flg)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidBoolPointer(flg, 2);
  *flg = snes->checkjacdomainerror;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESGetFunctionDomainError - Gets the status of the domain error after a call to `SNESComputeFunction()`;

   Logically Collective

   Input Parameters:
.  snes - the `SNES` context

   Output Parameters:
.  domainerror - Set to `PETSC_TRUE` if there's a domain error; `PETSC_FALSE` otherwise.

   Level: developer

.seealso: [](chapter_snes), `SNES`, `SNESSetFunctionDomainError()`, `SNESComputeFunction()`
@*/
PetscErrorCode SNESGetFunctionDomainError(SNES snes, PetscBool *domainerror)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidBoolPointer(domainerror, 2);
  *domainerror = snes->domainerror;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESGetJacobianDomainError - Gets the status of the Jacobian domain error after a call to `SNESComputeJacobian()`;

   Logically Collective

   Input Parameters:
.  snes - the `SNES` context

   Output Parameters:
.  domainerror - Set to `PETSC_TRUE` if there's a Jacobian domain error; `PETSC_FALSE` otherwise.

   Level: advanced

.seealso: [](chapter_snes), `SNES`, `SNESSetFunctionDomainError()`, `SNESComputeFunction()`, `SNESGetFunctionDomainError()`
@*/
PetscErrorCode SNESGetJacobianDomainError(SNES snes, PetscBool *domainerror)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidBoolPointer(domainerror, 2);
  *domainerror = snes->jacobiandomainerror;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
  SNESLoad - Loads a `SNES` that has been stored in `PETSCVIEWERBINARY` with `SNESView()`.

  Collective

  Input Parameters:
+ newdm - the newly loaded `SNES`, this needs to have been created with `SNESCreate()` or
           some related function before a call to `SNESLoad()`.
- viewer - binary file viewer, obtained from `PetscViewerBinaryOpen()`

   Level: intermediate

  Note:
   The type is determined by the data in the file, any type set into the `SNES` before this call is ignored.

.seealso: [](chapter_snes), `SNES`, `PetscViewer`, `SNESCreate()`, `SNESType`, `PetscViewerBinaryOpen()`, `SNESView()`, `MatLoad()`, `VecLoad()`
@*/
PetscErrorCode SNESLoad(SNES snes, PetscViewer viewer)
{
  PetscBool isbinary;
  PetscInt  classid;
  char      type[256];
  KSP       ksp;
  DM        dm;
  DMSNES    dmsnes;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidHeaderSpecific(viewer, PETSC_VIEWER_CLASSID, 2);
  PetscCall(PetscObjectTypeCompare((PetscObject)viewer, PETSCVIEWERBINARY, &isbinary));
  PetscCheck(isbinary, PETSC_COMM_SELF, PETSC_ERR_ARG_WRONG, "Invalid viewer; open viewer with PetscViewerBinaryOpen()");

  PetscCall(PetscViewerBinaryRead(viewer, &classid, 1, NULL, PETSC_INT));
  PetscCheck(classid == SNES_FILE_CLASSID, PetscObjectComm((PetscObject)snes), PETSC_ERR_ARG_WRONG, "Not SNES next in file");
  PetscCall(PetscViewerBinaryRead(viewer, type, 256, NULL, PETSC_CHAR));
  PetscCall(SNESSetType(snes, type));
  PetscTryTypeMethod(snes, load, viewer);
  PetscCall(SNESGetDM(snes, &dm));
  PetscCall(DMGetDMSNES(dm, &dmsnes));
  PetscCall(DMSNESLoad(dmsnes, viewer));
  PetscCall(SNESGetKSP(snes, &ksp));
  PetscCall(KSPLoad(ksp, viewer));
  PetscFunctionReturn(PETSC_SUCCESS);
}

#include <petscdraw.h>
#if defined(PETSC_HAVE_SAWS)
  #include <petscviewersaws.h>
#endif

/*@C
   SNESViewFromOptions - View a `SNES` based on values in the options database

   Collective

   Input Parameters:
+  A - the `SNES` context
.  obj - Optional object that provides the options prefix for the checks
-  name - command line option

   Level: intermediate

.seealso: [](chapter_snes), `SNES`, `SNESView`, `PetscObjectViewFromOptions()`, `SNESCreate()`
@*/
PetscErrorCode SNESViewFromOptions(SNES A, PetscObject obj, const char name[])
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(A, SNES_CLASSID, 1);
  PetscCall(PetscObjectViewFromOptions((PetscObject)A, obj, name));
  PetscFunctionReturn(PETSC_SUCCESS);
}

PETSC_EXTERN PetscErrorCode SNESComputeJacobian_DMDA(SNES, Vec, Mat, Mat, void *);

/*@C
   SNESView - Prints or visualizes the `SNES` data structure.

   Collective

   Input Parameters:
+  snes - the `SNES` context
-  viewer - the `PetscViewer`

   Options Database Key:
.  -snes_view - Calls `SNESView()` at end of `SNESSolve()`

   Level: beginner

   Notes:
   The available visualization contexts include
+     `PETSC_VIEWER_STDOUT_SELF` - standard output (default)
-     `PETSC_VIEWER_STDOUT_WORLD` - synchronized standard
         output where only the first processor opens
         the file.  All other processors send their
         data to the first processor to print.

   The available formats include
+     `PETSC_VIEWER_DEFAULT` - standard output (default)
-     `PETSC_VIEWER_ASCII_INFO_DETAIL` - more verbose output for `SNESNASM`

   The user can open an alternative visualization context with
   `PetscViewerASCIIOpen()` - output to a specified file.

  In the debugger you can do "call `SNESView`(snes,0)" to display the `SNES` solver. (The same holds for any PETSc object viewer).

.seealso: [](chapter_snes), `SNES`, `SNESLoad()`, `SNESCreate()`, `PetscViewerASCIIOpen()`
@*/
PetscErrorCode SNESView(SNES snes, PetscViewer viewer)
{
  SNESKSPEW     *kctx;
  KSP            ksp;
  SNESLineSearch linesearch;
  PetscBool      iascii, isstring, isbinary, isdraw;
  DMSNES         dmsnes;
#if defined(PETSC_HAVE_SAWS)
  PetscBool issaws;
#endif

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  if (!viewer) PetscCall(PetscViewerASCIIGetStdout(PetscObjectComm((PetscObject)snes), &viewer));
  PetscValidHeaderSpecific(viewer, PETSC_VIEWER_CLASSID, 2);
  PetscCheckSameComm(snes, 1, viewer, 2);

  PetscCall(PetscObjectTypeCompare((PetscObject)viewer, PETSCVIEWERASCII, &iascii));
  PetscCall(PetscObjectTypeCompare((PetscObject)viewer, PETSCVIEWERSTRING, &isstring));
  PetscCall(PetscObjectTypeCompare((PetscObject)viewer, PETSCVIEWERBINARY, &isbinary));
  PetscCall(PetscObjectTypeCompare((PetscObject)viewer, PETSCVIEWERDRAW, &isdraw));
#if defined(PETSC_HAVE_SAWS)
  PetscCall(PetscObjectTypeCompare((PetscObject)viewer, PETSCVIEWERSAWS, &issaws));
#endif
  if (iascii) {
    SNESNormSchedule normschedule;
    DM               dm;
    PetscErrorCode (*cJ)(SNES, Vec, Mat, Mat, void *);
    void       *ctx;
    const char *pre = "";

    PetscCall(PetscObjectPrintClassNamePrefixType((PetscObject)snes, viewer));
    if (!snes->setupcalled) PetscCall(PetscViewerASCIIPrintf(viewer, "  SNES has not been set up so information may be incomplete\n"));
    if (snes->ops->view) {
      PetscCall(PetscViewerASCIIPushTab(viewer));
      PetscUseTypeMethod(snes, view, viewer);
      PetscCall(PetscViewerASCIIPopTab(viewer));
    }
    PetscCall(PetscViewerASCIIPrintf(viewer, "  maximum iterations=%" PetscInt_FMT ", maximum function evaluations=%" PetscInt_FMT "\n", snes->max_its, snes->max_funcs));
    PetscCall(PetscViewerASCIIPrintf(viewer, "  tolerances: relative=%g, absolute=%g, solution=%g\n", (double)snes->rtol, (double)snes->abstol, (double)snes->stol));
    if (snes->usesksp) PetscCall(PetscViewerASCIIPrintf(viewer, "  total number of linear solver iterations=%" PetscInt_FMT "\n", snes->linear_its));
    PetscCall(PetscViewerASCIIPrintf(viewer, "  total number of function evaluations=%" PetscInt_FMT "\n", snes->nfuncs));
    PetscCall(SNESGetNormSchedule(snes, &normschedule));
    if (normschedule > 0) PetscCall(PetscViewerASCIIPrintf(viewer, "  norm schedule %s\n", SNESNormSchedules[normschedule]));
    if (snes->gridsequence) PetscCall(PetscViewerASCIIPrintf(viewer, "  total number of grid sequence refinements=%" PetscInt_FMT "\n", snes->gridsequence));
    if (snes->ksp_ewconv) {
      kctx = (SNESKSPEW *)snes->kspconvctx;
      if (kctx) {
        PetscCall(PetscViewerASCIIPrintf(viewer, "  Eisenstat-Walker computation of KSP relative tolerance (version %" PetscInt_FMT ")\n", kctx->version));
        PetscCall(PetscViewerASCIIPrintf(viewer, "    rtol_0=%g, rtol_max=%g, threshold=%g\n", (double)kctx->rtol_0, (double)kctx->rtol_max, (double)kctx->threshold));
        PetscCall(PetscViewerASCIIPrintf(viewer, "    gamma=%g, alpha=%g, alpha2=%g\n", (double)kctx->gamma, (double)kctx->alpha, (double)kctx->alpha2));
      }
    }
    if (snes->lagpreconditioner == -1) {
      PetscCall(PetscViewerASCIIPrintf(viewer, "  Preconditioned is never rebuilt\n"));
    } else if (snes->lagpreconditioner > 1) {
      PetscCall(PetscViewerASCIIPrintf(viewer, "  Preconditioned is rebuilt every %" PetscInt_FMT " new Jacobians\n", snes->lagpreconditioner));
    }
    if (snes->lagjacobian == -1) {
      PetscCall(PetscViewerASCIIPrintf(viewer, "  Jacobian is never rebuilt\n"));
    } else if (snes->lagjacobian > 1) {
      PetscCall(PetscViewerASCIIPrintf(viewer, "  Jacobian is rebuilt every %" PetscInt_FMT " SNES iterations\n", snes->lagjacobian));
    }
    PetscCall(SNESGetDM(snes, &dm));
    PetscCall(DMSNESGetJacobian(dm, &cJ, &ctx));
    if (snes->mf_operator) {
      PetscCall(PetscViewerASCIIPrintf(viewer, "  Jacobian is applied matrix-free with differencing\n"));
      pre = "Preconditioning ";
    }
    if (cJ == SNESComputeJacobianDefault) {
      PetscCall(PetscViewerASCIIPrintf(viewer, "  %sJacobian is built using finite differences one column at a time\n", pre));
    } else if (cJ == SNESComputeJacobianDefaultColor) {
      PetscCall(PetscViewerASCIIPrintf(viewer, "  %sJacobian is built using finite differences with coloring\n", pre));
      /* it slightly breaks data encapsulation for access the DMDA information directly */
    } else if (cJ == SNESComputeJacobian_DMDA) {
      MatFDColoring fdcoloring;
      PetscCall(PetscObjectQuery((PetscObject)dm, "DMDASNES_FDCOLORING", (PetscObject *)&fdcoloring));
      if (fdcoloring) {
        PetscCall(PetscViewerASCIIPrintf(viewer, "  %sJacobian is built using colored finite differences on a DMDA\n", pre));
      } else {
        PetscCall(PetscViewerASCIIPrintf(viewer, "  %sJacobian is built using a DMDA local Jacobian\n", pre));
      }
    } else if (snes->mf) {
      PetscCall(PetscViewerASCIIPrintf(viewer, "  Jacobian is applied matrix-free with differencing, no explicit Jacobian\n"));
    }
  } else if (isstring) {
    const char *type;
    PetscCall(SNESGetType(snes, &type));
    PetscCall(PetscViewerStringSPrintf(viewer, " SNESType: %-7.7s", type));
    PetscTryTypeMethod(snes, view, viewer);
  } else if (isbinary) {
    PetscInt    classid = SNES_FILE_CLASSID;
    MPI_Comm    comm;
    PetscMPIInt rank;
    char        type[256];

    PetscCall(PetscObjectGetComm((PetscObject)snes, &comm));
    PetscCallMPI(MPI_Comm_rank(comm, &rank));
    if (rank == 0) {
      PetscCall(PetscViewerBinaryWrite(viewer, &classid, 1, PETSC_INT));
      PetscCall(PetscStrncpy(type, ((PetscObject)snes)->type_name, sizeof(type)));
      PetscCall(PetscViewerBinaryWrite(viewer, type, sizeof(type), PETSC_CHAR));
    }
    PetscTryTypeMethod(snes, view, viewer);
  } else if (isdraw) {
    PetscDraw draw;
    char      str[36];
    PetscReal x, y, bottom, h;

    PetscCall(PetscViewerDrawGetDraw(viewer, 0, &draw));
    PetscCall(PetscDrawGetCurrentPoint(draw, &x, &y));
    PetscCall(PetscStrncpy(str, "SNES: ", sizeof(str)));
    PetscCall(PetscStrlcat(str, ((PetscObject)snes)->type_name, sizeof(str)));
    PetscCall(PetscDrawStringBoxed(draw, x, y, PETSC_DRAW_BLUE, PETSC_DRAW_BLACK, str, NULL, &h));
    bottom = y - h;
    PetscCall(PetscDrawPushCurrentPoint(draw, x, bottom));
    PetscTryTypeMethod(snes, view, viewer);
#if defined(PETSC_HAVE_SAWS)
  } else if (issaws) {
    PetscMPIInt rank;
    const char *name;

    PetscCall(PetscObjectGetName((PetscObject)snes, &name));
    PetscCallMPI(MPI_Comm_rank(PETSC_COMM_WORLD, &rank));
    if (!((PetscObject)snes)->amsmem && rank == 0) {
      char dir[1024];

      PetscCall(PetscObjectViewSAWs((PetscObject)snes, viewer));
      PetscCall(PetscSNPrintf(dir, 1024, "/PETSc/Objects/%s/its", name));
      PetscCallSAWs(SAWs_Register, (dir, &snes->iter, 1, SAWs_READ, SAWs_INT));
      if (!snes->conv_hist) PetscCall(SNESSetConvergenceHistory(snes, NULL, NULL, PETSC_DECIDE, PETSC_TRUE));
      PetscCall(PetscSNPrintf(dir, 1024, "/PETSc/Objects/%s/conv_hist", name));
      PetscCallSAWs(SAWs_Register, (dir, snes->conv_hist, 10, SAWs_READ, SAWs_DOUBLE));
    }
#endif
  }
  if (snes->linesearch) {
    PetscCall(SNESGetLineSearch(snes, &linesearch));
    PetscCall(PetscViewerASCIIPushTab(viewer));
    PetscCall(SNESLineSearchView(linesearch, viewer));
    PetscCall(PetscViewerASCIIPopTab(viewer));
  }
  if (snes->npc && snes->usesnpc) {
    PetscCall(PetscViewerASCIIPushTab(viewer));
    PetscCall(SNESView(snes->npc, viewer));
    PetscCall(PetscViewerASCIIPopTab(viewer));
  }
  PetscCall(PetscViewerASCIIPushTab(viewer));
  PetscCall(DMGetDMSNES(snes->dm, &dmsnes));
  PetscCall(DMSNESView(dmsnes, viewer));
  PetscCall(PetscViewerASCIIPopTab(viewer));
  if (snes->usesksp) {
    PetscCall(SNESGetKSP(snes, &ksp));
    PetscCall(PetscViewerASCIIPushTab(viewer));
    PetscCall(KSPView(ksp, viewer));
    PetscCall(PetscViewerASCIIPopTab(viewer));
  }
  if (isdraw) {
    PetscDraw draw;
    PetscCall(PetscViewerDrawGetDraw(viewer, 0, &draw));
    PetscCall(PetscDrawPopCurrentPoint(draw));
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*
  We retain a list of functions that also take SNES command
  line options. These are called at the end SNESSetFromOptions()
*/
#define MAXSETFROMOPTIONS 5
static PetscInt numberofsetfromoptions;
static PetscErrorCode (*othersetfromoptions[MAXSETFROMOPTIONS])(SNES);

/*@C
  SNESAddOptionsChecker - Adds an additional function to check for `SNES` options.

  Not Collective

  Input Parameter:
. snescheck - function that checks for options

  Level: developer

.seealso: [](chapter_snes), `SNES`, `SNESSetFromOptions()`
@*/
PetscErrorCode SNESAddOptionsChecker(PetscErrorCode (*snescheck)(SNES))
{
  PetscFunctionBegin;
  PetscCheck(numberofsetfromoptions < MAXSETFROMOPTIONS, PETSC_COMM_SELF, PETSC_ERR_ARG_OUTOFRANGE, "Too many options checkers, only %d allowed", MAXSETFROMOPTIONS);
  othersetfromoptions[numberofsetfromoptions++] = snescheck;
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode SNESSetUpMatrixFree_Private(SNES snes, PetscBool hasOperator, PetscInt version)
{
  Mat          J;
  MatNullSpace nullsp;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);

  if (!snes->vec_func && (snes->jacobian || snes->jacobian_pre)) {
    Mat A = snes->jacobian, B = snes->jacobian_pre;
    PetscCall(MatCreateVecs(A ? A : B, NULL, &snes->vec_func));
  }

  PetscCheck(version == 1 || version == 2, PETSC_COMM_SELF, PETSC_ERR_ARG_OUTOFRANGE, "matrix-free operator routines, only version 1 and 2");
  if (version == 1) {
    PetscCall(MatCreateSNESMF(snes, &J));
    PetscCall(MatMFFDSetOptionsPrefix(J, ((PetscObject)snes)->prefix));
    PetscCall(MatSetFromOptions(J));
    /* TODO: the version 2 code should be merged into the MatCreateSNESMF() and MatCreateMFFD() infrastructure and then removed */
  } else /* if (version == 2) */ {
    PetscCheck(snes->vec_func, PETSC_COMM_SELF, PETSC_ERR_ARG_WRONGSTATE, "SNESSetFunction() must be called first");
#if !defined(PETSC_USE_COMPLEX) && !defined(PETSC_USE_REAL_SINGLE) && !defined(PETSC_USE_REAL___FLOAT128) && !defined(PETSC_USE_REAL___FP16)
    PetscCall(MatCreateSNESMFMore(snes, snes->vec_func, &J));
#else
    SETERRQ(PETSC_COMM_SELF, PETSC_ERR_SUP, "matrix-free operator routines (version 2)");
#endif
  }

  /* attach any user provided null space that was on Amat to the newly created matrix free matrix */
  if (snes->jacobian) {
    PetscCall(MatGetNullSpace(snes->jacobian, &nullsp));
    if (nullsp) PetscCall(MatSetNullSpace(J, nullsp));
  }

  PetscCall(PetscInfo(snes, "Setting default matrix-free operator routines (version %" PetscInt_FMT ")\n", version));
  if (hasOperator) {
    /* This version replaces the user provided Jacobian matrix with a
       matrix-free version but still employs the user-provided preconditioner matrix. */
    PetscCall(SNESSetJacobian(snes, J, NULL, NULL, NULL));
  } else {
    /* This version replaces both the user-provided Jacobian and the user-
     provided preconditioner Jacobian with the default matrix free version. */
    if (snes->npcside == PC_LEFT && snes->npc) {
      if (!snes->jacobian) PetscCall(SNESSetJacobian(snes, J, NULL, NULL, NULL));
    } else {
      KSP       ksp;
      PC        pc;
      PetscBool match;

      PetscCall(SNESSetJacobian(snes, J, J, MatMFFDComputeJacobian, NULL));
      /* Force no preconditioner */
      PetscCall(SNESGetKSP(snes, &ksp));
      PetscCall(KSPGetPC(ksp, &pc));
      PetscCall(PetscObjectTypeCompareAny((PetscObject)pc, &match, PCSHELL, PCH2OPUS, ""));
      if (!match) {
        PetscCall(PetscInfo(snes, "Setting default matrix-free preconditioner routines\nThat is no preconditioner is being used\n"));
        PetscCall(PCSetType(pc, PCNONE));
      }
    }
  }
  PetscCall(MatDestroy(&J));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode DMRestrictHook_SNESVecSol(DM dmfine, Mat Restrict, Vec Rscale, Mat Inject, DM dmcoarse, void *ctx)
{
  SNES snes = (SNES)ctx;
  Vec  Xfine, Xfine_named = NULL, Xcoarse;

  PetscFunctionBegin;
  if (PetscLogPrintInfo) {
    PetscInt finelevel, coarselevel, fineclevel, coarseclevel;
    PetscCall(DMGetRefineLevel(dmfine, &finelevel));
    PetscCall(DMGetCoarsenLevel(dmfine, &fineclevel));
    PetscCall(DMGetRefineLevel(dmcoarse, &coarselevel));
    PetscCall(DMGetCoarsenLevel(dmcoarse, &coarseclevel));
    PetscCall(PetscInfo(dmfine, "Restricting SNES solution vector from level %" PetscInt_FMT "-%" PetscInt_FMT " to level %" PetscInt_FMT "-%" PetscInt_FMT "\n", finelevel, fineclevel, coarselevel, coarseclevel));
  }
  if (dmfine == snes->dm) Xfine = snes->vec_sol;
  else {
    PetscCall(DMGetNamedGlobalVector(dmfine, "SNESVecSol", &Xfine_named));
    Xfine = Xfine_named;
  }
  PetscCall(DMGetNamedGlobalVector(dmcoarse, "SNESVecSol", &Xcoarse));
  if (Inject) {
    PetscCall(MatRestrict(Inject, Xfine, Xcoarse));
  } else {
    PetscCall(MatRestrict(Restrict, Xfine, Xcoarse));
    PetscCall(VecPointwiseMult(Xcoarse, Xcoarse, Rscale));
  }
  PetscCall(DMRestoreNamedGlobalVector(dmcoarse, "SNESVecSol", &Xcoarse));
  if (Xfine_named) PetscCall(DMRestoreNamedGlobalVector(dmfine, "SNESVecSol", &Xfine_named));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode DMCoarsenHook_SNESVecSol(DM dm, DM dmc, void *ctx)
{
  PetscFunctionBegin;
  PetscCall(DMCoarsenHookAdd(dmc, DMCoarsenHook_SNESVecSol, DMRestrictHook_SNESVecSol, ctx));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/* This may be called to rediscretize the operator on levels of linear multigrid. The DM shuffle is so the user can
 * safely call SNESGetDM() in their residual evaluation routine. */
static PetscErrorCode KSPComputeOperators_SNES(KSP ksp, Mat A, Mat B, void *ctx)
{
  SNES  snes = (SNES)ctx;
  Vec   X, Xnamed = NULL;
  DM    dmsave;
  void *ctxsave;
  PetscErrorCode (*jac)(SNES, Vec, Mat, Mat, void *) = NULL;

  PetscFunctionBegin;
  dmsave = snes->dm;
  PetscCall(KSPGetDM(ksp, &snes->dm));
  if (dmsave == snes->dm) X = snes->vec_sol; /* We are on the finest level */
  else { /* We are on a coarser level, this vec was initialized using a DM restrict hook */ PetscCall(DMGetNamedGlobalVector(snes->dm, "SNESVecSol", &Xnamed));
    X = Xnamed;
    PetscCall(SNESGetJacobian(snes, NULL, NULL, &jac, &ctxsave));
    /* If the DM's don't match up, the MatFDColoring context needed for the jacobian won't match up either -- fixit. */
    if (jac == SNESComputeJacobianDefaultColor) PetscCall(SNESSetJacobian(snes, NULL, NULL, SNESComputeJacobianDefaultColor, NULL));
  }
  /* Make sure KSP DM has the Jacobian computation routine */
  {
    DMSNES sdm;

    PetscCall(DMGetDMSNES(snes->dm, &sdm));
    if (!sdm->ops->computejacobian) PetscCall(DMCopyDMSNES(dmsave, snes->dm));
  }
  /* Compute the operators */
  PetscCall(SNESComputeJacobian(snes, X, A, B));
  /* Put the previous context back */
  if (snes->dm != dmsave && jac == SNESComputeJacobianDefaultColor) PetscCall(SNESSetJacobian(snes, NULL, NULL, jac, ctxsave));

  if (Xnamed) PetscCall(DMRestoreNamedGlobalVector(snes->dm, "SNESVecSol", &Xnamed));
  snes->dm = dmsave;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESSetUpMatrices - ensures that matrices are available for `SNES` Newton-like methods, this is called by `SNESSetUp_XXX()`

   Collective

   Input Parameter:
.  snes - `SNES` object to configure

   Level: developer

   Note:
   If the matrices do not yet exist it attempts to create them based on options previously set for the `SNES` such as `-snes_mf`

.seealso: [](chapter_snes), `SNES`, `SNESSetUp()`
@*/
PetscErrorCode SNESSetUpMatrices(SNES snes)
{
  DM     dm;
  DMSNES sdm;

  PetscFunctionBegin;
  PetscCall(SNESGetDM(snes, &dm));
  PetscCall(DMGetDMSNES(dm, &sdm));
  if (!snes->jacobian && snes->mf) {
    Mat   J;
    void *functx;
    PetscCall(MatCreateSNESMF(snes, &J));
    PetscCall(MatMFFDSetOptionsPrefix(J, ((PetscObject)snes)->prefix));
    PetscCall(MatSetFromOptions(J));
    PetscCall(SNESGetFunction(snes, NULL, NULL, &functx));
    PetscCall(SNESSetJacobian(snes, J, J, NULL, NULL));
    PetscCall(MatDestroy(&J));
  } else if (snes->mf_operator && !snes->jacobian_pre && !snes->jacobian) {
    Mat J, B;
    PetscCall(MatCreateSNESMF(snes, &J));
    PetscCall(MatMFFDSetOptionsPrefix(J, ((PetscObject)snes)->prefix));
    PetscCall(MatSetFromOptions(J));
    PetscCall(DMCreateMatrix(snes->dm, &B));
    /* sdm->computejacobian was already set to reach here */
    PetscCall(SNESSetJacobian(snes, J, B, NULL, NULL));
    PetscCall(MatDestroy(&J));
    PetscCall(MatDestroy(&B));
  } else if (!snes->jacobian_pre) {
    PetscDS   prob;
    Mat       J, B;
    PetscBool hasPrec = PETSC_FALSE;

    J = snes->jacobian;
    PetscCall(DMGetDS(dm, &prob));
    if (prob) PetscCall(PetscDSHasJacobianPreconditioner(prob, &hasPrec));
    if (J) PetscCall(PetscObjectReference((PetscObject)J));
    else if (hasPrec) PetscCall(DMCreateMatrix(snes->dm, &J));
    PetscCall(DMCreateMatrix(snes->dm, &B));
    PetscCall(SNESSetJacobian(snes, J ? J : B, B, NULL, NULL));
    PetscCall(MatDestroy(&J));
    PetscCall(MatDestroy(&B));
  }
  {
    KSP ksp;
    PetscCall(SNESGetKSP(snes, &ksp));
    PetscCall(KSPSetComputeOperators(ksp, KSPComputeOperators_SNES, snes));
    PetscCall(DMCoarsenHookAdd(snes->dm, DMCoarsenHook_SNESVecSol, DMRestrictHook_SNESVecSol, snes));
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode SNESMonitorPauseFinal_Internal(SNES snes)
{
  PetscInt i;

  PetscFunctionBegin;
  if (!snes->pauseFinal) PetscFunctionReturn(PETSC_SUCCESS);
  for (i = 0; i < snes->numbermonitors; ++i) {
    PetscViewerAndFormat *vf = (PetscViewerAndFormat *)snes->monitorcontext[i];
    PetscDraw             draw;
    PetscReal             lpause;

    if (!vf) continue;
    if (vf->lg) {
      if (!PetscCheckPointer(vf->lg, PETSC_OBJECT)) continue;
      if (((PetscObject)vf->lg)->classid != PETSC_DRAWLG_CLASSID) continue;
      PetscCall(PetscDrawLGGetDraw(vf->lg, &draw));
      PetscCall(PetscDrawGetPause(draw, &lpause));
      PetscCall(PetscDrawSetPause(draw, -1.0));
      PetscCall(PetscDrawPause(draw));
      PetscCall(PetscDrawSetPause(draw, lpause));
    } else {
      PetscBool isdraw;

      if (!PetscCheckPointer(vf->viewer, PETSC_OBJECT)) continue;
      if (((PetscObject)vf->viewer)->classid != PETSC_VIEWER_CLASSID) continue;
      PetscCall(PetscObjectTypeCompare((PetscObject)vf->viewer, PETSCVIEWERDRAW, &isdraw));
      if (!isdraw) continue;
      PetscCall(PetscViewerDrawGetDraw(vf->viewer, 0, &draw));
      PetscCall(PetscDrawGetPause(draw, &lpause));
      PetscCall(PetscDrawSetPause(draw, -1.0));
      PetscCall(PetscDrawPause(draw));
      PetscCall(PetscDrawSetPause(draw, lpause));
    }
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
   SNESMonitorSetFromOptions - Sets a monitor function and viewer appropriate for the type indicated by the user

   Collective

   Input Parameters:
+  snes - `SNES` object you wish to monitor
.  name - the monitor type one is seeking
.  help - message indicating what monitoring is done
.  manual - manual page for the monitor
.  monitor - the monitor function
-  monitorsetup - a function that is called once ONLY if the user selected this monitor that may set additional features of the `SNES` or `PetscViewer` objects

   Options Database Key:
.  -name - trigger the use of this monitor in `SNESSetFromOptions()`

   Level: advanced

.seealso: [](chapter_snes), `PetscOptionsGetViewer()`, `PetscOptionsGetReal()`, `PetscOptionsHasName()`, `PetscOptionsGetString()`,
          `PetscOptionsGetIntArray()`, `PetscOptionsGetRealArray()`, `PetscOptionsBool()`
          `PetscOptionsInt()`, `PetscOptionsString()`, `PetscOptionsReal()`, `PetscOptionsBool()`,
          `PetscOptionsName()`, `PetscOptionsBegin()`, `PetscOptionsEnd()`, `PetscOptionsHeadBegin()`,
          `PetscOptionsStringArray()`, `PetscOptionsRealArray()`, `PetscOptionsScalar()`,
          `PetscOptionsBoolGroupBegin()`, `PetscOptionsBoolGroup()`, `PetscOptionsBoolGroupEnd()`,
          `PetscOptionsFList()`, `PetscOptionsEList()`
@*/
PetscErrorCode SNESMonitorSetFromOptions(SNES snes, const char name[], const char help[], const char manual[], PetscErrorCode (*monitor)(SNES, PetscInt, PetscReal, PetscViewerAndFormat *), PetscErrorCode (*monitorsetup)(SNES, PetscViewerAndFormat *))
{
  PetscViewer       viewer;
  PetscViewerFormat format;
  PetscBool         flg;

  PetscFunctionBegin;
  PetscCall(PetscOptionsGetViewer(PetscObjectComm((PetscObject)snes), ((PetscObject)snes)->options, ((PetscObject)snes)->prefix, name, &viewer, &format, &flg));
  if (flg) {
    PetscViewerAndFormat *vf;
    PetscCall(PetscViewerAndFormatCreate(viewer, format, &vf));
    PetscCall(PetscObjectDereference((PetscObject)viewer));
    if (monitorsetup) PetscCall((*monitorsetup)(snes, vf));
    PetscCall(SNESMonitorSet(snes, (PetscErrorCode(*)(SNES, PetscInt, PetscReal, void *))monitor, vf, (PetscErrorCode(*)(void **))PetscViewerAndFormatDestroy));
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

PetscErrorCode SNESEWSetFromOptions_Private(SNESKSPEW *kctx, MPI_Comm comm, const char *prefix)
{
  PetscFunctionBegin;
  PetscOptionsBegin(comm, prefix, "Eisenstat and Walker type forcing options", "KSP");
  PetscCall(PetscOptionsInt("-ksp_ew_version", "Version 1, 2 or 3", NULL, kctx->version, &kctx->version, NULL));
  PetscCall(PetscOptionsReal("-ksp_ew_rtol0", "0 <= rtol0 < 1", NULL, kctx->rtol_0, &kctx->rtol_0, NULL));
  PetscCall(PetscOptionsReal("-ksp_ew_rtolmax", "0 <= rtolmax < 1", NULL, kctx->rtol_max, &kctx->rtol_max, NULL));
  PetscCall(PetscOptionsReal("-ksp_ew_gamma", "0 <= gamma <= 1", NULL, kctx->gamma, &kctx->gamma, NULL));
  PetscCall(PetscOptionsReal("-ksp_ew_alpha", "1 < alpha <= 2", NULL, kctx->alpha, &kctx->alpha, NULL));
  PetscCall(PetscOptionsReal("-ksp_ew_alpha2", "alpha2", NULL, kctx->alpha2, &kctx->alpha2, NULL));
  PetscCall(PetscOptionsReal("-ksp_ew_threshold", "0 < threshold < 1", NULL, kctx->threshold, &kctx->threshold, NULL));
  PetscCall(PetscOptionsReal("-ksp_ew_v4_p1", "p1", NULL, kctx->v4_p1, &kctx->v4_p1, NULL));
  PetscCall(PetscOptionsReal("-ksp_ew_v4_p2", "p2", NULL, kctx->v4_p2, &kctx->v4_p2, NULL));
  PetscCall(PetscOptionsReal("-ksp_ew_v4_p3", "p3", NULL, kctx->v4_p3, &kctx->v4_p3, NULL));
  PetscCall(PetscOptionsReal("-ksp_ew_v4_m1", "Scaling when rk-1 in [p2,p3)", NULL, kctx->v4_m1, &kctx->v4_m1, NULL));
  PetscCall(PetscOptionsReal("-ksp_ew_v4_m2", "Scaling when rk-1 in [p3,+infty)", NULL, kctx->v4_m2, &kctx->v4_m2, NULL));
  PetscCall(PetscOptionsReal("-ksp_ew_v4_m3", "Threshold for successive rtol (0.1 in Eq.7)", NULL, kctx->v4_m3, &kctx->v4_m3, NULL));
  PetscCall(PetscOptionsReal("-ksp_ew_v4_m4", "Adaptation scaling (0.5 in Eq.7)", NULL, kctx->v4_m4, &kctx->v4_m4, NULL));
  PetscOptionsEnd();
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESSetFromOptions - Sets various `SNES` and `KSP` parameters from user options.

   Collective

   Input Parameter:
.  snes - the `SNES` context

   Options Database Keys:
+  -snes_type <type> - newtonls, newtontr, ngmres, ncg, nrichardson, qn, vi, fas, `SNESType` for complete list
.  -snes_stol - convergence tolerance in terms of the norm
                of the change in the solution between steps
.  -snes_atol <abstol> - absolute tolerance of residual norm
.  -snes_rtol <rtol> - relative decrease in tolerance norm from initial
.  -snes_divergence_tolerance <divtol> - if the residual goes above divtol*rnorm0, exit with divergence
.  -snes_force_iteration <force> - force SNESSolve() to take at least one iteration
.  -snes_max_it <max_it> - maximum number of iterations
.  -snes_max_funcs <max_funcs> - maximum number of function evaluations
.  -snes_max_fail <max_fail> - maximum number of line search failures allowed before stopping, default is none
.  -snes_max_linear_solve_fail - number of linear solver failures before SNESSolve() stops
.  -snes_lag_preconditioner <lag> - how often preconditioner is rebuilt (use -1 to never rebuild)
.  -snes_lag_preconditioner_persists <true,false> - retains the -snes_lag_preconditioner information across multiple SNESSolve()
.  -snes_lag_jacobian <lag> - how often Jacobian is rebuilt (use -1 to never rebuild)
.  -snes_lag_jacobian_persists <true,false> - retains the -snes_lag_jacobian information across multiple SNESSolve()
.  -snes_tr_tol <trtol> - trust region tolerance
.  -snes_convergence_test - <default,skip,correct_pressure> convergence test in nonlinear solver.
                               default `SNESConvergedDefault()`. skip `SNESConvergedSkip()` means continue iterating until max_it or some other criterion is reached, saving expense
                               of convergence test. correct_pressure S`NESConvergedCorrectPressure()` has special handling of a pressure null space.
.  -snes_monitor [ascii][:filename][:viewer format] - prints residual norm at each iteration. if no filename given prints to stdout
.  -snes_monitor_solution [ascii binary draw][:filename][:viewer format] - plots solution at each iteration
.  -snes_monitor_residual [ascii binary draw][:filename][:viewer format] - plots residual (not its norm) at each iteration
.  -snes_monitor_solution_update [ascii binary draw][:filename][:viewer format] - plots update to solution at each iteration
.  -snes_monitor_lg_residualnorm - plots residual norm at each iteration
.  -snes_monitor_lg_range - plots residual norm at each iteration
.  -snes_monitor_pause_final - Pauses all monitor drawing after the solver ends
.  -snes_fd - use finite differences to compute Jacobian; very slow, only for testing
.  -snes_fd_color - use finite differences with coloring to compute Jacobian
.  -snes_mf_ksp_monitor - if using matrix-free multiply then print h at each KSP iteration
.  -snes_converged_reason - print the reason for convergence/divergence after each solve
.  -npc_snes_type <type> - the SNES type to use as a nonlinear preconditioner
.   -snes_test_jacobian <optional threshold> - compare the user provided Jacobian with one computed via finite differences to check for errors.  If a threshold is given, display only those entries whose difference is greater than the threshold.
-   -snes_test_jacobian_view - display the user provided Jacobian, the finite difference Jacobian and the difference between them to help users detect the location of errors in the user provided Jacobian.

    Options Database Keys for Eisenstat-Walker method:
+  -snes_ksp_ew - use Eisenstat-Walker method for determining linear system convergence
.  -snes_ksp_ew_version ver - version of  Eisenstat-Walker method
.  -snes_ksp_ew_rtol0 <rtol0> - Sets rtol0
.  -snes_ksp_ew_rtolmax <rtolmax> - Sets rtolmax
.  -snes_ksp_ew_gamma <gamma> - Sets gamma
.  -snes_ksp_ew_alpha <alpha> - Sets alpha
.  -snes_ksp_ew_alpha2 <alpha2> - Sets alpha2
-  -snes_ksp_ew_threshold <threshold> - Sets threshold

   Level: beginner

   Notes:
   To see all options, run your program with the -help option or consult the users manual

   `SNES` supports three approaches for computing (approximate) Jacobians: user provided via `SNESSetJacobian()`, matrix free, and computing explicitly with
   finite differences and coloring using `MatFDColoring`. It is also possible to use automatic differentiation and the `MatFDColoring` object.

.seealso: [](chapter_snes), `SNESType`, `SNESSetOptionsPrefix()`, `SNESResetFromOptions()`, `SNES`, `SNESCreate()`
@*/
PetscErrorCode SNESSetFromOptions(SNES snes)
{
  PetscBool   flg, pcset, persist, set;
  PetscInt    i, indx, lag, grids;
  const char *deft        = SNESNEWTONLS;
  const char *convtests[] = {"default", "skip", "correct_pressure"};
  SNESKSPEW  *kctx        = NULL;
  char        type[256], monfilename[PETSC_MAX_PATH_LEN], ewprefix[256];
  PCSide      pcside;
  const char *optionsprefix;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscCall(SNESRegisterAll());
  PetscObjectOptionsBegin((PetscObject)snes);
  if (((PetscObject)snes)->type_name) deft = ((PetscObject)snes)->type_name;
  PetscCall(PetscOptionsFList("-snes_type", "Nonlinear solver method", "SNESSetType", SNESList, deft, type, 256, &flg));
  if (flg) {
    PetscCall(SNESSetType(snes, type));
  } else if (!((PetscObject)snes)->type_name) {
    PetscCall(SNESSetType(snes, deft));
  }
  PetscCall(PetscOptionsReal("-snes_stol", "Stop if step length less than", "SNESSetTolerances", snes->stol, &snes->stol, NULL));
  PetscCall(PetscOptionsReal("-snes_atol", "Stop if function norm less than", "SNESSetTolerances", snes->abstol, &snes->abstol, NULL));

  PetscCall(PetscOptionsReal("-snes_rtol", "Stop if decrease in function norm less than", "SNESSetTolerances", snes->rtol, &snes->rtol, NULL));
  PetscCall(PetscOptionsReal("-snes_divergence_tolerance", "Stop if residual norm increases by this factor", "SNESSetDivergenceTolerance", snes->divtol, &snes->divtol, NULL));
  PetscCall(PetscOptionsInt("-snes_max_it", "Maximum iterations", "SNESSetTolerances", snes->max_its, &snes->max_its, NULL));
  PetscCall(PetscOptionsInt("-snes_max_funcs", "Maximum function evaluations", "SNESSetTolerances", snes->max_funcs, &snes->max_funcs, NULL));
  PetscCall(PetscOptionsInt("-snes_max_fail", "Maximum nonlinear step failures", "SNESSetMaxNonlinearStepFailures", snes->maxFailures, &snes->maxFailures, NULL));
  PetscCall(PetscOptionsInt("-snes_max_linear_solve_fail", "Maximum failures in linear solves allowed", "SNESSetMaxLinearSolveFailures", snes->maxLinearSolveFailures, &snes->maxLinearSolveFailures, NULL));
  PetscCall(PetscOptionsBool("-snes_error_if_not_converged", "Generate error if solver does not converge", "SNESSetErrorIfNotConverged", snes->errorifnotconverged, &snes->errorifnotconverged, NULL));
  PetscCall(PetscOptionsBool("-snes_force_iteration", "Force SNESSolve() to take at least one iteration", "SNESSetForceIteration", snes->forceiteration, &snes->forceiteration, NULL));
  PetscCall(PetscOptionsBool("-snes_check_jacobian_domain_error", "Check Jacobian domain error after Jacobian evaluation", "SNESCheckJacobianDomainError", snes->checkjacdomainerror, &snes->checkjacdomainerror, NULL));

  PetscCall(PetscOptionsInt("-snes_lag_preconditioner", "How often to rebuild preconditioner", "SNESSetLagPreconditioner", snes->lagpreconditioner, &lag, &flg));
  if (flg) {
    PetscCheck(lag != -1, PetscObjectComm((PetscObject)snes), PETSC_ERR_USER, "Cannot set the lag to -1 from the command line since the preconditioner must be built as least once, perhaps you mean -2");
    PetscCall(SNESSetLagPreconditioner(snes, lag));
  }
  PetscCall(PetscOptionsBool("-snes_lag_preconditioner_persists", "Preconditioner lagging through multiple SNES solves", "SNESSetLagPreconditionerPersists", snes->lagjac_persist, &persist, &flg));
  if (flg) PetscCall(SNESSetLagPreconditionerPersists(snes, persist));
  PetscCall(PetscOptionsInt("-snes_lag_jacobian", "How often to rebuild Jacobian", "SNESSetLagJacobian", snes->lagjacobian, &lag, &flg));
  if (flg) {
    PetscCheck(lag != -1, PetscObjectComm((PetscObject)snes), PETSC_ERR_USER, "Cannot set the lag to -1 from the command line since the Jacobian must be built as least once, perhaps you mean -2");
    PetscCall(SNESSetLagJacobian(snes, lag));
  }
  PetscCall(PetscOptionsBool("-snes_lag_jacobian_persists", "Jacobian lagging through multiple SNES solves", "SNESSetLagJacobianPersists", snes->lagjac_persist, &persist, &flg));
  if (flg) PetscCall(SNESSetLagJacobianPersists(snes, persist));

  PetscCall(PetscOptionsInt("-snes_grid_sequence", "Use grid sequencing to generate initial guess", "SNESSetGridSequence", snes->gridsequence, &grids, &flg));
  if (flg) PetscCall(SNESSetGridSequence(snes, grids));

  PetscCall(PetscOptionsEList("-snes_convergence_test", "Convergence test", "SNESSetConvergenceTest", convtests, sizeof(convtests) / sizeof(char *), "default", &indx, &flg));
  if (flg) {
    switch (indx) {
    case 0:
      PetscCall(SNESSetConvergenceTest(snes, SNESConvergedDefault, NULL, NULL));
      break;
    case 1:
      PetscCall(SNESSetConvergenceTest(snes, SNESConvergedSkip, NULL, NULL));
      break;
    case 2:
      PetscCall(SNESSetConvergenceTest(snes, SNESConvergedCorrectPressure, NULL, NULL));
      break;
    }
  }

  PetscCall(PetscOptionsEList("-snes_norm_schedule", "SNES Norm schedule", "SNESSetNormSchedule", SNESNormSchedules, 5, "function", &indx, &flg));
  if (flg) PetscCall(SNESSetNormSchedule(snes, (SNESNormSchedule)indx));

  PetscCall(PetscOptionsEList("-snes_function_type", "SNES Norm schedule", "SNESSetFunctionType", SNESFunctionTypes, 2, "unpreconditioned", &indx, &flg));
  if (flg) PetscCall(SNESSetFunctionType(snes, (SNESFunctionType)indx));

  kctx = (SNESKSPEW *)snes->kspconvctx;

  PetscCall(PetscOptionsBool("-snes_ksp_ew", "Use Eisentat-Walker linear system convergence test", "SNESKSPSetUseEW", snes->ksp_ewconv, &snes->ksp_ewconv, NULL));

  PetscCall(SNESGetOptionsPrefix(snes, &optionsprefix));
  PetscCall(PetscSNPrintf(ewprefix, sizeof(ewprefix), "%s%s", optionsprefix ? optionsprefix : "", "snes_"));
  PetscCall(SNESEWSetFromOptions_Private(kctx, PetscObjectComm((PetscObject)snes), ewprefix));

  PetscCall(PetscOptionsInt("-snes_ksp_ew_version", "Version 1, 2 or 3", "SNESKSPSetParametersEW", kctx->version, &kctx->version, NULL));
  PetscCall(PetscOptionsReal("-snes_ksp_ew_rtol0", "0 <= rtol0 < 1", "SNESKSPSetParametersEW", kctx->rtol_0, &kctx->rtol_0, NULL));
  PetscCall(PetscOptionsReal("-snes_ksp_ew_rtolmax", "0 <= rtolmax < 1", "SNESKSPSetParametersEW", kctx->rtol_max, &kctx->rtol_max, NULL));
  PetscCall(PetscOptionsReal("-snes_ksp_ew_gamma", "0 <= gamma <= 1", "SNESKSPSetParametersEW", kctx->gamma, &kctx->gamma, NULL));
  PetscCall(PetscOptionsReal("-snes_ksp_ew_alpha", "1 < alpha <= 2", "SNESKSPSetParametersEW", kctx->alpha, &kctx->alpha, NULL));
  PetscCall(PetscOptionsReal("-snes_ksp_ew_alpha2", "alpha2", "SNESKSPSetParametersEW", kctx->alpha2, &kctx->alpha2, NULL));
  PetscCall(PetscOptionsReal("-snes_ksp_ew_threshold", "0 < threshold < 1", "SNESKSPSetParametersEW", kctx->threshold, &kctx->threshold, NULL));

  flg = PETSC_FALSE;
  PetscCall(PetscOptionsBool("-snes_monitor_cancel", "Remove all monitors", "SNESMonitorCancel", flg, &flg, &set));
  if (set && flg) PetscCall(SNESMonitorCancel(snes));

  PetscCall(SNESMonitorSetFromOptions(snes, "-snes_monitor", "Monitor norm of function", "SNESMonitorDefault", SNESMonitorDefault, SNESMonitorDefaultSetUp));
  PetscCall(SNESMonitorSetFromOptions(snes, "-snes_monitor_short", "Monitor norm of function with fewer digits", "SNESMonitorDefaultShort", SNESMonitorDefaultShort, NULL));
  PetscCall(SNESMonitorSetFromOptions(snes, "-snes_monitor_range", "Monitor range of elements of function", "SNESMonitorRange", SNESMonitorRange, NULL));

  PetscCall(SNESMonitorSetFromOptions(snes, "-snes_monitor_ratio", "Monitor ratios of the norm of function for consecutive steps", "SNESMonitorRatio", SNESMonitorRatio, SNESMonitorRatioSetUp));
  PetscCall(SNESMonitorSetFromOptions(snes, "-snes_monitor_field", "Monitor norm of function (split into fields)", "SNESMonitorDefaultField", SNESMonitorDefaultField, NULL));
  PetscCall(SNESMonitorSetFromOptions(snes, "-snes_monitor_solution", "View solution at each iteration", "SNESMonitorSolution", SNESMonitorSolution, NULL));
  PetscCall(SNESMonitorSetFromOptions(snes, "-snes_monitor_solution_update", "View correction at each iteration", "SNESMonitorSolutionUpdate", SNESMonitorSolutionUpdate, NULL));
  PetscCall(SNESMonitorSetFromOptions(snes, "-snes_monitor_residual", "View residual at each iteration", "SNESMonitorResidual", SNESMonitorResidual, NULL));
  PetscCall(SNESMonitorSetFromOptions(snes, "-snes_monitor_jacupdate_spectrum", "Print the change in the spectrum of the Jacobian", "SNESMonitorJacUpdateSpectrum", SNESMonitorJacUpdateSpectrum, NULL));
  PetscCall(SNESMonitorSetFromOptions(snes, "-snes_monitor_fields", "Monitor norm of function per field", "SNESMonitorSet", SNESMonitorFields, NULL));
  PetscCall(PetscOptionsBool("-snes_monitor_pause_final", "Pauses all draw monitors at the final iterate", "SNESMonitorPauseFinal_Internal", PETSC_FALSE, &snes->pauseFinal, NULL));

  PetscCall(PetscOptionsString("-snes_monitor_python", "Use Python function", "SNESMonitorSet", NULL, monfilename, sizeof(monfilename), &flg));
  if (flg) PetscCall(PetscPythonMonitorSet((PetscObject)snes, monfilename));

  flg = PETSC_FALSE;
  PetscCall(PetscOptionsBool("-snes_monitor_lg_range", "Plot function range at each iteration", "SNESMonitorLGRange", flg, &flg, NULL));
  if (flg) {
    PetscViewer ctx;

    PetscCall(PetscViewerDrawOpen(PetscObjectComm((PetscObject)snes), NULL, NULL, PETSC_DECIDE, PETSC_DECIDE, 400, 300, &ctx));
    PetscCall(SNESMonitorSet(snes, SNESMonitorLGRange, ctx, (PetscErrorCode(*)(void **))PetscViewerDestroy));
  }

  flg = PETSC_FALSE;
  PetscCall(PetscOptionsBool("-snes_converged_reason_view_cancel", "Remove all converged reason viewers", "SNESConvergedReasonViewCancel", flg, &flg, &set));
  if (set && flg) PetscCall(SNESConvergedReasonViewCancel(snes));

  flg = PETSC_FALSE;
  PetscCall(PetscOptionsBool("-snes_fd", "Use finite differences (slow) to compute Jacobian", "SNESComputeJacobianDefault", flg, &flg, NULL));
  if (flg) {
    void *functx;
    DM    dm;
    PetscCall(SNESGetDM(snes, &dm));
    PetscCall(DMSNESUnsetJacobianContext_Internal(dm));
    PetscCall(SNESGetFunction(snes, NULL, NULL, &functx));
    PetscCall(SNESSetJacobian(snes, snes->jacobian, snes->jacobian_pre, SNESComputeJacobianDefault, functx));
    PetscCall(PetscInfo(snes, "Setting default finite difference Jacobian matrix\n"));
  }

  flg = PETSC_FALSE;
  PetscCall(PetscOptionsBool("-snes_fd_function", "Use finite differences (slow) to compute function from user objective", "SNESObjectiveComputeFunctionDefaultFD", flg, &flg, NULL));
  if (flg) PetscCall(SNESSetFunction(snes, NULL, SNESObjectiveComputeFunctionDefaultFD, NULL));

  flg = PETSC_FALSE;
  PetscCall(PetscOptionsBool("-snes_fd_color", "Use finite differences with coloring to compute Jacobian", "SNESComputeJacobianDefaultColor", flg, &flg, NULL));
  if (flg) {
    DM dm;
    PetscCall(SNESGetDM(snes, &dm));
    PetscCall(DMSNESUnsetJacobianContext_Internal(dm));
    PetscCall(SNESSetJacobian(snes, snes->jacobian, snes->jacobian_pre, SNESComputeJacobianDefaultColor, NULL));
    PetscCall(PetscInfo(snes, "Setting default finite difference coloring Jacobian matrix\n"));
  }

  flg = PETSC_FALSE;
  PetscCall(PetscOptionsBool("-snes_mf_operator", "Use a Matrix-Free Jacobian with user-provided preconditioner matrix", "SNESSetUseMatrixFree", PETSC_FALSE, &snes->mf_operator, &flg));
  if (flg && snes->mf_operator) {
    snes->mf_operator = PETSC_TRUE;
    snes->mf          = PETSC_TRUE;
  }
  flg = PETSC_FALSE;
  PetscCall(PetscOptionsBool("-snes_mf", "Use a Matrix-Free Jacobian with no preconditioner matrix", "SNESSetUseMatrixFree", PETSC_FALSE, &snes->mf, &flg));
  if (!flg && snes->mf_operator) snes->mf = PETSC_TRUE;
  PetscCall(PetscOptionsInt("-snes_mf_version", "Matrix-Free routines version 1 or 2", "None", snes->mf_version, &snes->mf_version, NULL));

  flg = PETSC_FALSE;
  PetscCall(SNESGetNPCSide(snes, &pcside));
  PetscCall(PetscOptionsEnum("-snes_npc_side", "SNES nonlinear preconditioner side", "SNESSetNPCSide", PCSides, (PetscEnum)pcside, (PetscEnum *)&pcside, &flg));
  if (flg) PetscCall(SNESSetNPCSide(snes, pcside));

#if defined(PETSC_HAVE_SAWS)
  /*
    Publish convergence information using SAWs
  */
  flg = PETSC_FALSE;
  PetscCall(PetscOptionsBool("-snes_monitor_saws", "Publish SNES progress using SAWs", "SNESMonitorSet", flg, &flg, NULL));
  if (flg) {
    void *ctx;
    PetscCall(SNESMonitorSAWsCreate(snes, &ctx));
    PetscCall(SNESMonitorSet(snes, SNESMonitorSAWs, ctx, SNESMonitorSAWsDestroy));
  }
#endif
#if defined(PETSC_HAVE_SAWS)
  {
    PetscBool set;
    flg = PETSC_FALSE;
    PetscCall(PetscOptionsBool("-snes_saws_block", "Block for SAWs at end of SNESSolve", "PetscObjectSAWsBlock", ((PetscObject)snes)->amspublishblock, &flg, &set));
    if (set) PetscCall(PetscObjectSAWsSetBlock((PetscObject)snes, flg));
  }
#endif

  for (i = 0; i < numberofsetfromoptions; i++) PetscCall((*othersetfromoptions[i])(snes));

  PetscTryTypeMethod(snes, setfromoptions, PetscOptionsObject);

  /* process any options handlers added with PetscObjectAddOptionsHandler() */
  PetscCall(PetscObjectProcessOptionsHandlers((PetscObject)snes, PetscOptionsObject));
  PetscOptionsEnd();

  if (snes->linesearch) {
    PetscCall(SNESGetLineSearch(snes, &snes->linesearch));
    PetscCall(SNESLineSearchSetFromOptions(snes->linesearch));
  }

  if (snes->usesksp) {
    if (!snes->ksp) PetscCall(SNESGetKSP(snes, &snes->ksp));
    PetscCall(KSPSetOperators(snes->ksp, snes->jacobian, snes->jacobian_pre));
    PetscCall(KSPSetFromOptions(snes->ksp));
  }

  /* if user has set the SNES NPC type via options database, create it. */
  PetscCall(SNESGetOptionsPrefix(snes, &optionsprefix));
  PetscCall(PetscOptionsHasName(((PetscObject)snes)->options, optionsprefix, "-npc_snes_type", &pcset));
  if (pcset && (!snes->npc)) PetscCall(SNESGetNPC(snes, &snes->npc));
  if (snes->npc) PetscCall(SNESSetFromOptions(snes->npc));
  snes->setfromoptionscalled++;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESResetFromOptions - Sets various `SNES` and `KSP` parameters from user options ONLY if the `SNESSetFromOptions()` was previously set from options

   Collective

   Input Parameter:
.  snes - the `SNES` context

   Level: beginner

.seealso: [](chapter_snes), `SNES`, `SNESSetFromOptions()`, `SNESSetOptionsPrefix()`
@*/
PetscErrorCode SNESResetFromOptions(SNES snes)
{
  PetscFunctionBegin;
  if (snes->setfromoptionscalled) PetscCall(SNESSetFromOptions(snes));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
   SNESSetComputeApplicationContext - Sets an optional function to compute a user-defined context for
   the nonlinear solvers.

   Logically Collective; No Fortran Support

   Input Parameters:
+  snes - the `SNES` context
.  compute - function to compute the context
-  destroy - function to destroy the context

   Level: intermediate

   Note:
   This routine is useful if you are performing grid sequencing or using `SNESFAS` and need the appropriate context generated for each level.

   Use `SNESSetApplicationContext()` to see the context immediately

.seealso: [](chapter_snes), `SNESGetApplicationContext()`, `SNESSetComputeApplicationContext()`, `SNESSetApplicationContext()`
@*/
PetscErrorCode SNESSetComputeApplicationContext(SNES snes, PetscErrorCode (*compute)(SNES, void **), PetscErrorCode (*destroy)(void **))
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  snes->ops->usercompute = compute;
  snes->ops->userdestroy = destroy;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESSetApplicationContext - Sets the optional user-defined context for the nonlinear solvers.

   Logically Collective

   Input Parameters:
+  snes - the `SNES` context
-  usrP - optional user context

   Level: intermediate

   Notes:
   Users can provide a context when constructing the `SNES` options and then access it inside their function, Jacobian, or other evaluation function
   with `SNESGetApplicationContext()`

   To provide a function that computes the context for you use `SNESSetComputeApplicationContext()`

   Fortran Note:
    You must write a Fortran interface definition for this
    function that tells Fortran the Fortran derived data type that you are passing in as the ctx argument.

.seealso: [](chapter_snes), `SNES`, `SNESSetComputeApplicationContext()`, `SNESGetApplicationContext()`
@*/
PetscErrorCode SNESSetApplicationContext(SNES snes, void *usrP)
{
  KSP ksp;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscCall(SNESGetKSP(snes, &ksp));
  PetscCall(KSPSetApplicationContext(ksp, usrP));
  snes->user = usrP;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESGetApplicationContext - Gets the user-defined context for the
   nonlinear solvers set with `SNESGetApplicationContext()` or with `SNESSetComputeApplicationContext()`

   Not Collective

   Input Parameter:
.  snes - `SNES` context

   Output Parameter:
.  usrP - user context

   Level: intermediate

   Fortran Note:
   You must write a Fortran interface definition for this
   function that tells Fortran the Fortran derived data type that you are passing in as the ctx argument.

.seealso: [](chapter_snes), `SNESSetApplicationContext()`
@*/
PetscErrorCode SNESGetApplicationContext(SNES snes, void *usrP)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  *(void **)usrP = snes->user;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESSetUseMatrixFree - indicates that `SNES` should use matrix free finite difference matrix vector products to apply the Jacobian.

   Logically Collective

   Input Parameters:
+  snes - `SNES` context
.  mf_operator - use matrix-free only for the Amat used by `SNESSetJacobian()`, this means the user provided Pmat will continue to be used
-  mf - use matrix-free for both the Amat and Pmat used by `SNESSetJacobian()`, both the Amat and Pmat set in `SNESSetJacobian()` will be ignored. With
   this option no matrix element based preconditioners can be used in the linear solve since the matrix won't be explicitly available

   Options Database Keys:
+ -snes_mf_operator - use matrix free only for the mat operator
. -snes_mf - use matrix-free for both the mat and pmat operator
. -snes_fd_color - compute the Jacobian via coloring and finite differences.
- -snes_fd - compute the Jacobian via finite differences (slow)

   Level: intermediate

   Note:
   `SNES` supports three approaches for computing (approximate) Jacobians: user provided via `SNESSetJacobian()`, matrix-free, and computing explicitly with
   finite differences and coloring using `MatFDColoring`. It is also possible to use automatic differentiation and the `MatFDColoring` object.

.seealso: [](chapter_snes), `SNES`, `SNESGetUseMatrixFree()`, `MatCreateSNESMF()`, `SNESComputeJacobianDefaultColor()`
@*/
PetscErrorCode SNESSetUseMatrixFree(SNES snes, PetscBool mf_operator, PetscBool mf)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidLogicalCollectiveBool(snes, mf_operator, 2);
  PetscValidLogicalCollectiveBool(snes, mf, 3);
  snes->mf          = mf_operator ? PETSC_TRUE : mf;
  snes->mf_operator = mf_operator;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESGetUseMatrixFree - indicates if the `SNES` uses matrix-free finite difference matrix vector products to apply the Jacobian.

   Not Collective, but the resulting flags will be the same on all MPI ranks

   Input Parameter:
.  snes - `SNES` context

   Output Parameters:
+  mf_operator - use matrix-free only for the Amat used by `SNESSetJacobian()`, this means the user provided Pmat will continue to be used
-  mf - use matrix-free for both the Amat and Pmat used by `SNESSetJacobian()`, both the Amat and Pmat set in `SNESSetJacobian()` will be ignored

   Level: intermediate

.seealso: [](chapter_snes), `SNES`, `SNESSetUseMatrixFree()`, `MatCreateSNESMF()`
@*/
PetscErrorCode SNESGetUseMatrixFree(SNES snes, PetscBool *mf_operator, PetscBool *mf)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  if (mf) *mf = snes->mf;
  if (mf_operator) *mf_operator = snes->mf_operator;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESGetIterationNumber - Gets the number of nonlinear iterations completed
   at this time.

   Not Collective

   Input Parameter:
.  snes - `SNES` context

   Output Parameter:
.  iter - iteration number

   Level: intermediate

   Notes:
   For example, during the computation of iteration 2 this would return 1.

   This is useful for using lagged Jacobians (where one does not recompute the
   Jacobian at each `SNES` iteration). For example, the code
.vb
      ierr = SNESGetIterationNumber(snes,&it);
      if (!(it % 2)) {
        [compute Jacobian here]
      }
.ve
   can be used in your function that computes the Jacobian to cause the Jacobian to be
   recomputed every second `SNES` iteration. See also `SNESSetLagJacobian()`

   After the `SNES` solve is complete this will return the number of nonlinear iterations used.

.seealso: [](chapter_snes), `SNES`, `SNESSolve()`, `SNESSetLagJacobian()`, `SNESGetLinearSolveIterations()`
@*/
PetscErrorCode SNESGetIterationNumber(SNES snes, PetscInt *iter)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidIntPointer(iter, 2);
  *iter = snes->iter;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESSetIterationNumber - Sets the current iteration number.

   Not Collective

   Input Parameters:
+  snes - `SNES` context
-  iter - iteration number

   Level: developer

.seealso: [](chapter_snes), `SNESGetLinearSolveIterations()`
@*/
PetscErrorCode SNESSetIterationNumber(SNES snes, PetscInt iter)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscCall(PetscObjectSAWsTakeAccess((PetscObject)snes));
  snes->iter = iter;
  PetscCall(PetscObjectSAWsGrantAccess((PetscObject)snes));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESGetNonlinearStepFailures - Gets the number of unsuccessful steps
   attempted by the nonlinear solver.

   Not Collective

   Input Parameter:
.  snes - `SNES` context

   Output Parameter:
.  nfails - number of unsuccessful steps attempted

   Level: intermediate

   Note:
   This counter is reset to zero for each successive call to `SNESSolve()`.

.seealso: [](chapter_snes), `SNES`, `SNESGetMaxLinearSolveFailures()`, `SNESGetLinearSolveIterations()`, `SNESSetMaxLinearSolveFailures()`, `SNESGetLinearSolveFailures()`,
          `SNESSetMaxNonlinearStepFailures()`, `SNESGetMaxNonlinearStepFailures()`
@*/
PetscErrorCode SNESGetNonlinearStepFailures(SNES snes, PetscInt *nfails)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidIntPointer(nfails, 2);
  *nfails = snes->numFailures;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESSetMaxNonlinearStepFailures - Sets the maximum number of unsuccessful steps
   attempted by the nonlinear solver before it gives up and generates an error

   Not Collective

   Input Parameters:
+  snes     - `SNES` context
-  maxFails - maximum of unsuccessful steps

   Level: intermediate

.seealso: [](chapter_snes), `SNESSetErrorIfNotConverged()`, `SNESGetMaxLinearSolveFailures()`, `SNESGetLinearSolveIterations()`, `SNESSetMaxLinearSolveFailures()`, `SNESGetLinearSolveFailures()`,
          `SNESGetMaxNonlinearStepFailures()`, `SNESGetNonlinearStepFailures()`
@*/
PetscErrorCode SNESSetMaxNonlinearStepFailures(SNES snes, PetscInt maxFails)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  snes->maxFailures = maxFails;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESGetMaxNonlinearStepFailures - Gets the maximum number of unsuccessful steps
   attempted by the nonlinear solver before it gives up and generates an error

   Not Collective

   Input Parameter:
.  snes     - `SNES` context

   Output Parameter:
.  maxFails - maximum of unsuccessful steps

   Level: intermediate

.seealso: [](chapter_snes), `SNESSetErrorIfNotConverged()`, `SNESGetMaxLinearSolveFailures()`, `SNESGetLinearSolveIterations()`, `SNESSetMaxLinearSolveFailures()`, `SNESGetLinearSolveFailures()`,
          `SNESSetMaxNonlinearStepFailures()`, `SNESGetNonlinearStepFailures()`
@*/
PetscErrorCode SNESGetMaxNonlinearStepFailures(SNES snes, PetscInt *maxFails)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidIntPointer(maxFails, 2);
  *maxFails = snes->maxFailures;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESGetNumberFunctionEvals - Gets the number of user provided function evaluations
     done by the `SNES` object

   Not Collective

   Input Parameter:
.  snes     - `SNES` context

   Output Parameter:
.  nfuncs - number of evaluations

   Level: intermediate

   Note:
    Reset every time `SNESSolve()` is called unless `SNESSetCountersReset()` is used.

.seealso: [](chapter_snes), `SNES`, `SNESGetMaxLinearSolveFailures()`, `SNESGetLinearSolveIterations()`, `SNESSetMaxLinearSolveFailures()`, `SNESGetLinearSolveFailures()`, `SNESSetCountersReset()`
@*/
PetscErrorCode SNESGetNumberFunctionEvals(SNES snes, PetscInt *nfuncs)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidIntPointer(nfuncs, 2);
  *nfuncs = snes->nfuncs;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESGetLinearSolveFailures - Gets the number of failed (non-converged)
   linear solvers.

   Not Collective

   Input Parameter:
.  snes - `SNES` context

   Output Parameter:
.  nfails - number of failed solves

   Options Database Key:
. -snes_max_linear_solve_fail <num> - The number of failures before the solve is terminated

   Level: intermediate

   Note:
   This counter is reset to zero for each successive call to `SNESSolve()`.

.seealso: [](chapter_snes), `SNESGetMaxLinearSolveFailures()`, `SNESGetLinearSolveIterations()`, `SNESSetMaxLinearSolveFailures()`
@*/
PetscErrorCode SNESGetLinearSolveFailures(SNES snes, PetscInt *nfails)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidIntPointer(nfails, 2);
  *nfails = snes->numLinearSolveFailures;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESSetMaxLinearSolveFailures - the number of failed linear solve attempts
   allowed before `SNES` returns with a diverged reason of `SNES_DIVERGED_LINEAR_SOLVE`

   Logically Collective

   Input Parameters:
+  snes     - `SNES` context
-  maxFails - maximum allowed linear solve failures

   Options Database Key:
. -snes_max_linear_solve_fail <num> - The number of failures before the solve is terminated

   Level: intermediate

   Note:
    By default this is 0; that is `SNES` returns on the first failed linear solve

.seealso: [](chapter_snes), `SNESSetErrorIfNotConverged()`, `SNESGetLinearSolveFailures()`, `SNESGetMaxLinearSolveFailures()`, `SNESGetLinearSolveIterations()`
@*/
PetscErrorCode SNESSetMaxLinearSolveFailures(SNES snes, PetscInt maxFails)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidLogicalCollectiveInt(snes, maxFails, 2);
  snes->maxLinearSolveFailures = maxFails;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESGetMaxLinearSolveFailures - gets the maximum number of linear solve failures that
     are allowed before `SNES` returns as unsuccessful

   Not Collective

   Input Parameter:
.  snes     - `SNES` context

   Output Parameter:
.  maxFails - maximum of unsuccessful solves allowed

   Level: intermediate

   Note:
    By default this is 1; that is `SNES` returns on the first failed linear solve

.seealso: [](chapter_snes), `SNESSetErrorIfNotConverged()`, `SNESGetLinearSolveFailures()`, `SNESGetLinearSolveIterations()`, `SNESSetMaxLinearSolveFailures()`,
@*/
PetscErrorCode SNESGetMaxLinearSolveFailures(SNES snes, PetscInt *maxFails)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidIntPointer(maxFails, 2);
  *maxFails = snes->maxLinearSolveFailures;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESGetLinearSolveIterations - Gets the total number of linear iterations
   used by the nonlinear solver.

   Not Collective

   Input Parameter:
.  snes - `SNES` context

   Output Parameter:
.  lits - number of linear iterations

   Level: intermediate

   Notes:
   This counter is reset to zero for each successive call to `SNESSolve()` unless `SNESSetCountersReset()` is used.

   If the linear solver fails inside the `SNESSolve()` the iterations for that call to the linear solver are not included. If you wish to count them
   then call `KSPGetIterationNumber()` after the failed solve.

.seealso: [](chapter_snes), `SNES`, `SNESGetIterationNumber()`, `SNESGetLinearSolveFailures()`, `SNESGetMaxLinearSolveFailures()`, `SNESSetCountersReset()`
@*/
PetscErrorCode SNESGetLinearSolveIterations(SNES snes, PetscInt *lits)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidIntPointer(lits, 2);
  *lits = snes->linear_its;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESSetCountersReset - Sets whether or not the counters for linear iterations and function evaluations
   are reset every time `SNESSolve()` is called.

   Logically Collective

   Input Parameters:
+  snes - `SNES` context
-  reset - whether to reset the counters or not, defaults to `PETSC_TRUE`

   Level: developer

.seealso: [](chapter_snes), `SNESGetNumberFunctionEvals()`, `SNESGetLinearSolveIterations()`, `SNESGetNPC()`
@*/
PetscErrorCode SNESSetCountersReset(SNES snes, PetscBool reset)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidLogicalCollectiveBool(snes, reset, 2);
  snes->counters_reset = reset;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESSetKSP - Sets a `KSP` context for the `SNES` object to use

   Not Collective, but the `SNES` and `KSP` objects must live on the same MPI_Comm

   Input Parameters:
+  snes - the `SNES` context
-  ksp - the `KSP` context

   Level: developer

   Notes:
   The `SNES` object already has its `KSP` object, you can obtain with `SNESGetKSP()`
   so this routine is rarely needed.

   The `KSP` object that is already in the `SNES` object has its reference count
   decreased by one.

.seealso: [](chapter_snes), `SNES`, `KSP`, `KSPGetPC()`, `SNESCreate()`, `KSPCreate()`, `SNESSetKSP()`
@*/
PetscErrorCode SNESSetKSP(SNES snes, KSP ksp)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidHeaderSpecific(ksp, KSP_CLASSID, 2);
  PetscCheckSameComm(snes, 1, ksp, 2);
  PetscCall(PetscObjectReference((PetscObject)ksp));
  if (snes->ksp) PetscCall(PetscObjectDereference((PetscObject)snes->ksp));
  snes->ksp = ksp;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESCreate - Creates a nonlinear solver context used to manage a set of nonlinear solves

   Collective

   Input Parameter:
.  comm - MPI communicator

   Output Parameter:
.  outsnes - the new `SNES` context

   Options Database Keys:
+   -snes_mf - Activates default matrix-free Jacobian-vector products, and no preconditioning matrix
.   -snes_mf_operator - Activates default matrix-free Jacobian-vector products, and a user-provided preconditioning matrix
               as set by `SNESSetJacobian()`
.   -snes_fd_coloring - uses a relative fast computation of the Jacobian using finite differences and a graph coloring
-   -snes_fd - Uses (slow!) finite differences to compute Jacobian

   Level: beginner

   Developer Notes:
   `SNES` always creates a `KSP` object even though many `SNES` methods do not use it. This is
   unfortunate and should be fixed at some point. The flag snes->usesksp indicates if the
   particular method does use `KSP` and regulates if the information about the `KSP` is printed
   in `SNESView()`.

   `TSSetFromOptions()` does call `SNESSetFromOptions()` which can lead to users being confused
   by help messages about meaningless `SNES` options.

   `SNES` always creates the snes->kspconvctx even though it is used by only one type. This should be fixed.

.seealso: [](chapter_snes), `SNES`, `SNESSolve()`, `SNESDestroy()`, `SNES`, `SNESSetLagPreconditioner()`, `SNESSetLagJacobian()`
@*/
PetscErrorCode SNESCreate(MPI_Comm comm, SNES *outsnes)
{
  SNES       snes;
  SNESKSPEW *kctx;

  PetscFunctionBegin;
  PetscValidPointer(outsnes, 2);
  *outsnes = NULL;
  PetscCall(SNESInitializePackage());

  PetscCall(PetscHeaderCreate(snes, SNES_CLASSID, "SNES", "Nonlinear solver", "SNES", comm, SNESDestroy, SNESView));

  snes->ops->converged       = SNESConvergedDefault;
  snes->usesksp              = PETSC_TRUE;
  snes->tolerancesset        = PETSC_FALSE;
  snes->max_its              = 50;
  snes->max_funcs            = 10000;
  snes->norm                 = 0.0;
  snes->xnorm                = 0.0;
  snes->ynorm                = 0.0;
  snes->normschedule         = SNES_NORM_ALWAYS;
  snes->functype             = SNES_FUNCTION_DEFAULT;
  snes->rtol                 = PetscDefined(USE_REAL_SINGLE) ? 1.e-5 : 1.e-8;
  snes->ttol                 = 0.0;
  snes->abstol               = PetscDefined(USE_REAL_SINGLE) ? 1.e-25 : 1.e-50;
  snes->stol                 = PetscDefined(USE_REAL_SINGLE) ? 1.e-5 : 1.e-8;
  snes->deltatol             = PetscDefined(USE_REAL_SINGLE) ? 1.e-6 : 1.e-12;
  snes->divtol               = 1.e4;
  snes->rnorm0               = 0;
  snes->nfuncs               = 0;
  snes->numFailures          = 0;
  snes->maxFailures          = 1;
  snes->linear_its           = 0;
  snes->lagjacobian          = 1;
  snes->jac_iter             = 0;
  snes->lagjac_persist       = PETSC_FALSE;
  snes->lagpreconditioner    = 1;
  snes->pre_iter             = 0;
  snes->lagpre_persist       = PETSC_FALSE;
  snes->numbermonitors       = 0;
  snes->numberreasonviews    = 0;
  snes->data                 = NULL;
  snes->setupcalled          = PETSC_FALSE;
  snes->ksp_ewconv           = PETSC_FALSE;
  snes->nwork                = 0;
  snes->work                 = NULL;
  snes->nvwork               = 0;
  snes->vwork                = NULL;
  snes->conv_hist_len        = 0;
  snes->conv_hist_max        = 0;
  snes->conv_hist            = NULL;
  snes->conv_hist_its        = NULL;
  snes->conv_hist_reset      = PETSC_TRUE;
  snes->counters_reset       = PETSC_TRUE;
  snes->vec_func_init_set    = PETSC_FALSE;
  snes->reason               = SNES_CONVERGED_ITERATING;
  snes->npcside              = PC_RIGHT;
  snes->setfromoptionscalled = 0;

  snes->mf          = PETSC_FALSE;
  snes->mf_operator = PETSC_FALSE;
  snes->mf_version  = 1;

  snes->numLinearSolveFailures = 0;
  snes->maxLinearSolveFailures = 1;

  snes->vizerotolerance     = 1.e-8;
  snes->checkjacdomainerror = PetscDefined(USE_DEBUG) ? PETSC_TRUE : PETSC_FALSE;

  /* Set this to true if the implementation of SNESSolve_XXX does compute the residual at the final solution. */
  snes->alwayscomputesfinalresidual = PETSC_FALSE;

  /* Create context to compute Eisenstat-Walker relative tolerance for KSP */
  PetscCall(PetscNew(&kctx));

  snes->kspconvctx  = (void *)kctx;
  kctx->version     = 2;
  kctx->rtol_0      = 0.3; /* Eisenstat and Walker suggest rtol_0=.5, but
                             this was too large for some test cases */
  kctx->rtol_last   = 0.0;
  kctx->rtol_max    = 0.9;
  kctx->gamma       = 1.0;
  kctx->alpha       = 0.5 * (1.0 + PetscSqrtReal(5.0));
  kctx->alpha2      = kctx->alpha;
  kctx->threshold   = 0.1;
  kctx->lresid_last = 0.0;
  kctx->norm_last   = 0.0;

  kctx->rk_last     = 0.0;
  kctx->rk_last_2   = 0.0;
  kctx->rtol_last_2 = 0.0;
  kctx->v4_p1       = 0.1;
  kctx->v4_p2       = 0.4;
  kctx->v4_p3       = 0.7;
  kctx->v4_m1       = 0.8;
  kctx->v4_m2       = 0.5;
  kctx->v4_m3       = 0.1;
  kctx->v4_m4       = 0.5;

  *outsnes = snes;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*MC
    SNESFunction - Functional form used to convey the nonlinear function to `SNES` in `SNESSetFunction()`

     Synopsis:
     #include "petscsnes.h"
     PetscErrorCode SNESFunction(SNES snes,Vec x,Vec f,void *ctx);

     Collective

     Input Parameters:
+     snes - the `SNES` context
.     x    - state at which to evaluate residual
-     ctx     - optional user-defined function context, passed in with `SNESSetFunction()`

     Output Parameter:
.     f  - vector to put residual (function value)

   Level: intermediate

.seealso: [](chapter_snes), `SNESSetFunction()`, `SNESGetFunction()`
M*/

/*@C
   SNESSetFunction - Sets the function evaluation routine and function
   vector for use by the `SNES` routines in solving systems of nonlinear
   equations.

   Logically Collective

   Input Parameters:
+  snes - the `SNES` context
.  r - vector to store function values, may be `NULL`
.  f - function evaluation routine;  for calling sequence see `SNESFunction`
-  ctx - [optional] user-defined context for private data for the
         function evaluation routine (may be `NULL`)

   Level: beginner

.seealso: [](chapter_snes), `SNES`, `SNESGetFunction()`, `SNESComputeFunction()`, `SNESSetJacobian()`, `SNESSetPicard()`, `SNESFunction`
@*/
PetscErrorCode SNESSetFunction(SNES snes, Vec r, PetscErrorCode (*f)(SNES, Vec, Vec, void *), void *ctx)
{
  DM dm;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  if (r) {
    PetscValidHeaderSpecific(r, VEC_CLASSID, 2);
    PetscCheckSameComm(snes, 1, r, 2);
    PetscCall(PetscObjectReference((PetscObject)r));
    PetscCall(VecDestroy(&snes->vec_func));
    snes->vec_func = r;
  }
  PetscCall(SNESGetDM(snes, &dm));
  PetscCall(DMSNESSetFunction(dm, f, ctx));
  if (f == SNESPicardComputeFunction) PetscCall(DMSNESSetMFFunction(dm, SNESPicardComputeMFFunction, ctx));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
   SNESSetInitialFunction - Sets the function vector to be used as the
   initial function value at the initialization of the method.  In some
   instances, the user has precomputed the function before calling
   `SNESSolve()`.  This function allows one to avoid a redundant call
   to `SNESComputeFunction()` in that case.

   Logically Collective

   Input Parameters:
+  snes - the `SNES` context
-  f - vector to store function value

   Level: developer

   Notes:
   This should not be modified during the solution procedure.

   This is used extensively in the `SNESFAS` hierarchy and in nonlinear preconditioning.

.seealso: [](chapter_snes), `SNES`, `SNESFAS`, `SNESSetFunction()`, `SNESComputeFunction()`, `SNESSetInitialFunctionNorm()`
@*/
PetscErrorCode SNESSetInitialFunction(SNES snes, Vec f)
{
  Vec vec_func;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidHeaderSpecific(f, VEC_CLASSID, 2);
  PetscCheckSameComm(snes, 1, f, 2);
  if (snes->npcside == PC_LEFT && snes->functype == SNES_FUNCTION_PRECONDITIONED) {
    snes->vec_func_init_set = PETSC_FALSE;
    PetscFunctionReturn(PETSC_SUCCESS);
  }
  PetscCall(SNESGetFunction(snes, &vec_func, NULL, NULL));
  PetscCall(VecCopy(f, vec_func));

  snes->vec_func_init_set = PETSC_TRUE;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESSetNormSchedule - Sets the `SNESNormSchedule` used in convergence and monitoring
   of the `SNES` method, when norms are computed in the solving process

   Logically Collective

   Input Parameters:
+  snes - the `SNES` context
-  normschedule - the frequency of norm computation

   Options Database Key:
.  -snes_norm_schedule <none, always, initialonly, finalonly, initialfinalonly> - set the schedule

   Level: advanced

   Notes:
   Only certain `SNES` methods support certain `SNESNormSchedules`.  Most require evaluation
   of the nonlinear function and the taking of its norm at every iteration to
   even ensure convergence at all.  However, methods such as custom Gauss-Seidel methods
   `SNESNGS` and the like do not require the norm of the function to be computed, and therefore
   may either be monitored for convergence or not.  As these are often used as nonlinear
   preconditioners, monitoring the norm of their error is not a useful enterprise within
   their solution.

.seealso: [](chapter_snes), `SNESNormSchedule`, `SNESGetNormSchedule()`, `SNESComputeFunction()`, `VecNorm()`, `SNESSetFunction()`, `SNESSetInitialFunction()`, `SNESNormSchedule`
@*/
PetscErrorCode SNESSetNormSchedule(SNES snes, SNESNormSchedule normschedule)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  snes->normschedule = normschedule;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESGetNormSchedule - Gets the `SNESNormSchedule` used in convergence and monitoring
   of the `SNES` method.

   Logically Collective

   Input Parameters:
+  snes - the `SNES` context
-  normschedule - the type of the norm used

   Level: advanced

.seealso: [](chapter_snes), `SNES`, `SNESSetNormSchedule()`, `SNESComputeFunction()`, `VecNorm()`, `SNESSetFunction()`, `SNESSetInitialFunction()`, `SNESNormSchedule`
@*/
PetscErrorCode SNESGetNormSchedule(SNES snes, SNESNormSchedule *normschedule)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  *normschedule = snes->normschedule;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  SNESSetFunctionNorm - Sets the last computed residual norm.

  Logically Collective

  Input Parameters:
+  snes - the `SNES` context
-  norm - the value of the norm

  Level: developer

.seealso: [](chapter_snes), `SNES`, `SNESGetNormSchedule()`, `SNESComputeFunction()`, `VecNorm()`, `SNESSetFunction()`, `SNESSetInitialFunction()`, `SNESNormSchedule`
@*/
PetscErrorCode SNESSetFunctionNorm(SNES snes, PetscReal norm)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  snes->norm = norm;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  SNESGetFunctionNorm - Gets the last computed norm of the residual

  Not Collective

  Input Parameter:
. snes - the `SNES` context

  Output Parameter:
. norm - the last computed residual norm

  Level: developer

.seealso: [](chapter_snes), `SNES`, `SNESSetNormSchedule()`, `SNESComputeFunction()`, `VecNorm()`, `SNESSetFunction()`, `SNESSetInitialFunction()`, `SNESNormSchedule`
@*/
PetscErrorCode SNESGetFunctionNorm(SNES snes, PetscReal *norm)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidRealPointer(norm, 2);
  *norm = snes->norm;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  SNESGetUpdateNorm - Gets the last computed norm of the solution update

  Not Collective

  Input Parameter:
. snes - the `SNES` context

  Output Parameter:
. ynorm - the last computed update norm

  Level: developer

  Note:
  The new solution is the current solution plus the update, so this norm is an indication of the size of the update

.seealso: [](chapter_snes), `SNES`, `SNESSetNormSchedule()`, `SNESComputeFunction()`, `SNESGetFunctionNorm()`
@*/
PetscErrorCode SNESGetUpdateNorm(SNES snes, PetscReal *ynorm)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidRealPointer(ynorm, 2);
  *ynorm = snes->ynorm;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  SNESGetSolutionNorm - Gets the last computed norm of the solution

  Not Collective

  Input Parameter:
. snes - the `SNES` context

  Output Parameter:
. xnorm - the last computed solution norm

  Level: developer

.seealso: [](chapter_snes), `SNES`, `SNESSetNormSchedule()`, `SNESComputeFunction()`, `SNESGetFunctionNorm()`, `SNESGetUpdateNorm()`
@*/
PetscErrorCode SNESGetSolutionNorm(SNES snes, PetscReal *xnorm)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidRealPointer(xnorm, 2);
  *xnorm = snes->xnorm;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
   SNESSetFunctionType - Sets the `SNESFunctionType`
   of the `SNES` method.

   Logically Collective

   Input Parameters:
+  snes - the `SNES` context
-  type - the function type

   Level: developer

   Notes:
   Possible values of the function type
+  `SNES_FUNCTION_DEFAULT` - the default for the given `SNESType`
.  `SNES_FUNCTION_UNPRECONDITIONED` - an unpreconditioned function evaluation (this is the function provided with `SNESSetFunction()`
-  `SNES_FUNCTION_PRECONDITIONED` - a transformation of the function provided with `SNESSetFunction()`

   Different `SNESType`s use this value in different ways

.seealso: [](chapter_snes), `SNES`, `SNESFunctionType`, `SNESGetNormSchedule()`, `SNESComputeFunction()`, `VecNorm()`, `SNESSetFunction()`, `SNESSetInitialFunction()`, `SNESNormSchedule`
@*/
PetscErrorCode SNESSetFunctionType(SNES snes, SNESFunctionType type)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  snes->functype = type;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
   SNESGetFunctionType - Gets the `SNESFunctionType` used in convergence and monitoring set with `SNESSetFunctionType()`
   of the SNES method.

   Logically Collective

   Input Parameters:
+  snes - the `SNES` context
-  type - the type of the function evaluation, see `SNESSetFunctionType()`

   Level: advanced

.seealso: [](chapter_snes), `SNESSetFunctionType()`, `SNESFunctionType`, `SNESSetNormSchedule()`, `SNESComputeFunction()`, `VecNorm()`, `SNESSetFunction()`, `SNESSetInitialFunction()`, `SNESNormSchedule`
@*/
PetscErrorCode SNESGetFunctionType(SNES snes, SNESFunctionType *type)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  *type = snes->functype;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*MC
    SNESNGSFunction - function used to apply a Gauss-Seidel sweep on the nonlinear function

     Synopsis:
     #include <petscsnes.h>
$    SNESNGSFunction(SNES snes,Vec x,Vec b,void *ctx);

     Collective

     Input Parameters:
+  X   - solution vector
.  B   - RHS vector
-  ctx - optional user-defined Gauss-Seidel context

     Output Parameter:
.  X   - solution vector

   Level: intermediate

.seealso: [](chapter_snes), `SNESNGS`, `SNESSetNGS()`, `SNESGetNGS()`
M*/

/*@C
   SNESSetNGS - Sets the user nonlinear Gauss-Seidel routine for
   use with composed nonlinear solvers.

   Input Parameters:
+  snes   - the `SNES` context
.  f - function evaluation routine to apply Gauss-Seidel see `SNESNGSFunction`
-  ctx    - [optional] user-defined context for private data for the
            smoother evaluation routine (may be `NULL`)

   Calling sequence of `f`:
$  PetscErrorCode f(SNES snes, Vec X, Vec B, void *ctx)
+  snes - the `SNES` context
.  X - the current solution
.  B - the right hand side vector (which may be `NULL`)
-  ctx - a user provided context

   Level: intermediate

   Note:
   The `SNESNGS` routines are used by the composed nonlinear solver to generate
    a problem appropriate update to the solution, particularly `SNESFAS`.

.seealso: [](chapter_snes), `SNESGetNGS()`, `SNESNGSFunction`, `SNESNCG`, `SNESGetFunction()`, `SNESComputeNGS()`
@*/
PetscErrorCode SNESSetNGS(SNES snes, PetscErrorCode (*f)(SNES, Vec, Vec, void *), void *ctx)
{
  DM dm;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscCall(SNESGetDM(snes, &dm));
  PetscCall(DMSNESSetNGS(dm, f, ctx));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*
     This is used for -snes_mf_operator; it uses a duplicate of snes->jacobian_pre because snes->jacobian_pre cannot be
   changed during the KSPSolve()
*/
PetscErrorCode SNESPicardComputeMFFunction(SNES snes, Vec x, Vec f, void *ctx)
{
  DM     dm;
  DMSNES sdm;

  PetscFunctionBegin;
  PetscCall(SNESGetDM(snes, &dm));
  PetscCall(DMGetDMSNES(dm, &sdm));
  /*  A(x)*x - b(x) */
  if (sdm->ops->computepfunction) {
    PetscCallBack("SNES Picard callback function", (*sdm->ops->computepfunction)(snes, x, f, sdm->pctx));
    PetscCall(VecScale(f, -1.0));
    /* Cannot share nonzero pattern because of the possible use of SNESComputeJacobianDefault() */
    if (!snes->picard) PetscCall(MatDuplicate(snes->jacobian_pre, MAT_DO_NOT_COPY_VALUES, &snes->picard));
    PetscCallBack("SNES Picard callback Jacobian", (*sdm->ops->computepjacobian)(snes, x, snes->picard, snes->picard, sdm->pctx));
    PetscCall(MatMultAdd(snes->picard, x, f, f));
  } else {
    PetscCallBack("SNES Picard callback Jacobian", (*sdm->ops->computepjacobian)(snes, x, snes->picard, snes->picard, sdm->pctx));
    PetscCall(MatMult(snes->picard, x, f));
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

PetscErrorCode SNESPicardComputeFunction(SNES snes, Vec x, Vec f, void *ctx)
{
  DM     dm;
  DMSNES sdm;

  PetscFunctionBegin;
  PetscCall(SNESGetDM(snes, &dm));
  PetscCall(DMGetDMSNES(dm, &sdm));
  /*  A(x)*x - b(x) */
  if (sdm->ops->computepfunction) {
    PetscCallBack("SNES Picard callback function", (*sdm->ops->computepfunction)(snes, x, f, sdm->pctx));
    PetscCall(VecScale(f, -1.0));
    PetscCallBack("SNES Picard callback Jacobian", (*sdm->ops->computepjacobian)(snes, x, snes->jacobian, snes->jacobian_pre, sdm->pctx));
    PetscCall(MatMultAdd(snes->jacobian_pre, x, f, f));
  } else {
    PetscCallBack("SNES Picard callback Jacobian", (*sdm->ops->computepjacobian)(snes, x, snes->jacobian, snes->jacobian_pre, sdm->pctx));
    PetscCall(MatMult(snes->jacobian_pre, x, f));
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

PetscErrorCode SNESPicardComputeJacobian(SNES snes, Vec x1, Mat J, Mat B, void *ctx)
{
  PetscFunctionBegin;
  /* the jacobian matrix should be pre-filled in SNESPicardComputeFunction */
  /* must assembly if matrix-free to get the last SNES solution */
  PetscCall(MatAssemblyBegin(J, MAT_FINAL_ASSEMBLY));
  PetscCall(MatAssemblyEnd(J, MAT_FINAL_ASSEMBLY));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
   SNESSetPicard - Use `SNES` to solve the system A(x) x = bp(x) + b via a Picard type iteration (Picard linearization)

   Logically Collective

   Input Parameters:
+  snes - the `SNES` context
.  r - vector to store function values, may be `NULL`
.  bp - function evaluation routine, may be `NULL`
.  Amat - matrix with which A(x) x - bp(x) - b is to be computed
.  Pmat - matrix from which preconditioner is computed (usually the same as `Amat`)
.  J  - function to compute matrix values, for the calling sequence see `SNESJacobianFunction()`
-  ctx - [optional] user-defined context for private data for the function evaluation routine (may be `NULL`)

   Level: intermediate

   Notes:
    It is often better to provide the nonlinear function F() and some approximation to its Jacobian directly and use
    an approximate Newton solver. This interface is provided to allow porting/testing a previous Picard based code in PETSc before converting it to approximate Newton.

    One can call `SNESSetPicard()` or `SNESSetFunction()` (and possibly `SNESSetJacobian()`) but cannot call both

     Solves the equation A(x) x = bp(x) - b via the defect correction algorithm A(x^{n}) (x^{n+1} - x^{n}) = bp(x^{n}) + b - A(x^{n})x^{n}.
     When an exact solver is used this corresponds to the "classic" Picard A(x^{n}) x^{n+1} = bp(x^{n}) + b iteration.

     Run with `-snes_mf_operator` to solve the system with Newton's method using A(x^{n}) to construct the preconditioner.

   We implement the defect correction form of the Picard iteration because it converges much more generally when inexact linear solvers are used then
   the direct Picard iteration A(x^n) x^{n+1} = bp(x^n) + b

   There is some controversity over the definition of a Picard iteration for nonlinear systems but almost everyone agrees that it involves a linear solve and some
   believe it is the iteration  A(x^{n}) x^{n+1} = b(x^{n}) hence we use the name Picard. If anyone has an authoritative  reference that defines the Picard iteration
   different please contact us at petsc-dev@mcs.anl.gov and we'll have an entirely new argument :-).

   When used with `-snes_mf_operator` this will run matrix-free Newton's method where the matrix-vector product is of the true Jacobian of A(x)x - bp(x) -b and
    A(x^{n}) is used to build the preconditioner

   When used with `-snes_fd` this will compute the true Jacobian (very slowly one column at at time) and thus represent Newton's method.

   When used with `-snes_fd_coloring` this will compute the Jacobian via coloring and thus represent a faster implementation of Newton's method. But the
   the nonzero structure of the Jacobian is, in general larger than that of the Picard matrix A so you must provide in A the needed nonzero structure for the correct
   coloring. When using `DMDA` this may mean creating the matrix A with `DMCreateMatrix()` using a wider stencil than strictly needed for A or with a `DMDA_STENCIL_BOX`.
   See the commment in src/snes/tutorials/ex15.c.

.seealso: [](chapter_snes), `SNES`, `SNESGetFunction()`, `SNESSetFunction()`, `SNESComputeFunction()`, `SNESSetJacobian()`, `SNESGetPicard()`, `SNESLineSearchPreCheckPicard()`, `SNESJacobianFunction`
@*/
PetscErrorCode SNESSetPicard(SNES snes, Vec r, PetscErrorCode (*bp)(SNES, Vec, Vec, void *), Mat Amat, Mat Pmat, PetscErrorCode (*J)(SNES, Vec, Mat, Mat, void *), void *ctx)
{
  DM dm;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscCall(SNESGetDM(snes, &dm));
  PetscCall(DMSNESSetPicard(dm, bp, J, ctx));
  PetscCall(DMSNESSetMFFunction(dm, SNESPicardComputeMFFunction, ctx));
  PetscCall(SNESSetFunction(snes, r, SNESPicardComputeFunction, ctx));
  PetscCall(SNESSetJacobian(snes, Amat, Pmat, SNESPicardComputeJacobian, ctx));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
   SNESGetPicard - Returns the context for the Picard iteration

   Not Collective, but `Vec` is parallel if `SNES` is parallel. Collective if `Vec` is requested, but has not been created yet.

   Input Parameter:
.  snes - the `SNES` context

   Output Parameters:
+  r - the function (or `NULL`)
.  f - the function (or `NULL`);  for calling sequence see `SNESFunction`
.  Amat - the matrix used to defined the operation A(x) x - b(x) (or `NULL`)
.  Pmat  - the matrix from which the preconditioner will be constructed (or `NULL`)
.  J - the function for matrix evaluation (or `NULL`);  for calling sequence see `SNESJacobianFunction`
-  ctx - the function context (or `NULL`)

   Level: advanced

.seealso: [](chapter_snes), `SNESSetFunction()`, `SNESSetPicard()`, `SNESGetFunction()`, `SNESGetJacobian()`, `SNESGetDM()`, `SNESFunction`, `SNESJacobianFunction`
@*/
PetscErrorCode SNESGetPicard(SNES snes, Vec *r, PetscErrorCode (**f)(SNES, Vec, Vec, void *), Mat *Amat, Mat *Pmat, PetscErrorCode (**J)(SNES, Vec, Mat, Mat, void *), void **ctx)
{
  DM dm;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscCall(SNESGetFunction(snes, r, NULL, NULL));
  PetscCall(SNESGetJacobian(snes, Amat, Pmat, NULL, NULL));
  PetscCall(SNESGetDM(snes, &dm));
  PetscCall(DMSNESGetPicard(dm, f, J, ctx));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
   SNESSetComputeInitialGuess - Sets a routine used to compute an initial guess for the nonlinear problem

   Logically Collective

   Input Parameters:
+  snes - the `SNES` context
.  func - function evaluation routine
-  ctx - [optional] user-defined context for private data for the
         function evaluation routine (may be `NULL`)

   Calling sequence of `func`:
$    PetscErrorCode func(SNES snes, Vec x, void *ctx);
+  snes - the `SNES` solver
.  x - vector to put initial guess
-  ctx - optional user-defined function context

   Level: intermediate

.seealso: [](chapter_snes), `SNES`, `SNESSolve()`, `SNESGetFunction()`, `SNESComputeFunction()`, `SNESSetJacobian()`
@*/
PetscErrorCode SNESSetComputeInitialGuess(SNES snes, PetscErrorCode (*func)(SNES, Vec, void *), void *ctx)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  if (func) snes->ops->computeinitialguess = func;
  if (ctx) snes->initialguessP = ctx;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
   SNESGetRhs - Gets the vector for solving F(x) = `rhs`. If `rhs` is not set
   it assumes a zero right hand side.

   Logically Collective

   Input Parameter:
.  snes - the `SNES` context

   Output Parameter:
.  rhs - the right hand side vector or `NULL` if the right hand side vector is null

   Level: intermediate

.seealso: [](chapter_snes), `SNES`, `SNESGetSolution()`, `SNESGetFunction()`, `SNESComputeFunction()`, `SNESSetJacobian()`, `SNESSetFunction()`
@*/
PetscErrorCode SNESGetRhs(SNES snes, Vec *rhs)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidPointer(rhs, 2);
  *rhs = snes->vec_rhs;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESComputeFunction - Calls the function that has been set with `SNESSetFunction()`.

   Collective

   Input Parameters:
+  snes - the `SNES` context
-  x - input vector

   Output Parameter:
.  y - function vector, as set by `SNESSetFunction()`

   Level: developer

   Note:
   `SNESComputeFunction()` is typically used within nonlinear solvers
   implementations, so users would not generally call this routine themselves.

.seealso: [](chapter_snes), `SNES`, `SNESSetFunction()`, `SNESGetFunction()`, `SNESComputeMFFunction()`
@*/
PetscErrorCode SNESComputeFunction(SNES snes, Vec x, Vec y)
{
  DM     dm;
  DMSNES sdm;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidHeaderSpecific(x, VEC_CLASSID, 2);
  PetscValidHeaderSpecific(y, VEC_CLASSID, 3);
  PetscCheckSameComm(snes, 1, x, 2);
  PetscCheckSameComm(snes, 1, y, 3);
  PetscCall(VecValidValues_Internal(x, 2, PETSC_TRUE));

  PetscCall(SNESGetDM(snes, &dm));
  PetscCall(DMGetDMSNES(dm, &sdm));
  PetscCheck(sdm->ops->computefunction || snes->vec_rhs, PETSC_COMM_SELF, PETSC_ERR_ARG_WRONGSTATE, "Must call SNESSetFunction() or SNESSetDM() before SNESComputeFunction(), likely called from SNESSolve().");
  if (sdm->ops->computefunction) {
    if (sdm->ops->computefunction != SNESObjectiveComputeFunctionDefaultFD) PetscCall(PetscLogEventBegin(SNES_FunctionEval, snes, x, y, 0));
    PetscCall(VecLockReadPush(x));
    /* ensure domainerror is false prior to computefunction evaluation (may not have been reset) */
    snes->domainerror = PETSC_FALSE;
    {
      void *ctx;
      PetscErrorCode (*computefunction)(SNES, Vec, Vec, void *);
      PetscCall(DMSNESGetFunction(dm, &computefunction, &ctx));
      PetscCallBack("SNES callback function", (*computefunction)(snes, x, y, ctx));
    }
    PetscCall(VecLockReadPop(x));
    if (sdm->ops->computefunction != SNESObjectiveComputeFunctionDefaultFD) PetscCall(PetscLogEventEnd(SNES_FunctionEval, snes, x, y, 0));
  } else /* if (snes->vec_rhs) */ {
    PetscCall(MatMult(snes->jacobian, x, y));
  }
  if (snes->vec_rhs) PetscCall(VecAXPY(y, -1.0, snes->vec_rhs));
  snes->nfuncs++;
  /*
     domainerror might not be set on all processes; so we tag vector locally with Inf and the next inner product or norm will
     propagate the value to all processes
  */
  if (snes->domainerror) PetscCall(VecSetInf(y));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESComputeMFFunction - Calls the function that has been set with `SNESSetMFFunction()`.

   Collective

   Input Parameters:
+  snes - the `SNES` context
-  x - input vector

   Output Parameter:
.  y - function vector, as set by `SNESSetMFFunction()`

   Level: developer

   Notes:
   `SNESComputeMFFunction()` is used within the matrix vector products called by the matrix created with `MatCreateSNESMF()`
   so users would not generally call this routine themselves.

    Since this function is intended for use with finite differencing it does not subtract the right hand side vector provided with `SNESSolve()`
    while `SNESComputeFunction()` does. As such, this routine cannot be used with  `MatMFFDSetBase()` with a provided F function value even if it applies the
    same function as `SNESComputeFunction()` if a `SNESSolve()` right hand side vector is use because the two functions difference would include this right hand side function.

.seealso: [](chapter_snes), `SNES`, `SNESSetFunction()`, `SNESGetFunction()`, `SNESComputeFunction()`, `MatCreateSNESMF`
@*/
PetscErrorCode SNESComputeMFFunction(SNES snes, Vec x, Vec y)
{
  DM     dm;
  DMSNES sdm;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidHeaderSpecific(x, VEC_CLASSID, 2);
  PetscValidHeaderSpecific(y, VEC_CLASSID, 3);
  PetscCheckSameComm(snes, 1, x, 2);
  PetscCheckSameComm(snes, 1, y, 3);
  PetscCall(VecValidValues_Internal(x, 2, PETSC_TRUE));

  PetscCall(SNESGetDM(snes, &dm));
  PetscCall(DMGetDMSNES(dm, &sdm));
  PetscCall(PetscLogEventBegin(SNES_FunctionEval, snes, x, y, 0));
  PetscCall(VecLockReadPush(x));
  /* ensure domainerror is false prior to computefunction evaluation (may not have been reset) */
  snes->domainerror = PETSC_FALSE;
  PetscCallBack("SNES callback function", (*sdm->ops->computemffunction)(snes, x, y, sdm->mffunctionctx));
  PetscCall(VecLockReadPop(x));
  PetscCall(PetscLogEventEnd(SNES_FunctionEval, snes, x, y, 0));
  snes->nfuncs++;
  /*
     domainerror might not be set on all processes; so we tag vector locally with Inf and the next inner product or norm will
     propagate the value to all processes
  */
  if (snes->domainerror) PetscCall(VecSetInf(y));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESComputeNGS - Calls the Gauss-Seidel function that has been set with  `SNESSetNGS()`.

   Collective

   Input Parameters:
+  snes - the `SNES` context
.  x - input vector
-  b - rhs vector

   Output Parameter:
.  x - new solution vector

   Level: developer

   Note:
   `SNESComputeNGS()` is typically used within composed nonlinear solver
   implementations, so most users would not generally call this routine
   themselves.

.seealso: [](chapter_snes), `SNESNGS`, `SNESSetNGS()`, `SNESComputeFunction()`
@*/
PetscErrorCode SNESComputeNGS(SNES snes, Vec b, Vec x)
{
  DM     dm;
  DMSNES sdm;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidHeaderSpecific(x, VEC_CLASSID, 3);
  if (b) PetscValidHeaderSpecific(b, VEC_CLASSID, 2);
  PetscCheckSameComm(snes, 1, x, 3);
  if (b) PetscCheckSameComm(snes, 1, b, 2);
  if (b) PetscCall(VecValidValues_Internal(b, 2, PETSC_TRUE));
  PetscCall(PetscLogEventBegin(SNES_NGSEval, snes, x, b, 0));
  PetscCall(SNESGetDM(snes, &dm));
  PetscCall(DMGetDMSNES(dm, &sdm));
  PetscCheck(sdm->ops->computegs, PETSC_COMM_SELF, PETSC_ERR_ARG_WRONGSTATE, "Must call SNESSetNGS() before SNESComputeNGS(), likely called from SNESSolve().");
  if (b) PetscCall(VecLockReadPush(b));
  PetscCallBack("SNES callback NGS", (*sdm->ops->computegs)(snes, x, b, sdm->gsctx));
  if (b) PetscCall(VecLockReadPop(b));
  PetscCall(PetscLogEventEnd(SNES_NGSEval, snes, x, b, 0));
  PetscFunctionReturn(PETSC_SUCCESS);
}

PetscErrorCode SNESTestJacobian(SNES snes)
{
  Mat               A, B, C, D, jacobian;
  Vec               x = snes->vec_sol, f = snes->vec_func;
  PetscReal         nrm, gnorm;
  PetscReal         threshold = 1.e-5;
  MatType           mattype;
  PetscInt          m, n, M, N;
  void             *functx;
  PetscBool         complete_print = PETSC_FALSE, threshold_print = PETSC_FALSE, test = PETSC_FALSE, flg, istranspose;
  PetscViewer       viewer, mviewer;
  MPI_Comm          comm;
  PetscInt          tabs;
  static PetscBool  directionsprinted = PETSC_FALSE;
  PetscViewerFormat format;

  PetscFunctionBegin;
  PetscObjectOptionsBegin((PetscObject)snes);
  PetscCall(PetscOptionsName("-snes_test_jacobian", "Compare hand-coded and finite difference Jacobians", "None", &test));
  PetscCall(PetscOptionsReal("-snes_test_jacobian", "Threshold for element difference between hand-coded and finite difference being meaningful", "None", threshold, &threshold, NULL));
  PetscCall(PetscOptionsViewer("-snes_test_jacobian_view", "View difference between hand-coded and finite difference Jacobians element entries", "None", &mviewer, &format, &complete_print));
  if (!complete_print) {
    PetscCall(PetscOptionsDeprecated("-snes_test_jacobian_display", "-snes_test_jacobian_view", "3.13", NULL));
    PetscCall(PetscOptionsViewer("-snes_test_jacobian_display", "Display difference between hand-coded and finite difference Jacobians", "None", &mviewer, &format, &complete_print));
  }
  /* for compatibility with PETSc 3.9 and older. */
  PetscCall(PetscOptionsDeprecated("-snes_test_jacobian_display_threshold", "-snes_test_jacobian", "3.13", "-snes_test_jacobian accepts an optional threshold (since v3.10)"));
  PetscCall(PetscOptionsReal("-snes_test_jacobian_display_threshold", "Display difference between hand-coded and finite difference Jacobians which exceed input threshold", "None", threshold, &threshold, &threshold_print));
  PetscOptionsEnd();
  if (!test) PetscFunctionReturn(PETSC_SUCCESS);

  PetscCall(PetscObjectGetComm((PetscObject)snes, &comm));
  PetscCall(PetscViewerASCIIGetStdout(comm, &viewer));
  PetscCall(PetscViewerASCIIGetTab(viewer, &tabs));
  PetscCall(PetscViewerASCIISetTab(viewer, ((PetscObject)snes)->tablevel));
  PetscCall(PetscViewerASCIIPrintf(viewer, "  ---------- Testing Jacobian -------------\n"));
  if (!complete_print && !directionsprinted) {
    PetscCall(PetscViewerASCIIPrintf(viewer, "  Run with -snes_test_jacobian_view and optionally -snes_test_jacobian <threshold> to show difference\n"));
    PetscCall(PetscViewerASCIIPrintf(viewer, "    of hand-coded and finite difference Jacobian entries greater than <threshold>.\n"));
  }
  if (!directionsprinted) {
    PetscCall(PetscViewerASCIIPrintf(viewer, "  Testing hand-coded Jacobian, if (for double precision runs) ||J - Jfd||_F/||J||_F is\n"));
    PetscCall(PetscViewerASCIIPrintf(viewer, "    O(1.e-8), the hand-coded Jacobian is probably correct.\n"));
    directionsprinted = PETSC_TRUE;
  }
  if (complete_print) PetscCall(PetscViewerPushFormat(mviewer, format));

  PetscCall(PetscObjectTypeCompare((PetscObject)snes->jacobian, MATMFFD, &flg));
  if (!flg) jacobian = snes->jacobian;
  else jacobian = snes->jacobian_pre;

  if (!x) {
    PetscCall(MatCreateVecs(jacobian, &x, NULL));
  } else {
    PetscCall(PetscObjectReference((PetscObject)x));
  }
  if (!f) {
    PetscCall(VecDuplicate(x, &f));
  } else {
    PetscCall(PetscObjectReference((PetscObject)f));
  }
  /* evaluate the function at this point because SNESComputeJacobianDefault() assumes that the function has been evaluated and put into snes->vec_func */
  PetscCall(SNESComputeFunction(snes, x, f));
  PetscCall(VecDestroy(&f));
  PetscCall(PetscObjectTypeCompare((PetscObject)snes, SNESKSPTRANSPOSEONLY, &istranspose));
  while (jacobian) {
    Mat JT = NULL, Jsave = NULL;

    if (istranspose) {
      PetscCall(MatCreateTranspose(jacobian, &JT));
      Jsave    = jacobian;
      jacobian = JT;
    }
    PetscCall(PetscObjectBaseTypeCompareAny((PetscObject)jacobian, &flg, MATSEQAIJ, MATMPIAIJ, MATSEQDENSE, MATMPIDENSE, MATSEQBAIJ, MATMPIBAIJ, MATSEQSBAIJ, MATMPISBAIJ, ""));
    if (flg) {
      A = jacobian;
      PetscCall(PetscObjectReference((PetscObject)A));
    } else {
      PetscCall(MatComputeOperator(jacobian, MATAIJ, &A));
    }

    PetscCall(MatGetType(A, &mattype));
    PetscCall(MatGetSize(A, &M, &N));
    PetscCall(MatGetLocalSize(A, &m, &n));
    PetscCall(MatCreate(PetscObjectComm((PetscObject)A), &B));
    PetscCall(MatSetType(B, mattype));
    PetscCall(MatSetSizes(B, m, n, M, N));
    PetscCall(MatSetBlockSizesFromMats(B, A, A));
    PetscCall(MatSetUp(B));
    PetscCall(MatSetOption(B, MAT_NEW_NONZERO_ALLOCATION_ERR, PETSC_FALSE));

    PetscCall(SNESGetFunction(snes, NULL, NULL, &functx));
    PetscCall(SNESComputeJacobianDefault(snes, x, B, B, functx));

    PetscCall(MatDuplicate(B, MAT_COPY_VALUES, &D));
    PetscCall(MatAYPX(D, -1.0, A, DIFFERENT_NONZERO_PATTERN));
    PetscCall(MatNorm(D, NORM_FROBENIUS, &nrm));
    PetscCall(MatNorm(A, NORM_FROBENIUS, &gnorm));
    PetscCall(MatDestroy(&D));
    if (!gnorm) gnorm = 1; /* just in case */
    PetscCall(PetscViewerASCIIPrintf(viewer, "  ||J - Jfd||_F/||J||_F = %g, ||J - Jfd||_F = %g\n", (double)(nrm / gnorm), (double)nrm));

    if (complete_print) {
      PetscCall(PetscViewerASCIIPrintf(viewer, "  Hand-coded Jacobian ----------\n"));
      PetscCall(MatView(A, mviewer));
      PetscCall(PetscViewerASCIIPrintf(viewer, "  Finite difference Jacobian ----------\n"));
      PetscCall(MatView(B, mviewer));
    }

    if (threshold_print || complete_print) {
      PetscInt           Istart, Iend, *ccols, bncols, cncols, j, row;
      PetscScalar       *cvals;
      const PetscInt    *bcols;
      const PetscScalar *bvals;

      PetscCall(MatCreate(PetscObjectComm((PetscObject)A), &C));
      PetscCall(MatSetType(C, mattype));
      PetscCall(MatSetSizes(C, m, n, M, N));
      PetscCall(MatSetBlockSizesFromMats(C, A, A));
      PetscCall(MatSetUp(C));
      PetscCall(MatSetOption(C, MAT_NEW_NONZERO_ALLOCATION_ERR, PETSC_FALSE));

      PetscCall(MatAYPX(B, -1.0, A, DIFFERENT_NONZERO_PATTERN));
      PetscCall(MatGetOwnershipRange(B, &Istart, &Iend));

      for (row = Istart; row < Iend; row++) {
        PetscCall(MatGetRow(B, row, &bncols, &bcols, &bvals));
        PetscCall(PetscMalloc2(bncols, &ccols, bncols, &cvals));
        for (j = 0, cncols = 0; j < bncols; j++) {
          if (PetscAbsScalar(bvals[j]) > threshold) {
            ccols[cncols] = bcols[j];
            cvals[cncols] = bvals[j];
            cncols += 1;
          }
        }
        if (cncols) PetscCall(MatSetValues(C, 1, &row, cncols, ccols, cvals, INSERT_VALUES));
        PetscCall(MatRestoreRow(B, row, &bncols, &bcols, &bvals));
        PetscCall(PetscFree2(ccols, cvals));
      }
      PetscCall(MatAssemblyBegin(C, MAT_FINAL_ASSEMBLY));
      PetscCall(MatAssemblyEnd(C, MAT_FINAL_ASSEMBLY));
      PetscCall(PetscViewerASCIIPrintf(viewer, "  Hand-coded minus finite-difference Jacobian with tolerance %g ----------\n", (double)threshold));
      PetscCall(MatView(C, complete_print ? mviewer : viewer));
      PetscCall(MatDestroy(&C));
    }
    PetscCall(MatDestroy(&A));
    PetscCall(MatDestroy(&B));
    PetscCall(MatDestroy(&JT));
    if (Jsave) jacobian = Jsave;
    if (jacobian != snes->jacobian_pre) {
      jacobian = snes->jacobian_pre;
      PetscCall(PetscViewerASCIIPrintf(viewer, "  ---------- Testing Jacobian for preconditioner -------------\n"));
    } else jacobian = NULL;
  }
  PetscCall(VecDestroy(&x));
  if (complete_print) PetscCall(PetscViewerPopFormat(mviewer));
  if (mviewer) PetscCall(PetscViewerDestroy(&mviewer));
  PetscCall(PetscViewerASCIISetTab(viewer, tabs));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESComputeJacobian - Computes the Jacobian matrix that has been set with `SNESSetJacobian()`.

   Collective

   Input Parameters:
+  snes - the `SNES` context
-  x - input vector

   Output Parameters:
+  A - Jacobian matrix
-  B - optional matrix for building the preconditioner

  Options Database Keys:
+    -snes_lag_preconditioner <lag> - how often to rebuild preconditioner
.    -snes_lag_jacobian <lag> - how often to rebuild Jacobian
.    -snes_test_jacobian <optional threshold> - compare the user provided Jacobian with one compute via finite differences to check for errors.  If a threshold is given, display only those entries whose difference is greater than the threshold.
.    -snes_test_jacobian_view - display the user provided Jacobian, the finite difference Jacobian and the difference between them to help users detect the location of errors in the user provided Jacobian
.    -snes_compare_explicit - Compare the computed Jacobian to the finite difference Jacobian and output the differences
.    -snes_compare_explicit_draw  - Compare the computed Jacobian to the finite difference Jacobian and draw the result
.    -snes_compare_explicit_contour  - Compare the computed Jacobian to the finite difference Jacobian and draw a contour plot with the result
.    -snes_compare_operator  - Make the comparison options above use the operator instead of the preconditioning matrix
.    -snes_compare_coloring - Compute the finite difference Jacobian using coloring and display norms of difference
.    -snes_compare_coloring_display - Compute the finite difference Jacobian using coloring and display verbose differences
.    -snes_compare_coloring_threshold - Display only those matrix entries that differ by more than a given threshold
.    -snes_compare_coloring_threshold_atol - Absolute tolerance for difference in matrix entries to be displayed by `-snes_compare_coloring_threshold`
.    -snes_compare_coloring_threshold_rtol - Relative tolerance for difference in matrix entries to be displayed by `-snes_compare_coloring_threshold`
.    -snes_compare_coloring_draw - Compute the finite difference Jacobian using coloring and draw differences
-    -snes_compare_coloring_draw_contour - Compute the finite difference Jacobian using coloring and show contours of matrices and differences

   Level: developer

   Note:
   Most users should not need to explicitly call this routine, as it
   is used internally within the nonlinear solvers.

   Developer Note:
    This has duplicative ways of checking the accuracy of the user provided Jacobian (see the options above). This is for historical reasons, the routine `SNESTestJacobian()` use to used
      for with the `SNESType` of test that has been removed.

.seealso: [](chapter_snes), `SNESSetJacobian()`, `KSPSetOperators()`, `MatStructure`, `SNESSetLagPreconditioner()`, `SNESSetLagJacobian()`
@*/
PetscErrorCode SNESComputeJacobian(SNES snes, Vec X, Mat A, Mat B)
{
  PetscBool flag;
  DM        dm;
  DMSNES    sdm;
  KSP       ksp;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidHeaderSpecific(X, VEC_CLASSID, 2);
  PetscCheckSameComm(snes, 1, X, 2);
  PetscCall(VecValidValues_Internal(X, 2, PETSC_TRUE));
  PetscCall(SNESGetDM(snes, &dm));
  PetscCall(DMGetDMSNES(dm, &sdm));

  /* make sure that MatAssemblyBegin/End() is called on A matrix if it is matrix free */
  if (snes->lagjacobian == -2) {
    snes->lagjacobian = -1;

    PetscCall(PetscInfo(snes, "Recomputing Jacobian/preconditioner because lag is -2 (means compute Jacobian, but then never again) \n"));
  } else if (snes->lagjacobian == -1) {
    PetscCall(PetscInfo(snes, "Reusing Jacobian/preconditioner because lag is -1\n"));
    PetscCall(PetscObjectTypeCompare((PetscObject)A, MATMFFD, &flag));
    if (flag) {
      PetscCall(MatAssemblyBegin(A, MAT_FINAL_ASSEMBLY));
      PetscCall(MatAssemblyEnd(A, MAT_FINAL_ASSEMBLY));
    }
    PetscFunctionReturn(PETSC_SUCCESS);
  } else if (snes->lagjacobian > 1 && (snes->iter + snes->jac_iter) % snes->lagjacobian) {
    PetscCall(PetscInfo(snes, "Reusing Jacobian/preconditioner because lag is %" PetscInt_FMT " and SNES iteration is %" PetscInt_FMT "\n", snes->lagjacobian, snes->iter));
    PetscCall(PetscObjectTypeCompare((PetscObject)A, MATMFFD, &flag));
    if (flag) {
      PetscCall(MatAssemblyBegin(A, MAT_FINAL_ASSEMBLY));
      PetscCall(MatAssemblyEnd(A, MAT_FINAL_ASSEMBLY));
    }
    PetscFunctionReturn(PETSC_SUCCESS);
  }
  if (snes->npc && snes->npcside == PC_LEFT) {
    PetscCall(MatAssemblyBegin(A, MAT_FINAL_ASSEMBLY));
    PetscCall(MatAssemblyEnd(A, MAT_FINAL_ASSEMBLY));
    PetscFunctionReturn(PETSC_SUCCESS);
  }

  PetscCall(PetscLogEventBegin(SNES_JacobianEval, snes, X, A, B));
  PetscCall(VecLockReadPush(X));
  {
    void *ctx;
    PetscErrorCode (*J)(SNES, Vec, Mat, Mat, void *);
    PetscCall(DMSNESGetJacobian(dm, &J, &ctx));
    PetscCallBack("SNES callback Jacobian", (*J)(snes, X, A, B, ctx));
  }
  PetscCall(VecLockReadPop(X));
  PetscCall(PetscLogEventEnd(SNES_JacobianEval, snes, X, A, B));

  /* attach latest linearization point to the preconditioning matrix */
  PetscCall(PetscObjectCompose((PetscObject)B, "__SNES_latest_X", (PetscObject)X));

  /* the next line ensures that snes->ksp exists */
  PetscCall(SNESGetKSP(snes, &ksp));
  if (snes->lagpreconditioner == -2) {
    PetscCall(PetscInfo(snes, "Rebuilding preconditioner exactly once since lag is -2\n"));
    PetscCall(KSPSetReusePreconditioner(snes->ksp, PETSC_FALSE));
    snes->lagpreconditioner = -1;
  } else if (snes->lagpreconditioner == -1) {
    PetscCall(PetscInfo(snes, "Reusing preconditioner because lag is -1\n"));
    PetscCall(KSPSetReusePreconditioner(snes->ksp, PETSC_TRUE));
  } else if (snes->lagpreconditioner > 1 && (snes->iter + snes->pre_iter) % snes->lagpreconditioner) {
    PetscCall(PetscInfo(snes, "Reusing preconditioner because lag is %" PetscInt_FMT " and SNES iteration is %" PetscInt_FMT "\n", snes->lagpreconditioner, snes->iter));
    PetscCall(KSPSetReusePreconditioner(snes->ksp, PETSC_TRUE));
  } else {
    PetscCall(PetscInfo(snes, "Rebuilding preconditioner\n"));
    PetscCall(KSPSetReusePreconditioner(snes->ksp, PETSC_FALSE));
  }

  PetscCall(SNESTestJacobian(snes));
  /* make sure user returned a correct Jacobian and preconditioner */
  /* PetscValidHeaderSpecific(A,MAT_CLASSID,3);
    PetscValidHeaderSpecific(B,MAT_CLASSID,4);   */
  {
    PetscBool flag = PETSC_FALSE, flag_draw = PETSC_FALSE, flag_contour = PETSC_FALSE, flag_operator = PETSC_FALSE;
    PetscCall(PetscOptionsGetViewer(PetscObjectComm((PetscObject)snes), ((PetscObject)snes)->options, ((PetscObject)snes)->prefix, "-snes_compare_explicit", NULL, NULL, &flag));
    PetscCall(PetscOptionsGetViewer(PetscObjectComm((PetscObject)snes), ((PetscObject)snes)->options, ((PetscObject)snes)->prefix, "-snes_compare_explicit_draw", NULL, NULL, &flag_draw));
    PetscCall(PetscOptionsGetViewer(PetscObjectComm((PetscObject)snes), ((PetscObject)snes)->options, ((PetscObject)snes)->prefix, "-snes_compare_explicit_draw_contour", NULL, NULL, &flag_contour));
    PetscCall(PetscOptionsGetViewer(PetscObjectComm((PetscObject)snes), ((PetscObject)snes)->options, ((PetscObject)snes)->prefix, "-snes_compare_operator", NULL, NULL, &flag_operator));
    if (flag || flag_draw || flag_contour) {
      Mat         Bexp_mine = NULL, Bexp, FDexp;
      PetscViewer vdraw, vstdout;
      PetscBool   flg;
      if (flag_operator) {
        PetscCall(MatComputeOperator(A, MATAIJ, &Bexp_mine));
        Bexp = Bexp_mine;
      } else {
        /* See if the preconditioning matrix can be viewed and added directly */
        PetscCall(PetscObjectBaseTypeCompareAny((PetscObject)B, &flg, MATSEQAIJ, MATMPIAIJ, MATSEQDENSE, MATMPIDENSE, MATSEQBAIJ, MATMPIBAIJ, MATSEQSBAIJ, MATMPIBAIJ, ""));
        if (flg) Bexp = B;
        else {
          /* If the "preconditioning" matrix is itself MATSHELL or some other type without direct support */
          PetscCall(MatComputeOperator(B, MATAIJ, &Bexp_mine));
          Bexp = Bexp_mine;
        }
      }
      PetscCall(MatConvert(Bexp, MATSAME, MAT_INITIAL_MATRIX, &FDexp));
      PetscCall(SNESComputeJacobianDefault(snes, X, FDexp, FDexp, NULL));
      PetscCall(PetscViewerASCIIGetStdout(PetscObjectComm((PetscObject)snes), &vstdout));
      if (flag_draw || flag_contour) {
        PetscCall(PetscViewerDrawOpen(PetscObjectComm((PetscObject)snes), NULL, "Explicit Jacobians", PETSC_DECIDE, PETSC_DECIDE, 300, 300, &vdraw));
        if (flag_contour) PetscCall(PetscViewerPushFormat(vdraw, PETSC_VIEWER_DRAW_CONTOUR));
      } else vdraw = NULL;
      PetscCall(PetscViewerASCIIPrintf(vstdout, "Explicit %s\n", flag_operator ? "Jacobian" : "preconditioning Jacobian"));
      if (flag) PetscCall(MatView(Bexp, vstdout));
      if (vdraw) PetscCall(MatView(Bexp, vdraw));
      PetscCall(PetscViewerASCIIPrintf(vstdout, "Finite difference Jacobian\n"));
      if (flag) PetscCall(MatView(FDexp, vstdout));
      if (vdraw) PetscCall(MatView(FDexp, vdraw));
      PetscCall(MatAYPX(FDexp, -1.0, Bexp, SAME_NONZERO_PATTERN));
      PetscCall(PetscViewerASCIIPrintf(vstdout, "User-provided matrix minus finite difference Jacobian\n"));
      if (flag) PetscCall(MatView(FDexp, vstdout));
      if (vdraw) { /* Always use contour for the difference */
        PetscCall(PetscViewerPushFormat(vdraw, PETSC_VIEWER_DRAW_CONTOUR));
        PetscCall(MatView(FDexp, vdraw));
        PetscCall(PetscViewerPopFormat(vdraw));
      }
      if (flag_contour) PetscCall(PetscViewerPopFormat(vdraw));
      PetscCall(PetscViewerDestroy(&vdraw));
      PetscCall(MatDestroy(&Bexp_mine));
      PetscCall(MatDestroy(&FDexp));
    }
  }
  {
    PetscBool flag = PETSC_FALSE, flag_display = PETSC_FALSE, flag_draw = PETSC_FALSE, flag_contour = PETSC_FALSE, flag_threshold = PETSC_FALSE;
    PetscReal threshold_atol = PETSC_SQRT_MACHINE_EPSILON, threshold_rtol = 10 * PETSC_SQRT_MACHINE_EPSILON;
    PetscCall(PetscOptionsGetViewer(PetscObjectComm((PetscObject)snes), ((PetscObject)snes)->options, ((PetscObject)snes)->prefix, "-snes_compare_coloring", NULL, NULL, &flag));
    PetscCall(PetscOptionsGetViewer(PetscObjectComm((PetscObject)snes), ((PetscObject)snes)->options, ((PetscObject)snes)->prefix, "-snes_compare_coloring_display", NULL, NULL, &flag_display));
    PetscCall(PetscOptionsGetViewer(PetscObjectComm((PetscObject)snes), ((PetscObject)snes)->options, ((PetscObject)snes)->prefix, "-snes_compare_coloring_draw", NULL, NULL, &flag_draw));
    PetscCall(PetscOptionsGetViewer(PetscObjectComm((PetscObject)snes), ((PetscObject)snes)->options, ((PetscObject)snes)->prefix, "-snes_compare_coloring_draw_contour", NULL, NULL, &flag_contour));
    PetscCall(PetscOptionsGetViewer(PetscObjectComm((PetscObject)snes), ((PetscObject)snes)->options, ((PetscObject)snes)->prefix, "-snes_compare_coloring_threshold", NULL, NULL, &flag_threshold));
    if (flag_threshold) {
      PetscCall(PetscOptionsGetReal(((PetscObject)snes)->options, ((PetscObject)snes)->prefix, "-snes_compare_coloring_threshold_rtol", &threshold_rtol, NULL));
      PetscCall(PetscOptionsGetReal(((PetscObject)snes)->options, ((PetscObject)snes)->prefix, "-snes_compare_coloring_threshold_atol", &threshold_atol, NULL));
    }
    if (flag || flag_display || flag_draw || flag_contour || flag_threshold) {
      Mat           Bfd;
      PetscViewer   vdraw, vstdout;
      MatColoring   coloring;
      ISColoring    iscoloring;
      MatFDColoring matfdcoloring;
      PetscErrorCode (*func)(SNES, Vec, Vec, void *);
      void     *funcctx;
      PetscReal norm1, norm2, normmax;

      PetscCall(MatDuplicate(B, MAT_DO_NOT_COPY_VALUES, &Bfd));
      PetscCall(MatColoringCreate(Bfd, &coloring));
      PetscCall(MatColoringSetType(coloring, MATCOLORINGSL));
      PetscCall(MatColoringSetFromOptions(coloring));
      PetscCall(MatColoringApply(coloring, &iscoloring));
      PetscCall(MatColoringDestroy(&coloring));
      PetscCall(MatFDColoringCreate(Bfd, iscoloring, &matfdcoloring));
      PetscCall(MatFDColoringSetFromOptions(matfdcoloring));
      PetscCall(MatFDColoringSetUp(Bfd, iscoloring, matfdcoloring));
      PetscCall(ISColoringDestroy(&iscoloring));

      /* This method of getting the function is currently unreliable since it doesn't work for DM local functions. */
      PetscCall(SNESGetFunction(snes, NULL, &func, &funcctx));
      PetscCall(MatFDColoringSetFunction(matfdcoloring, (PetscErrorCode(*)(void))func, funcctx));
      PetscCall(PetscObjectSetOptionsPrefix((PetscObject)matfdcoloring, ((PetscObject)snes)->prefix));
      PetscCall(PetscObjectAppendOptionsPrefix((PetscObject)matfdcoloring, "coloring_"));
      PetscCall(MatFDColoringSetFromOptions(matfdcoloring));
      PetscCall(MatFDColoringApply(Bfd, matfdcoloring, X, snes));
      PetscCall(MatFDColoringDestroy(&matfdcoloring));

      PetscCall(PetscViewerASCIIGetStdout(PetscObjectComm((PetscObject)snes), &vstdout));
      if (flag_draw || flag_contour) {
        PetscCall(PetscViewerDrawOpen(PetscObjectComm((PetscObject)snes), NULL, "Colored Jacobians", PETSC_DECIDE, PETSC_DECIDE, 300, 300, &vdraw));
        if (flag_contour) PetscCall(PetscViewerPushFormat(vdraw, PETSC_VIEWER_DRAW_CONTOUR));
      } else vdraw = NULL;
      PetscCall(PetscViewerASCIIPrintf(vstdout, "Explicit preconditioning Jacobian\n"));
      if (flag_display) PetscCall(MatView(B, vstdout));
      if (vdraw) PetscCall(MatView(B, vdraw));
      PetscCall(PetscViewerASCIIPrintf(vstdout, "Colored Finite difference Jacobian\n"));
      if (flag_display) PetscCall(MatView(Bfd, vstdout));
      if (vdraw) PetscCall(MatView(Bfd, vdraw));
      PetscCall(MatAYPX(Bfd, -1.0, B, SAME_NONZERO_PATTERN));
      PetscCall(MatNorm(Bfd, NORM_1, &norm1));
      PetscCall(MatNorm(Bfd, NORM_FROBENIUS, &norm2));
      PetscCall(MatNorm(Bfd, NORM_MAX, &normmax));
      PetscCall(PetscViewerASCIIPrintf(vstdout, "User-provided matrix minus finite difference Jacobian, norm1=%g normFrob=%g normmax=%g\n", (double)norm1, (double)norm2, (double)normmax));
      if (flag_display) PetscCall(MatView(Bfd, vstdout));
      if (vdraw) { /* Always use contour for the difference */
        PetscCall(PetscViewerPushFormat(vdraw, PETSC_VIEWER_DRAW_CONTOUR));
        PetscCall(MatView(Bfd, vdraw));
        PetscCall(PetscViewerPopFormat(vdraw));
      }
      if (flag_contour) PetscCall(PetscViewerPopFormat(vdraw));

      if (flag_threshold) {
        PetscInt bs, rstart, rend, i;
        PetscCall(MatGetBlockSize(B, &bs));
        PetscCall(MatGetOwnershipRange(B, &rstart, &rend));
        for (i = rstart; i < rend; i++) {
          const PetscScalar *ba, *ca;
          const PetscInt    *bj, *cj;
          PetscInt           bn, cn, j, maxentrycol = -1, maxdiffcol = -1, maxrdiffcol = -1;
          PetscReal          maxentry = 0, maxdiff = 0, maxrdiff = 0;
          PetscCall(MatGetRow(B, i, &bn, &bj, &ba));
          PetscCall(MatGetRow(Bfd, i, &cn, &cj, &ca));
          PetscCheck(bn == cn, ((PetscObject)A)->comm, PETSC_ERR_PLIB, "Unexpected different nonzero pattern in -snes_compare_coloring_threshold");
          for (j = 0; j < bn; j++) {
            PetscReal rdiff = PetscAbsScalar(ca[j]) / (threshold_atol + threshold_rtol * PetscAbsScalar(ba[j]));
            if (PetscAbsScalar(ba[j]) > PetscAbs(maxentry)) {
              maxentrycol = bj[j];
              maxentry    = PetscRealPart(ba[j]);
            }
            if (PetscAbsScalar(ca[j]) > PetscAbs(maxdiff)) {
              maxdiffcol = bj[j];
              maxdiff    = PetscRealPart(ca[j]);
            }
            if (rdiff > maxrdiff) {
              maxrdiffcol = bj[j];
              maxrdiff    = rdiff;
            }
          }
          if (maxrdiff > 1) {
            PetscCall(PetscViewerASCIIPrintf(vstdout, "row %" PetscInt_FMT " (maxentry=%g at %" PetscInt_FMT ", maxdiff=%g at %" PetscInt_FMT ", maxrdiff=%g at %" PetscInt_FMT "):", i, (double)maxentry, maxentrycol, (double)maxdiff, maxdiffcol, (double)maxrdiff, maxrdiffcol));
            for (j = 0; j < bn; j++) {
              PetscReal rdiff;
              rdiff = PetscAbsScalar(ca[j]) / (threshold_atol + threshold_rtol * PetscAbsScalar(ba[j]));
              if (rdiff > 1) PetscCall(PetscViewerASCIIPrintf(vstdout, " (%" PetscInt_FMT ",%g:%g)", bj[j], (double)PetscRealPart(ba[j]), (double)PetscRealPart(ca[j])));
            }
            PetscCall(PetscViewerASCIIPrintf(vstdout, "\n"));
          }
          PetscCall(MatRestoreRow(B, i, &bn, &bj, &ba));
          PetscCall(MatRestoreRow(Bfd, i, &cn, &cj, &ca));
        }
      }
      PetscCall(PetscViewerDestroy(&vdraw));
      PetscCall(MatDestroy(&Bfd));
    }
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*MC
    SNESJacobianFunction - Function used by `SNES` to compute the nonlinear Jacobian of the function to be solved by `SNES`

     Synopsis:
     #include "petscsnes.h"
     PetscErrorCode SNESJacobianFunction(SNES snes,Vec x,Mat Amat,Mat Pmat,void *ctx);

     Collective

    Input Parameters:
+  x - input vector, the Jacobian is to be computed at this value
-  ctx - [optional] user-defined Jacobian context

    Output Parameters:
+  Amat - the matrix that defines the (approximate) Jacobian
-  Pmat - the matrix to be used in constructing the preconditioner, usually the same as `Amat`.

   Level: intermediate

.seealso: [](chapter_snes), `SNES`, `SNESSetFunction()`, `SNESGetFunction()`, `SNESSetJacobian()`, `SNESGetJacobian()`
M*/

/*@C
   SNESSetJacobian - Sets the function to compute Jacobian as well as the
   location to store the matrix.

   Logically Collective

   Input Parameters:
+  snes - the `SNES` context
.  Amat - the matrix that defines the (approximate) Jacobian
.  Pmat - the matrix to be used in constructing the preconditioner, usually the same as `Amat`.
.  J - Jacobian evaluation routine (if `NULL` then `SNES` retains any previously set value), see `SNESJacobianFunction` for details
-  ctx - [optional] user-defined context for private data for the
         Jacobian evaluation routine (may be `NULL`) (if `NULL` then `SNES` retains any previously set value)

   Level: beginner

   Notes:
   If the `Amat` matrix and `Pmat` matrix are different you must call `MatAssemblyBegin()`/`MatAssemblyEnd()` on
   each matrix.

   If you know the operator `Amat` has a null space you can use `MatSetNullSpace()` and `MatSetTransposeNullSpace()` to supply the null
   space to `Amat` and the `KSP` solvers will automatically use that null space as needed during the solution process.

   If using `SNESComputeJacobianDefaultColor()` to assemble a Jacobian, the `ctx` argument
   must be a `MatFDColoring`.

   Other defect-correction schemes can be used by computing a different matrix in place of the Jacobian.  One common
   example is to use the "Picard linearization" which only differentiates through the highest order parts of each term using `SNESSetPicard()`

.seealso: [](chapter_snes), `SNES`, `KSPSetOperators()`, `SNESSetFunction()`, `MatMFFDComputeJacobian()`, `SNESComputeJacobianDefaultColor()`, `MatStructure`,
          `SNESSetPicard()`, `SNESJacobianFunction`
@*/
PetscErrorCode SNESSetJacobian(SNES snes, Mat Amat, Mat Pmat, PetscErrorCode (*J)(SNES, Vec, Mat, Mat, void *), void *ctx)
{
  DM dm;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  if (Amat) PetscValidHeaderSpecific(Amat, MAT_CLASSID, 2);
  if (Pmat) PetscValidHeaderSpecific(Pmat, MAT_CLASSID, 3);
  if (Amat) PetscCheckSameComm(snes, 1, Amat, 2);
  if (Pmat) PetscCheckSameComm(snes, 1, Pmat, 3);
  PetscCall(SNESGetDM(snes, &dm));
  PetscCall(DMSNESSetJacobian(dm, J, ctx));
  if (Amat) {
    PetscCall(PetscObjectReference((PetscObject)Amat));
    PetscCall(MatDestroy(&snes->jacobian));

    snes->jacobian = Amat;
  }
  if (Pmat) {
    PetscCall(PetscObjectReference((PetscObject)Pmat));
    PetscCall(MatDestroy(&snes->jacobian_pre));

    snes->jacobian_pre = Pmat;
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
   SNESGetJacobian - Returns the Jacobian matrix and optionally the user
   provided context for evaluating the Jacobian.

   Not Collective, but `Mat` object will be parallel if `SNES` object is

   Input Parameter:
.  snes - the nonlinear solver context

   Output Parameters:
+  Amat - location to stash (approximate) Jacobian matrix (or `NULL`)
.  Pmat - location to stash matrix used to compute the preconditioner (or `NULL`)
.  J - location to put Jacobian function (or `NULL`), for calling sequence see `SNESJacobianFunction`
-  ctx - location to stash Jacobian ctx (or `NULL`)

   Level: advanced

.seealso: [](chapter_snes), `SNES`, `Mat`, `SNESSetJacobian()`, `SNESComputeJacobian()`, `SNESJacobianFunction`, `SNESGetFunction()`
@*/
PetscErrorCode SNESGetJacobian(SNES snes, Mat *Amat, Mat *Pmat, PetscErrorCode (**J)(SNES, Vec, Mat, Mat, void *), void **ctx)
{
  DM dm;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  if (Amat) *Amat = snes->jacobian;
  if (Pmat) *Pmat = snes->jacobian_pre;
  PetscCall(SNESGetDM(snes, &dm));
  PetscCall(DMSNESGetJacobian(dm, J, ctx));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode SNESSetDefaultComputeJacobian(SNES snes)
{
  DM     dm;
  DMSNES sdm;

  PetscFunctionBegin;
  PetscCall(SNESGetDM(snes, &dm));
  PetscCall(DMGetDMSNES(dm, &sdm));
  if (!sdm->ops->computejacobian && snes->jacobian_pre) {
    DM        dm;
    PetscBool isdense, ismf;

    PetscCall(SNESGetDM(snes, &dm));
    PetscCall(PetscObjectTypeCompareAny((PetscObject)snes->jacobian_pre, &isdense, MATSEQDENSE, MATMPIDENSE, MATDENSE, NULL));
    PetscCall(PetscObjectTypeCompareAny((PetscObject)snes->jacobian_pre, &ismf, MATMFFD, MATSHELL, NULL));
    if (isdense) {
      PetscCall(DMSNESSetJacobian(dm, SNESComputeJacobianDefault, NULL));
    } else if (!ismf) {
      PetscCall(DMSNESSetJacobian(dm, SNESComputeJacobianDefaultColor, NULL));
    }
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESSetUp - Sets up the internal data structures for the later use
   of a nonlinear solver.

   Collective

   Input Parameters:
.  snes - the `SNES` context

   Level: advanced

   Note:
   For basic use of the `SNES` solvers the user need not explicitly call
   `SNESSetUp()`, since these actions will automatically occur during
   the call to `SNESSolve()`.  However, if one wishes to control this
   phase separately, `SNESSetUp()` should be called after `SNESCreate()`
   and optional routines of the form SNESSetXXX(), but before `SNESSolve()`.

.seealso: [](chapter_snes), `SNES`, `SNESCreate()`, `SNESSolve()`, `SNESDestroy()`
@*/
PetscErrorCode SNESSetUp(SNES snes)
{
  DM             dm;
  DMSNES         sdm;
  SNESLineSearch linesearch, pclinesearch;
  void          *lsprectx, *lspostctx;
  PetscBool      mf_operator, mf;
  Vec            f, fpc;
  void          *funcctx;
  void          *jacctx, *appctx;
  Mat            j, jpre;
  PetscErrorCode (*precheck)(SNESLineSearch, Vec, Vec, PetscBool *, void *);
  PetscErrorCode (*postcheck)(SNESLineSearch, Vec, Vec, Vec, PetscBool *, PetscBool *, void *);
  PetscErrorCode (*func)(SNES, Vec, Vec, void *);
  PetscErrorCode (*jac)(SNES, Vec, Mat, Mat, void *);

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  if (snes->setupcalled) PetscFunctionReturn(PETSC_SUCCESS);
  PetscCall(PetscLogEventBegin(SNES_SetUp, snes, 0, 0, 0));

  if (!((PetscObject)snes)->type_name) PetscCall(SNESSetType(snes, SNESNEWTONLS));

  PetscCall(SNESGetFunction(snes, &snes->vec_func, NULL, NULL));

  PetscCall(SNESGetDM(snes, &dm));
  PetscCall(DMGetDMSNES(dm, &sdm));
  PetscCall(SNESSetDefaultComputeJacobian(snes));

  if (!snes->vec_func) PetscCall(DMCreateGlobalVector(dm, &snes->vec_func));

  if (!snes->ksp) PetscCall(SNESGetKSP(snes, &snes->ksp));

  if (snes->linesearch) {
    PetscCall(SNESGetLineSearch(snes, &snes->linesearch));
    PetscCall(SNESLineSearchSetFunction(snes->linesearch, SNESComputeFunction));
  }

  PetscCall(SNESGetUseMatrixFree(snes, &mf_operator, &mf));
  if (snes->npc && snes->npcside == PC_LEFT) {
    snes->mf          = PETSC_TRUE;
    snes->mf_operator = PETSC_FALSE;
  }

  if (snes->npc) {
    /* copy the DM over */
    PetscCall(SNESGetDM(snes, &dm));
    PetscCall(SNESSetDM(snes->npc, dm));

    PetscCall(SNESGetFunction(snes, &f, &func, &funcctx));
    PetscCall(VecDuplicate(f, &fpc));
    PetscCall(SNESSetFunction(snes->npc, fpc, func, funcctx));
    PetscCall(SNESGetJacobian(snes, &j, &jpre, &jac, &jacctx));
    PetscCall(SNESSetJacobian(snes->npc, j, jpre, jac, jacctx));
    PetscCall(SNESGetApplicationContext(snes, &appctx));
    PetscCall(SNESSetApplicationContext(snes->npc, appctx));
    PetscCall(SNESSetUseMatrixFree(snes->npc, mf_operator, mf));
    PetscCall(VecDestroy(&fpc));

    /* copy the function pointers over */
    PetscCall(PetscObjectCopyFortranFunctionPointers((PetscObject)snes, (PetscObject)snes->npc));

    /* default to 1 iteration */
    PetscCall(SNESSetTolerances(snes->npc, 0.0, 0.0, 0.0, 1, snes->npc->max_funcs));
    if (snes->npcside == PC_RIGHT) {
      PetscCall(SNESSetNormSchedule(snes->npc, SNES_NORM_FINAL_ONLY));
    } else {
      PetscCall(SNESSetNormSchedule(snes->npc, SNES_NORM_NONE));
    }
    PetscCall(SNESSetFromOptions(snes->npc));

    /* copy the line search context over */
    if (snes->linesearch && snes->npc->linesearch) {
      PetscCall(SNESGetLineSearch(snes, &linesearch));
      PetscCall(SNESGetLineSearch(snes->npc, &pclinesearch));
      PetscCall(SNESLineSearchGetPreCheck(linesearch, &precheck, &lsprectx));
      PetscCall(SNESLineSearchGetPostCheck(linesearch, &postcheck, &lspostctx));
      PetscCall(SNESLineSearchSetPreCheck(pclinesearch, precheck, lsprectx));
      PetscCall(SNESLineSearchSetPostCheck(pclinesearch, postcheck, lspostctx));
      PetscCall(PetscObjectCopyFortranFunctionPointers((PetscObject)linesearch, (PetscObject)pclinesearch));
    }
  }
  if (snes->mf) PetscCall(SNESSetUpMatrixFree_Private(snes, snes->mf_operator, snes->mf_version));
  if (snes->ops->usercompute && !snes->user) PetscCall((*snes->ops->usercompute)(snes, (void **)&snes->user));

  snes->jac_iter = 0;
  snes->pre_iter = 0;

  PetscTryTypeMethod(snes, setup);

  PetscCall(SNESSetDefaultComputeJacobian(snes));

  if (snes->npc && snes->npcside == PC_LEFT) {
    if (snes->functype == SNES_FUNCTION_PRECONDITIONED) {
      if (snes->linesearch) {
        PetscCall(SNESGetLineSearch(snes, &linesearch));
        PetscCall(SNESLineSearchSetFunction(linesearch, SNESComputeFunctionDefaultNPC));
      }
    }
  }
  PetscCall(PetscLogEventEnd(SNES_SetUp, snes, 0, 0, 0));
  snes->setupcalled = PETSC_TRUE;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESReset - Resets a `SNES` context to the snessetupcalled = 0 state and removes any allocated `Vec`s and `Mat`s

   Collective

   Input Parameter:
.  snes - iterative context obtained from `SNESCreate()`

   Level: intermediate

   Notes:
   Call this if you wish to reuse a `SNES` but with different size vectors

   Also calls the application context destroy routine set with `SNESSetComputeApplicationContext()`

.seealso: [](chapter_snes), `SNES`, `SNESDestroy()`, `SNESCreate()`, `SNESSetUp()`, `SNESSolve()`
@*/
PetscErrorCode SNESReset(SNES snes)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  if (snes->ops->userdestroy && snes->user) {
    PetscCall((*snes->ops->userdestroy)((void **)&snes->user));
    snes->user = NULL;
  }
  if (snes->npc) PetscCall(SNESReset(snes->npc));

  PetscTryTypeMethod(snes, reset);
  if (snes->ksp) PetscCall(KSPReset(snes->ksp));

  if (snes->linesearch) PetscCall(SNESLineSearchReset(snes->linesearch));

  PetscCall(VecDestroy(&snes->vec_rhs));
  PetscCall(VecDestroy(&snes->vec_sol));
  PetscCall(VecDestroy(&snes->vec_sol_update));
  PetscCall(VecDestroy(&snes->vec_func));
  PetscCall(MatDestroy(&snes->jacobian));
  PetscCall(MatDestroy(&snes->jacobian_pre));
  PetscCall(MatDestroy(&snes->picard));
  PetscCall(VecDestroyVecs(snes->nwork, &snes->work));
  PetscCall(VecDestroyVecs(snes->nvwork, &snes->vwork));

  snes->alwayscomputesfinalresidual = PETSC_FALSE;

  snes->nwork = snes->nvwork = 0;
  snes->setupcalled          = PETSC_FALSE;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESConvergedReasonViewCancel - Clears all the reason view functions for a `SNES` object.

   Collective

   Input Parameter:
.  snes - iterative context obtained from `SNESCreate()`

   Level: intermediate

.seealso: [](chapter_snes), `SNES`, `SNESCreate()`, `SNESDestroy()`, `SNESReset()`
@*/
PetscErrorCode SNESConvergedReasonViewCancel(SNES snes)
{
  PetscInt i;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  for (i = 0; i < snes->numberreasonviews; i++) {
    if (snes->reasonviewdestroy[i]) PetscCall((*snes->reasonviewdestroy[i])(&snes->reasonviewcontext[i]));
  }
  snes->numberreasonviews = 0;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
   SNESDestroy - Destroys the nonlinear solver context that was created
   with `SNESCreate()`.

   Collective

   Input Parameter:
.  snes - the `SNES` context

   Level: beginner

.seealso: [](chapter_snes), `SNES`, `SNESCreate()`, `SNESSolve()`
@*/
PetscErrorCode SNESDestroy(SNES *snes)
{
  PetscFunctionBegin;
  if (!*snes) PetscFunctionReturn(PETSC_SUCCESS);
  PetscValidHeaderSpecific((*snes), SNES_CLASSID, 1);
  if (--((PetscObject)(*snes))->refct > 0) {
    *snes = NULL;
    PetscFunctionReturn(PETSC_SUCCESS);
  }

  PetscCall(SNESReset((*snes)));
  PetscCall(SNESDestroy(&(*snes)->npc));

  /* if memory was published with SAWs then destroy it */
  PetscCall(PetscObjectSAWsViewOff((PetscObject)*snes));
  PetscTryTypeMethod((*snes), destroy);

  if ((*snes)->dm) PetscCall(DMCoarsenHookRemove((*snes)->dm, DMCoarsenHook_SNESVecSol, DMRestrictHook_SNESVecSol, *snes));
  PetscCall(DMDestroy(&(*snes)->dm));
  PetscCall(KSPDestroy(&(*snes)->ksp));
  PetscCall(SNESLineSearchDestroy(&(*snes)->linesearch));

  PetscCall(PetscFree((*snes)->kspconvctx));
  if ((*snes)->ops->convergeddestroy) PetscCall((*(*snes)->ops->convergeddestroy)((*snes)->cnvP));
  if ((*snes)->conv_hist_alloc) PetscCall(PetscFree2((*snes)->conv_hist, (*snes)->conv_hist_its));
  PetscCall(SNESMonitorCancel((*snes)));
  PetscCall(SNESConvergedReasonViewCancel((*snes)));
  PetscCall(PetscHeaderDestroy(snes));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/* ----------- Routines to set solver parameters ---------- */

/*@
   SNESSetLagPreconditioner - Determines when the preconditioner is rebuilt in the nonlinear solve.

   Logically Collective

   Input Parameters:
+  snes - the `SNES` context
-  lag - 1 means rebuild every time the Jacobian is computed within a single nonlinear solve, 2 means every second time
         the Jacobian is built etc. -2 indicates rebuild preconditioner at next chance but then never rebuild after that

   Options Database Keys:
+    -snes_lag_jacobian_persists <true,false> - sets the persistence
.    -snes_lag_jacobian <-2,1,2,...> - sets the lag
.    -snes_lag_preconditioner_persists <true,false> - sets the persistence
-    -snes_lag_preconditioner <-2,1,2,...> - sets the lag

   Notes:
   Level: intermediate

   The default is 1
   The preconditioner is ALWAYS built in the first iteration of a nonlinear solve unless lag is -1 or `SNESSetLagPreconditionerPersists()` was called

   `SNESSetLagPreconditionerPersists()` allows using the same uniform lagging (for example every second linear solve) across multiple nonlinear solves.

.seealso: [](chapter_snes), `SNESSetTrustRegionTolerance()`, `SNESGetLagPreconditioner()`, `SNESSetLagJacobian()`, `SNESGetLagJacobian()`, `SNESSetLagPreconditionerPersists()`,
          `SNESSetLagJacobianPersists()`, `SNES`, `SNESSolve()`
@*/
PetscErrorCode SNESSetLagPreconditioner(SNES snes, PetscInt lag)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscCheck(lag >= -2, PETSC_COMM_SELF, PETSC_ERR_ARG_OUTOFRANGE, "Lag must be -2, -1, 1 or greater");
  PetscCheck(lag, PETSC_COMM_SELF, PETSC_ERR_ARG_OUTOFRANGE, "Lag cannot be 0");
  PetscValidLogicalCollectiveInt(snes, lag, 2);
  snes->lagpreconditioner = lag;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESSetGridSequence - sets the number of steps of grid sequencing that `SNES` will do

   Logically Collective

   Input Parameters:
+  snes - the `SNES` context
-  steps - the number of refinements to do, defaults to 0

   Options Database Key:
.    -snes_grid_sequence <steps> - Use grid sequencing to generate initial guess

   Level: intermediate

   Note:
   Use `SNESGetSolution()` to extract the fine grid solution after grid sequencing.

.seealso: [](chapter_snes), `SNES`, `SNESSetTrustRegionTolerance()`, `SNESGetLagPreconditioner()`, `SNESSetLagJacobian()`, `SNESGetLagJacobian()`, `SNESGetGridSequence()`
@*/
PetscErrorCode SNESSetGridSequence(SNES snes, PetscInt steps)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidLogicalCollectiveInt(snes, steps, 2);
  snes->gridsequence = steps;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESGetGridSequence - gets the number of steps of grid sequencing that `SNES` will do

   Logically Collective

   Input Parameter:
.  snes - the `SNES` context

   Output Parameter:
.  steps - the number of refinements to do, defaults to 0

   Options Database Key:
.    -snes_grid_sequence <steps> - set number of refinements

   Level: intermediate

   Note:
   Use `SNESGetSolution()` to extract the fine grid solution after grid sequencing.

.seealso: [](chapter_snes), `SNESSetTrustRegionTolerance()`, `SNESGetLagPreconditioner()`, `SNESSetLagJacobian()`, `SNESGetLagJacobian()`, `SNESSetGridSequence()`
@*/
PetscErrorCode SNESGetGridSequence(SNES snes, PetscInt *steps)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  *steps = snes->gridsequence;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESGetLagPreconditioner - Return how often the preconditioner is rebuilt

   Not Collective

   Input Parameter:
.  snes - the `SNES` context

   Output Parameter:
.   lag - -1 indicates NEVER rebuild, 1 means rebuild every time the Jacobian is computed within a single nonlinear solve, 2 means every second time
         the Jacobian is built etc. -2 indicates rebuild preconditioner at next chance but then never rebuild after that

   Options Database Keys:
+    -snes_lag_jacobian_persists <true,false> - sets the persistence
.    -snes_lag_jacobian <-2,1,2,...> - sets the lag
.    -snes_lag_preconditioner_persists <true,false> - sets the persistence
-    -snes_lag_preconditioner <-2,1,2,...> - sets the lag

   Level: intermediate

   Notes:
   The default is 1

   The preconditioner is ALWAYS built in the first iteration of a nonlinear solve unless lag is -1

.seealso: [](chapter_snes), `SNES`, `SNESSetTrustRegionTolerance()`, `SNESSetLagPreconditioner()`, `SNESSetLagJacobianPersists()`, `SNESSetLagPreconditionerPersists()`
@*/
PetscErrorCode SNESGetLagPreconditioner(SNES snes, PetscInt *lag)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  *lag = snes->lagpreconditioner;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESSetLagJacobian - Set when the Jacobian is rebuilt in the nonlinear solve. See `SNESSetLagPreconditioner()` for determining how
     often the preconditioner is rebuilt.

   Logically Collective

   Input Parameters:
+  snes - the `SNES` context
-  lag - -1 indicates NEVER rebuild, 1 means rebuild every time the Jacobian is computed within a single nonlinear solve, 2 means every second time
         the Jacobian is built etc. -2 means rebuild at next chance but then never again

   Options Database Keys:
+    -snes_lag_jacobian_persists <true,false> - sets the persistence
.    -snes_lag_jacobian <-2,1,2,...> - sets the lag
.    -snes_lag_preconditioner_persists <true,false> - sets the persistence
-    -snes_lag_preconditioner <-2,1,2,...> - sets the lag.

   Level: intermediate

   Notes:
   The default is 1

   The Jacobian is ALWAYS built in the first iteration of a nonlinear solve unless lag is -1

   If  -1 is used before the very first nonlinear solve the CODE WILL FAIL! because no Jacobian is used, use -2 to indicate you want it recomputed
   at the next Newton step but never again (unless it is reset to another value)

.seealso: [](chapter_snes), `SNES`, `SNESSetTrustRegionTolerance()`, `SNESGetLagPreconditioner()`, `SNESSetLagPreconditioner()`, `SNESGetLagJacobianPersists()`, `SNESSetLagPreconditionerPersists()`
@*/
PetscErrorCode SNESSetLagJacobian(SNES snes, PetscInt lag)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscCheck(lag >= -2, PETSC_COMM_SELF, PETSC_ERR_ARG_OUTOFRANGE, "Lag must be -2, -1, 1 or greater");
  PetscCheck(lag, PETSC_COMM_SELF, PETSC_ERR_ARG_OUTOFRANGE, "Lag cannot be 0");
  PetscValidLogicalCollectiveInt(snes, lag, 2);
  snes->lagjacobian = lag;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESGetLagJacobian - Get how often the Jacobian is rebuilt. See `SNESGetLagPreconditioner()` to determine when the preconditioner is rebuilt

   Not Collective

   Input Parameter:
.  snes - the `SNES` context

   Output Parameter:
.   lag - -1 indicates NEVER rebuild, 1 means rebuild every time the Jacobian is computed within a single nonlinear solve, 2 means every second time
         the Jacobian is built etc.

   Level: intermediate

   Notes:
   The default is 1

   The jacobian is ALWAYS built in the first iteration of a nonlinear solve unless lag is -1 or `SNESSetLagJacobianPersists()` was called.

.seealso: [](chapter_snes), `SNES`, `SNESSetTrustRegionTolerance()`, `SNESSetLagJacobian()`, `SNESSetLagPreconditioner()`, `SNESGetLagPreconditioner()`, `SNESSetLagJacobianPersists()`, `SNESSetLagPreconditionerPersists()`

@*/
PetscErrorCode SNESGetLagJacobian(SNES snes, PetscInt *lag)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  *lag = snes->lagjacobian;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESSetLagJacobianPersists - Set whether or not the Jacobian lagging persists through multiple nonlinear solves

   Logically collective

   Input Parameters:
+  snes - the `SNES` context
-   flg - jacobian lagging persists if true

   Options Database Keys:
+    -snes_lag_jacobian_persists <true,false> - sets the persistence
.    -snes_lag_jacobian <-2,1,2,...> - sets the lag
.    -snes_lag_preconditioner_persists <true,false> - sets the persistence
-    -snes_lag_preconditioner <-2,1,2,...> - sets the lag

   Level: advanced

   Notes:
    Normally when `SNESSetLagJacobian()` is used, the Jacobian is always rebuilt at the beginning of each new nonlinear solve, this removes that.

    This is useful both for nonlinear preconditioning, where it's appropriate to have the Jacobian be stale by
   several solves, and for implicit time-stepping, where Jacobian lagging in the inner nonlinear solve over several
   timesteps may present huge efficiency gains.

.seealso: [](chapter_snes), `SNES, `SNESSetLagPreconditionerPersists()`, `SNESSetLagJacobian()`, `SNESGetLagJacobian()`, `SNESGetNPC()`, `SNESSetLagJacobianPersists()`
@*/
PetscErrorCode SNESSetLagJacobianPersists(SNES snes, PetscBool flg)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidLogicalCollectiveBool(snes, flg, 2);
  snes->lagjac_persist = flg;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESSetLagPreconditionerPersists - Set whether or not the preconditioner lagging persists through multiple nonlinear solves

   Logically Collective

   Input Parameters:
+  snes - the `SNES` context
-   flg - preconditioner lagging persists if true

   Options Database Keys:
+    -snes_lag_jacobian_persists <true,false> - sets the persistence
.    -snes_lag_jacobian <-2,1,2,...> - sets the lag
.    -snes_lag_preconditioner_persists <true,false> - sets the persistence
-    -snes_lag_preconditioner <-2,1,2,...> - sets the lag

   Level: developer

   Notes:
    Normally when `SNESSetLagPreconditioner()` is used, the preconditioner is always rebuilt at the beginning of each new nonlinear solve, this removes that.

   This is useful both for nonlinear preconditioning, where it's appropriate to have the preconditioner be stale
   by several solves, and for implicit time-stepping, where preconditioner lagging in the inner nonlinear solve over
   several timesteps may present huge efficiency gains.

.seealso: [](chapter_snes), `SNES`, `SNESSetLagJacobianPersists()`, `SNESSetLagJacobian()`, `SNESGetLagJacobian()`, `SNESGetNPC()`, `SNESSetLagPreconditioner()`
@*/
PetscErrorCode SNESSetLagPreconditionerPersists(SNES snes, PetscBool flg)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidLogicalCollectiveBool(snes, flg, 2);
  snes->lagpre_persist = flg;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESSetForceIteration - force `SNESSolve()` to take at least one iteration regardless of the initial residual norm

   Logically Collective

   Input Parameters:
+  snes - the `SNES` context
-  force - `PETSC_TRUE` require at least one iteration

   Options Database Key:
.    -snes_force_iteration <force> - Sets forcing an iteration

   Level: intermediate

   Note:
   This is used sometimes with `TS` to prevent `TS` from detecting a false steady state solution

.seealso: [](chapter_snes), `SNES`, `TS`, `SNESSetTrustRegionTolerance()`, `SNESSetDivergenceTolerance()`
@*/
PetscErrorCode SNESSetForceIteration(SNES snes, PetscBool force)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  snes->forceiteration = force;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESGetForceIteration - Check whether or not `SNESSolve()` take at least one iteration regardless of the initial residual norm

   Logically Collective

   Input Parameters:
.  snes - the `SNES` context

   Output Parameter:
.  force - `PETSC_TRUE` requires at least one iteration.

   Level: intermediate

.seealso: [](chapter_snes), `SNES`, `SNESSetForceIteration()`, `SNESSetTrustRegionTolerance()`, `SNESSetDivergenceTolerance()`
@*/
PetscErrorCode SNESGetForceIteration(SNES snes, PetscBool *force)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  *force = snes->forceiteration;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESSetTolerances - Sets `SNES` various parameters used in convergence tests.

   Logically Collective

   Input Parameters:
+  snes - the `SNES` context
.  abstol - absolute convergence tolerance
.  rtol - relative convergence tolerance
.  stol -  convergence tolerance in terms of the norm of the change in the solution between steps,  || delta x || < stol*|| x ||
.  maxit - maximum number of iterations, default 50.
-  maxf - maximum number of function evaluations (-1 indicates no limit), default 1000

   Options Database Keys:
+    -snes_atol <abstol> - Sets abstol
.    -snes_rtol <rtol> - Sets rtol
.    -snes_stol <stol> - Sets stol
.    -snes_max_it <maxit> - Sets maxit
-    -snes_max_funcs <maxf> - Sets maxf

   Level: intermediate

.seealso: [](chapter_snes), `SNESolve()`, `SNES`, `SNESSetTrustRegionTolerance()`, `SNESSetDivergenceTolerance()`, `SNESSetForceIteration()`
@*/
PetscErrorCode SNESSetTolerances(SNES snes, PetscReal abstol, PetscReal rtol, PetscReal stol, PetscInt maxit, PetscInt maxf)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidLogicalCollectiveReal(snes, abstol, 2);
  PetscValidLogicalCollectiveReal(snes, rtol, 3);
  PetscValidLogicalCollectiveReal(snes, stol, 4);
  PetscValidLogicalCollectiveInt(snes, maxit, 5);
  PetscValidLogicalCollectiveInt(snes, maxf, 6);

  if (abstol != (PetscReal)PETSC_DEFAULT) {
    PetscCheck(abstol >= 0.0, PetscObjectComm((PetscObject)snes), PETSC_ERR_ARG_OUTOFRANGE, "Absolute tolerance %g must be non-negative", (double)abstol);
    snes->abstol = abstol;
  }
  if (rtol != (PetscReal)PETSC_DEFAULT) {
    PetscCheck(rtol >= 0.0 && 1.0 > rtol, PetscObjectComm((PetscObject)snes), PETSC_ERR_ARG_OUTOFRANGE, "Relative tolerance %g must be non-negative and less than 1.0", (double)rtol);
    snes->rtol = rtol;
  }
  if (stol != (PetscReal)PETSC_DEFAULT) {
    PetscCheck(stol >= 0.0, PetscObjectComm((PetscObject)snes), PETSC_ERR_ARG_OUTOFRANGE, "Step tolerance %g must be non-negative", (double)stol);
    snes->stol = stol;
  }
  if (maxit != PETSC_DEFAULT) {
    PetscCheck(maxit >= 0, PetscObjectComm((PetscObject)snes), PETSC_ERR_ARG_OUTOFRANGE, "Maximum number of iterations %" PetscInt_FMT " must be non-negative", maxit);
    snes->max_its = maxit;
  }
  if (maxf != PETSC_DEFAULT) {
    PetscCheck(maxf >= -1, PetscObjectComm((PetscObject)snes), PETSC_ERR_ARG_OUTOFRANGE, "Maximum number of function evaluations %" PetscInt_FMT " must be -1 or nonnegative", maxf);
    snes->max_funcs = maxf;
  }
  snes->tolerancesset = PETSC_TRUE;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESSetDivergenceTolerance - Sets the divergence tolerance used for the `SNES` divergence test.

   Logically Collective

   Input Parameters:
+  snes - the `SNES` context
-  divtol - the divergence tolerance. Use -1 to deactivate the test, default is 1e4

   Options Database Key:
.    -snes_divergence_tolerance <divtol> - Sets `divtol`

   Level: intermediate

.seealso: [](chapter_snes), `SNES`, `SNESSolve()`, `SNESSetTolerances()`, `SNESGetDivergenceTolerance`
@*/
PetscErrorCode SNESSetDivergenceTolerance(SNES snes, PetscReal divtol)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidLogicalCollectiveReal(snes, divtol, 2);

  if (divtol != (PetscReal)PETSC_DEFAULT) {
    snes->divtol = divtol;
  } else {
    snes->divtol = 1.0e4;
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESGetTolerances - Gets various parameters used in convergence tests.

   Not Collective

   Input Parameters:
+  snes - the `SNES` context
.  atol - absolute convergence tolerance
.  rtol - relative convergence tolerance
.  stol -  convergence tolerance in terms of the norm
           of the change in the solution between steps
.  maxit - maximum number of iterations
-  maxf - maximum number of function evaluations

   Level: intermediate

   Note:
   The user can specify `NULL` for any parameter that is not needed.

.seealso: [](chapter_snes), `SNES`, `SNESSetTolerances()`
@*/
PetscErrorCode SNESGetTolerances(SNES snes, PetscReal *atol, PetscReal *rtol, PetscReal *stol, PetscInt *maxit, PetscInt *maxf)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  if (atol) *atol = snes->abstol;
  if (rtol) *rtol = snes->rtol;
  if (stol) *stol = snes->stol;
  if (maxit) *maxit = snes->max_its;
  if (maxf) *maxf = snes->max_funcs;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESGetDivergenceTolerance - Gets divergence tolerance used in divergence test.

   Not Collective

   Input Parameters:
+  snes - the `SNES` context
-  divtol - divergence tolerance

   Level: intermediate

.seealso: [](chapter_snes), `SNES`, `SNESSetDivergenceTolerance()`
@*/
PetscErrorCode SNESGetDivergenceTolerance(SNES snes, PetscReal *divtol)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  if (divtol) *divtol = snes->divtol;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESSetTrustRegionTolerance - Sets the trust region parameter tolerance.

   Logically Collective

   Input Parameters:
+  snes - the `SNES` context
-  tol - tolerance

   Options Database Key:
.  -snes_tr_tol <tol> - Sets tol

   Level: intermediate

.seealso: [](chapter_snes), `SNES`, `SNESNEWTONTR`, `SNESSetTolerances()`
@*/
PetscErrorCode SNESSetTrustRegionTolerance(SNES snes, PetscReal tol)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidLogicalCollectiveReal(snes, tol, 2);
  snes->deltatol = tol;
  PetscFunctionReturn(PETSC_SUCCESS);
}

PETSC_INTERN PetscErrorCode SNESMonitorRange_Private(SNES, PetscInt, PetscReal *);

PetscErrorCode SNESMonitorLGRange(SNES snes, PetscInt n, PetscReal rnorm, void *monctx)
{
  PetscDrawLG      lg;
  PetscReal        x, y, per;
  PetscViewer      v = (PetscViewer)monctx;
  static PetscReal prev; /* should be in the context */
  PetscDraw        draw;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(v, PETSC_VIEWER_CLASSID, 4);
  PetscCall(PetscViewerDrawGetDrawLG(v, 0, &lg));
  if (!n) PetscCall(PetscDrawLGReset(lg));
  PetscCall(PetscDrawLGGetDraw(lg, &draw));
  PetscCall(PetscDrawSetTitle(draw, "Residual norm"));
  x = (PetscReal)n;
  if (rnorm > 0.0) y = PetscLog10Real(rnorm);
  else y = -15.0;
  PetscCall(PetscDrawLGAddPoint(lg, &x, &y));
  if (n < 20 || !(n % 5) || snes->reason) {
    PetscCall(PetscDrawLGDraw(lg));
    PetscCall(PetscDrawLGSave(lg));
  }

  PetscCall(PetscViewerDrawGetDrawLG(v, 1, &lg));
  if (!n) PetscCall(PetscDrawLGReset(lg));
  PetscCall(PetscDrawLGGetDraw(lg, &draw));
  PetscCall(PetscDrawSetTitle(draw, "% elemts > .2*max elemt"));
  PetscCall(SNESMonitorRange_Private(snes, n, &per));
  x = (PetscReal)n;
  y = 100.0 * per;
  PetscCall(PetscDrawLGAddPoint(lg, &x, &y));
  if (n < 20 || !(n % 5) || snes->reason) {
    PetscCall(PetscDrawLGDraw(lg));
    PetscCall(PetscDrawLGSave(lg));
  }

  PetscCall(PetscViewerDrawGetDrawLG(v, 2, &lg));
  if (!n) {
    prev = rnorm;
    PetscCall(PetscDrawLGReset(lg));
  }
  PetscCall(PetscDrawLGGetDraw(lg, &draw));
  PetscCall(PetscDrawSetTitle(draw, "(norm -oldnorm)/oldnorm"));
  x = (PetscReal)n;
  y = (prev - rnorm) / prev;
  PetscCall(PetscDrawLGAddPoint(lg, &x, &y));
  if (n < 20 || !(n % 5) || snes->reason) {
    PetscCall(PetscDrawLGDraw(lg));
    PetscCall(PetscDrawLGSave(lg));
  }

  PetscCall(PetscViewerDrawGetDrawLG(v, 3, &lg));
  if (!n) PetscCall(PetscDrawLGReset(lg));
  PetscCall(PetscDrawLGGetDraw(lg, &draw));
  PetscCall(PetscDrawSetTitle(draw, "(norm -oldnorm)/oldnorm*(% > .2 max)"));
  x = (PetscReal)n;
  y = (prev - rnorm) / (prev * per);
  if (n > 2) { /*skip initial crazy value */
    PetscCall(PetscDrawLGAddPoint(lg, &x, &y));
  }
  if (n < 20 || !(n % 5) || snes->reason) {
    PetscCall(PetscDrawLGDraw(lg));
    PetscCall(PetscDrawLGSave(lg));
  }
  prev = rnorm;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESMonitor - runs the user provided monitor routines, if they exist

   Collective

   Input Parameters:
+  snes - nonlinear solver context obtained from `SNESCreate()`
.  iter - iteration number
-  rnorm - relative norm of the residual

   Level: developer

   Note:
   This routine is called by the `SNES` implementations.
   It does not typically need to be called by the user.

.seealso: [](chapter_snes), `SNES`, `SNESMonitorSet()`
@*/
PetscErrorCode SNESMonitor(SNES snes, PetscInt iter, PetscReal rnorm)
{
  PetscInt i, n = snes->numbermonitors;

  PetscFunctionBegin;
  PetscCall(VecLockReadPush(snes->vec_sol));
  for (i = 0; i < n; i++) PetscCall((*snes->monitor[i])(snes, iter, rnorm, snes->monitorcontext[i]));
  PetscCall(VecLockReadPop(snes->vec_sol));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/* ------------ Routines to set performance monitoring options ----------- */

/*MC
    SNESMonitorFunction - functional form passed to `SNESMonitorSet()` to monitor convergence of nonlinear solver

     Synopsis:
     #include <petscsnes.h>
$    PetscErrorCode SNESMonitorFunction(SNES snes,PetscInt its, PetscReal norm,void *mctx)

     Collective

    Input Parameters:
+    snes - the `SNES` context
.    its - iteration number
.    norm - 2-norm function value (may be estimated)
-    mctx - [optional] monitoring context

   Level: advanced

.seealso: [](chapter_snes), `SNESMonitorSet()`, `SNESMonitorSet()`, `SNESMonitorGet()`
M*/

/*@C
   SNESMonitorSet - Sets an ADDITIONAL function that is to be used at every
   iteration of the nonlinear solver to display the iteration's
   progress.

   Logically Collective

   Input Parameters:
+  snes - the `SNES` context
.  f - the monitor function,  for the calling sequence see `SNESMonitorFunction`
.  mctx - [optional] user-defined context for private data for the
          monitor routine (use `NULL` if no context is desired)
-  monitordestroy - [optional] routine that frees monitor context (may be `NULL`)

   Options Database Keys:
+    -snes_monitor        - sets `SNESMonitorDefault()`
.    -snes_monitor draw::draw_lg - sets line graph monitor,
-    -snes_monitor_cancel - cancels all monitors that have been hardwired into a code by calls to `SNESMonitorSet()`, but does not cancel those set via
                            the options database.

   Level: intermediate

   Note:
   Several different monitoring routines may be set by calling
   `SNESMonitorSet()` multiple times; all will be called in the
   order in which they were set.

   Fortran Note:
   Only a single monitor function can be set for each `SNES` object

.seealso: [](chapter_snes), `SNES`, `SNESSolve()`, `SNESMonitorDefault()`, `SNESMonitorCancel()`, `SNESMonitorFunction`
@*/
PetscErrorCode SNESMonitorSet(SNES snes, PetscErrorCode (*f)(SNES, PetscInt, PetscReal, void *), void *mctx, PetscErrorCode (*monitordestroy)(void **))
{
  PetscInt  i;
  PetscBool identical;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  for (i = 0; i < snes->numbermonitors; i++) {
    PetscCall(PetscMonitorCompare((PetscErrorCode(*)(void))f, mctx, monitordestroy, (PetscErrorCode(*)(void))snes->monitor[i], snes->monitorcontext[i], snes->monitordestroy[i], &identical));
    if (identical) PetscFunctionReturn(PETSC_SUCCESS);
  }
  PetscCheck(snes->numbermonitors < MAXSNESMONITORS, PETSC_COMM_SELF, PETSC_ERR_ARG_OUTOFRANGE, "Too many monitors set");
  snes->monitor[snes->numbermonitors]          = f;
  snes->monitordestroy[snes->numbermonitors]   = monitordestroy;
  snes->monitorcontext[snes->numbermonitors++] = (void *)mctx;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESMonitorCancel - Clears all the monitor functions for a `SNES` object.

   Logically Collective

   Input Parameters:
.  snes - the `SNES` context

   Options Database Key:
.  -snes_monitor_cancel - cancels all monitors that have been hardwired
    into a code by calls to `SNESMonitorSet()`, but does not cancel those
    set via the options database

   Level: intermediate

   Note:
   There is no way to clear one specific monitor from a `SNES` object.

.seealso: [](chapter_snes), `SNES`, `SNESMonitorGet()`, `SNESMonitorDefault()`, `SNESMonitorSet()`
@*/
PetscErrorCode SNESMonitorCancel(SNES snes)
{
  PetscInt i;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  for (i = 0; i < snes->numbermonitors; i++) {
    if (snes->monitordestroy[i]) PetscCall((*snes->monitordestroy[i])(&snes->monitorcontext[i]));
  }
  snes->numbermonitors = 0;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*MC
    SNESConvergenceTestFunction - functional form used for testing of convergence of nonlinear solver

     Synopsis:
     #include <petscsnes.h>
$     PetscErrorCode SNESConvergenceTest(SNES snes,PetscInt it,PetscReal xnorm,PetscReal gnorm,PetscReal f,SNESConvergedReason *reason,void *cctx)

     Collective

    Input Parameters:
+    snes - the `SNES` context
.    it - current iteration (0 is the first and is before any Newton step)
.    xnorm - 2-norm of current iterate
.    gnorm - 2-norm of current step
.    f - 2-norm of function
-    cctx - [optional] convergence context

    Output Parameter:
.    reason - reason for convergence/divergence, only needs to be set when convergence or divergence is detected

   Level: intermediate

.seealso: [](chapter_snes), `SNES`, `SNESSolve`, `SNESSetConvergenceTest()`, `SNESGetConvergenceTest()`
M*/

/*@C
   SNESSetConvergenceTest - Sets the function that is to be used
   to test for convergence of the nonlinear iterative solution.

   Logically Collective

   Input Parameters:
+  snes - the `SNES` context
.  `SNESConvergenceTestFunction` - routine to test for convergence
.  cctx - [optional] context for private data for the convergence routine  (may be `NULL`)
-  destroy - [optional] destructor for the context (may be `NULL`; `PETSC_NULL_FUNCTION` in Fortran)

   Level: advanced

.seealso: [](chapter_snes), `SNES`, `SNESConvergedDefault()`, `SNESConvergedSkip()`, `SNESConvergenceTestFunction`
@*/
PetscErrorCode SNESSetConvergenceTest(SNES snes, PetscErrorCode (*SNESConvergenceTestFunction)(SNES, PetscInt, PetscReal, PetscReal, PetscReal, SNESConvergedReason *, void *), void *cctx, PetscErrorCode (*destroy)(void *))
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  if (!SNESConvergenceTestFunction) SNESConvergenceTestFunction = SNESConvergedSkip;
  if (snes->ops->convergeddestroy) PetscCall((*snes->ops->convergeddestroy)(snes->cnvP));
  snes->ops->converged        = SNESConvergenceTestFunction;
  snes->ops->convergeddestroy = destroy;
  snes->cnvP                  = cctx;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESGetConvergedReason - Gets the reason the `SNES` iteration was stopped.

   Not Collective

   Input Parameter:
.  snes - the `SNES` context

   Output Parameter:
.  reason - negative value indicates diverged, positive value converged, see `SNESConvergedReason` for the individual convergence tests for complete lists

   Options Database Key:
.   -snes_converged_reason - prints the reason to standard out

   Level: intermediate

   Note:
    Should only be called after the call the `SNESSolve()` is complete, if it is called earlier it returns the value `SNES__CONVERGED_ITERATING`.

.seealso: [](chapter_snes), `SNESSolve()`, `SNESSetConvergenceTest()`, `SNESSetConvergedReason()`, `SNESConvergedReason`, `SNESGetConvergedReasonString()`
@*/
PetscErrorCode SNESGetConvergedReason(SNES snes, SNESConvergedReason *reason)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidPointer(reason, 2);
  *reason = snes->reason;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
   SNESGetConvergedReasonString - Return a human readable string for `SNESConvergedReason`

   Not Collective

   Input Parameter:
.  snes - the `SNES` context

   Output Parameter:
.  strreason - a human readable string that describes `SNES` converged reason

   Level: beginner

.seealso: [](chapter_snes), `SNES`, `SNESGetConvergedReason()`
@*/
PetscErrorCode SNESGetConvergedReasonString(SNES snes, const char **strreason)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidPointer(strreason, 2);
  *strreason = SNESConvergedReasons[snes->reason];
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESSetConvergedReason - Sets the reason the `SNES` iteration was stopped.

   Not Collective

   Input Parameters:
+  snes - the `SNES` context
-  reason - negative value indicates diverged, positive value converged, see `SNESConvergedReason` or the
            manual pages for the individual convergence tests for complete lists

   Level: developer

   Developer Note:
   Called inside the various `SNESSolve()` implementations

.seealso: [](chapter_snes), `SNESGetConvergedReason()`, `SNESSetConvergenceTest()`, `SNESConvergedReason`
@*/
PetscErrorCode SNESSetConvergedReason(SNES snes, SNESConvergedReason reason)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  snes->reason = reason;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESSetConvergenceHistory - Sets the array used to hold the convergence history.

   Logically Collective

   Input Parameters:
+  snes - iterative context obtained from `SNESCreate()`
.  a   - array to hold history, this array will contain the function norms computed at each step
.  its - integer array holds the number of linear iterations for each solve.
.  na  - size of a and its
-  reset - `PETSC_TRUE` indicates each new nonlinear solve resets the history counter to zero,
           else it continues storing new values for new nonlinear solves after the old ones

   Level: intermediate

   Notes:
   If 'a' and 'its' are `NULL` then space is allocated for the history. If 'na' `PETSC_DECIDE` or `PETSC_DEFAULT` then a
   default array of length 10000 is allocated.

   This routine is useful, e.g., when running a code for purposes
   of accurate performance monitoring, when no I/O should be done
   during the section of code that is being timed.

.seealso: [](chapter_snes), `SNES`, `SNESSolve()`, `SNESGetConvergenceHistory()`
@*/
PetscErrorCode SNESSetConvergenceHistory(SNES snes, PetscReal a[], PetscInt its[], PetscInt na, PetscBool reset)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  if (a) PetscValidRealPointer(a, 2);
  if (its) PetscValidIntPointer(its, 3);
  if (!a) {
    if (na == PETSC_DECIDE || na == PETSC_DEFAULT) na = 1000;
    PetscCall(PetscCalloc2(na, &a, na, &its));
    snes->conv_hist_alloc = PETSC_TRUE;
  }
  snes->conv_hist       = a;
  snes->conv_hist_its   = its;
  snes->conv_hist_max   = (size_t)na;
  snes->conv_hist_len   = 0;
  snes->conv_hist_reset = reset;
  PetscFunctionReturn(PETSC_SUCCESS);
}

#if defined(PETSC_HAVE_MATLAB)
  #include <engine.h> /* MATLAB include file */
  #include <mex.h>    /* MATLAB include file */

PETSC_EXTERN mxArray *SNESGetConvergenceHistoryMatlab(SNES snes)
{
  mxArray   *mat;
  PetscInt   i;
  PetscReal *ar;

  mat = mxCreateDoubleMatrix(snes->conv_hist_len, 1, mxREAL);
  ar  = (PetscReal *)mxGetData(mat);
  for (i = 0; i < snes->conv_hist_len; i++) ar[i] = snes->conv_hist[i];
  return mat;
}
#endif

/*@C
   SNESGetConvergenceHistory - Gets the array used to hold the convergence history.

   Not Collective

   Input Parameter:
.  snes - iterative context obtained from `SNESCreate()`

   Output Parameters:
+  a   - array to hold history, usually was set with `SNESSetConvergenceHistory()`
.  its - integer array holds the number of linear iterations (or
         negative if not converged) for each solve.
-  na  - size of `a` and `its`

   Level: intermediate

   Note:
   This routine is useful, e.g., when running a code for purposes
   of accurate performance monitoring, when no I/O should be done
   during the section of code that is being timed.

   Fortran Note:
    The calling sequence for this routine in Fortran is
.vb
    call SNESGetConvergenceHistory(SNES snes, integer na, integer ierr)
.ve

.seealso: [](chapter_snes), `SNES`, `SNESSolve()`, `SNESSetConvergenceHistory()`
@*/
PetscErrorCode SNESGetConvergenceHistory(SNES snes, PetscReal *a[], PetscInt *its[], PetscInt *na)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  if (a) *a = snes->conv_hist;
  if (its) *its = snes->conv_hist_its;
  if (na) *na = (PetscInt)snes->conv_hist_len;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
  SNESSetUpdate - Sets the general-purpose update function called
  at the beginning of every iteration of the nonlinear solve. Specifically
  it is called just before the Jacobian is "evaluated".

  Logically Collective

  Input Parameters:
+ snes - The nonlinear solver context
- func - The function

  Calling sequence of `func`:
$ PetscErrorCode func(SNES snes, PetscInt step);
+ snes - the nonlinear solver context
- step - The current step of the iteration

  Level: advanced

  Note:
     This is NOT what one uses to update the ghost points before a function evaluation, that should be done at the beginning of your function provided
     to `SNESSetFunction()`, or `SNESSetPicard()`
     This is not used by most users.

     There are a varity of function hooks one many set that are called at different stages of the nonlinear solution process, see the functions listed below.

.seealso: [](chapter_snes), `SNES`, `SNESSolve()`, `SNESSetJacobian()`, `SNESSolve()`, `SNESLineSearchSetPreCheck()`, `SNESLineSearchSetPostCheck()`, `SNESNewtonTRSetPreCheck()`, `SNESNewtonTRSetPostCheck()`,
         `SNESMonitorSet()`, `SNESSetDivergenceTest()`
@*/
PetscErrorCode SNESSetUpdate(SNES snes, PetscErrorCode (*func)(SNES, PetscInt))
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  snes->ops->update = func;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*
   SNESScaleStep_Private - Scales a step so that its length is less than the
   positive parameter delta.

    Input Parameters:
+   snes - the `SNES` context
.   y - approximate solution of linear system
.   fnorm - 2-norm of current function
-   delta - trust region size

    Output Parameters:
+   gpnorm - predicted function norm at the new point, assuming local
    linearization.  The value is zero if the step lies within the trust
    region, and exceeds zero otherwise.
-   ynorm - 2-norm of the step

    Level: developer

    Note:
    For non-trust region methods such as `SNESNEWTONLS`, the parameter delta
    is set to be the maximum allowable step size.
*/
PetscErrorCode SNESScaleStep_Private(SNES snes, Vec y, PetscReal *fnorm, PetscReal *delta, PetscReal *gpnorm, PetscReal *ynorm)
{
  PetscReal   nrm;
  PetscScalar cnorm;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidHeaderSpecific(y, VEC_CLASSID, 2);
  PetscCheckSameComm(snes, 1, y, 2);

  PetscCall(VecNorm(y, NORM_2, &nrm));
  if (nrm > *delta) {
    nrm     = *delta / nrm;
    *gpnorm = (1.0 - nrm) * (*fnorm);
    cnorm   = nrm;
    PetscCall(VecScale(y, cnorm));
    *ynorm = *delta;
  } else {
    *gpnorm = 0.0;
    *ynorm  = nrm;
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
   SNESConvergedReasonView - Displays the reason a `SNES` solve converged or diverged to a viewer

   Collective

   Parameter:
+  snes - iterative context obtained from `SNESCreate()`
-  viewer - the viewer to display the reason

   Options Database Keys:
+  -snes_converged_reason - print reason for converged or diverged, also prints number of iterations
-  -snes_converged_reason ::failed - only print reason and number of iterations when diverged

  Note:
     To change the format of the output call `PetscViewerPushFormat`(viewer,format) before this call. Use `PETSC_VIEWER_DEFAULT` for the default,
     use `PETSC_VIEWER_FAILED` to only display a reason if it fails.

   Level: beginner

.seealso: [](chapter_snes), `SNESConvergedReason`, `PetscViewer`, `SNES`,
          `SNESCreate()`, `SNESSetUp()`, `SNESDestroy()`, `SNESSetTolerances()`, `SNESConvergedDefault()`, `SNESGetConvergedReason()`,
          `SNESConvergedReasonViewFromOptions()`,
          `PetscViewerPushFormat()`, `PetscViewerPopFormat()`
@*/
PetscErrorCode SNESConvergedReasonView(SNES snes, PetscViewer viewer)
{
  PetscViewerFormat format;
  PetscBool         isAscii;

  PetscFunctionBegin;
  if (!viewer) viewer = PETSC_VIEWER_STDOUT_(PetscObjectComm((PetscObject)snes));
  PetscCall(PetscObjectTypeCompare((PetscObject)viewer, PETSCVIEWERASCII, &isAscii));
  if (isAscii) {
    PetscCall(PetscViewerGetFormat(viewer, &format));
    PetscCall(PetscViewerASCIIAddTab(viewer, ((PetscObject)snes)->tablevel));
    if (format == PETSC_VIEWER_ASCII_INFO_DETAIL) {
      DM       dm;
      Vec      u;
      PetscDS  prob;
      PetscInt Nf, f;
      PetscErrorCode (**exactSol)(PetscInt, PetscReal, const PetscReal[], PetscInt, PetscScalar[], void *);
      void    **exactCtx;
      PetscReal error;

      PetscCall(SNESGetDM(snes, &dm));
      PetscCall(SNESGetSolution(snes, &u));
      PetscCall(DMGetDS(dm, &prob));
      PetscCall(PetscDSGetNumFields(prob, &Nf));
      PetscCall(PetscMalloc2(Nf, &exactSol, Nf, &exactCtx));
      for (f = 0; f < Nf; ++f) PetscCall(PetscDSGetExactSolution(prob, f, &exactSol[f], &exactCtx[f]));
      PetscCall(DMComputeL2Diff(dm, 0.0, exactSol, exactCtx, u, &error));
      PetscCall(PetscFree2(exactSol, exactCtx));
      if (error < 1.0e-11) PetscCall(PetscViewerASCIIPrintf(viewer, "L_2 Error: < 1.0e-11\n"));
      else PetscCall(PetscViewerASCIIPrintf(viewer, "L_2 Error: %g\n", (double)error));
    }
    if (snes->reason > 0 && format != PETSC_VIEWER_FAILED) {
      if (((PetscObject)snes)->prefix) {
        PetscCall(PetscViewerASCIIPrintf(viewer, "Nonlinear %s solve converged due to %s iterations %" PetscInt_FMT "\n", ((PetscObject)snes)->prefix, SNESConvergedReasons[snes->reason], snes->iter));
      } else {
        PetscCall(PetscViewerASCIIPrintf(viewer, "Nonlinear solve converged due to %s iterations %" PetscInt_FMT "\n", SNESConvergedReasons[snes->reason], snes->iter));
      }
    } else if (snes->reason <= 0) {
      if (((PetscObject)snes)->prefix) {
        PetscCall(PetscViewerASCIIPrintf(viewer, "Nonlinear %s solve did not converge due to %s iterations %" PetscInt_FMT "\n", ((PetscObject)snes)->prefix, SNESConvergedReasons[snes->reason], snes->iter));
      } else {
        PetscCall(PetscViewerASCIIPrintf(viewer, "Nonlinear solve did not converge due to %s iterations %" PetscInt_FMT "\n", SNESConvergedReasons[snes->reason], snes->iter));
      }
    }
    PetscCall(PetscViewerASCIISubtractTab(viewer, ((PetscObject)snes)->tablevel));
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
   SNESConvergedReasonViewSet - Sets an ADDITIONAL function that is to be used at the
    end of the nonlinear solver to display the conver reason of the nonlinear solver.

   Logically Collective

   Input Parameters:
+  snes - the `SNES` context
.  f - the snes converged reason view function
.  vctx - [optional] user-defined context for private data for the
          snes converged reason view routine (use `NULL` if no context is desired)
-  reasonviewdestroy - [optional] routine that frees reasonview context (may be `NULL`)

   Options Database Keys:
+    -snes_converged_reason        - sets a default `SNESConvergedReasonView()`
-    -snes_converged_reason_view_cancel - cancels all converged reason viewers that have
                            been hardwired into a code by
                            calls to `SNESConvergedReasonViewSet()`, but
                            does not cancel those set via
                            the options database.

   Level: intermediate

   Note:
   Several different converged reason view routines may be set by calling
   `SNESConvergedReasonViewSet()` multiple times; all will be called in the
   order in which they were set.

.seealso: [](chapter_snes), `SNES`, `SNESSolve()`, `SNESConvergedReason`, `SNESGetConvergedReason()`, `SNESConvergedReasonView()`, `SNESConvergedReasonViewCancel()`
@*/
PetscErrorCode SNESConvergedReasonViewSet(SNES snes, PetscErrorCode (*f)(SNES, void *), void *vctx, PetscErrorCode (*reasonviewdestroy)(void **))
{
  PetscInt  i;
  PetscBool identical;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  for (i = 0; i < snes->numberreasonviews; i++) {
    PetscCall(PetscMonitorCompare((PetscErrorCode(*)(void))f, vctx, reasonviewdestroy, (PetscErrorCode(*)(void))snes->reasonview[i], snes->reasonviewcontext[i], snes->reasonviewdestroy[i], &identical));
    if (identical) PetscFunctionReturn(PETSC_SUCCESS);
  }
  PetscCheck(snes->numberreasonviews < MAXSNESREASONVIEWS, PETSC_COMM_SELF, PETSC_ERR_ARG_OUTOFRANGE, "Too many SNES reasonview set");
  snes->reasonview[snes->numberreasonviews]          = f;
  snes->reasonviewdestroy[snes->numberreasonviews]   = reasonviewdestroy;
  snes->reasonviewcontext[snes->numberreasonviews++] = (void *)vctx;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  SNESConvergedReasonViewFromOptions - Processes command line options to determine if/how a `SNESConvergedReason` is to be viewed.
                                       All the user-provided convergedReasonView routines will be involved as well, if they exist.

  Collective

  Input Parameters:
. snes   - the `SNES` object

  Level: advanced

.seealso: [](chapter_snes), `SNES`, `SNESConvergedReason`, `SNESConvergedReasonViewSet()`, `SNESCreate()`, `SNESSetUp()`, `SNESDestroy()`,
          `SNESSetTolerances()`, `SNESConvergedDefault()`, `SNESGetConvergedReason()`, `SNESConvergedReasonView()`
@*/
PetscErrorCode SNESConvergedReasonViewFromOptions(SNES snes)
{
  PetscViewer       viewer;
  PetscBool         flg;
  static PetscBool  incall = PETSC_FALSE;
  PetscViewerFormat format;
  PetscInt          i;

  PetscFunctionBegin;
  if (incall) PetscFunctionReturn(PETSC_SUCCESS);
  incall = PETSC_TRUE;

  /* All user-provided viewers are called first, if they exist. */
  for (i = 0; i < snes->numberreasonviews; i++) PetscCall((*snes->reasonview[i])(snes, snes->reasonviewcontext[i]));

  /* Call PETSc default routine if users ask for it */
  PetscCall(PetscOptionsGetViewer(PetscObjectComm((PetscObject)snes), ((PetscObject)snes)->options, ((PetscObject)snes)->prefix, "-snes_converged_reason", &viewer, &format, &flg));
  if (flg) {
    PetscCall(PetscViewerPushFormat(viewer, format));
    PetscCall(SNESConvergedReasonView(snes, viewer));
    PetscCall(PetscViewerPopFormat(viewer));
    PetscCall(PetscViewerDestroy(&viewer));
  }
  incall = PETSC_FALSE;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESSolve - Solves a nonlinear system F(x) = b.
   Call `SNESSolve()` after calling `SNESCreate()` and optional routines of the form `SNESSetXXX()`.

   Collective

   Input Parameters:
+  snes - the `SNES` context
.  b - the constant part of the equation F(x) = b, or `NULL` to use zero.
-  x - the solution vector.

   Level: beginner

   Note:
   The user should initialize the vector,x, with the initial guess
   for the nonlinear solve prior to calling `SNESSolve()`.  In particular,
   to employ an initial guess of zero, the user should explicitly set
   this vector to zero by calling `VecSet()`.

.seealso: [](chapter_snes), `SNES`, `SNESCreate()`, `SNESDestroy()`, `SNESSetFunction()`, `SNESSetJacobian()`, `SNESSetGridSequence()`, `SNESGetSolution()`,
          `SNESNewtonTRSetPreCheck()`, `SNESNewtonTRGetPreCheck()`, `SNESNewtonTRSetPostCheck()`, `SNESNewtonTRGetPostCheck()`,
          `SNESLineSearchSetPostCheck()`, `SNESLineSearchGetPostCheck()`, `SNESLineSearchSetPreCheck()`, `SNESLineSearchGetPreCheck()`
@*/
PetscErrorCode SNESSolve(SNES snes, Vec b, Vec x)
{
  PetscBool flg;
  PetscInt  grid;
  Vec       xcreated = NULL;
  DM        dm;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  if (x) PetscValidHeaderSpecific(x, VEC_CLASSID, 3);
  if (x) PetscCheckSameComm(snes, 1, x, 3);
  if (b) PetscValidHeaderSpecific(b, VEC_CLASSID, 2);
  if (b) PetscCheckSameComm(snes, 1, b, 2);

  /* High level operations using the nonlinear solver */
  {
    PetscViewer       viewer;
    PetscViewerFormat format;
    PetscInt          num;
    PetscBool         flg;
    static PetscBool  incall = PETSC_FALSE;

    if (!incall) {
      /* Estimate the convergence rate of the discretization */
      PetscCall(PetscOptionsGetViewer(PetscObjectComm((PetscObject)snes), ((PetscObject)snes)->options, ((PetscObject)snes)->prefix, "-snes_convergence_estimate", &viewer, &format, &flg));
      if (flg) {
        PetscConvEst conv;
        DM           dm;
        PetscReal   *alpha; /* Convergence rate of the solution error for each field in the L_2 norm */
        PetscInt     Nf;

        incall = PETSC_TRUE;
        PetscCall(SNESGetDM(snes, &dm));
        PetscCall(DMGetNumFields(dm, &Nf));
        PetscCall(PetscCalloc1(Nf, &alpha));
        PetscCall(PetscConvEstCreate(PetscObjectComm((PetscObject)snes), &conv));
        PetscCall(PetscConvEstSetSolver(conv, (PetscObject)snes));
        PetscCall(PetscConvEstSetFromOptions(conv));
        PetscCall(PetscConvEstSetUp(conv));
        PetscCall(PetscConvEstGetConvRate(conv, alpha));
        PetscCall(PetscViewerPushFormat(viewer, format));
        PetscCall(PetscConvEstRateView(conv, alpha, viewer));
        PetscCall(PetscViewerPopFormat(viewer));
        PetscCall(PetscViewerDestroy(&viewer));
        PetscCall(PetscConvEstDestroy(&conv));
        PetscCall(PetscFree(alpha));
        incall = PETSC_FALSE;
      }
      /* Adaptively refine the initial grid */
      num = 1;
      PetscCall(PetscOptionsGetInt(NULL, ((PetscObject)snes)->prefix, "-snes_adapt_initial", &num, &flg));
      if (flg) {
        DMAdaptor adaptor;

        incall = PETSC_TRUE;
        PetscCall(DMAdaptorCreate(PetscObjectComm((PetscObject)snes), &adaptor));
        PetscCall(DMAdaptorSetSolver(adaptor, snes));
        PetscCall(DMAdaptorSetSequenceLength(adaptor, num));
        PetscCall(DMAdaptorSetFromOptions(adaptor));
        PetscCall(DMAdaptorSetUp(adaptor));
        PetscCall(DMAdaptorAdapt(adaptor, x, DM_ADAPTATION_INITIAL, &dm, &x));
        PetscCall(DMAdaptorDestroy(&adaptor));
        incall = PETSC_FALSE;
      }
      /* Use grid sequencing to adapt */
      num = 0;
      PetscCall(PetscOptionsGetInt(NULL, ((PetscObject)snes)->prefix, "-snes_adapt_sequence", &num, NULL));
      if (num) {
        DMAdaptor adaptor;

        incall = PETSC_TRUE;
        PetscCall(DMAdaptorCreate(PetscObjectComm((PetscObject)snes), &adaptor));
        PetscCall(DMAdaptorSetSolver(adaptor, snes));
        PetscCall(DMAdaptorSetSequenceLength(adaptor, num));
        PetscCall(DMAdaptorSetFromOptions(adaptor));
        PetscCall(DMAdaptorSetUp(adaptor));
        PetscCall(DMAdaptorAdapt(adaptor, x, DM_ADAPTATION_SEQUENTIAL, &dm, &x));
        PetscCall(DMAdaptorDestroy(&adaptor));
        incall = PETSC_FALSE;
      }
    }
  }
  if (!x) x = snes->vec_sol;
  if (!x) {
    PetscCall(SNESGetDM(snes, &dm));
    PetscCall(DMCreateGlobalVector(dm, &xcreated));
    x = xcreated;
  }
  PetscCall(SNESViewFromOptions(snes, NULL, "-snes_view_pre"));

  for (grid = 0; grid < snes->gridsequence; grid++) PetscCall(PetscViewerASCIIPushTab(PETSC_VIEWER_STDOUT_(PetscObjectComm((PetscObject)snes))));
  for (grid = 0; grid < snes->gridsequence + 1; grid++) {
    /* set solution vector */
    if (!grid) PetscCall(PetscObjectReference((PetscObject)x));
    PetscCall(VecDestroy(&snes->vec_sol));
    snes->vec_sol = x;
    PetscCall(SNESGetDM(snes, &dm));

    /* set affine vector if provided */
    if (b) PetscCall(PetscObjectReference((PetscObject)b));
    PetscCall(VecDestroy(&snes->vec_rhs));
    snes->vec_rhs = b;

    if (snes->vec_rhs) PetscCheck(snes->vec_func != snes->vec_rhs, PETSC_COMM_SELF, PETSC_ERR_ARG_IDN, "Right hand side vector cannot be function vector");
    PetscCheck(snes->vec_func != snes->vec_sol, PETSC_COMM_SELF, PETSC_ERR_ARG_IDN, "Solution vector cannot be function vector");
    PetscCheck(snes->vec_rhs != snes->vec_sol, PETSC_COMM_SELF, PETSC_ERR_ARG_IDN, "Solution vector cannot be right hand side vector");
    if (!snes->vec_sol_update /* && snes->vec_sol */) PetscCall(VecDuplicate(snes->vec_sol, &snes->vec_sol_update));
    PetscCall(DMShellSetGlobalVector(dm, snes->vec_sol));
    PetscCall(SNESSetUp(snes));

    if (!grid) {
      if (snes->ops->computeinitialguess) PetscCallBack("SNES callback initial guess", (*snes->ops->computeinitialguess)(snes, snes->vec_sol, snes->initialguessP));
    }

    if (snes->conv_hist_reset) snes->conv_hist_len = 0;
    if (snes->counters_reset) {
      snes->nfuncs      = 0;
      snes->linear_its  = 0;
      snes->numFailures = 0;
    }

    PetscCall(PetscLogEventBegin(SNES_Solve, snes, 0, 0, 0));
    PetscUseTypeMethod(snes, solve);
    PetscCall(PetscLogEventEnd(SNES_Solve, snes, 0, 0, 0));
    PetscCheck(snes->reason, PETSC_COMM_SELF, PETSC_ERR_PLIB, "Internal error, solver returned without setting converged reason");
    snes->domainerror = PETSC_FALSE; /* clear the flag if it has been set */

    if (snes->lagjac_persist) snes->jac_iter += snes->iter;
    if (snes->lagpre_persist) snes->pre_iter += snes->iter;

    PetscCall(PetscOptionsGetViewer(PetscObjectComm((PetscObject)snes), ((PetscObject)snes)->options, ((PetscObject)snes)->prefix, "-snes_test_local_min", NULL, NULL, &flg));
    if (flg && !PetscPreLoadingOn) PetscCall(SNESTestLocalMin(snes));
    /* Call converged reason views. This may involve user-provided viewers as well */
    PetscCall(SNESConvergedReasonViewFromOptions(snes));

    if (snes->errorifnotconverged) PetscCheck(snes->reason >= 0, PetscObjectComm((PetscObject)snes), PETSC_ERR_NOT_CONVERGED, "SNESSolve has not converged");
    if (snes->reason < 0) break;
    if (grid < snes->gridsequence) {
      DM  fine;
      Vec xnew;
      Mat interp;

      PetscCall(DMRefine(snes->dm, PetscObjectComm((PetscObject)snes), &fine));
      PetscCheck(fine, PetscObjectComm((PetscObject)snes), PETSC_ERR_ARG_INCOMP, "DMRefine() did not perform any refinement, cannot continue grid sequencing");
      PetscCall(DMCreateInterpolation(snes->dm, fine, &interp, NULL));
      PetscCall(DMCreateGlobalVector(fine, &xnew));
      PetscCall(MatInterpolate(interp, x, xnew));
      PetscCall(DMInterpolate(snes->dm, interp, fine));
      PetscCall(MatDestroy(&interp));
      x = xnew;

      PetscCall(SNESReset(snes));
      PetscCall(SNESSetDM(snes, fine));
      PetscCall(SNESResetFromOptions(snes));
      PetscCall(DMDestroy(&fine));
      PetscCall(PetscViewerASCIIPopTab(PETSC_VIEWER_STDOUT_(PetscObjectComm((PetscObject)snes))));
    }
  }
  PetscCall(SNESViewFromOptions(snes, NULL, "-snes_view"));
  PetscCall(VecViewFromOptions(snes->vec_sol, (PetscObject)snes, "-snes_view_solution"));
  PetscCall(DMMonitor(snes->dm));
  PetscCall(SNESMonitorPauseFinal_Internal(snes));

  PetscCall(VecDestroy(&xcreated));
  PetscCall(PetscObjectSAWsBlock((PetscObject)snes));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/* --------- Internal routines for SNES Package --------- */

/*@C
   SNESSetType - Sets the method for the nonlinear solver.

   Collective

   Input Parameters:
+  snes - the `SNES` context
-  type - a known method

   Options Database Key:
.  -snes_type <type> - Sets the method; use -help for a list
   of available methods (for instance, newtonls or newtontr)

  Level: intermediate

   Notes:
   See "petsc/include/petscsnes.h" for available methods (for instance)
+    `SNESNEWTONLS` - Newton's method with line search
     (systems of nonlinear equations)
-    `SNESNEWTONTR` - Newton's method with trust region
     (systems of nonlinear equations)

  Normally, it is best to use the `SNESSetFromOptions()` command and then
  set the `SNES` solver type from the options database rather than by using
  this routine.  Using the options database provides the user with
  maximum flexibility in evaluating the many nonlinear solvers.
  The `SNESSetType()` routine is provided for those situations where it
  is necessary to set the nonlinear solver independently of the command
  line or options database.  This might be the case, for example, when
  the choice of solver changes during the execution of the program,
  and the user's application is taking responsibility for choosing the
  appropriate method.

    Developer Note:
    `SNESRegister()` adds a constructor for a new `SNESType` to `SNESList`, `SNESSetType()` locates
    the constructor in that list and calls it to create the specific object.

.seealso: [](chapter_snes), `SNES`, `SNESSolve()`, `SNESType`, `SNESCreate()`, `SNESDestroy()`, `SNESGetType()`, `SNESSetFromOptions()`
@*/
PetscErrorCode SNESSetType(SNES snes, SNESType type)
{
  PetscBool match;
  PetscErrorCode (*r)(SNES);

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidCharPointer(type, 2);

  PetscCall(PetscObjectTypeCompare((PetscObject)snes, type, &match));
  if (match) PetscFunctionReturn(PETSC_SUCCESS);

  PetscCall(PetscFunctionListFind(SNESList, type, &r));
  PetscCheck(r, PETSC_COMM_SELF, PETSC_ERR_ARG_UNKNOWN_TYPE, "Unable to find requested SNES type %s", type);
  /* Destroy the previous private SNES context */
  PetscTryTypeMethod(snes, destroy);
  /* Reinitialize function pointers in SNESOps structure */
  snes->ops->setup          = NULL;
  snes->ops->solve          = NULL;
  snes->ops->view           = NULL;
  snes->ops->setfromoptions = NULL;
  snes->ops->destroy        = NULL;

  /* It may happen the user has customized the line search before calling SNESSetType */
  if (((PetscObject)snes)->type_name) PetscCall(SNESLineSearchDestroy(&snes->linesearch));

  /* Call the SNESCreate_XXX routine for this particular Nonlinear solver */
  snes->setupcalled = PETSC_FALSE;

  PetscCall(PetscObjectChangeTypeName((PetscObject)snes, type));
  PetscCall((*r)(snes));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
   SNESGetType - Gets the `SNES` method type and name (as a string).

   Not Collective

   Input Parameter:
.  snes - nonlinear solver context

   Output Parameter:
.  type - `SNES` method (a character string)

   Level: intermediate

.seealso: [](chapter_snes), `SNESSetType()`, `SNESType`, `SNESSetFromOptions()`, `SNES`
@*/
PetscErrorCode SNESGetType(SNES snes, SNESType *type)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidPointer(type, 2);
  *type = ((PetscObject)snes)->type_name;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  SNESSetSolution - Sets the solution vector for use by the `SNES` routines.

  Logically Collective

  Input Parameters:
+ snes - the `SNES` context obtained from `SNESCreate()`
- u    - the solution vector

  Level: beginner

.seealso: [](chapter_snes), `SNES`, `SNESSolve()`, `SNESGetSolution()`, `Vec`
@*/
PetscErrorCode SNESSetSolution(SNES snes, Vec u)
{
  DM dm;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidHeaderSpecific(u, VEC_CLASSID, 2);
  PetscCall(PetscObjectReference((PetscObject)u));
  PetscCall(VecDestroy(&snes->vec_sol));

  snes->vec_sol = u;

  PetscCall(SNESGetDM(snes, &dm));
  PetscCall(DMShellSetGlobalVector(dm, u));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESGetSolution - Returns the vector where the approximate solution is
   stored. This is the fine grid solution when using `SNESSetGridSequence()`.

   Not Collective, but x is parallel if snes is parallel

   Input Parameter:
.  snes - the `SNES` context

   Output Parameter:
.  x - the solution

   Level: intermediate

.seealso: [](chapter_snes), `SNESSetSolution()`, `SNESSolve()`, `SNES`, `SNESGetSolutionUpdate()`, `SNESGetFunction()`
@*/
PetscErrorCode SNESGetSolution(SNES snes, Vec *x)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidPointer(x, 2);
  *x = snes->vec_sol;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESGetSolutionUpdate - Returns the vector where the solution update is
   stored.

   Not Collective, but x is parallel if snes is parallel

   Input Parameter:
.  snes - the `SNES` context

   Output Parameter:
.  x - the solution update

   Level: advanced

.seealso: [](chapter_snes), `SNES`, `SNESGetSolution()`, `SNESGetFunction()`
@*/
PetscErrorCode SNESGetSolutionUpdate(SNES snes, Vec *x)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidPointer(x, 2);
  *x = snes->vec_sol_update;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
   SNESGetFunction - Returns the function that defines the nonlinear system set with `SNESSetFunction()`

   Not Collective, but r is parallel if snes is parallel. Collective if r is requested, but has not been created yet.

   Input Parameter:
.  snes - the `SNES` context

   Output Parameters:
+  r - the vector that is used to store residuals (or `NULL` if you don't want it)
.  f - the function (or `NULL` if you don't want it);  for calling sequence see `SNESFunction`
-  ctx - the function context (or `NULL` if you don't want it)

   Level: advanced

    Note:
   The vector `r` DOES NOT, in general, contain the current value of the `SNES` nonlinear function

.seealso: [](chapter_snes), `SNES, `SNESSolve()`, `SNESSetFunction()`, `SNESGetSolution()`, `SNESFunction`
@*/
PetscErrorCode SNESGetFunction(SNES snes, Vec *r, PetscErrorCode (**f)(SNES, Vec, Vec, void *), void **ctx)
{
  DM dm;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  if (r) {
    if (!snes->vec_func) {
      if (snes->vec_rhs) {
        PetscCall(VecDuplicate(snes->vec_rhs, &snes->vec_func));
      } else if (snes->vec_sol) {
        PetscCall(VecDuplicate(snes->vec_sol, &snes->vec_func));
      } else if (snes->dm) {
        PetscCall(DMCreateGlobalVector(snes->dm, &snes->vec_func));
      }
    }
    *r = snes->vec_func;
  }
  PetscCall(SNESGetDM(snes, &dm));
  PetscCall(DMSNESGetFunction(dm, f, ctx));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
   SNESGetNGS - Returns the `SNESNGS` function and context set with `SNESSetNGS()`

   Input Parameter:
.  snes - the `SNES` context

   Output Parameters:
+  f - the function (or `NULL`) see `SNESNGSFunction` for details
-  ctx    - the function context (or `NULL`)

   Level: advanced

.seealso: [](chapter_snes), `SNESSetNGS()`, `SNESGetFunction()`
@*/

PetscErrorCode SNESGetNGS(SNES snes, PetscErrorCode (**f)(SNES, Vec, Vec, void *), void **ctx)
{
  DM dm;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscCall(SNESGetDM(snes, &dm));
  PetscCall(DMSNESGetNGS(dm, f, ctx));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
   SNESSetOptionsPrefix - Sets the prefix used for searching for all
   `SNES` options in the database.

   Logically Collective

   Input Parameters:
+  snes - the `SNES` context
-  prefix - the prefix to prepend to all option names

   Level: advanced

   Note:
   A hyphen (-) must NOT be given at the beginning of the prefix name.
   The first character of all runtime options is AUTOMATICALLY the hyphen.

.seealso: [](chapter_snes), `SNES`, `SNESSetFromOptions()`, `SNESAppendOptionsPrefix()`
@*/
PetscErrorCode SNESSetOptionsPrefix(SNES snes, const char prefix[])
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscCall(PetscObjectSetOptionsPrefix((PetscObject)snes, prefix));
  if (!snes->ksp) PetscCall(SNESGetKSP(snes, &snes->ksp));
  if (snes->linesearch) {
    PetscCall(SNESGetLineSearch(snes, &snes->linesearch));
    PetscCall(PetscObjectSetOptionsPrefix((PetscObject)snes->linesearch, prefix));
  }
  PetscCall(KSPSetOptionsPrefix(snes->ksp, prefix));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
   SNESAppendOptionsPrefix - Appends to the prefix used for searching for all
   `SNES` options in the database.

   Logically Collective

   Input Parameters:
+  snes - the `SNES` context
-  prefix - the prefix to prepend to all option names

   Level: advanced

   Note:
   A hyphen (-) must NOT be given at the beginning of the prefix name.
   The first character of all runtime options is AUTOMATICALLY the hyphen.

.seealso: [](chapter_snes), `SNESGetOptionsPrefix()`, `SNESSetOptionsPrefix()`
@*/
PetscErrorCode SNESAppendOptionsPrefix(SNES snes, const char prefix[])
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscCall(PetscObjectAppendOptionsPrefix((PetscObject)snes, prefix));
  if (!snes->ksp) PetscCall(SNESGetKSP(snes, &snes->ksp));
  if (snes->linesearch) {
    PetscCall(SNESGetLineSearch(snes, &snes->linesearch));
    PetscCall(PetscObjectAppendOptionsPrefix((PetscObject)snes->linesearch, prefix));
  }
  PetscCall(KSPAppendOptionsPrefix(snes->ksp, prefix));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
   SNESGetOptionsPrefix - Gets the prefix used for searching for all
   `SNES` options in the database.

   Not Collective

   Input Parameter:
.  snes - the `SNES` context

   Output Parameter:
.  prefix - pointer to the prefix string used

   Level: advanced

   Fortran Note:
    The user should pass in a string 'prefix' of
   sufficient length to hold the prefix.

.seealso: [](chapter_snes), `SNES`, `SNESSetOptionsPrefix()`, `SNESAppendOptionsPrefix()`
@*/
PetscErrorCode SNESGetOptionsPrefix(SNES snes, const char *prefix[])
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscCall(PetscObjectGetOptionsPrefix((PetscObject)snes, prefix));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
  SNESRegister - Adds a method to the nonlinear solver package.

   Not Collective

   Input Parameters:
+  sname - name of a new user-defined solver
-  function - routine to create method context

   Level: advanced

   Note:
   `SNESRegister()` may be called multiple times to add several user-defined solvers.

   Sample usage:
.vb
   SNESRegister("my_solver",MySolverCreate);
.ve

   Then, your solver can be chosen with the procedural interface via
$     SNESSetType(snes,"my_solver")
   or at runtime via the option
$     -snes_type my_solver

.seealso: [](chapter_snes), `SNESRegisterAll()`, `SNESRegisterDestroy()`
@*/
PetscErrorCode SNESRegister(const char sname[], PetscErrorCode (*function)(SNES))
{
  PetscFunctionBegin;
  PetscCall(SNESInitializePackage());
  PetscCall(PetscFunctionListAdd(&SNESList, sname, function));
  PetscFunctionReturn(PETSC_SUCCESS);
}

PetscErrorCode SNESTestLocalMin(SNES snes)
{
  PetscInt    N, i, j;
  Vec         u, uh, fh;
  PetscScalar value;
  PetscReal   norm;

  PetscFunctionBegin;
  PetscCall(SNESGetSolution(snes, &u));
  PetscCall(VecDuplicate(u, &uh));
  PetscCall(VecDuplicate(u, &fh));

  /* currently only works for sequential */
  PetscCall(PetscPrintf(PetscObjectComm((PetscObject)snes), "Testing FormFunction() for local min\n"));
  PetscCall(VecGetSize(u, &N));
  for (i = 0; i < N; i++) {
    PetscCall(VecCopy(u, uh));
    PetscCall(PetscPrintf(PetscObjectComm((PetscObject)snes), "i = %" PetscInt_FMT "\n", i));
    for (j = -10; j < 11; j++) {
      value = PetscSign(j) * PetscExpReal(PetscAbs(j) - 10.0);
      PetscCall(VecSetValue(uh, i, value, ADD_VALUES));
      PetscCall(SNESComputeFunction(snes, uh, fh));
      PetscCall(VecNorm(fh, NORM_2, &norm));
      PetscCall(PetscPrintf(PetscObjectComm((PetscObject)snes), "       j norm %" PetscInt_FMT " %18.16e\n", j, (double)norm));
      value = -value;
      PetscCall(VecSetValue(uh, i, value, ADD_VALUES));
    }
  }
  PetscCall(VecDestroy(&uh));
  PetscCall(VecDestroy(&fh));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESKSPSetUseEW - Sets `SNES` to the use Eisenstat-Walker method for
   computing relative tolerance for linear solvers within an inexact
   Newton method.

   Logically Collective

   Input Parameters:
+  snes - `SNES` context
-  flag - `PETSC_TRUE` or `PETSC_FALSE`

    Options Database Keys:
+  -snes_ksp_ew - use Eisenstat-Walker method for determining linear system convergence
.  -snes_ksp_ew_version ver - version of  Eisenstat-Walker method
.  -snes_ksp_ew_rtol0 <rtol0> - Sets rtol0
.  -snes_ksp_ew_rtolmax <rtolmax> - Sets rtolmax
.  -snes_ksp_ew_gamma <gamma> - Sets gamma
.  -snes_ksp_ew_alpha <alpha> - Sets alpha
.  -snes_ksp_ew_alpha2 <alpha2> - Sets alpha2
-  -snes_ksp_ew_threshold <threshold> - Sets threshold

   Level: advanced

   Note:
   The default is to use a constant relative tolerance for
   the inner linear solvers.  Alternatively, one can use the
   Eisenstat-Walker method, where the relative convergence tolerance
   is reset at each Newton iteration according progress of the nonlinear
   solver.

   Reference:
.  - * S. C. Eisenstat and H. F. Walker, "Choosing the forcing terms in an inexact Newton method", SISC 17 (1), pp.16-32, 1996.

.seealso: [](chapter_snes), `KSP`, `SNES`, `SNESKSPGetUseEW()`, `SNESKSPGetParametersEW()`, `SNESKSPSetParametersEW()`
@*/
PetscErrorCode SNESKSPSetUseEW(SNES snes, PetscBool flag)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidLogicalCollectiveBool(snes, flag, 2);
  snes->ksp_ewconv = flag;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESKSPGetUseEW - Gets if `SNES` is using Eisenstat-Walker method
   for computing relative tolerance for linear solvers within an
   inexact Newton method.

   Not Collective

   Input Parameter:
.  snes - `SNES` context

   Output Parameter:
.  flag - `PETSC_TRUE` or `PETSC_FALSE`

   Level: advanced

.seealso: [](chapter_snes), `SNESKSPSetUseEW()`, `SNESKSPGetParametersEW()`, `SNESKSPSetParametersEW()`
@*/
PetscErrorCode SNESKSPGetUseEW(SNES snes, PetscBool *flag)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidBoolPointer(flag, 2);
  *flag = snes->ksp_ewconv;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESKSPSetParametersEW - Sets parameters for Eisenstat-Walker
   convergence criteria for the linear solvers within an inexact
   Newton method.

   Logically Collective

   Input Parameters:
+    snes - `SNES` context
.    version - version 1, 2 (default is 2), 3 or 4
.    rtol_0 - initial relative tolerance (0 <= rtol_0 < 1)
.    rtol_max - maximum relative tolerance (0 <= rtol_max < 1)
.    gamma - multiplicative factor for version 2 rtol computation
             (0 <= gamma2 <= 1)
.    alpha - power for version 2 rtol computation (1 < alpha <= 2)
.    alpha2 - power for safeguard
-    threshold - threshold for imposing safeguard (0 < threshold < 1)

   Level: advanced

   Notes:
   Version 3 was contributed by Luis Chacon, June 2006.

   Use `PETSC_DEFAULT` to retain the default for any of the parameters.

.seealso: [](chapter_snes), `SNES`, `SNESKSPSetUseEW()`, `SNESKSPGetUseEW()`, `SNESKSPGetParametersEW()`
@*/
PetscErrorCode SNESKSPSetParametersEW(SNES snes, PetscInt version, PetscReal rtol_0, PetscReal rtol_max, PetscReal gamma, PetscReal alpha, PetscReal alpha2, PetscReal threshold)
{
  SNESKSPEW *kctx;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  kctx = (SNESKSPEW *)snes->kspconvctx;
  PetscCheck(kctx, PETSC_COMM_SELF, PETSC_ERR_ARG_WRONGSTATE, "No Eisenstat-Walker context existing");
  PetscValidLogicalCollectiveInt(snes, version, 2);
  PetscValidLogicalCollectiveReal(snes, rtol_0, 3);
  PetscValidLogicalCollectiveReal(snes, rtol_max, 4);
  PetscValidLogicalCollectiveReal(snes, gamma, 5);
  PetscValidLogicalCollectiveReal(snes, alpha, 6);
  PetscValidLogicalCollectiveReal(snes, alpha2, 7);
  PetscValidLogicalCollectiveReal(snes, threshold, 8);

  if (version != PETSC_DEFAULT) kctx->version = version;
  if (rtol_0 != (PetscReal)PETSC_DEFAULT) kctx->rtol_0 = rtol_0;
  if (rtol_max != (PetscReal)PETSC_DEFAULT) kctx->rtol_max = rtol_max;
  if (gamma != (PetscReal)PETSC_DEFAULT) kctx->gamma = gamma;
  if (alpha != (PetscReal)PETSC_DEFAULT) kctx->alpha = alpha;
  if (alpha2 != (PetscReal)PETSC_DEFAULT) kctx->alpha2 = alpha2;
  if (threshold != (PetscReal)PETSC_DEFAULT) kctx->threshold = threshold;

  PetscCheck(kctx->version >= 1 && kctx->version <= 4, PETSC_COMM_SELF, PETSC_ERR_ARG_OUTOFRANGE, "Only versions 1 to 4 are supported: %" PetscInt_FMT, kctx->version);
  PetscCheck(kctx->rtol_0 >= 0.0 && kctx->rtol_0 < 1.0, PETSC_COMM_SELF, PETSC_ERR_ARG_OUTOFRANGE, "0.0 <= rtol_0 < 1.0: %g", (double)kctx->rtol_0);
  PetscCheck(kctx->rtol_max >= 0.0 && kctx->rtol_max < 1.0, PETSC_COMM_SELF, PETSC_ERR_ARG_OUTOFRANGE, "0.0 <= rtol_max (%g) < 1.0", (double)kctx->rtol_max);
  PetscCheck(kctx->gamma >= 0.0 && kctx->gamma <= 1.0, PETSC_COMM_SELF, PETSC_ERR_ARG_OUTOFRANGE, "0.0 <= gamma (%g) <= 1.0", (double)kctx->gamma);
  PetscCheck(kctx->alpha > 1.0 && kctx->alpha <= 2.0, PETSC_COMM_SELF, PETSC_ERR_ARG_OUTOFRANGE, "1.0 < alpha (%g) <= 2.0", (double)kctx->alpha);
  PetscCheck(kctx->threshold > 0.0 && kctx->threshold < 1.0, PETSC_COMM_SELF, PETSC_ERR_ARG_OUTOFRANGE, "0.0 < threshold (%g) < 1.0", (double)kctx->threshold);
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESKSPGetParametersEW - Gets parameters for Eisenstat-Walker
   convergence criteria for the linear solvers within an inexact
   Newton method.

   Not Collective

   Input Parameter:
.    snes - `SNES` context

   Output Parameters:
+    version - version 1, 2 (default is 2), 3 or 4
.    rtol_0 - initial relative tolerance (0 <= rtol_0 < 1)
.    rtol_max - maximum relative tolerance (0 <= rtol_max < 1)
.    gamma - multiplicative factor for version 2 rtol computation (0 <= gamma2 <= 1)
.    alpha - power for version 2 rtol computation (1 < alpha <= 2)
.    alpha2 - power for safeguard
-    threshold - threshold for imposing safeguard (0 < threshold < 1)

   Level: advanced

.seealso: [](chapter_snes), `SNES`, `SNESKSPSetUseEW()`, `SNESKSPGetUseEW()`, `SNESKSPSetParametersEW()`
@*/
PetscErrorCode SNESKSPGetParametersEW(SNES snes, PetscInt *version, PetscReal *rtol_0, PetscReal *rtol_max, PetscReal *gamma, PetscReal *alpha, PetscReal *alpha2, PetscReal *threshold)
{
  SNESKSPEW *kctx;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  kctx = (SNESKSPEW *)snes->kspconvctx;
  PetscCheck(kctx, PETSC_COMM_SELF, PETSC_ERR_ARG_WRONGSTATE, "No Eisenstat-Walker context existing");
  if (version) *version = kctx->version;
  if (rtol_0) *rtol_0 = kctx->rtol_0;
  if (rtol_max) *rtol_max = kctx->rtol_max;
  if (gamma) *gamma = kctx->gamma;
  if (alpha) *alpha = kctx->alpha;
  if (alpha2) *alpha2 = kctx->alpha2;
  if (threshold) *threshold = kctx->threshold;
  PetscFunctionReturn(PETSC_SUCCESS);
}

PetscErrorCode KSPPreSolve_SNESEW(KSP ksp, Vec b, Vec x, SNES snes)
{
  SNESKSPEW *kctx = (SNESKSPEW *)snes->kspconvctx;
  PetscReal  rtol = PETSC_DEFAULT, stol;

  PetscFunctionBegin;
  if (!snes->ksp_ewconv) PetscFunctionReturn(PETSC_SUCCESS);
  if (!snes->iter) {
    rtol = kctx->rtol_0; /* first time in, so use the original user rtol */
    PetscCall(VecNorm(snes->vec_func, NORM_2, &kctx->norm_first));
  } else {
    PetscCheck(kctx->version >= 1 && kctx->version <= 4, PETSC_COMM_SELF, PETSC_ERR_ARG_OUTOFRANGE, "Only versions 1-4 are supported: %" PetscInt_FMT, kctx->version);
    if (kctx->version == 1) {
      rtol = PetscAbsReal(snes->norm - kctx->lresid_last) / kctx->norm_last;
      stol = PetscPowReal(kctx->rtol_last, kctx->alpha2);
      if (stol > kctx->threshold) rtol = PetscMax(rtol, stol);
    } else if (kctx->version == 2) {
      rtol = kctx->gamma * PetscPowReal(snes->norm / kctx->norm_last, kctx->alpha);
      stol = kctx->gamma * PetscPowReal(kctx->rtol_last, kctx->alpha);
      if (stol > kctx->threshold) rtol = PetscMax(rtol, stol);
    } else if (kctx->version == 3) { /* contributed by Luis Chacon, June 2006. */
      rtol = kctx->gamma * PetscPowReal(snes->norm / kctx->norm_last, kctx->alpha);
      /* safeguard: avoid sharp decrease of rtol */
      stol = kctx->gamma * PetscPowReal(kctx->rtol_last, kctx->alpha);
      stol = PetscMax(rtol, stol);
      rtol = PetscMin(kctx->rtol_0, stol);
      /* safeguard: avoid oversolving */
      stol = kctx->gamma * (kctx->norm_first * snes->rtol) / snes->norm;
      stol = PetscMax(rtol, stol);
      rtol = PetscMin(kctx->rtol_0, stol);
    } else /* if (kctx->version == 4) */ {
      /* H.-B. An et al. Journal of Computational and Applied Mathematics 200 (2007) 47-60 */
      PetscReal ared = PetscAbsReal(kctx->norm_last - snes->norm);
      PetscReal pred = PetscAbsReal(kctx->norm_last - kctx->lresid_last);
      PetscReal rk   = ared / pred;
      if (rk < kctx->v4_p1) rtol = 1. - 2. * kctx->v4_p1;
      else if (rk < kctx->v4_p2) rtol = kctx->rtol_last;
      else if (rk < kctx->v4_p3) rtol = kctx->v4_m1 * kctx->rtol_last;
      else rtol = kctx->v4_m2 * kctx->rtol_last;

      if (kctx->rtol_last_2 > kctx->v4_m3 && kctx->rtol_last > kctx->v4_m3 && kctx->rk_last_2 < kctx->v4_p1 && kctx->rk_last < kctx->v4_p1) {
        rtol = kctx->v4_m4 * kctx->rtol_last;
        //printf("iter %" PetscInt_FMT ", Eisenstat-Walker (version %" PetscInt_FMT ") KSP rtol=%g (rk %g ps %g %g %g) (AD)\n",snes->iter,kctx->version,(double)rtol,rk,kctx->v4_p1,kctx->v4_p2,kctx->v4_p3);
      } else {
        //printf("iter %" PetscInt_FMT ", Eisenstat-Walker (version %" PetscInt_FMT ") KSP rtol=%g (rk %g ps %g %g %g)\n",snes->iter,kctx->version,(double)rtol,rk,kctx->v4_p1,kctx->v4_p2,kctx->v4_p3);
      }
      kctx->rtol_last_2 = kctx->rtol_last;
      kctx->rk_last_2   = kctx->rk_last;
      kctx->rk_last     = rk;
    }
  }
  /* safeguard: avoid rtol greater than rtol_max */
  rtol = PetscMin(rtol, kctx->rtol_max);
  PetscCall(KSPSetTolerances(ksp, rtol, PETSC_DEFAULT, PETSC_DEFAULT, PETSC_DEFAULT));
  PetscCall(PetscInfo(snes, "iter %" PetscInt_FMT ", Eisenstat-Walker (version %" PetscInt_FMT ") KSP rtol=%g\n", snes->iter, kctx->version, (double)rtol));
  PetscFunctionReturn(PETSC_SUCCESS);
}

PetscErrorCode KSPPostSolve_SNESEW(KSP ksp, Vec b, Vec x, SNES snes)
{
  SNESKSPEW *kctx = (SNESKSPEW *)snes->kspconvctx;
  PCSide     pcside;
  Vec        lres;

  PetscFunctionBegin;
  if (!snes->ksp_ewconv) PetscFunctionReturn(PETSC_SUCCESS);
  PetscCall(KSPGetTolerances(ksp, &kctx->rtol_last, NULL, NULL, NULL));
  kctx->norm_last = snes->norm;
  if (kctx->version == 1 || kctx->version == 4) {
    PC        pc;
    PetscBool getRes;

    PetscCall(KSPGetPC(ksp, &pc));
    PetscCall(PetscObjectTypeCompare((PetscObject)pc, PCNONE, &getRes));
    if (!getRes) {
      KSPNormType normtype;

      PetscCall(KSPGetNormType(ksp, &normtype));
      getRes = (PetscBool)(normtype == KSP_NORM_UNPRECONDITIONED);
    }
    PetscCall(KSPGetPCSide(ksp, &pcside));
    if (pcside == PC_RIGHT || getRes) { /* KSP residual is true linear residual */
      PetscCall(KSPGetResidualNorm(ksp, &kctx->lresid_last));
    } else {
      /* KSP residual is preconditioned residual */
      /* compute true linear residual norm */
      Mat J;
      PetscCall(KSPGetOperators(ksp, &J, NULL));
      PetscCall(VecDuplicate(b, &lres));
      PetscCall(MatMult(J, x, lres));
      PetscCall(VecAYPX(lres, -1.0, b));
      PetscCall(VecNorm(lres, NORM_2, &kctx->lresid_last));
      PetscCall(VecDestroy(&lres));
    }
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESGetKSP - Returns the `KSP` context for a `SNES` solver.

   Not Collective, but if snes is parallel, then ksp is parallel

   Input Parameter:
.  snes - the `SNES` context

   Output Parameter:
.  ksp - the `KSP` context

   Level: beginner

   Notes:
   The user can then directly manipulate the `KSP` context to set various
   options, etc.  Likewise, the user can then extract and manipulate the
   `PC` contexts as well.

   Some `SNESType`s do not use a `KSP` but a `KSP` is still returned by this function

.seealso: [](chapter_snes), `SNES`, `KSP`, `PC`, `KSPGetPC()`, `SNESCreate()`, `KSPCreate()`, `SNESSetKSP()`
@*/
PetscErrorCode SNESGetKSP(SNES snes, KSP *ksp)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidPointer(ksp, 2);

  if (!snes->ksp) {
    PetscCall(KSPCreate(PetscObjectComm((PetscObject)snes), &snes->ksp));
    PetscCall(PetscObjectIncrementTabLevel((PetscObject)snes->ksp, (PetscObject)snes, 1));

    PetscCall(KSPSetPreSolve(snes->ksp, (PetscErrorCode(*)(KSP, Vec, Vec, void *))KSPPreSolve_SNESEW, snes));
    PetscCall(KSPSetPostSolve(snes->ksp, (PetscErrorCode(*)(KSP, Vec, Vec, void *))KSPPostSolve_SNESEW, snes));

    PetscCall(KSPMonitorSetFromOptions(snes->ksp, "-snes_monitor_ksp", "snes_preconditioned_residual", snes));
    PetscCall(PetscObjectSetOptions((PetscObject)snes->ksp, ((PetscObject)snes)->options));
  }
  *ksp = snes->ksp;
  PetscFunctionReturn(PETSC_SUCCESS);
}

#include <petsc/private/dmimpl.h>
/*@
   SNESSetDM - Sets the `DM` that may be used by some nonlinear solvers or their underlying preconditioners

   Logically Collective

   Input Parameters:
+  snes - the nonlinear solver context
-  dm - the dm, cannot be `NULL`

   Level: intermediate

   Note:
   A `DM` can only be used for solving one problem at a time because information about the problem is stored on the `DM`,
   even when not using interfaces like `DMSNESSetFunction()`.  Use `DMClone()` to get a distinct `DM` when solving different
   problems using the same function space.

.seealso: [](chapter_snes), `DM`, `SNESGetDM()`, `KSPSetDM()`, `KSPGetDM()`
@*/
PetscErrorCode SNESSetDM(SNES snes, DM dm)
{
  KSP    ksp;
  DMSNES sdm;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidHeaderSpecific(dm, DM_CLASSID, 2);
  PetscCall(PetscObjectReference((PetscObject)dm));
  if (snes->dm) { /* Move the DMSNES context over to the new DM unless the new DM already has one */
    if (snes->dm->dmsnes && !dm->dmsnes) {
      PetscCall(DMCopyDMSNES(snes->dm, dm));
      PetscCall(DMGetDMSNES(snes->dm, &sdm));
      if (sdm->originaldm == snes->dm) sdm->originaldm = dm; /* Grant write privileges to the replacement DM */
    }
    PetscCall(DMCoarsenHookRemove(snes->dm, DMCoarsenHook_SNESVecSol, DMRestrictHook_SNESVecSol, snes));
    PetscCall(DMDestroy(&snes->dm));
  }
  snes->dm     = dm;
  snes->dmAuto = PETSC_FALSE;

  PetscCall(SNESGetKSP(snes, &ksp));
  PetscCall(KSPSetDM(ksp, dm));
  PetscCall(KSPSetDMActive(ksp, PETSC_FALSE));
  if (snes->npc) {
    PetscCall(SNESSetDM(snes->npc, snes->dm));
    PetscCall(SNESSetNPCSide(snes, snes->npcside));
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
   SNESGetDM - Gets the `DM` that may be used by some preconditioners

   Not Collective but dm obtained is parallel on snes

   Input Parameter:
. snes - the preconditioner context

   Output Parameter:
.  dm - the dm

   Level: intermediate

.seealso: [](chapter_snes), `DM`, `SNESSetDM()`, `KSPSetDM()`, `KSPGetDM()`
@*/
PetscErrorCode SNESGetDM(SNES snes, DM *dm)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  if (!snes->dm) {
    PetscCall(DMShellCreate(PetscObjectComm((PetscObject)snes), &snes->dm));
    snes->dmAuto = PETSC_TRUE;
  }
  *dm = snes->dm;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  SNESSetNPC - Sets the nonlinear preconditioner to be used.

  Collective

  Input Parameters:
+ snes - iterative context obtained from `SNESCreate()`
- npc   - the preconditioner object

  Level: developer

  Notes:
  Use `SNESGetNPC()` to retrieve the preconditioner context (for example,
  to configure it using the API).

  Only some `SNESType` can use a nonlinear preconditioner

.seealso: [](chapter_snes), `SNESNGS`, `SNESFAS`, `SNESGetNPC()`, `SNESHasNPC()`
@*/
PetscErrorCode SNESSetNPC(SNES snes, SNES npc)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidHeaderSpecific(npc, SNES_CLASSID, 2);
  PetscCheckSameComm(snes, 1, npc, 2);
  PetscCall(PetscObjectReference((PetscObject)npc));
  PetscCall(SNESDestroy(&snes->npc));
  snes->npc = npc;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  SNESGetNPC - Gets a nonlinear preconditioning solver SNES` to be used to precondition the original nonlinear solver.

  Not Collective; but any changes to the obtained the npc object must be applied collectively

  Input Parameter:
. snes - iterative context obtained from `SNESCreate()`

  Output Parameter:
. npc - preconditioner context

  Options Database Key:
. -npc_snes_type <type> - set the type of the `SNES` to use as the nonlinear preconditioner

  Level: developer

  Notes:
    If a `SNES` was previously set with `SNESSetNPC()` then that value is returned, otherwise a new `SNES` object is created.

    The (preconditioner) `SNES` returned automatically inherits the same nonlinear function and Jacobian supplied to the original
    `SNES`

.seealso: [](chapter_snes), `SNESSetNPC()`, `SNESHasNPC()`, `SNES`, `SNESCreate()`
@*/
PetscErrorCode SNESGetNPC(SNES snes, SNES *pc)
{
  const char *optionsprefix;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidPointer(pc, 2);
  if (!snes->npc) {
    PetscCall(SNESCreate(PetscObjectComm((PetscObject)snes), &snes->npc));
    PetscCall(PetscObjectIncrementTabLevel((PetscObject)snes->npc, (PetscObject)snes, 1));
    PetscCall(SNESGetOptionsPrefix(snes, &optionsprefix));
    PetscCall(SNESSetOptionsPrefix(snes->npc, optionsprefix));
    PetscCall(SNESAppendOptionsPrefix(snes->npc, "npc_"));
    PetscCall(SNESSetCountersReset(snes->npc, PETSC_FALSE));
  }
  *pc = snes->npc;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  SNESHasNPC - Returns whether a nonlinear preconditioner exists

  Not Collective

  Input Parameter:
. snes - iterative context obtained from `SNESCreate()`

  Output Parameter:
. has_npc - whether the `SNES` has an NPC or not

  Level: developer

.seealso: [](chapter_snes), `SNESSetNPC()`, `SNESGetNPC()`
@*/
PetscErrorCode SNESHasNPC(SNES snes, PetscBool *has_npc)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  *has_npc = (PetscBool)(snes->npc ? PETSC_TRUE : PETSC_FALSE);
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
    SNESSetNPCSide - Sets the preconditioning side.

    Logically Collective

    Input Parameter:
.   snes - iterative context obtained from `SNESCreate()`

    Output Parameter:
.   side - the preconditioning side, where side is one of
.vb
      PC_LEFT - left preconditioning
      PC_RIGHT - right preconditioning (default for most nonlinear solvers)
.ve

    Options Database Key:
.   -snes_npc_side <right,left> - nonlinear preconditioner side

    Level: intermediate

    Note:
    `SNESNRICHARDSON` and `SNESNCG` only support left preconditioning.

.seealso: [](chapter_snes), `SNESType`, `SNESGetNPCSide()`, `KSPSetPCSide()`
@*/
PetscErrorCode SNESSetNPCSide(SNES snes, PCSide side)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidLogicalCollectiveEnum(snes, side, 2);
  if (side == PC_SIDE_DEFAULT) side = PC_RIGHT;
  PetscCheck((side == PC_LEFT) || (side == PC_RIGHT), PetscObjectComm((PetscObject)snes), PETSC_ERR_ARG_WRONG, "Only PC_LEFT and PC_RIGHT are supported");
  snes->npcside = side;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
    SNESGetNPCSide - Gets the preconditioning side.

    Not Collective

    Input Parameter:
.   snes - iterative context obtained from `SNESCreate()`

    Output Parameter:
.   side - the preconditioning side, where side is one of
.vb
      `PC_LEFT` - left preconditioning
      `PC_RIGHT` - right preconditioning (default for most nonlinear solvers)
.ve

    Level: intermediate

.seealso: [](chapter_snes), `SNES`, `SNESSetNPCSide()`, `KSPGetPCSide()`
@*/
PetscErrorCode SNESGetNPCSide(SNES snes, PCSide *side)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidPointer(side, 2);
  *side = snes->npcside;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  SNESSetLineSearch - Sets the linesearch on the `SNES` instance.

  Collective

  Input Parameters:
+ snes - iterative context obtained from `SNESCreate()`
- linesearch   - the linesearch object

  Level: developer

  Note:
  Use `SNESGetLineSearch()` to retrieve the preconditioner context (for example,
  to configure it using the API).

.seealso: [](chapter_snes), `SNESGetLineSearch()`
@*/
PetscErrorCode SNESSetLineSearch(SNES snes, SNESLineSearch linesearch)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidHeaderSpecific(linesearch, SNESLINESEARCH_CLASSID, 2);
  PetscCheckSameComm(snes, 1, linesearch, 2);
  PetscCall(PetscObjectReference((PetscObject)linesearch));
  PetscCall(SNESLineSearchDestroy(&snes->linesearch));

  snes->linesearch = linesearch;

  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@
  SNESGetLineSearch - Returns the line search context set with `SNESSetLineSearch()`
  or creates a default line search instance associated with the `SNES` and returns it.

  Not Collective

  Input Parameter:
. snes - iterative context obtained from `SNESCreate()`

  Output Parameter:
. linesearch - linesearch context

  Level: beginner

.seealso: [](chapter_snes), `SNESLineSearch`, `SNESSetLineSearch()`, `SNESLineSearchCreate()`
@*/
PetscErrorCode SNESGetLineSearch(SNES snes, SNESLineSearch *linesearch)
{
  const char *optionsprefix;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidPointer(linesearch, 2);
  if (!snes->linesearch) {
    PetscCall(SNESGetOptionsPrefix(snes, &optionsprefix));
    PetscCall(SNESLineSearchCreate(PetscObjectComm((PetscObject)snes), &snes->linesearch));
    PetscCall(SNESLineSearchSetSNES(snes->linesearch, snes));
    PetscCall(SNESLineSearchAppendOptionsPrefix(snes->linesearch, optionsprefix));
    PetscCall(PetscObjectIncrementTabLevel((PetscObject)snes->linesearch, (PetscObject)snes, 1));
  }
  *linesearch = snes->linesearch;
  PetscFunctionReturn(PETSC_SUCCESS);
}