xref: /petsc/src/snes/mf/snesmfj.c (revision 5a655dc69978bfa5d89a399bdbc880d673ee91a3)

#ifdef PETSC_RCS_HEADER
static char vcid[] = "$Id: snesmfj.c,v 1.80 1999/03/17 23:24:23 bsmith Exp bsmith $";
#endif

#include "src/snes/snesimpl.h"
#include "src/snes/mf/snesmfj.h"   /*I  "snes.h"   I*/

FList MatSNESMFList              = 0;
int   MatSNESMFRegisterAllCalled = 0;


#undef __FUNC__
#define __FUNC__ "MatSNESMFSetType"
/*@
      MatSNESMFSetType - Sets the method that is used to compute the h in the
            finite difference matrix free formulation.

   Input Parameters:
+     mat - the matrix free matrix created via MatCreateSNESMF()
-     ftype - the type requested

   Level: advanced

.seealso: MatCreateSNESMF(), MatSNESMFRegister()
@*/
int MatSNESMFSetType(Mat mat,char *ftype)
{
  int          ierr, (*r)(MatSNESMFCtx);
  MatSNESMFCtx ctx;

  PetscFunctionBegin;
  ierr = MatShellGetContext(mat,(void **)&ctx);CHKERRQ(ierr);

  /* already set, so just return */
  if (PetscTypeCompare(ctx->type_name,ftype)) PetscFunctionReturn(0);

  /* destroy the old one if it exists */
  if (ctx->ops->destroy) {
    ierr = (*ctx->ops->destroy)(ctx);CHKERRQ(ierr);
  }

  /* Get the function pointers for the iterative method requested */
  if (!MatSNESMFRegisterAllCalled) {ierr = MatSNESMFRegisterAll(PETSC_NULL); CHKERRQ(ierr);}

  ierr =  FListFind(ctx->comm, MatSNESMFList, ftype,(int (**)(void *)) &r );CHKERRQ(ierr);

  if (!r) SETERRQ(1,1,"Unknown MatSNESMF type given");

  ierr = (*r)(ctx); CHKERRQ(ierr);
  ierr = PetscStrncpy(ctx->type_name,ftype,256);CHKERRQ(ierr);

  PetscFunctionReturn(0);
}

/*MC
   MatSNESMFRegister - Adds a method to the MatSNESMF registry

   Synopsis:
   MatSNESMFRegister(char *name_solver,char *path,char *name_create,int (*routine_create)(MatSNESMF))

   Not Collective

   Input Parameters:
+  name_solver - name of a new user-defined compute-h module
.  path - path (either absolute or relative) the library containing this solver
.  name_create - name of routine to create method context
-  routine_create - routine to create method context

   Level: developer

   Notes:
   MatSNESMFRegister() may be called multiple times to add several user-defined solvers.

   If dynamic libraries are used, then the fourth input argument (routine_create)
   is ignored.

   Sample usage:
.vb
   MatSNESMFRegister("my_h",/home/username/my_lib/lib/libO/solaris/mylib.a,
               "MyHCreate",MyHCreate);
.ve

   Then, your solver can be chosen with the procedural interface via
$     MatSNESMFSetType(mfctx,"my_h")
   or at runtime via the option
$     -snes_mf_type my_h

.keywords: MatSNESMF, register

.seealso: MatSNESMFRegisterAll(), MatSNESMFRegisterDestroy()
M*/

#undef __FUNC__
#define __FUNC__ "MatSNESMFRegister_Private"
int MatSNESMFRegister_Private(char *sname,char *path,char *name,int (*function)(MatSNESMFCtx))
{
  int ierr;
  char fullname[256];

  PetscFunctionBegin;
  PetscStrcpy(fullname,path); PetscStrcat(fullname,":");PetscStrcat(fullname,name);
  ierr = FListAdd_Private(&MatSNESMFList,sname,fullname,(int (*)(void*))function);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}


#undef __FUNC__
#define __FUNC__ "MatSNESMFRegisterDestroy"
/*@C
   MatSNESMFRegisterDestroy - Frees the list of MatSNESMF methods that were
   registered by MatSNESMFRegister().

   Not Collective

   Level: developer

.keywords: MatSNESMF, register, destroy

.seealso: MatSNESMFRegister(), MatSNESMFRegisterAll()
@*/
int MatSNESMFRegisterDestroy(void)
{
  int ierr;

  PetscFunctionBegin;
  if (MatSNESMFList) {
    ierr = FListDestroy( MatSNESMFList );CHKERRQ(ierr);
    MatSNESMFList = 0;
  }
  MatSNESMFRegisterAllCalled = 0;
  PetscFunctionReturn(0);
}

/* ----------------------------------------------------------------------------------------*/
#undef __FUNC__
#define __FUNC__ "MatSNESMFDestroy_Private"
int MatSNESMFDestroy_Private(Mat mat)
{
  int            ierr;
  MatSNESMFCtx ctx;

  PetscFunctionBegin;
  ierr = MatShellGetContext(mat,(void **)&ctx);CHKERRQ(ierr);
  ierr = VecDestroy(ctx->w); CHKERRQ(ierr);
  if (ctx->ops->destroy) {ierr = (*ctx->ops->destroy)(ctx);CHKERRQ(ierr);}
  if (ctx->sp) {ierr = PCNullSpaceDestroy(ctx->sp);CHKERRQ(ierr);}
  PetscFree(ctx->ops);
  PetscFree(ctx);
  PetscFunctionReturn(0);
}

#undef __FUNC__
#define __FUNC__ "MatSNESMFView_Private"
/*
   MatSNESMFView_Private - Views matrix-free parameters.

*/
int MatSNESMFView_Private(Mat J,Viewer viewer)
{
  int            ierr;
  MatSNESMFCtx ctx;
  MPI_Comm       comm;
  FILE           *fd;
  ViewerType     vtype;

  PetscFunctionBegin;
  ierr = PetscObjectGetComm((PetscObject)J,&comm);CHKERRQ(ierr);
  ierr = MatShellGetContext(J,(void **)&ctx); CHKERRQ(ierr);
  ierr = ViewerGetType(viewer,&vtype); CHKERRQ(ierr);
  ierr = ViewerASCIIGetPointer(viewer,&fd); CHKERRQ(ierr);
  if (PetscTypeCompare(vtype,ASCII_VIEWER)) {
     PetscFPrintf(comm,fd,"  SNES matrix-free approximation:\n");
     PetscFPrintf(comm,fd,"    err=%g (relative error in function evaluation)\n",ctx->error_rel);
     PetscFPrintf(ctx->comm,fd,"    Using %s compute h routine\n",ctx->type_name);
     if (ctx->ops->view) {
       ierr = (*ctx->ops->view)(ctx,viewer);CHKERRQ(ierr);
     }
  } else {
    SETERRQ(1,1,"Viewer type not supported for this object");
  }
  PetscFunctionReturn(0);
}

#undef __FUNC__
#define __FUNC__ "MatSNESMFAssemblyEnd_Private"
/*
   MatSNESMFAssemblyEnd_Private - Resets the ctx->ncurrenth to zero. This
     allows the user to indicate the beginning of a new linear solve by call
     MatAssemblyXXX() on the matrix free matrix. This then allows the
     MatSNESMFCreate_WP() to properly compute the || U|| only the first
     time in the linear solver rather than every time

*/
int MatSNESMFAssemblyEnd_Private(Mat J)
{
  int            ierr;

  PetscFunctionBegin;
  ierr = MatSNESMFResetHHistory(J);CHKERRQ(ierr);
  PetscFunctionReturn(0);
}


#undef __FUNC__
#define __FUNC__ "MatSNESMFMult_Private"
/*
  MatSNESMFMult_Private - Default matrix-free form for Jacobian-vector
  product, y = F'(u)*a:

        y ~= ( F(u + ha) - F(u) )/h,
  where F = nonlinear function, as set by SNESSetFunction()
        u = current iterate
        h = difference interval
*/
int MatSNESMFMult_Private(Mat mat,Vec a,Vec y)
{
  MatSNESMFCtx ctx;
  SNES           snes;
  Scalar         h, mone = -1.0;
  Vec            w,U,F;
  int            ierr, (*eval_fct)(SNES,Vec,Vec)=0;

  PetscFunctionBegin;
  /* We log matrix-free matrix-vector products separately, so that we can
     separate the performance monitoring from the cases that use conventional
     storage.  We may eventually modify event logging to associate events
     with particular objects, hence alleviating the more general problem. */
  PLogEventBegin(MAT_MatrixFreeMult,a,y,0,0);

  ierr = MatShellGetContext(mat,(void **)&ctx); CHKERRQ(ierr);
  snes = ctx->snes;
  w    = ctx->w;

  ierr = SNESGetSolution(snes,&U); CHKERRQ(ierr);
  if (snes->method_class == SNES_NONLINEAR_EQUATIONS) {
    eval_fct = SNESComputeFunction;
    ierr = SNESGetFunction(snes,&F); CHKERRQ(ierr);
  } else if (snes->method_class == SNES_UNCONSTRAINED_MINIMIZATION) {
    eval_fct = SNESComputeGradient;
    ierr = SNESGetGradient(snes,&F); CHKERRQ(ierr);
  } else SETERRQ(PETSC_ERR_ARG_OUTOFRANGE,0,"Invalid method class");

  if (!ctx->ops->compute) {
    ierr = MatSNESMFSetType(mat,"default");CHKERRQ(ierr);
    ierr = MatSNESMFSetFromOptions(mat);CHKERRQ(ierr);
  }
  ierr = (*ctx->ops->compute)(ctx,U,a,&h); CHKERRQ(ierr);

  /* keep a record of the current differencing parameter h */
  ctx->currenth = h;
#if defined(USE_PETSC_COMPLEX)
  PLogInfo(mat,"Current differencing parameter: %g + %g i\n",PetscReal(h),PetscImaginary(h));
#else
  PLogInfo(mat,"Current differencing parameter: %g\n",h);
#endif
  if (ctx->historyh && ctx->ncurrenth < ctx->maxcurrenth) {
    ctx->historyh[ctx->ncurrenth++] = h;
  } else {
    ctx->ncurrenth++;
  }

  /* Evaluate function at F(u + ha) */
  ierr = VecWAXPY(&h,a,U,w); CHKERRQ(ierr);
  ierr = eval_fct(snes,w,y); CHKERRQ(ierr);

  ierr = VecAXPY(&mone,F,y); CHKERRQ(ierr);
  h = 1.0/h;
  ierr = VecScale(&h,y); CHKERRQ(ierr);
  if (ctx->sp) {ierr = PCNullSpaceRemove(ctx->sp,y); CHKERRQ(ierr);}

  PLogEventEnd(MAT_MatrixFreeMult,a,y,0,0);
  PetscFunctionReturn(0);
}

#undef __FUNC__
#define __FUNC__ "MatCreateSNESMF"
/*@C
   MatCreateSNESMF - Creates a matrix-free matrix
   context for use with a SNES solver.  This matrix can be used as
   the Jacobian argument for the routine SNESSetJacobian().

   Collective on SNES and Vec

   Input Parameters:
+  snes - the SNES context
-  x - vector where SNES solution is to be stored.

   Output Parameter:
.  J - the matrix-free matrix

   Level: advanced

   Notes:
   The matrix-free matrix context merely contains the function pointers
   and work space for performing finite difference approximations of
   Jacobian-vector products, J(u)*a,

   The default code uses the following approach to compute h

.vb
     J(u)*a = [J(u+h*a) - J(u)]/h where
     h = error_rel*u'a/||a||^2                        if  |u'a| > umin*||a||_{1}
       = error_rel*umin*sign(u'a)*||a||_{1}/||a||^2   otherwise
 where
     error_rel = square root of relative error in function evaluation
     umin = minimum iterate parameter
.ve

   The user can set the error_rel via MatSNESMFSetFunctionError() and
   umin via MatSNESMFDefaultSetUmin()
   See the nonlinear solvers chapter of the users manual for details.

   The user should call MatDestroy() when finished with the matrix-free
   matrix context.

   Options Database Keys:
+  -snes_mf_err <error_rel> - Sets error_rel
.  -snes_mf_unim <umin> - Sets umin (for default PETSc routine that computes h only)
-  -snes_mf_ksp_monitor - KSP monitor routine that prints differencing h

.keywords: SNES, default, matrix-free, create, matrix

.seealso: MatDestroy(), MatSNESMFSetFunctionError(), MatSNESMFDefaultSetUmin()
          MatSNESMFSetHHistory(), MatSNESMFResetHHistory(),
          MatSNESMFGetH(),MatSNESMFKSPMonitor(), MatSNESMFRegister()

@*/
int MatCreateSNESMF(SNES snes,Vec x, Mat *J)
{
  MPI_Comm       comm;
  MatSNESMFCtx mfctx;
  int            n, nloc, ierr;

  PetscFunctionBegin;
  mfctx = (MatSNESMFCtx) PetscMalloc(sizeof(struct _p_MatSNESMFCtx)); CHKPTRQ(mfctx);
  PLogObjectMemory(snes,sizeof(MatSNESMFCtx));
  mfctx->comm         = snes->comm;
  mfctx->sp           = 0;
  mfctx->snes         = snes;
  mfctx->error_rel    = 1.e-8; /* assumes double precision */
  mfctx->currenth     = 0.0;
  mfctx->historyh     = PETSC_NULL;
  mfctx->ncurrenth    = 0;
  mfctx->maxcurrenth  = 0;

  /*
     Create the empty data structure to contain compute-h routines.
     These will be filled in below from the command line options or
     a later call with MatSNESMFSetType() or if that is not called
     then it will default in the first use of MatSNESMFMult_private()
  */
  mfctx->ops                 = (MFOps *)PetscMalloc(sizeof(MFOps)); CHKPTRQ(mfctx->ops);
  mfctx->ops->compute        = 0;
  mfctx->ops->destroy        = 0;
  mfctx->ops->view           = 0;
  mfctx->ops->printhelp      = 0;
  mfctx->ops->setfromoptions = 0;
  mfctx->hctx                = 0;

  ierr = VecDuplicate(x,&mfctx->w); CHKERRQ(ierr);
  ierr = PetscObjectGetComm((PetscObject)x,&comm); CHKERRQ(ierr);
  ierr = VecGetSize(x,&n); CHKERRQ(ierr);
  ierr = VecGetLocalSize(x,&nloc); CHKERRQ(ierr);
  ierr = MatCreateShell(comm,nloc,nloc,n,n,mfctx,J); CHKERRQ(ierr);
  ierr = MatShellSetOperation(*J,MATOP_MULT,(void*)MatSNESMFMult_Private);CHKERRQ(ierr);
  ierr = MatShellSetOperation(*J,MATOP_DESTROY,(void *)MatSNESMFDestroy_Private);CHKERRQ(ierr);
  ierr = MatShellSetOperation(*J,MATOP_VIEW,(void *)MatSNESMFView_Private); CHKERRQ(ierr);
  ierr = MatShellSetOperation(*J,MATOP_ASSEMBLY_END,(void *)MatSNESMFAssemblyEnd_Private);CHKERRQ(ierr);
  PLogObjectParent(*J,mfctx->w);
  PLogObjectParent(snes,*J);

  mfctx->mat = *J;


  PetscFunctionReturn(0);
}

#undef __FUNC__
#define __FUNC__ "MatSNESMFSetFromOptions"
/*@
   MatSNESMFSetFromOptions - Sets the MatSNESMF options from the command line
     parameter.

   Collective on Mat

   Input Parameters:
.   mat - the matrix obtained with MatCreateSNESMF()

      Options Database Keys:
+   -snes_mf_type - <default,wp>
-   -snes_mf_err - square root of estimated relative error in function evaluation

   Level: advanced

.keywords: SNES, matrix-free, parameters

.seealso: MatCreateSNESMF(),MatSNESMFSetHHistory(),
          MatSNESMFResetHHistory(),MatSNESMFKSPMonitor()
@*/
int MatSNESMFSetFromOptions(Mat mat)
{
  MatSNESMFCtx mfctx;
  int            ierr,flg;
  char           ftype[256],p[64];

  PetscFunctionBegin;
  ierr = MatShellGetContext(mat,(void **)&mfctx); CHKERRQ(ierr);
  if (mfctx) {
    /* allow user to set the type */
    ierr = OptionsGetString(mfctx->snes->prefix,"-snes_mf_type",ftype,256,&flg);CHKERRQ(ierr);
    if (flg) {
      ierr = MatSNESMFSetType(mat,ftype);CHKERRQ(ierr);
    }

    ierr = OptionsGetDouble(mfctx->snes->prefix,"-snes_mf_err",&mfctx->error_rel,&flg);CHKERRQ(ierr);
    if (mfctx->ops->setfromoptions) {
      ierr = (*mfctx->ops->setfromoptions)(mfctx);CHKERRQ(ierr);
    }

    ierr = OptionsHasName(PETSC_NULL,"-help",&flg); CHKERRQ(ierr);
    PetscStrcpy(p,"-");
    if (mfctx->snes->prefix) PetscStrcat(p,mfctx->snes->prefix);
    if (flg) {
      (*PetscHelpPrintf)(mfctx->snes->comm,"   %ssnes_mf_err <err>: set sqrt rel error in function (default %g)\n",p,mfctx->error_rel);
      if (mfctx->ops->printhelp) {
        (*mfctx->ops->printhelp)(mfctx);CHKERRQ(ierr);
      }
    }
  }
  PetscFunctionReturn(0);
}

#undef __FUNC__
#define __FUNC__ "MatSNESMFGetH"
/*@
   MatSNESMFGetH - Gets the last h that was used as the differencing
     parameter.

   Not Collective

   Input Parameters:
.   mat - the matrix obtained with MatCreateSNESMF()

   Output Paramter:
.  h - the differencing step size

   Level: advanced

.keywords: SNES, matrix-free, parameters

.seealso: MatCreateSNESMF(),MatSNESMFSetHHistory(),
          MatSNESMFResetHHistory(),MatSNESMFKSPMonitor()
@*/
int MatSNESMFGetH(Mat mat,Scalar *h)
{
  MatSNESMFCtx ctx;
  int            ierr;

  PetscFunctionBegin;
  ierr = MatShellGetContext(mat,(void **)&ctx); CHKERRQ(ierr);
  if (ctx) {
    *h = ctx->currenth;
  }
  PetscFunctionReturn(0);
}

#undef __FUNC__
#define __FUNC__ "MatSNESMFKSPMonitor"
/*
   MatSNESMFKSPMonitor - A KSP monitor for use with the default PETSc
      SNES matrix free routines. Prints the h differencing parameter used at each
      timestep.

*/
int MatSNESMFKSPMonitor(KSP ksp,int n,double rnorm,void *dummy)
{
  PC             pc;
  MatSNESMFCtx ctx;
  int            ierr;
  Mat            mat;
  MPI_Comm       comm;
  PetscTruth     nonzeroinitialguess;

  PetscFunctionBegin;
  ierr = PetscObjectGetComm((PetscObject)ksp,&comm);CHKERRQ(ierr);
  ierr = KSPGetPC(ksp,&pc); CHKERRQ(ierr);
  ierr = KSPGetInitialGuessNonzero(ksp,&nonzeroinitialguess);CHKERRQ(ierr);
  ierr = PCGetOperators(pc,&mat,PETSC_NULL,PETSC_NULL); CHKERRQ(ierr);
  ierr = MatShellGetContext(mat,(void **)&ctx); CHKERRQ(ierr);
  if (!ctx) {
    SETERRQ(1,1,"Matrix is not a matrix free shell matrix");
  }
  if (n > 0 || nonzeroinitialguess) {
#if defined(USE_PETSC_COMPLEX)
    PetscPrintf(comm,"%d KSP Residual norm %14.12e h %g + %g i\n",n,rnorm,
                PetscReal(ctx->currenth),PetscImaginary(ctx->currenth));
#else
    PetscPrintf(comm,"%d KSP Residual norm %14.12e h %g \n",n,rnorm,ctx->currenth);
#endif
  } else {
    PetscPrintf(comm,"%d KSP Residual norm %14.12e\n",n,rnorm);
  }
  PetscFunctionReturn(0);
}

#undef __FUNC__
#define __FUNC__ "MatSNESMFSetFunctionError"
/*@
   MatSNESMFSetFunctionError - Sets the error_rel for the approximation of
   matrix-vector products using finite differences.

   Collective on Mat

   Input Parameters:
+  mat - the matrix free matrix created via MatCreateSNESMF()
-  error_rel - relative error (should be set to the square root of
               the relative error in the function evaluations)

   Options Database Keys:
+  -snes_mf_err <error_rel> - Sets error_rel

   Level: advanced

   Notes:
   The default matrix-free matrix-vector product routine computes
.vb
     J(u)*a = [J(u+h*a) - J(u)]/h where
     h = error_rel*u'a/||a||^2                        if  |u'a| > umin*||a||_{1}
       = error_rel*umin*sign(u'a)*||a||_{1}/||a||^2   else
.ve


.keywords: SNES, matrix-free, parameters

.seealso: MatCreateSNESMF(),MatSNESMFGetH(),
          MatSNESMFSetHHistory(), MatSNESMFResetHHistory(),
          MatSNESMFKSPMonitor()
@*/
int MatSNESMFSetFunctionError(Mat mat,double error)
{
  MatSNESMFCtx ctx;
  int            ierr;

  PetscFunctionBegin;
  ierr = MatShellGetContext(mat,(void **)&ctx); CHKERRQ(ierr);
  if (ctx) {
    if (error != PETSC_DEFAULT) ctx->error_rel = error;
  }
  PetscFunctionReturn(0);
}

#undef __FUNC__
#define __FUNC__ "MatSNESMFAddNullSpace"
/*@
   MatSNESMFAddNullSpace - Provides a null space that
   an operator is supposed to have.  Since roundoff will create a
   small component in the null space, if you know the null space
   you may have it automatically removed.

   Collective on Mat

   Input Parameters:
+  J - the matrix-free matrix context
.  has_cnst - PETSC_TRUE or PETSC_FALSE, indicating if null space has constants
.  n - number of vectors (excluding constant vector) in null space
-  vecs - the vectors that span the null space (excluding the constant vector);
          these vectors must be orthonormal

   Level: advanced

.keywords: SNES, matrix-free, null space

.seealso: MatSNESMFGetH(), MatCreateSNESMF(),
          MatSNESMFSetHHistory(), MatSNESMFResetHHistory(),
          MatSNESMFKSPMonitor(), MatSNESMFErrorRel()

@*/
int MatSNESMFAddNullSpace(Mat J,int has_cnst,int n,Vec *vecs)
{
  int            ierr;
  MatSNESMFCtx ctx;
  MPI_Comm       comm;

  PetscFunctionBegin;
  ierr = PetscObjectGetComm((PetscObject)J,&comm);CHKERRQ(ierr);

  ierr = MatShellGetContext(J,(void **)&ctx); CHKERRQ(ierr);
  /* no context indicates that it is not the "matrix free" matrix type */
  if (!ctx) PetscFunctionReturn(0);
  ierr = PCNullSpaceCreate(comm,has_cnst,n,vecs,&ctx->sp); CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

#undef __FUNC__
#define __FUNC__ "MatSNESMFSetHHistory"
/*@
   MatSNESMFSetHHistory - Sets an array to collect a history
      of the differencing values h computed for the matrix free product

   Collective on Mat

   Input Parameters:
+  J - the matrix-free matrix context
.  histroy - space to hold the h history
-  nhistory - number of entries in history, if more h are generated than
              nhistory the later ones are discarded

   Level: advanced

   Notes:
    Use MatSNESMFResetHHistory() to reset the history counter
    and collect a new batch of h.

.keywords: SNES, matrix-free, h history, differencing history

.seealso: MatSNESMFGetH(), MatCreateSNESMF(),
          MatSNESMFResetHHistory(),
          MatSNESMFKSPMonitor(), MatSNESMFSetFunctionError()

@*/
int MatSNESMFSetHHistory(Mat J,Scalar *history,int nhistory)
{
  int            ierr;
  MatSNESMFCtx ctx;

  PetscFunctionBegin;

  ierr = MatShellGetContext(J,(void **)&ctx); CHKERRQ(ierr);
  /* no context indicates that it is not the "matrix free" matrix type */
  if (!ctx) PetscFunctionReturn(0);
  ctx->historyh    = history;
  ctx->maxcurrenth = nhistory;
  ctx->currenth    = 0;

  PetscFunctionReturn(0);
}

#undef __FUNC__
#define __FUNC__ "MatSNESMFResetHHistory"
/*@
   MatSNESMFResetHHistory - Resets the counter to zero to begin
      collecting a new set of differencing histories.

   Collective on Mat

   Input Parameters:
.  J - the matrix-free matrix context

   Level: advanced

   Notes:
    Use MatSNESMFSetHHistory() to create the original history counter

.keywords: SNES, matrix-free, h history, differencing history

.seealso: MatSNESMFGetH(), MatCreateSNESMF(),
          MatSNESMFSetHHistory(),
          MatSNESMFKSPMonitor(), MatSNESMFSetFunctionError()

@*/
int MatSNESMFResetHHistory(Mat J)
{
  int            ierr;
  MatSNESMFCtx ctx;

  PetscFunctionBegin;

  ierr = MatShellGetContext(J,(void **)&ctx); CHKERRQ(ierr);
  /* no context indicates that it is not the "matrix free" matrix type */
  if (!ctx) PetscFunctionReturn(0);
  ctx->ncurrenth    = 0;

  PetscFunctionReturn(0);
}