static char help[] = "Test DMStag ghosted boundaries in 1d\n\n";
/* This solves a very contrived problem - the "pressure" terms are set to a constant function
   and the "velocity" terms are just the sum of neighboring values of these, hence twice the
   constant */
#include <petscdm.h>
#include <petscksp.h>
#include <petscdmstag.h>

#define LEFT    DMSTAG_LEFT
#define RIGHT   DMSTAG_RIGHT
#define ELEMENT DMSTAG_ELEMENT

#define PRESSURE_CONST 2.0

PetscErrorCode ApplyOperator(Mat, Vec, Vec);

int main(int argc, char **argv)
{
  DM            dmSol;
  Vec           sol, solRef, solRefLocal, rhs, rhsLocal;
  Mat           A;
  KSP           ksp;
  PC            pc;
  PetscInt      start, n, e, nExtra;
  PetscInt      iu, ip;
  PetscScalar **arrSol, **arrRHS;

  PetscFunctionBeginUser;
  PetscCall(PetscInitialize(&argc, &argv, NULL, help));
  PetscCall(DMStagCreate1d(PETSC_COMM_WORLD, DM_BOUNDARY_GHOSTED, 3, 1, 1, DMSTAG_STENCIL_BOX, 1, NULL, &dmSol));
  PetscCall(DMSetFromOptions(dmSol));
  PetscCall(DMSetUp(dmSol));

  /* Compute reference solution on the grid, using direct array access */
  PetscCall(DMCreateGlobalVector(dmSol, &rhs));
  PetscCall(DMCreateGlobalVector(dmSol, &solRef));
  PetscCall(DMGetLocalVector(dmSol, &solRefLocal));
  PetscCall(DMGetLocalVector(dmSol, &rhsLocal));
  PetscCall(DMStagVecGetArray(dmSol, solRefLocal, &arrSol));

  PetscCall(DMStagGetCorners(dmSol, &start, NULL, NULL, &n, NULL, NULL, &nExtra, NULL, NULL));
  PetscCall(DMStagVecGetArray(dmSol, rhsLocal, &arrRHS));

  /* Get the correct entries for each of our variables in local element-wise storage */
  PetscCall(DMStagGetLocationSlot(dmSol, LEFT, 0, &iu));
  PetscCall(DMStagGetLocationSlot(dmSol, ELEMENT, 0, &ip));
  for (e = start; e < start + n + nExtra; ++e) {
    {
      arrSol[e][iu] = 2 * PRESSURE_CONST;
      arrRHS[e][iu] = 0.0;
    }
    if (e < start + n) {
      arrSol[e][ip] = PRESSURE_CONST;
      arrRHS[e][ip] = PRESSURE_CONST;
    }
  }
  PetscCall(DMStagVecRestoreArray(dmSol, rhsLocal, &arrRHS));
  PetscCall(DMLocalToGlobalBegin(dmSol, rhsLocal, INSERT_VALUES, rhs));
  PetscCall(DMLocalToGlobalEnd(dmSol, rhsLocal, INSERT_VALUES, rhs));
  PetscCall(DMStagVecRestoreArray(dmSol, solRefLocal, &arrSol));
  PetscCall(DMLocalToGlobalBegin(dmSol, solRefLocal, INSERT_VALUES, solRef));
  PetscCall(DMLocalToGlobalEnd(dmSol, solRefLocal, INSERT_VALUES, solRef));
  PetscCall(DMRestoreLocalVector(dmSol, &solRefLocal));
  PetscCall(DMRestoreLocalVector(dmSol, &rhsLocal));

  /* Matrix-free Operator */
  PetscCall(DMSetMatType(dmSol, MATSHELL));
  PetscCall(DMCreateMatrix(dmSol, &A));
  PetscCall(MatShellSetOperation(A, MATOP_MULT, (PetscErrorCodeFn *)ApplyOperator));

  /* Solve */
  PetscCall(DMCreateGlobalVector(dmSol, &sol));
  PetscCall(KSPCreate(PETSC_COMM_WORLD, &ksp));
  PetscCall(KSPSetOperators(ksp, A, A));
  PetscCall(KSPGetPC(ksp, &pc));
  PetscCall(PCSetType(pc, PCNONE));
  PetscCall(KSPSetFromOptions(ksp));
  PetscCall(KSPSolve(ksp, rhs, sol));

  /* Check Solution */
  {
    Vec       diff;
    PetscReal normsolRef, errAbs, errRel;

    PetscCall(VecDuplicate(sol, &diff));
    PetscCall(VecCopy(sol, diff));
    PetscCall(VecAXPY(diff, -1.0, solRef));
    PetscCall(VecNorm(diff, NORM_2, &errAbs));
    PetscCall(VecNorm(solRef, NORM_2, &normsolRef));
    errRel = errAbs / normsolRef;
    if (errAbs > 1e14 || errRel > 1e14) {
      PetscCall(PetscPrintf(PetscObjectComm((PetscObject)dmSol), "Error (abs): %g\nError (rel): %g\n", (double)errAbs, (double)errRel));
      PetscCall(PetscPrintf(PetscObjectComm((PetscObject)dmSol), "Non-zero error. Probable failure.\n"));
    }
    PetscCall(VecDestroy(&diff));
  }

  PetscCall(KSPDestroy(&ksp));
  PetscCall(VecDestroy(&sol));
  PetscCall(VecDestroy(&solRef));
  PetscCall(VecDestroy(&rhs));
  PetscCall(MatDestroy(&A));
  PetscCall(DMDestroy(&dmSol));
  PetscCall(PetscFinalize());
  return 0;
}

PetscErrorCode ApplyOperator(Mat A, Vec in, Vec out)
{
  DM             dm;
  Vec            inLocal, outLocal;
  PetscScalar  **arrIn;
  PetscScalar  **arrOut;
  PetscInt       start, n, nExtra, ex, idxP, idxU, startGhost, nGhost;
  DMBoundaryType boundaryType;
  PetscBool      isFirst, isLast;

  PetscFunctionBeginUser;
  PetscCall(MatGetDM(A, &dm));
  PetscCall(DMStagGetBoundaryTypes(dm, &boundaryType, NULL, NULL));
  PetscCheck(boundaryType == DM_BOUNDARY_GHOSTED, PetscObjectComm((PetscObject)dm), PETSC_ERR_ARG_INCOMP, "Ghosted boundaries required");
  PetscCall(DMGetLocalVector(dm, &inLocal));
  PetscCall(DMGetLocalVector(dm, &outLocal));
  PetscCall(DMGlobalToLocalBegin(dm, in, INSERT_VALUES, inLocal));
  PetscCall(DMGlobalToLocalEnd(dm, in, INSERT_VALUES, inLocal));
  PetscCall(DMStagGetCorners(dm, &start, NULL, NULL, &n, NULL, NULL, &nExtra, NULL, NULL));
  PetscCall(DMStagGetGhostCorners(dm, &startGhost, NULL, NULL, &nGhost, NULL, NULL));
  PetscCall(DMStagVecGetArrayRead(dm, inLocal, &arrIn));
  PetscCall(DMStagVecGetArray(dm, outLocal, &arrOut));
  PetscCall(DMStagGetLocationSlot(dm, LEFT, 0, &idxU));
  PetscCall(DMStagGetLocationSlot(dm, ELEMENT, 0, &idxP));
  PetscCall(DMStagGetIsFirstRank(dm, &isFirst, NULL, NULL));
  PetscCall(DMStagGetIsLastRank(dm, &isLast, NULL, NULL));

  /* Set "pressures" on ghost boundaries by copying neighboring values*/
  if (isFirst) arrIn[-1][idxP] = arrIn[0][idxP];
  if (isLast) arrIn[start + n][idxP] = arrIn[start + n - 1][idxP];

  /* Apply operator on physical points */
  for (ex = start; ex < start + n + nExtra; ++ex) {
    if (ex < start + n) { /* Don't compute pressure outside domain */
      arrOut[ex][idxP] = arrIn[ex][idxP];
    }
    arrOut[ex][idxU] = arrIn[ex][idxP] + arrIn[ex - 1][idxP] - arrIn[ex][idxU];
  }
  PetscCall(DMStagVecRestoreArrayRead(dm, inLocal, &arrIn));
  PetscCall(DMStagVecRestoreArray(dm, outLocal, &arrOut));
  PetscCall(DMLocalToGlobalBegin(dm, outLocal, INSERT_VALUES, out));
  PetscCall(DMLocalToGlobalEnd(dm, outLocal, INSERT_VALUES, out));
  PetscCall(DMRestoreLocalVector(dm, &inLocal));
  PetscCall(DMRestoreLocalVector(dm, &outLocal));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*TEST

   test:
      suffix: 1
      nsize: 1
      output_file: output/empty.out

   test:
      suffix: 2
      nsize: 2
      output_file: output/empty.out

   test:
      suffix: 3
      nsize: 3
      args: -stag_grid_x 19
      output_file: output/empty.out

   test:
      suffix: 4
      nsize: 5
      args: -stag_grid_x 21 -stag_stencil_width 2
      output_file: output/empty.out

TEST*/