static char help[] = "Test DMStag ghosted boundaries in 2d\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 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       startx, starty, nx, ny, ex, ey, nExtrax, nExtray;
  PetscInt       iux, iuy, ip;
  PetscInt       dof0, dof1, dof2;
  PetscScalar ***arrSol, ***arrRHS;

  PetscFunctionBeginUser;
  PetscCall(PetscInitialize(&argc, &argv, NULL, help));
  /* Note: these defaults are chosen to suit the problem. We allow adjusting
     them to check that things still work when you add unused extra dof */
  dof0 = 0;
  dof1 = 1;
  dof2 = 1;
  PetscCall(PetscOptionsGetInt(NULL, NULL, "-dof2", &dof2, NULL));
  PetscCall(DMStagCreate2d(PETSC_COMM_WORLD, DM_BOUNDARY_GHOSTED, DM_BOUNDARY_GHOSTED, 3, 3, PETSC_DECIDE, PETSC_DECIDE, dof0, dof1, dof2, DMSTAG_STENCIL_BOX, 1, NULL, 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, &startx, &starty, NULL, &nx, &ny, NULL, &nExtrax, &nExtray, NULL));
  PetscCall(DMStagVecGetArray(dmSol, rhsLocal, &arrRHS));

  /* Get the correct entries for each of our variables in local element-wise storage */
  PetscCall(DMStagGetLocationSlot(dmSol, DMSTAG_LEFT, 0, &iux));
  PetscCall(DMStagGetLocationSlot(dmSol, DMSTAG_DOWN, 0, &iuy));
  PetscCall(DMStagGetLocationSlot(dmSol, DMSTAG_ELEMENT, 0, &ip));
  for (ey = starty; ey < starty + ny + nExtray; ++ey) {
    for (ex = startx; ex < startx + nx + nExtrax; ++ex) {
      arrSol[ey][ex][iux] = 2 * PRESSURE_CONST;
      arrRHS[ey][ex][iux] = 0.0;
      arrSol[ey][ex][iuy] = 2 * PRESSURE_CONST;
      arrRHS[ey][ex][iuy] = 0.0;
      if (ex < startx + nx && ey < starty + ny) {
        arrSol[ey][ex][ip] = PRESSURE_CONST;
        arrRHS[ey][ex][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       startx, starty, nx, ny, nExtrax, nExtray, ex, ey, idxP, idxUx, idxUy, startGhostx, startGhosty, nGhostx, nGhosty;
  PetscBool      isFirstx, isFirsty, isFirstz, isLastx, isLasty, isLastz;

  PetscFunctionBeginUser;
  PetscCall(MatGetDM(A, &dm));
  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, &startx, &starty, NULL, &nx, &ny, NULL, &nExtrax, &nExtray, NULL));
  PetscCall(DMStagGetGhostCorners(dm, &startGhostx, &startGhosty, NULL, &nGhostx, &nGhosty, NULL));
  PetscCall(DMStagVecGetArrayRead(dm, inLocal, &arrIn));
  PetscCall(DMStagVecGetArray(dm, outLocal, &arrOut));
  PetscCall(DMStagGetLocationSlot(dm, DMSTAG_LEFT, 0, &idxUx));
  PetscCall(DMStagGetLocationSlot(dm, DMSTAG_DOWN, 0, &idxUy));
  PetscCall(DMStagGetLocationSlot(dm, DMSTAG_ELEMENT, 0, &idxP));
  PetscCall(DMStagGetIsFirstRank(dm, &isFirstx, &isFirsty, &isFirstz));
  PetscCall(DMStagGetIsLastRank(dm, &isLastx, &isLasty, &isLastz));

  /* Set "pressures" on ghost boundaries by copying neighboring values*/
  if (isFirstx) {
    for (ey = starty; ey < starty + ny + nExtray; ++ey) arrIn[ey][-1][idxP] = arrIn[ey][0][idxP];
  }
  if (isLastx) {
    for (ey = starty; ey < starty + ny + nExtray; ++ey) arrIn[ey][startx + nx][idxP] = arrIn[ey][startx + nx - 1][idxP];
  }
  if (isFirsty) {
    for (ex = startx; ex < startx + nx + nExtrax; ++ex) arrIn[-1][ex][idxP] = arrIn[0][ex][idxP];
  }
  if (isLasty) {
    for (ex = startx; ex < startx + nx + nExtrax; ++ex) arrIn[starty + ny][ex][idxP] = arrIn[starty + ny - 1][ex][idxP];
  }

  /* Apply operator on physical points */
  for (ey = starty; ey < starty + ny + nExtray; ++ey) {
    for (ex = startx; ex < startx + nx + nExtrax; ++ex) {
      if (ex < startx + nx && ey < starty + ny) { /* Don't compute pressure outside domain */
        arrOut[ey][ex][idxP] = arrIn[ey][ex][idxP];
      }
      arrOut[ey][ex][idxUx] = arrIn[ey][ex][idxP] + arrIn[ey][ex - 1][idxP] - arrIn[ey][ex][idxUx];
      arrOut[ey][ex][idxUy] = arrIn[ey][ex][idxP] + arrIn[ey - 1][ex][idxP] - arrIn[ey][ex][idxUy];
    }
  }
  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: 4
      output_file: output/empty.out

   test:
      suffix: 3
      nsize: 1
      args: -stag_stencil_width 2
      output_file: output/empty.out

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

   test:
      suffix: 5
      nsize: 4
      args: -stag_dof_0 3 -stag_dof_1 2 -stag_dof_2 4 -stag_stencil_width 3 -stag_grid_x 6 -stag_grid_y 6
      output_file: output/empty.out

TEST*/