static char help[] = "Partition tiny grid using hierarchical partitioning and increase overlap using MatIncreaseOverlapSplit.\n\n";

/*
  Include "petscmat.h" so that we can use matrices.  Note that this file
  automatically includes:
     petscsys.h       - base PETSc routines   petscvec.h - vectors
     petscmat.h - matrices
     petscis.h     - index sets
     petscviewer.h - viewers
*/
#include <petscmat.h>

int main(int argc, char **args)
{
  Mat             A, B;
  PetscMPIInt     rank, size, membershipKey;
  PetscInt       *ia, *ja, *indices_sc, isrows_localsize;
  const PetscInt *indices;
  MatPartitioning part;
  IS              is, isrows, isrows_sc;
  IS              coarseparts, fineparts;
  MPI_Comm        comm, scomm;

  PetscFunctionBeginUser;
  PetscCall(PetscInitialize(&argc, &args, NULL, help));
  comm = PETSC_COMM_WORLD;
  PetscCallMPI(MPI_Comm_size(comm, &size));
  PetscCheck(size == 4, comm, PETSC_ERR_WRONG_MPI_SIZE, "Must run with 4 processors ");
  PetscCallMPI(MPI_Comm_rank(comm, &rank));
  /*set a small matrix */
  PetscCall(PetscMalloc1(5, &ia));
  PetscCall(PetscMalloc1(16, &ja));
  if (rank == 0) {
    ja[0]         = 1;
    ja[1]         = 4;
    ja[2]         = 0;
    ja[3]         = 2;
    ja[4]         = 5;
    ja[5]         = 1;
    ja[6]         = 3;
    ja[7]         = 6;
    ja[8]         = 2;
    ja[9]         = 7;
    ia[0]         = 0;
    ia[1]         = 2;
    ia[2]         = 5;
    ia[3]         = 8;
    ia[4]         = 10;
    membershipKey = 0;
  } else if (rank == 1) {
    ja[0]         = 0;
    ja[1]         = 5;
    ja[2]         = 8;
    ja[3]         = 1;
    ja[4]         = 4;
    ja[5]         = 6;
    ja[6]         = 9;
    ja[7]         = 2;
    ja[8]         = 5;
    ja[9]         = 7;
    ja[10]        = 10;
    ja[11]        = 3;
    ja[12]        = 6;
    ja[13]        = 11;
    ia[0]         = 0;
    ia[1]         = 3;
    ia[2]         = 7;
    ia[3]         = 11;
    ia[4]         = 14;
    membershipKey = 0;
  } else if (rank == 2) {
    ja[0]         = 4;
    ja[1]         = 9;
    ja[2]         = 12;
    ja[3]         = 5;
    ja[4]         = 8;
    ja[5]         = 10;
    ja[6]         = 13;
    ja[7]         = 6;
    ja[8]         = 9;
    ja[9]         = 11;
    ja[10]        = 14;
    ja[11]        = 7;
    ja[12]        = 10;
    ja[13]        = 15;
    ia[0]         = 0;
    ia[1]         = 3;
    ia[2]         = 7;
    ia[3]         = 11;
    ia[4]         = 14;
    membershipKey = 1;
  } else {
    ja[0]         = 8;
    ja[1]         = 13;
    ja[2]         = 9;
    ja[3]         = 12;
    ja[4]         = 14;
    ja[5]         = 10;
    ja[6]         = 13;
    ja[7]         = 15;
    ja[8]         = 11;
    ja[9]         = 14;
    ia[0]         = 0;
    ia[1]         = 2;
    ia[2]         = 5;
    ia[3]         = 8;
    ia[4]         = 10;
    membershipKey = 1;
  }
  PetscCall(MatCreateMPIAdj(comm, 4, 16, ia, ja, NULL, &A));
  PetscCall(MatView(A, PETSC_VIEWER_STDOUT_WORLD));
  /*
   Partition the graph of the matrix
  */
  PetscCall(MatPartitioningCreate(comm, &part));
  PetscCall(MatPartitioningSetAdjacency(part, A));
  PetscCall(MatPartitioningSetType(part, MATPARTITIONINGHIERARCH));
  PetscCall(MatPartitioningHierarchicalSetNcoarseparts(part, 2));
  PetscCall(MatPartitioningHierarchicalSetNfineparts(part, 2));
  PetscCall(MatPartitioningSetFromOptions(part));
  /* get new processor owner number of each vertex */
  PetscCall(MatPartitioningApply(part, &is));
  /* coarse parts */
  PetscCall(MatPartitioningHierarchicalGetCoarseparts(part, &coarseparts));
  PetscCall(ISView(coarseparts, PETSC_VIEWER_STDOUT_WORLD));
  /* fine parts */
  PetscCall(MatPartitioningHierarchicalGetFineparts(part, &fineparts));
  PetscCall(ISView(fineparts, PETSC_VIEWER_STDOUT_WORLD));
  /* partitioning */
  PetscCall(ISView(is, PETSC_VIEWER_STDOUT_WORLD));
  /* compute coming rows */
  PetscCall(ISBuildTwoSided(is, NULL, &isrows));
  PetscCall(ISView(isrows, PETSC_VIEWER_STDOUT_WORLD));
  /*create a sub-communicator */
  PetscCallMPI(MPI_Comm_split(comm, membershipKey, rank, &scomm));
  PetscCall(ISGetLocalSize(isrows, &isrows_localsize));
  PetscCall(PetscMalloc1(isrows_localsize, &indices_sc));
  PetscCall(ISGetIndices(isrows, &indices));
  PetscCall(PetscArraycpy(indices_sc, indices, isrows_localsize));
  PetscCall(ISRestoreIndices(isrows, &indices));
  PetscCall(ISDestroy(&is));
  PetscCall(ISDestroy(&coarseparts));
  PetscCall(ISDestroy(&fineparts));
  PetscCall(ISDestroy(&isrows));
  PetscCall(MatPartitioningDestroy(&part));
  /*create a sub-IS on the sub communicator  */
  PetscCall(ISCreateGeneral(scomm, isrows_localsize, indices_sc, PETSC_OWN_POINTER, &isrows_sc));
  PetscCall(MatConvert(A, MATMPIAIJ, MAT_INITIAL_MATRIX, &B));
#if 1
  PetscCall(MatView(B, PETSC_VIEWER_STDOUT_WORLD));
#endif
  /*increase overlap */
  PetscCall(MatIncreaseOverlapSplit(B, 1, &isrows_sc, 1));
  PetscCall(ISView(isrows_sc, NULL));
  PetscCall(ISDestroy(&isrows_sc));
  /*
    Free work space.  All PETSc objects should be destroyed when they
    are no longer needed.
  */
  PetscCall(MatDestroy(&A));
  PetscCall(MatDestroy(&B));
  PetscCall(PetscFinalize());
  return 0;
}