static char help[] = "Tests MatCreateSubmatrix() in parallel.";

#include <petscmat.h>

int main(int argc, char **args)
{
  Mat         C, A;
  PetscInt    i, j, m = 3, n = 2, rstart, rend;
  PetscMPIInt size, rank;
  PetscScalar v;
  IS          isrow, iscol;
  PetscBool   test_matmatmult = PETSC_FALSE;

  PetscFunctionBeginUser;
  PetscCall(PetscInitialize(&argc, &args, NULL, help));
  PetscCall(PetscOptionsGetBool(NULL, NULL, "-test_matmatmult", &test_matmatmult, NULL));

  PetscCallMPI(MPI_Comm_rank(PETSC_COMM_WORLD, &rank));
  PetscCallMPI(MPI_Comm_size(PETSC_COMM_WORLD, &size));
  n = 2 * size;

  PetscCall(MatCreate(PETSC_COMM_WORLD, &C));
  PetscCall(MatSetSizes(C, PETSC_DECIDE, PETSC_DECIDE, m * n, m * n));
  PetscCall(MatSetFromOptions(C));
  PetscCall(MatSetUp(C));

  /*
        This is JUST to generate a nice test matrix, all processors fill up
    the entire matrix. This is not something one would ever do in practice.
  */
  PetscCall(MatGetOwnershipRange(C, &rstart, &rend));
  for (i = rstart; i < rend; i++) {
    for (j = 0; j < m * n; j++) {
      v = i + j + 1;
      PetscCall(MatSetValues(C, 1, &i, 1, &j, &v, INSERT_VALUES));
    }
  }

  PetscCall(MatAssemblyBegin(C, MAT_FINAL_ASSEMBLY));
  PetscCall(MatAssemblyEnd(C, MAT_FINAL_ASSEMBLY));
  PetscCall(PetscViewerPushFormat(PETSC_VIEWER_STDOUT_WORLD, PETSC_VIEWER_ASCII_COMMON));
  PetscCall(MatView(C, PETSC_VIEWER_STDOUT_WORLD));

  /*
     Generate a new matrix consisting of every second row and column of
   the original matrix
  */
  PetscCall(MatGetOwnershipRange(C, &rstart, &rend));
  /* Create parallel IS with the rows we want on THIS processor */
  PetscCall(ISCreateStride(PETSC_COMM_WORLD, (rend - rstart) / 2, rstart, 2, &isrow));
  /* Create parallel IS with the rows we want on THIS processor (same as rows for now) */
  PetscCall(ISCreateStride(PETSC_COMM_WORLD, (rend - rstart) / 2, rstart, 2, &iscol));

  PetscCall(MatCreateSubMatrix(C, isrow, iscol, MAT_INITIAL_MATRIX, &A));
  PetscCall(MatCreateSubMatrix(C, isrow, iscol, MAT_REUSE_MATRIX, &A));
  PetscCall(MatView(A, PETSC_VIEWER_STDOUT_WORLD));

  if (test_matmatmult) {
    PetscCall(MatDestroy(&C));
    PetscCall(MatMatMult(A, A, MAT_INITIAL_MATRIX, PETSC_DETERMINE, &C));
    PetscCall(MatView(C, PETSC_VIEWER_STDOUT_WORLD));
  }

  PetscCall(ISDestroy(&isrow));
  PetscCall(ISDestroy(&iscol));
  PetscCall(MatDestroy(&A));
  PetscCall(MatDestroy(&C));
  PetscCall(PetscFinalize());
  return 0;
}

/*TEST

   test:

   test:
      suffix: 2
      nsize: 3

   test:
      suffix: 2_baij
      nsize: 3
      args: -mat_type baij

   test:
      suffix: 2_sbaij
      nsize: 3
      args: -mat_type sbaij

   test:
      suffix: baij
      args: -mat_type baij
      output_file: output/ex59_1_baij.out

   test:
      suffix: sbaij
      args: -mat_type sbaij
      output_file: output/ex59_1_sbaij.out

   test:
      suffix: kok
      nsize: 3
      requires: kokkos_kernels
      args: -mat_type aijkokkos -test_matmatmult
      filter: grep -v -i type
      output_file: output/ex59_kok.out

TEST*/