static char help[] = "Tests MatCreateSubmatrix() with certain entire rows of matrix, modified from ex181.c.";

#include <petscmat.h>

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

  PetscFunctionBeginUser;
  PetscCall(PetscInitialize(&argc, &args, (char *)0, help));
  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));
  PetscCall(PetscViewerPopFormat(PETSC_VIEWER_STDOUT_WORLD));

  /*
     Generate a new matrix consisting every column of the original matrix
  */
  PetscCall(MatGetOwnershipRange(C, &rstart, &rend));
  PetscCall(MatGetOwnershipRangeColumn(C, &cstart, &cend));

  PetscCall(ISCreateStride(PETSC_COMM_WORLD, rend - rstart > 0 ? rend - rstart - 1 : 0, rstart, 1, &isrow));
  PetscCall(ISCreateStride(PETSC_COMM_WORLD, cend - cstart, cstart, 1, &iscol));
  PetscCall(MatCreateSubMatrix(C, isrow, NULL, MAT_INITIAL_MATRIX, &A));

  /* Change C to test the case MAT_REUSE_MATRIX */
  if (rank == 0) {
    i = 0;
    j = 0;
    v = 100;
    PetscCall(MatSetValues(C, 1, &i, 1, &j, &v, INSERT_VALUES));
  }
  PetscCall(MatAssemblyBegin(C, MAT_FINAL_ASSEMBLY));
  PetscCall(MatAssemblyEnd(C, MAT_FINAL_ASSEMBLY));

  PetscCall(MatCreateSubMatrix(C, isrow, NULL, MAT_REUSE_MATRIX, &A));
  PetscCall(PetscViewerPushFormat(PETSC_VIEWER_STDOUT_WORLD, PETSC_VIEWER_ASCII_COMMON));
  PetscCall(MatView(A, PETSC_VIEWER_STDOUT_WORLD));
  PetscCall(PetscViewerPopFormat(PETSC_VIEWER_STDOUT_WORLD));

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

/*TEST

   test:
      nsize: 2

TEST*/
