#include <petscmat.h>
#include <petsc/private/matorderimpl.h>

/*
    MatGetOrdering_ND - Find the nested dissection ordering of a given matrix.
*/
PETSC_INTERN PetscErrorCode MatGetOrdering_ND(Mat mat, MatOrderingType type, IS *row, IS *col)
{
  PetscInt        i, *mask, *xls, *ls, nrow, *perm;
  const PetscInt *ia, *ja;
  PetscBool       done;
  Mat             B = NULL;

  PetscFunctionBegin;
  PetscCall(MatGetRowIJ(mat, 1, PETSC_TRUE, PETSC_TRUE, &nrow, &ia, &ja, &done));
  if (!done) {
    PetscCall(MatConvert(mat, MATAIJ, MAT_INITIAL_MATRIX, &B));
    PetscCall(MatGetRowIJ(B, 1, PETSC_TRUE, PETSC_TRUE, &nrow, &ia, &ja, &done));
  }

  PetscCall(PetscMalloc4(nrow, &mask, nrow, &perm, nrow + 1, &xls, nrow, &ls));
  PetscCall(SPARSEPACKgennd(&nrow, ia, ja, mask, perm, xls, ls));
  if (B) {
    PetscCall(MatRestoreRowIJ(B, 1, PETSC_TRUE, PETSC_TRUE, NULL, &ia, &ja, &done));
    PetscCall(MatDestroy(&B));
  } else {
    PetscCall(MatRestoreRowIJ(mat, 1, PETSC_TRUE, PETSC_TRUE, NULL, &ia, &ja, &done));
  }

  /* shift because Sparsepack indices start at one */
  for (i = 0; i < nrow; i++) perm[i]--;

  PetscCall(ISCreateGeneral(PETSC_COMM_SELF, nrow, perm, PETSC_COPY_VALUES, row));
  PetscCall(ISCreateGeneral(PETSC_COMM_SELF, nrow, perm, PETSC_COPY_VALUES, col));
  PetscCall(PetscFree4(mask, perm, xls, ls));
  PetscFunctionReturn(PETSC_SUCCESS);
}