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

/*
    MatGetOrdering_RCM - Find the Reverse Cuthill-McKee ordering of a given matrix.
*/
PETSC_INTERN PetscErrorCode MatGetOrdering_RCM(Mat mat, MatOrderingType type, IS *row, IS *col)
{
  PetscInt        i, *mask, *xls, nrow, *perm;
  const PetscInt *ia, *ja;
  PetscBool       done;

  PetscFunctionBegin;
  PetscCall(MatGetRowIJ(mat, 1, PETSC_TRUE, PETSC_TRUE, &nrow, &ia, &ja, &done));
  PetscCheck(done, PetscObjectComm((PetscObject)mat), PETSC_ERR_SUP, "Cannot get rows for matrix");

  PetscCall(PetscMalloc3(nrow, &mask, nrow, &perm, 2 * nrow, &xls));
  PetscCall(SPARSEPACKgenrcm(&nrow, ia, ja, perm, mask, xls));
  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(PetscFree3(mask, perm, xls));
  PetscFunctionReturn(PETSC_SUCCESS);
}