1: #include <petsc/private/matimpl.h>       /*I  "petscmat.h"  I*/

  5: /*@
  6:   MatComputeBandwidth - Calculate the full bandwidth of the matrix, meaning the width 2k+1 where k diagonals on either side are sufficient to contain all the matrix nonzeros.

  8:   Collective on Mat

 10:   Input Parameters:
 11: + A - The Mat
 12: - fraction - An optional percentage of the Frobenius norm of the matrix that the bandwidth should enclose

 14:   Output Parameter:
 15: . bw - The matrix bandwidth

 17:   Level: beginner

 19: .seealso: DMPlexCreate(), DMPlexSetConeSize(), DMPlexSetChart()
 20: @*/
 21: PetscErrorCode MatComputeBandwidth(Mat A, PetscReal fraction, PetscInt *bw)
 22: {
 23:   PetscInt       lbw[2] = {0, 0}, gbw[2];
 24:   PetscInt       rStart, rEnd, r;

 31:   if ((fraction > 0.0) && (fraction < 1.0)) SETERRQ(PetscObjectComm((PetscObject) A), PETSC_ERR_SUP, "We do not yet support a fractional bandwidth");
 32:   MatGetOwnershipRange(A, &rStart, &rEnd);
 33:   for (r = rStart; r < rEnd; ++r) {
 34:     const PetscInt *cols;
 35:     PetscInt        ncols;

 37:     MatGetRow(A, r, &ncols, &cols, NULL);
 38:     if (ncols) {
 39:       lbw[0] = PetscMax(lbw[0], r - cols[0]);
 40:       lbw[1] = PetscMax(lbw[1], cols[ncols-1] - r);
 41:     }
 42:     MatRestoreRow(A, r, &ncols, &cols, NULL);
 43:   }
 44:   MPI_Allreduce(lbw, gbw, 2, MPIU_INT, MPI_MAX, PetscObjectComm((PetscObject) A));
 45:   *bw = 2*PetscMax(gbw[0], gbw[1]) + 1;
 46:   return(0);
 47: }