Actual source code: bandwidth.c
petsc-3.10.5 2019-03-28
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      1:  #include <petsc/private/matimpl.h>

      3: /*@
      4:   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.

      6:   Collective on Mat

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

     12:   Output Parameter:
     13: . bw - The matrix bandwidth

     15:   Level: beginner

     17: .seealso: DMPlexCreate(), DMPlexSetConeSize(), DMPlexSetChart()
     18: @*/
     19: PetscErrorCode MatComputeBandwidth(Mat A, PetscReal fraction, PetscInt *bw)
     20: {
     21:   PetscInt       lbw[2] = {0, 0}, gbw[2];
     22:   PetscInt       rStart, rEnd, r;

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

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