static char help[] = "Reads a matrix from PETSc binary file. Use for view or investigating matrix data structure. \n\n";
/*
 Example:
      ./ex16 -f <matrix file> -a_mat_view draw -draw_pause -1
      ./ex16 -f <matrix file> -a_mat_view ascii::ascii_info
 */

#include <petscmat.h>
int main(int argc, char **args)
{
  Mat                A, Asp;
  PetscViewer        fd;                       /* viewer */
  char               file[PETSC_MAX_PATH_LEN]; /* input file name */
  PetscInt           m, n, rstart, rend;
  PetscBool          flg;
  PetscInt           row, ncols, j, nrows, nnzA = 0, nnzAsp = 0;
  const PetscInt    *cols;
  const PetscScalar *vals;
  PetscReal          norm, percent, val, dtol = 1.e-16;
  PetscMPIInt        rank;
  MatInfo            matinfo;
  PetscInt           Dnnz, Onnz;

  PetscFunctionBeginUser;
  PetscCall(PetscInitialize(&argc, &args, NULL, help));
  PetscCallMPI(MPI_Comm_rank(PETSC_COMM_WORLD, &rank));

  /* Determine files from which we read the linear systems. */
  PetscCall(PetscOptionsGetString(NULL, NULL, "-f", file, sizeof(file), &flg));
  PetscCheck(flg, PETSC_COMM_WORLD, PETSC_ERR_USER, "Must indicate binary file with the -f option");

  /* Open binary file.  Note that we use FILE_MODE_READ to indicate
     reading from this file. */
  PetscCall(PetscViewerBinaryOpen(PETSC_COMM_WORLD, file, FILE_MODE_READ, &fd));

  /* Load the matrix; then destroy the viewer. */
  PetscCall(MatCreate(PETSC_COMM_WORLD, &A));
  PetscCall(MatSetOptionsPrefix(A, "a_"));
  PetscCall(MatSetFromOptions(A));
  PetscCall(MatLoad(A, fd));
  PetscCall(PetscViewerDestroy(&fd));
  PetscCall(MatGetSize(A, &m, &n));
  PetscCall(MatGetInfo(A, MAT_LOCAL, &matinfo));
  /*printf("matinfo.nz_used %g\n",matinfo.nz_used);*/

  /* Get a sparse matrix Asp by dumping zero entries of A */
  PetscCall(MatCreate(PETSC_COMM_WORLD, &Asp));
  PetscCall(MatSetSizes(Asp, m, n, PETSC_DECIDE, PETSC_DECIDE));
  PetscCall(MatSetOptionsPrefix(Asp, "asp_"));
  PetscCall(MatSetFromOptions(Asp));
  Dnnz = (PetscInt)matinfo.nz_used / m + 1;
  Onnz = Dnnz / 2;
  printf("Dnnz %d %d\n", Dnnz, Onnz);
  PetscCall(MatSeqAIJSetPreallocation(Asp, Dnnz, NULL));
  PetscCall(MatMPIAIJSetPreallocation(Asp, Dnnz, NULL, Onnz, NULL));
  /* The allocation above is approximate so we must set this option to be permissive.
   * Real code should preallocate exactly. */
  PetscCall(MatSetOption(Asp, MAT_NEW_NONZERO_LOCATION_ERR, PETSC_FALSE));

  /* Check zero rows */
  PetscCall(MatGetOwnershipRange(A, &rstart, &rend));
  nrows = 0;
  for (row = rstart; row < rend; row++) {
    PetscCall(MatGetRow(A, row, &ncols, &cols, &vals));
    nnzA += ncols;
    norm = 0.0;
    for (j = 0; j < ncols; j++) {
      val = PetscAbsScalar(vals[j]);
      if (norm < val) norm = norm;
      if (val > dtol) {
        PetscCall(MatSetValues(Asp, 1, &row, 1, &cols[j], &vals[j], INSERT_VALUES));
        nnzAsp++;
      }
    }
    if (!norm) nrows++;
    PetscCall(MatRestoreRow(A, row, &ncols, &cols, &vals));
  }
  PetscCall(MatAssemblyBegin(Asp, MAT_FINAL_ASSEMBLY));
  PetscCall(MatAssemblyEnd(Asp, MAT_FINAL_ASSEMBLY));

  percent = (PetscReal)nnzA * 100 / (m * n);
  PetscCall(PetscPrintf(PETSC_COMM_SELF, " [%d] Matrix A local size %d,%d; nnzA %d, %g percent; No. of zero rows: %d\n", rank, m, n, nnzA, percent, nrows));
  percent = (PetscReal)nnzAsp * 100 / (m * n);
  PetscCall(PetscPrintf(PETSC_COMM_SELF, " [%d] Matrix Asp nnzAsp %d, %g percent\n", rank, nnzAsp, percent));

  /* investigate matcoloring for Asp */
  PetscBool Asp_coloring = PETSC_FALSE;
  PetscCall(PetscOptionsHasName(NULL, NULL, "-Asp_color", &Asp_coloring));
  if (Asp_coloring) {
    MatColoring   mc;
    ISColoring    iscoloring;
    MatFDColoring matfdcoloring;
    PetscCall(PetscPrintf(PETSC_COMM_WORLD, " Create coloring of Asp...\n"));
    PetscCall(MatColoringCreate(Asp, &mc));
    PetscCall(MatColoringSetType(mc, MATCOLORINGSL));
    PetscCall(MatColoringSetFromOptions(mc));
    PetscCall(MatColoringApply(mc, &iscoloring));
    PetscCall(MatColoringDestroy(&mc));
    PetscCall(MatFDColoringCreate(Asp, iscoloring, &matfdcoloring));
    PetscCall(MatFDColoringSetFromOptions(matfdcoloring));
    PetscCall(MatFDColoringSetUp(Asp, iscoloring, matfdcoloring));
    /*PetscCall(MatFDColoringView(matfdcoloring,PETSC_VIEWER_STDOUT_WORLD));*/
    PetscCall(ISColoringDestroy(&iscoloring));
    PetscCall(MatFDColoringDestroy(&matfdcoloring));
  }

  /* Write Asp in binary for study - see ~petsc/src/mat/tests/ex124.c */
  PetscBool Asp_write = PETSC_FALSE;
  PetscCall(PetscOptionsHasName(NULL, NULL, "-Asp_write", &Asp_write));
  if (Asp_write) {
    PetscViewer viewer;
    PetscCall(PetscPrintf(PETSC_COMM_SELF, "Write Asp into file Asp.dat ...\n"));
    PetscCall(PetscViewerBinaryOpen(PETSC_COMM_WORLD, "Asp.dat", FILE_MODE_WRITE, &viewer));
    PetscCall(MatView(Asp, viewer));
    PetscCall(PetscViewerDestroy(&viewer));
  }

  PetscCall(MatDestroy(&A));
  PetscCall(MatDestroy(&Asp));
  PetscCall(PetscFinalize());
  return 0;
}
