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 */
  PetscErrorCode    ierr;
  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;

  ierr = PetscInitialize(&argc,&args,(char*)0,help);if (ierr) return ierr;
  ierr = MPI_Comm_rank(PETSC_COMM_WORLD,&rank);CHKERRMPI(ierr);

  /* Determine files from which we read the linear systems. */
  ierr = PetscOptionsGetString(NULL,NULL,"-f",file,sizeof(file),&flg);CHKERRQ(ierr);
  PetscCheckFalse(!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. */
  ierr = PetscViewerBinaryOpen(PETSC_COMM_WORLD,file,FILE_MODE_READ,&fd);CHKERRQ(ierr);

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

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

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

  percent=(PetscReal)nnzA*100/(m*n);
  ierr   = 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);CHKERRQ(ierr);
  percent=(PetscReal)nnzAsp*100/(m*n);
  ierr   = PetscPrintf(PETSC_COMM_SELF," [%d] Matrix Asp nnzAsp %d, %g percent\n",rank,nnzAsp,percent);CHKERRQ(ierr);

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

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

  ierr = MatDestroy(&A);CHKERRQ(ierr);
  ierr = MatDestroy(&Asp);CHKERRQ(ierr);
  ierr = PetscFinalize();
  return ierr;
}