#include <stdio.h>
#include "mpi.h"

/* 

Exercise 6.9.3, see Chap 6, p. 108 in PPMPI.
by John Weathewax

*/ 

main( int argc, char* argv[] ){
    int             my_rank;
    int             p;
    int             i,j;
    float           value; 
    MPI_Datatype    sparse_m_elt_t; 

    /* declarations: */ 
    void Build_sparse_matrix_entry(int* i_ptr, int* j_ptr, float* value_ptr,
				   MPI_Datatype* sparse_m_elt_t_ptr);

    MPI_Init(&argc,&argv); 
    MPI_Comm_size(MPI_COMM_WORLD,&p);
    MPI_Comm_rank(MPI_COMM_WORLD,&my_rank); 

    if( my_rank==1 ){ /* create a sparse_matrix_entry and send to process 0 */
      i=2; j=3; value=3.14159;
    }

    /* Create a sparse matrix type... */ 
    Build_sparse_matrix_entry(&i,&j,&value,&sparse_m_elt_t);

    MPI_Bcast(&i,1,sparse_m_elt_t,1,MPI_COMM_WORLD); 

    if( my_rank==0 ){ /* print the sparse element type */ 
      printf("Process 0:  Now i=%d\n",i);
      printf("Process 0:  Now j=%d\n",j);
      printf("Process 0:  Now value=%f\n",value);
    }

    MPI_Finalize(); 
}


/*------------------------------------------------------------------------
 * Create a sparse MPI type...
 */
void Build_sparse_matrix_entry(
                     int*                     i_ptr,     /* in */ 
                     int*                     j_ptr,     /* in */
                     float*               value_ptr,     /* in */ 
                     MPI_Datatype*   sparse_m_elt_t_ptr  /* out */){

  int block_lengths[3];
  MPI_Aint displacements[3];
  MPI_Datatype typelist[3];
  MPI_Aint start_address, address;

  block_lengths[0] = block_lengths[1] = block_lengths[2] = 1;

  /* Build a derived type consisting of two ints and a float */ 
  typelist[0] = MPI_INT; 
  typelist[1] = MPI_INT; 
  typelist[2] = MPI_FLOAT; 

  /* First element value "i" is at displacement 0 */ 
  displacements[0] = 0; 
  /* Calculate the other displacements relative to "i" */ 
  MPI_Address(i_ptr,&start_address); 
  /* Calculate the displacement of "j" relative to "i" */ 
  MPI_Address(j_ptr,&address); 
  displacements[1] = address-start_address;
  /* Calculate the displacement of "value" relative to "i" */ 
  MPI_Address(value_ptr,&address); 
  displacements[2] = address-start_address;
  
  /* Build the derived type */ 
  MPI_Type_struct(3,block_lengths,displacements,typelist,
		  sparse_m_elt_t_ptr);

  /* Commit this type */ 
  MPI_Type_commit(sparse_m_elt_t_ptr);
}