/*
 * Open Systems Laboratory
 * http://www.lam-mpi.org/tutorials/
 * Indiana University
 *
 * MPI Tutorial
 *
 * Mail questions regarding tutorial material to lam at lam dash mpi dot org
 */

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


int
main(int argc, char** argv)
{
  int rank, size;
  int done;
  int left_dest, right_dest;
  const int tag = 4;
  double x, left_x, right_x;
  MPI_Request req[4];

  MPI_Init(&argc, &argv);
  MPI_Comm_rank(MPI_COMM_WORLD, &rank);
  MPI_Comm_size(MPI_COMM_WORLD, &size);

  if (rank == 0) {
    printf("Enter the number of iterations: ");
    scanf("%d", &done);
  }

  MPI_Bcast(&done, 1, MPI_INT, 0, MPI_COMM_WORLD);

  x = (rank == 0 || rank == size - 1) ? 1 : 2; 
  left_x = (rank == 0) ? 1 : 2; 
  right_x = (rank == size - 1) ? 1 : 2; 
  left_dest = (rank == 0) ? MPI_PROC_NULL : rank - 1;
  right_dest = (rank == size - 1) ? MPI_PROC_NULL : rank + 1;

  while (--done > 0) {

    /* Post receives from neighbors */

    MPI_Irecv(&left_x, 1, MPI_DOUBLE, left_dest, tag,
	      MPI_COMM_WORLD, &req[0]);
    
    MPI_Irecv(&right_x, 1, MPI_DOUBLE, right_dest, tag,
	      MPI_COMM_WORLD, &req[1]);
    
    /* Post sends to neighbors */

    MPI_Isend(&x, 1, MPI_DOUBLE, left_dest, tag, 
	      MPI_COMM_WORLD, &req[2]);
    
    MPI_Isend(&x, 1, MPI_DOUBLE, right_dest, tag,
	      MPI_COMM_WORLD, &req[3]);
    
    /* Do local computation.  Unfortunately, there is none.  But this
       is where you would do it. */

    /* Wait for all requests to complete */

    MPI_Waitall(4, req, MPI_STATUSES_IGNORE);
    
    /* Calculate the average (using left_x and right_x) */ 

    x = (right_x + left_x) / 2.0;
  }

  if (rank == 0)
    printf("Final average is: %lf\n", x);

  MPI_Finalize();
  return 0;
}