/*
 * Open Systems Laboratory
 * http://www.lam-mpi.org/tutorials/
 * Indiana University
 *
 * MPI Tutorial Lab
 * Rewrite the ring program using persistent communication
 *
 * 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 num; 
  int rank, size, tag, next, from;
  MPI_Request send_request, recv_request;

  MPI_Init(&argc, &argv);
  MPI_Comm_rank( MPI_COMM_WORLD, &rank);
  MPI_Comm_size( MPI_COMM_WORLD, &size);
 
  tag = 201;
  next = (rank + 1) % size;
  from = MPI_ANY_SOURCE;

  if (rank == 0) {
    printf("Enter the number of times around the ring: ");
    scanf("%d", &num);

    printf("Process %d sending %d to %d\n", rank, num, next);
    MPI_Send(&num, 1, MPI_INT, next, tag, MPI_COMM_WORLD); 
  }

  MPI_Send_init(&num, 1, MPI_INT, next, tag, MPI_COMM_WORLD, &send_request);
  MPI_Recv_init(&num, 1, MPI_INT, from, tag, MPI_COMM_WORLD, &recv_request);

  do {
    MPI_Start(&recv_request);
    MPI_Wait(&recv_request, MPI_STATUS_IGNORE);

    printf("Process %d received %d\n", rank, num);

    if (rank == 0) {
      --num;
      printf("Process 0 decremented num\n");
    }

    printf("Process %d sending %d to %d\n", rank, num, next);

    MPI_Start(&send_request);
    MPI_Wait(&send_request, MPI_STATUS_IGNORE);
  } while (num > 0);
  printf("Process %d exiting\n", rank);

  /* The last process does one extra send to process 0, which needs */
  /* to be received before the program can exit */

  if (rank == 0)
    MPI_Recv(&num, 1, MPI_INT, from, tag, MPI_COMM_WORLD, MPI_STATUS_IGNORE);

  /* Now free the persistent requests */

  MPI_Request_free(&send_request);
  MPI_Request_free(&recv_request);

  /* Quit */

  MPI_Finalize();
  return 0;
}