/*
 * 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 <iostream>
#include <mpi.h>

using namespace std;


int 
main(int argc, char *argv[])
{
  int num; 
  int rank, size, tag, next, from;
  MPI::Prequest send_request, recv_request;

  MPI::Init(argc, argv);
  rank = MPI::COMM_WORLD.Get_rank();
  size = MPI::COMM_WORLD.Get_size();
 
  tag = 201;
  next = (rank + 1) % size;
  from = MPI::ANY_SOURCE;

  if (rank == 0) {
    cout << "Enter the number of times around the ring: ";
    cin >> num;

    cout << "Process " << rank << " sending " << num << " to " << next << endl;
    MPI::COMM_WORLD.Send(&num, 1, MPI::INT, next, tag);
  }

  send_request = MPI::COMM_WORLD.Send_init(&num, 1, MPI::INT, next, tag);
  recv_request = MPI::COMM_WORLD.Recv_init(&num, 1, MPI::INT, from, tag);

  do {
    recv_request.Start();
    recv_request.Wait();

    cout << "Process " << rank << " received " << num << endl;

    if (rank == 0) {
      --num;
      cout << "Process 0 decremented num" << endl;;
    }

    cout << "Process " << rank << " sending " << num << " to " << next << endl;

    send_request.Start();
    send_request.Wait();
  } while (num > 0);
  cout << "Process " << rank << " exiting" << endl;

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

  if (rank == 0)
    MPI::COMM_WORLD.Recv(&num, 1, MPI::INT, from, tag);

  /* Now free the persistent requests */

  send_request.Free();
  recv_request.Free();

  // Quit

  MPI::Finalize();
  return 0;
}