//
// Open Systems Laboratory
// http://www.lam-mpi.org/tutorials/
// Indiana University
//
// MPI Tutorial
// Lab 2: The cannonical ring program
//
// 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::Status status;

  // Start up MPI 

  MPI::Init(argc, argv);
  rank = MPI::COMM_WORLD.Get_rank();
  size = MPI::COMM_WORLD.Get_size();
 
  // Arbitrarily choose 201 to be our tag.  Calculate the rank of the
  // next process in the ring.  Use the modulus operator so that the
  // last process "wraps around" to rank zero.

  tag = 201;
  next = (rank + 1) % size;
  from = (rank + size - 1) % size;

  // If we are the "console" process, get a integer from the user to
  // specify how many times we want to go around the ring

  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); 
  }

  // Pass the message around the ring.  The exit mechanism works as
  // follows: the message (a positive integer) is passed around the
  // ring.  Each time is passes rank 0, it is decremented.  When each
  // processes receives the 0 message, it passes it on to the next
  // process and then quits.  By passing the 0 first, every process
  // gets the 0 message and can quit normally.

  do {

    MPI::COMM_WORLD.Recv(&num, 1, MPI::INT, from, tag, status);
    cout << "Process " << rank << " received " << num << endl;

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

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

  // Quit

  MPI::Finalize();
  return 0;
}