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

int main(int argc, char *argv[])
{
  MPI_Status status;
  int num, rank, size, tag, next, from;

  // Start up MPI
  MPI_Init(&argc, &argv);

  // how many process do we have & which numer is mine?
  MPI_Comm_rank(MPI_COMM_WORLD, &rank);
  MPI_Comm_size(MPI_COMM_WORLD, &size);
 
  // Arbitrarily choose 42 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 = 42;
  next = ...;
  from = ...;

  // 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) {
    std::cout << "Enter the number of times around the ring: ";
    std::cin >> num;
    --num;

    std::cout << "Process " << rank << " sending " << num << " to " << next << std::endl;
    // Send integer 'num' to 'next' process
    ...
  }

  // 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.
  while (1) {

    // get integer from 'from' process and store it to 'num'
    ...
    std::cout << "Process " << rank << " received " << num << std::endl;

    if (rank == 0) {
      num--;
      std::cout << "Process " << rank << " depremented num to " << num << std::endl;
    }

    std::cout << "Process " << rank << " sending " << num << " to " << next << std::endl;
    // Send integer 'num' to 'next' process
    ...

    if (num == 0) {
      std::cout << "Process " << rank << " exiting" << std::endl;
      break;
    }
  }

  // The last process does one extra send to process 0, which needs to
  // be received before the program can exit
  if (rank == 0)
    // get integer from 'from' process and store it to 'num'
    ...

  // cleanup
  MPI_Finalize();
  return 0;
}