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

using namespace std;


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);
  rank = MPI::COMM_WORLD.Get_rank();
  size = MPI::COMM_WORLD.Get_size();

  if (rank == 0) {
    cout << "Enter the number of iterations: ";
    cin >> done;
  }

  MPI::COMM_WORLD.Bcast(&done, 1, MPI::INT, 0);

  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

    req[0] = MPI::COMM_WORLD.Irecv(&left_x, 1, MPI::DOUBLE, 
				   left_dest, tag);
    
    req[1] = MPI::COMM_WORLD.Irecv(&right_x, 1, MPI::DOUBLE,
				   right_dest, tag);
    
    // Post sends to neighbors

    req[2] = MPI::COMM_WORLD.Isend(&x, 1, MPI::DOUBLE, left_dest, tag);
    
    req[3] = MPI::COMM_WORLD.Isend(&x, 1, MPI::DOUBLE, right_dest, tag);
    
    // Do local computation.  Unfortunately, there is none.  But this
    // is where you would do it.

    // Wait for the receives to complete

    MPI::Request::Waitall(4, req);
    
    // Calculate the average (using left_x and right_x)

    x = (right_x + left_x) / 2.0;
  }

  if (rank == 0)
    cout << "Final average is: " << x << endl;

  MPI::Finalize();
  return 0;
}