!************************************************************************
! * This file has been written as a sample solution to an exercise in a 
! * course given at the Edinburgh Parallel Computing Centre. It is made
! * freely available with the understanding that every copy of this file
! * must include this header and that EPCC takes no responsibility for
! * the use of the enclosed teaching material.
! *
! * Author:      Joel Malard     
! *
! * Contact:     epcc-tec@epcc.ed.ac.uk
! *
! * Purpose:     A program to try out non-blocking point-to-point 
! *              communications.
! *
! * Contents:    F90 source code.
! *
! ************************************************************************/

      program ring
      include "mpif.h"
      integer to_right, to_left
      parameter (to_right = 201, to_left = 102)
      integer ierror, rank, my_rank, size
      integer right, left
      integer other, sum, i
      
      integer send_status(MPI_STATUS_SIZE), recv_status(MPI_STATUS_SIZE)
      integer request
      
      call MPI_Init(ierror)
      
      call MPI_Comm_rank(MPI_COMM_WORLD, my_rank, ierror)
      call MPI_Comm_size(MPI_COMM_WORLD, size, ierror)
      
      right = my_rank + 1
      if (right == size) right = 0    
      
      left = my_rank - 1
      if (left == -1) left = size-1
      
      sum = 0
      rank = my_rank
      
      do i = 0, size-1
    
         call MPI_Issend(rank, 1, MPI_INTEGER, right, to_right, 
     .        MPI_COMM_WORLD, request, ierror)
    
         call MPI_Recv(other, 1, MPI_INTEGER, left, to_right, 
     .        MPI_COMM_WORLD, recv_status, ierror)
    
         call MPI_Wait(request, send_status, ierror)
    
         sum = sum + other
         rank = other
    
      enddo
  
      write(*, '("PE",i1,": Sum = ",i4)') rank, sum
      
      call MPI_Finalize(ierror)
      
      stop
      end