C-----------------------------------------------------------------------------
C MPI tutorial example code: Non-Blocking Send/Receive
C FILE: mpi_ringtopo.f
C AUTHOR: Blaise Barney
C LAST REVISED:
C-----------------------------------------------------------------------------
      program ringtopo
      include 'mpif.h'

      integer numtasks, rank, next, prev, buf(2), tag1, tag2, ierr
      integer stats(MPI_STATUS_SIZE,4), reqs(4)
      tag1 = 1
      tag2 = 2

      call MPI_INIT(ierr)
      call MPI_COMM_RANK(MPI_COMM_WORLD, rank, ierr)
      call MPI_COMM_SIZE(MPI_COMM_WORLD, numtasks, ierr)

      prev = rank - 1
      next = rank + 1
      if (rank .eq. 0) then
         prev = numtasks - 1
      endif
      if (rank .eq. numtasks - 1) then
         next = 0
      endif

      call MPI_IRECV(buf(1), 1, MPI_INTEGER, prev, tag1, 
     &          MPI_COMM_WORLD, reqs(1), ierr)
      call MPI_IRECV(buf(2), 1, MPI_INTEGER, next, tag2, 
     &          MPI_COMM_WORLD, reqs(2), ierr)

      call MPI_ISEND(rank, 1, MPI_INTEGER, prev, tag2,
     &          MPI_COMM_WORLD, reqs(3), ierr)
      call MPI_ISEND(rank, 1, MPI_INTEGER, next, tag1,
     &          MPI_COMM_WORLD, reqs(4), ierr)

      call MPI_WAITALL(4, reqs, stats, ierr);
      print *, 'Task ',rank,' communicated with tasks ',prev,' & ',next

      call MPI_FINALIZE(ierr)

      end