C
C Copyright (c) 2004-2006 The Trustees of Indiana University and Indiana
C                         University Research and Technology
C                         Corporation.  All rights reserved.
C Copyright (c) 2006-2012 Cisco Systems, Inc.  All rights reserved.
C $COPYRIGHT$
C
C Simple ring test program
C
      program ring_f77
      implicit none
      include 'mpif.h'
      integer rank, size, tag, next, from, message, ierr

C     Start up MPI */

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

C     Calculate the rank of the next process in the ring.  Use the
C     modulus operator so that the last process "wraps around" to rank
C     zero.

      tag = 201
      next = mod((rank + 1), size)
      from = mod((rank + size - 1), size)
      
C     If we are the "master" process (i.e., MPI_COMM_WORLD rank 0), put
C     the number of times to go around the ring in the message.

      if (rank .eq. 0) then
         message = 10

         write(*, '("Process 0 sending ", i2, " to ", i2, " tag ",
     &        i3, " (", i2, " processes in ring)")')
     &        message, next, tag, size
         call MPI_SEND(message, 1, MPI_INTEGER, next, tag, 
     &        MPI_COMM_WORLD, ierr)
         write(*, '("Process 0 sent to ", i2)')
     &        next
      endif

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

 10   call MPI_RECV(message, 1, MPI_INTEGER, from, tag, 
     &     MPI_COMM_WORLD, MPI_STATUS_IGNORE, ierr)

      if (rank .eq. 0) then
         message = message - 1
         write(*, '("Process 0 decremented value: ", i2)') message
      endif

      call MPI_SEND(message, 1, MPI_INTEGER, next, tag, 
     &     MPI_COMM_WORLD, ierr)
      
      if (message .eq. 0) then
         write(*, '("Process ", i2, " exiting")') rank
         goto 20
      endif
      goto 10

C     The last process does one extra send to process 0, which needs to
C     be received before the program can exit

 20   if (rank .eq. 0) then
         call MPI_RECV(message, 1, MPI_INTEGER, from, tag, 
     &        MPI_COMM_WORLD, MPI_STATUS_IGNORE, ierr)
      endif

C     All done

      call MPI_FINALIZE(ierr)
      end
      
