C
C Copyright (c) 2013      Mellanox Technologies, Inc.
C                         All rights reserved.
C $COPYRIGHT$
C
C Additional copyrights may follow
C
C $HEADER$
C

      program ring_shmem_f77
      implicit none
      include 'shmem.fh'

      integer*8 rbuf
      save rbuf
      integer*8 message
      integer proc, nproc, next
C       func definitions
      integer my_pe, num_pes

      rbuf = -1
      message = 10

      call start_pes(0)
      proc = my_pe()
      nproc = num_pes()

C   Calculate the PE number of the next process in the ring.  Use the
C   modulus operator so that the last process "wraps around" to PE 0.

      next = mod((proc + 1), nproc)

      if (proc .eq. 0) then
          write(*, '("Process 0 sending ", i2, " to ", i2,
     &        " (", i2, " processes in ring)")')
     &        message, next, nproc
         call shmem_put8(rbuf, message, 1, next)
      write(*, '("Process 0 sent to ", i2)') next
      end if

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 PE 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 shmem_int8_wait_until(rbuf, SHMEM_CMP_EQ, message)

      if (proc .eq. 0) then
         message = message - 1
	        write(*, '("Process 0 decremented value:", i2)') message
      else
         message = rbuf
      end if

      call shmem_put8(rbuf, message, 1, next)

      if (message .gt. 0) then
         goto 10
      else
         goto 20
      end if

C 	All done

 20   write(*, '("Process", i2," exiting.")') proc

      end