! ring_ex.f90
! Example from Rutgers
include "mpiSim.f90"
program ring
include "useSim.f90"

!
! This example program uses MPI blocking and nonblocking point-to-point
! communication calls to send and receive message in the ring where node
! with rank i sends a message to the node with rank i+1 and receives a
! message from the node with rank i-1
!**************************************************************
implicit none
!include 'mpif.h'
integer ntasks, taskid, right, left, inmsg, outmsg, mtype, &
        msgid, rbytes, sbytes, i
integer ierr,status(MPI_STATUS_SIZE),tag,requests(2)
outmsg = 0
i = 1
! $$ Error: tag has not been defined before calling MPI_Send.
!
! learn number of tasks in partition and task ID
call mpi_init(ierr)
call mpi_comm_size(MPI_COMM_WORLD,ntasks,ierr)
call mpi_comm_rank(MPI_COMM_WORLD,taskid,ierr)
! compute source and destination for messages
if (taskid.eq.0) then
  left = ntasks - 1
else
  left = taskid - 1
endif
if (taskid.eq.ntasks-1) then
  right = 0
else
  right = taskid + 1
endif
do while (outmsg .ne. -1)
  mtype = i
!node 0 queries user for message, sends it to the right,
! then waits for its return
  if (taskid .eq. 0) then
    write (*,*) 'Enter integer value to be passed along ring'
    write (*,*) 'A value of -1 ends the program'
    read (*,*) outmsg
!$$ tag has never been initialized; for example tag = MPI_Any_tag.
    call mpi_send(outmsg,1,MPI_INTEGER,right,tag,  &
                  MPI_COMM_WORLD,ierr)
    call mpi_irecv(inmsg,1,MPI_INTEGER,left,tag,  &
                   MPI_COMM_WORLD,requests(1:1),ierr)
    call mpi_wait(requests(1),status,ierr)
    write (*,*) 'node 0 received message content is ', inmsg
! the rest of the nodes in the group read the message and pass it on
  else
    call mpi_recv (inmsg,1,MPI_INTEGER,left,tag,  &
                   MPI_COMM_WORLD, status,ierr)
    outmsg = inmsg
    call mpi_isend (outmsg,1,MPI_INTEGER,right,tag,  &
                    MPI_COMM_WORLD, requests(2),ierr)
    call mpi_wait (requests(2),status,ierr)
    write (*,*) taskid, 'processed message ', mtype, &
               'content is ', outmsg
  end if
  i = i + 1
end do
end