! FILE: pingpong.f90 from www.nersc.gov
!
! This is an example program intended to demonstrate
! the use of simple send and receive commands.  Each
! of 2 CPUs sends its ID number to the other one and
! at the end they both print the values of what they
! sent as well as what they received.
!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

include "mpiSim.f90"
program pingpong
include "useSim.f90"

      IMPLICIT NONE

!     INCLUDE "mpif.h"

      INTEGER :: me, sbuf, rbuf, ierr, status(MPI_STATUS_SIZE)
      INTEGER :: count, dest, source, tag

! initialize MPI

      CALL MPI_INIT(ierr)

! find the current task

        CALL MPI_COMM_RANK( MPI_COMM_WORLD, me, ierr )

! set an ID value for the message tag

      tag=99

! have the first task [task zero] send then receive

      IF (me.EQ.0) THEN
         write(*,*) "Task 0"
! set values for the send command
!    dest is where the message is going
!    count is the number of items being sent

         dest=1
         count=1

! set values for the recv command
!     source is where to expect a message from

         source=1

! send a message containing the task number to task 1
! place the task ID in the variable sbuf
         sbuf = me

         CALL MPI_SEND(sbuf, count, MPI_INTEGER, dest, tag,  &
              MPI_COMM_WORLD, ierr)

! receive the message from task 1 into rbuf

         CALL MPI_RECV(rbuf, count, MPI_INTEGER, source, tag, &
              MPI_COMM_WORLD, status, ierr)

      END IF

! have the second task [task one] receive then send.  This constitutes
!    "blocking" message passing.

      IF (me.EQ.1) THEN
         write(*,*) "Task 1"

! set values for the send command
!    dest is where the message is going
!    count is the number of items being sent

         dest=0
         count=1

! set values for the recv command
!     source is where to expect a message from

         source=0

! Receive the message from task 0 into rbuf

         CALL MPI_RECV(rbuf, count, MPI_INTEGER, source, tag,  &
              MPI_COMM_WORLD, status, ierr)

! Send a message containing the task number to task 0
! place the task ID in the variable sbuf

         sbuf = me
         CALL MPI_SEND(sbuf, count, MPI_INTEGER, dest, tag,   &
              MPI_COMM_WORLD, ierr)

      END IF

! have each task print both what it sent and received

      PRINT*, "TASK #", me, " sent ", sbuf
      PRINT*, "TASK #", me, " received ", rbuf
! output with 2 processors:
!Task 0
!Task 1
!TASK #           0  sent            0
!TASK #           0  received        1
!TASK #           1  sent            1
!TASK #           1  received        0

! close out MPI

      CALL MPI_FINALIZE(ierr)

      END
    1
!TASK #           1  received        0

! close out MPI

      CALL MPI_FINALIZE(ierr)

      END