program ping_pong
	use mpi
	implicit none
	integer :: rank, nb_mpi_processes, ierror, tag, statu(MPI_STATUS_SIZE), n, ball
	integer :: niter = 6
 
	! Ping Pong program

	tag = 7777

	call MPI_INIT( ierror )
	call MPI_COMM_SIZE( MPI_COMM_WORLD , nb_mpi_processes , ierror )
	call MPI_COMM_RANK( MPI_COMM_WORLD , rank , ierror )

	if(nb_mpi_processes /= 2) stop 'This program is design to be run with 2 processes only'

	ball = 0
	do n=1,niter

		if(rank==0) then
			call MPI_SEND ( ball , 1 , MPI_INTEGER , 1 , tag , MPI_COMM_WORLD , MPI_STATUS_IGNORE , ierror ) ! 0 send ball to 1, and wait for transfer to be finished
			call MPI_RECV ( ball , 1 , MPI_INTEGER , 1 , tag , MPI_COMM_WORLD , MPI_STATUS_IGNORE , ierror ) ! 0 receive ball from 1, and wait for transfer to be finished
			ball = ball + 2
		end if

		if(rank==1) then
			call MPI_RECV ( ball , 1 , MPI_INTEGER , 0 , tag , MPI_COMM_WORLD , MPI_STATUS_IGNORE , ierror )
			ball = ball + 1
			call MPI_SEND ( ball , 1 , MPI_INTEGER , 0 , tag , MPI_COMM_WORLD , MPI_STATUS_IGNORE , ierror )
		end if

		print*, 'Process',rank,'iter',n,'ball value is :',ball

		call MPI_BARRIER(MPI_COMM_WORLD,ierror) ! A barrier. processes stop here, and can pass it only if ALL processes are here. Useful for debug, can impact performances

	end do

	call MPI_FINALIZE ( ierror ) ! Close MPI

end program ping_pong