C	/* 
C	 * The root node sends out a message to the next node in the ring and each node then passes the 
C	 * message along to the next node.  The root node times how long it takes for the message to get back to it.
C	 */

	program main
	include 'mpif.h'

	double precision start, finish
	double precision buf(64)
	integer my_rank
	integer n_processes
	integer next
	integer ierr
	integer BUFSIZE = 64
	integer tag = 2001

	call MPI_INIT(ierr)
	call MPI_COMM_RANK(MPI_COMM_WORLD, my_rank, ierr)
	call MPI_COMM_SIZE(MPI_COMM_WORLD, n_processes, ierr)

C	/* 
C	* If this process is the root process send a messege to the next node
C	* and wait to recieve one from the last node.  Time how long it takes
C	* for the messege to get around the ring.  If this process is not the 
C	* root node, wait to recieve a messege from the previous node and
C	* then send it to the next node.
C	*/

	start = MPI_Wtime()

	print *, 'Hello world! I am ', my_rank, ' of ', n_processes

	if (my_rank .eq. 0) then
C		/* send to the next node */
		call MPI_SEND(buf, BUFSIZE, MPI_DOUBLE_PRECISION, my_rank+1,
	1	tag, MPI_COMM_WORLD, ierr)
C		/* recieve from the last node */
		call MPI_RECV(buf, BUFSIZE, MPI_DOUBLE_PRECISION, n_processes-1,
	1	tag, MPI_COMM_WORLD, status, ierr)
	else
C		/* recieve from the previous node */
		call MPI_RECV(buf, BUFSIZE, MPI_DOUBLE_PRECISION, my_rank-1, tag,
	1	MPI_COMM_WORLD, status, ierr)

C		/* send to the next node */
		if (my_rank .eq. (n_processes-1)) then 
			next=0
		else
			next=my_rank+1
		endif
		print *, 'P',my_rank,': next= ',next
		call MPI_SEND(buf, BUFSIZE, MPI_DOUBLE_PRECISION, next, tag,
	1	MPI_COMM_WORLD, ierr)
	endif

	finish=MPI_Wtime()

C	/* Print out the results. */
	if (my_rank .eq. 0) then
		print *, 'Total time used was ', finish-start, ' seconds'
	endif

	call MPI_FINALIZE(ierr)
	end