c******************************************************************
c
c                        Example 4 ( ring_topology.f )
c
c  Objective           : MPI program to calculate sum of 'n' integers 
c                        using Point to Point Communication and ring 
c                        topology. 
c                        This example demonstrates the use of
c                        MPI_Init
c                        MPI_Comm_rank
c                        MPI_Comm_size
c                        MPI_Send
c                        MPI_Recv
c                        MPI_Finalize
c
c  Input               : Automatic input data generation 
c                        The rank of each process is input on each process. 
c
c  Output              : Process with rank 0 prints sum of 'n'values  
c
c  Necessary Condition : Number of Processes should be 
c                        less than or equal to 8.
c
c*******************************************************************



       program main
 
       include "mpif.h"

       integer myrank, MyRank, Numprocs
       integer status(MPI_STATUS_SIZE)
       integer value, sum, Root 
       integer Source, Source_tag, Destination, Destination_tag

       call MPI_INIT(ierror)
       call MPI_COMM_SIZE(MPI_COMM_WORLD, Numprocs, ierror)
       call MPI_COMM_RANK(MPI_COMM_WORLD, MyRank, ierror)

       Root = 0
       if( MyRank .eq. Root) then
           Destination = 1
           Destination_tag = 0
           call MPI_SEND(MyRank, 1, MPI_INTEGER, Destination,
     &                   Destination_tag, MPI_COMM_WORLD, ierror)

	else 
         if( MyRank .lt. Numprocs - 1) then
            Source = MyRank - 1
            Source_tag = 0
            call MPI_RECV(value,1,MPI_INTEGER,Source,Source_tag,
     &                    MPI_COMM_WORLD,status,ierror)
            sum = MyRank + value
            Destination = MyRank +1
            Destination_tag = 0
            call MPI_SEND(sum, 1, MPI_INTEGER,Destination,
     &			Destination_tag, MPI_COMM_WORLD,ierror)
          else
            Source = MyRank - 1
            Source_tag = 0
            call MPI_RECV(value,1,MPI_INTEGER,Source,Source_tag,
     &		MPI_COMM_WORLD ,status,ierror)
            sum = MyRank + value
          endif
        endif
          
c	 .........Process with Rank 0 receives the result from 
c	          Process wth Rank "Numprocs-1".
	 if(MyRank .eq. Root) then
            Source = Numprocs - 1 
            Source_tag = 0
            call MPI_RECV(sum,1,MPI_INTEGER,Source,Source_tag,
     &                    MPI_COMM_WORLD,status,ierror)
            write(6,*) 'MyRank', MyRank,    'SUM ',sum
	 endif
c
	 if(MyRank .eq. Numprocs-1) then
            Destination = 0 
            Destination_tag = 0
            call MPI_SEND(sum, 1, MPI_INTEGER,Destination,
     &			Destination_tag, MPI_COMM_WORLD,ierror)
	endif

        call MPI_FINALIZE(ierror)

        stop
        end