/* compile: /usr/local/mpich-1.2.0/bin/mpicc mess.passing.example.c
   run : nice /usr/local/mpich-1.2.0/bin/mpirun -np 5 a.out

   To understand the program, draw the processors and slowly draw each
   message.

   All statements after MPI_Init are executed by (-np) 5 processes,
   PIDs are 0..P-1.

*/
   
#include <stdio.h>
#include <mpi.h>

/* a variable needeed for call, result not used */
static MPI_Status tmpstat;

/* logical directions */
#define Left (PID-1)
#define Right (PID+1)
#define Any MPI_ANY_SOURCE

/* Send and Receive operations
   DEST & SOURCE can be Left/Right or a PID (int 0..P-1)
   SOURCE can be Any
*/

/* Send the value of an int _variable_ VAR to destination process DEST */
#define Send(DEST, VAR) MPI_Send(&VAR, 1, MPI_INT, DEST, 0, MPI_COMM_WORLD)

/* Receive an integer to an int variable VAR from destination process SOURCE */
#define Receive(SOURCE, VAR) MPI_Recv(&VAR, 1, MPI_INT, SOURCE, 0, MPI_COMM_WORLD, &tmpstat)



int main(int argc, char **argv)
{
  int PID,	/* process-ID */
	  P;	/* number of processes */ 

  /* place own variables here */
  int num, mess;

  /* keep these 3 calls */
  MPI_Init(&argc, &argv);
  MPI_Comm_rank(MPI_COMM_WORLD, &PID);
  MPI_Comm_size(MPI_COMM_WORLD, &P);

  /* actual code starts here */

  /* Send a number right, return back left */

  if (PID == 0) {
	  /* 0 |-> */
	  num = 42;
  } else {
	  /* ->| */
	  Receive(Left, num);
  }

  if (PID != P-1) {
	  /* |-> */
	  Send(Right, num);
	  /* |<- */
	  Receive(Right, mess);
  } else {
	  /* Rightmost one does not send further right */
	  /* just senpar the same back */
	  mess = num;
  }

  /* All print just for debugging */
  printf("Pros: %d, num: %d mess: %d \n", PID, num, mess);

  if (PID != 0) {
	  /* <-| */
	  Send(Left, mess);
  }

  /* keep this call */
  MPI_Finalize();

  exit(0);

}