/*
  nearly the simplest possible program to time message-passing:

  process 0 sends a message, process 1 receives it and sends it
    back, process 0 receives the result and prints timing info.
*/

#include <stdio.h>
#include <stdlib.h>
#include "mpi.h"		/* MPI header file */

int main(int argc, char *argv[]) {

  int nprocs;
  int myid;
  int buff[2];
  int bufflen = 2;
  int mytag = 0;
  int other_id;
  MPI_Status status;
  double start_time, end_time;

  /* initialize for MPI */
  if (MPI_Init(&argc, &argv) != MPI_SUCCESS) {
    printf("MPI initialization error\n"); exit(EXIT_FAILURE);
  }

  /* get number of processes */
  (void) MPI_Comm_size(MPI_COMM_WORLD, &nprocs);
  if (nprocs != 2) {
    printf("number of processes must be 2\n"); exit(EXIT_FAILURE);
  }

  /* get this process's number (ranges from 0 to nprocs - 1) */
  (void) MPI_Comm_rank(MPI_COMM_WORLD, &myid);

  if (myid == 0) {
    /* in process 0:  send message to process 1 and wait for echo */
    buff[0] = 10;
    buff[1] = 20;
    other_id = 1;
    start_time = MPI_Wtime();
    if (MPI_Send(buff, bufflen, MPI_INTEGER, other_id, mytag, 
		 MPI_COMM_WORLD) != MPI_SUCCESS) {
      printf("error in send in process 0\n"); exit(EXIT_FAILURE);
    }
    if (MPI_Recv(buff, bufflen, MPI_INTEGER, other_id, mytag,
		 MPI_COMM_WORLD, &status) != MPI_SUCCESS) {
      printf("error in receive in process 0\n"); exit(EXIT_FAILURE);
    }
    end_time = MPI_Wtime();
    printf("starting, ending times %g %g\n", start_time, end_time);
    printf("difference %g\n", end_time - start_time);
  }
  else {
    /* in process 1:  receive message from process 0 and send back */
    other_id = 0;
    if (MPI_Recv(buff, bufflen, MPI_INTEGER, other_id, mytag,
		 MPI_COMM_WORLD, &status) != MPI_SUCCESS) {
      printf("error in receive in process 1\n"); exit(EXIT_FAILURE);
    }
    if (MPI_Send(buff, bufflen, MPI_INTEGER, other_id, mytag, 
		 MPI_COMM_WORLD) != MPI_SUCCESS) {
      printf("error in send in process 1\n"); exit(EXIT_FAILURE);
    }
  }

  /* clean up for MPI */
  (void) MPI_Finalize();

  return EXIT_SUCCESS;
}