#include <stdio.h>
#include <stdlib.h>
#include "mpi.h"

/* This program tests ping-pong performance between
   nprocs/2 pairs of processeces (nprocs must be even):

   process 2k sends/recvs 'msgcount' messages of length 'msgsize' to 2k+1
*/

int main(int argc, char *argv[])
{
    int i, msgsize, msgcount, myrank, nprocs;
    char *buffer;
    MPI_Status status;
    double start_time, end_time;
		
    MPI_Init(&argc, &argv);
    MPI_Comm_size(MPI_COMM_WORLD, &nprocs);
    if (nprocs%2) {
	fprintf(stderr, "Error: This program requires an even number of processes.\n");
	MPI_Abort(MPI_COMM_WORLD, 1);
	exit(1);
    }
    MPI_Comm_rank(MPI_COMM_WORLD, &myrank);

    if (argc!=3) {
	fprintf(stderr, "Usage: %s msgsize msgcount\n", argv[0]);
	MPI_Abort(MPI_COMM_WORLD, 2);
	exit(2);
    }

    msgsize=atoi(argv[1]);
    msgcount=atoi(argv[2]);

#ifdef DEBUG
    fprintf(stderr, "Proc %d: size=%d, count=%d\n", myrank, msgsize, msgcount);
#endif

    /* Allocate a buffer of size msgsize */
    if ( (buffer=malloc(msgsize)) == NULL ) {
	fprintf(stderr, "Cannot allocate buffer!\n");
	MPI_Abort(MPI_COMM_WORLD, 3);
	exit(3);
    }

    /* Synchronization */
    MPI_Barrier(MPI_COMM_WORLD);
    MPI_Barrier(MPI_COMM_WORLD);
    MPI_Barrier(MPI_COMM_WORLD);
    MPI_Barrier(MPI_COMM_WORLD);
    MPI_Barrier(MPI_COMM_WORLD);

    start_time=MPI_Wtime();

    if ( myrank < nprocs/2 ) {
	/* receive from myrank-1 */
	for (i=0; i<msgcount; i++) {
	    MPI_Recv(buffer, msgsize, MPI_BYTE, myrank+nprocs/2, 111,
			MPI_COMM_WORLD, &status);
	    MPI_Send(buffer, msgsize, MPI_BYTE, myrank+nprocs/2, 111,
			MPI_COMM_WORLD);
	}
    } else {
	/* send to myrank+1 */
	for (i=0; i<msgcount; i++) {
	    MPI_Send(buffer, msgsize, MPI_BYTE, myrank-nprocs/2, 111,
			MPI_COMM_WORLD);
	    MPI_Recv(buffer, msgsize, MPI_BYTE, myrank-nprocs/2, 111,
			MPI_COMM_WORLD, &status);
	}
    }

    /* Synchronization */
    MPI_Barrier(MPI_COMM_WORLD);

    end_time=MPI_Wtime();

    MPI_Finalize();

    if (!myrank) {
	printf("Wall time: %0.4lf\n", end_time-start_time);
    }

    exit(0);
}