//
// 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 <iostream>
#include <cstdlib>		// has exit(), etc.
#include <mpi++.h>		// MPI header file

// a short function to print a message, clean up, and exit

void errorExit(const char msg[]) {
  cerr << msg;
  MPI_Finalize();
  exit(EXIT_FAILURE);
}

// main program

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

  // initialize for MPI (should come before any other calls to
  //     MPI routines)
  if (MPI_Init(&argc, &argv) != MPI_SUCCESS) 
    errorExit("MPI initialization error\n");

  // get number of processes
  int nprocs;
  MPI_Comm_size(MPI_COMM_WORLD, &nprocs);
  if (nprocs != 2) 
    errorExit("number of processes must be 2\n");

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

  const int bufflen = 2;
  int buff[bufflen];
  int mytag = 0;
  MPI_Status status;

  if (myid == 0) {
    // in process 0:  send message to process 1 and wait for echo
    buff[0] = 10;
    buff[1] = 20;
    int other_id = 1;
    double start_time = MPI_Wtime();
    if (MPI_Send(buff, bufflen, MPI_INT, other_id, mytag, 
		 MPI_COMM_WORLD) != MPI_SUCCESS) 
      errorExit("error in send in process 0\n");
    if (MPI_Recv(buff, bufflen, MPI_INT, other_id, mytag,
		 MPI_COMM_WORLD, &status) != MPI_SUCCESS) 
      errorExit("error in receive in process 0\n");
    double end_time = MPI_Wtime();
    cout << "starting, ending times " << start_time << " " << end_time 
	 << endl;
    cout << "difference " << end_time - start_time << endl;
  }
  else {
    // in process 1:  receive message from process 0 and send back
    int other_id = 0;
    if (MPI_Recv(buff, bufflen, MPI_INT, other_id, mytag,
		 MPI_COMM_WORLD, &status) != MPI_SUCCESS) 
      errorExit("error in receive in process 1\n");
    if (MPI_Send(buff, bufflen, MPI_INT, other_id, mytag, 
		 MPI_COMM_WORLD) != MPI_SUCCESS) 
      errorExit("error in send in process 1\n");
  }

  // clean up for MPI 
  MPI_Finalize();

  return EXIT_SUCCESS;
}