/* problem 3.7.1 in PPMPI
 *
 * Send a message from all processes with rank != 0 to process 0.
 *    Process 0 prints the messages received.
 *
 * Input: none.
 * Output: contents of messages received by process 0.
 *
 * See Chapter 3, pp. 41 & ff in PPMPI.
 *
 * by John Weatherwax
 *
 * Try:
 * 1) Run MPI_Send then MPI_Recv
 *    -Random ordering of printing statements
 * 2) Run MPI_Recv the MPI_Send
 *    -Every process hangs waiting on the message it is supposed to recieve
 * 3) Alternate which process even send then recieve
 *                            odd  are recieving and then sending.
 *    -Random ordering of print statements.
 * 
 */
#include <stdio.h>
#include <string.h>
#include "mpi.h"

main(int argc, char* argv[]) {
    int         my_rank;       /* rank of process      */
    int         p;             /* number of processes  */
    int         dest;          /* rank of destination  */
    int         recv;          /* rank of receiver     */
    int         tag = 0;       /* tag for messages     */
    char        messageS[100];  /* storage for message sent  */
    char        messageR[100];  /* storage for message recieved  */
    MPI_Status  status;        /* return status for    */
                               /* receive              */

    /* Start up MPI */
    MPI_Init(&argc, &argv);

    /* Find out process rank  */
    MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);

    /* Find out number of processes */
    MPI_Comm_size(MPI_COMM_WORLD, &p);

    /* Calculate who to SEND a message to: */ 
    dest = (my_rank+1) % p;
    /* Calculate who to RECIEVE a message from: */ 
    recv = ( (my_rank==0) ? (p-1) : (my_rank-1) );

    /* Create message */
    sprintf(messageS, "Greetings from process %d!",my_rank);

    if( my_rank % 2 == 0 ){
      /* Use strlen+1 so that '\0' gets transmitted */
      printf("Process %d sending: %s\n", my_rank, messageS); 
      MPI_Send(messageS, strlen(messageS)+1, MPI_CHAR, 
	       dest, tag, MPI_COMM_WORLD);

      MPI_Recv(messageR, 100, MPI_CHAR, recv, tag, 
	       MPI_COMM_WORLD, &status);
      printf("Process %d recieved: %s\n", my_rank, messageR);
    }else{
      MPI_Recv(messageR, 100, MPI_CHAR, recv, tag, 
	       MPI_COMM_WORLD, &status);
      printf("Process %d recieved: %s\n", my_rank, messageR);

      /* Use strlen+1 so that '\0' gets transmitted */
      printf("Process %d sending: %s\n", my_rank, messageS); 
      MPI_Send(messageS, strlen(messageS)+1, MPI_CHAR, 
	       dest, tag, MPI_COMM_WORLD);
    }

    /* Shut down MPI */
    MPI_Finalize();
} /* main */