#include <stdlib.h>
#include <stdio.h>  
int main(int argc, char *argv[])
{
/*
   this program demonstrates a very simple MPI program illustrating
   communication between two nodes.  Node 0 sends a message consisting
   of 3 integers to node 1.  Upon receipt, node 1 replies with another
   message consisting of 3 integers.
*/
/* get definition of MPI constants */
#include "mpi.h"

/* define variables
ierror = error indicator
nproc = number of processors participating
idproc = each processor's id number (0 <= idproc < nproc)
len = length of message (in words)
tag = message tag, used to distinguish between messages  */

   int ierror, nproc, idproc, len=3, tag=1;

/* status = status array returned by MPI_Recv */

   MPI_Status status;

/* sendmsg = message being sent */
/* recvmsg = message being received */
/* replymsg = reply message being sent */

   int recvmsg[3];
   int sendmsg[3] = {1802399587,1798073198,1868786465};
   int replymsg[3] = {2003332903,1931506792,1701995839};
   
   printf("Knock program initializing...\n"); 
   
/* initialize the MPI execution environment */
   ierror = MPI_Init(&argc, &argv);
/* stop if MPI could not be initialized */
   if (ierror)
      exit(1);
/* determine nproc, number of participating processors */
   ierror = MPI_Comm_size(MPI_COMM_WORLD,&nproc);
/* determine idproc, the processor's id */
   ierror = MPI_Comm_rank(MPI_COMM_WORLD,&idproc);

/* use only even number of processors */
      nproc = 2*(nproc/2);
      if (idproc < nproc) {
/* even processor sends and prints message,
   then receives and prints reply           */

      if (idproc%2==0) {
         ierror = MPI_Send(&sendmsg,len,MPI_INT,idproc+1,tag,
                           MPI_COMM_WORLD);
         printf("proc %d sent: %.12s\n",idproc,sendmsg);
         ierror = MPI_Recv(&recvmsg,len,MPI_INT,idproc+1,tag+1,
                           MPI_COMM_WORLD,&status);
         printf("proc %d received: %.12s\n",idproc,recvmsg);
      }

/* odd processor receives and prints message,
   then sends reply and prints it             */

      else {
         ierror = MPI_Recv(&recvmsg,len,MPI_INT,idproc-1,tag,
                           MPI_COMM_WORLD,&status);
         printf("proc %d received: %.12s\n",idproc,recvmsg);
         ierror = MPI_Send(&replymsg,len,MPI_INT,idproc-1,tag+1,
                           MPI_COMM_WORLD);
         printf("proc %d sent: %.12s\n",idproc,replymsg);
      }
   }

/* terminate MPI execution environment */
   ierror = MPI_Finalize();

   if (idproc < nproc) {
      printf("hit carriage return to continue\n");
      getchar();
   }
   return 0;
}