/* ---------------------------------------------------------------
 *  pi_reduce.c
 *  FILES: pi_reduce.c, dboard.c, make.pi.c
 *  DESCRIPTION:  MPI pi calculation example program.  C Version.
 *    This program calculates pi using a "dartboard" algorithm.  See
 *    Fox et al.(1988) Solving Problems on Concurrent Processors, vol.1
 *    page 207.  All processes contribute to the calculation, with the
 *    master averaging the values for pi.
 *
 *    SPMD version:  Conditional statements check if the process  
 *    is the master or a worker.  
 *
 *    This version uses MPI_Reduce to collect results
 *
 *  AUTHOR: Roslyn Leibensperger
 *  REVISED: 05/24/93 for API by Roslyn Leibensperger
 *           09/14/93 for latest API changes  Blaise Barney
 *           01/10/94 changed API to MPL  Stacy Pendell
 *           05/18/94 corrections to comments Roslyn Leibensperger
 *  CONVERTED TO MPI: 11/12/94 by  Xianneng Shen
 * --------------------------------------------------------------- */

#include <stdlib.h>
#include <stdio.h>
#include "mpi.h"
double dboard (int darts);

#define DARTS 5000      /* number of throws at dartboard */
#define ROUNDS 10       /* number of times "darts" is iterated */
#define MASTER 0        /* task ID of master task */

main(int argc, char **argv) 
{  
double homepi,          /* value of pi calculated by current task */
       pisum,           /* sum of tasks' pi values */
       pi,              /* average of pi after "darts" is thrown */
       avepi;           /* average pi value for all iterations */
int mytid,              /* task ID - also used as seed number */
    nproc,              /* number of tasks */
    rcode,              /* return code */
    i;

/* Obtain number of tasks and task ID */
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &mytid);
MPI_Comm_size(MPI_COMM_WORLD, &nproc);
printf ("MPI task ID = %d\n", mytid);

/* Set seed for random number generator equal to task ID */
srand (mytid);

avepi = 0;
for (i = 0; i < ROUNDS; i++) {
     /* All tasks calculate pi using dartboard algorithm */
     homepi = dboard(DARTS);

     /* Use MPI_Reduce to sum values of homepi across all tasks  
      * Master will store the accumulated value in pisum  
      * - homepi is the send buffer
      * - pisum is the receive buffer (used by the receiving task only)
      * - the size of the message is sizeof(double)
      * - MASTER is the task that will receive the result of the reduction
      *   operation
      */
      rcode = MPI_Reduce(&homepi, &pisum, 1, MPI_DOUBLE, MPI_SUM, MASTER,
                MPI_COMM_WORLD);
     if (rcode != 0)
              printf("%d: failure on MPI_Reduce\n", mytid);

     /* Master computes average for this iteration and all iterations */
     if (mytid == MASTER) {
          pi = pisum/nproc;
          avepi = ((avepi * i) + pi)/(i + 1);  
          printf("   After %3d throws, average value of pi = %10.8f\n",
                 (DARTS * (i + 1)),avepi);
          }     
    }  
    MPI_Finalize();
}