/* ---------------------------------------------------------------
 * MPI pi Calculation Example - C Version 
 * Collective Communication example
 * FILE: mpi_pi_reduce.c
 * OTHER FILES: dboard.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 mpc_reduce to collect results
 *
 * AUTHOR: Roslyn Leibensperger. Converted to MPI: George L. Gusciora 
 *   (1/25/95)
 * LAST REVISED: 06/07/96 Blaise Barney
 * --------------------------------------------------------------- */
#include "mpi.h"
#include <stdlib.h>
#include <stdio.h>
void srandom (unsigned seed);
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 */

int main(argc,argv)
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	taskid,	        /* task ID - also used as seed number */
	numtasks,       /* number of tasks */
	rc,             /* return code */
	i;
MPI_Status status;

   /* Obtain number of tasks and task ID */
   rc = MPI_Init(&argc,&argv);
   rc|= MPI_Comm_size(MPI_COMM_WORLD,&numtasks);
   rc|= MPI_Comm_rank(MPI_COMM_WORLD,&taskid);
   if (rc != MPI_SUCCESS)
      printf ("error initializing MPI and obtaining task ID information\n");
   else
      printf ("task ID = %d\n", taskid);

   /* Set seed for random number generator equal to task ID */
   srandom (taskid);

   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
       * - MPI_SUM is a pre-defined reduction function (double-precision
       *   floating-point vector addition).  Must be declared extern.
       * - MPI_COMM_WORLD is the group of tasks that will participate.
       */

      rc = MPI_Reduce(&homepi, &pisum, 1, MPI_DOUBLE, MPI_SUM,
                         MASTER, MPI_COMM_WORLD);
      if (rc != MPI_SUCCESS)
         printf("%d: failure on mpc_reduce\n", taskid);

      /* Master computes average for this iteration and all iterations */
      if (taskid == MASTER)
      {
         pi = pisum/numtasks;
         avepi = ((avepi * i) + pi)/(i + 1); 
         printf("   After %3d throws, average value of pi = %10.8f\n",
                   (DARTS * (i + 1)),avepi);
      }    
   } 
   MPI_Finalize();
   return 0;
}