#include "mpi.h"
#include<stdio.h>
#include<math.h>
 double f(double);
 double f(double a)
 {
     return (4.0 / (1.0 + a*a));
 }
 int main(int argc,char *argv[])
 {
    int done = 0, n, myid, numprocs, i;
    double PI25DT = 3.141592653589793238462643;
    double mypi, pi, h, sum, x;
    double startwtime = 0.0, endwtime;
    int  namelen;
    char processor_name[MPI_MAX_PROCESSOR_NAME];
    MPI_Init(&argc,&argv);
    MPI_Comm_size(MPI_COMM_WORLD,&numprocs);
    MPI_Comm_rank(MPI_COMM_WORLD,&myid);
    MPI_Get_processor_name(processor_name,&namelen);
    fprintf(stdout,"Process %d of %d on %s\n",
            myid, numprocs, processor_name);
    n = 0;
    while (!done)
    {
        if (myid ==  0)
        {
 /*
            printf("Enter the number of intervals: (0 quits) ");
            scanf("%d",&n);
 */
            if (n == 0) n=10000; else n=0;
            startwtime = MPI_Wtime();
        }
        MPI_Bcast(&n, 1, MPI_INT, 0, MPI_COMM_WORLD);
        if (n == 0)
            done = 1;
        else
        {
            h   = 1.0 / (double) n;
            sum = 0.0;
            /* A slightly better approach starts from large i and works back */
            for (i = myid + 1; i <= n; i += numprocs)
            {
                x = h * ((double)i - 0.5);
                sum += f(x);
            }
            mypi = h * sum;
            MPI_Reduce(&mypi, &pi, 1, MPI_DOUBLE, MPI_SUM, 0, MPI_COMM_WORLD);
            if (myid == 0)
            {
                printf("pi is approximately %.16f, Error is %.16f\n",
                       pi, fabs(pi - PI25DT));
                endwtime = MPI_Wtime();
                printf("wall clock time = %f\n", endwtime-startwtime);
                fflush( stdout );
            }
        }
     }
     MPI_Finalize();
     return 0;
 }