/* ³ÌÐòÀ´Ô´£ºMPICH examples/cpi.c */
#include "mpi.h"
#include <stdio.h>

double f( double a ) { return (4.0 / (1.0 + a*a)); }

int main( int argc, char *argv[])
{
    int n, myid, numprocs, i, namelen;
    double PI25DT = 3.141592653589793238462643;
    double mypi, pi, h, sum, x;
    double startwtime, endwtime;
    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(stderr,"Process %d on %s\n", myid, processor_name);
    if (myid == 0) {
	n=10000;
	startwtime = MPI_Wtime();
    }
    MPI_Bcast(&n, 1, MPI_INT, 0, MPI_COMM_WORLD);
    h   = 1.0 / (double) n;
    sum = 0.0;
    for (i = myid; 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) {
	endwtime = MPI_Wtime();
        printf("pi is approximately %.16f, error is %.16f\n", pi, pi - PI25DT);
	printf("wall clock time = %f\n", endwtime-startwtime);	       
    }
    MPI_Finalize();

    return 0;
}