/* -*- Mode: C; c-basic-offset:4 ; indent-tabs-mode:nil ; -*- */
/*
 *  (C) 2001 by Argonne National Laboratory.
 *      See COPYRIGHT in top-level directory.
 */
#include "mpi.h"
#include "stdio.h"
#include <math.h>

/* From Using MPI-2 */

int main(int argc, char *argv[])
{
    int n, myid, numprocs, i;
    double PI25DT = 3.141592653589793238462643;
    double mypi, pi, h, sum, x;
    MPI_Win nwin, piwin;

    MPI_Init(&argc,&argv);
    MPI_Comm_size(MPI_COMM_WORLD,&numprocs);
    MPI_Comm_rank(MPI_COMM_WORLD,&myid);

    if (myid == 0) {
        MPI_Win_create(&n, sizeof(int), 1, MPI_INFO_NULL,
                       MPI_COMM_WORLD, &nwin);
        MPI_Win_create(&pi, sizeof(double), 1, MPI_INFO_NULL,
                       MPI_COMM_WORLD, &piwin);
    }
    else {
        MPI_Win_create(MPI_BOTTOM, 0, 1, MPI_INFO_NULL,
                       MPI_COMM_WORLD, &nwin);
        MPI_Win_create(MPI_BOTTOM, 0, 1, MPI_INFO_NULL,
                       MPI_COMM_WORLD, &piwin);
    }
    while (1) {
        if (myid == 0) {
            fprintf(stdout, "Enter the number of intervals: (0 quits) ");
            fflush(stdout);
            scanf("%d",&n);
            pi = 0.0;
        }
        MPI_Win_fence(0, nwin);
        if (myid != 0)
            MPI_Get(&n, 1, MPI_INT, 0, 0, 1, MPI_INT, nwin);
        MPI_Win_fence(0, nwin);
        if (n == 0)
            break;
        else {
            h = 1.0 / (double) n;
            sum = 0.0;
            for (i = myid + 1; i <= n; i += numprocs) {
                x = h * ((double)i - 0.5);
                sum += (4.0 / (1.0 + x*x));
            }
            mypi = h * sum;
            MPI_Win_fence( 0, piwin);
            MPI_Accumulate(&mypi, 1, MPI_DOUBLE, 0, 0, 1, MPI_DOUBLE,
                           MPI_SUM, piwin);
            MPI_Win_fence(0, piwin);
            if (myid == 0) {
                fprintf(stdout, "pi is approximately %.16f, Error is %.16f\n",
                        pi, fabs(pi - PI25DT));
                fflush(stdout);
            }
        }
    }
    MPI_Win_free(&nwin);
    MPI_Win_free(&piwin);
    MPI_Finalize();
    return 0;
}