/*    Program pilul.c                  */
#include "mpi.h"
#include <stdio.h>
#include <math.h>
int main( int argc, char *argv[] )
{
    int n, myid, numprocs, i;
    double PI25DT = 3.141592653589793238462643;
    double mypi, pi, h, sum, x;
    MPI_Init(&argc,&argv);
    MPI_Comm_size(MPI_COMM_WORLD,&numprocs);
    MPI_Comm_rank(MPI_COMM_WORLD,&myid);
    MPI_Win winn, winpi;

    if (myid == 0) {
        /* Two windows, one for n and one for pi */
        MPI_Win_create(&n,sizeof(int),sizeof(int),
            MPI_INFO_NULL,MPI_COMM_WORLD,&winn);
        MPI_Win_create(&pi,sizeof(double),
            sizeof(double),MPI_INFO_NULL,MPI_COMM_WORLD,&winpi);
    }
    else {
        MPI_Win_create(NULL,0,sizeof(int),
            MPI_INFO_NULL,MPI_COMM_WORLD,&winn);
        MPI_Win_create(NULL,0,sizeof(double),
            MPI_INFO_NULL,MPI_COMM_WORLD,&winpi);
    }

    while (1) {
        if (myid == 0) {
            printf("Enter the number of intervals: (0 quits) ");
            scanf("%d",&n);
            pi = 0;
        }
        /* Sync here so all processes get n AFTER it is input */
        MPI_Barrier(MPI_COMM_WORLD);

        /* (Alternative: rank 0 could do a put to each rank here) */
        if (myid > 0) {
            MPI_Win_lock(MPI_LOCK_SHARED,0,0,winn);
            MPI_Get(&n,1,MPI_INT,0,0,1,MPI_INT,winn);
            MPI_Win_unlock(0,winn);
        }
        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;
            if (myid > 0) {
                /* Send pi for summing into buffer on rank 0 */
                MPI_Win_lock(MPI_LOCK_SHARED,0,0,winpi);
                MPI_Accumulate(&mypi,1,MPI_DOUBLE,0,0,1,MPI_DOUBLE,
                               MPI_SUM,winpi);
                MPI_Win_unlock(0,winpi);
            }
            /* Sync here so rank 0 adds mypi to pi last of all */
            MPI_Barrier(MPI_COMM_WORLD);

            if (myid == 0) {
                pi += mypi;
                printf("pi is approximately %.16f, Error is %.16f\n",
                       pi, fabs(pi - PI25DT));
            }
        }
    }
    MPI_Finalize();
    return 0;
}