#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);
    fflush(stdout);
    
    n = 0;
    
    if (myid == 0) {
      if (argc != 2) {
	fprintf(stdout,"usage: mpirun -np <num_p> ./cpi <num_intervals>\n");   
	fflush(stdout);
      } else {
	n = atoi(argv[1]);
	printf("Number of intervals: %d\n", n);
	
	startwtime = MPI_Wtime();
      }
    }

    MPI_Bcast(&n, 1, MPI_INT, 0, MPI_COMM_WORLD);

    if (n!=0) {
      h   = 1.0 / (double) n;
      sum = 0.0;
      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;
}