// Compile with:  g++ -o PiOMP -fopenmp  PiOMP.cpp

#include <iostream>
#include <vector>
#include <cstdlib>
#include <ctime>
#include  <omp.h>

using namespace std;

int piRnd(long n) {
    long count = 0;


    // use re-entrant random gen
    drand48_data dbuf;
    // init random seed differently for each thread
    srand48_r(time(0) + omp_get_thread_num(), &dbuf); 

    double x, y;

    for ( long i = 0; i<n; i++ ) {
        drand48_r( &dbuf, &x);
        drand48_r( &dbuf, &y);
        if ( x*x + y*y < 1 ) count ++;
    }        
    return count;
}

int main() {

    long nrnd = 100000000;
    long ntask = 20;
    long ctot = 0;
    #pragma omp parallel for
    for (int i = 0; i < ntask; i++) {
        int count = piRnd( nrnd );
        cout << " OpenMP ID: " << omp_get_thread_num() << " count "<< count << endl;
        #pragma omp critical // protects ctot for concurrent write
        ctot += count;

    }
    cout << "ctot = " << ctot << " Pi = " << 4.*ctot / (ntask * nrnd) << endl;
    return 0;
}