#include <iostream>
#include <thread>
#include <atomic>
#include <sys/time.h>
#include <ctime>
#include <cmath>
#include <random>

static const int num_threads = 8;
static const int max_ = 100000;

void slice_of_pi(int tries, int my_id, std::atomic<int> & in_circle) {
    struct timeval tv;
    gettimeofday(&tv, NULL);
    srand(tv.tv_usec + my_id);

    std::uniform_int_distribution<int> distribution {0, max_};
    std::mt19937 engine; // Mersenne twister MT19937
    auto generator = std::bind(distribution, engine);

    double x, y, d;
    int _in_circle = 0;
    for(int i = 0; i < tries; i++) {
        x = (float)generator() / (float)max_;
        y = (float)generator() / (float)max_;
        d = sqrt(x * x + y * y);
        if(d < 1.0)
            _in_circle++;
    }
    in_circle += _in_circle;
}

int main(int argc, char * argv[]) {

    if(argc != 2) {
        std::cerr << "pi <number of attempts>" << std::endl;
        return 1;
    }

    std::atomic<int> successes {0};
    int attempts = atoi(argv[1]);

    std::thread t[num_threads];

    for (int i = 0; i < num_threads; ++i) {
        t[i] = std::thread(slice_of_pi, attempts, i, std::ref(successes));
    }


    for (int i = 0; i < num_threads; ++i) {
        t[i].join();
    }

    float pi = 4 * float(successes) / float(attempts * num_threads);
    printf("%0.8f\n", pi);

    return 0;
}