π day 2017 (https://www.youtube.com/watch?v=RZBhSi_PwHU)

main.cpp

#include <iostream>
#include <math.h>
#include <stdint.h>
#include <random>
#include <chrono>
#include <stdlib.h>

using namespace std;

uint64_t gcd(uint64_t a, uint64_t b);

int main(int argc, char* argv[]){
	if(argc > 2){
		cerr<<"\n\tJust one option specifying number of iterations please. Leave blank for default (50,000,000).\n"<<endl;
		return 1;
	}

	uint64_t iterations = 50000000;
	if(argv[1]){
		iterations = strtoul(argv[1], NULL, 0);
	}

	cout<<"Calculating PI..."<<endl;
	unsigned seed1 = chrono::system_clock::now().time_since_epoch().count();
	mt19937_64 g1 (seed1);

	uint64_t i = 1;
	uint64_t count = 0;
	while(i < iterations){
		uint64_t n1 = g1();
		uint64_t n2 = g1();

		if(gcd(n1, n2) == 1){
			count++;
		}

		i++;
	}

	long double probability = (long double)count/(long double)iterations;
	long double pi = sqrt(6/probability);

	cout<<pi<<endl;

	return 0;
}

uint64_t gcd(uint64_t a, uint64_t b) {
    return b == 0 ? a : gcd(b, a % b);
}