montcarlo_pi.ex

defmodule Montecarlo.PI do
  def sample(_, 0, acc) do
    acc
  end

  def sample(state, n, acc) do
    {x, state} = :random.uniform_s(state)
    {y, state} = :random.uniform_s(state)

    res = x*x+y*y
    if res < 1 do
      sample(state, n-1, acc+1)
    else
      sample(state, n-1, acc)
    end
  end

  def sample(n, sender) do
    :random.seed(:erlang.now())
    result = sample(:random.seed(), n, 0)
    send sender, result
  end

  def run(samples, c) when is_integer(c) do
    n = trunc(samples / c)

    for _ <- 1..c do
      spawn(__MODULE__, :sample, [n, self])
    end

    results = for _ <- 1..c do
      receive do
        result -> result
      end
    end

    result = Enum.reduce(results, fn(acc, x) -> acc + x end)

    result / samples * 4
  end
end

Montecarlo.PI.run(1000000000, 4)

MontecarloPI.java

import java.util.stream.IntStream;
import java.util.Random;

public class MontecarloPI {
    static IntStream repeat(int value, int times) {
        int[] tab = new int[times];
        for (int i = 0; i < times; i++) {
            tab[i] = value;
        }
        return IntStream.of(tab);
    }

    static int sample(int iters) {
        Random r = new Random(System.nanoTime());
        int inside = 0;
        for (int i = 0; i < iters; i++) {
            double x = r.nextDouble(),
                   y = r.nextDouble();

            if (x*x+y*y < 1.0) {
                inside++;
            }
        }
        return inside;
    }

    public static void main(String[] args) {
        final int precision   = Integer.parseInt(args[0]);
        final int concurrency = Integer.parseInt(args[1]);
        final int iters       = precision / concurrency;

        int result = repeat(iters, concurrency)
            .parallel()
            .map(MontecarloPI::sample)
            .reduce(0, Integer::sum);

        double pi = (double) result / precision * 4;
        System.out.println(pi);
    }
}

results

$ time java MontecarloPI 1000000000 4
3.141590992
java MontecarloPI 1000000000 4  105.96s user 0.06s system 380% cpu 27.828 total

$ time mix run            
Compiled lib/montecarlo_pi.ex
Generated montecarlo_pi.app
mix run  1591.47s user 1.79s system 383% cpu 6:55.77 total