extern crate rand;
extern crate rayon;

use rand::{
    distributions::{Distribution, Uniform},
    thread_rng,
};
use rayon::prelude::*;
use std::f64::consts::PI;

fn buffon(number: usize, length: f64, spacing: f64) -> usize {
    assert!(length > 0.0);
    assert!(spacing > 0.0);

    let center = length / 2.0;
    let angles = Uniform::new_inclusive(0.0, PI / 2.0);
    let positions = Uniform::new_inclusive(0.0, spacing / 2.0);

    (0..number)
        .into_par_iter()
        .map(|_| {
            let mut r = thread_rng();
            let a = angles.sample(&mut r);
            let p = positions.sample(&mut r);
            (a, p)
        })
        .filter(|&(a, p)| p < center * a.cos())
        .count()
}

fn main() {
    let number = 4_000_000;
    let length = 2.0;
    let spacing = 4.0;

    println!("--------------------------");
    println!("Simulador Agulha de Buffon");
    println!("--------------------------");
    println!("Nr. Agulhas: {}", number);
    println!("Comprimento: {}", length);
    println!("Espaçamento: {}", spacing);
    println!("--------------------------");

    let hits = buffon(number, length, spacing);
    let pi = number as f64 / hits as f64;

    println!("  Resultado: {}", hits);
    println!(" Estimativa: {:.8}", pi);
    println!(" Erro Total: {:+.4e}", pi - PI);
    println!("--------------------------");
}