extern crate rayon;

use rayon::prelude::*;

fn semicircle(x: f64) -> f64 {
    (1.0 - x * x).sqrt()
}

fn parallel_integrate<F>(f: F, points: usize) -> f64
where
    F: Fn(f64) -> f64 + Sync,
{
    let delta = 1f64 / points as f64;
    (0..points)
        .into_par_iter()
        .map(|i| {
            let a = i as f64 * delta;
            delta * (f(a) + f(a + delta)) / 2f64
        })
        .sum()
}

fn main() {
    let pi = 4.0 * parallel_integrate(semicircle, 1_000_000_000_000);

    println!("Pi: {}", pi);
}