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+// Copyright 2018 Developers of the Rand project.
+// Copyright 2013-2018 The Rust Project Developers.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! # Monte Carlo estimation of π
+//!
+//! Imagine that we have a square with sides of length 2 and a unit circle
+//! (radius = 1), both centered at the origin. The areas are:
+//! 
+//! ```text
+//!     area of circle  = πr² = π * r * r = π
+//!     area of square  = 2² = 4
+//! ```
+//!
+//! The circle is entirely within the square, so if we sample many points
+//! randomly from the square, roughly π / 4 of them should be inside the circle.
+//!
+//! We can use the above fact to estimate the value of π: pick many points in
+//! the square at random, calculate the fraction that fall within the circle,
+//! and multiply this fraction by 4.
+
+#![cfg(feature="std")]
+
+
+extern crate rand;
+
+use rand::distributions::{Distribution, Uniform};
+
+fn main() {
+   let range = Uniform::new(-1.0f64, 1.0);
+   let mut rng = rand::thread_rng();
+
+   let total = 1_000_000;
+   let mut in_circle = 0;
+
+   for _ in 0..total {
+       let a = range.sample(&mut rng);
+       let b = range.sample(&mut rng);
+       if a*a + b*b <= 1.0 {
+           in_circle += 1;
+       }
+   }
+
+   // prints something close to 3.14159...
+   println!("π is approximately {}", 4. * (in_circle as f64) / (total as f64));
+}