1 files changed, 69 insertions, 0 deletions
diff --git a/rand/rand_distr/src/unit_ball.rs b/rand/rand_distr/src/unit_ball.rs
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+++ b/rand/rand_distr/src/unit_ball.rs
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+// Copyright 2019 Developers of the Rand project.
+//
+// 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.
+
+use rand::Rng;
+use crate::{Distribution, Uniform, uniform::SampleUniform};
+use crate::utils::Float;
+
+/// Samples uniformly from the unit ball (surface and interior) in three
+/// dimensions.
+///
+/// Implemented via rejection sampling.
+///
+///
+/// # Example
+///
+/// ```
+/// use rand_distr::{UnitBall, Distribution};
+///
+/// let v: [f64; 3] = UnitBall.sample(&mut rand::thread_rng());
+/// println!("{:?} is from the unit ball.", v)
+/// ```
+#[derive(Clone, Copy, Debug)]
+pub struct UnitBall;
+
+impl<N: Float + SampleUniform> Distribution<[N; 3]> for UnitBall {
+    #[inline]
+    fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> [N; 3] {
+        let uniform = Uniform::new(N::from(-1.), N::from(1.));
+        let mut x1;
+        let mut x2;
+        let mut x3;
+        loop {
+            x1 = uniform.sample(rng);
+            x2 = uniform.sample(rng);
+            x3 = uniform.sample(rng);
+            if x1*x1 + x2*x2 + x3*x3 <= N::from(1.) {
+                break;
+            }
+        }
+        [x1, x2, x3]
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::Distribution;
+    use super::UnitBall;
+
+    #[test]
+    fn value_stability() {
+        let mut rng = crate::test::rng(2);
+        let expected = [
+                [0.018035709265959987, -0.4348771383120438, -0.07982762085055706],
+                [0.10588569388223945, -0.4734350111375454, -0.7392104908825501],
+                [0.11060237642041049, -0.16065642822852677, -0.8444043930440075]
+            ];
+        let samples: [[f64; 3]; 3] = [
+                UnitBall.sample(&mut rng),
+                UnitBall.sample(&mut rng),
+                UnitBall.sample(&mut rng),
+            ];
+        assert_eq!(samples, expected);
+    }
+}