Add uniformity test for 65355u16

Cargo.toml

@@ -73,6 +73,8 @@ rand_isaac = { path = "rand_isaac", version = "0.1" }
 rand_chacha = { path = "rand_chacha", version = "0.2" }
 rand_xorshift = { path = "rand_xorshift", version = "0.1" }
 rand_distr = { path = "rand_distr", version = "0.1" }
+# Histogram implementation for testing uniformity
+average = "0.9.2"
 
 [build-dependencies]
 autocfg = "0.1"

tests/uniformity.rs

@@ -0,0 +1,56 @@
+// Copyright 2018 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.
+
+#[macro_use]
+extern crate average;
+extern crate rand;
+extern crate core;
+
+use average::Histogram;
+use rand::prelude::*;
+
+// 65355 = 3 * 5 * 4357 produced the zone with largest deviation from a multiple
+// of itself (deviation = 32579).
+const RANGE: u16 = 65355;
+// We want a reasonably small number of bins which is a divisor of the range
+const N_BINS: usize = 15;
+// Even with 100M samples no pattern is obvious!
+const N_SAMPLES: u32 = 1_000_000;
+const SCALE: f64 = 0.05;   // adjust with N_SAMPLES to scale bar lengths
+define_histogram!(hist, 15);
+use hist::Histogram as Histogram15;
+
+#[test]
+fn uniform() {
+    // 65175 = 3*25*11*79 has the largest difference in 
+    let distr = rand::distributions::Uniform::new(0, RANGE);
+    let mut rng = rand::rngs::SmallRng::from_entropy();
+    let mut h = Histogram15::with_const_width(0., RANGE as f64);
+    for _ in 0..N_SAMPLES {
+        let x = distr.sample(&mut rng);
+        h.add(x as f64).unwrap();
+    }
+
+    let expected = (N_SAMPLES as f64) / (N_BINS as f64);
+    let spaces = "                                                           ";
+    let stars  = "***********************************************************";
+    assert_eq!(spaces.len(), stars.len());
+    let len = stars.len() as isize;
+    println!("Deviations per bin:");
+    for &b in h.bins() {
+        let diff = (b as f64) - expected;
+        let n = (diff * SCALE).round() as isize;
+        if n < 0 {
+            println!("{}\t{}{}|", diff.abs(), &spaces[0..((len + n) as usize)], &stars[0..(-n as usize)]);
+        } else {
+            println!("{}\t{}|{}", diff.abs(), spaces, &stars[0..(n as usize)]);
+        }
+    }
+    // TODO: results are too variable to make any useful assertions?
+    // assert!(false);
+}