Use proptest to test reasonable bounds of mean

Cargo.toml

@@ -43,3 +43,4 @@ rand = "0.6"
 serde_json = "1"
 streaming-stats = "0.2"
 quantiles = "0.7"
+proptest = "0.9"

tests/proptest.rs

@@ -0,0 +1,28 @@
+#![cfg_attr(feature = "cargo-clippy", allow(float_cmp, map_clone))]
+
+#[macro_use] extern crate average;
+#[macro_use] extern crate proptest;
+
+extern crate core;
+
+use core::iter::Iterator;
+
+use average::Mean;
+use proptest::prelude::*;
+use prop::num::f64;
+
+proptest! {
+    #![proptest_config(ProptestConfig::with_cases(10000))]
+    #[test]
+    fn reasonable_bounds(s in prop::collection::vec(
+        f64::POSITIVE | f64::NEGATIVE | f64::SUBNORMAL | f64::ZERO, 1..100usize)) {
+        // See https://hypothesis.works/articles/calculating-the-mean/.
+        let max = s.iter().cloned().fold(0./0., f64::max);
+        let min = s.iter().cloned().fold(0./0., f64::min);
+        let a: Mean = s.iter().collect();
+        let mean = a.mean();
+        println!("min: {}  mean: {}  max: {}", min, mean, max);
+        assert!(min <= mean);
+        assert!(mean <= max);
+    }
+}