Test and fix choose_multiple_weighted with very small probabilities: #1476

Also improves choose_two_weighted_indexed time by 23% (excluding new test)

Author: dhardy

distr_test/tests/weighted.rs

@@ -181,6 +181,7 @@ fn choose_two_weighted_indexed() {
     test_weights(100, |i| i as f64);
     test_weights(100, |i| (i as f64).powi(3));
     test_weights(100, |i| 1.0 / ((i + 1) as f64));
+    test_weights(10, |i| ((i + 1) as f64).powi(-8));
 }
 
 #[test]

src/seq/index.rs

@@ -392,7 +392,7 @@ where
     while index < length {
         let weight = weight(index.as_usize()).into();
         if weight > 0.0 {
-            let key = rng.random::<f64>().powf(1.0 / weight);
+            let key = rng.random::<f64>().ln() / weight;
             candidates.push(Element { index, key });
         } else if !(weight >= 0.0) {
             return Err(WeightError::InvalidWeight);