Add MAD=0 unit tests for outlier detection, see #329

src/hyperfine/outlier_detection.rs

@@ -29,6 +29,9 @@ pub fn modified_zscores(xs: &[f64]) -> Vec<f64> {
     // Compute median absolute deviation:
     let mad = median(&deviations);
 
+    // Handle MAD == 0 case
+    let mad = if mad > 0.0 { mad } else { f64::EPSILON };
+
     // Compute modified Z-scores (x_i - x_median) / MAD
     xs.iter().map(|&x| (x - x_median) / mad).collect()
 }
@@ -99,3 +102,13 @@ fn test_detect_outliers() {
     ];
     assert_eq!(2, num_outliers(&xs));
 }
+
+#[test]
+fn test_detect_outliers_if_mad_becomes_0() {
+    // See https://stats.stackexchange.com/q/339932
+    let xs = [10.0, 10.0, 10.0, 10.0, 10.0, 10.0, 10.0, 100.0];
+    assert_eq!(1, num_outliers(&xs));
+
+    let xs = [10.0, 10.0, 10.0, 10.0, 10.0, 10.0, 10.0, 100.0, 100.0];
+    assert_eq!(2, num_outliers(&xs));
+}