Fix overflow for Midpoint

src/quantile.rs

@@ -9,7 +9,7 @@ pub mod interpolate {
     use ndarray::azip;
     use ndarray::prelude::*;
     use num_traits::{FromPrimitive, ToPrimitive};
-    use std::ops::{Add, Div};
+    use std::ops::{Add, Div, Sub};
 
     /// Used to provide an interpolation strategy to [`quantile_axis_mut`].
     ///
@@ -116,7 +116,7 @@ pub mod interpolate {
 
     impl<T> Interpolate<T> for Midpoint
     where
-        T: Add<T, Output = T> + Div<T, Output = T> + Clone + FromPrimitive,
+        T: Add<T, Output = T> + Sub<T, Output=T> + Div<T, Output = T> + Clone + FromPrimitive,
     {
         fn needs_lower(_q: f64, _len: usize) -> bool {
             true
@@ -134,7 +134,14 @@ pub mod interpolate {
             D: Dimension,
         {
             let denom = T::from_u8(2).unwrap();
-            (lower.unwrap() + higher.unwrap()).mapv_into(|x| x / denom.clone())
+            let mut lower = lower.unwrap();
+            let higher = higher.unwrap();
+            azip!(
+                mut lower, ref higher in {
+                    *lower = lower.clone() + (higher.clone() - lower.clone()) / denom.clone()
+                }
+            );
+            lower
         }
     }
 

tests/quantile.rs

@@ -6,6 +6,7 @@ use ndarray::prelude::*;
 use ndarray_stats::{
     interpolate::{Higher, Linear, Lower, Midpoint, Nearest},
     QuantileExt,
+    Quantile1dExt,
 };
 
 #[test]
@@ -148,3 +149,13 @@ fn test_quantile_axis_skipnan_mut_linear_opt_i32() {
     assert_eq!(q[0], Some(3));
     assert!(q[1].is_none());
 }
+
+#[test]
+fn test_midpoint_overflow() {
+    // Regression test
+    // This triggered an overflow panic with a naive Midpoint implementation: (a+b)/2
+    let mut a: Array1<u8> = array![129, 130, 130, 131];
+    let median = a.quantile_mut::<Midpoint>(0.5).unwrap();
+    let expected_median = 130;
+    assert_eq!(median, expected_median);
+}