Change Poisson algorithm threshold to >=10 per Ahrens-Dieter paper

CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixes
 - Avoid returning negative values in `InverseGaussian::sample`; this is a Value-breaking change ([#56])
 - Fix Zipf returning values larger than `n` in rare cases ([#57])
+- Changed threshold for Ahrens-Dieter PD algorithm to >=10, matching the suggested value in the original 1982 paper
 
 ## [0.6.0] — 2026-02-10
 - Bump to MSRV 1.85.0 and Edition 2024 in line with `rand` ([#28])

src/poisson.rs

@@ -161,7 +161,9 @@ where
         }
 
         // Use the Knuth method only for low expected values
-        let method = if lambda < F::from(12.0).unwrap() {
+        // Per Ahrens-Dieter, the rejection sampling algorithm is
+        // only designed for values >= 10
+        let method = if lambda < F::from(10.0).unwrap() {
             Method::Knuth(KnuthMethod::new(lambda))
         } else {
             if lambda > F::from(Self::MAX_LAMBDA).unwrap() {