From da0e67acfa099f7a342fe48c9fc2d68c5db02fb7 Mon Sep 17 00:00:00 2001
From: k88hudson <k88hudson@gmail.com>
Date: Wed, 29 Apr 2026 20:53:00 -0400
Subject: [PATCH] Change Poisson algorithm threshold to >=10 per Ahrens-Dieter
 paper

---
 CHANGELOG.md   | 1 +
 src/poisson.rs | 4 +++-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index b8e3044..e34d32e 100644
--- a/CHANGELOG.md
+++ b/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])
diff --git a/src/poisson.rs b/src/poisson.rs
index 69dd655..041a4c1 100644
--- a/src/poisson.rs
+++ b/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() {