Poisson distribution falls into an infinite loop for parameter λ=∞ (#…

…1290)

rand_distr/CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   This breaks serialization compatibility with older versions.
 - Upgrade Rand
 - Fix Knuth's method so `Poisson` doesn't return -1.0 for small lambda
+- Fix `Poisson` distribution instantiation so it return an error if lambda is infinite
 
 ## [0.4.3] - 2021-12-30
 - Fix `no_std` build (#1208)

rand_distr/src/poisson.rs

@@ -46,12 +46,15 @@ where F: Float + FloatConst, Standard: Distribution<F>
 pub enum Error {
     /// `lambda <= 0` or `nan`.
     ShapeTooSmall,
+    /// `lambda == +∞`
+    ParameterInfinite,
 }
 
 impl fmt::Display for Error {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.write_str(match self {
             Error::ShapeTooSmall => "lambda is not positive in Poisson distribution",
+            Error::ParameterInfinite => "lambda is infinite in Poisson distribution",
         })
     }
 }
@@ -69,6 +72,9 @@ where F: Float + FloatConst, Standard: Distribution<F>
         if !(lambda > F::zero()) {
             return Err(Error::ShapeTooSmall);
         }
+        if lambda.is_infinite() {
+            return Err(Error::ParameterInfinite);
+        }
         let log_lambda = lambda.ln();
         Ok(Poisson {
             lambda,
@@ -163,7 +169,7 @@ mod test {
     fn test_poisson_avg() {
         test_poisson_avg_gen::<f64>(10.0, 0.1);
         test_poisson_avg_gen::<f64>(15.0, 0.1);
-        
+
         test_poisson_avg_gen::<f32>(10.0, 0.1);
         test_poisson_avg_gen::<f32>(15.0, 0.1);
 
@@ -178,6 +184,12 @@ mod test {
         Poisson::new(0.0).unwrap();
     }
 
+    #[test]
+    #[should_panic]
+    fn test_poisson_invalid_lambda_infinity() {
+        Poisson::new(f64::INFINITY).unwrap();
+    }
+
     #[test]
     #[should_panic]
     fn test_poisson_invalid_lambda_neg() {
@@ -188,4 +200,4 @@ mod test {
     fn poisson_distributions_can_be_compared() {
         assert_eq!(Poisson::new(1.0), Poisson::new(1.0));
     }
-}
+}