Avoid panics on invalid parameters for Zipf

The generalized Zipf distribution is not well defined when its
normalization constant is 0 (when s is infinite) or is infinity
(when n is infinite and s is <= 1).

Author: mstoeckl

CHANGELOG.md

@@ -19,6 +19,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixes
 - Fix `Geometric::new` for small `p > 0` where `1 - p` rounds to 1 (#36)
 - Fix panic in `FisherF::new` on almost zero parameters (#39)
+- Fix panic in `Zipf::new` by rejecting more invalid parameters (#41)
 
 ## [0.5.2]
 

src/zipf.rs

@@ -66,17 +66,20 @@ where
 /// Error type returned from [`Zipf::new`].
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum Error {
-    /// `s < 0` or `nan`.
+    /// `s < 0` or `s` is `nan` or `inf`
     STooSmall,
-    /// `n < 1`.
+    /// `n < 1` or `n` is `nan`
     NTooSmall,
+    /// `n = inf` and `s <= 1`
+    IllDefined,
 }
 
 impl fmt::Display for Error {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.write_str(match self {
-            Error::STooSmall => "s < 0 or is NaN in Zipf distribution",
-            Error::NTooSmall => "n < 1 in Zipf distribution",
+            Error::STooSmall => "s < 0 or is inf or NaN in Zipf distribution",
+            Error::NTooSmall => "n < 1 or is NaN in Zipf distribution",
+            Error::IllDefined => "n = inf and s <= 1 in Zipf distribution",
         })
     }
 }
@@ -97,12 +100,15 @@ where
     /// and since our implementation requires a floating-point type.
     #[inline]
     pub fn new(n: F, s: F) -> Result<Zipf<F>, Error> {
-        if !(s >= F::zero()) {
+        if !(s >= F::zero()) || s.is_infinite() {
             return Err(Error::STooSmall);
         }
         if n < F::one() {
             return Err(Error::NTooSmall);
         }
+        if n.is_infinite() && s <= F::one() {
+            return Err(Error::IllDefined);
+        }
         let q = if s != F::one() {
             // Make sure to calculate the division only once.
             F::one() / (F::one() - s)