fix: mode for gamma distribution is 0 for shape<=1

src/distribution/gamma.rs

@@ -133,17 +133,13 @@ impl ContinuousCDF<f64, f64> for Gamma {
     fn sf(&self, x: f64) -> f64 {
         if x <= 0.0 {
             1.0
-        }
-        else if ulps_eq!(x, self.shape) && self.rate.is_infinite() {
+        } else if ulps_eq!(x, self.shape) && self.rate.is_infinite() {
             0.0
-        }
-        else if self.rate.is_infinite() {
+        } else if self.rate.is_infinite() {
             1.0
-        }
-        else if x.is_infinite() {
+        } else if x.is_infinite() {
             0.0
-        }
-        else {
+        } else {
             gamma::gamma_ur(self.shape, x * self.rate)
         }
     }
@@ -239,13 +235,17 @@ impl Mode<Option<f64>> for Gamma {
     ///
     /// # Formula
     ///
-    /// ```ignore
-    /// (α - 1) / β
+    /// ```text
+    /// max{(α - 1) / β, 0}
     /// ```
     ///
     /// where `α` is the shape and `β` is the rate
     fn mode(&self) -> Option<f64> {
-        Some((self.shape - 1.0) / self.rate)
+        if self.shape < 1.0 {
+            Some((self.shape - 1.0) / self.rate)
+        } else {
+            Some(0.0)
+        }
     }
 }