Handle a==b==0 case when it always generates NaN

Author: 埃博拉酱

src/Numerics/Distributions/Beta.cs

@@ -393,11 +393,20 @@ public IEnumerable<double> Samples()
         internal static double SampleUnchecked(System.Random rnd, double a, double b)
         {
             double x, y;
-            do
+            if (a == b)
             {
                 x = Gamma.SampleUnchecked(rnd, a, 1.0);
                 y = Gamma.SampleUnchecked(rnd, b, 1.0);
-            } while (x == 0 && y == 0);
+                //When a==b (and possibly a==b==0), return value is equally possible to be 0 or 1
+                if (x == 0 && y == 0)
+                    return Bernoulli.Sample(0.5);//In particular, when a==b==0, Beta distribution degradates to Bernoulli distribution.
+            }
+            else
+                do
+                {
+                    x = Gamma.SampleUnchecked(rnd, a, 1.0);
+                    y = Gamma.SampleUnchecked(rnd, b, 1.0);
+                } while (x == 0 && y == 0);//When a!=b, return value is not equally possible to be 0 or 1. Regenerate.
             return x / (x + y);
         }