Make Beta result equally possible to be 0 or 1 but not NaN when a and b are both very small

Author: 埃博拉酱

src/Numerics/Distributions/Beta.cs

@@ -392,9 +392,13 @@ public IEnumerable<double> Samples()
         /// <returns>a random number from the Beta distribution.</returns>
         internal static double SampleUnchecked(System.Random rnd, double a, double b)
         {
-            var x = Gamma.SampleUnchecked(rnd, a, 1.0);
-            var y = Gamma.SampleUnchecked(rnd, b, 1.0);
-            return x == 0 ? 0 : x / (x + y);
+            double x, y;
+            do
+            {
+                x = Gamma.SampleUnchecked(rnd, a, 1.0);
+                y = Gamma.SampleUnchecked(rnd, b, 1.0);
+            } while (x == 0 && y == 0);
+            return x / (x + y);
         }
 
         internal static void SamplesUnchecked(System.Random rnd, double[] values, double a, double b)