MATH-1356

Handle special cases for which the computation would otherwise produce NaN.
Thanks to Thomas Lacroix for the report.

src/main/java/org/apache/commons/math4/distribution/SaddlePointExpansion.java

@@ -159,6 +159,9 @@ final class SaddlePointExpansion {
             }
             ret = s1;
         } else {
+            if (x == 0) {
+                return mu;
+            }
             ret = x * FastMath.log(x / mu) + mu - x;
         }
         return ret;
@@ -180,6 +183,9 @@ final class SaddlePointExpansion {
             if (p < 0.1) {
                 ret = -getDeviancePart(n, n * q) - n * p;
             } else {
+                if (n == 0) {
+                    return 0;
+                }
                 ret = n * FastMath.log(q);
             }
         } else if (x == n) {

src/test/java/org/apache/commons/math4/distribution/HypergeometricDistributionTest.java

@@ -311,6 +311,32 @@ public class HypergeometricDistributionTest extends IntegerDistributionAbstractT
         Assert.assertTrue(Precision.compareTo(1.0, upper, 1) == 0);
     }
 
+    @Test
+    public void testZeroTrial() {
+        final int n = 11; // population
+        final int m = 4;  // successes in population
+        final int s = 0;  // number of trials
+
+        final HypergeometricDistribution dist = new HypergeometricDistribution(n, m, 0);
+
+        for (int i = 1; i <= n; i++) {
+            final double p = dist.probability(i);
+            Assert.assertEquals("p=" + p, 0, p, 0d);
+        }
+    }
+
+    @Test
+    public void testMath1356() {
+        final int n = 11;  // population
+        final int m = 11;  // successes in population
+
+        for (int s = 0; s <= n; s++) {
+            final HypergeometricDistribution dist = new HypergeometricDistribution(n, m, s);
+            final double p = dist.probability(s);
+            Assert.assertEquals("p=" + p, 1, p, 0d);
+        }
+    }
+
     @Test
     public void testMath1021() {
         final int N = 43130568;