MATH-1401: Branch added to prevent passing invalid input to "FDistribution".

Thanks to Michele De Stefano for circumscribing the issue.

src/changes/changes.xml

@@ -54,6 +54,9 @@ If the output is not quite correct, check for invisible trailing spaces!
     </release>
 
     <release version="4.0" date="XXXX-XX-XX" description="">
+      <action dev="erans" type="fix" issue="MATH-1401">
+        "ClopperPearsonInterval": Fixed case where number of trials equals number of successes.
+      </action>
       <action dev="erans" type="add" issue="MATH-1477" due-to="Chee Sing Lee">
         "MillerUpdatingRegression": Fixed "ArrayIndexOutOfBounds" exception.
       </action>

src/main/java/org/apache/commons/math4/stat/interval/ClopperPearsonInterval.java

@@ -46,11 +46,15 @@ public class ClopperPearsonInterval implements BinomialConfidenceInterval {
             lowerBound = numberOfSuccesses /
                 (numberOfSuccesses + (numberOfTrials - numberOfSuccesses + 1) * fValueLowerBound);
 
+            if (numberOfSuccesses != numberOfTrials) {
                 final FDistribution distributionUpperBound = new FDistribution(2 * (numberOfSuccesses + 1),
                                                                                2 * (numberOfTrials - numberOfSuccesses));
                 final double fValueUpperBound = distributionUpperBound.inverseCumulativeProbability(1 - alpha);
                 upperBound = (numberOfSuccesses + 1) * fValueUpperBound /
                     (numberOfTrials - numberOfSuccesses + (numberOfSuccesses + 1) * fValueUpperBound);
+            } else {
+                upperBound = 1;
+            }
         }
 
         return new ConfidenceInterval(lowerBound, upperBound, confidenceLevel);

src/test/java/org/apache/commons/math4/stat/interval/ClopperPearsonIntervalTest.java

@@ -40,4 +40,10 @@ public class ClopperPearsonIntervalTest extends BinomialConfidenceIntervalAbstra
         Assert.assertEquals(0.1248658, confidenceInterval.getUpperBound(), 1E-5);
     }
 
+    @Test
+    public void testMath1401() {
+        ConfidenceInterval interval = new ClopperPearsonInterval().createInterval(1, 1, 0.95);
+        Assert.assertEquals(0.025, interval.getLowerBound(), 1e-16);
+        Assert.assertEquals(1, interval.getUpperBound(), 0d);
+    }
 }