From 53d9c7fbfbe9cbd7e11cb872ef87e00462fa3eca Mon Sep 17 00:00:00 2001
From: Gilles Sadowski <gilles@harfang.homelinux.org>
Date: Tue, 16 Apr 2019 12:54:04 +0200
Subject: [PATCH] MATH-1401: Branch added to prevent passing invalid input to
 "FDistribution".

Thanks to Michele De Stefano for circumscribing the issue.
---
 src/changes/changes.xml                            |  3 +++
 .../stat/interval/ClopperPearsonInterval.java      | 14 +++++++++-----
 .../stat/interval/ClopperPearsonIntervalTest.java  |  6 ++++++
 3 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/src/changes/changes.xml b/src/changes/changes.xml
index 24012826f..8b5f8e539 100644
--- a/src/changes/changes.xml
+++ b/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>
diff --git a/src/main/java/org/apache/commons/math4/stat/interval/ClopperPearsonInterval.java b/src/main/java/org/apache/commons/math4/stat/interval/ClopperPearsonInterval.java
index b6a72ff4e..5bf642c5c 100644
--- a/src/main/java/org/apache/commons/math4/stat/interval/ClopperPearsonInterval.java
+++ b/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);
 
-            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);
+            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);
diff --git a/src/test/java/org/apache/commons/math4/stat/interval/ClopperPearsonIntervalTest.java b/src/test/java/org/apache/commons/math4/stat/interval/ClopperPearsonIntervalTest.java
index e50ab581b..a8a1db1a3 100644
--- a/src/test/java/org/apache/commons/math4/stat/interval/ClopperPearsonIntervalTest.java
+++ b/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);
+    }
 }