gistfile1.diff

Index: src/main/java/org/apache/commons/math3/stat/inference/MannWhitneyUTest.java
===================================================================
--- src/main/java/org/apache/commons/math3/stat/inference/MannWhitneyUTest.java (revision 1616791)
+++ src/main/java/org/apache/commons/math3/stat/inference/MannWhitneyUTest.java (working copy)
@@ -145,12 +145,12 @@
          * U1 = R1 - (n1 * (n1 + 1)) / 2 where R1 is sum of ranks for sample 1,
          * e.g. x, n1 is the number of observations in sample 1.
          */
-        final double U1 = sumRankX - (x.length * (x.length + 1)) / 2;
+        final double U1 = sumRankX - ((double) x.length * (x.length + 1)) / 2;
 
         /*
          * It can be shown that U1 + U2 = n1 * n2
          */
-        final double U2 = x.length * y.length - U1;
+        final double U2 = (double) x.length * y.length - U1;
 
         return FastMath.max(U1, U2);
     }
@@ -230,7 +230,7 @@
         /*
          * It can be shown that U1 + U2 = n1 * n2
          */
-        final double Umin = x.length * y.length - Umax;
+        final double Umin = (double) x.length * y.length - Umax;
 
         return calculateAsymptoticPValue(Umin, x.length, y.length);
     }
Index: src/test/java/org/apache/commons/math3/stat/inference/MannWhitneyUTestTest.java
===================================================================
--- src/test/java/org/apache/commons/math3/stat/inference/MannWhitneyUTestTest.java     (revision 1616791)
+++ src/test/java/org/apache/commons/math3/stat/inference/MannWhitneyUTestTest.java     (working copy)
@@ -112,4 +112,16 @@
         double result = testStatistic.mannWhitneyUTest(d1, d2);
         Assert.assertTrue(result > 0.1);
     }
+
+    @Test
+    public void testReallyBigDataSetSameValues() {
+        double[] d1 = new double[110000];
+        double[] d2 = new double[110000];
+        for (int i = 0; i < 110000; i++) {
+            d1[i] = i; 
+            d2[i] = i;
+        }
+        double result = testStatistic.mannWhitneyUTest(d1, d2);
+        Assert.assertTrue(result == 1.0);      
+    }
 }