fix percentile reordering data in DescriptiveStatistics and StatUtils

commons-math-legacy/src/main/java/org/apache/commons/math4/legacy/stat/StatUtils.java

@@ -41,7 +41,7 @@
 public final class StatUtils {
 
     /** percentile. */
-    private static final Quantile QUANTILE = Quantile.withDefaults().with(EstimationMethod.HF6).withCopy(false);
+    private static final Quantile QUANTILE = Quantile.withDefaults().with(EstimationMethod.HF6).withCopy(true);
 
     /**
      * Private Constructor.

commons-math-legacy/src/main/java/org/apache/commons/math4/legacy/stat/descriptive/Statistics.java

@@ -316,7 +316,7 @@ static Sum getInstance() {
     static final class Percentile implements UnivariateStatistic {
         /** Delegate percentile implementation. */
         private static final Quantile QUANTILE =
-            Quantile.withDefaults().with(EstimationMethod.HF6).withCopy(false);
+            Quantile.withDefaults().with(EstimationMethod.HF6).withCopy(true);
 
         /** Probability. */
         private double p;

commons-math-legacy/src/test/java/org/apache/commons/math4/legacy/stat/StatUtilsTest.java

@@ -456,6 +456,14 @@ public void testPercentile() {
         TestUtils.assertEquals(2.5, StatUtils.percentile(x, 1, 3, 62.5), TOLERANCE);
     }
 
+    @Test
+    public void testPercentileDoesNotModifyInput() {
+        final double[] x = {THREE, ONE, TWO, TWO};
+        StatUtils.percentile(x, 50);
+        StatUtils.percentile(x, 1, 3, 50);
+        Assert.assertArrayEquals(new double[] {THREE, ONE, TWO, TWO}, x, 0);
+    }
+
     @Test
     public void testDifferenceStats() {
         double sample1[] = {1d, 2d, 3d, 4d};

commons-math-legacy/src/test/java/org/apache/commons/math4/legacy/stat/descriptive/DescriptiveStatisticsTest.java

@@ -125,6 +125,22 @@ public void testGetValues() {
         Assert.assertEquals(12.0, stats.getElement(88), 1.0e-10);
     }
 
+    @Test
+    public void testPercentileDoesNotReorderValues() {
+        DescriptiveStatistics stats = createDescriptiveStatistics();
+        stats.setWindowSize(5);
+        for (double v : new double[] {5, 4, 3, 2, 1}) {
+            stats.addValue(v);
+        }
+        Assert.assertEquals(3, stats.getPercentile(50), 0);
+        // toString() reports the median
+        Assert.assertNotNull(stats.toString());
+        Assert.assertArrayEquals(new double[] {5, 4, 3, 2, 1}, stats.getValues(), 0);
+        // The oldest value is discarded
+        stats.addValue(100);
+        Assert.assertArrayEquals(new double[] {4, 3, 2, 1, 100}, stats.getValues(), 0);
+    }
+
     @Test
     public void testQuadraticMean() {
         final double[] values = { 1.2, 3.4, 5.6, 7.89 };