1 files changed, 63 insertions, 0 deletions
diff --git a/src/main/java/org/apache/commons/math3/util/MedianOf3PivotingStrategy.java b/src/main/java/org/apache/commons/math3/util/MedianOf3PivotingStrategy.java
new file mode 100644
index 0000000..205f114
--- /dev/null
+++ b/src/main/java/org/apache/commons/math3/util/MedianOf3PivotingStrategy.java
@@ -0,0 +1,63 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.commons.math3.util;
+
+import org.apache.commons.math3.exception.MathIllegalArgumentException;
+
+import java.io.Serializable;
+
+/**
+ * Classic median of 3 strategy given begin and end indices.
+ *
+ * @since 3.4
+ */
+public class MedianOf3PivotingStrategy implements PivotingStrategyInterface, Serializable {
+
+    /** Serializable UID. */
+    private static final long serialVersionUID = 20140713L;
+
+    /**
+     * {@inheritDoc} This in specific makes use of median of 3 pivoting.
+     *
+     * @return The index corresponding to a pivot chosen between the first, middle and the last
+     *     indices of the array slice
+     * @throws MathIllegalArgumentException when indices exceeds range
+     */
+    public int pivotIndex(final double[] work, final int begin, final int end)
+            throws MathIllegalArgumentException {
+        MathArrays.verifyValues(work, begin, end - begin);
+        final int inclusiveEnd = end - 1;
+        final int middle = begin + (inclusiveEnd - begin) / 2;
+        final double wBegin = work[begin];
+        final double wMiddle = work[middle];
+        final double wEnd = work[inclusiveEnd];
+
+        if (wBegin < wMiddle) {
+            if (wMiddle < wEnd) {
+                return middle;
+            } else {
+                return wBegin < wEnd ? inclusiveEnd : begin;
+            }
+        } else {
+            if (wBegin < wEnd) {
+                return begin;
+            } else {
+                return wMiddle < wEnd ? inclusiveEnd : middle;
+            }
+        }
+    }
+}