1 /*
2 * Licensed to the Apache Software Foundation (ASF) under one or more
3 * contributor license agreements. See the NOTICE file distributed with
4 * this work for additional information regarding copyright ownership.
5 * The ASF licenses this file to You under the Apache License, Version 2.0
6 * (the "License"); you may not use this file except in compliance with
7 * the License. You may obtain a copy of the License at
8 *
9 * http://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 */
17 package org.apache.commons.math4.legacy.stat.descriptive.rank;
18
19 import org.apache.commons.math4.legacy.core.MathArrays;
20
21 /**
22 * Classic median of 3 strategy given begin and end indices.
23 * @since 3.4
24 */
25 public class MedianOf3PivotingStrategy implements PivotingStrategy {
26 /**{@inheritDoc}
27 * This in specific makes use of median of 3 pivoting.
28 * @return The index corresponding to a pivot chosen between the
29 * first, middle and the last indices of the array slice
30 * @throws org.apache.commons.math4.legacy.exception.MathIllegalArgumentException MathIllegalArgumentException when indices exceeds range
31 */
32 @Override
33 public int pivotIndex(final double[] work, final int begin, final int end) {
34 MathArrays.verifyValues(work, begin, end - begin);
35 final int inclusiveEnd = end - 1;
36 final int middle = begin + (inclusiveEnd - begin) / 2;
37 final double wBegin = work[begin];
38 final double wMiddle = work[middle];
39 final double wEnd = work[inclusiveEnd];
40
41 if (wBegin < wMiddle) {
42 if (wMiddle < wEnd) {
43 return middle;
44 } else {
45 return wBegin < wEnd ? inclusiveEnd : begin;
46 }
47 } else {
48 if (wBegin < wEnd) {
49 return begin;
50 } else {
51 return wMiddle < wEnd ? inclusiveEnd : middle;
52 }
53 }
54 }
55 }