001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *      http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.commons.math4.util;
018
019import java.io.Serializable;
020
021import org.apache.commons.math4.exception.MathIllegalArgumentException;
022
023
024/**
025 * Classic median of 3 strategy given begin and end indices.
026 * @since 3.4
027 */
028public class MedianOf3PivotingStrategy implements PivotingStrategyInterface, Serializable {
029
030    /** Serializable UID. */
031    private static final long serialVersionUID = 20140713L;
032
033    /**{@inheritDoc}
034     * This in specific makes use of median of 3 pivoting.
035     * @return The index corresponding to a pivot chosen between the
036     * first, middle and the last indices of the array slice
037     * @throws MathIllegalArgumentException when indices exceeds range
038     */
039    @Override
040    public int pivotIndex(final double[] work, final int begin, final int end)
041        throws MathIllegalArgumentException {
042        MathArrays.verifyValues(work, begin, end-begin);
043        final int inclusiveEnd = end - 1;
044        final int middle = begin + (inclusiveEnd - begin) / 2;
045        final double wBegin = work[begin];
046        final double wMiddle = work[middle];
047        final double wEnd = work[inclusiveEnd];
048
049        if (wBegin < wMiddle) {
050            if (wMiddle < wEnd) {
051                return middle;
052            } else {
053                return wBegin < wEnd ? inclusiveEnd : begin;
054            }
055        } else {
056            if (wBegin < wEnd) {
057                return begin;
058            } else {
059                return wMiddle < wEnd ? inclusiveEnd : middle;
060            }
061        }
062    }
063
064}