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.math3.util; 018 019import java.io.Serializable; 020 021import org.apache.commons.math3.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 public int pivotIndex(final double[] work, final int begin, final int end) 040 throws MathIllegalArgumentException { 041 MathArrays.verifyValues(work, begin, end-begin); 042 final int inclusiveEnd = end - 1; 043 final int middle = begin + (inclusiveEnd - begin) / 2; 044 final double wBegin = work[begin]; 045 final double wMiddle = work[middle]; 046 final double wEnd = work[inclusiveEnd]; 047 048 if (wBegin < wMiddle) { 049 if (wMiddle < wEnd) { 050 return middle; 051 } else { 052 return wBegin < wEnd ? inclusiveEnd : begin; 053 } 054 } else { 055 if (wBegin < wEnd) { 056 return begin; 057 } else { 058 return wMiddle < wEnd ? inclusiveEnd : middle; 059 } 060 } 061 } 062 063}