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.legacy.stat.descriptive.rank; 018 019import org.apache.commons.math4.legacy.core.MathArrays; 020 021/** 022 * Classic median of 3 strategy given begin and end indices. 023 * @since 3.4 024 */ 025public class MedianOf3PivotingStrategy implements PivotingStrategy { 026 /**{@inheritDoc} 027 * This in specific makes use of median of 3 pivoting. 028 * @return The index corresponding to a pivot chosen between the 029 * first, middle and the last indices of the array slice 030 * @throws org.apache.commons.math4.legacy.exception.MathIllegalArgumentException MathIllegalArgumentException when indices exceeds range 031 */ 032 @Override 033 public int pivotIndex(final double[] work, final int begin, final int end) { 034 MathArrays.verifyValues(work, begin, end - begin); 035 final int inclusiveEnd = end - 1; 036 final int middle = begin + (inclusiveEnd - begin) / 2; 037 final double wBegin = work[begin]; 038 final double wMiddle = work[middle]; 039 final double wEnd = work[inclusiveEnd]; 040 041 if (wBegin < wMiddle) { 042 if (wMiddle < wEnd) { 043 return middle; 044 } else { 045 return wBegin < wEnd ? inclusiveEnd : begin; 046 } 047 } else { 048 if (wBegin < wEnd) { 049 return begin; 050 } else { 051 return wMiddle < wEnd ? inclusiveEnd : middle; 052 } 053 } 054 } 055}