org.hipparchus.util.PivotingStrategy Maven / Gradle / Ivy
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* 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.hipparchus.util;
import org.hipparchus.exception.MathIllegalArgumentException;
/**
* A strategy to pick a pivoting index of an array for doing partitioning.
*/
public enum PivotingStrategy {
/**
* A mid point strategy based on the average of begin and end indices.
*/
CENTRAL {
/**
* {@inheritDoc}
* This in particular picks a average of begin and end indices
* @return The index corresponding to a simple average of
* the first and the last element indices of the array slice
* @throws MathIllegalArgumentException when indices exceeds range
*/
@Override
public int pivotIndex(final double[] work, final int begin, final int end)
throws MathIllegalArgumentException {
MathArrays.verifyValues(work, begin, end-begin);
return begin + (end - begin)/2;
}
},
/**
* Classic median of 3 strategy given begin and end indices.
*/
MEDIAN_OF_3 {
/**
* {@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
*/
@Override
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;
}
}
}
};
/**
* Find pivot index of the array so that partition and Kth
* element selection can be made
* @param work data array
* @param begin index of the first element of the slice
* @param end index after the last element of the slice
* @return the index of the pivot element chosen between the
* first and the last element of the array slice
* @throws MathIllegalArgumentException when indices exceeds range
*/
public abstract int pivotIndex(double[] work, int begin, int end)
throws MathIllegalArgumentException;
}