101 lines
3.1 KiB
Java
101 lines
3.1 KiB
Java
/*******************************************************************************
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* Copyright 2011 LibGDX.
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* Mario Zechner <badlogicgames@gmail.com>
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* Nathan Sweet <nathan.sweet@gmail.com>
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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******************************************************************************/
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package dorkbox.collections;
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import java.util.Comparator;
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/** Implementation of Tony Hoare's quickselect algorithm. Running time is generally O(n), but worst case is O(n^2) Pivot choice is
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* median of three method, providing better performance than a random pivot for partially sorted data.
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* http://en.wikipedia.org/wiki/Quickselect
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* @author Jon Renner */
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public class QuickSelect<T> {
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private T[] array;
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private Comparator<? super T> comp;
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public int select (T[] items, Comparator<T> comp, int n, int size) {
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this.array = items;
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this.comp = comp;
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return recursiveSelect(0, size - 1, n);
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}
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private int partition (int left, int right, int pivot) {
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T pivotValue = array[pivot];
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swap(right, pivot);
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int storage = left;
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for (int i = left; i < right; i++) {
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if (comp.compare(array[i], pivotValue) < 0) {
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swap(storage, i);
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storage++;
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}
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}
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swap(right, storage);
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return storage;
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}
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private int recursiveSelect (int left, int right, int k) {
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if (left == right) return left;
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int pivotIndex = medianOfThreePivot(left, right);
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int pivotNewIndex = partition(left, right, pivotIndex);
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int pivotDist = (pivotNewIndex - left) + 1;
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int result;
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if (pivotDist == k) {
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result = pivotNewIndex;
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} else if (k < pivotDist) {
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result = recursiveSelect(left, pivotNewIndex - 1, k);
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} else {
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result = recursiveSelect(pivotNewIndex + 1, right, k - pivotDist);
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}
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return result;
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}
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/** Median of Three has the potential to outperform a random pivot, especially for partially sorted arrays */
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private int medianOfThreePivot (int leftIdx, int rightIdx) {
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T left = array[leftIdx];
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int midIdx = (leftIdx + rightIdx) / 2;
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T mid = array[midIdx];
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T right = array[rightIdx];
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// spaghetti median of three algorithm
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// does at most 3 comparisons
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if (comp.compare(left, mid) > 0) {
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if (comp.compare(mid, right) > 0) {
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return midIdx;
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} else if (comp.compare(left, right) > 0) {
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return rightIdx;
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} else {
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return leftIdx;
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}
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} else {
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if (comp.compare(left, right) > 0) {
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return leftIdx;
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} else if (comp.compare(mid, right) > 0) {
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return rightIdx;
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} else {
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return midIdx;
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}
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}
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}
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private void swap (int left, int right) {
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T tmp = array[left];
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array[left] = array[right];
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array[right] = tmp;
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}
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}
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