/*******************************************************************************
* Copyright 2011 See AUTHORS file.
*
* Licensed 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 com.badlogic.gdx.utils;
import java.util.Comparator;
/** Implementation of Tony Hoare's quickselect algorithm. Running time is generally O(n), but worst case is O(n^2) Pivot choice is
* median of three method, providing better performance than a random pivot for partially sorted data.
* http://en.wikipedia.org/wiki/Quickselect
* @author Jon Renner */
public class QuickSelect<T> {
private T[] array;
private Comparator<? super T> comp;
public int select (T[] items, Comparator<T> comp, int n, int size) {
this.array = items;
this.comp = comp;
return recursiveSelect(0, size - 1, n);
}
private int partition (int left, int right, int pivot) {
T pivotValue = array[pivot];
swap(right, pivot);
int storage = left;
for (int i = left; i < right; i++) {
if (comp.compare(array[i], pivotValue) < 0) {
swap(storage, i);
storage++;
}
}
swap(right, storage);
return storage;
}
private int recursiveSelect (int left, int right, int k) {
if (left == right) return left;
int pivotIndex = medianOfThreePivot(left, right);
int pivotNewIndex = partition(left, right, pivotIndex);
int pivotDist = (pivotNewIndex - left) + 1;
int result;
if (pivotDist == k) {
result = pivotNewIndex;
} else if (k < pivotDist) {
result = recursiveSelect(left, pivotNewIndex - 1, k);
} else {
result = recursiveSelect(pivotNewIndex + 1, right, k - pivotDist);
}
return result;
}
/** Median of Three has the potential to outperform a random pivot, especially for partially sorted arrays */
private int medianOfThreePivot (int leftIdx, int rightIdx) {
T left = array[leftIdx];
int midIdx = (leftIdx + rightIdx) / 2;
T mid = array[midIdx];
T right = array[rightIdx];
// spaghetti median of three algorithm
// does at most 3 comparisons
if (comp.compare(left, mid) > 0) {
if (comp.compare(mid, right) > 0) {
return midIdx;
} else if (comp.compare(left, right) > 0) {
return rightIdx;
} else {
return leftIdx;
}
} else {
if (comp.compare(left, right) > 0) {
return leftIdx;
} else if (comp.compare(mid, right) > 0) {
return rightIdx;
} else {
return midIdx;
}
}
}
private void swap (int left, int right) {
T tmp = array[left];
array[left] = array[right];
array[right] = tmp;
}
}