Arrays.java

/**
 * Copyright 2003-2016 SSHTOOLS Limited. All Rights Reserved.
 *
 * For product documentation visit https://www.sshtools.com/
 *
 * This file is part of J2SSH Maverick.
 *
 * J2SSH Maverick is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * J2SSH Maverick is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with J2SSH Maverick.  If not, see <http://www.gnu.org/licenses/>.
 */
package com.sshtools.util;

public class Arrays {

	/**
	 * Returns the index of the median of the three indexed integers.
	 */
	private static int med3(int x[], int a, int b, int c) {
		return (x[a] < x[b] ? (x[b] < x[c] ? b : x[a] < x[c] ? c : a)
				: (x[b] > x[c] ? b : x[a] > x[c] ? c : a));
	}

	/**
	 * Swaps x[a] with x[b].
	 */
	private static void swap(int x[], int a, int b) {
		int t = x[a];
		x[a] = x[b];
		x[b] = t;
	}

	/**
	 * Sorts the specified array of ints into ascending numerical order. The
	 * sorting algorithm is a tuned quicksort, adapted from Jon L. Bentley and
	 * M. Douglas McIlroy's "Engineering a Sort Function", Software-Practice and
	 * Experience, Vol. 23(11) P. 1249-1265 (November 1993). This algorithm
	 * offers n*log(n) performance on many data sets that cause other quicksorts
	 * to degrade to quadratic performance.
	 * 
	 * @param a
	 *            the array to be sorted
	 */
	public static void sort(int[] a) {
		sort1(a, 0, a.length);
	}

	private static void sort1(int x[], int off, int len) {
		// Insertion sort on smallest arrays
		if (len < 7) {
			for (int i = off; i < len + off; i++)
				for (int j = i; j > off && x[j - 1] > x[j]; j--)
					swap(x, j, j - 1);
			return;
		}

		// Choose a partition element, v
		int m = off + (len >> 1); // Small arrays, middle element
		if (len > 7) {
			int l = off;
			int n = off + len - 1;
			if (len > 40) { // Big arrays, pseudomedian of 9
				int s = len / 8;
				l = med3(x, l, l + s, l + 2 * s);
				m = med3(x, m - s, m, m + s);
				n = med3(x, n - 2 * s, n - s, n);
			}
			m = med3(x, l, m, n); // Mid-size, med of 3
		}
		int v = x[m];

		// Establish Invariant: v* (<v)* (>v)* v*
		int a = off, b = a, c = off + len - 1, d = c;
		while (true) {
			while (b <= c && x[b] <= v) {
				if (x[b] == v)
					swap(x, a++, b);
				b++;
			}
			while (c >= b && x[c] >= v) {
				if (x[c] == v)
					swap(x, c, d--);
				c--;
			}
			if (b > c)
				break;
			swap(x, b++, c--);
		}
	}
}