Repository /Debellor/debellor-1.0.jar:org.debellor.core.util.Permute

/*
 *  Debellor
 *
 *  Copyright (C) 2008-2009 by Marcin Wojnarski
 *
 *  This program is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  This program 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 this program.  If not, see http://www.gnu.org/licenses/.
 */
package org.debellor.core.util;

import java.util.Arrays;
import java.util.Random;

/**
 * @author Marcin Wojnarski
 *
 */
public class Permute {

	/**
	 * Generates random combination of integers from [0,<code>range</code>), 
	 * of length <code>length</code>, without repetitions.
	 * @param range		Values will be picked randomly from [0,range) interval
	 * @param length	Number of returned values. Must be <= <code>range</code>
	 * @param random	Random number generator to be used
	 * @return	Random combination of integers from [0,<code>range</code>), 
	 * of length <code>length</code>, without repetitions. 
	 */
	public static int[] indices(int range, int length, Random random) {
		if((length > range) || (length < 0))
			return null;
		
		int[] a = new int[range];
		for(int i = 0; i < range; i++)
			a[i] = i;

		// after this loop, a[0..(length-1)] will contain the desired combination
		for(int i = 0; i < length; i++) {
			int j = random.nextInt(range - i) + i;
			if(i != j) {
				int tmp = a[i];
				a[i] = a[j];
				a[j] = tmp;
			}
		}
		
		if(length == range)
			return a;
		
		return Arrays.copyOf(a, length);
	}

	public static int[] indices(int range, Random random) {
		return indices(range, range, random);
	}	

	public static int[] indices(int range) {
		return indices(range, new Random());
	}	
}