/*
 *  RapidMiner
 *
 *  Copyright (C) 2001-2008 by Rapid-I and the contributors
 *
 *  Complete list of developers available at our web site:
 *
 *       http://rapid-i.com
 *
 *  This program is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU Affero 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 Affero General Public License for more details.
 *
 *  You should have received a copy of the GNU Affero General Public License
 *  along with this program.  If not, see http://www.gnu.org/licenses/.
 */
package com.rapidminer.tools.math.sampling;

import com.rapidminer.tools.RandomGenerator;


/**
 * Implements iterative, linear sampling without replacement. The
 * size of the population and the size or fraction of the sample
 * have to be given as parameters.
 * 
 * The algorithm is based upon A.F. Bissell (1986): Ordered Random
 * Selection Without Replacement. In: Applied Statistics, 35 (1),
 * pp. 73-75.
 * 
 * @author Tobias Malbrecht
 * @version $Id: OrderedSamplingWithoutReplacement.java,v 1.4 2008/07/15 15:07:24 ingomierswa Exp $
 */
public class OrderedSamplingWithoutReplacement {

	/**
	 * The random generator which delivers random numbers
	 * between 0 and 1.
	 */ 
	private RandomGenerator randomGenerator;
	
	/**
	 * Number of those population elements which are still
	 * under consideration and from which the elements are
	 * drawn.
	 */
	private int populationCounter;

	/**
	 * Number of elements in the elements under consideration
	 * which will be not included in the sample.
	 */
	private int notRequiredElementsCounter;

	/**
	 * Probability that no element of the first values are
	 * drawn from the population.
	 */
	private double probability;
	
	/**
	 * A random value.
	 */
	private double randomValue;

	/**
	 * Constructor for an absolute number of elements.
	 * 
	 * @param randomGenerator A RandomGenerator.
	 * @param populationSize The size of the population.
	 * @param sampleSize The size of the sample.
	 */
	public OrderedSamplingWithoutReplacement(RandomGenerator randomGenerator, int populationSize, int sampleSize) {
		this.randomGenerator = randomGenerator;
		this.populationCounter = populationSize;
		this.notRequiredElementsCounter = populationSize - sampleSize;
		this.probability = 1;
		this.randomValue = 1 - randomGenerator.nextDouble();
	}
	
	/**
	 * Constructor for a relative fraction of elements.
	 * 
	 * @param randomGenerator A RandomGenerator.
	 * @param populationSize The size of the sample relative to the population size.
	 * @param sampleRatio The ratio of the sample.
	 */
	public OrderedSamplingWithoutReplacement(RandomGenerator randomGenerator, int populationSize, double sampleRatio) {
		this(randomGenerator, populationSize, (int) Math.round(populationSize * sampleRatio));
	}

	public static int[] getSampledIndices(RandomGenerator randomGenerator, int populationSize, int sampleSize) {
		OrderedSamplingWithoutReplacement sampling = new OrderedSamplingWithoutReplacement(randomGenerator, populationSize, sampleSize);
		int[] result = new int[sampleSize];
		int rCounter = 0;
		for (int i = 0; i < populationSize; i++) {
			if (sampling.acceptElement()) {
				result[rCounter++] = i;
			}
		}
		return result;
	}
	
	/**
	 * Include element in the sample.
	 * 
	 * @return flag whether to include an element in the sample
	 */
	public boolean acceptElement() {
		probability *= ((double) notRequiredElementsCounter) / ((double) populationCounter);
		if (probability > randomValue) {
			populationCounter--;
			notRequiredElementsCounter--;
			return false;
		} else {
			populationCounter--;
			probability = 1;
			randomValue = 1 - randomGenerator.nextDouble();
			return true;
		}
	}
}