001package net.sf.cpsolver.studentsct.filter;
002
003import java.util.HashSet;
004
005import net.sf.cpsolver.ifs.util.ToolBox;
006import net.sf.cpsolver.studentsct.model.Student;
007
008/**
009 * This student filter accepts every student with the given probability. The
010 * choice for each student is remembered, i.e., if the student is passed to the
011 * filter multiple times the same answer is returned.
012 * 
013 * @version StudentSct 1.2 (Student Sectioning)<br>
014 *          Copyright (C) 2007 - 2010 Tomáš Müller<br>
015 *          <a href="mailto:muller@unitime.org">muller@unitime.org</a><br>
016 *          <a href="http://muller.unitime.org">http://muller.unitime.org</a><br>
017 * <br>
018 *          This library is free software; you can redistribute it and/or modify
019 *          it under the terms of the GNU Lesser General Public License as
020 *          published by the Free Software Foundation; either version 3 of the
021 *          License, or (at your option) any later version. <br>
022 * <br>
023 *          This library is distributed in the hope that it will be useful, but
024 *          WITHOUT ANY WARRANTY; without even the implied warranty of
025 *          MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
026 *          Lesser General Public License for more details. <br>
027 * <br>
028 *          You should have received a copy of the GNU Lesser General Public
029 *          License along with this library; if not see
030 *          <a href='http://www.gnu.org/licenses/'>http://www.gnu.org/licenses/</a>.
031 */
032public class RandomStudentFilter implements StudentFilter {
033    private double iProb = 1.0;
034    private HashSet<Long> iAcceptedStudentIds = new HashSet<Long>();
035    private HashSet<Long> iRejectedStudentIds = new HashSet<Long>();
036
037    /**
038     * Constructor
039     * 
040     * @param prob
041     *            probability of acceptance of a student
042     */
043    public RandomStudentFilter(double prob) {
044        iProb = prob;
045    }
046
047    /**
048     * A student is accepted with the given probability
049     */
050    @Override
051    public boolean accept(Student student) {
052        Long studentId = new Long(student.getId());
053        if (iAcceptedStudentIds.contains(studentId))
054            return true;
055        if (iRejectedStudentIds.contains(studentId))
056            return false;
057        boolean accept = (Math.random() < iProb);
058        if (accept)
059            iAcceptedStudentIds.add(studentId);
060        else
061            iRejectedStudentIds.add(studentId);
062        return accept;
063    }
064
065    /**
066     * Set acceptance probability. Update the sets of accepted and rejected
067     * students accordingly.
068     * 
069     * @param prob
070     *            new acceptance probability
071     */
072    public void setProbability(double prob) {
073        iProb = prob;
074        int accept = (int) Math.round(prob * (iAcceptedStudentIds.size() + iRejectedStudentIds.size()));
075        while (iAcceptedStudentIds.size() < accept && !iRejectedStudentIds.isEmpty()) {
076            Long studentId = ToolBox.random(iRejectedStudentIds);
077            iRejectedStudentIds.remove(studentId);
078            iAcceptedStudentIds.add(studentId);
079        }
080        while (iAcceptedStudentIds.size() > accept && !iAcceptedStudentIds.isEmpty()) {
081            Long studentId = ToolBox.random(iAcceptedStudentIds);
082            iRejectedStudentIds.add(studentId);
083            iAcceptedStudentIds.remove(studentId);
084        }
085    }
086
087}