    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}