001    package net.sf.cpsolver.ifs.model;
002    
003    import net.sf.cpsolver.ifs.model.Model;
004    import net.sf.cpsolver.ifs.model.Neighbour;
005    import net.sf.cpsolver.ifs.model.Value;
006    import net.sf.cpsolver.ifs.model.Variable;
007    
008    /**
009     * Lazy neigbour (a change of the overall solution value is unknown before
010     * the neighbour is assigned, it is possible to undo the neighbour instead). 
011     * This neighbour is useful when it is 
012     * two expensive to compute change of overall solution value before the 
013     * variable is reassigned. It is possible to undo the neighbour instead.
014     * Search strategy has to implement {@link LazyNeighbourAcceptanceCriterion}.
015     *  
016     * @version IFS 1.2 (Iterative Forward Search)<br>
017     *          Copyright (C) 2013 Tomas Muller<br>
018     *          <a href="mailto:muller@unitime.org">muller@unitime.org</a><br>
019     *          <a href="http://muller.unitime.org">http://muller.unitime.org</a><br>
020     * <br>
021     *          This library is free software; you can redistribute it and/or modify
022     *          it under the terms of the GNU Lesser General Public License as
023     *          published by the Free Software Foundation; either version 3 of the
024     *          License, or (at your option) any later version. <br>
025     * <br>
026     *          This library is distributed in the hope that it will be useful, but
027     *          WITHOUT ANY WARRANTY; without even the implied warranty of
028     *          MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
029     *          Lesser General Public License for more details. <br>
030     * <br>
031     *          You should have received a copy of the GNU Lesser General Public
032     *          License along with this library; if not see
033     *          <a href='http://www.gnu.org/licenses/'>http://www.gnu.org/licenses/</a>.
034     */
035    public abstract class LazyNeighbour<V extends Variable<V, T>, T extends Value<V, T>> extends Neighbour<V,T> {
036        private LazyNeighbourAcceptanceCriterion<V,T> iCriterion = null;
037        
038        /**
039         * Set acceptance criterion (to be used by a search strategy before the 
040         * neighbour is accepted, so that it can be undone if desired)  
041         */
042        public void setAcceptanceCriterion(LazyNeighbourAcceptanceCriterion<V,T> criterion) {
043            iCriterion = criterion;
044        }
045        
046        /**
047         * Assign neighbour, check given acceptance criterion, and undo
048         * assignment if the change is not accepted. 
049         */
050        @Override
051        public void assign(long iteration) {
052            double before = getModel().getTotalValue();
053            doAssign(iteration);
054            double after = getModel().getTotalValue();
055            if (!iCriterion.accept(this, after - before)) undoAssign(iteration);
056        }
057        /**
058         * Return -1 (neighbour is always accepted). The search strategy that
059         * is using this neighbour must implement {@link LazyNeighbourAcceptanceCriterion}.
060         */
061        @Override
062        public double value() { return -1; }
063        
064        /** Perform assignment */
065        protected abstract void doAssign(long iteration);
066        /** Undo assignment */
067        protected abstract void undoAssign(long iteration);
068        /** Return problem model (it is needed in order to be able to get
069         * overall solution value before and after the assignment of this neighbour) */
070        public abstract Model<V,T> getModel();
071        
072        /** Neighbour acceptance criterion interface (to be implemented
073         * by search strategies that are using {@link LazyNeighbour}. 
074         * It is also required to call {@link LazyNeighbour#setAcceptanceCriterion(LazyNeighbour.LazyNeighbourAcceptanceCriterion)}
075         * before the neighbour is accepted by the search strategy. 
076         */ 
077        public static interface LazyNeighbourAcceptanceCriterion<V extends Variable<V, T>, T extends Value<V, T>> {
078            /** True when the currently assigned neighbour should be accepted (false means
079             * that the change will be undone
080             * @param neighbour neighbour that was assigned
081             * @param value change in overall solution value
082             * @return true if the neighbour can be accepted (false to undo the assignment)
083             */
084            public boolean accept(LazyNeighbour<V,T> neighbour, double value);
085        }
086    }