001package org.cpsolver.ifs.heuristics;
002
003import java.util.ArrayList;
004import java.util.Collection;
005import java.util.HashMap;
006import java.util.Iterator;
007import java.util.List;
008import java.util.Map;
009import java.util.Set;
010import java.util.concurrent.locks.Lock;
011
012
013import org.apache.log4j.Logger;
014import org.cpsolver.ifs.assignment.Assignment;
015import org.cpsolver.ifs.assignment.context.AssignmentContext;
016import org.cpsolver.ifs.constant.ConstantVariable;
017import org.cpsolver.ifs.extension.ConflictStatistics;
018import org.cpsolver.ifs.extension.Extension;
019import org.cpsolver.ifs.model.Model;
020import org.cpsolver.ifs.model.Neighbour;
021import org.cpsolver.ifs.model.Value;
022import org.cpsolver.ifs.model.Variable;
023import org.cpsolver.ifs.solution.Solution;
024import org.cpsolver.ifs.solver.Solver;
025import org.cpsolver.ifs.util.DataProperties;
026import org.cpsolver.ifs.util.JProf;
027
028/**
029 * Backtracking-based neighbour selection. A best neighbour that is found by a
030 * backtracking-based algorithm within a limited depth from a selected variable
031 * is returned. <br>
032 * <br>
033 * Parameters: <br>
034 * <table border='1' summary='Related Solver Parameters'>
035 * <tr>
036 * <th>Parameter</th>
037 * <th>Type</th>
038 * <th>Comment</th>
039 * </tr>
040 * <tr>
041 * <td>Neighbour.BackTrackTimeout</td>
042 * <td>{@link Integer}</td>
043 * <td>Timeout for each neighbour selection (in milliseconds).</td>
044 * </tr>
045 * <tr>
046 * <td>Neighbour.BackTrackDepth</td>
047 * <td>{@link Integer}</td>
048 * <td>Limit of search depth.</td>
049 * </tr>
050 * </table>
051 * 
052 * @version StudentSct 1.3 (Student Sectioning)<br>
053 *          Copyright (C) 2007 - 2014 Tomas Muller<br>
054 *          <a href="mailto:muller@unitime.org">muller@unitime.org</a><br>
055 *          <a href="http://muller.unitime.org">http://muller.unitime.org</a><br>
056 * <br>
057 *          This library is free software; you can redistribute it and/or modify
058 *          it under the terms of the GNU Lesser General Public License as
059 *          published by the Free Software Foundation; either version 3 of the
060 *          License, or (at your option) any later version. <br>
061 * <br>
062 *          This library is distributed in the hope that it will be useful, but
063 *          WITHOUT ANY WARRANTY; without even the implied warranty of
064 *          MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
065 *          Lesser General Public License for more details. <br>
066 * <br>
067 *          You should have received a copy of the GNU Lesser General Public
068 *          License along with this library; if not see
069 *          <a href='http://www.gnu.org/licenses/'>http://www.gnu.org/licenses/</a>.
070 * 
071 * @param <V> Variable 
072 * @param <T> Value
073 */
074public class BacktrackNeighbourSelection<V extends Variable<V, T>, T extends Value<V, T>> extends StandardNeighbourSelection<V, T> {
075    private ConflictStatistics<V, T> iStat = null;
076    private static Logger sLog = Logger.getLogger(BacktrackNeighbourSelection.class);
077    private int iTimeout = 5000;
078    private int iDepth = 4;
079    private int iMaxIters = -1;
080
081    /**
082     * Constructor
083     * 
084     * @param properties
085     *            configuration
086     * @throws Exception thrown when initialization fails
087     */
088    public BacktrackNeighbourSelection(DataProperties properties) throws Exception {
089        super(properties);
090        iTimeout = properties.getPropertyInt("Neighbour.BackTrackTimeout", iTimeout);
091        iDepth = properties.getPropertyInt("Neighbour.BackTrackDepth", iDepth);
092        iMaxIters = properties.getPropertyInt("Neighbour.BackTrackMaxIters", iMaxIters);
093    }
094
095    /** Solver initialization */
096    @Override
097    public void init(Solver<V, T> solver) {
098        super.init(solver);
099        for (Extension<V, T> extension : solver.getExtensions()) {
100            if (ConflictStatistics.class.isInstance(extension))
101                iStat = (ConflictStatistics<V, T>) extension;
102        }
103    }
104
105    /**
106     * Select neighbour. The standard variable selection (see
107     * {@link StandardNeighbourSelection#getVariableSelection()}) is used to
108     * select a variable. A backtracking of a limited depth is than employed
109     * from this variable. The best assignment found is returned (see
110     * {@link BackTrackNeighbour}).
111     **/
112    @Override
113    public Neighbour<V, T> selectNeighbour(Solution<V, T> solution) {
114        return selectNeighbour(solution, getVariableSelection().selectVariable(solution));
115    }
116
117    /**
118     * Select neighbour -- starts from the provided variable. A backtracking of
119     * a limited depth is employed from the given variable. The best assignment
120     * found is returned (see {@link BackTrackNeighbour}).
121     * @param solution current solution
122     * @param variable selected variable
123     * @return a neighbour, null if not found
124     **/
125    public Neighbour<V, T> selectNeighbour(Solution<V, T> solution, V variable) {
126        if (variable == null)
127            return null;
128
129        BacktrackNeighbourSelectionContext context = new BacktrackNeighbourSelectionContext(solution);
130        
131        Lock lock = solution.getLock().writeLock();
132        lock.lock();
133        try {
134            if (sLog.isDebugEnabled())
135                sLog.debug("-- before BT (" + variable.getName() + "): nrAssigned=" + solution.getAssignment().nrAssignedVariables() + ",  value=" + solution.getModel().getTotalValue(solution.getAssignment()));
136
137            List<V> variables2resolve = new ArrayList<V>(1);
138            variables2resolve.add(variable);
139            backtrack(context, variables2resolve, 0, iDepth);
140
141            if (sLog.isDebugEnabled())
142                sLog.debug("-- after  BT (" + variable.getName() + "): nrAssigned=" + solution.getAssignment().nrAssignedVariables() + ",  value=" + solution.getModel().getTotalValue(solution.getAssignment()));
143        } finally {
144            lock.unlock();
145        }
146
147        if (sLog.isDebugEnabled())
148            sLog.debug("-- selected neighbour: " + context.getBackTrackNeighbour());
149        return context.getBackTrackNeighbour();
150    }
151
152    private boolean containsConstantValues(Collection<T> values) {
153        for (T value : values) {
154            if (value.variable() instanceof ConstantVariable && ((ConstantVariable<?>) value.variable()).isConstant())
155                return true;
156        }
157        return false;
158    }
159
160    /** List of values of the given variable that will be considered 
161     * @param context assignment context
162     * @param variable given variable
163     * @return values of the given variable that will be considered
164     **/
165    protected Iterator<T> values(BacktrackNeighbourSelectionContext context, V variable) {
166        return variable.values(context.getAssignment()).iterator();
167    }
168
169    /** Check bound 
170     * @param variables2resolve unassigned variables that are in conflict with the current solution
171     * @param idx position in variables2resolve
172     * @param depth current depth
173     * @param value value to check
174     * @param conflicts conflicting values
175     * @return bound (best possible improvement)
176     **/
177    protected boolean checkBound(List<V> variables2resolve, int idx, int depth, T value, Set<T> conflicts) {
178        int nrUnassigned = variables2resolve.size() - idx;
179        if ((nrUnassigned + conflicts.size() > depth)) {
180            if (sLog.isDebugEnabled())
181                sLog.debug("        -- too deap");
182            return false;
183        }
184        if (containsConstantValues(conflicts)) {
185            if (sLog.isDebugEnabled())
186                sLog.debug("        -- contains constants values");
187            return false;
188        }
189        boolean containAssigned = false;
190        for (Iterator<T> i = conflicts.iterator(); !containAssigned && i.hasNext();) {
191            T conflict = i.next();
192            int confIdx = variables2resolve.indexOf(conflict.variable());
193            if (confIdx >= 0 && confIdx <= idx) {
194                if (sLog.isDebugEnabled())
195                    sLog.debug("        -- contains resolved variable " + conflict.variable());
196                containAssigned = true;
197            }
198        }
199        if (containAssigned)
200            return false;
201        return true;
202    }
203
204    /** Check whether backtrack can continue 
205     * @param context assignment context
206     * @param variables2resolve unassigned variables that are in conflict with the current solution
207     * @param idx position in variables2resolve
208     * @param depth current depth
209     * @return true if the search can continue
210     **/
211    protected boolean canContinue(BacktrackNeighbourSelectionContext context, List<V> variables2resolve, int idx, int depth) {
212        if (depth <= 0) {
213            if (sLog.isDebugEnabled())
214                sLog.debug("    -- depth reached");
215            return false;
216        }
217        if (context.isTimeoutReached()) {
218            if (sLog.isDebugEnabled())
219                sLog.debug("    -- timeout reached");
220            return false;
221        }
222        if (context.isMaxItersReached()) {
223            if (sLog.isDebugEnabled())
224                sLog.debug("    -- max number of iterations reached");
225            return false;
226        }
227        return true;
228    }
229
230    protected boolean canContinueEvaluation(BacktrackNeighbourSelectionContext context) {
231        return !context.isMaxItersReached() && !context.isTimeoutReached();
232    }
233
234    /** Backtracking 
235     * @param context assignment context
236     * @param variables2resolve unassigned variables that are in conflict with the current solution
237     * @param idx position in variables2resolve
238     * @param depth current depth
239     **/
240    protected void backtrack(BacktrackNeighbourSelectionContext context, List<V> variables2resolve, int idx, int depth) {
241        if (sLog.isDebugEnabled())
242            sLog.debug("  -- bt[" + depth + "]: " + idx + " of " + variables2resolve.size() + " " + variables2resolve);
243        context.incIteration();
244        int nrUnassigned = variables2resolve.size() - idx;
245        if (nrUnassigned == 0) {
246            context.saveBest(variables2resolve);
247            return;
248        }
249        if (!canContinue(context, variables2resolve, idx, depth))
250            return;
251        V variable = variables2resolve.get(idx);
252        if (sLog.isDebugEnabled())
253            sLog.debug("    -- variable " + variable);
254        for (Iterator<T> e = values(context, variable); canContinueEvaluation(context) && e.hasNext();) {
255            T value = e.next();
256            T current = context.getAssignment().getValue(variable);
257            if (value.equals(current))
258                continue;
259            if (sLog.isDebugEnabled())
260                sLog.debug("      -- value " + value);
261            Set<T> conflicts = context.getModel().conflictValues(context.getAssignment(), value);
262            if (sLog.isDebugEnabled())
263                sLog.debug("      -- conflicts " + conflicts);
264            if (!checkBound(variables2resolve, idx, depth, value, conflicts))
265                continue;
266            List<V> newVariables2resolve = new ArrayList<V>(variables2resolve);
267            for (Iterator<T> i = conflicts.iterator(); i.hasNext();) {
268                T conflict = i.next();
269                context.getAssignment().unassign(0, conflict.variable());
270                if (!newVariables2resolve.contains(conflict.variable()))
271                    newVariables2resolve.add(conflict.variable());
272            }
273            if (current != null)
274                context.getAssignment().unassign(0, current.variable());
275            context.getAssignment().assign(0, value);
276            backtrack(context, newVariables2resolve, idx + 1, depth - 1);
277            if (current == null)
278                context.getAssignment().unassign(0, variable);
279            else
280                context.getAssignment().assign(0, current);
281            for (Iterator<T> i = conflicts.iterator(); i.hasNext();) {
282                T conflict = i.next();
283                context.getAssignment().assign(0, conflict); 
284            }
285        }
286    }
287
288    /** Backtracking neighbour */
289    public class BackTrackNeighbour implements Neighbour<V, T> {
290        private double iTotalValue = 0;
291        private double iValue = 0;
292        private List<T> iDifferentAssignments = null;
293        private Model<V, T> iModel = null;
294
295        /**
296         * Constructor
297         * 
298         * @param context assignment context
299         * @param resolvedVariables
300         *            variables that has been changed
301         */
302        public BackTrackNeighbour(BacktrackNeighbourSelectionContext context, List<V> resolvedVariables) {
303            iTotalValue = context.getModel().getTotalValue(context.getAssignment());
304            iValue = 0;
305            iDifferentAssignments = new ArrayList<T>();
306            for (V variable : resolvedVariables) {
307                T value = variable.getAssignment(context.getAssignment());
308                iDifferentAssignments.add(value);
309                iValue += value.toDouble(context.getAssignment());
310            }
311            if (sLog.isDebugEnabled())
312                iModel = context.getModel();
313        }
314
315        /** Neighbour value (solution total value if the neighbour is applied). 
316         * @return value of the new solution
317         **/
318        public double getTotalValue() {
319            return iTotalValue;
320        }
321
322        /**
323         * Sum of values of variables from the neighbour that change their
324         * values
325         */
326        @Override
327        public double value(Assignment<V, T> assignment) {
328            return iValue;
329        }
330
331        /** Neighbour assignments 
332         * @return list of assignments in this neighbour
333         **/
334        public List<T> getAssignments() {
335            return iDifferentAssignments;
336        }
337
338        /**
339         * Assign the neighbour
340         */
341        @Override
342        public void assign(Assignment<V, T> assignment, long iteration) {
343            if (sLog.isDebugEnabled())
344                sLog.debug("-- before assignment: nrAssigned=" + assignment.nrAssignedVariables() + ",  value=" + iModel.getTotalValue(assignment));
345            if (sLog.isDebugEnabled())
346                sLog.debug("  " + this);
347            int idx = 0;
348            for (Iterator<T> e = iDifferentAssignments.iterator(); e.hasNext(); idx++) {
349                T p = e.next();
350                T o = assignment.getValue(p.variable());
351                if (o != null) {
352                    if (idx > 0 && iStat != null)
353                        iStat.variableUnassigned(iteration, o, iDifferentAssignments.get(0));
354                    assignment.unassign(iteration, p.variable());
355                }
356            }
357            for (T p : iDifferentAssignments) {
358                assignment.assign(iteration, p);
359            }
360            if (sLog.isDebugEnabled())
361                sLog.debug("-- after assignment: nrAssigned=" + assignment.nrAssignedVariables() + ",  value=" + iModel.getTotalValue(assignment));
362        }
363
364        /**
365         * Compare two neighbours
366         * @param solution current solution
367         * @return comparison
368         */
369        public int compareTo(Solution<V, T> solution) {
370            return Double.compare(iTotalValue, solution.getModel().getTotalValue(solution.getAssignment()));
371        }
372
373        @Override
374        public String toString() {
375            StringBuffer sb = new StringBuffer("BT{value=" + iTotalValue + ": ");
376            for (Iterator<T> e = iDifferentAssignments.iterator(); e.hasNext();) {
377                T p = e.next();
378                sb.append("\n    " + p.variable().getName() + " " + p.getName() + (e.hasNext() ? "," : ""));
379            }
380            sb.append("}");
381            return sb.toString();
382        }
383
384        @Override
385        public Map<V, T> assignments() {
386            Map<V, T> ret = new HashMap<V, T>();
387            for (T p : iDifferentAssignments)
388                ret.put(p.variable(), p);
389            return ret;
390        }
391    }
392
393    /** Return maximal depth 
394     * @return maximal search depth
395     **/
396    public int getDepth() {
397        return iDepth;
398    }
399
400    /** Set maximal depth 
401     * @param depth maximal search depth
402     **/
403    public void setDepth(int depth) {
404        iDepth = depth;
405    }
406
407    /** Return time limit 
408     * @return time limit 
409     **/
410    public int getTimeout() {
411        return iTimeout;
412    }
413
414    /** Set time limit 
415     * @param timeout time limit
416     **/
417    public void setTimeout(int timeout) {
418        iTimeout = timeout;
419    }
420
421    /** Return maximal number of iterations 
422     * @return maximal number of iterations
423     **/
424    public int getMaxIters() {
425        return iMaxIters;
426    }
427
428    /** Set maximal number of iterations 
429     * @param maxIters maximal number of iterations
430     **/
431    public void setMaxIters(int maxIters) {
432        iMaxIters = maxIters;
433    }
434    
435    public class BacktrackNeighbourSelectionContext implements AssignmentContext {
436        private long iT0, iT1;
437        private boolean iTimeoutReached = false;
438        private int iMaxIters = -1, iNrIters = 0;
439        protected Solution<V, T> iSolution = null;
440        protected BackTrackNeighbour iBackTrackNeighbour = null;
441        protected double iValue = 0;
442        private int iNrAssigned = 0;
443        private boolean iMaxItersReached = false;
444        
445        public BacktrackNeighbourSelectionContext(Solution<V, T> solution) {
446            iSolution = solution;
447            iBackTrackNeighbour = null;
448            iValue = solution.getModel().getTotalValue(iSolution.getAssignment());
449            iNrAssigned = iSolution.getAssignment().nrAssignedVariables();
450            iT0 = JProf.currentTimeMillis();
451            iNrIters = 0;
452            iTimeoutReached = false;
453            iMaxItersReached = false;
454        }
455
456        /** Time needed to find a neighbour (last call of selectNeighbour method) 
457         * @return search time
458         **/
459        public long getTime() {
460            return iT1 - iT0;
461        }
462
463        /**
464         * True, if timeout was reached during the last call of selectNeighbour
465         * method
466         * @return true if the timeout was reached
467         */
468        public boolean isTimeoutReached() {
469            return iTimeoutReached;
470        }
471
472        /**
473         * True, if the maximum number of iterations was reached by the last call of
474         * selectNeighbour method
475         * @return true if the maximum number of iterations was reached
476         */
477        public boolean isMaxItersReached() {
478            return iMaxItersReached;
479        }
480        
481        public BackTrackNeighbour getBackTrackNeighbour() { return iBackTrackNeighbour; }
482        
483        public void incIteration() {
484            iT1 = JProf.currentTimeMillis();
485            if (!iTimeoutReached && iTimeout > 0 && iT1 - iT0 > iTimeout)
486                iTimeoutReached = true;
487            if (!iMaxItersReached && iMaxIters > 0 && iNrIters++ > iMaxIters)
488                iMaxItersReached = true;
489        }
490        
491        public void saveBest(List<V> variables2resolve) {
492            if (sLog.isDebugEnabled())
493                sLog.debug("    -- all assigned");
494            if (iSolution.getAssignment().nrAssignedVariables() > iNrAssigned || (iSolution.getAssignment().nrAssignedVariables() == iNrAssigned && iValue > iSolution.getModel().getTotalValue(iSolution.getAssignment()))) {
495                if (sLog.isDebugEnabled())
496                    sLog.debug("    -- better than current");
497                if (iBackTrackNeighbour == null || iBackTrackNeighbour.compareTo(iSolution) >= 0) {
498                    if (sLog.isDebugEnabled())
499                        sLog.debug("      -- better than best");
500                    iBackTrackNeighbour = new BackTrackNeighbour(this, variables2resolve);
501                }
502            }
503        }
504        
505        public Model<V, T> getModel() { return iSolution.getModel();}
506        
507        public Assignment<V, T> getAssignment() { return iSolution.getAssignment(); }
508    }
509}