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Open-source constraint solver.
/*
* This file is part of choco-solver, http://choco-solver.org/
*
* Copyright (c) 2022, IMT Atlantique. All rights reserved.
*
* Licensed under the BSD 4-clause license.
*
* See LICENSE file in the project root for full license information.
*/
package org.chocosolver.solver.search.loop.move;
import org.chocosolver.solver.Solution;
import org.chocosolver.solver.Solver;
import org.chocosolver.solver.constraints.Constraint;
import org.chocosolver.solver.constraints.Propagator;
import org.chocosolver.solver.exception.ContradictionException;
import org.chocosolver.solver.learn.ExplanationForSignedClause;
import org.chocosolver.solver.search.limits.ICounter;
import org.chocosolver.solver.search.loop.lns.neighbors.INeighbor;
import org.chocosolver.solver.search.strategy.decision.RootDecision;
import org.chocosolver.solver.search.strategy.strategy.AbstractStrategy;
import org.chocosolver.solver.variables.IntVar;
import org.chocosolver.solver.variables.Variable;
import org.chocosolver.solver.variables.events.IntEventType;
import org.chocosolver.util.ESat;
import org.chocosolver.util.objects.setDataStructures.iterable.IntIterableRangeSet;
import org.chocosolver.util.objects.setDataStructures.iterable.IntIterableSetUtils;
import java.util.Collections;
import java.util.List;
/**
* This {@link Move}'s implementation defines a Large Neighborhood Search.
* Created by cprudhom on 03/09/15.
* Project: choco.
* @author Charles Prud'homme
* @since 03/09/2015
*/
public class MoveLNS implements Move {
/**
* the strategy required to complete the generated fragment
*/
protected Move move;
/**
* IntNeighbor to used
*/
protected INeighbor neighbor;
/**
* Number of solutions found so far
*/
protected long solutions;
/**
* Indicates if a solution has been loaded
*/
protected boolean solutionLoaded;
/**
* Indicate a restart has been triggered
*/
private boolean freshRestart;
/**
* Restart counter
*/
protected ICounter counter;
/**
* For restart strategy
*/
private final long frequency;
protected PropLNS prop;
private boolean canApplyNeighborhood;
/**
* Create a move which defines a Large Neighborhood Search.
* @param move how the subtree is explored
* @param neighbor how the fragment are computed
* @param restartCounter when a restart should occur
*/
public MoveLNS(Move move, INeighbor neighbor, ICounter restartCounter) {
this.move = move;
this.neighbor = neighbor;
this.counter = restartCounter;
this.frequency = counter.getLimitValue();
this.solutions = 0;
this.freshRestart = false;
this.solutionLoaded = false;
}
@Override
public boolean init() {
neighbor.init();
return move.init();
}
/**
* Return false when:
*
* -
* the underlying search has no more decision to provide,
*
*
*
* Return true when:
*
* -
* a new neighbor is provided,
*
* -
* or a new decision is provided by the underlying decision
*
* -
* or the fast restart criterion is met.
*
*
*
* Restart when:
*
* -
* a restart criterion is met
*
*
*
* @param solver SearchLoop
* @return true if the decision path is extended
*/
@Override
public boolean extend(Solver solver) {
boolean extend;
// when a new fragment is needed (condition: at least one solution has been found)
if (solutions > 0 || solutionLoaded) {
if (freshRestart) {
assert solver.getDecisionPath().size() == 1;
assert solver.getDecisionPath().getDecision(0) == RootDecision.ROOT;
solver.getEnvironment().worldPush();
if(prop == null){
prop = new PropLNS(solver.getModel().intVar(2));
new Constraint("LNS", prop).post();
}
solver.getEngine().propagateOnBacktrack(prop);
canApplyNeighborhood = true;
freshRestart = false;
extend = true;
} else {
// if fast restart is on
if (counter.isMet()) {
// then is restart is triggered
doRestart(solver);
extend = true;
} else {
extend = move.extend(solver);
}
}
} else {
extend = move.extend(solver);
}
return extend;
}
/**
* Return false when :
*
* -
* move.repair(searchLoop) returns false and neighbor is complete.
*
* -
* posting the cut at root node fails
*
*
* Return true when:
*
* -
* move.repair(searchLoop) returns true,
*
* -
* or move.repair(searchLoop) returns false and neighbor is not complete,
*
*
*
* Restart when:
*
* -
* a new solution has been found
*
* -
* move.repair(searchLoop) returns false and neighbor is not complete,
*
* -
* or the fast restart criterion is met
*
*
*
* @param solver SearchLoop
* @return true if the decision path is repaired
*/
@Override
public boolean repair(Solver solver) {
boolean repair = true;
if(solutions > 0
// the second condition is only here for intiale calls, when solutions is not already up to date
|| solver.getSolutionCount() > 0
// the third condition is true when a solution was given as input
|| solutionLoaded) {
// the detection of a new solution can only be met here
if (solutions < solver.getSolutionCount()) {
assert solutions == solver.getSolutionCount() - 1;
solutions++;
solutionLoaded = false;
neighbor.recordSolution();
doRestart(solver);
}
// when posting the cut directly at root node fails
else if (freshRestart) {
repair = false;
}
// the current sub-tree has been entirely explored
else if (!(repair = move.repair(solver))) {
// but the neighbor cannot ensure completeness
if (!neighbor.isSearchComplete()) {
// then a restart is triggered
doRestart(solver);
repair = true;
}
}
// or a fast restart is on
else if (counter.isMet()) {
// then is restart is triggered
doRestart(solver);
}
}else{
repair = move.repair(solver);
}
return repair;
}
/**
* Give an initial solution to begin with if called before executing the solving process
* or erase the last recorded one otherwise.
* @param solution a solution to record
* @param solver that manages the LNS
*/
public void loadFromSolution(Solution solution, Solver solver){
neighbor.loadFromSolution(solution);
solutionLoaded = true;
if(solutions == 0){
freshRestart = true;
}else{
doRestart(solver);
}
}
@Override
public void setTopDecisionPosition(int position) {
move.setTopDecisionPosition(position);
}
@Override
public AbstractStrategy getStrategy() {
return move.getStrategy();
}
@Override
public void setStrategy(AbstractStrategy aStrategy) {
move.setStrategy(aStrategy);
}
@Override
public void removeStrategy() {
move.removeStrategy();
}
/**
* Extend the neighbor when conditions are met and do the restart
*
* @param solver SearchLoop
*/
private void doRestart(Solver solver) {
if (!freshRestart) {
neighbor.restrictLess();
}
freshRestart = true;
counter.overrideLimit(counter.currentValue() + frequency);
solver.restart();
}
@Override
public List getChildMoves() {
return Collections.singletonList(move);
}
@Override
public void setChildMoves(List someMoves) {
if(someMoves.size() == 1) {
this.move = someMoves.get(0);
}else{
throw new UnsupportedOperationException("Only one child move can be attached to it.");
}
}
class PropLNS extends Propagator{
PropLNS(IntVar var) {
super(var);
this.vars = new IntVar[0];
}
@Override
public int getPropagationConditions(int vIdx) {
return IntEventType.VOID.getMask();
}
@Override
public void propagate(int evtmask) throws ContradictionException {
if(canApplyNeighborhood) {
canApplyNeighborhood = false;
neighbor.fixSomeVariables();
}
}
@Override
public ESat isEntailed() {
return ESat.TRUE;
}
@Override
public void explain(int p, ExplanationForSignedClause explanation) {
IntVar pivot = explanation.readVar(p);
IntIterableRangeSet dom = explanation.complement(pivot);
IntIterableSetUtils.unionOf(dom, explanation.readDom(p));
pivot.intersectLit(dom, explanation);
}
}
}