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Open-source constraint solver.
There is a newer version: 4.10.17
/*
 * 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.constraints.binary;

import org.chocosolver.solver.constraints.Propagator;
import org.chocosolver.solver.constraints.PropagatorPriority;
import org.chocosolver.solver.exception.ContradictionException;
import org.chocosolver.solver.variables.IntVar;
import org.chocosolver.solver.variables.events.IntEventType;
import org.chocosolver.util.ESat;
import org.chocosolver.util.tools.ArrayUtils;

/**
 * Enforces X = |Y|
 * 

 *
 * @author Charles Prud'homme
 * @author Jean-Guillaume Fages
 * @since 18/05/11
 */
public class PropAbsolute extends Propagator {

    private final IntVar X;
    private final IntVar Y;
    private final boolean bothEnumerated;

    public PropAbsolute(IntVar X, IntVar Y) {
        super(ArrayUtils.toArray(X, Y), PropagatorPriority.BINARY, true);
        this.X = vars[0];
        this.Y = vars[1];
        bothEnumerated = X.hasEnumeratedDomain() && Y.hasEnumeratedDomain();
    }

    @Override
    public int getPropagationConditions(int vIdx) {
        if (vars[0].hasEnumeratedDomain() && vars[1].hasEnumeratedDomain()) {
            return IntEventType.all();
        } else {
            return IntEventType.boundAndInst();
        }
    }

    @Override
    public ESat isEntailed() {
        if (vars[0].getUB() < 0) {
            return ESat.FALSE;
        } else if (vars[0].isInstantiated()) {
            if (vars[1].isInstantiated()) {
                return ESat.eval(vars[0].getValue() == Math.abs(vars[1].getValue()));
            } else if (vars[1].getDomainSize() == 2 &&
                    vars[1].contains(vars[0].getValue()) &&
                    vars[1].contains(-vars[0].getValue())) {
                return ESat.TRUE;
            } else if (!vars[1].contains(vars[0].getValue()) &&
                    !vars[1].contains(-vars[0].getValue())) {
                return ESat.FALSE;
            } else {
                return ESat.UNDEFINED;
            }
        } else {
            return ESat.UNDEFINED;
        }
    }

    @Override
    public String toString() {
        return String.format("%s = |%s|", vars[0].toString(), vars[1].toString());
    }

    //***********************************************************************************
    // FILTERING
    //***********************************************************************************

    @Override
    public void propagate(int evtmask) throws ContradictionException {
        X.updateLowerBound(0, this);
        setBounds();
        if (bothEnumerated) {
            enumeratedFiltering();
        }
    }

    @Override
    public void propagate(int varIdx, int mask) throws ContradictionException {
        if (IntEventType.isInstantiate(mask)){
            if(varIdx == 1){
                X.instantiateTo(Math.abs(Y.getValue()), this);
                setPassive();
            }else if(Y.hasEnumeratedDomain()){
                int val = X.getValue();
                Y.updateBounds(-val, val, this);
                val--;
                Y.removeInterval(-val, val, this);
                setPassive();
            }else{
                setBounds();
            }
        }else {
            if(IntEventType.isBound(mask)) {
                setBounds();
            }
            if (IntEventType.isRemove(mask) && bothEnumerated) {
                enumeratedFiltering();
            }
        }
    }

    private void setBounds() throws ContradictionException {
        // X = |Y|
        int max = X.getUB();
        int min = X.getLB();
        Y.updateBounds(-max, max, this);
        Y.removeInterval(1 - min, min - 1, this);
        /////////////////////////////////////////////////
        int prevLB = X.getLB();
        int prevUB = X.getUB();
        min = Y.getLB();
        max = Y.getUB();
        if (max <= 0) {
            X.updateBounds(-max, -min, this);
        } else if (min >= 0) {
            X.updateBounds(min, max, this);
        } else {
            if (Y.hasEnumeratedDomain()) {
                int mP = Y.nextValue(-1);
                int mN = -Y.previousValue(1);
                X.updateLowerBound(Math.min(mP, mN), this);
            }
            X.updateUpperBound(Math.max(-min, max), this);
        }
        if (prevLB != X.getLB() || prevUB != X.getUB()) setBounds();
    }

    private void enumeratedFiltering() throws ContradictionException {
        int min = X.getLB();
        int max = X.getUB();
        for (int v = min; v <= max; v = X.nextValue(v)) {
            if (!(Y.contains(v) || Y.contains(-v))) {
                X.removeValue(v, this);
            }
        }
        min = Y.getLB();
        max = Y.getUB();
        for (int v = min; v <= max; v = Y.nextValue(v)) {
            if (!(X.contains(Math.abs(v)))) {
                Y.removeValue(v, this);
            }
        }
    }

    //***********************************************************************************
    // EXPLANATIONS
    //***********************************************************************************

}