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/*
* This file is part of choco-solver, http://choco-solver.org/
*
* Copyright (c) 2023, 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.objective;
import org.chocosolver.solver.Model;
import org.chocosolver.solver.Solution;
import org.chocosolver.solver.constraints.Propagator;
import org.chocosolver.solver.constraints.PropagatorPriority;
import org.chocosolver.solver.exception.ContradictionException;
import org.chocosolver.solver.search.loop.monitors.IMonitorSolution;
import org.chocosolver.solver.variables.IntVar;
import org.chocosolver.util.ESat;
import java.util.LinkedList;
import java.util.List;
import java.util.stream.Stream;
/**
* Class to store the pareto front (multi-objective optimization).
*
* Based on "Multi-Objective Large Neighborhood Search", P. Schaus , R. Hartert (CP'2013)
*
*
* @author Charles Vernerey
* @author Charles Prud'homme
* @author Jean-Guillaume Fages
*/
public class ParetoMaximizer extends Propagator implements IMonitorSolution {
//***********************************************************************************
// VARIABLES
//***********************************************************************************
// Set of incomparable and Pareto-best solutions
private final LinkedList paretoSolutions;
private final LinkedList paretoFront;
private final Model model;
// Allow to recycle (dominated) Solution objects
private final LinkedList poolSols = new LinkedList<>();
// objective function
private final IntVar[] objectives;
private final int n;
//private final int[] vals;
//***********************************************************************************
// CONSTRUCTOR
//***********************************************************************************
/**
* Create an object to compute the Pareto front of a multi-objective problem.
* Objectives are expected to be maximized (use {@link org.chocosolver.solver.variables.IViewFactory#intMinusView(IntVar)} in case of minimisation).
*
* Maintain the set of dominating solutions and
* posts constraints dynamically to prevent search from computing dominated ones.
*
* The Solutions store decision variables (those declared in the search strategy)
* BEWARE: requires the objectives to be declared in the search strategy
*
* @param objectives objective variables (must all be optimized in the same direction)
*/
public ParetoMaximizer(final IntVar[] objectives) {
super(objectives, PropagatorPriority.QUADRATIC, false);
this.paretoSolutions = new LinkedList<>();
this.paretoFront = new LinkedList<>();
this.objectives = objectives.clone();
n = objectives.length;
model = objectives[0].getModel();
//vals = new int[n];
}
//***********************************************************************************
// METHODS
//***********************************************************************************
/**
* @return the set of Pareto-best (possibly optimal) solutions found so far
*/
public List getParetoFront() {
return paretoSolutions;
}
@Override
public void onSolution() {
// get objective values
int[] vals = Stream.of(objectives).mapToInt(IntVar::getValue).toArray();
// remove dominated solutions
for (int i = paretoFront.size() - 1; i >= 0; i--) {
if (isDominated(paretoSolutions.get(i), vals)) {
poolSols.add(paretoSolutions.remove(i));
paretoFront.remove(i);
}
}
// store current solution
Solution solution;
if (poolSols.isEmpty()) {
solution = new Solution(model);
} else {
solution = poolSols.remove();
}
solution.record();
paretoSolutions.add(solution);
paretoFront.add(vals);
}
private boolean isDominated(Solution solution, int[] vals) {
for (int i = 0; i < n; i++) {
int delta = solution.getIntVal(objectives[i]) - vals[i];
if (delta > 0) {
return false;
}
}
return true;
}
@Override
public void propagate(int evtmask) throws ContradictionException {
for (int i = 0; i < objectives.length; i++) {
computeTightestPoint(i);
}
}
/**
* Compute tightest point for objective i
* i.e. the point that dominates DP_i and has the biggest obj_i
*
* @param i index of the variable
*/
private void computeTightestPoint(int i) throws ContradictionException {
int tightestPoint = Integer.MIN_VALUE;
int[] dominatedPoint = computeDominatedPoint(i);
for (int[] sol : paretoFront) {
int dominates = dominates(sol, dominatedPoint, i);
if (dominates > 0) {
int currentPoint = dominates == 1 ? sol[i] : sol[i] + 1;
if (tightestPoint < currentPoint) {
tightestPoint = currentPoint;
}
}
}
if (tightestPoint > Integer.MIN_VALUE) {
objectives[i].updateLowerBound(tightestPoint, this);
}
}
/**
* Compute dominated point for objective i,
* i.e. DP_i = (obj_1_max,...,obj_i_min,...,obj_m_max)
*
* @param i index of the variable
* @return dominated point
*/
private int[] computeDominatedPoint(int i) {
int[] dp = Stream.of(objectives).mapToInt(IntVar::getUB).toArray();
dp[i] = objectives[i].getLB();
return dp;
}
/**
* Return an int :
* 0 if a doesn't dominate b
* 1 if a dominates b and a = b if we don't take into account index i
* 2 if a dominates b and a dominates b if we don't take into account index i
*
* @param a vector
* @param b vector
* @param i index
* @return an int representing the fact that a dominates b
*/
private int dominates(int[] a, int[] b, int i) {
int dominates = 0;
for (int j = 0; j < objectives.length; j++) {
if (a[j] < b[j]) return 0;
if (a[j] > b[j]) {
if (dominates == 0) dominates = 1;
if (j != i) dominates = 2;
}
}
return dominates;
}
@Override
public ESat isEntailed() {
return ESat.TRUE;
}
}