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The James core module is part of the James framework for optimization using local search metaheuristics in Java. The core contains general components to model problems, objectives and constraints, as well as generic algorithms to solve the problems. Moreover, the core provides implementations of specific utilities for subset selection.

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//  Copyright 2014 Herman De Beukelaer
//
//  Licensed under the Apache License, Version 2.0 (the "License");
//  you may not use this file except in compliance with the License.
//  You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
//  Unless required by applicable law or agreed to in writing, software
//  distributed under the License is distributed on an "AS IS" BASIS,
//  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
//  See the License for the specific language governing permissions and
//  limitations under the License.

package org.jamesframework.core.search.algo;

import org.jamesframework.core.exceptions.JamesRuntimeException;
import org.jamesframework.core.problems.Problem;
import org.jamesframework.core.problems.solutions.Solution;
import org.jamesframework.core.search.SingleNeighbourhoodSearch;
import org.jamesframework.core.search.neigh.Move;
import org.jamesframework.core.search.neigh.Neighbourhood;

/**
 * Basic random descent algorithm. In every search step, a random neighbour is created by applying a random move to the current solution.
 * If this neighbour is an improvement over the current solution, it is accepted as the new current solution; else, it is rejected. The
 * random descent algorithm usually does not come to a natural end, and therefore fully depends on stop criteria for termination, except
 * in the unusual case when the current solution does not have any neighbours.
 * 
 * @param  solution type of the problems that may be solved using this search, required to extend {@link Solution}
 * @author Herman De Beukelaer
 */
public class RandomDescent extends SingleNeighbourhoodSearch {

    /**
     * Creates a new random descent search, specifying the problem to solve and the neighbourhood used to
     * modify the current solution. Neither arguments can be null. The search name defaults
     * to "RandomDescent".
     * 
     * @throws NullPointerException if problem or neighbourhood are null
     * @param problem problem to solve
     * @param neighbourhood neighbourhood used to create neighbouring solutions
     */
    public RandomDescent(Problem problem, Neighbourhood neighbourhood){
        this(null, problem, neighbourhood);
    }
    
    /**
     * Creates a new random descent search, specifying the problem to solve, the neighbourhood used to
     * modify the current solution, and a custom search name. The problem and neighbourhood can not be
     * null. The search name can be null in which case the default name
     * "RandomDescent" is assigned.
     * 
     * @throws NullPointerException if problem or neighbourhood are null
     * @param problem problem to solve
     * @param neighbourhood neighbourhood used to create neighbouring solutions
     * @param name custom search name
     */
    public RandomDescent(String name, Problem problem, Neighbourhood neighbourhood){
        super(name != null ? name : "RandomDescent", problem, neighbourhood);
    }

    /**
     * Creates a random neighbour of the current solution and accepts it if it improves over the current solution.
     * 
     * @throws JamesRuntimeException if depending on malfunctioning components (problem, neighbourhood, ...)
     */
    @Override
    protected void searchStep() {
        // get random move
        Move move = getNeighbourhood().getRandomMove(getCurrentSolution());
        // got move ?
        if(move != null){
            // accept if improvement
            if(isImprovement(move)){
                // accept move
                acceptMove(move);
            } else {
                // no improvement
                rejectMove();
            }
        } else {
            // no move/neighbour reported by neighbourhood
            stop();
        }
    }
    
}