001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *      http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.commons.math3.genetics;
018
019/**
020 * Individual in a population. Chromosomes are compared based on their fitness.
021 * <p>
022 * The chromosomes are IMMUTABLE, and so their fitness is also immutable and
023 * therefore it can be cached.
024 *
025 * @since 2.0
026 * @version $Id: Chromosome.java 1416643 2012-12-03 19:37:14Z tn $
027 */
028public abstract class Chromosome implements Comparable<Chromosome>,Fitness {
029    /** Value assigned when no fitness has been computed yet. */
030    private static final double NO_FITNESS = Double.NEGATIVE_INFINITY;
031
032    /** Cached value of the fitness of this chromosome. */
033    private double fitness = NO_FITNESS;
034
035    /**
036     * Access the fitness of this chromosome. The bigger the fitness, the better the chromosome.
037     * <p>
038     * Computation of fitness is usually very time-consuming task, therefore the fitness is cached.
039     *
040     * @return the fitness
041     */
042    public double getFitness() {
043        if (this.fitness == NO_FITNESS) {
044            // no cache - compute the fitness
045            this.fitness = fitness();
046        }
047        return this.fitness;
048    }
049
050    /**
051     * Compares two chromosomes based on their fitness. The bigger the fitness, the better the chromosome.
052     *
053     * @param another another chromosome to compare
054     * @return
055     * <ul>
056     *   <li>-1 if <code>another</code> is better than <code>this</code></li>
057     *   <li>1 if <code>another</code> is worse than <code>this</code></li>
058     *   <li>0 if the two chromosomes have the same fitness</li>
059     * </ul>
060     */
061    public int compareTo(final Chromosome another) {
062        return ((Double)this.getFitness()).compareTo(another.getFitness());
063    }
064
065    /**
066     * Returns <code>true</code> iff <code>another</code> has the same representation and therefore the same fitness. By
067     * default, it returns false -- override it in your implementation if you need it.
068     *
069     * @param another chromosome to compare
070     * @return true if <code>another</code> is equivalent to this chromosome
071     */
072    protected boolean isSame(final Chromosome another) {
073        return false;
074    }
075
076    /**
077     * Searches the <code>population</code> for another chromosome with the same representation. If such chromosome is
078     * found, it is returned, if no such chromosome exists, returns <code>null</code>.
079     *
080     * @param population Population to search
081     * @return Chromosome with the same representation, or <code>null</code> if no such chromosome exists.
082     */
083    protected Chromosome findSameChromosome(final Population population) {
084        for (Chromosome anotherChr : population) {
085            if (this.isSame(anotherChr)) {
086                return anotherChr;
087            }
088        }
089        return null;
090    }
091
092    /**
093     * Searches the population for a chromosome representing the same solution, and if it finds one,
094     * updates the fitness to its value.
095     *
096     * @param population Population to search
097     */
098    public void searchForFitnessUpdate(final Population population) {
099        Chromosome sameChromosome = findSameChromosome(population);
100        if (sameChromosome != null) {
101            fitness = sameChromosome.getFitness();
102        }
103    }
104
105}