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 */
027public abstract class Chromosome implements Comparable<Chromosome>,Fitness {
028    /** Value assigned when no fitness has been computed yet. */
029    private static final double NO_FITNESS = Double.NEGATIVE_INFINITY;
030
031    /** Cached value of the fitness of this chromosome. */
032    private double fitness = NO_FITNESS;
033
034    /**
035     * Access the fitness of this chromosome. The bigger the fitness, the better the chromosome.
036     * <p>
037     * Computation of fitness is usually very time-consuming task, therefore the fitness is cached.
038     *
039     * @return the fitness
040     */
041    public double getFitness() {
042        if (this.fitness == NO_FITNESS) {
043            // no cache - compute the fitness
044            this.fitness = fitness();
045        }
046        return this.fitness;
047    }
048
049    /**
050     * Compares two chromosomes based on their fitness. The bigger the fitness, the better the chromosome.
051     *
052     * @param another another chromosome to compare
053     * @return
054     * <ul>
055     *   <li>-1 if <code>another</code> is better than <code>this</code></li>
056     *   <li>1 if <code>another</code> is worse than <code>this</code></li>
057     *   <li>0 if the two chromosomes have the same fitness</li>
058     * </ul>
059     */
060    public int compareTo(final Chromosome another) {
061        return Double.compare(getFitness(), another.getFitness());
062    }
063
064    /**
065     * Returns <code>true</code> iff <code>another</code> has the same representation and therefore the same fitness. By
066     * default, it returns false -- override it in your implementation if you need it.
067     *
068     * @param another chromosome to compare
069     * @return true if <code>another</code> is equivalent to this chromosome
070     */
071    protected boolean isSame(final Chromosome another) {
072        return false;
073    }
074
075    /**
076     * Searches the <code>population</code> for another chromosome with the same representation. If such chromosome is
077     * found, it is returned, if no such chromosome exists, returns <code>null</code>.
078     *
079     * @param population Population to search
080     * @return Chromosome with the same representation, or <code>null</code> if no such chromosome exists.
081     */
082    protected Chromosome findSameChromosome(final Population population) {
083        for (Chromosome anotherChr : population) {
084            if (this.isSame(anotherChr)) {
085                return anotherChr;
086            }
087        }
088        return null;
089    }
090
091    /**
092     * Searches the population for a chromosome representing the same solution, and if it finds one,
093     * updates the fitness to its value.
094     *
095     * @param population Population to search
096     */
097    public void searchForFitnessUpdate(final Population population) {
098        Chromosome sameChromosome = findSameChromosome(population);
099        if (sameChromosome != null) {
100            fitness = sameChromosome.getFitness();
101        }
102    }
103
104}