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 */
017 package 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 */
028 public 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 }