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.math.genetics;
018
019 /**
020 * Individual in a population. Chromosomes are compared based on their fitness.
021 *
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 $Revision: 811685 $ $Date: 2009-09-05 13:36:48 -0400 (Sat, 05 Sep 2009) $
027 */
028 public abstract class Chromosome implements Comparable<Chromosome>,Fitness {
029
030 /**
031 * Cached value of the fitness of this chromosome.
032 */
033 private double fitness = Double.MIN_VALUE;
034
035 /**
036 * Access the fitness of this chromosome. The bigger the fitness, the better
037 * the chromosome.
038 *
039 * Computation of fitness is usually very time-consuming task, therefore the
040 * fitness is cached.
041 *
042 * @return the fitness.
043 */
044 public double getFitness() {
045 if (this.fitness == Double.MIN_VALUE) {
046 // no cache - compute the fitness
047 this.fitness = fitness();
048 }
049 return this.fitness;
050 }
051
052 /**
053 * Compares two chromosomes based on their fitness. The bigger the fitness,
054 * the better the chromosome.
055 *
056 * @param another another chromosome to compare
057 * @return
058 * <ul>
059 * <li>-1 if <code>another</code> is better than <code>this</code></li>
060 * <li>1 if <code>another</code> is worse than <code>this</code></li>
061 * <li>0 if the two chromosomes have the same fitness</li>
062 * </ul>
063 */
064 public int compareTo(Chromosome another) {
065 return ((Double)this.getFitness()).compareTo(another.getFitness());
066 }
067
068 /**
069 * Returns <code>true<code> iff <code>another</code> has the same
070 * representation and therefore the same fitness. By default, it returns
071 * false -- override it in your implementation if you need it.
072 * @param another chromosome to compare
073 * @return true if <code>another</code> is equivalent to this chromosome
074 */
075 protected boolean isSame(Chromosome another) {
076 return false;
077 }
078
079 /**
080 * Searches the <code>population</code> for another chromosome with the same
081 * representation. If such chromosome is found, it is returned, if no such
082 * chromosome exists, returns <code>null</code>.
083 *
084 * @param population
085 * Population to search
086 * @return Chromosome with the same representation, or <code>null</code> if
087 * no such chromosome exists.
088 */
089 protected Chromosome findSameChromosome(Population population) {
090 for (Chromosome anotherChr : population) {
091 if (this.isSame(anotherChr))
092 return anotherChr;
093 }
094 return null;
095 }
096
097 /**
098 * Searches the population for a chromosome representing the same solution,
099 * and if it finds one, updates the fitness to its value.
100 *
101 * @param population
102 * Population to search
103 */
104 public void searchForFitnessUpdate(Population population) {
105 Chromosome sameChromosome = findSameChromosome(population);
106 if (sameChromosome != null) {
107 fitness = sameChromosome.getFitness();
108 }
109 }
110
111 }