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A Java's Collaborative Filtering library to carry out experiments in research of Collaborative Filtering based Recommender Systems. The library has been designed from researchers to researchers.
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/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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.apache.commons.math3.genetics;

/**
 * Individual in a population. Chromosomes are compared based on their fitness.
 * 
 * The chromosomes are IMMUTABLE, and so their fitness is also immutable and
 * therefore it can be cached.
 *
 * @since 2.0
 */
public abstract class Chromosome implements Comparable,Fitness {
    /** Value assigned when no fitness has been computed yet. */
    private static final double NO_FITNESS = Double.NEGATIVE_INFINITY;

    /** Cached value of the fitness of this chromosome. */
    private double fitness = NO_FITNESS;

    /**
     * Access the fitness of this chromosome. The bigger the fitness, the better the chromosome.
     * 

     * Computation of fitness is usually very time-consuming task, therefore the fitness is cached.
     *
     * @return the fitness
     */
    public double getFitness() {
        if (this.fitness == NO_FITNESS) {
            // no cache - compute the fitness
            this.fitness = fitness();
        }
        return this.fitness;
    }

    /**
     * Compares two chromosomes based on their fitness. The bigger the fitness, the better the chromosome.
     *
     * @param another another chromosome to compare
     * @return
     * 

     *   -1 if another is better than this
     *   1 if another is worse than this
     *   0 if the two chromosomes have the same fitness
     * 
     */
    public int compareTo(final Chromosome another) {
        return Double.compare(getFitness(), another.getFitness());
    }

    /**
     * Returns true iff another has the same representation and therefore the same fitness. By
     * default, it returns false -- override it in your implementation if you need it.
     *
     * @param another chromosome to compare
     * @return true if another is equivalent to this chromosome
     */
    protected boolean isSame(final Chromosome another) {
        return false;
    }

    /**
     * Searches the population for another chromosome with the same representation. If such chromosome is
     * found, it is returned, if no such chromosome exists, returns null.
     *
     * @param population Population to search
     * @return Chromosome with the same representation, or null if no such chromosome exists.
     */
    protected Chromosome findSameChromosome(final Population population) {
        for (Chromosome anotherChr : population) {
            if (this.isSame(anotherChr)) {
                return anotherChr;
            }
        }
        return null;
    }

    /**
     * Searches the population for a chromosome representing the same solution, and if it finds one,
     * updates the fitness to its value.
     *
     * @param population Population to search
     */
    public void searchForFitnessUpdate(final Population population) {
        Chromosome sameChromosome = findSameChromosome(population);
        if (sameChromosome != null) {
            fitness = sameChromosome.getFitness();
        }
    }

}