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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;
+
+import org.apache.commons.math3.exception.NotPositiveException;
+import org.apache.commons.math3.exception.NullArgumentException;
+import org.apache.commons.math3.exception.NumberIsTooLargeException;
+import org.apache.commons.math3.exception.OutOfRangeException;
+import org.apache.commons.math3.exception.util.LocalizedFormats;
+import org.apache.commons.math3.util.FastMath;
+
+import java.util.Collections;
+import java.util.List;
+
+/**
+ * Population of chromosomes which uses elitism (certain percentage of the best chromosomes is
+ * directly copied to the next generation).
+ *
+ * @since 2.0
+ */
+public class ElitisticListPopulation extends ListPopulation {
+
+ /** percentage of chromosomes copied to the next generation */
+ private double elitismRate = 0.9;
+
+ /**
+ * Creates a new {@link ElitisticListPopulation} instance.
+ *
+ * @param chromosomes list of chromosomes in the population
+ * @param populationLimit maximal size of the population
+ * @param elitismRate how many best chromosomes will be directly transferred to the next
+ * generation [in %]
+ * @throws NullArgumentException if the list of chromosomes is {@code null}
+ * @throws NotPositiveException if the population limit is not a positive number (< 1)
+ * @throws NumberIsTooLargeException if the list of chromosomes exceeds the population limit
+ * @throws OutOfRangeException if the elitism rate is outside the [0, 1] range
+ */
+ public ElitisticListPopulation(
+ final List<Chromosome> chromosomes, final int populationLimit, final double elitismRate)
+ throws NullArgumentException,
+ NotPositiveException,
+ NumberIsTooLargeException,
+ OutOfRangeException {
+
+ super(chromosomes, populationLimit);
+ setElitismRate(elitismRate);
+ }
+
+ /**
+ * Creates a new {@link ElitisticListPopulation} instance and initializes its inner chromosome
+ * list.
+ *
+ * @param populationLimit maximal size of the population
+ * @param elitismRate how many best chromosomes will be directly transferred to the next
+ * generation [in %]
+ * @throws NotPositiveException if the population limit is not a positive number (&lt; 1)
+ * @throws OutOfRangeException if the elitism rate is outside the [0, 1] range
+ */
+ public ElitisticListPopulation(final int populationLimit, final double elitismRate)
+ throws NotPositiveException, OutOfRangeException {
+
+ super(populationLimit);
+ setElitismRate(elitismRate);
+ }
+
+ /**
+ * Start the population for the next generation. The <code>{@link #elitismRate}</code> percents
+ * of the best chromosomes are directly copied to the next generation.
+ *
+ * @return the beginnings of the next generation.
+ */
+ public Population nextGeneration() {
+ // initialize a new generation with the same parameters
+ ElitisticListPopulation nextGeneration =
+ new ElitisticListPopulation(getPopulationLimit(), getElitismRate());
+
+ final List<Chromosome> oldChromosomes = getChromosomeList();
+ Collections.sort(oldChromosomes);
+
+ // index of the last "not good enough" chromosome
+ int boundIndex = (int) FastMath.ceil((1.0 - getElitismRate()) * oldChromosomes.size());
+ for (int i = boundIndex; i < oldChromosomes.size(); i++) {
+ nextGeneration.addChromosome(oldChromosomes.get(i));
+ }
+ return nextGeneration;
+ }
+
+ /**
+ * Sets the elitism rate, i.e. how many best chromosomes will be directly transferred to the
+ * next generation [in %].
+ *
+ * @param elitismRate how many best chromosomes will be directly transferred to the next
+ * generation [in %]
+ * @throws OutOfRangeException if the elitism rate is outside the [0, 1] range
+ */
+ public void setElitismRate(final double elitismRate) throws OutOfRangeException {
+ if (elitismRate < 0 || elitismRate > 1) {
+ throw new OutOfRangeException(LocalizedFormats.ELITISM_RATE, elitismRate, 0, 1);
+ }
+ this.elitismRate = elitismRate;
+ }
+
+ /**
+ * Access the elitism rate.
+ *
+ * @return the elitism rate
+ */
+ public double getElitismRate() {
+ return this.elitismRate;
+ }
+}