001 /*
002 * Java Genetic Algorithm Library (jenetics-3.0.0).
003 * Copyright (c) 2007-2014 Franz Wilhelmstötter
004 *
005 * Licensed under the Apache License, Version 2.0 (the "License");
006 * you may not use this file except in compliance with the License.
007 * 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 * Author:
018 * Franz Wilhelmstötter (franz.wilhelmstoetter@gmx.at)
019 */
020 package org.jenetics;
021
022 import static java.lang.String.format;
023 import static java.util.Objects.requireNonNull;
024
025 import org.jenetics.internal.util.Equality;
026 import org.jenetics.internal.util.Hash;
027
028 /**
029 * In truncation selection individuals are sorted according to their fitness.
030 * Only the n best individuals are selected. The truncation selection is a very
031 * basic selection algorithm. It has it's strength in fast selecting individuals
032 * in large populations, but is not very often used in practice.
033 *
034 * @see <a href="http://en.wikipedia.org/wiki/Truncation_selection">
035 * Wikipedia: Truncation selection
036 * </a>
037 *
038 * @author <a href="mailto:franz.wilhelmstoetter@gmx.at">Franz Wilhelmstötter</a>
039 * @since 1.0
040 * @version 2.0 — <em>$Date: 2014-12-28 $</em>
041 */
042 public final class TruncationSelector<
043 G extends Gene<?, G>,
044 C extends Comparable<? super C>
045 >
046 implements Selector<G, C>
047 {
048
049 /**
050 * Create a new TruncationSelector object.
051 */
052 public TruncationSelector() {
053 }
054
055 /**
056 * This method sorts the population in descending order while calculating
057 * the selection probabilities. (The method
058 * {@link Population#sortWith(java.util.Comparator)} )} is called by this
059 * method.) If the selection size is greater the the population size, the
060 * whole population is duplicated until the desired sample size is reached.
061 *
062 * @throws NullPointerException if the {@code population} is {@code null}.
063 */
064 @Override
065 public Population<G, C> select(
066 final Population<G, C> population,
067 final int count,
068 final Optimize opt
069 ) {
070 requireNonNull(population, "Population");
071 requireNonNull(opt, "Optimization");
072 if (count < 0) {
073 throw new IllegalArgumentException(format(
074 "Selection count must be greater or equal then zero, but was %s",
075 count
076 ));
077 }
078
079 population.sortWith(opt.<C>descending());
080 final Population<G, C> selection = new Population<>(count);
081 int size = count;
082 do {
083 final int length = Math.min(population.size(), size);
084 selection.addAll(population.subList(0, length));
085 size -= length;
086 } while (size > 0);
087
088 return selection;
089 }
090
091 @Override
092 public int hashCode() {
093 return Hash.of(getClass()).value();
094 }
095
096 @Override
097 public boolean equals(final Object obj) {
098 return Equality.ofType(this, obj);
099 }
100
101 @Override
102 public String toString() {
103 return getClass().getName();
104 }
105
106 }
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