001 /*
002 * Java Genetic Algorithm Library (jenetics-5.1.0).
003 * Copyright (c) 2007-2019 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@gmail.com)
019 */
020 package io.jenetics.ext.moea;
021
022 import static java.util.Objects.requireNonNull;
023 import static io.jenetics.internal.util.array.revert;
024
025 import java.util.AbstractSet;
026 import java.util.ArrayList;
027 import java.util.Collection;
028 import java.util.Comparator;
029 import java.util.Iterator;
030 import java.util.List;
031 import java.util.Objects;
032 import java.util.Set;
033 import java.util.function.BiPredicate;
034 import java.util.function.ToIntFunction;
035 import java.util.stream.Collector;
036 import java.util.stream.Collectors;
037 import java.util.stream.IntStream;
038
039 import io.jenetics.util.ISeq;
040 import io.jenetics.util.ProxySorter;
041 import io.jenetics.util.Seq;
042
043 /**
044 * This class only contains non-dominate (Pareto-optimal) elements according to
045 * a given <em>dominance</em> measure. Like a {@link Set}, it only contains no
046 * duplicate entries. Unlike the usual set implementation, the iteration order
047 * is deterministic.
048 * <p>
049 * You can create a new {@code ParetoFront} for {@link Vec} objects
050 * <pre>{@code
051 * final ParetoFront<Vec<double[]>> front = new ParetoFront<>(Vec::dominance);
052 * front.add(Vec.of(1.0, 2.0));
053 * front.add(Vec.of(1.1, 2.5));
054 * front.add(Vec.of(0.9, 2.1));
055 * front.add(Vec.of(0.0, 2.9));
056 * }</pre>
057 *
058 * or directly for {@code double[]} array objects
059 * <pre>{@code
060 * final ParetoFront<double[]> front = new ParetoFront<>(Pareto::dominance);
061 * front.add(new double[]{1.0, 2.0});
062 * front.add(new double[]{1.1, 2.5});
063 * front.add(new double[]{0.9, 2.1});
064 * front.add(new double[]{0.0, 2.9});
065 * }</pre>
066 *
067 * You only have to specify the <a href="https://en.wikipedia.org/wiki/Pareto_efficiency">
068 * Pareto dominance/efficiency</a> measure.
069 *
070 * @see Pareto
071 *
072 * @apiNote
073 * Inserting a new element has a time complexity of {@code O(n)}.
074 *
075 * @author <a href="mailto:franz.wilhelmstoetter@gmail.com">Franz Wilhelmstötter</a>
076 * @version 5.1
077 * @since 4.1
078 */
079 public final class ParetoFront<T> extends AbstractSet<T> {
080
081 private final Comparator<? super T> _dominance;
082 private final BiPredicate<? super T, ? super T> _equals;
083 private final List<T> _population = new ArrayList<>();
084
085 /**
086 * Create a new {@code ParetoSet} with the given {@code dominance} measure.
087 *
088 * @since 5.1
089 *
090 * @param dominance the <em>Pareto</em> dominance measure
091 * @param equals the equals predicate used for keeping the set distinct
092 * @throws NullPointerException if the given {@code dominance} measure is
093 * {@code null}
094 */
095 public ParetoFront(
096 final Comparator<? super T> dominance,
097 final BiPredicate<? super T, ? super T> equals
098 ) {
099 _dominance = requireNonNull(dominance);
100 _equals = requireNonNull(equals);
101 }
102
103 /**
104 * Create a new {@code ParetoSet} with the given {@code dominance} measure.
105 *
106 * @param dominance the <em>Pareto</em> dominance measure
107 * @throws NullPointerException if the given {@code dominance} measure is
108 * {@code null}
109 */
110 public ParetoFront(final Comparator<? super T> dominance) {
111 this(dominance, Objects::equals);
112 }
113
114 /**
115 * Inserts an {@code element} to this pareto front.
116 *
117 * @implNote
118 * Inserting a new element has a time complexity of {@code O(n)}, where
119 * <em>n</em> is the number of elements of {@code this} pareto-front.
120 *
121 * @param element the element to add
122 * @return {@code true} if this set did not already contain the specified
123 * element
124 */
125 @Override
126 public boolean add(final T element) {
127 requireNonNull(element);
128
129 boolean updated = false;
130 final Iterator<T> iterator = _population.iterator();
131 while (iterator.hasNext()) {
132 final T existing = iterator.next();
133
134 int cmp = _dominance.compare(element, existing);
135 if (cmp > 0) {
136 iterator.remove();
137 updated = true;
138 } else if (cmp < 0 || _equals.test(element, existing)) {
139 return updated;
140 }
141 }
142
143 _population.add(element);
144 return true;
145 }
146
147 @Override
148 public boolean addAll(final Collection<? extends T> elements) {
149 final int sum = elements.stream()
150 .mapToInt(e -> add(e) ? 1 : 0)
151 .sum();
152 return sum > 0;
153 }
154
155 /**
156 * Add the all {@code elements} to {@code this} pareto-set.
157 *
158 * @apiNote
159 * Merging two pareto fronts has a time complexity of {@code O(n*m)}.
160 *
161 * @param elements the elements to add
162 * @return {@code this} pareto-set
163 * @throws NullPointerException if the given parameter is {@code null}
164 */
165 public ParetoFront<T> merge(final ParetoFront<? extends T> elements) {
166 addAll(elements);
167 return this;
168 }
169
170 /**
171 * Trims {@code this} pareto front to the given size. The front elements are
172 * sorted according its crowding distance and the elements which have smaller
173 * distance to its neighbors are removed first.
174 *
175 * <pre>{@code
176 * final ParetoFront<Vec<double[]>> front = new ParetoFront<>(Vec::dominance);
177 * front.trim(10, Vec::compare, Vec::distance, Vec::length);
178 * }</pre>
179 * The example above reduces the given front to 10 elements.
180 *
181 * @param size the number of front elements after the trim. If
182 * {@code size() <= size}, nothing is trimmed.
183 * @param comparator the element comparator used for calculating the
184 * crowded distance
185 * @param distance the element distance measure
186 * @param dimension the number of vector elements of {@code T}
187 * @return {@code this} trimmed pareto front
188 * @throws NullPointerException if one of the objects is {@code null}
189 */
190 public ParetoFront<T> trim(
191 final int size,
192 final ElementComparator<? super T> comparator,
193 final ElementDistance<? super T> distance,
194 final ToIntFunction<? super T> dimension
195 ) {
196 requireNonNull(comparator);
197 requireNonNull(distance);
198 requireNonNull(dimension);
199
200 if (size() > size) {
201 final double[] distances = Pareto.crowdingDistance(
202 Seq.viewOf(_population),
203 comparator,
204 distance,
205 dimension
206 );
207 final int[] indexes = ProxySorter.sort(distances);
208 revert(indexes);
209
210 final List<T> list = IntStream.of(indexes)
211 .limit(size)
212 .mapToObj(_population::get)
213 .collect(Collectors.toList());
214
215 _population.clear();
216 _population.addAll(list);
217 }
218
219 return this;
220 }
221
222 @Override
223 public Iterator<T> iterator() {
224 return _population.iterator();
225 }
226
227 @Override
228 public int size() {
229 return _population.size();
230 }
231
232 @Override
233 public boolean isEmpty() {
234 return _population.isEmpty();
235 }
236
237 /**
238 * Return the elements of {@code this} pareto-front as {@link ISeq}.
239 *
240 * @return the elements of {@code this} pareto-front as {@link ISeq}
241 */
242 public ISeq<T> toISeq() {
243 return ISeq.of(_population);
244 }
245
246 /**
247 * Return a pareto-front collector. The natural order of the elements is
248 * used as pareto-dominance order.
249 *
250 * @param <C> the element type
251 * @return a new pareto-front collector
252 */
253 public static <C extends Comparable<? super C>>
254 Collector<C, ?, ParetoFront<C>> toParetoFront() {
255 return toParetoFront(Comparator.naturalOrder());
256 }
257
258 /**
259 * Return a pareto-front collector with the given pareto {@code dominance}
260 * measure.
261 *
262 * @param dominance the pareto dominance comparator
263 * @param <T> the element type
264 * @return a new pareto-front collector
265 * @throws NullPointerException if the given {@code dominance} collector is
266 * {@code null}
267 */
268 public static <T> Collector<T, ?, ParetoFront<T>>
269 toParetoFront(final Comparator<? super T> dominance) {
270 return Collector.of(
271 () -> new ParetoFront<>(dominance),
272 ParetoFront::add,
273 ParetoFront::merge
274 );
275 }
276
277 }
|