001 /*
002 * Java Genetic Algorithm Library (jenetics-7.1.2).
003 * Copyright (c) 2007-2023 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.Arrays.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.IntStream;
037
038 import io.jenetics.util.ISeq;
039 import io.jenetics.util.ProxySorter;
040 import io.jenetics.util.Seq;
041
042 /**
043 * This class only contains non-dominate (Pareto-optimal) elements according to
044 * a given <em>dominance</em> measure. Like a {@link Set}, it only contains no
045 * duplicate entries. Unlike the usual set implementation, the iteration order
046 * is deterministic.
047 * <p>
048 * You can create a new {@code ParetoFront} for {@link Vec} objects
049 * <pre>{@code
050 * final ParetoFront<Vec<double[]>> front = new ParetoFront<>(Vec::dominance);
051 * front.add(Vec.of(1.0, 2.0));
052 * front.add(Vec.of(1.1, 2.5));
053 * front.add(Vec.of(0.9, 2.1));
054 * front.add(Vec.of(0.0, 2.9));
055 * }</pre>
056 *
057 * or directly for {@code double[]} array objects
058 * <pre>{@code
059 * final ParetoFront<double[]> front = new ParetoFront<>(Pareto::dominance);
060 * front.add(new double[]{1.0, 2.0});
061 * front.add(new double[]{1.1, 2.5});
062 * front.add(new double[]{0.9, 2.1});
063 * front.add(new double[]{0.0, 2.9});
064 * }</pre>
065 *
066 * You only have to specify the <a href="https://en.wikipedia.org/wiki/Pareto_efficiency">
067 * Pareto dominance/efficiency</a> measure.
068 *
069 * @see Pareto
070 *
071 * @apiNote
072 * Inserting a new element has a time complexity of {@code O(n)}.
073 *
074 * @author <a href="mailto:franz.wilhelmstoetter@gmail.com">Franz Wilhelmstötter</a>
075 * @version 5.1
076 * @since 4.1
077 */
078 public final class ParetoFront<T> extends AbstractSet<T> {
079
080 private final List<T> _population = new ArrayList<>();
081
082 private final Comparator<? super T> _dominance;
083 private final BiPredicate<? super T, ? super T> _equals;
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(this.size())},
119 * where <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 /**
148 * Adds all elements of the given collection to {@code this} pareto front.
149 *
150 * @implNote
151 * The runtime complexity of this operation is
152 * {@code O(elements.size()*this.size())}.
153 *
154 * @param elements the elements to add to {@code this} pareto front
155 * @return {@code true} if {@code this} pareto front has been changed,
156 * {@code false} otherwise
157 */
158 @Override
159 public boolean addAll(final Collection<? extends T> elements) {
160 final int sum = elements.stream()
161 .mapToInt(e -> add(e) ? 1 : 0)
162 .sum();
163 return sum > 0;
164 }
165
166 /**
167 * Add the all {@code elements} to {@code this} pareto-set.
168 *
169 * @implNote
170 * Merging two pareto fronts has a time complexity of
171 * {@code O(elements.size()*this.size())}.
172 *
173 * @param elements the elements to add
174 * @return {@code this} pareto-set
175 * @throws NullPointerException if the given parameter is {@code null}
176 */
177 public ParetoFront<T> merge(final ParetoFront<? extends T> elements) {
178 addAll(elements);
179 return this;
180 }
181
182 /**
183 * Trims {@code this} pareto front to the given size. The front elements are
184 * sorted according its crowding distance and the elements which have smaller
185 * distance to its neighbors are removed first.
186 *
187 * <pre>{@code
188 * final ParetoFront<Vec<double[]>> front = new ParetoFront<>(Vec::dominance);
189 * front.trim(10, Vec::compare, Vec::distance, Vec::length);
190 * }</pre>
191 * The example above reduces the given front to 10 elements.
192 *
193 * @param size the number of front elements after the trim. If
194 * {@code size() <= size}, nothing is trimmed.
195 * @param comparator the element comparator used for calculating the
196 * crowded distance
197 * @param distance the element distance measure
198 * @param dimension the number of vector elements of {@code T}
199 * @return {@code this} trimmed pareto front
200 * @throws NullPointerException if one of the objects is {@code null}
201 */
202 public ParetoFront<T> trim(
203 final int size,
204 final ElementComparator<? super T> comparator,
205 final ElementDistance<? super T> distance,
206 final ToIntFunction<? super T> dimension
207 ) {
208 requireNonNull(comparator);
209 requireNonNull(distance);
210 requireNonNull(dimension);
211
212 if (size() > size) {
213 final double[] distances = Pareto.crowdingDistance(
214 Seq.viewOf(_population),
215 comparator,
216 distance,
217 dimension
218 );
219 final int[] indexes = ProxySorter.sort(distances);
220 revert(indexes);
221
222 final List<T> list = IntStream.of(indexes)
223 .limit(size)
224 .mapToObj(_population::get)
225 .toList();
226
227 _population.clear();
228 _population.addAll(list);
229 }
230
231 return this;
232 }
233
234 @Override
235 public Iterator<T> iterator() {
236 return _population.iterator();
237 }
238
239 @Override
240 public int size() {
241 return _population.size();
242 }
243
244 @Override
245 public boolean isEmpty() {
246 return _population.isEmpty();
247 }
248
249 /**
250 * Return the elements of {@code this} pareto-front as {@link ISeq}.
251 *
252 * @return the elements of {@code this} pareto-front as {@link ISeq}
253 */
254 public ISeq<T> toISeq() {
255 return ISeq.of(_population);
256 }
257
258 /**
259 * Return a pareto-front collector. The natural order of the elements is
260 * used as pareto-dominance order.
261 *
262 * @param <C> the element type
263 * @return a new pareto-front collector
264 */
265 public static <C extends Comparable<? super C>>
266 Collector<C, ?, ParetoFront<C>> toParetoFront() {
267 return toParetoFront(Comparator.naturalOrder());
268 }
269
270 /**
271 * Return a pareto-front collector with the given pareto {@code dominance}
272 * measure.
273 *
274 * @param dominance the pareto dominance comparator
275 * @param <T> the element type
276 * @return a new pareto-front collector
277 * @throws NullPointerException if the given {@code dominance} collector is
278 * {@code null}
279 */
280 public static <T> Collector<T, ?, ParetoFront<T>>
281 toParetoFront(final Comparator<? super T> dominance) {
282 requireNonNull(dominance);
283
284 return Collector.of(
285 () -> new ParetoFront<>(dominance),
286 ParetoFront::add,
287 ParetoFront::merge
288 );
289 }
290
291 }
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