001 /*
002 * Java Genetic Algorithm Library (jenetics-6.1.0).
003 * Copyright (c) 2007-2020 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.prog.regression;
021
022 import static java.lang.Math.min;
023 import static java.lang.Math.sqrt;
024
025 import io.jenetics.ext.util.Tree;
026
027 import io.jenetics.prog.op.Op;
028
029 /**
030 * Represents a complexity <em>measure</em> if a given program tree. The
031 * program complexity ensures, that simpler programs with similar loss function
032 * values are preferred. It is part of the <em>overall</em> {@link Error}
033 * function.
034 *
035 * <pre>{@code
036 * final Error<Double> error = Error.of(LossFunction::mse, Complexity.ofMaxNodeCount(50));
037 * }</pre>
038 *
039 * @see LossFunction
040 * @see Error
041 *
042 * @param <T> the sample type
043 *
044 * @author <a href="mailto:franz.wilhelmstoetter@gmail.com">Franz Wilhelmstötter</a>
045 * @version 5.0
046 * @since 5.0
047 */
048 @FunctionalInterface
049 public interface Complexity<T> {
050
051 /**
052 * Calculates the complexity of the current program (possibly) relative
053 * to the actual error value.
054 *
055 * @param program the actual program
056 * @return the measure of the program complexity
057 */
058 double apply(final Tree<? extends Op<T>, ?> program);
059
060 /**
061 * Return a complexity measure which counts the number of nodes of a program.
062 *
063 * @see #count(Tree, int)
064 *
065 * @param <T> the sample type
066 * @param maxNodeCount the maximal node count. The returned complexity will
067 * be one if the program node count is greater or equal the given
068 * {@code count}
069 * @return a program node count complexity measure
070 * @throws IllegalArgumentException if the max node {@code count} is smaller
071 * than one
072 */
073 static <T> Complexity<T> ofNodeCount(final int maxNodeCount) {
074 if (maxNodeCount < 1) {
075 throw new IllegalArgumentException(
076 "Max node count must be greater than zero: " + maxNodeCount
077 );
078 }
079
080 return p -> count(p, maxNodeCount);
081 }
082
083 /**
084 * This method uses the node count of a program tree for calculating its
085 * complexity. The returned node count <em>measure</em> is within the range
086 * of {@code [0, 1]}. If the program contains only one node, zero is returned.
087 * If the node count is bigger or equal {@code maxNodes}, one is returned.
088 * <p>
089 * The complexity is calculated in the following way:
090 * <pre>{@code
091 * final double cc = min(program.size() - 1, maxNodes);
092 * return 1.0 - sqrt(1.0 - (cc*cc)/(maxNodes*maxNodes));
093 * }</pre>
094 *
095 * @see #ofNodeCount(int)
096 *
097 * @param program the program used for the complexity measure
098 * @param maxNodes the maximal expected node count
099 * @return the complexity measure of the given {@code program}
100 * @throws NullPointerException if the given {@code program} is {@code null}
101 * @throws IllegalArgumentException if {@code maxNodes} is smaller than one
102 */
103 static double count(final Tree<?, ?> program, final int maxNodes) {
104 if (maxNodes < 1) {
105 throw new IllegalArgumentException(
106 "Max node count must be greater than zero: " + maxNodes
107 );
108 }
109
110 final double cc = min(program.size() - 1, maxNodes);
111 return 1.0 - sqrt(1.0 - (cc*cc)/(maxNodes*maxNodes));
112 }
113
114 }
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