001 /*
002 * Java Genetic Algorithm Library (jenetics-4.4.0).
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.prog.op;
021
022 import static java.util.Objects.requireNonNull;
023 import static io.jenetics.internal.util.Hashes.hash;
024
025 import java.io.Serializable;
026 import java.util.Objects;
027
028 /**
029 * Represents the program variables. The {@code Var} operation is a
030 * <em>terminal</em> operation, which just returns the value with the defined
031 * index of the input variable array. It is essentially an orthogonal projection
032 * of the <em>n</em>-dimensional input space to the <em>1</em>-dimensional
033 * result space.
034 *
035 * <pre>{@code
036 * final ISeq<? extends Op<Double>> operations = ISeq.of(...);
037 * final ISeq<? extends Op<Double>> terminals = ISeq.of(
038 * Var.of("x", 0), Var.of("y", 1)
039 * );
040 * }</pre>
041 *
042 * The example above shows how to define the terminal operations for a GP, which
043 * tries to optimize a 2-dimensional function.
044 *
045 * <pre>{@code
046 * static double error(final ProgramChromosome<Double> program) {
047 * final double x = ...;
048 * final double y = ...;
049 * final double result = program.apply(x, y);
050 * ...
051 *
052 * return ...;
053 * }
054 * }</pre>
055 *
056 * @author <a href="mailto:franz.wilhelmstoetter@gmail.com">Franz Wilhelmstötter</a>
057 * @version 4.1
058 * @since 3.9
059 */
060 public final class Var<T> implements Op<T>, Serializable {
061
062 private static final long serialVersionUID = 1L;
063
064 private final String _name;
065 private final int _index;
066
067 /**
068 * Create a new variable with the given {@code name} and projection
069 * {@code index}.
070 *
071 * @param name the variable name. Used when printing the operation tree
072 * (program)
073 * @param index the projection index
074 * @throws IllegalArgumentException if the projection {@code index} is
075 * smaller than zero
076 * @throws NullPointerException if the given variable {@code name} is
077 * {@code null}
078 */
079 private Var(final String name, final int index) {
080 _name = requireNonNull(name);
081 if (index < 0) {
082 throw new IndexOutOfBoundsException(
083 "Index smaller than zero: " + index
084 );
085 }
086 _index = index;
087 }
088
089 /**
090 * The projection index of the variable.
091 *
092 * @return the projection index of the variable
093 */
094 public int index() {
095 return _index;
096 }
097
098 @Override
099 public String name() {
100 return _name;
101 }
102
103 @Override
104 public int arity() {
105 return 0;
106 }
107
108 @Override
109 public T apply(final T[] variables) {
110 return variables[_index];
111 }
112
113 @Override
114 public int hashCode() {
115 return hash(_name, hash(_index));
116 }
117
118 @Override
119 public boolean equals(final Object obj) {
120 return obj == this ||
121 obj instanceof Var &&
122 Objects.equals(((Var)obj)._name, _name) &&
123 ((Var)obj)._index == _index;
124 }
125
126 @Override
127 public String toString() {
128 return _name;
129 }
130
131 /**
132 * Create a new variable with the given {@code name} and projection
133 * {@code index}.
134 *
135 * @param name the variable name. Used when printing the operation tree
136 * (program)
137 * @param index the projection index
138 * @param <T> the variable type
139 * @return a new variable with the given {@code name} and projection
140 * {@code index}
141 */
142 public static <T> Var<T> of(final String name, final int index) {
143 return new Var<>(name, index);
144 }
145
146 }
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