001/*
002 * Copyright (C) 2016 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package com.google.common.graph;
018
019import com.google.common.annotations.Beta;
020import java.util.Optional;
021import java.util.Set;
022import javax.annotation.Nullable;
023
024/**
025 * An interface for <a
026 * href="https://en.wikipedia.org/wiki/Graph_(discrete_mathematics)">graph</a>-structured data,
027 * whose edges have associated non-unique values.
028 *
029 * <p>A graph is composed of a set of nodes and a set of edges connecting pairs of nodes.
030 *
031 * <p>There are three primary interfaces provided to represent graphs. In order of increasing
032 * complexity they are: {@link Graph}, {@link ValueGraph}, and {@link Network}. You should generally
033 * prefer the simplest interface that satisfies your use case. See the <a
034 * href="https://github.com/google/guava/wiki/GraphsExplained#choosing-the-right-graph-type">
035 * "Choosing the right graph type"</a> section of the Guava User Guide for more details.
036 *
037 * <h3>Capabilities</h3>
038 *
039 * <p>{@code ValueGraph} supports the following use cases (<a
040 * href="https://github.com/google/guava/wiki/GraphsExplained#definitions">definitions of
041 * terms</a>):
042 *
043 * <ul>
044 *   <li>directed graphs
045 *   <li>undirected graphs
046 *   <li>graphs that do/don't allow self-loops
047 *   <li>graphs whose nodes/edges are insertion-ordered, sorted, or unordered
048 *   <li>graphs whose edges have associated values
049 * </ul>
050 *
051 * <p>{@code ValueGraph}, as a subtype of {@code Graph}, explicitly does not support parallel edges,
052 * and forbids implementations or extensions with parallel edges. If you need parallel edges, use
053 * {@link Network}. (You can use a positive {@code Integer} edge value as a loose representation of
054 * edge multiplicity, but the {@code *degree()} and mutation methods will not reflect your
055 * interpretation of the edge value as its multiplicity.)
056 *
057 * <h3>Building a {@code ValueGraph}</h3>
058 *
059 * <p>The implementation classes that `common.graph` provides are not public, by design. To create
060 * an instance of one of the built-in implementations of {@code ValueGraph}, use the {@link
061 * ValueGraphBuilder} class:
062 *
063 * <pre>{@code
064 *   MutableValueGraph<Integer, Double> graph = ValueGraphBuilder.directed().build();
065 * }</pre>
066 *
067 * <p>{@link ValueGraphBuilder#build()} returns an instance of {@link MutableValueGraph}, which is a
068 * subtype of {@code ValueGraph} that provides methods for adding and removing nodes and edges. If
069 * you do not need to mutate a graph (e.g. if you write a method than runs a read-only algorithm on
070 * the graph), you should use the non-mutating {@link ValueGraph} interface, or an {@link
071 * ImmutableValueGraph}.
072 *
073 * <p>You can create an immutable copy of an existing {@code ValueGraph} using {@link
074 * ImmutableValueGraph#copyOf(ValueGraph)}:
075 *
076 * <pre>{@code
077 *   ImmutableValueGraph<Integer, Double> immutableGraph = ImmutableValueGraph.copyOf(graph);
078 * }</pre>
079 *
080 * <p>Instances of {@link ImmutableValueGraph} do not implement {@link MutableValueGraph}
081 * (obviously!) and are contractually guaranteed to be unmodifiable and thread-safe.
082 *
083 * <p>The Guava User Guide has <a
084 * href="https://github.com/google/guava/wiki/GraphsExplained#building-graph-instances">more
085 * information on (and examples of) building graphs</a>.
086 *
087 * <h3>Additional documentation</h3>
088 *
089 * <p>See the Guava User Guide for the {@code common.graph} package (<a
090 * href="https://github.com/google/guava/wiki/GraphsExplained">"Graphs Explained"</a>) for
091 * additional documentation, including:
092 *
093 * <ul>
094 *   <li><a
095 *       href="https://github.com/google/guava/wiki/GraphsExplained#equals-hashcode-and-graph-equivalence">
096 *       {@code equals()}, {@code hashCode()}, and graph equivalence</a>
097 *   <li><a href="https://github.com/google/guava/wiki/GraphsExplained#synchronization">
098 *       Synchronization policy</a>
099 *   <li><a href="https://github.com/google/guava/wiki/GraphsExplained#notes-for-implementors">Notes
100 *       for implementors</a>
101 * </ul>
102 *
103 * @author James Sexton
104 * @author Joshua O'Madadhain
105 * @param <N> Node parameter type
106 * @param <V> Value parameter type
107 * @since 20.0
108 */
109// TODO(b/35456940): Update the documentation to reflect the new interfaces
110@Beta
111public interface ValueGraph<N, V> extends BaseGraph<N> {
112  //
113  // ValueGraph-level accessors
114  //
115
116  /** {@inheritDoc} */
117  @Override
118  Set<N> nodes();
119
120  /** {@inheritDoc} */
121  @Override
122  Set<EndpointPair<N>> edges();
123
124  /**
125   * Returns a live view of this graph as a {@link Graph}. The resulting {@link Graph} will have an
126   * edge connecting node A to node B if this {@link ValueGraph} has an edge connecting A to B.
127   */
128  Graph<N> asGraph();
129
130  //
131  // ValueGraph properties
132  //
133
134  /** {@inheritDoc} */
135  @Override
136  boolean isDirected();
137
138  /** {@inheritDoc} */
139  @Override
140  boolean allowsSelfLoops();
141
142  /** {@inheritDoc} */
143  @Override
144  ElementOrder<N> nodeOrder();
145
146  //
147  // Element-level accessors
148  //
149
150  /** {@inheritDoc} */
151  @Override
152  Set<N> adjacentNodes(N node);
153
154  /** {@inheritDoc} */
155  @Override
156  Set<N> predecessors(N node);
157
158  /** {@inheritDoc} */
159  @Override
160  Set<N> successors(N node);
161
162  /** {@inheritDoc} */
163  @Override
164  int degree(N node);
165
166  /** {@inheritDoc} */
167  @Override
168  int inDegree(N node);
169
170  /** {@inheritDoc} */
171  @Override
172  int outDegree(N node);
173
174  /** {@inheritDoc} */
175  @Override
176  boolean hasEdgeConnecting(N nodeU, N nodeV);
177
178  /**
179   * Returns the value of the edge connecting {@code nodeU} to {@code nodeV}, if one is present;
180   * otherwise, returns {@code Optional.empty()}.
181   *
182   * <p>In an undirected graph, this is equal to {@code edgeValue(nodeV, nodeU)}.
183   *
184   * @throws IllegalArgumentException if {@code nodeU} or {@code nodeV} is not an element of this
185   *     graph
186   * @since 23.0 (since 20.0 with return type {@code V})
187   */
188  Optional<V> edgeValue(N nodeU, N nodeV);
189
190  /**
191   * Returns the value of the edge connecting {@code nodeU} to {@code nodeV}, if one is present;
192   * otherwise, returns {@code defaultValue}.
193   *
194   * <p>In an undirected graph, this is equal to {@code edgeValueOrDefault(nodeV, nodeU,
195   * defaultValue)}.
196   *
197   * @throws IllegalArgumentException if {@code nodeU} or {@code nodeV} is not an element of this
198   *     graph
199   */
200  @Nullable
201  V edgeValueOrDefault(N nodeU, N nodeV, @Nullable V defaultValue);
202
203  //
204  // ValueGraph identity
205  //
206
207  /**
208   * Returns {@code true} iff {@code object} is a {@link ValueGraph} that has the same elements and
209   * the same structural relationships as those in this graph.
210   *
211   * <p>Thus, two value graphs A and B are equal if <b>all</b> of the following are true:
212   *
213   * <ul>
214   * <li>A and B have equal {@link #isDirected() directedness}.
215   * <li>A and B have equal {@link #nodes() node sets}.
216   * <li>A and B have equal {@link #edges() edge sets}.
217   * <li>The {@link #edgeValue(Object, Object) value} of a given edge is the same in both A and B.
218   * </ul>
219   *
220   * <p>Graph properties besides {@link #isDirected() directedness} do <b>not</b> affect equality.
221   * For example, two graphs may be considered equal even if one allows self-loops and the other
222   * doesn't. Additionally, the order in which nodes or edges are added to the graph, and the order
223   * in which they are iterated over, are irrelevant.
224   *
225   * <p>A reference implementation of this is provided by {@link AbstractValueGraph#equals(Object)}.
226   */
227  @Override
228  boolean equals(@Nullable Object object);
229
230  /**
231   * Returns the hash code for this graph. The hash code of a graph is defined as the hash code of a
232   * map from each of its {@link #edges() edges} to the associated {@link #edgeValue(Object, Object)
233   * edge value}.
234   *
235   * <p>A reference implementation of this is provided by {@link AbstractValueGraph#hashCode()}.
236   */
237  @Override
238  int hashCode();
239}