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 org.checkerframework.checker.nullness.compatqual.NullableDecl;
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 {@code common.graph} provides are not public, by design. To
060 * create 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@Beta
110public interface ValueGraph<N, V> extends BaseGraph<N> {
111  //
112  // ValueGraph-level accessors
113  //
114
115  /** {@inheritDoc} */
116  @Override
117  Set<N> nodes();
118
119  /** {@inheritDoc} */
120  @Override
121  Set<EndpointPair<N>> edges();
122
123  /**
124   * Returns a live view of this graph as a {@link Graph}. The resulting {@link Graph} will have an
125   * edge connecting node A to node B if this {@link ValueGraph} has an edge connecting A to B.
126   */
127  Graph<N> asGraph();
128
129  //
130  // ValueGraph properties
131  //
132
133  /** {@inheritDoc} */
134  @Override
135  boolean isDirected();
136
137  /** {@inheritDoc} */
138  @Override
139  boolean allowsSelfLoops();
140
141  /** {@inheritDoc} */
142  @Override
143  ElementOrder<N> nodeOrder();
144
145  //
146  // Element-level accessors
147  //
148
149  /** {@inheritDoc} */
150  @Override
151  Set<N> adjacentNodes(N node);
152
153  /** {@inheritDoc} */
154  @Override
155  Set<N> predecessors(N node);
156
157  /** {@inheritDoc} */
158  @Override
159  Set<N> successors(N node);
160
161  /** {@inheritDoc} */
162  @Override
163  int degree(N node);
164
165  /** {@inheritDoc} */
166  @Override
167  int inDegree(N node);
168
169  /** {@inheritDoc} */
170  @Override
171  int outDegree(N node);
172
173  /** {@inheritDoc} */
174  @Override
175  boolean hasEdgeConnecting(N nodeU, N nodeV);
176
177  /**
178   * Returns the value of the edge connecting {@code nodeU} to {@code nodeV}, if one is present;
179   * otherwise, returns {@code Optional.empty()}.
180   *
181   * <p>In an undirected graph, this is equal to {@code edgeValue(nodeV, nodeU)}.
182   *
183   * @throws IllegalArgumentException if {@code nodeU} or {@code nodeV} is not an element of this
184   *     graph
185   * @since 23.0 (since 20.0 with return type {@code V})
186   */
187  Optional<V> edgeValue(N nodeU, N nodeV);
188
189  /**
190   * Returns the value of the edge connecting {@code nodeU} to {@code nodeV}, if one is present;
191   * otherwise, returns {@code defaultValue}.
192   *
193   * <p>In an undirected graph, this is equal to {@code edgeValueOrDefault(nodeV, nodeU,
194   * defaultValue)}.
195   *
196   * @throws IllegalArgumentException if {@code nodeU} or {@code nodeV} is not an element of this
197   *     graph
198   */
199  @NullableDecl
200  V edgeValueOrDefault(N nodeU, N nodeV, @NullableDecl V defaultValue);
201
202  //
203  // ValueGraph identity
204  //
205
206  /**
207   * Returns {@code true} iff {@code object} is a {@link ValueGraph} that has the same elements and
208   * the same structural relationships as those in this graph.
209   *
210   * <p>Thus, two value graphs A and B are equal if <b>all</b> of the following are true:
211   *
212   * <ul>
213   *   <li>A and B have equal {@link #isDirected() directedness}.
214   *   <li>A and B have equal {@link #nodes() node sets}.
215   *   <li>A and B have equal {@link #edges() edge sets}.
216   *   <li>The {@link #edgeValue(Object, Object) value} of a given edge is the same in both A and B.
217   * </ul>
218   *
219   * <p>Graph properties besides {@link #isDirected() directedness} do <b>not</b> affect equality.
220   * For example, two graphs may be considered equal even if one allows self-loops and the other
221   * doesn't. Additionally, the order in which nodes or edges are added to the graph, and the order
222   * in which they are iterated over, are irrelevant.
223   *
224   * <p>A reference implementation of this is provided by {@link AbstractValueGraph#equals(Object)}.
225   */
226  @Override
227  boolean equals(@NullableDecl Object object);
228
229  /**
230   * Returns the hash code for this graph. The hash code of a graph is defined as the hash code of a
231   * map from each of its {@link #edges() edges} to the associated {@link #edgeValue(Object, Object)
232   * edge value}.
233   *
234   * <p>A reference implementation of this is provided by {@link AbstractValueGraph#hashCode()}.
235   */
236  @Override
237  int hashCode();
238}