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.Set;
021import org.checkerframework.checker.nullness.compatqual.NullableDecl;
022
023/**
024 * An interface for <a
025 * href="https://en.wikipedia.org/wiki/Graph_(discrete_mathematics)">graph</a>-structured data,
026 * whose edges have associated non-unique values.
027 *
028 * <p>A graph is composed of a set of nodes and a set of edges connecting pairs of nodes.
029 *
030 * <p>There are three primary interfaces provided to represent graphs. In order of increasing
031 * complexity they are: {@link Graph}, {@link ValueGraph}, and {@link Network}. You should generally
032 * prefer the simplest interface that satisfies your use case. See the <a
033 * href="https://github.com/google/guava/wiki/GraphsExplained#choosing-the-right-graph-type">
034 * "Choosing the right graph type"</a> section of the Guava User Guide for more details.
035 *
036 * <h3>Capabilities</h3>
037 *
038 * <p>{@code ValueGraph} supports the following use cases (<a
039 * href="https://github.com/google/guava/wiki/GraphsExplained#definitions">definitions of
040 * terms</a>):
041 *
042 * <ul>
043 *   <li>directed graphs
044 *   <li>undirected graphs
045 *   <li>graphs that do/don't allow self-loops
046 *   <li>graphs whose nodes/edges are insertion-ordered, sorted, or unordered
047 *   <li>graphs whose edges have associated values
048 * </ul>
049 *
050 * <p>{@code ValueGraph}, as a subtype of {@code Graph}, explicitly does not support parallel edges,
051 * and forbids implementations or extensions with parallel edges. If you need parallel edges, use
052 * {@link Network}. (You can use a positive {@code Integer} edge value as a loose representation of
053 * edge multiplicity, but the {@code *degree()} and mutation methods will not reflect your
054 * interpretation of the edge value as its multiplicity.)
055 *
056 * <h3>Building a {@code ValueGraph}</h3>
057 *
058 * <p>The implementation classes that {@code common.graph} provides are not public, by design. To
059 * create an instance of one of the built-in implementations of {@code ValueGraph}, use the {@link
060 * ValueGraphBuilder} class:
061 *
062 * <pre>{@code
063 * MutableValueGraph<Integer, Double> graph = ValueGraphBuilder.directed().build();
064 * }</pre>
065 *
066 * <p>{@link ValueGraphBuilder#build()} returns an instance of {@link MutableValueGraph}, which is a
067 * subtype of {@code ValueGraph} that provides methods for adding and removing nodes and edges. If
068 * you do not need to mutate a graph (e.g. if you write a method than runs a read-only algorithm on
069 * the graph), you should use the non-mutating {@link ValueGraph} interface, or an {@link
070 * ImmutableValueGraph}.
071 *
072 * <p>You can create an immutable copy of an existing {@code ValueGraph} using {@link
073 * ImmutableValueGraph#copyOf(ValueGraph)}:
074 *
075 * <pre>{@code
076 * ImmutableValueGraph<Integer, Double> immutableGraph = ImmutableValueGraph.copyOf(graph);
077 * }</pre>
078 *
079 * <p>Instances of {@link ImmutableValueGraph} do not implement {@link MutableValueGraph}
080 * (obviously!) and are contractually guaranteed to be unmodifiable and thread-safe.
081 *
082 * <p>The Guava User Guide has <a
083 * href="https://github.com/google/guava/wiki/GraphsExplained#building-graph-instances">more
084 * information on (and examples of) building graphs</a>.
085 *
086 * <h3>Additional documentation</h3>
087 *
088 * <p>See the Guava User Guide for the {@code common.graph} package (<a
089 * href="https://github.com/google/guava/wiki/GraphsExplained">"Graphs Explained"</a>) for
090 * additional documentation, including:
091 *
092 * <ul>
093 *   <li><a
094 *       href="https://github.com/google/guava/wiki/GraphsExplained#equals-hashcode-and-graph-equivalence">
095 *       {@code equals()}, {@code hashCode()}, and graph equivalence</a>
096 *   <li><a href="https://github.com/google/guava/wiki/GraphsExplained#synchronization">
097 *       Synchronization policy</a>
098 *   <li><a href="https://github.com/google/guava/wiki/GraphsExplained#notes-for-implementors">Notes
099 *       for implementors</a>
100 * </ul>
101 *
102 * @author James Sexton
103 * @author Joshua O'Madadhain
104 * @param <N> Node parameter type
105 * @param <V> Value parameter type
106 * @since 20.0
107 */
108@Beta
109public interface ValueGraph<N, V> extends BaseGraph<N> {
110  //
111  // ValueGraph-level accessors
112  //
113
114  /** Returns all nodes in this graph, in the order specified by {@link #nodeOrder()}. */
115  @Override
116  Set<N> nodes();
117
118  /** Returns all edges in this graph. */
119  @Override
120  Set<EndpointPair<N>> edges();
121
122  /**
123   * Returns a live view of this graph as a {@link Graph}. The resulting {@link Graph} will have an
124   * edge connecting node A to node B if this {@link ValueGraph} has an edge connecting A to B.
125   */
126  Graph<N> asGraph();
127
128  //
129  // ValueGraph properties
130  //
131
132  /**
133   * Returns true if the edges in this graph are directed. Directed edges connect a {@link
134   * EndpointPair#source() source node} to a {@link EndpointPair#target() target node}, while
135   * undirected edges connect a pair of nodes to each other.
136   */
137  @Override
138  boolean isDirected();
139
140  /**
141   * Returns true if this graph allows self-loops (edges that connect a node to itself). Attempting
142   * to add a self-loop to a graph that does not allow them will throw an {@link
143   * IllegalArgumentException}.
144   */
145  @Override
146  boolean allowsSelfLoops();
147
148  /** Returns the order of iteration for the elements of {@link #nodes()}. */
149  @Override
150  ElementOrder<N> nodeOrder();
151
152  //
153  // Element-level accessors
154  //
155
156  /**
157   * Returns the nodes which have an incident edge in common with {@code node} in this graph.
158   *
159   * @throws IllegalArgumentException if {@code node} is not an element of this graph
160   */
161  @Override
162  Set<N> adjacentNodes(N node);
163
164  /**
165   * Returns all nodes in this graph adjacent to {@code node} which can be reached by traversing
166   * {@code node}'s incoming edges <i>against</i> the direction (if any) of the edge.
167   *
168   * <p>In an undirected graph, this is equivalent to {@link #adjacentNodes(Object)}.
169   *
170   * @throws IllegalArgumentException if {@code node} is not an element of this graph
171   */
172  @Override
173  Set<N> predecessors(N node);
174
175  /**
176   * Returns all nodes in this graph adjacent to {@code node} which can be reached by traversing
177   * {@code node}'s outgoing edges in the direction (if any) of the edge.
178   *
179   * <p>In an undirected graph, this is equivalent to {@link #adjacentNodes(Object)}.
180   *
181   * <p>This is <i>not</i> the same as "all nodes reachable from {@code node} by following outgoing
182   * edges". For that functionality, see {@link Graphs#reachableNodes(Graph, Object)}.
183   *
184   * @throws IllegalArgumentException if {@code node} is not an element of this graph
185   */
186  @Override
187  Set<N> successors(N node);
188
189  /**
190   * Returns the edges in this graph whose endpoints include {@code node}.
191   *
192   * @throws IllegalArgumentException if {@code node} is not an element of this graph
193   * @since 24.0
194   */
195  @Override
196  Set<EndpointPair<N>> incidentEdges(N node);
197
198  /**
199   * Returns the count of {@code node}'s incident edges, counting self-loops twice (equivalently,
200   * the number of times an edge touches {@code node}).
201   *
202   * <p>For directed graphs, this is equal to {@code inDegree(node) + outDegree(node)}.
203   *
204   * <p>For undirected graphs, this is equal to {@code incidentEdges(node).size()} + (number of
205   * self-loops incident to {@code node}).
206   *
207   * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}.
208   *
209   * @throws IllegalArgumentException if {@code node} is not an element of this graph
210   */
211  @Override
212  int degree(N node);
213
214  /**
215   * Returns the count of {@code node}'s incoming edges (equal to {@code predecessors(node).size()})
216   * in a directed graph. In an undirected graph, returns the {@link #degree(Object)}.
217   *
218   * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}.
219   *
220   * @throws IllegalArgumentException if {@code node} is not an element of this graph
221   */
222  @Override
223  int inDegree(N node);
224
225  /**
226   * Returns the count of {@code node}'s outgoing edges (equal to {@code successors(node).size()})
227   * in a directed graph. In an undirected graph, returns the {@link #degree(Object)}.
228   *
229   * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}.
230   *
231   * @throws IllegalArgumentException if {@code node} is not an element of this graph
232   */
233  @Override
234  int outDegree(N node);
235
236  /**
237   * Returns true if there is an edge directly connecting {@code nodeU} to {@code nodeV}. This is
238   * equivalent to {@code nodes().contains(nodeU) && successors(nodeU).contains(nodeV)}.
239   *
240   * <p>In an undirected graph, this is equal to {@code hasEdgeConnecting(nodeV, nodeU)}.
241   *
242   * @since 23.0
243   */
244  @Override
245  boolean hasEdgeConnecting(N nodeU, N nodeV);
246
247  /**
248   * Returns the value of the edge connecting {@code nodeU} to {@code nodeV}, if one is present;
249   * otherwise, returns {@code defaultValue}.
250   *
251   * <p>In an undirected graph, this is equal to {@code edgeValueOrDefault(nodeV, nodeU,
252   * defaultValue)}.
253   *
254   * @throws IllegalArgumentException if {@code nodeU} or {@code nodeV} is not an element of this
255   *     graph
256   */
257  @NullableDecl
258  V edgeValueOrDefault(N nodeU, N nodeV, @NullableDecl V defaultValue);
259
260  //
261  // ValueGraph identity
262  //
263
264  /**
265   * Returns {@code true} iff {@code object} is a {@link ValueGraph} that has the same elements and
266   * the same structural relationships as those in this graph.
267   *
268   * <p>Thus, two value graphs A and B are equal if <b>all</b> of the following are true:
269   *
270   * <ul>
271   *   <li>A and B have equal {@link #isDirected() directedness}.
272   *   <li>A and B have equal {@link #nodes() node sets}.
273   *   <li>A and B have equal {@link #edges() edge sets}.
274   *   <li>The {@link #edgeValue(Object, Object) value} of a given edge is the same in both A and B.
275   * </ul>
276   *
277   * <p>Graph properties besides {@link #isDirected() directedness} do <b>not</b> affect equality.
278   * For example, two graphs may be considered equal even if one allows self-loops and the other
279   * doesn't. Additionally, the order in which nodes or edges are added to the graph, and the order
280   * in which they are iterated over, are irrelevant.
281   *
282   * <p>A reference implementation of this is provided by {@link AbstractValueGraph#equals(Object)}.
283   */
284  @Override
285  boolean equals(@NullableDecl Object object);
286
287  /**
288   * Returns the hash code for this graph. The hash code of a graph is defined as the hash code of a
289   * map from each of its {@link #edges() edges} to the associated {@link #edgeValue(Object, Object)
290   * edge value}.
291   *
292   * <p>A reference implementation of this is provided by {@link AbstractValueGraph#hashCode()}.
293   */
294  @Override
295  int hashCode();
296}