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.Collection;
021import java.util.Optional;
022import java.util.Set;
023import org.checkerframework.checker.nullness.qual.Nullable;
024
025/**
026 * An interface for <a
027 * href="https://en.wikipedia.org/wiki/Graph_(discrete_mathematics)">graph</a>-structured data,
028 * whose edges have associated non-unique values.
029 *
030 * <p>A graph is composed of a set of nodes and a set of edges connecting pairs of nodes.
031 *
032 * <p>There are three primary interfaces provided to represent graphs. In order of increasing
033 * complexity they are: {@link Graph}, {@link ValueGraph}, and {@link Network}. You should generally
034 * prefer the simplest interface that satisfies your use case. See the <a
035 * href="https://github.com/google/guava/wiki/GraphsExplained#choosing-the-right-graph-type">
036 * "Choosing the right graph type"</a> section of the Guava User Guide for more details.
037 *
038 * <h3>Capabilities</h3>
039 *
040 * <p>{@code ValueGraph} supports the following use cases (<a
041 * href="https://github.com/google/guava/wiki/GraphsExplained#definitions">definitions of
042 * terms</a>):
043 *
044 * <ul>
045 *   <li>directed graphs
046 *   <li>undirected graphs
047 *   <li>graphs that do/don't allow self-loops
048 *   <li>graphs whose nodes/edges are insertion-ordered, sorted, or unordered
049 *   <li>graphs whose edges have associated values
050 * </ul>
051 *
052 * <p>{@code ValueGraph}, as a subtype of {@code Graph}, explicitly does not support parallel edges,
053 * and forbids implementations or extensions with parallel edges. If you need parallel edges, use
054 * {@link Network}. (You can use a positive {@code Integer} edge value as a loose representation of
055 * edge multiplicity, but the {@code *degree()} and mutation methods will not reflect your
056 * interpretation of the edge value as its multiplicity.)
057 *
058 * <h3>Building a {@code ValueGraph}</h3>
059 *
060 * <p>The implementation classes that {@code common.graph} provides are not public, by design. To
061 * create an instance of one of the built-in implementations of {@code ValueGraph}, use the {@link
062 * ValueGraphBuilder} class:
063 *
064 * <pre>{@code
065 * MutableValueGraph<Integer, Double> graph = ValueGraphBuilder.directed().build();
066 * }</pre>
067 *
068 * <p>{@link ValueGraphBuilder#build()} returns an instance of {@link MutableValueGraph}, which is a
069 * subtype of {@code ValueGraph} that provides methods for adding and removing nodes and edges. If
070 * you do not need to mutate a graph (e.g. if you write a method than runs a read-only algorithm on
071 * the graph), you should use the non-mutating {@link ValueGraph} interface, or an {@link
072 * ImmutableValueGraph}.
073 *
074 * <p>You can create an immutable copy of an existing {@code ValueGraph} using {@link
075 * ImmutableValueGraph#copyOf(ValueGraph)}:
076 *
077 * <pre>{@code
078 * ImmutableValueGraph<Integer, Double> immutableGraph = ImmutableValueGraph.copyOf(graph);
079 * }</pre>
080 *
081 * <p>Instances of {@link ImmutableValueGraph} do not implement {@link MutableValueGraph}
082 * (obviously!) and are contractually guaranteed to be unmodifiable and thread-safe.
083 *
084 * <p>The Guava User Guide has <a
085 * href="https://github.com/google/guava/wiki/GraphsExplained#building-graph-instances">more
086 * information on (and examples of) building graphs</a>.
087 *
088 * <h3>Additional documentation</h3>
089 *
090 * <p>See the Guava User Guide for the {@code common.graph} package (<a
091 * href="https://github.com/google/guava/wiki/GraphsExplained">"Graphs Explained"</a>) for
092 * additional documentation, including:
093 *
094 * <ul>
095 *   <li><a
096 *       href="https://github.com/google/guava/wiki/GraphsExplained#equals-hashcode-and-graph-equivalence">
097 *       {@code equals()}, {@code hashCode()}, and graph equivalence</a>
098 *   <li><a href="https://github.com/google/guava/wiki/GraphsExplained#synchronization">
099 *       Synchronization policy</a>
100 *   <li><a href="https://github.com/google/guava/wiki/GraphsExplained#notes-for-implementors">Notes
101 *       for implementors</a>
102 * </ul>
103 *
104 * @author James Sexton
105 * @author Joshua O'Madadhain
106 * @param <N> Node parameter type
107 * @param <V> Value parameter type
108 * @since 20.0
109 */
110@Beta
111public interface ValueGraph<N, V> extends BaseGraph<N> {
112  //
113  // ValueGraph-level accessors
114  //
115
116  /** Returns all nodes in this graph, in the order specified by {@link #nodeOrder()}. */
117  @Override
118  Set<N> nodes();
119
120  /** Returns all edges in this graph. */
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  /**
135   * Returns true if the edges in this graph are directed. Directed edges connect a {@link
136   * EndpointPair#source() source node} to a {@link EndpointPair#target() target node}, while
137   * undirected edges connect a pair of nodes to each other.
138   */
139  @Override
140  boolean isDirected();
141
142  /**
143   * Returns true if this graph allows self-loops (edges that connect a node to itself). Attempting
144   * to add a self-loop to a graph that does not allow them will throw an {@link
145   * IllegalArgumentException}.
146   */
147  @Override
148  boolean allowsSelfLoops();
149
150  /** Returns the order of iteration for the elements of {@link #nodes()}. */
151  @Override
152  ElementOrder<N> nodeOrder();
153
154  /**
155   * Returns an {@link ElementOrder} that specifies the order of iteration for the elements of
156   * {@link #edges()}, {@link #adjacentNodes(Object)}, {@link #predecessors(Object)}, {@link
157   * #successors(Object)} and {@link #incidentEdges(Object)}.
158   *
159   * @since 29.0
160   */
161  @Override
162  ElementOrder<N> incidentEdgeOrder();
163
164  //
165  // Element-level accessors
166  //
167
168  /**
169   * Returns the nodes which have an incident edge in common with {@code node} in this graph.
170   *
171   * <p>This is equal to the union of {@link #predecessors(Object)} and {@link #successors(Object)}.
172   *
173   * @throws IllegalArgumentException if {@code node} is not an element of this graph
174   */
175  @Override
176  Set<N> adjacentNodes(N node);
177
178  /**
179   * Returns all nodes in this graph adjacent to {@code node} which can be reached by traversing
180   * {@code node}'s incoming edges <i>against</i> the direction (if any) of the edge.
181   *
182   * <p>In an undirected graph, this is equivalent to {@link #adjacentNodes(Object)}.
183   *
184   * @throws IllegalArgumentException if {@code node} is not an element of this graph
185   */
186  @Override
187  Set<N> predecessors(N node);
188
189  /**
190   * Returns all nodes in this graph adjacent to {@code node} which can be reached by traversing
191   * {@code node}'s outgoing edges in the direction (if any) of the edge.
192   *
193   * <p>In an undirected graph, this is equivalent to {@link #adjacentNodes(Object)}.
194   *
195   * <p>This is <i>not</i> the same as "all nodes reachable from {@code node} by following outgoing
196   * edges". For that functionality, see {@link Graphs#reachableNodes(Graph, Object)}.
197   *
198   * @throws IllegalArgumentException if {@code node} is not an element of this graph
199   */
200  @Override
201  Set<N> successors(N node);
202
203  /**
204   * Returns the edges in this graph whose endpoints include {@code node}.
205   *
206   * <p>This is equal to the union of incoming and outgoing edges.
207   *
208   * @throws IllegalArgumentException if {@code node} is not an element of this graph
209   * @since 24.0
210   */
211  @Override
212  Set<EndpointPair<N>> incidentEdges(N node);
213
214  /**
215   * Returns the count of {@code node}'s incident edges, counting self-loops twice (equivalently,
216   * the number of times an edge touches {@code node}).
217   *
218   * <p>For directed graphs, this is equal to {@code inDegree(node) + outDegree(node)}.
219   *
220   * <p>For undirected graphs, this is equal to {@code incidentEdges(node).size()} + (number of
221   * self-loops incident to {@code node}).
222   *
223   * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}.
224   *
225   * @throws IllegalArgumentException if {@code node} is not an element of this graph
226   */
227  @Override
228  int degree(N node);
229
230  /**
231   * Returns the count of {@code node}'s incoming edges (equal to {@code predecessors(node).size()})
232   * in a directed graph. In an undirected graph, returns the {@link #degree(Object)}.
233   *
234   * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}.
235   *
236   * @throws IllegalArgumentException if {@code node} is not an element of this graph
237   */
238  @Override
239  int inDegree(N node);
240
241  /**
242   * Returns the count of {@code node}'s outgoing edges (equal to {@code successors(node).size()})
243   * in a directed graph. In an undirected graph, returns the {@link #degree(Object)}.
244   *
245   * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}.
246   *
247   * @throws IllegalArgumentException if {@code node} is not an element of this graph
248   */
249  @Override
250  int outDegree(N node);
251
252  /**
253   * Returns true if there is an edge that directly connects {@code nodeU} to {@code nodeV}. This is
254   * equivalent to {@code nodes().contains(nodeU) && successors(nodeU).contains(nodeV)}.
255   *
256   * <p>In an undirected graph, this is equal to {@code hasEdgeConnecting(nodeV, nodeU)}.
257   *
258   * @since 23.0
259   */
260  @Override
261  boolean hasEdgeConnecting(N nodeU, N nodeV);
262
263  /**
264   * Returns true if there is an edge that directly connects {@code endpoints} (in the order, if
265   * any, specified by {@code endpoints}). This is equivalent to {@code
266   * edges().contains(endpoints)}.
267   *
268   * <p>Unlike the other {@code EndpointPair}-accepting methods, this method does not throw if the
269   * endpoints are unordered and the graph is directed; it simply returns {@code false}. This is for
270   * consistency with the behavior of {@link Collection#contains(Object)} (which does not generally
271   * throw if the object cannot be present in the collection), and the desire to have this method's
272   * behavior be compatible with {@code edges().contains(endpoints)}.
273   *
274   * @since 27.1
275   */
276  @Override
277  boolean hasEdgeConnecting(EndpointPair<N> endpoints);
278
279  /**
280   * Returns the value of the edge that connects {@code nodeU} to {@code nodeV} (in the order, if
281   * any, specified by {@code endpoints}), if one is present; otherwise, returns {@code
282   * Optional.empty()}.
283   *
284   * @throws IllegalArgumentException if {@code nodeU} or {@code nodeV} is not an element of this
285   *     graph
286   * @since 23.0 (since 20.0 with return type {@code V})
287   */
288  Optional<V> edgeValue(N nodeU, N nodeV);
289
290  /**
291   * Returns the value of the edge that connects {@code endpoints} (in the order, if any, specified
292   * by {@code endpoints}), if one is present; otherwise, returns {@code Optional.empty()}.
293   *
294   * <p>If this graph is directed, the endpoints must be ordered.
295   *
296   * @throws IllegalArgumentException if either endpoint is not an element of this graph
297   * @throws IllegalArgumentException if the endpoints are unordered and the graph is directed
298   * @since 27.1
299   */
300  Optional<V> edgeValue(EndpointPair<N> endpoints);
301
302  /**
303   * Returns the value of the edge that connects {@code nodeU} to {@code nodeV}, if one is present;
304   * otherwise, returns {@code defaultValue}.
305   *
306   * <p>In an undirected graph, this is equal to {@code edgeValueOrDefault(nodeV, nodeU,
307   * defaultValue)}.
308   *
309   * @throws IllegalArgumentException if {@code nodeU} or {@code nodeV} is not an element of this
310   *     graph
311   */
312  @Nullable
313  V edgeValueOrDefault(N nodeU, N nodeV, @Nullable V defaultValue);
314
315  /**
316   * Returns the value of the edge that connects {@code endpoints} (in the order, if any, specified
317   * by {@code endpoints}), if one is present; otherwise, returns {@code defaultValue}.
318   *
319   * <p>If this graph is directed, the endpoints must be ordered.
320   *
321   * @throws IllegalArgumentException if either endpoint is not an element of this graph
322   * @throws IllegalArgumentException if the endpoints are unordered and the graph is directed
323   * @since 27.1
324   */
325  @Nullable
326  V edgeValueOrDefault(EndpointPair<N> endpoints, @Nullable V defaultValue);
327
328  //
329  // ValueGraph identity
330  //
331
332  /**
333   * Returns {@code true} iff {@code object} is a {@link ValueGraph} that has the same elements and
334   * the same structural relationships as those in this graph.
335   *
336   * <p>Thus, two value graphs A and B are equal if <b>all</b> of the following are true:
337   *
338   * <ul>
339   *   <li>A and B have equal {@link #isDirected() directedness}.
340   *   <li>A and B have equal {@link #nodes() node sets}.
341   *   <li>A and B have equal {@link #edges() edge sets}.
342   *   <li>The {@link #edgeValue(Object, Object) value} of a given edge is the same in both A and B.
343   * </ul>
344   *
345   * <p>Graph properties besides {@link #isDirected() directedness} do <b>not</b> affect equality.
346   * For example, two graphs may be considered equal even if one allows self-loops and the other
347   * doesn't. Additionally, the order in which nodes or edges are added to the graph, and the order
348   * in which they are iterated over, are irrelevant.
349   *
350   * <p>A reference implementation of this is provided by {@link AbstractValueGraph#equals(Object)}.
351   */
352  @Override
353  boolean equals(@Nullable Object object);
354
355  /**
356   * Returns the hash code for this graph. The hash code of a graph is defined as the hash code of a
357   * map from each of its {@link #edges() edges} to the associated {@link #edgeValue(Object, Object)
358   * edge value}.
359   *
360   * <p>A reference implementation of this is provided by {@link AbstractValueGraph#hashCode()}.
361   */
362  @Override
363  int hashCode();
364}