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.CheckForNull;
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
110@ElementTypesAreNonnullByDefault
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 a live view of the nodes which have an incident edge in common with {@code node} in
170   * this graph.
171   *
172   * <p>This is equal to the union of {@link #predecessors(Object)} and {@link #successors(Object)}.
173   *
174   * <p>If {@code node} is removed from the graph after this method is called, the {@code Set}
175   * {@code view} returned by this method will be invalidated, and will throw {@code
176   * IllegalStateException} if it is accessed in any way, with the following exceptions:
177   *
178   * <ul>
179   *   <li>{@code view.equals(view)} evaluates to {@code true} (but any other `equals()` expression
180   *       involving {@code view} will throw)
181   *   <li>{@code hashCode()} does not throw
182   *   <li>if {@code node} is re-added to the graph after having been removed, {@code view}'s
183   *       behavior is undefined
184   * </ul>
185   *
186   * @throws IllegalArgumentException if {@code node} is not an element of this graph
187   */
188  @Override
189  Set<N> adjacentNodes(N node);
190
191  /**
192   * Returns a live view of all nodes in this graph adjacent to {@code node} which can be reached by
193   * traversing {@code node}'s incoming edges <i>against</i> the direction (if any) of the edge.
194   *
195   * <p>In an undirected graph, this is equivalent to {@link #adjacentNodes(Object)}.
196   *
197   * <p>If {@code node} is removed from the graph after this method is called, the `Set` returned by
198   * this method will be invalidated, and will throw `IllegalStateException` if it is accessed in
199   * any way.
200   *
201   * @throws IllegalArgumentException if {@code node} is not an element of this graph
202   */
203  @Override
204  Set<N> predecessors(N node);
205
206  /**
207   * Returns a live view of all nodes in this graph adjacent to {@code node} which can be reached by
208   * traversing {@code node}'s outgoing edges in the direction (if any) of the edge.
209   *
210   * <p>In an undirected graph, this is equivalent to {@link #adjacentNodes(Object)}.
211   *
212   * <p>This is <i>not</i> the same as "all nodes reachable from {@code node} by following outgoing
213   * edges". For that functionality, see {@link Graphs#reachableNodes(Graph, Object)}.
214   *
215   * <p>If {@code node} is removed from the graph after this method is called, the {@code Set}
216   * {@code view} returned by this method will be invalidated, and will throw {@code
217   * IllegalStateException} if it is accessed in any way, with the following exceptions:
218   *
219   * <ul>
220   *   <li>{@code view.equals(view)} evaluates to {@code true} (but any other `equals()` expression
221   *       involving {@code view} will throw)
222   *   <li>{@code hashCode()} does not throw
223   *   <li>if {@code node} is re-added to the graph after having been removed, {@code view}'s
224   *       behavior is undefined
225   * </ul>
226   *
227   * @throws IllegalArgumentException if {@code node} is not an element of this graph
228   */
229  @Override
230  Set<N> successors(N node);
231
232  /**
233   * Returns a live view of the edges in this graph whose endpoints include {@code node}.
234   *
235   * <p>This is equal to the union of incoming and outgoing edges.
236   *
237   * <p>If {@code node} is removed from the graph after this method is called, the {@code Set}
238   * {@code view} returned by this method will be invalidated, and will throw {@code
239   * IllegalStateException} if it is accessed in any way, with the following exceptions:
240   *
241   * <ul>
242   *   <li>{@code view.equals(view)} evaluates to {@code true} (but any other `equals()` expression
243   *       involving {@code view} will throw)
244   *   <li>{@code hashCode()} does not throw
245   *   <li>if {@code node} is re-added to the graph after having been removed, {@code view}'s
246   *       behavior is undefined
247   * </ul>
248   *
249   * @throws IllegalArgumentException if {@code node} is not an element of this graph
250   * @since 24.0
251   */
252  @Override
253  Set<EndpointPair<N>> incidentEdges(N node);
254
255  /**
256   * Returns the count of {@code node}'s incident edges, counting self-loops twice (equivalently,
257   * the number of times an edge touches {@code node}).
258   *
259   * <p>For directed graphs, this is equal to {@code inDegree(node) + outDegree(node)}.
260   *
261   * <p>For undirected graphs, this is equal to {@code incidentEdges(node).size()} + (number of
262   * self-loops incident to {@code node}).
263   *
264   * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}.
265   *
266   * @throws IllegalArgumentException if {@code node} is not an element of this graph
267   */
268  @Override
269  int degree(N node);
270
271  /**
272   * Returns the count of {@code node}'s incoming edges (equal to {@code predecessors(node).size()})
273   * in a directed graph. In an undirected graph, returns the {@link #degree(Object)}.
274   *
275   * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}.
276   *
277   * @throws IllegalArgumentException if {@code node} is not an element of this graph
278   */
279  @Override
280  int inDegree(N node);
281
282  /**
283   * Returns the count of {@code node}'s outgoing edges (equal to {@code successors(node).size()})
284   * in a directed graph. In an undirected graph, returns the {@link #degree(Object)}.
285   *
286   * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}.
287   *
288   * @throws IllegalArgumentException if {@code node} is not an element of this graph
289   */
290  @Override
291  int outDegree(N node);
292
293  /**
294   * Returns true if there is an edge that directly connects {@code nodeU} to {@code nodeV}. This is
295   * equivalent to {@code nodes().contains(nodeU) && successors(nodeU).contains(nodeV)}.
296   *
297   * <p>In an undirected graph, this is equal to {@code hasEdgeConnecting(nodeV, nodeU)}.
298   *
299   * @since 23.0
300   */
301  @Override
302  boolean hasEdgeConnecting(N nodeU, N nodeV);
303
304  /**
305   * Returns true if there is an edge that directly connects {@code endpoints} (in the order, if
306   * any, specified by {@code endpoints}). This is equivalent to {@code
307   * edges().contains(endpoints)}.
308   *
309   * <p>Unlike the other {@code EndpointPair}-accepting methods, this method does not throw if the
310   * endpoints are unordered and the graph is directed; it simply returns {@code false}. This is for
311   * consistency with the behavior of {@link Collection#contains(Object)} (which does not generally
312   * throw if the object cannot be present in the collection), and the desire to have this method's
313   * behavior be compatible with {@code edges().contains(endpoints)}.
314   *
315   * @since 27.1
316   */
317  @Override
318  boolean hasEdgeConnecting(EndpointPair<N> endpoints);
319
320  /**
321   * Returns the value of the edge that connects {@code nodeU} to {@code nodeV} (in the order, if
322   * any, specified by {@code endpoints}), if one is present; otherwise, returns {@code
323   * Optional.empty()}.
324   *
325   * @throws IllegalArgumentException if {@code nodeU} or {@code nodeV} is not an element of this
326   *     graph
327   * @since 23.0 (since 20.0 with return type {@code V})
328   */
329  Optional<V> edgeValue(N nodeU, N nodeV);
330
331  /**
332   * Returns the value of the edge that connects {@code endpoints} (in the order, if any, specified
333   * by {@code endpoints}), if one is present; otherwise, returns {@code Optional.empty()}.
334   *
335   * <p>If this graph is directed, the endpoints must be ordered.
336   *
337   * @throws IllegalArgumentException if either endpoint is not an element of this graph
338   * @throws IllegalArgumentException if the endpoints are unordered and the graph is directed
339   * @since 27.1
340   */
341  Optional<V> edgeValue(EndpointPair<N> endpoints);
342
343  /**
344   * Returns the value of the edge that connects {@code nodeU} to {@code nodeV}, if one is present;
345   * otherwise, returns {@code defaultValue}.
346   *
347   * <p>In an undirected graph, this is equal to {@code edgeValueOrDefault(nodeV, nodeU,
348   * defaultValue)}.
349   *
350   * @throws IllegalArgumentException if {@code nodeU} or {@code nodeV} is not an element of this
351   *     graph
352   */
353  @CheckForNull
354  V edgeValueOrDefault(N nodeU, N nodeV, @CheckForNull V defaultValue);
355
356  /**
357   * Returns the value of the edge that connects {@code endpoints} (in the order, if any, specified
358   * by {@code endpoints}), if one is present; otherwise, returns {@code defaultValue}.
359   *
360   * <p>If this graph is directed, the endpoints must be ordered.
361   *
362   * @throws IllegalArgumentException if either endpoint is not an element of this graph
363   * @throws IllegalArgumentException if the endpoints are unordered and the graph is directed
364   * @since 27.1
365   */
366  @CheckForNull
367  V edgeValueOrDefault(EndpointPair<N> endpoints, @CheckForNull V defaultValue);
368
369  //
370  // ValueGraph identity
371  //
372
373  /**
374   * Returns {@code true} iff {@code object} is a {@link ValueGraph} that has the same elements and
375   * the same structural relationships as those in this graph.
376   *
377   * <p>Thus, two value graphs A and B are equal if <b>all</b> of the following are true:
378   *
379   * <ul>
380   *   <li>A and B have equal {@link #isDirected() directedness}.
381   *   <li>A and B have equal {@link #nodes() node sets}.
382   *   <li>A and B have equal {@link #edges() edge sets}.
383   *   <li>The {@link #edgeValue(N, N) value} of a given edge is the same in both A and B.
384   * </ul>
385   *
386   * <p>Graph properties besides {@link #isDirected() directedness} do <b>not</b> affect equality.
387   * For example, two graphs may be considered equal even if one allows self-loops and the other
388   * doesn't. Additionally, the order in which nodes or edges are added to the graph, and the order
389   * in which they are iterated over, are irrelevant.
390   *
391   * <p>A reference implementation of this is provided by {@link AbstractValueGraph#equals(Object)}.
392   */
393  @Override
394  boolean equals(@CheckForNull Object object);
395
396  /**
397   * Returns the hash code for this graph. The hash code of a graph is defined as the hash code of a
398   * map from each of its {@link #edges() edges} to the associated {@link #edgeValue(N, N) edge
399   * value}.
400   *
401   * <p>A reference implementation of this is provided by {@link AbstractValueGraph#hashCode()}.
402   */
403  @Override
404  int hashCode();
405}