001/*
002 * Copyright (C) 2014 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 com.google.errorprone.annotations.DoNotMock;
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 are unique objects.
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 Network} 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 parallel edges
048 *   <li>graphs that do/don't allow self-loops
049 *   <li>graphs whose nodes/edges are insertion-ordered, sorted, or unordered
050 *   <li>graphs whose edges are unique objects
051 * </ul>
052 *
053 * <h3>Building a {@code Network}</h3>
054 *
055 * <p>The implementation classes that {@code common.graph} provides are not public, by design. To
056 * create an instance of one of the built-in implementations of {@code Network}, use the {@link
057 * NetworkBuilder} class:
058 *
059 * <pre>{@code
060 * MutableNetwork<Integer, MyEdge> graph = NetworkBuilder.directed().build();
061 * }</pre>
062 *
063 * <p>{@link NetworkBuilder#build()} returns an instance of {@link MutableNetwork}, which is a
064 * subtype of {@code Network} that provides methods for adding and removing nodes and edges. If you
065 * do not need to mutate a graph (e.g. if you write a method than runs a read-only algorithm on the
066 * graph), you should use the non-mutating {@link Network} interface, or an {@link
067 * ImmutableNetwork}.
068 *
069 * <p>You can create an immutable copy of an existing {@code Network} using {@link
070 * ImmutableNetwork#copyOf(Network)}:
071 *
072 * <pre>{@code
073 * ImmutableNetwork<Integer, MyEdge> immutableGraph = ImmutableNetwork.copyOf(graph);
074 * }</pre>
075 *
076 * <p>Instances of {@link ImmutableNetwork} do not implement {@link MutableNetwork} (obviously!) and
077 * are contractually guaranteed to be unmodifiable and thread-safe.
078 *
079 * <p>The Guava User Guide has <a
080 * href="https://github.com/google/guava/wiki/GraphsExplained#building-graph-instances">more
081 * information on (and examples of) building graphs</a>.
082 *
083 * <h3>Additional documentation</h3>
084 *
085 * <p>See the Guava User Guide for the {@code common.graph} package (<a
086 * href="https://github.com/google/guava/wiki/GraphsExplained">"Graphs Explained"</a>) for
087 * additional documentation, including:
088 *
089 * <ul>
090 *   <li><a
091 *       href="https://github.com/google/guava/wiki/GraphsExplained#equals-hashcode-and-graph-equivalence">
092 *       {@code equals()}, {@code hashCode()}, and graph equivalence</a>
093 *   <li><a href="https://github.com/google/guava/wiki/GraphsExplained#synchronization">
094 *       Synchronization policy</a>
095 *   <li><a href="https://github.com/google/guava/wiki/GraphsExplained#notes-for-implementors">Notes
096 *       for implementors</a>
097 * </ul>
098 *
099 * @author James Sexton
100 * @author Joshua O'Madadhain
101 * @param <N> Node parameter type
102 * @param <E> Edge parameter type
103 * @since 20.0
104 */
105@Beta
106@DoNotMock("Use NetworkBuilder to create a real instance")
107public interface Network<N, E> extends SuccessorsFunction<N>, PredecessorsFunction<N> {
108  //
109  // Network-level accessors
110  //
111
112  /** Returns all nodes in this network, in the order specified by {@link #nodeOrder()}. */
113  Set<N> nodes();
114
115  /** Returns all edges in this network, in the order specified by {@link #edgeOrder()}. */
116  Set<E> edges();
117
118  /**
119   * Returns a live view of this network as a {@link Graph}. The resulting {@link Graph} will have
120   * an edge connecting node A to node B if this {@link Network} has an edge connecting A to B.
121   *
122   * <p>If this network {@link #allowsParallelEdges() allows parallel edges}, parallel edges will be
123   * treated as if collapsed into a single edge. For example, the {@link #degree(Object)} of a node
124   * in the {@link Graph} view may be less than the degree of the same node in this {@link Network}.
125   */
126  Graph<N> asGraph();
127
128  //
129  // Network properties
130  //
131
132  /**
133   * Returns true if the edges in this network 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  boolean isDirected();
138
139  /**
140   * Returns true if this network allows parallel edges. Attempting to add a parallel edge to a
141   * network that does not allow them will throw an {@link IllegalArgumentException}.
142   */
143  boolean allowsParallelEdges();
144
145  /**
146   * Returns true if this network allows self-loops (edges that connect a node to itself).
147   * Attempting to add a self-loop to a network that does not allow them will throw an {@link
148   * IllegalArgumentException}.
149   */
150  boolean allowsSelfLoops();
151
152  /** Returns the order of iteration for the elements of {@link #nodes()}. */
153  ElementOrder<N> nodeOrder();
154
155  /** Returns the order of iteration for the elements of {@link #edges()}. */
156  ElementOrder<E> edgeOrder();
157
158  //
159  // Element-level accessors
160  //
161
162  /**
163   * Returns the nodes which have an incident edge in common with {@code node} in this network.
164   *
165   * <p>This is equal to the union of {@link #predecessors(Object)} and {@link #successors(Object)}.
166   *
167   * @throws IllegalArgumentException if {@code node} is not an element of this network
168   */
169  Set<N> adjacentNodes(N node);
170
171  /**
172   * Returns all nodes in this network adjacent to {@code node} which can be reached by traversing
173   * {@code node}'s incoming edges <i>against</i> the direction (if any) of the edge.
174   *
175   * <p>In an undirected network, this is equivalent to {@link #adjacentNodes(Object)}.
176   *
177   * @throws IllegalArgumentException if {@code node} is not an element of this network
178   */
179  @Override
180  Set<N> predecessors(N node);
181
182  /**
183   * Returns all nodes in this network adjacent to {@code node} which can be reached by traversing
184   * {@code node}'s outgoing edges in the direction (if any) of the edge.
185   *
186   * <p>In an undirected network, this is equivalent to {@link #adjacentNodes(Object)}.
187   *
188   * <p>This is <i>not</i> the same as "all nodes reachable from {@code node} by following outgoing
189   * edges". For that functionality, see {@link Graphs#reachableNodes(Graph, Object)}.
190   *
191   * @throws IllegalArgumentException if {@code node} is not an element of this network
192   */
193  @Override
194  Set<N> successors(N node);
195
196  /**
197   * Returns the edges whose {@link #incidentNodes(Object) incident nodes} in this network include
198   * {@code node}.
199   *
200   * <p>This is equal to the union of {@link #inEdges(Object)} and {@link #outEdges(Object)}.
201   *
202   * @throws IllegalArgumentException if {@code node} is not an element of this network
203   */
204  Set<E> incidentEdges(N node);
205
206  /**
207   * Returns all edges in this network which can be traversed in the direction (if any) of the edge
208   * to end at {@code node}.
209   *
210   * <p>In a directed network, an incoming edge's {@link EndpointPair#target()} equals {@code node}.
211   *
212   * <p>In an undirected network, this is equivalent to {@link #incidentEdges(Object)}.
213   *
214   * @throws IllegalArgumentException if {@code node} is not an element of this network
215   */
216  Set<E> inEdges(N node);
217
218  /**
219   * Returns all edges in this network which can be traversed in the direction (if any) of the edge
220   * starting from {@code node}.
221   *
222   * <p>In a directed network, an outgoing edge's {@link EndpointPair#source()} equals {@code node}.
223   *
224   * <p>In an undirected network, this is equivalent to {@link #incidentEdges(Object)}.
225   *
226   * @throws IllegalArgumentException if {@code node} is not an element of this network
227   */
228  Set<E> outEdges(N node);
229
230  /**
231   * Returns the count of {@code node}'s {@link #incidentEdges(Object) incident edges}, counting
232   * self-loops twice (equivalently, the number of times an edge touches {@code node}).
233   *
234   * <p>For directed networks, this is equal to {@code inDegree(node) + outDegree(node)}.
235   *
236   * <p>For undirected networks, this is equal to {@code incidentEdges(node).size()} + (number of
237   * self-loops incident to {@code node}).
238   *
239   * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}.
240   *
241   * @throws IllegalArgumentException if {@code node} is not an element of this network
242   */
243  int degree(N node);
244
245  /**
246   * Returns the count of {@code node}'s {@link #inEdges(Object) incoming edges} in a directed
247   * network. In an undirected network, returns the {@link #degree(Object)}.
248   *
249   * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}.
250   *
251   * @throws IllegalArgumentException if {@code node} is not an element of this network
252   */
253  int inDegree(N node);
254
255  /**
256   * Returns the count of {@code node}'s {@link #outEdges(Object) outgoing edges} in a directed
257   * network. In an undirected network, returns the {@link #degree(Object)}.
258   *
259   * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}.
260   *
261   * @throws IllegalArgumentException if {@code node} is not an element of this network
262   */
263  int outDegree(N node);
264
265  /**
266   * Returns the nodes which are the endpoints of {@code edge} in this network.
267   *
268   * @throws IllegalArgumentException if {@code edge} is not an element of this network
269   */
270  EndpointPair<N> incidentNodes(E edge);
271
272  /**
273   * Returns the edges which have an {@link #incidentNodes(Object) incident node} in common with
274   * {@code edge}. An edge is not considered adjacent to itself.
275   *
276   * @throws IllegalArgumentException if {@code edge} is not an element of this network
277   */
278  Set<E> adjacentEdges(E edge);
279
280  /**
281   * Returns the set of edges that each directly connect {@code nodeU} to {@code nodeV}.
282   *
283   * <p>In an undirected network, this is equal to {@code edgesConnecting(nodeV, nodeU)}.
284   *
285   * <p>The resulting set of edges will be parallel (i.e. have equal {@link #incidentNodes(Object)}.
286   * If this network does not {@link #allowsParallelEdges() allow parallel edges}, the resulting set
287   * will contain at most one edge (equivalent to {@code edgeConnecting(nodeU, nodeV).asSet()}).
288   *
289   * @throws IllegalArgumentException if {@code nodeU} or {@code nodeV} is not an element of this
290   *     network
291   */
292  Set<E> edgesConnecting(N nodeU, N nodeV);
293
294  /**
295   * Returns the set of edges that each directly connect {@code endpoints} (in the order, if any,
296   * specified by {@code endpoints}).
297   *
298   * <p>The resulting set of edges will be parallel (i.e. have equal {@link #incidentNodes(Object)}.
299   * If this network does not {@link #allowsParallelEdges() allow parallel edges}, the resulting set
300   * will contain at most one edge (equivalent to {@code edgeConnecting(endpoints).asSet()}).
301   *
302   * <p>If this network is directed, {@code endpoints} must be ordered.
303   *
304   * @throws IllegalArgumentException if either endpoint is not an element of this network
305   * @throws IllegalArgumentException if the endpoints are unordered and the graph is directed
306   * @since 27.1
307   */
308  Set<E> edgesConnecting(EndpointPair<N> endpoints);
309
310  /**
311   * Returns the single edge that directly connects {@code nodeU} to {@code nodeV}, if one is
312   * present, or {@code Optional.empty()} if no such edge exists.
313   *
314   * <p>In an undirected network, this is equal to {@code edgeConnecting(nodeV, nodeU)}.
315   *
316   * @throws IllegalArgumentException if there are multiple parallel edges connecting {@code nodeU}
317   *     to {@code nodeV}
318   * @throws IllegalArgumentException if {@code nodeU} or {@code nodeV} is not an element of this
319   *     network
320   * @since 23.0
321   */
322  Optional<E> edgeConnecting(N nodeU, N nodeV);
323
324  /**
325   * Returns the single edge that directly connects {@code endpoints} (in the order, if any,
326   * specified by {@code endpoints}), if one is present, or {@code Optional.empty()} if no such edge
327   * exists.
328   *
329   * <p>If this graph is directed, the endpoints must be ordered.
330   *
331   * @throws IllegalArgumentException if there are multiple parallel edges connecting {@code nodeU}
332   *     to {@code nodeV}
333   * @throws IllegalArgumentException if either endpoint is not an element of this network
334   * @throws IllegalArgumentException if the endpoints are unordered and the graph is directed
335   * @since 27.1
336   */
337  Optional<E> edgeConnecting(EndpointPair<N> endpoints);
338
339  /**
340   * Returns the single edge that directly connects {@code nodeU} to {@code nodeV}, if one is
341   * present, or {@code null} if no such edge exists.
342   *
343   * <p>In an undirected network, this is equal to {@code edgeConnectingOrNull(nodeV, nodeU)}.
344   *
345   * @throws IllegalArgumentException if there are multiple parallel edges connecting {@code nodeU}
346   *     to {@code nodeV}
347   * @throws IllegalArgumentException if {@code nodeU} or {@code nodeV} is not an element of this
348   *     network
349   * @since 23.0
350   */
351  @Nullable
352  E edgeConnectingOrNull(N nodeU, N nodeV);
353
354  /**
355   * Returns the single edge that directly connects {@code endpoints} (in the order, if any,
356   * specified by {@code endpoints}), if one is present, or {@code null} if no such edge exists.
357   *
358   * <p>If this graph is directed, the endpoints must be ordered.
359   *
360   * @throws IllegalArgumentException if there are multiple parallel edges connecting {@code nodeU}
361   *     to {@code nodeV}
362   * @throws IllegalArgumentException if either endpoint is not an element of this network
363   * @throws IllegalArgumentException if the endpoints are unordered and the graph is directed
364   * @since 27.1
365   */
366  @Nullable
367  E edgeConnectingOrNull(EndpointPair<N> endpoints);
368
369  /**
370   * Returns true if there is an edge that directly connects {@code nodeU} to {@code nodeV}. This is
371   * equivalent to {@code nodes().contains(nodeU) && successors(nodeU).contains(nodeV)}, and to
372   * {@code edgeConnectingOrNull(nodeU, nodeV) != null}.
373   *
374   * <p>In an undirected graph, this is equal to {@code hasEdgeConnecting(nodeV, nodeU)}.
375   *
376   * @since 23.0
377   */
378  boolean hasEdgeConnecting(N nodeU, N nodeV);
379
380  /**
381   * Returns true if there is an edge that directly connects {@code endpoints} (in the order, if
382   * any, specified by {@code endpoints}).
383   *
384   * <p>Unlike the other {@code EndpointPair}-accepting methods, this method does not throw if the
385   * endpoints are unordered and the graph is directed; it simply returns {@code false}. This is for
386   * consistency with {@link Graph#hasEdgeConnecting(EndpointPair)} and {@link
387   * ValueGraph#hasEdgeConnecting(EndpointPair)}.
388   *
389   * @since 27.1
390   */
391  boolean hasEdgeConnecting(EndpointPair<N> endpoints);
392
393  //
394  // Network identity
395  //
396
397  /**
398   * Returns {@code true} iff {@code object} is a {@link Network} that has the same elements and the
399   * same structural relationships as those in this network.
400   *
401   * <p>Thus, two networks A and B are equal if <b>all</b> of the following are true:
402   *
403   * <ul>
404   *   <li>A and B have equal {@link #isDirected() directedness}.
405   *   <li>A and B have equal {@link #nodes() node sets}.
406   *   <li>A and B have equal {@link #edges() edge sets}.
407   *   <li>Every edge in A and B connects the same nodes in the same direction (if any).
408   * </ul>
409   *
410   * <p>Network properties besides {@link #isDirected() directedness} do <b>not</b> affect equality.
411   * For example, two networks may be considered equal even if one allows parallel edges and the
412   * other doesn't. Additionally, the order in which nodes or edges are added to the network, and
413   * the order in which they are iterated over, are irrelevant.
414   *
415   * <p>A reference implementation of this is provided by {@link AbstractNetwork#equals(Object)}.
416   */
417  @Override
418  boolean equals(@Nullable Object object);
419
420  /**
421   * Returns the hash code for this network. The hash code of a network is defined as the hash code
422   * of a map from each of its {@link #edges() edges} to their {@link #incidentNodes(Object)
423   * incident nodes}.
424   *
425   * <p>A reference implementation of this is provided by {@link AbstractNetwork#hashCode()}.
426   */
427  @Override
428  int hashCode();
429}