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 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 are unique objects.
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 Network} 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 parallel edges
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 are unique objects
049 * </ul>
050 *
051 * <h3>Building a {@code Network}</h3>
052 *
053 * <p>The implementation classes that {@code common.graph} provides are not public, by design. To
054 * create an instance of one of the built-in implementations of {@code Network}, use the {@link
055 * NetworkBuilder} class:
056 *
057 * <pre>{@code
058 * MutableNetwork<Integer, MyEdge> graph = NetworkBuilder.directed().build();
059 * }</pre>
060 *
061 * <p>{@link NetworkBuilder#build()} returns an instance of {@link MutableNetwork}, which is a
062 * subtype of {@code Network} that provides methods for adding and removing nodes and edges. If you
063 * do not need to mutate a graph (e.g. if you write a method than runs a read-only algorithm on the
064 * graph), you should use the non-mutating {@link Network} interface, or an {@link
065 * ImmutableNetwork}.
066 *
067 * <p>You can create an immutable copy of an existing {@code Network} using {@link
068 * ImmutableNetwork#copyOf(Network)}:
069 *
070 * <pre>{@code
071 * ImmutableNetwork<Integer, MyEdge> immutableGraph = ImmutableNetwork.copyOf(graph);
072 * }</pre>
073 *
074 * <p>Instances of {@link ImmutableNetwork} do not implement {@link MutableNetwork} (obviously!) and
075 * are contractually guaranteed to be unmodifiable and thread-safe.
076 *
077 * <p>The Guava User Guide has <a
078 * href="https://github.com/google/guava/wiki/GraphsExplained#building-graph-instances">more
079 * information on (and examples of) building graphs</a>.
080 *
081 * <h3>Additional documentation</h3>
082 *
083 * <p>See the Guava User Guide for the {@code common.graph} package (<a
084 * href="https://github.com/google/guava/wiki/GraphsExplained">"Graphs Explained"</a>) for
085 * additional documentation, including:
086 *
087 * <ul>
088 *   <li><a
089 *       href="https://github.com/google/guava/wiki/GraphsExplained#equals-hashcode-and-graph-equivalence">
090 *       {@code equals()}, {@code hashCode()}, and graph equivalence</a>
091 *   <li><a href="https://github.com/google/guava/wiki/GraphsExplained#synchronization">
092 *       Synchronization policy</a>
093 *   <li><a href="https://github.com/google/guava/wiki/GraphsExplained#notes-for-implementors">Notes
094 *       for implementors</a>
095 * </ul>
096 *
097 * @author James Sexton
098 * @author Joshua O'Madadhain
099 * @param <N> Node parameter type
100 * @param <E> Edge parameter type
101 * @since 20.0
102 */
103@Beta
104public interface Network<N, E> extends SuccessorsFunction<N>, PredecessorsFunction<N> {
105  //
106  // Network-level accessors
107  //
108
109  /** Returns all nodes in this network, in the order specified by {@link #nodeOrder()}. */
110  Set<N> nodes();
111
112  /** Returns all edges in this network, in the order specified by {@link #edgeOrder()}. */
113  Set<E> edges();
114
115  /**
116   * Returns a live view of this network as a {@link Graph}. The resulting {@link Graph} will have
117   * an edge connecting node A to node B if this {@link Network} has an edge connecting A to B.
118   *
119   * <p>If this network {@link #allowsParallelEdges() allows parallel edges}, parallel edges will be
120   * treated as if collapsed into a single edge. For example, the {@link #degree(Object)} of a node
121   * in the {@link Graph} view may be less than the degree of the same node in this {@link Network}.
122   */
123  Graph<N> asGraph();
124
125  //
126  // Network properties
127  //
128
129  /**
130   * Returns true if the edges in this network are directed. Directed edges connect a {@link
131   * EndpointPair#source() source node} to a {@link EndpointPair#target() target node}, while
132   * undirected edges connect a pair of nodes to each other.
133   */
134  boolean isDirected();
135
136  /**
137   * Returns true if this network allows parallel edges. Attempting to add a parallel edge to a
138   * network that does not allow them will throw an {@link IllegalArgumentException}.
139   */
140  boolean allowsParallelEdges();
141
142  /**
143   * Returns true if this network allows self-loops (edges that connect a node to itself).
144   * Attempting to add a self-loop to a network that does not allow them will throw an {@link
145   * IllegalArgumentException}.
146   */
147  boolean allowsSelfLoops();
148
149  /** Returns the order of iteration for the elements of {@link #nodes()}. */
150  ElementOrder<N> nodeOrder();
151
152  /** Returns the order of iteration for the elements of {@link #edges()}. */
153  ElementOrder<E> edgeOrder();
154
155  //
156  // Element-level accessors
157  //
158
159  /**
160   * Returns the nodes which have an incident edge in common with {@code node} in this network.
161   *
162   * @throws IllegalArgumentException if {@code node} is not an element of this network
163   */
164  Set<N> adjacentNodes(N node);
165
166  /**
167   * Returns all nodes in this network adjacent to {@code node} which can be reached by traversing
168   * {@code node}'s incoming edges <i>against</i> the direction (if any) of the edge.
169   *
170   * <p>In an undirected network, this is equivalent to {@link #adjacentNodes(Object)}.
171   *
172   * @throws IllegalArgumentException if {@code node} is not an element of this network
173   */
174  @Override
175  Set<N> predecessors(N node);
176
177  /**
178   * Returns all nodes in this network adjacent to {@code node} which can be reached by traversing
179   * {@code node}'s outgoing edges in the direction (if any) of the edge.
180   *
181   * <p>In an undirected network, this is equivalent to {@link #adjacentNodes(Object)}.
182   *
183   * <p>This is <i>not</i> the same as "all nodes reachable from {@code node} by following outgoing
184   * edges". For that functionality, see {@link Graphs#reachableNodes(Graph, Object)}.
185   *
186   * @throws IllegalArgumentException if {@code node} is not an element of this network
187   */
188  @Override
189  Set<N> successors(N node);
190
191  /**
192   * Returns the edges whose {@link #incidentNodes(Object) incident nodes} in this network include
193   * {@code node}.
194   *
195   * @throws IllegalArgumentException if {@code node} is not an element of this network
196   */
197  Set<E> incidentEdges(N node);
198
199  /**
200   * Returns all edges in this network which can be traversed in the direction (if any) of the edge
201   * to end at {@code node}.
202   *
203   * <p>In a directed network, an incoming edge's {@link EndpointPair#target()} equals {@code node}.
204   *
205   * <p>In an undirected network, this is equivalent to {@link #incidentEdges(Object)}.
206   *
207   * @throws IllegalArgumentException if {@code node} is not an element of this network
208   */
209  Set<E> inEdges(N node);
210
211  /**
212   * Returns all edges in this network which can be traversed in the direction (if any) of the edge
213   * starting from {@code node}.
214   *
215   * <p>In a directed network, an outgoing edge's {@link EndpointPair#source()} equals {@code node}.
216   *
217   * <p>In an undirected network, this is equivalent to {@link #incidentEdges(Object)}.
218   *
219   * @throws IllegalArgumentException if {@code node} is not an element of this network
220   */
221  Set<E> outEdges(N node);
222
223  /**
224   * Returns the count of {@code node}'s {@link #incidentEdges(Object) incident edges}, counting
225   * self-loops twice (equivalently, the number of times an edge touches {@code node}).
226   *
227   * <p>For directed networks, this is equal to {@code inDegree(node) + outDegree(node)}.
228   *
229   * <p>For undirected networks, this is equal to {@code incidentEdges(node).size()} + (number of
230   * self-loops incident to {@code node}).
231   *
232   * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}.
233   *
234   * @throws IllegalArgumentException if {@code node} is not an element of this network
235   */
236  int degree(N node);
237
238  /**
239   * Returns the count of {@code node}'s {@link #inEdges(Object) incoming edges} in a directed
240   * network. In an undirected network, returns the {@link #degree(Object)}.
241   *
242   * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}.
243   *
244   * @throws IllegalArgumentException if {@code node} is not an element of this network
245   */
246  int inDegree(N node);
247
248  /**
249   * Returns the count of {@code node}'s {@link #outEdges(Object) outgoing edges} in a directed
250   * network. In an undirected network, returns the {@link #degree(Object)}.
251   *
252   * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}.
253   *
254   * @throws IllegalArgumentException if {@code node} is not an element of this network
255   */
256  int outDegree(N node);
257
258  /**
259   * Returns the nodes which are the endpoints of {@code edge} in this network.
260   *
261   * @throws IllegalArgumentException if {@code edge} is not an element of this network
262   */
263  EndpointPair<N> incidentNodes(E edge);
264
265  /**
266   * Returns the edges which have an {@link #incidentNodes(Object) incident node} in common with
267   * {@code edge}. An edge is not considered adjacent to itself.
268   *
269   * @throws IllegalArgumentException if {@code edge} is not an element of this network
270   */
271  Set<E> adjacentEdges(E edge);
272
273  /**
274   * Returns the set of edges directly connecting {@code nodeU} to {@code nodeV}.
275   *
276   * <p>In an undirected network, this is equal to {@code edgesConnecting(nodeV, nodeU)}.
277   *
278   * <p>The resulting set of edges will be parallel (i.e. have equal {@link #incidentNodes(Object)}.
279   * If this network does not {@link #allowsParallelEdges() allow parallel edges}, the resulting set
280   * will contain at most one edge (equivalent to {@code edgeConnecting(nodeU, nodeV).asSet()}).
281   *
282   * @throws IllegalArgumentException if {@code nodeU} or {@code nodeV} is not an element of this
283   *     network
284   */
285  Set<E> edgesConnecting(N nodeU, N nodeV);
286
287  /**
288   * Returns the single edge directly connecting {@code nodeU} to {@code nodeV}, if one is present,
289   * or {@code null} if no such edge exists.
290   *
291   * <p>In an undirected network, this is equal to {@code edgeConnectingOrNull(nodeV, nodeU)}.
292   *
293   * @throws IllegalArgumentException if there are multiple parallel edges connecting {@code nodeU}
294   *     to {@code nodeV}
295   * @throws IllegalArgumentException if {@code nodeU} or {@code nodeV} is not an element of this
296   *     network
297   * @since 23.0
298   */
299  @NullableDecl
300  E edgeConnectingOrNull(N nodeU, N nodeV);
301
302  /**
303   * Returns true if there is an edge directly connecting {@code nodeU} to {@code nodeV}. This is
304   * equivalent to {@code nodes().contains(nodeU) && successors(nodeU).contains(nodeV)}, and to
305   * {@code edgeConnectingOrNull(nodeU, nodeV) != null}.
306   *
307   * <p>In an undirected graph, this is equal to {@code hasEdgeConnecting(nodeV, nodeU)}.
308   *
309   * @since 23.0
310   */
311  boolean hasEdgeConnecting(N nodeU, N nodeV);
312
313  //
314  // Network identity
315  //
316
317  /**
318   * Returns {@code true} iff {@code object} is a {@link Network} that has the same elements and the
319   * same structural relationships as those in this network.
320   *
321   * <p>Thus, two networks A and B are equal if <b>all</b> of the following are true:
322   *
323   * <ul>
324   *   <li>A and B have equal {@link #isDirected() directedness}.
325   *   <li>A and B have equal {@link #nodes() node sets}.
326   *   <li>A and B have equal {@link #edges() edge sets}.
327   *   <li>Every edge in A and B connects the same nodes in the same direction (if any).
328   * </ul>
329   *
330   * <p>Network properties besides {@link #isDirected() directedness} do <b>not</b> affect equality.
331   * For example, two networks may be considered equal even if one allows parallel edges and the
332   * other doesn't. Additionally, the order in which nodes or edges are added to the network, and
333   * the order in which they are iterated over, are irrelevant.
334   *
335   * <p>A reference implementation of this is provided by {@link AbstractNetwork#equals(Object)}.
336   */
337  @Override
338  boolean equals(@NullableDecl Object object);
339
340  /**
341   * Returns the hash code for this network. The hash code of a network is defined as the hash code
342   * of a map from each of its {@link #edges() edges} to their {@link #incidentNodes(Object)
343   * incident nodes}.
344   *
345   * <p>A reference implementation of this is provided by {@link AbstractNetwork#hashCode()}.
346   */
347  @Override
348  int hashCode();
349}