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 javax.annotation.Nullable;
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 main 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 `common.graph` provides are not public, by design. To create
054 * 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> {
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 UnsupportedOperationException}.
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   * UnsupportedOperationException}.
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(Object 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  Set<N> predecessors(Object node);
175
176  /**
177   * Returns all nodes in this network adjacent to {@code node} which can be reached by traversing
178   * {@code node}'s outgoing edges in the direction (if any) of the edge.
179   *
180   * <p>In an undirected network, this is equivalent to {@link #adjacentNodes(Object)}.
181   *
182   * <p>This is <i>not</i> the same as "all nodes reachable from {@code node} by following outgoing
183   * edges". For that functionality, see {@link Graphs#reachableNodes(Graph, Object)}.
184   *
185   * @throws IllegalArgumentException if {@code node} is not an element of this network
186   */
187  Set<N> successors(Object node);
188
189  /**
190   * Returns the edges whose {@link #incidentNodes(Object) incident nodes} in this network include
191   * {@code node}.
192   *
193   * @throws IllegalArgumentException if {@code node} is not an element of this network
194   */
195  Set<E> incidentEdges(Object node);
196
197  /**
198   * Returns all edges in this network which can be traversed in the direction (if any) of the edge
199   * to end at {@code node}.
200   *
201   * <p>In a directed network, an incoming edge's {@link EndpointPair#target()} equals {@code node}.
202   *
203   * <p>In an undirected network, this is equivalent to {@link #incidentEdges(Object)}.
204   *
205   * @throws IllegalArgumentException if {@code node} is not an element of this network
206   */
207  Set<E> inEdges(Object node);
208
209  /**
210   * Returns all edges in this network which can be traversed in the direction (if any) of the edge
211   * starting from {@code node}.
212   *
213   * <p>In a directed network, an outgoing edge's {@link EndpointPair#source()} equals {@code node}.
214   *
215   * <p>In an undirected network, this is equivalent to {@link #incidentEdges(Object)}.
216   *
217   * @throws IllegalArgumentException if {@code node} is not an element of this network
218   */
219  Set<E> outEdges(Object node);
220
221  /**
222   * Returns the count of {@code node}'s {@link #incidentEdges(Object) incident edges}, counting
223   * self-loops twice (equivalently, the number of times an edge touches {@code node}).
224   *
225   * <p>For directed networks, this is equal to {@code inDegree(node) + outDegree(node)}.
226   *
227   * <p>For undirected networks, this is equal to {@code incidentEdges(node).size()} + (number of
228   * self-loops incident to {@code node}).
229   *
230   * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}.
231   *
232   * @throws IllegalArgumentException if {@code node} is not an element of this network
233   */
234  int degree(Object node);
235
236  /**
237   * Returns the count of {@code node}'s {@link #inEdges(Object) incoming edges} in a directed
238   * network. In an undirected network, returns the {@link #degree(Object)}.
239   *
240   * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}.
241   *
242   * @throws IllegalArgumentException if {@code node} is not an element of this network
243   */
244  int inDegree(Object node);
245
246  /**
247   * Returns the count of {@code node}'s {@link #outEdges(Object) outgoing edges} in a directed
248   * network. In an undirected network, returns the {@link #degree(Object)}.
249   *
250   * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}.
251   *
252   * @throws IllegalArgumentException if {@code node} is not an element of this network
253   */
254  int outDegree(Object node);
255
256  /**
257   * Returns the nodes which are the endpoints of {@code edge} in this network.
258   *
259   * @throws IllegalArgumentException if {@code edge} is not an element of this network
260   */
261  EndpointPair<N> incidentNodes(Object edge);
262
263  /**
264   * Returns the edges which have an {@link #incidentNodes(Object) incident node} in common with
265   * {@code edge}. An edge is not considered adjacent to itself.
266   *
267   * @throws IllegalArgumentException if {@code edge} is not an element of this network
268   */
269  Set<E> adjacentEdges(Object edge);
270
271  /**
272   * Returns the set of edges directly connecting {@code nodeU} to {@code nodeV}.
273   *
274   * <p>In an undirected network, this is equal to {@code edgesConnecting(nodeV, nodeU)}.
275   *
276   * <p>The resulting set of edges will be parallel (i.e. have equal {@link #incidentNodes(Object)}.
277   * If this network does not {@link #allowsParallelEdges() allow parallel edges}, the resulting set
278   * will contain at most one edge.
279   *
280   * @throws IllegalArgumentException if {@code nodeU} or {@code nodeV} is not an element of this
281   *     network
282   */
283  Set<E> edgesConnecting(Object nodeU, Object nodeV);
284
285  //
286  // Network identity
287  //
288
289  /**
290   * For the default {@link Network} implementations, returns true if {@code this == object}
291   * (reference equality). External implementations are free to define this method as they see fit,
292   * as long as they satisfy the {@link Object#equals(Object)} contract.
293   *
294   * <p>To compare two {@link Network}s based on their contents rather than their references, see
295   * {@link Graphs#equivalent(Network, Network)}.
296   */
297  @Override
298  boolean equals(@Nullable Object object);
299
300  /**
301   * For the default {@link Network} implementations, returns {@code System.identityHashCode(this)}.
302   * External implementations are free to define this method as they see fit, as long as they
303   * satisfy the {@link Object#hashCode()} contract.
304   */
305  @Override
306  int hashCode();
307}