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.Optional; 021import java.util.Set; 022import org.checkerframework.checker.nullness.qual.Nullable; 023 024/** 025 * An interface for <a 026 * href="https://en.wikipedia.org/wiki/Graph_(discrete_mathematics)">graph</a>-structured data, 027 * whose edges are unique objects. 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 Network} 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 parallel edges 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 are unique objects 050 * </ul> 051 * 052 * <h3>Building a {@code Network}</h3> 053 * 054 * <p>The implementation classes that {@code common.graph} provides are not public, by design. To 055 * create an instance of one of the built-in implementations of {@code Network}, use the {@link 056 * NetworkBuilder} class: 057 * 058 * <pre>{@code 059 * MutableNetwork<Integer, MyEdge> graph = NetworkBuilder.directed().build(); 060 * }</pre> 061 * 062 * <p>{@link NetworkBuilder#build()} returns an instance of {@link MutableNetwork}, which is a 063 * subtype of {@code Network} that provides methods for adding and removing nodes and edges. If you 064 * do not need to mutate a graph (e.g. if you write a method than runs a read-only algorithm on the 065 * graph), you should use the non-mutating {@link Network} interface, or an {@link 066 * ImmutableNetwork}. 067 * 068 * <p>You can create an immutable copy of an existing {@code Network} using {@link 069 * ImmutableNetwork#copyOf(Network)}: 070 * 071 * <pre>{@code 072 * ImmutableNetwork<Integer, MyEdge> immutableGraph = ImmutableNetwork.copyOf(graph); 073 * }</pre> 074 * 075 * <p>Instances of {@link ImmutableNetwork} do not implement {@link MutableNetwork} (obviously!) and 076 * are contractually guaranteed to be unmodifiable and thread-safe. 077 * 078 * <p>The Guava User Guide has <a 079 * href="https://github.com/google/guava/wiki/GraphsExplained#building-graph-instances">more 080 * information on (and examples of) building graphs</a>. 081 * 082 * <h3>Additional documentation</h3> 083 * 084 * <p>See the Guava User Guide for the {@code common.graph} package (<a 085 * href="https://github.com/google/guava/wiki/GraphsExplained">"Graphs Explained"</a>) for 086 * additional documentation, including: 087 * 088 * <ul> 089 * <li><a 090 * href="https://github.com/google/guava/wiki/GraphsExplained#equals-hashcode-and-graph-equivalence"> 091 * {@code equals()}, {@code hashCode()}, and graph equivalence</a> 092 * <li><a href="https://github.com/google/guava/wiki/GraphsExplained#synchronization"> 093 * Synchronization policy</a> 094 * <li><a href="https://github.com/google/guava/wiki/GraphsExplained#notes-for-implementors">Notes 095 * for implementors</a> 096 * </ul> 097 * 098 * @author James Sexton 099 * @author Joshua O'Madadhain 100 * @param <N> Node parameter type 101 * @param <E> Edge parameter type 102 * @since 20.0 103 */ 104@Beta 105public interface Network<N, E> extends SuccessorsFunction<N>, PredecessorsFunction<N> { 106 // 107 // Network-level accessors 108 // 109 110 /** Returns all nodes in this network, in the order specified by {@link #nodeOrder()}. */ 111 Set<N> nodes(); 112 113 /** Returns all edges in this network, in the order specified by {@link #edgeOrder()}. */ 114 Set<E> edges(); 115 116 /** 117 * Returns a live view of this network as a {@link Graph}. The resulting {@link Graph} will have 118 * an edge connecting node A to node B if this {@link Network} has an edge connecting A to B. 119 * 120 * <p>If this network {@link #allowsParallelEdges() allows parallel edges}, parallel edges will be 121 * treated as if collapsed into a single edge. For example, the {@link #degree(Object)} of a node 122 * in the {@link Graph} view may be less than the degree of the same node in this {@link Network}. 123 */ 124 Graph<N> asGraph(); 125 126 // 127 // Network properties 128 // 129 130 /** 131 * Returns true if the edges in this network are directed. Directed edges connect a {@link 132 * EndpointPair#source() source node} to a {@link EndpointPair#target() target node}, while 133 * undirected edges connect a pair of nodes to each other. 134 */ 135 boolean isDirected(); 136 137 /** 138 * Returns true if this network allows parallel edges. Attempting to add a parallel edge to a 139 * network that does not allow them will throw an {@link IllegalArgumentException}. 140 */ 141 boolean allowsParallelEdges(); 142 143 /** 144 * Returns true if this network allows self-loops (edges that connect a node to itself). 145 * Attempting to add a self-loop to a network that does not allow them will throw an {@link 146 * IllegalArgumentException}. 147 */ 148 boolean allowsSelfLoops(); 149 150 /** Returns the order of iteration for the elements of {@link #nodes()}. */ 151 ElementOrder<N> nodeOrder(); 152 153 /** Returns the order of iteration for the elements of {@link #edges()}. */ 154 ElementOrder<E> edgeOrder(); 155 156 // 157 // Element-level accessors 158 // 159 160 /** 161 * Returns the nodes which have an incident edge in common with {@code node} in this network. 162 * 163 * <p>This is equal to the union of {@link #predecessors(Object)} and {@link #successors(Object)}. 164 * 165 * @throws IllegalArgumentException if {@code node} is not an element of this network 166 */ 167 Set<N> adjacentNodes(N node); 168 169 /** 170 * Returns all nodes in this network adjacent to {@code node} which can be reached by traversing 171 * {@code node}'s incoming edges <i>against</i> the direction (if any) of the edge. 172 * 173 * <p>In an undirected network, this is equivalent to {@link #adjacentNodes(Object)}. 174 * 175 * @throws IllegalArgumentException if {@code node} is not an element of this network 176 */ 177 @Override 178 Set<N> predecessors(N node); 179 180 /** 181 * Returns all nodes in this network adjacent to {@code node} which can be reached by traversing 182 * {@code node}'s outgoing edges in the direction (if any) of the edge. 183 * 184 * <p>In an undirected network, this is equivalent to {@link #adjacentNodes(Object)}. 185 * 186 * <p>This is <i>not</i> the same as "all nodes reachable from {@code node} by following outgoing 187 * edges". For that functionality, see {@link Graphs#reachableNodes(Graph, Object)}. 188 * 189 * @throws IllegalArgumentException if {@code node} is not an element of this network 190 */ 191 @Override 192 Set<N> successors(N node); 193 194 /** 195 * Returns the edges whose {@link #incidentNodes(Object) incident nodes} in this network include 196 * {@code node}. 197 * 198 * <p>This is equal to the union of {@link #inEdges(Object)} and {@link #outEdges(Object)}. 199 * 200 * @throws IllegalArgumentException if {@code node} is not an element of this network 201 */ 202 Set<E> incidentEdges(N node); 203 204 /** 205 * Returns all edges in this network which can be traversed in the direction (if any) of the edge 206 * to end at {@code node}. 207 * 208 * <p>In a directed network, an incoming edge's {@link EndpointPair#target()} equals {@code node}. 209 * 210 * <p>In an undirected network, this is equivalent to {@link #incidentEdges(Object)}. 211 * 212 * @throws IllegalArgumentException if {@code node} is not an element of this network 213 */ 214 Set<E> inEdges(N node); 215 216 /** 217 * Returns all edges in this network which can be traversed in the direction (if any) of the edge 218 * starting from {@code node}. 219 * 220 * <p>In a directed network, an outgoing edge's {@link EndpointPair#source()} equals {@code node}. 221 * 222 * <p>In an undirected network, this is equivalent to {@link #incidentEdges(Object)}. 223 * 224 * @throws IllegalArgumentException if {@code node} is not an element of this network 225 */ 226 Set<E> outEdges(N node); 227 228 /** 229 * Returns the count of {@code node}'s {@link #incidentEdges(Object) incident edges}, counting 230 * self-loops twice (equivalently, the number of times an edge touches {@code node}). 231 * 232 * <p>For directed networks, this is equal to {@code inDegree(node) + outDegree(node)}. 233 * 234 * <p>For undirected networks, this is equal to {@code incidentEdges(node).size()} + (number of 235 * self-loops incident to {@code node}). 236 * 237 * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}. 238 * 239 * @throws IllegalArgumentException if {@code node} is not an element of this network 240 */ 241 int degree(N node); 242 243 /** 244 * Returns the count of {@code node}'s {@link #inEdges(Object) incoming edges} in a directed 245 * network. In an undirected network, returns the {@link #degree(Object)}. 246 * 247 * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}. 248 * 249 * @throws IllegalArgumentException if {@code node} is not an element of this network 250 */ 251 int inDegree(N node); 252 253 /** 254 * Returns the count of {@code node}'s {@link #outEdges(Object) outgoing edges} in a directed 255 * network. In an undirected network, returns the {@link #degree(Object)}. 256 * 257 * <p>If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}. 258 * 259 * @throws IllegalArgumentException if {@code node} is not an element of this network 260 */ 261 int outDegree(N node); 262 263 /** 264 * Returns the nodes which are the endpoints of {@code edge} in this network. 265 * 266 * @throws IllegalArgumentException if {@code edge} is not an element of this network 267 */ 268 EndpointPair<N> incidentNodes(E edge); 269 270 /** 271 * Returns the edges which have an {@link #incidentNodes(Object) incident node} in common with 272 * {@code edge}. An edge is not considered adjacent to itself. 273 * 274 * @throws IllegalArgumentException if {@code edge} is not an element of this network 275 */ 276 Set<E> adjacentEdges(E edge); 277 278 /** 279 * Returns the set of edges that each directly connect {@code nodeU} to {@code nodeV}. 280 * 281 * <p>In an undirected network, this is equal to {@code edgesConnecting(nodeV, nodeU)}. 282 * 283 * <p>The resulting set of edges will be parallel (i.e. have equal {@link #incidentNodes(Object)}. 284 * If this network does not {@link #allowsParallelEdges() allow parallel edges}, the resulting set 285 * will contain at most one edge (equivalent to {@code edgeConnecting(nodeU, nodeV).asSet()}). 286 * 287 * @throws IllegalArgumentException if {@code nodeU} or {@code nodeV} is not an element of this 288 * network 289 */ 290 Set<E> edgesConnecting(N nodeU, N nodeV); 291 292 /** 293 * Returns the set of edges that each directly connect {@code endpoints} (in the order, if any, 294 * specified by {@code endpoints}). 295 * 296 * <p>The resulting set of edges will be parallel (i.e. have equal {@link #incidentNodes(Object)}. 297 * If this network does not {@link #allowsParallelEdges() allow parallel edges}, the resulting set 298 * will contain at most one edge (equivalent to {@code edgeConnecting(endpoints).asSet()}). 299 * 300 * <p>If this network is directed, {@code endpoints} must be ordered. 301 * 302 * @throws IllegalArgumentException if either endpoint is not an element of this network 303 * @throws IllegalArgumentException if the endpoints are unordered and the graph is directed 304 * @since 27.1 305 */ 306 Set<E> edgesConnecting(EndpointPair<N> endpoints); 307 308 /** 309 * Returns the single edge that directly connects {@code nodeU} to {@code nodeV}, if one is 310 * present, or {@code Optional.empty()} if no such edge exists. 311 * 312 * <p>In an undirected network, this is equal to {@code edgeConnecting(nodeV, nodeU)}. 313 * 314 * @throws IllegalArgumentException if there are multiple parallel edges connecting {@code nodeU} 315 * to {@code nodeV} 316 * @throws IllegalArgumentException if {@code nodeU} or {@code nodeV} is not an element of this 317 * network 318 * @since 23.0 319 */ 320 Optional<E> edgeConnecting(N nodeU, N nodeV); 321 322 /** 323 * Returns the single edge that directly connects {@code endpoints} (in the order, if any, 324 * specified by {@code endpoints}), if one is present, or {@code Optional.empty()} if no such edge 325 * exists. 326 * 327 * <p>If this graph is directed, the endpoints must be ordered. 328 * 329 * @throws IllegalArgumentException if there are multiple parallel edges connecting {@code nodeU} 330 * to {@code nodeV} 331 * @throws IllegalArgumentException if either endpoint is not an element of this network 332 * @throws IllegalArgumentException if the endpoints are unordered and the graph is directed 333 * @since 27.1 334 */ 335 Optional<E> edgeConnecting(EndpointPair<N> endpoints); 336 337 /** 338 * Returns the single edge that directly connects {@code nodeU} to {@code nodeV}, if one is 339 * present, or {@code null} if no such edge exists. 340 * 341 * <p>In an undirected network, this is equal to {@code edgeConnectingOrNull(nodeV, nodeU)}. 342 * 343 * @throws IllegalArgumentException if there are multiple parallel edges connecting {@code nodeU} 344 * to {@code nodeV} 345 * @throws IllegalArgumentException if {@code nodeU} or {@code nodeV} is not an element of this 346 * network 347 * @since 23.0 348 */ 349 @Nullable 350 E edgeConnectingOrNull(N nodeU, N nodeV); 351 352 /** 353 * Returns the single edge that directly connects {@code endpoints} (in the order, if any, 354 * specified by {@code endpoints}), if one is present, or {@code null} if no such edge exists. 355 * 356 * <p>If this graph is directed, the endpoints must be ordered. 357 * 358 * @throws IllegalArgumentException if there are multiple parallel edges connecting {@code nodeU} 359 * to {@code nodeV} 360 * @throws IllegalArgumentException if either endpoint is not an element of this network 361 * @throws IllegalArgumentException if the endpoints are unordered and the graph is directed 362 * @since 27.1 363 */ 364 @Nullable 365 E edgeConnectingOrNull(EndpointPair<N> endpoints); 366 367 /** 368 * Returns true if there is an edge that directly connects {@code nodeU} to {@code nodeV}. This is 369 * equivalent to {@code nodes().contains(nodeU) && successors(nodeU).contains(nodeV)}, and to 370 * {@code edgeConnectingOrNull(nodeU, nodeV) != null}. 371 * 372 * <p>In an undirected graph, this is equal to {@code hasEdgeConnecting(nodeV, nodeU)}. 373 * 374 * @since 23.0 375 */ 376 boolean hasEdgeConnecting(N nodeU, N nodeV); 377 378 /** 379 * Returns true if there is an edge that directly connects {@code endpoints} (in the order, if 380 * any, specified by {@code endpoints}). 381 * 382 * <p>Unlike the other {@code EndpointPair}-accepting methods, this method does not throw if the 383 * endpoints are unordered and the graph is directed; it simply returns {@code false}. This is for 384 * consistency with {@link Graph#hasEdgeConnecting(EndpointPair)} and {@link 385 * ValueGraph#hasEdgeConnecting(EndpointPair)}. 386 * 387 * @since 27.1 388 */ 389 boolean hasEdgeConnecting(EndpointPair<N> endpoints); 390 391 // 392 // Network identity 393 // 394 395 /** 396 * Returns {@code true} iff {@code object} is a {@link Network} that has the same elements and the 397 * same structural relationships as those in this network. 398 * 399 * <p>Thus, two networks A and B are equal if <b>all</b> of the following are true: 400 * 401 * <ul> 402 * <li>A and B have equal {@link #isDirected() directedness}. 403 * <li>A and B have equal {@link #nodes() node sets}. 404 * <li>A and B have equal {@link #edges() edge sets}. 405 * <li>Every edge in A and B connects the same nodes in the same direction (if any). 406 * </ul> 407 * 408 * <p>Network properties besides {@link #isDirected() directedness} do <b>not</b> affect equality. 409 * For example, two networks may be considered equal even if one allows parallel edges and the 410 * other doesn't. Additionally, the order in which nodes or edges are added to the network, and 411 * the order in which they are iterated over, are irrelevant. 412 * 413 * <p>A reference implementation of this is provided by {@link AbstractNetwork#equals(Object)}. 414 */ 415 @Override 416 boolean equals(@Nullable Object object); 417 418 /** 419 * Returns the hash code for this network. The hash code of a network is defined as the hash code 420 * of a map from each of its {@link #edges() edges} to their {@link #incidentNodes(Object) 421 * incident nodes}. 422 * 423 * <p>A reference implementation of this is provided by {@link AbstractNetwork#hashCode()}. 424 */ 425 @Override 426 int hashCode(); 427}