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/*
 * Copyright (C) 2014 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.graph;

import com.google.common.annotations.Beta;
import com.google.errorprone.annotations.DoNotMock;
import java.util.Collection;
import java.util.Set;
import javax.annotation.CheckForNull;

/**
 * An interface for graph-structured data,
 * whose edges are anonymous entities with no identity or information of their own.
 *
 * 

A graph is composed of a set of nodes and a set of edges connecting pairs of nodes. * *

There are three primary interfaces provided to represent graphs. In order of increasing * complexity they are: {@link Graph}, {@link ValueGraph}, and {@link Network}. You should generally * prefer the simplest interface that satisfies your use case. See the * "Choosing the right graph type" section of the Guava User Guide for more details. * *

Capabilities

* *

{@code Graph} supports the following use cases (definitions of * terms): * *

    *
  • directed graphs *
  • undirected graphs *
  • graphs that do/don't allow self-loops *
  • graphs whose nodes/edges are insertion-ordered, sorted, or unordered *
* *

{@code Graph} explicitly does not support parallel edges, and forbids implementations or * extensions with parallel edges. If you need parallel edges, use {@link Network}. * *

Building a {@code Graph}

* *

The implementation classes that {@code common.graph} provides are not public, by design. To * create an instance of one of the built-in implementations of {@code Graph}, use the {@link * GraphBuilder} class: * *

{@code
 * MutableGraph graph = GraphBuilder.undirected().build();
 * }
* *

{@link GraphBuilder#build()} returns an instance of {@link MutableGraph}, which is a subtype * of {@code Graph} that provides methods for adding and removing nodes and edges. If you do not * need to mutate a graph (e.g. if you write a method than runs a read-only algorithm on the graph), * you should use the non-mutating {@link Graph} interface, or an {@link ImmutableGraph}. * *

You can create an immutable copy of an existing {@code Graph} using {@link * ImmutableGraph#copyOf(Graph)}: * *

{@code
 * ImmutableGraph immutableGraph = ImmutableGraph.copyOf(graph);
 * }
* *

Instances of {@link ImmutableGraph} do not implement {@link MutableGraph} (obviously!) and are * contractually guaranteed to be unmodifiable and thread-safe. * *

The Guava User Guide has more * information on (and examples of) building graphs. * *

Additional documentation

* *

See the Guava User Guide for the {@code common.graph} package ("Graphs Explained") for * additional documentation, including: * *

* * @author James Sexton * @author Joshua O'Madadhain * @param Node parameter type * @since 20.0 */ @Beta @DoNotMock("Use GraphBuilder to create a real instance") @ElementTypesAreNonnullByDefault public interface Graph extends BaseGraph { // // Graph-level accessors // /** Returns all nodes in this graph, in the order specified by {@link #nodeOrder()}. */ @Override Set nodes(); /** Returns all edges in this graph. */ @Override Set> edges(); // // Graph properties // /** * Returns true if the edges in this graph are directed. Directed edges connect a {@link * EndpointPair#source() source node} to a {@link EndpointPair#target() target node}, while * undirected edges connect a pair of nodes to each other. */ @Override boolean isDirected(); /** * Returns true if this graph allows self-loops (edges that connect a node to itself). Attempting * to add a self-loop to a graph that does not allow them will throw an {@link * IllegalArgumentException}. */ @Override boolean allowsSelfLoops(); /** Returns the order of iteration for the elements of {@link #nodes()}. */ @Override ElementOrder nodeOrder(); /** * Returns an {@link ElementOrder} that specifies the order of iteration for the elements of * {@link #edges()}, {@link #adjacentNodes(Object)}, {@link #predecessors(Object)}, {@link * #successors(Object)} and {@link #incidentEdges(Object)}. * * @since 29.0 */ @Override ElementOrder incidentEdgeOrder(); // // Element-level accessors // /** * Returns the nodes which have an incident edge in common with {@code node} in this graph. * *

This is equal to the union of {@link #predecessors(Object)} and {@link #successors(Object)}. * * @throws IllegalArgumentException if {@code node} is not an element of this graph */ @Override Set adjacentNodes(N node); /** * Returns all nodes in this graph adjacent to {@code node} which can be reached by traversing * {@code node}'s incoming edges against the direction (if any) of the edge. * *

In an undirected graph, this is equivalent to {@link #adjacentNodes(Object)}. * * @throws IllegalArgumentException if {@code node} is not an element of this graph */ @Override Set predecessors(N node); /** * Returns all nodes in this graph adjacent to {@code node} which can be reached by traversing * {@code node}'s outgoing edges in the direction (if any) of the edge. * *

In an undirected graph, this is equivalent to {@link #adjacentNodes(Object)}. * *

This is not the same as "all nodes reachable from {@code node} by following outgoing * edges". For that functionality, see {@link Graphs#reachableNodes(Graph, Object)}. * * @throws IllegalArgumentException if {@code node} is not an element of this graph */ @Override Set successors(N node); /** * Returns the edges in this graph whose endpoints include {@code node}. * *

This is equal to the union of incoming and outgoing edges. * * @throws IllegalArgumentException if {@code node} is not an element of this graph * @since 24.0 */ @Override Set> incidentEdges(N node); /** * Returns the count of {@code node}'s incident edges, counting self-loops twice (equivalently, * the number of times an edge touches {@code node}). * *

For directed graphs, this is equal to {@code inDegree(node) + outDegree(node)}. * *

For undirected graphs, this is equal to {@code incidentEdges(node).size()} + (number of * self-loops incident to {@code node}). * *

If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}. * * @throws IllegalArgumentException if {@code node} is not an element of this graph */ @Override int degree(N node); /** * Returns the count of {@code node}'s incoming edges (equal to {@code predecessors(node).size()}) * in a directed graph. In an undirected graph, returns the {@link #degree(Object)}. * *

If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}. * * @throws IllegalArgumentException if {@code node} is not an element of this graph */ @Override int inDegree(N node); /** * Returns the count of {@code node}'s outgoing edges (equal to {@code successors(node).size()}) * in a directed graph. In an undirected graph, returns the {@link #degree(Object)}. * *

If the count is greater than {@code Integer.MAX_VALUE}, returns {@code Integer.MAX_VALUE}. * * @throws IllegalArgumentException if {@code node} is not an element of this graph */ @Override int outDegree(N node); /** * Returns true if there is an edge that directly connects {@code nodeU} to {@code nodeV}. This is * equivalent to {@code nodes().contains(nodeU) && successors(nodeU).contains(nodeV)}. * *

In an undirected graph, this is equal to {@code hasEdgeConnecting(nodeV, nodeU)}. * * @since 23.0 */ @Override boolean hasEdgeConnecting(N nodeU, N nodeV); /** * Returns true if there is an edge that directly connects {@code endpoints} (in the order, if * any, specified by {@code endpoints}). This is equivalent to {@code * edges().contains(endpoints)}. * *

Unlike the other {@code EndpointPair}-accepting methods, this method does not throw if the * endpoints are unordered and the graph is directed; it simply returns {@code false}. This is for * consistency with the behavior of {@link Collection#contains(Object)} (which does not generally * throw if the object cannot be present in the collection), and the desire to have this method's * behavior be compatible with {@code edges().contains(endpoints)}. * * @since 27.1 */ @Override boolean hasEdgeConnecting(EndpointPair endpoints); // // Graph identity // /** * Returns {@code true} iff {@code object} is a {@link Graph} that has the same elements and the * same structural relationships as those in this graph. * *

Thus, two graphs A and B are equal if all of the following are true: * *

    *
  • A and B have equal {@link #isDirected() directedness}. *
  • A and B have equal {@link #nodes() node sets}. *
  • A and B have equal {@link #edges() edge sets}. *
* *

Graph properties besides {@link #isDirected() directedness} do not affect equality. * For example, two graphs may be considered equal even if one allows self-loops and the other * doesn't. Additionally, the order in which nodes or edges are added to the graph, and the order * in which they are iterated over, are irrelevant. * *

A reference implementation of this is provided by {@link AbstractGraph#equals(Object)}. */ @Override boolean equals(@CheckForNull Object object); /** * Returns the hash code for this graph. The hash code of a graph is defined as the hash code of * the set returned by {@link #edges()}. * *

A reference implementation of this is provided by {@link AbstractGraph#hashCode()}. */ @Override int hashCode(); }





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