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;
020
021/**
022 * A functional interface for <a
023 * href="https://en.wikipedia.org/wiki/Graph_(discrete_mathematics)">graph</a>-structured data.
024 *
025 * <p>This interface is meant to be used as the type of a parameter to graph algorithms (such as
026 * topological sort) that only need a way of accessing the predecessors of a node in a graph.
027 *
028 * <h3>Usage</h3>
029 *
030 * Given an algorithm, for example:
031 *
032 * <pre>{@code
033 * public <N> someGraphAlgorithm(N startNode, PredecessorsFunction<N> predecessorsFunction);
034 * }</pre>
035 *
036 * you will invoke it depending on the graph representation you're using.
037 *
038 * <p>If you have an instance of one of the primary {@code common.graph} types ({@link Graph},
039 * {@link ValueGraph}, and {@link Network}):
040 *
041 * <pre>{@code
042 * someGraphAlgorithm(startNode, graph);
043 * }</pre>
044 *
045 * This works because those types each implement {@code PredecessorsFunction}. It will also work
046 * with any other implementation of this interface.
047 *
048 * <p>If you have your own graph implementation based around a custom node type {@code MyNode},
049 * which has a method {@code getParents()} that retrieves its predecessors in a graph:
050 *
051 * <pre>{@code
052 * someGraphAlgorithm(startNode, MyNode::getParents);
053 * }</pre>
054 *
055 * <p>If you have some other mechanism for returning the predecessors of a node, or one that doesn't
056 * return a {@code Iterable<? extends N>}, then you can use a lambda to perform a more general
057 * transformation:
058 *
059 * <pre>{@code
060 * someGraphAlgorithm(startNode, node -> ImmutableList.of(node.mother(), node.father()));
061 * }</pre>
062 *
063 * <p>Graph algorithms that need additional capabilities (accessing both predecessors and
064 * successors, iterating over the edges, etc.) should declare their input to be of a type that
065 * provides those capabilities, such as {@link Graph}, {@link ValueGraph}, or {@link Network}.
066 *
067 * <h3>Additional documentation</h3>
068 *
069 * <p>See the Guava User Guide for the {@code common.graph} package (<a
070 * href="https://github.com/google/guava/wiki/GraphsExplained">"Graphs Explained"</a>) for
071 * additional documentation, including <a
072 * href="https://github.com/google/guava/wiki/GraphsExplained#notes-for-implementors">notes for
073 * implementors</a>
074 *
075 * @author Joshua O'Madadhain
076 * @author Jens Nyman
077 * @param <N> Node parameter type
078 * @since 23.0
079 */
080@Beta
081public interface PredecessorsFunction<N> {
082
083  /**
084   * Returns all nodes in this graph adjacent to {@code node} which can be reached by traversing
085   * {@code node}'s incoming edges <i>against</i> the direction (if any) of the edge.
086   *
087   * <p>Some algorithms that operate on a {@code PredecessorsFunction} may produce undesired results
088   * if the returned {@link Iterable} contains duplicate elements. Implementations of such
089   * algorithms should document their behavior in the presence of duplicates.
090   *
091   * <p>The elements of the returned {@code Iterable} must each be:
092   *
093   * <ul>
094   *   <li>Non-null
095   *   <li>Usable as {@code Map} keys (see the Guava User Guide's section on <a
096   *       href="https://github.com/google/guava/wiki/GraphsExplained#graph-elements-nodes-and-edges">
097   *       graph elements</a> for details)
098   * </ul>
099   *
100   * @throws IllegalArgumentException if {@code node} is not an element of this graph
101   */
102  Iterable<? extends N> predecessors(N node);
103}