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/*
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you 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 org.apache.ofbiz.base.util;
import static java.util.stream.Collectors.toSet;
import java.util.ArrayList;
import java.util.Collection;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
/**
* A basic directed graph utilitary.
* <p>
* A directed graph is a data structure consisting of nodes and arrows connecting those nodes
* which are called <em>edges</em>. In a directed graph edges are ordered pairs of respectively
* source and target nodes.
* <p>
* This implementation is adapted to small in-memory graphs.
*
* @param <T> the type of the nodes
* @see <a href="https://www.wikipedia.org/wiki/Directed_graph">Directed graph</a>
*/
public class Digraph<T> {
/** The map associating source nodes to their adjacent target nodes. */
private final Map<T, Collection<T>> edges;
/** The set of nodes */
private final Set<T> nodes;
/**
* Constructs a directed graph from a specification Map.
* @param spec the map defining a set of source nodes (keys) that are linked to a collection
* of adjacent target nodes (values). Both keys and values must not be {@code null}.
* @throws IllegalArgumentException when a target node is not present in the sources nodes.
*/
public Digraph(Map<T, Collection<T>> spec) throws IllegalArgumentException {
this.edges = spec;
this.nodes = spec.keySet();
// Check that all adjacent nodes are present as keys in the map.
Set<T> undeclaredNodes = spec.values().stream()
.flatMap(Collection::stream)
.filter(child -> !nodes.contains(child))
.collect(toSet());
if (!undeclaredNodes.isEmpty()) {
String msg = String.format("%s nodes are not present in the graph", undeclaredNodes);
throw new IllegalArgumentException(msg);
}
}
/**
* Sort nodes in a topological ordering assuming that this graph is acyclic.
* <p>
* A graph without cycles is often called a <em>Directed Acyclic Graph</em> (DAG).
* @return a linear ordering of nodes such for every edge in the graph its target node
* is present before its source node.
* @throws IllegalStateException when this graph contains a cycle.
* @see <a href="https://www.wikipedia.org/wiki/Topological_sorting">topological sorting</a>
* @see <a href="https://www.wikipedia.org/wiki/Directed_acyclic_graph">DAG</a>
*/
public List<T> sort() throws IllegalStateException {
Set<T> permanents = new HashSet<>();
Set<T> temporaries = new HashSet<>();
List<T> result = new ArrayList<>();
for (T node : nodes) {
if (!permanents.contains(node)) {
visit(result, node, permanents, temporaries);
}
}
return result;
}
/**
* Traverses the graph using <em>Depth First Search</em> (DFS) to construct a topological ordering.
* @param res the ordered list that we are building
* @param root the current node we are visiting
* @param permanents the nodes that have been successfully been visited
* @param temporaries the nodes that we have started to visit but might contain cycles.
* @throws IllegalStateException when a cycle is found.
* @see #sort
* @see <a href="https://www.wikipedia.org/wiki/Depth-first_search">Depth Dirst Search</a>
*/
private void visit(List<T> res, T root, Set<T> permanents, Set<T> temporaries) throws IllegalStateException {
if (permanents.contains(root)) {
return;
} else if (temporaries.contains(root)) {
throw new IllegalStateException("A cycle has been found");
} else {
temporaries.add(root);
for (T next : edges.get(root)) {
visit(res, next, permanents, temporaries);
}
temporaries.remove(root);
permanents.add(root);
res.add(root);
}
}
}