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/*
* (C) Copyright 2017-2023, by Dimitrios Michail and Contributors.
*
* JGraphT : a free Java graph-theory library
*
* See the CONTRIBUTORS.md file distributed with this work for additional
* information regarding copyright ownership.
*
* This program and the accompanying materials are made available under the
* terms of the Eclipse Public License 2.0 which is available at
* http://www.eclipse.org/legal/epl-2.0, or the
* GNU Lesser General Public License v2.1 or later
* which is available at
* http://www.gnu.org/licenses/old-licenses/lgpl-2.1-standalone.html.
*
* SPDX-License-Identifier: EPL-2.0 OR LGPL-2.1-or-later
*/
package org.jgrapht.alg.cycle;
import org.jgrapht.*;
import org.jgrapht.graph.*;
import java.util.*;
/**
* Collection of helper methods related to cycles.
*
* @author Dimitrios Michail
*/
public abstract class Cycles
{
/**
* Transform a simple cycle from an edge set representation to a graph path. A simple cycle
* contains vertices with degrees either zero or two. This method treats directed graphs as
* undirected.
*
* @param graph the graph
* @param cycle the simple cycle
* @return the cycle as a graph path
* @param graph vertex type
* @param graph edge type
* @throws IllegalArgumentException if the provided edge set is not a simple cycle (circuit)
*/
public static GraphPath simpleCycleToGraphPath(Graph graph, List cycle)
{
Objects.requireNonNull(graph, "Graph cannot be null");
Objects.requireNonNull(cycle, "Cycle cannot be null");
if (cycle.isEmpty()) {
return null;
}
// index
Map firstEdge = new HashMap<>();
Map secondEdge = new HashMap<>();
for (E e : cycle) {
V s = graph.getEdgeSource(e);
if (!firstEdge.containsKey(s)) {
firstEdge.put(s, e);
} else {
if (!secondEdge.containsKey(s)) {
secondEdge.put(s, e);
} else {
throw new IllegalArgumentException("Not a simple cycle");
}
}
V t = graph.getEdgeTarget(e);
if (!firstEdge.containsKey(t)) {
firstEdge.put(t, e);
} else {
if (!secondEdge.containsKey(t)) {
secondEdge.put(t, e);
} else {
throw new IllegalArgumentException("Not a simple cycle");
}
}
}
// traverse
List edges = new ArrayList<>();
double weight = 0d;
E e = cycle.stream().findAny().get();
edges.add(e);
weight += graph.getEdgeWeight(e);
V start = graph.getEdgeSource(e);
V cur = Graphs.getOppositeVertex(graph, e, start);
while (!cur.equals(start)) {
E fe = firstEdge.get(cur);
if (fe == null) {
throw new IllegalArgumentException("Not a simple cycle");
}
E se = secondEdge.get(cur);
if (se == null) {
throw new IllegalArgumentException("Not a simple cycle");
}
if (fe.equals(e)) {
e = se;
} else if (se.equals(e)) {
e = fe;
} else {
throw new IllegalArgumentException("Not a simple cycle");
}
edges.add(e);
weight += graph.getEdgeWeight(e);
cur = Graphs.getOppositeVertex(graph, e, cur);
}
// return result
return new GraphWalk<>(graph, start, start, edges, weight);
}
}