GraphUtilities.java

/*******************************************************************************
 * Copyright (c) 2003, 2005 IBM Corporation and others.
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 *
 * Contributors:
 *     IBM Corporation - initial API and implementation
 *******************************************************************************/
package org.eclipse.draw2d.graph;

/**
 * Some utility methods for graphs.
 * @author Eric Bordeau
 * @since 2.1.2
 */
class GraphUtilities {

static Subgraph getCommonAncestor(Node left, Node right) {
	Subgraph parent;
	if (right instanceof Subgraph)
		parent = (Subgraph)right;
	else
		parent = right.getParent();
	while (parent != null) {
		if (parent.isNested(left))
			return parent;
		parent = parent.getParent();
	}
	return null;
}

/**
 * Returns <code>true</code> if the given graph contains at least one cycle.
 * @param graph the graph to test
 * @return whether the graph is cyclic
 */
public static boolean isCyclic(DirectedGraph graph) {
	return isCyclic(new NodeList(graph.nodes));
}

/**
 * Recursively removes leaf nodes from the list until there are no nodes remaining (acyclic)
 * or there are no leaf nodes but the list is not empty (cyclic), then returns the result.
 * @param nodes the list of nodes to test
 * @return whether the graph is cyclic
 */
public static boolean isCyclic(NodeList nodes) {
	if (nodes.isEmpty())
		return false;
	int size = nodes.size();
	// remove all the leaf nodes from the graph
	for (int i = 0; i < nodes.size(); i++) {
		Node node = nodes.getNode(i);
		if (node.outgoing == null || node.outgoing.isEmpty()) { // this is a leaf node
			nodes.remove(node);
			for (int j = 0; j < node.incoming.size(); j++) {
				Edge e = node.incoming.getEdge(j);
				e.source.outgoing.remove(e);
			}
		}
	}
	//	if no nodes were removed, that means there are no leaf nodes and the graph is cyclic
	if (nodes.size() == size) 
		return true;
	// leaf nodes were removed, so recursively call this method with the new list
	return isCyclic(nodes);
}

/**
 * Counts the number of edge crossings in a DirectedGraph
 * @param graph the graph whose crossed edges are counted
 * @return the number of edge crossings in the graph
 */
public static int numberOfCrossingsInGraph(DirectedGraph graph) {
	int crossings = 0;
	for (int i = 0; i < graph.ranks.size(); i++) {
		Rank rank = graph.ranks.getRank(i);
		crossings += numberOfCrossingsInRank(rank);
	}
	return crossings;
}

/**
 * Counts the number of edge crossings in a Rank
 * @param rank the rank whose crossed edges are counted
 * @return the number of edge crossings in the rank
 */
public static int numberOfCrossingsInRank(Rank rank) {
	int crossings = 0;
	for (int i = 0; i < rank.size() - 1; i++) {
		Node currentNode = rank.getNode(i);
		Node nextNode;
		for (int j = i + 1; j < rank.size(); j++) {
			nextNode = rank.getNode(j);
			EdgeList currentOutgoing = currentNode.outgoing;
			EdgeList nextOutgoing = nextNode.outgoing;
			for (int k = 0; k < currentOutgoing.size(); k++) {
				Edge currentEdge = currentOutgoing.getEdge(k);
				for (int l = 0; l < nextOutgoing.size(); l++) {
					if (nextOutgoing.getEdge(l).getIndexForRank(currentNode.rank + 1) 
						< currentEdge.getIndexForRank(currentNode.rank + 1))
						crossings++;	
				}
			}
		}
	}
	return crossings;
}

private static NodeList search(Node node, NodeList list) {
	if (node.flag)
		return list;
	node.flag = true;
	list.add(node);
	for (int i = 0; i < node.outgoing.size(); i++)
		search(node.outgoing.getEdge(i).target, list);
	return list;
}

/**
 * Returns <code>true</code> if adding an edge between the 2 given nodes will introduce a
 * cycle in the containing graph.
 * @param source the potential source node
 * @param target the potential target node
 * @return whether an edge between the 2 given nodes will introduce a cycle
 */
public static boolean willCauseCycle(Node source, Node target) {
	NodeList nodes = search(target, new NodeList());
	nodes.resetFlags();
	return nodes.contains(source);
}

static boolean isConstrained(Node left, Node right) {
	Subgraph common = left.getParent();
	while (common != null && !common.isNested(right)) {
		left = left.getParent();
		common = left.getParent();
	}
	while (right.getParent() != common)
		right = right.getParent();
	return (left.rowOrder != -1 && right.rowOrder != -1)
	  && left.rowOrder != right.rowOrder;
}

}