TopologicalSort.java

package org.mef.sprig.util;

import java.util.ArrayList;
import java.util.Collection;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

//http://chianti.ucsd.edu/svn/core3/model-impl/tags/model-impl-parent-3.0.0-alpha3/impl/src/main/java/org/cytoscape/model/internal/tsort/TopologicalSort.java
/**
 *  Implements topological sorting of nodes in a graph.
 *  See for example http://en.wikipedia.org/wiki/Topological_sorting (the Tarjan algorithm)
 */
public class TopologicalSort {
	/**
	 *  @param nodes the list of all nodes
	 *  @param edges the edges that connect the nodes that need to be sorted.
	 *  @return the topological order
	 *  @throws IllegalStateException if a cycle has been detected
	 *  N.B. it might be a good idea to make sure that whatever the concrete type of the nodes in
	 *  "nodes" are has a toString() method that returns the name of a node since this method
	 *  will be used if a cycle has been detected to report one of the nodes in the cycle.
	 */
	public static List<TSortNode> sort(final Collection<TSortNode> nodes)
			throws IllegalStateException
			{
		final List<TSortNode> order = new ArrayList<TSortNode>();
		final Set<TSortNode> visited = new HashSet<TSortNode>();

		final Set<TSortNode> alreadySeen = new HashSet<TSortNode>();
		for (final TSortNode n : nodes) {
			alreadySeen.clear();
			visit(n, alreadySeen, visited, order);
		}

		return order;
			}

	private static void visit(final TSortNode n, final Set<TSortNode> alreadySeen,
			final Set<TSortNode> visited, final List<TSortNode> order)
	{
		if (alreadySeen.contains(n))
			throw new IllegalStateException("cycle containing " + n + " found!");
		alreadySeen.add(n);

		if (!visited.contains(n)) {
			visited.add(n);
			for (final TSortNode m : n.getDependents())
				visit(m, alreadySeen, visited, order);
			order.add(n);
		}

		alreadySeen.remove(n);
	}
}