/*******************************************************************************
* Copyright (c) 2007 IBM Corporation.
* 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 com.ibm.wala.util.graph.traverse;
import java.util.Iterator;
import com.ibm.wala.util.collections.ReverseIterator;
import com.ibm.wala.util.graph.Graph;
import com.ibm.wala.util.graph.impl.GraphInverter;
/**
* Utilities for iterating over graphs in topological order.
*/
public class Topological {
/**
* Build an Iterator over all the nodes in the graph, in an order such that SCCs are visited in topological order.
*
* @throws IllegalArgumentException if graph == null
*/
public static <T> Iterator<T> makeTopologicalIter(Graph<T> graph) throws IllegalArgumentException {
// the following code ensures a topological order over SCCs.
// note that the first two lines of the following give a topological
// order for dags, but that can get screwed up by cycles. so
// instead, we use Tarjan's SCC algorithm, which happens to
// visit nodes in an order consistent with a top. order over SCCs.
if (graph == null) {
throw new IllegalArgumentException("graph == null");
}
// finish time is post-order
// note that if you pay attention only to the first representative
// of each SCC discovered, we have a top. order of these SCC
// representatives
Iterator<T> finishTime = DFS.iterateFinishTime(graph);
// reverse postorder is usual topological sort.
Iterator<T> rev = ReverseIterator.reverse(finishTime);
// the following statement helps out the GC; note that finishTime holds
// on to a large array
finishTime = null;
Graph<T> G_T = GraphInverter.invert(graph);
Iterator<T> order = DFS.iterateFinishTime(G_T, rev);
return order;
}
}