59 lines
2.3 KiB
Java
59 lines
2.3 KiB
Java
/*******************************************************************************
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* Copyright (c) 2007 IBM Corporation.
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* All rights reserved. This program and the accompanying materials
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* are made available under the terms of the Eclipse Public License v1.0
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* which accompanies this distribution, and is available at
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* http://www.eclipse.org/legal/epl-v10.html
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*
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* Contributors:
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* IBM Corporation - initial API and implementation
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*******************************************************************************/
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package com.ibm.wala.util.graph.traverse;
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import java.util.Iterator;
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import com.ibm.wala.util.collections.ReverseIterator;
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import com.ibm.wala.util.graph.Graph;
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import com.ibm.wala.util.graph.impl.GraphInverter;
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/**
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* Utilities for iterating over graphs in topological order.
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*/
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public class Topological {
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/**
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* Build an Iterator over all the nodes in the graph, in an order such that SCCs are visited in topological order.
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*
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* @throws IllegalArgumentException if graph == null
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*/
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public static <T> Iterable<T> makeTopologicalIter(final Graph<T> graph) throws IllegalArgumentException {
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if (graph == null) {
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throw new IllegalArgumentException("graph == null");
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}
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return new Iterable<T>() {
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@Override
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public Iterator<T> iterator() {
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// the following code ensures a topological order over SCCs.
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// note that the first two lines of the following give a topological
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// order for dags, but that can get screwed up by cycles. so
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// instead, we use Tarjan's SCC algorithm, which happens to
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// visit nodes in an order consistent with a top. order over SCCs.
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// finish time is post-order
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// note that if you pay attention only to the first representative
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// of each SCC discovered, we have a top. order of these SCC
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// representatives
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Iterator<T> finishTime = DFS.iterateFinishTime(graph);
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// reverse postorder is usual topological sort.
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Iterator<T> rev = ReverseIterator.reverse(finishTime);
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// the following statement helps out the GC; note that finishTime holds
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// on to a large array
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finishTime = null;
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Graph<T> G_T = GraphInverter.invert(graph);
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return DFS.iterateFinishTime(G_T, rev);
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}
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};
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}
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}
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