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/*
* Generic graph library
* Copyright (C) 2000,2003-2005 University of Maryland
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2.1 of the License, or (at your option) any later version.
*
* This library is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
*/
// $Revision: 1.17 $
package edu.umd.cs.findbugs.graph;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.Set;
import java.util.TreeSet;
/**
* Algorithm to find strongly connected components in a graph. Based on
* algorithm in Cormen et. al., Introduction to Algorithms, p. 489.
*/
public class StronglyConnectedComponents, EdgeType extends GraphEdge, VertexType extends GraphVertex> {
private final ArrayList> m_stronglyConnectedSearchTreeList;
private VertexChooser m_vertexChooser;
/**
* Constructor.
*/
public StronglyConnectedComponents() {
m_stronglyConnectedSearchTreeList = new ArrayList<>();
m_vertexChooser = null;
}
/**
* Specify a VertexChooser object to restrict which vertices are considered.
* This is useful if you only want to find strongly connected components
* among a particular category of vertices.
*/
public void setVertexChooser(VertexChooser vertexChooser) {
m_vertexChooser = vertexChooser;
}
/**
* Find the strongly connected components in given graph.
*
* @param g
* the graph
* @param toolkit
* a GraphToolkit, used to create temporary graphs used by the
* algorithm
*/
public void findStronglyConnectedComponents(GraphType g, GraphToolkit toolkit) {
// Perform the initial depth first search
DepthFirstSearch initialDFS = new DepthFirstSearch<>(g);
if (m_vertexChooser != null) {
initialDFS.setVertexChooser(m_vertexChooser);
}
initialDFS.search();
// Create a transposed graph
Transpose t = new Transpose<>();
GraphType transpose = t.transpose(g, toolkit);
// Create a set of vertices in the transposed graph,
// in descending order of finish time in the initial
// depth first search.
VisitationTimeComparator comparator = new VisitationTimeComparator<>(
initialDFS.getFinishTimeList(), VisitationTimeComparator.DESCENDING);
Set descendingByFinishTimeSet = new TreeSet<>(comparator);
Iterator i = transpose.vertexIterator();
while (i.hasNext()) {
descendingByFinishTimeSet.add(i.next());
}
// Create a SearchTreeBuilder for transposed DFS
SearchTreeBuilder searchTreeBuilder = new SearchTreeBuilder<>();
// Now perform a DFS on the transpose, choosing the vertices
// to visit in the main loop by descending finish time
final Iterator vertexIter = descendingByFinishTimeSet.iterator();
DepthFirstSearch transposeDFS = new DepthFirstSearch(
transpose) {
@Override
protected VertexType getNextSearchTreeRoot() {
while (vertexIter.hasNext()) {
VertexType vertex = vertexIter.next();
if (visitMe(vertex)) {
return vertex;
}
}
return null;
}
};
if (m_vertexChooser != null) {
transposeDFS.setVertexChooser(m_vertexChooser);
}
transposeDFS.setSearchTreeCallback(searchTreeBuilder);
transposeDFS.search();
// The search tree roots of the second DFS represent the
// strongly connected components. Note that we call copySearchTree()
// to make the returned search trees relative to the original
// graph, not the transposed graph (which would be very confusing).
Iterator> j = searchTreeBuilder.searchTreeIterator();
while (j.hasNext()) {
m_stronglyConnectedSearchTreeList.add(copySearchTree(j.next(), t));
}
}
/**
* Make a copy of given search tree (in the transposed graph) using vertices
* of the original graph.
*
* @param tree
* a search tree in the transposed graph
* @param t
* the Transpose object which performed the transposition of the
* original graph
*/
private SearchTree copySearchTree(SearchTree tree, Transpose t) {
// Copy this node
SearchTree copy = new SearchTree<>(t.getOriginalGraphVertex(tree.getVertex()));
// Copy children
Iterator> i = tree.childIterator();
while (i.hasNext()) {
SearchTree child = i.next();
copy.addChild(copySearchTree(child, t));
}
return copy;
}
/**
* Returns an iterator over the search trees containing the vertices of each
* strongly connected component.
*
* @return an Iterator over a sequence of SearchTree objects
*/
public Iterator> searchTreeIterator() {
return m_stronglyConnectedSearchTreeList.iterator();
}
/**
* Iterator for iterating over sets of vertices in strongly connected
* components.
*/
private class SCCSetIterator implements Iterator> {
private final Iterator> m_searchTreeIterator;
public SCCSetIterator() {
m_searchTreeIterator = searchTreeIterator();
}
@Override
public boolean hasNext() {
return m_searchTreeIterator.hasNext();
}
@Override
public Set next() {
SearchTree tree = m_searchTreeIterator.next();
TreeSet set = new TreeSet<>();
tree.addVerticesToSet(set);
return set;
}
@Override
public void remove() {
throw new UnsupportedOperationException();
}
}
/**
* Returns an iterator over the sets of vertices of each strongly connected
* component.
*
* @return an Iterator over a sequence of Set objects
*/
public Iterator> setIterator() {
return new SCCSetIterator();
}
}