org.jgrapht.traverse

Class TopologicalOrderIterator<V,E>

java.lang.Object

org.jgrapht.traverse.AbstractGraphIterator<V,E>

org.jgrapht.traverse.CrossComponentIterator<V,E,Object>

org.jgrapht.traverse.TopologicalOrderIterator<V,E>

Type Parameters:

V - the graph vertex type

E - the graph edge type

All Implemented Interfaces:

Iterator<V>, GraphIterator<V,E>

public class TopologicalOrderIterator<V,E>
extends CrossComponentIterator<V,E,Object>

Implements topological order traversal for a directed acyclic graph. A topological sort is a permutation p of the vertices of a graph such that an edge (i,j) implies that i appears before j in p (Skiena 1990, p. 208). See also http://mathworld.wolfram.com/TopologicalSort.html.

See "Algorithms in Java, Third Edition, Part 5: Graph Algorithms" by Robert Sedgewick and "Data Structures and Algorithms with Object-Oriented Design Patterns in Java" by Bruno R. Preiss for implementation alternatives. The latter can be found online at http://www.brpreiss.com/books/opus5/

For this iterator to work correctly the graph must be acyclic, and must not be modified during iteration. Currently there are no means to ensure that, nor to fail-fast; the results with cyclic input (including self-loops) or concurrent modifications are undefined. To precheck a graph for cycles, consider using CycleDetector or KosarajuStrongConnectivityInspector.

Since:

Dec 18, 2004

Author:

Marden Neubert

Nested Class Summary

Nested classes/interfaces inherited from class org.jgrapht.traverse.CrossComponentIterator

CrossComponentIterator.VisitColor

Field Summary

Fields inherited from class org.jgrapht.traverse.AbstractGraphIterator

nListeners, reusableEdgeEvent, reusableVertexEvent, specifics

Constructor Summary

Constructors

Constructor and Description
TopologicalOrderIterator(DirectedGraph<V,E> dg) Creates a new topological order iterator over the directed graph specified, with arbitrary tie-breaking in case of partial order.
TopologicalOrderIterator(DirectedGraph<V,E> dg, Queue<V> queue) Creates a new topological order iterator over the directed graph specified, with a user-supplied queue implementation to allow customized control over tie-breaking in case of partial order.

Method Summary

Modifier and Type	Method and Description
protected void	encounterVertex(V vertex, E edge) Update data structures the first time we see a vertex.
protected void	encounterVertexAgain(V vertex, E edge) Called whenever we re-encounter a vertex.
protected boolean	isConnectedComponentExhausted() Returns true if there are no more uniterated vertices in the currently iterated connected component; false otherwise.
protected V	provideNextVertex() Returns the vertex to be returned in the following call to the iterator next method.

Methods inherited from class org.jgrapht.traverse.CrossComponentIterator

finishVertex, getGraph, getSeenData, hasNext, isSeenVertex, next, putSeenData

Methods inherited from class org.jgrapht.traverse.AbstractGraphIterator

addTraversalListener, fireConnectedComponentFinished, fireConnectedComponentStarted, fireEdgeTraversed, fireVertexFinished, fireVertexTraversed, isCrossComponentTraversal, isReuseEvents, remove, removeTraversalListener, setCrossComponentTraversal, setReuseEvents

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Methods inherited from interface java.util.Iterator

forEachRemaining

Constructor Detail

TopologicalOrderIterator

public TopologicalOrderIterator(DirectedGraph<V,E> dg)

Creates a new topological order iterator over the directed graph specified, with arbitrary tie-breaking in case of partial order. Traversal will start at one of the graph's sources. See the definition of source at http://mathworld.wolfram.com/Source.html.

Parameters:

dg - the directed graph to be iterated.

TopologicalOrderIterator

public TopologicalOrderIterator(DirectedGraph<V,E> dg,
                                Queue<V> queue)

Creates a new topological order iterator over the directed graph specified, with a user-supplied queue implementation to allow customized control over tie-breaking in case of partial order. Traversal will start at one of the graph's sources. See the definition of source at http://mathworld.wolfram.com/Source.html.

Parameters:

dg - the directed graph to be iterated.

queue - queue to use for tie-break in case of partial order (e.g. a PriorityQueue can be used to break ties according to vertex priority); must be initially empty

Method Detail

isConnectedComponentExhausted

protected boolean isConnectedComponentExhausted()

Description copied from class: CrossComponentIterator

Returns true if there are no more uniterated vertices in the currently iterated connected component; false otherwise.

Specified by:

isConnectedComponentExhausted in class CrossComponentIterator<V,E,Object>

Returns:

true if there are no more uniterated vertices in the currently iterated connected component; false otherwise.

See Also:

CrossComponentIterator.isConnectedComponentExhausted()

encounterVertex

protected void encounterVertex(V vertex,
                               E edge)

Description copied from class: CrossComponentIterator

Update data structures the first time we see a vertex.

Specified by:

encounterVertex in class CrossComponentIterator<V,E,Object>

Parameters:

vertex - the vertex encountered

edge - the edge via which the vertex was encountered, or null if the vertex is a starting point

See Also:

CrossComponentIterator.encounterVertex(Object, Object)

encounterVertexAgain

protected void encounterVertexAgain(V vertex,
                                    E edge)

Description copied from class: CrossComponentIterator

Called whenever we re-encounter a vertex. The default implementation does nothing.

Specified by:

encounterVertexAgain in class CrossComponentIterator<V,E,Object>

Parameters:

vertex - the vertex re-encountered

edge - the edge via which the vertex was re-encountered

See Also:

CrossComponentIterator.encounterVertexAgain(Object, Object)

provideNextVertex

protected V provideNextVertex()

Description copied from class: CrossComponentIterator

Returns the vertex to be returned in the following call to the iterator next method.

Specified by:

provideNextVertex in class CrossComponentIterator<V,E,Object>

Returns:

the next vertex to be returned by this iterator.

See Also:

CrossComponentIterator.provideNextVertex()