/*
* The subroutine of DFS. NOTE: the set is used to distinguish between 1st
* and 2nd round of DFS. set == null: finished vertices are stored (1st
* round). set != null: all vertices found will be saved in the set (2nd
* round)
*/
private void dfsVisit(
DirectedGraph<V, E> visitedGraph, VertexData<V> vertexData, Set<V> vertices) {
Deque<VertexData<V>> stack = new ArrayDeque<VertexData<V>>();
stack.add(vertexData);
while (!stack.isEmpty()) {
VertexData<V> data = stack.removeLast();
if (!data.isDiscovered()) {
data.setDiscovered(true);
if (vertices != null) {
vertices.add(data.getVertex());
}
stack.add(new VertexData1<V>(data, true, true));
// follow all edges
for (E edge : visitedGraph.outgoingEdgesOf(data.getVertex())) {
VertexData<V> targetData = vertexToVertexData.get(visitedGraph.getEdgeTarget(edge));
if (!targetData.isDiscovered()) {
// the "recursion"
stack.add(targetData);
}
}
} else if (data.isFinished()) {
if (vertices == null) {
orderedVertices.addFirst(data.getFinishedData());
}
}
}
}
/**
* Computes a {@link List} of {@link Set}s, where each set contains vertices which together form a
* strongly connected component within the given graph.
*
* @return <code>List</code> of <code>Set</code> s containing the strongly connected components
*/
public List<Set<V>> stronglyConnectedSets() {
if (stronglyConnectedSets == null) {
orderedVertices = new LinkedList<VertexData<V>>();
stronglyConnectedSets = new Vector<Set<V>>();
// create VertexData objects for all vertices, store them
createVertexData();
// perform the first round of DFS, result is an ordering
// of the vertices by decreasing finishing time
for (VertexData<V> data : vertexToVertexData.values()) {
if (!data.isDiscovered()) {
dfsVisit(graph, data, null);
}
}
// 'create' inverse graph (i.e. every edge is reversed)
DirectedGraph<V, E> inverseGraph = new EdgeReversedGraph<V, E>(graph);
// get ready for next dfs round
resetVertexData();
// second dfs round: vertices are considered in decreasing
// finishing time order; every tree found is a strongly
// connected set
for (VertexData<V> data : orderedVertices) {
if (!data.isDiscovered()) {
// new strongly connected set
Set<V> set = new HashSet<V>();
stronglyConnectedSets.add(set);
dfsVisit(inverseGraph, data, set);
}
}
// clean up for garbage collection
orderedVertices = null;
vertexToVertexData = null;
}
return stronglyConnectedSets;
}
/*
* Resets all VertexData objects.
*/
private void resetVertexData() {
for (VertexData<V> data : vertexToVertexData.values()) {
data.setDiscovered(false);
data.setFinished(false);
}
}