package hudson.util;
import hudson.Util;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
import java.util.Stack;
/**
* Traverses a directed graph and if it contains any cycle, throw an exception.
*
* @author Kohsuke Kawaguchi
*/
public abstract class CyclicGraphDetector<N> {
private final Set<N> visited = new HashSet<N>();
private final Set<N> visiting = new HashSet<N>();
private final Stack<N> path = new Stack<N>();
public void run(Iterable<? extends N> allNodes) throws CycleDetectedException {
for (N n : allNodes)
visit(n);
}
/**
* List up edges from the given node (by listing nodes that those edges point to.)
*
* @return
* Never null.
*/
protected abstract Iterable<? extends N> getEdges(N n);
private void visit(N p) throws CycleDetectedException {
if (!visited.add(p)) return;
visiting.add(p);
path.push(p);
for (N q : getEdges(p)) {
if (q==null) continue; // ignore unresolved references
if (visiting.contains(q))
detectedCycle(q);
visit(q);
}
visiting.remove(p);
path.pop();
}
private void detectedCycle(N q) throws CycleDetectedException {
int i = path.indexOf(q);
path.push(q);
throw new CycleDetectedException(path.subList(i, path.size()));
}
public static final class CycleDetectedException extends Exception {
public final List cycle;
public CycleDetectedException(List cycle) {
super("Cycle detected: "+Util.join(cycle," -> "));
this.cycle = cycle;
}
}
}