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// Copyright Yahoo. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
package com.yahoo.container.di.componentgraph.cycle;
import java.util.ArrayList;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.logging.Logger;
import java.util.stream.Collectors;
import static com.yahoo.container.di.componentgraph.cycle.CycleFinder.State.BLACK;
import static com.yahoo.container.di.componentgraph.cycle.CycleFinder.State.GRAY;
import static com.yahoo.container.di.componentgraph.cycle.CycleFinder.State.WHITE;
import static java.util.logging.Level.FINE;
import static java.util.Collections.singletonList;
/**
* <p>Applies the
* <a href="https://www.geeksforgeeks.org/detect-cycle-direct-graph-using-colors/"> three-color algorithm</a>
* to detect a cycle in a directed graph. If there are multiple cycles, this implementation only detects one
* of them and does not guarantee that the shortest cycle is found.
* </p>
*
* @author gjoranv
*/
public class CycleFinder<T> {
private static final Logger log = Logger.getLogger(CycleFinder.class.getName());
enum State {
WHITE, GRAY, BLACK;
}
private final Graph<T> graph;
private Map<T, State> colors;
private List<T> cycle;
public CycleFinder(Graph<T> graph) {
this.graph = graph;
}
private void resetState() {
cycle = null;
colors = new LinkedHashMap<>();
graph.getVertices().forEach(v -> colors.put(v, WHITE));
}
/**
* Returns a list of vertices constituting a cycle in the graph, or an empty
* list if no cycle was found. Only the first encountered cycle is returned.
*/
public List<T> findCycle() {
resetState();
for (T vertex : graph.getVertices()) {
if (colors.get(vertex) == WHITE) {
if (visitDepthFirst(vertex, new ArrayList<>(singletonList(vertex)))) {
if (cycle == null) throw new IllegalStateException("Null cycle - this should never happen");
if (cycle.isEmpty()) throw new IllegalStateException("Empty cycle - this should never happen");
log.log(FINE, () -> "Cycle detected: " + cycle);
return cycle;
}
}
}
return new ArrayList<>();
}
private boolean visitDepthFirst(T vertex, List<T> path) {
colors.put(vertex, GRAY);
log.log(FINE, () -> "Vertex start " + vertex + " - colors: " + colors + " - path: " + path);
for (T adjacent : graph.getAdjacent(vertex)) {
path.add(adjacent);
if (colors.get(adjacent) == GRAY) {
cycle = removePathIntoCycle(path);
return true;
}
if (colors.get(adjacent) == WHITE && visitDepthFirst(adjacent, path)) {
return true;
}
path.remove(adjacent);
}
colors.put(vertex, BLACK);
log.log(FINE, () -> "Vertex end " + vertex + " - colors: " + colors + " - path: " + path);
return false;
}
private List<T> removePathIntoCycle(List<T> pathWithCycle) {
T cycleStart = pathWithCycle.get(pathWithCycle.size() - 1);
return pathWithCycle.stream()
.dropWhile(vertex -> ! vertex.equals(cycleStart))
.toList();
}
}
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