1 files changed, 98 insertions, 0 deletions
diff --git a/container-di/src/main/java/com/yahoo/container/di/componentgraph/cycle/CycleFinder.java b/container-di/src/main/java/com/yahoo/container/di/componentgraph/cycle/CycleFinder.java
new file mode 100644
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+++ b/container-di/src/main/java/com/yahoo/container/di/componentgraph/cycle/CycleFinder.java
@@ -0,0 +1,98 @@
+/*
+ * Copyright 2019 Oath Inc. 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 com.yahoo.log.LogLevel.DEBUG;
+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(DEBUG, "Cycle detected: " + cycle);
+                    return cycle;
+                }
+            }
+        }
+        return new ArrayList<>();
+    }
+
+    private boolean visitDepthFirst(T vertex, List<T> path) {
+        colors.put(vertex, GRAY);
+        log.log(DEBUG, "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(DEBUG, "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))
+                .collect(Collectors.toList());
+    }
+
+}