AU: Cycle Breaker for directed graphs.

src/platform/update_engine/cycle_breaker.h

Index: src/platform/update_engine/cycle_breaker.h
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+// Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CHROMEOS_PLATFORM_UPDATE_ENGINE_CYCLE_BREAKER_H__
+#define CHROMEOS_PLATFORM_UPDATE_ENGINE_CYCLE_BREAKER_H__
+
+// This is a modified implemenation of Donald B. Johnson's algorithm for

[Daniel Erat, 2010/03/10 22:50:01]

s/implemenation/implementation/

[adlr, 2010/03/11 00:41:42]

Done.

+// finding all elementary cycles (a.k.a. circuits) in a directed graph.
+// See the paper "Finding All the Elementary Circuits of a Directed Graph"
+// at http://dutta.csc.ncsu.edu/csc791_spring07/wrap/circuits_johnson.pdf
+// for reference.
+
+// Note: this version of the algorithm not only finds cycles, but breaks them.
+// It uses a simple greedy algorithm for cutting: when a cycle is discovered,
+// the edge with the least weight is cut. Longer term we may wish to do
+// something more intelligent, since the goal is (ideally) to minimize the
+// sum of the weights of all cut cycles. In practice, it's intractable
+// to consider all cycles before cutting any; there are simply too many.
+// In a sample graph representative of a typical workload, I found over
+// 5 * 10^15 cycles.
+
+#include <set>
+#include <vector>
+#include "update_engine/graph_types.h"
+
+namespace chromeos_update_engine {
+
+class CycleBreaker {
+ public:
+  // out_cut_edges is replaced with the cut edges.
+  void BreakCycles(const Graph& graph, std::set<Edge>* out_cut_edges);
+
+ private:
+  void HandleCircuit();
+  void Unblock(Vertex::Index u);
+  bool Circuit(Vertex::Index vertex);
+
+  std::vector<bool> blocked_;  // "blocked" in the paper
+  Vertex::Index current_vertex_;  // "s" in the paper
+  std::vector<Vertex::Index> stack_;  // the stack variable in the paper
+  Graph subgraph_;  // "A_K" in the paper
+  Graph blocked_graph_;  // "B" in the paper
+
+  std::set<Edge> cut_edges_;
+};
+
+}  // namespace chromeos_update_engine
+
+#endif  // CHROMEOS_PLATFORM_UPDATE_ENGINE_CYCLE_BREAKER_H__