cycle_breaker.h

// 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 implementation of Donald B. Johnson's algorithm for
// 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:
  CycleBreaker() : skipped_ops_(0) {}
  // out_cut_edges is replaced with the cut edges.
  void BreakCycles(const Graph& graph, std::set<Edge>* out_cut_edges);
  
  size_t skipped_ops() const { return skipped_ops_; }

 private:
  void HandleCircuit();
  void Unblock(Vertex::Index u);
  bool Circuit(Vertex::Index vertex, Vertex::Index depth);
  bool StackContainsCutEdge() const;

  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_;
  
  // Number of operations skipped b/c we know they don't have any
  // incoming edges.
  size_t skipped_ops_;
};

}  // namespace chromeos_update_engine

#endif  // CHROMEOS_PLATFORM_UPDATE_ENGINE_CYCLE_BREAKER_H__