//
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// Copyright (C) 2010 The Android Open Source Project
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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//
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#ifndef UPDATE_ENGINE_PAYLOAD_GENERATOR_CYCLE_BREAKER_H_
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#define UPDATE_ENGINE_PAYLOAD_GENERATOR_CYCLE_BREAKER_H_
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// This is a modified implementation of Donald B. Johnson's algorithm for
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// finding all elementary cycles (a.k.a. circuits) in a directed graph.
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// See the paper "Finding All the Elementary Circuits of a Directed Graph"
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// at http://dutta.csc.ncsu.edu/csc791_spring07/wrap/circuits_johnson.pdf
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// for reference.
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// Note: this version of the algorithm not only finds cycles, but breaks them.
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// It uses a simple greedy algorithm for cutting: when a cycle is discovered,
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// the edge with the least weight is cut. Longer term we may wish to do
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// something more intelligent, since the goal is (ideally) to minimize the
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// sum of the weights of all cut cycles. In practice, it's intractable
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// to consider all cycles before cutting any; there are simply too many.
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// In a sample graph representative of a typical workload, I found over
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// 5 * 10^15 cycles.
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#include <set>
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#include <vector>
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#include "update_engine/payload_generator/graph_types.h"
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namespace chromeos_update_engine {
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class CycleBreaker {
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public:
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CycleBreaker() : skipped_ops_(0) {}
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// out_cut_edges is replaced with the cut edges.
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void BreakCycles(const Graph& graph, std::set<Edge>* out_cut_edges);
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size_t skipped_ops() const { return skipped_ops_; }
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private:
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void HandleCircuit();
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void Unblock(Vertex::Index u);
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bool Circuit(Vertex::Index vertex, Vertex::Index depth);
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bool StackContainsCutEdge() const;
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std::vector<bool> blocked_; // "blocked" in the paper
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Vertex::Index current_vertex_; // "s" in the paper
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std::vector<Vertex::Index> stack_; // the stack variable in the paper
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Graph subgraph_; // "A_K" in the paper
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Graph blocked_graph_; // "B" in the paper
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std::set<Edge> cut_edges_;
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// Number of operations skipped b/c we know they don't have any
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// incoming edges.
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size_t skipped_ops_;
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};
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} // namespace chromeos_update_engine
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#endif // UPDATE_ENGINE_PAYLOAD_GENERATOR_CYCLE_BREAKER_H_
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