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1 /*
2  * Copyright (C) 2016 The Android Open Source Project
3  *
4  * Licensed under the Apache License, Version 2.0 (the "License");
5  * you may not use this file except in compliance with the License.
6  * You may obtain a copy of the License at
7  *
8  *      http://www.apache.org/licenses/LICENSE-2.0
9  *
10  * Unless required by applicable law or agreed to in writing, software
11  * distributed under the License is distributed on an "AS IS" BASIS,
12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13  * See the License for the specific language governing permissions and
14  * limitations under the License.
15  */
16 
17 // Based on system/update_engine/payload_generator/tarjan.cc
18 
19 #ifndef LIBMEMUNREACHABLE_TARJAN_H_
20 #define LIBMEMUNREACHABLE_TARJAN_H_
21 
22 #include <algorithm>
23 
24 #include "Allocator.h"
25 
26 template<class T>
27 class Node {
28  public:
29   allocator::set<Node<T>*> references_in;
30   allocator::set<Node<T>*> references_out;
31   size_t index;
32   size_t lowlink;
33 
34   T* ptr;
35 
Node(T * ptr,Allocator<Node> allocator)36   Node(T* ptr, Allocator<Node> allocator) : references_in(allocator), references_out(allocator),
37       ptr(ptr) {};
38   Node(Node&& rhs) = default;
Edge(Node<T> * ref)39   void Edge(Node<T>* ref) {
40     references_out.emplace(ref);
41     ref->references_in.emplace(this);
42   }
43   template<class F>
Foreach(F && f)44   void Foreach(F&& f) {
45     for (auto& node: references_out) {
46       f(node->ptr);
47     }
48   }
49  private:
50   DISALLOW_COPY_AND_ASSIGN(Node<T>);
51 };
52 
53 template<class T>
54 using Graph = allocator::vector<Node<T>*>;
55 
56 template<class T>
57 using SCC = allocator::vector<Node<T>*>;
58 
59 template<class T>
60 using SCCList = allocator::vector<SCC<T>>;
61 
62 template<class T>
63 class TarjanAlgorithm {
64  public:
TarjanAlgorithm(Allocator<void> allocator)65   TarjanAlgorithm(Allocator<void> allocator) : index_(0),
66     stack_(allocator), components_(allocator) {}
67 
68   void Execute(Graph<T>& graph, SCCList<T>& out);
69  private:
70   static constexpr size_t UNDEFINED_INDEX = static_cast<size_t>(-1);
71   void Tarjan(Node<T>* vertex, Graph<T>& graph);
72 
73   size_t index_;
74   allocator::vector<Node<T>*> stack_;
75   SCCList<T> components_;
76 };
77 
78 template<class T>
Execute(Graph<T> & graph,SCCList<T> & out)79 void TarjanAlgorithm<T>::Execute(Graph<T>& graph, SCCList<T>& out) {
80   stack_.clear();
81   components_.clear();
82   index_ = 0;
83   for (auto& it: graph) {
84     it->index = UNDEFINED_INDEX;
85     it->lowlink = UNDEFINED_INDEX;
86   }
87 
88   for (auto& it: graph) {
89     if (it->index == UNDEFINED_INDEX) {
90       Tarjan(it, graph);
91     }
92   }
93   out.swap(components_);
94 }
95 
96 template<class T>
Tarjan(Node<T> * vertex,Graph<T> & graph)97 void TarjanAlgorithm<T>::Tarjan(Node<T>* vertex, Graph<T>& graph) {
98   assert(vertex->index == UNDEFINED_INDEX);
99   vertex->index = index_;
100   vertex->lowlink = index_;
101   index_++;
102   stack_.push_back(vertex);
103   for (auto& it: vertex->references_out) {
104     Node<T>* vertex_next = it;
105     if (vertex_next->index == UNDEFINED_INDEX) {
106       Tarjan(vertex_next, graph);
107       vertex->lowlink = std::min(vertex->lowlink, vertex_next->lowlink);
108     } else if (std::find(stack_.begin(), stack_.end(), vertex_next) != stack_.end()) {
109       vertex->lowlink = std::min(vertex->lowlink, vertex_next->index);
110     }
111   }
112   if (vertex->lowlink == vertex->index) {
113     SCC<T> component{components_.get_allocator()};
114     Node<T>* other_vertex;
115     do {
116       other_vertex = stack_.back();
117       stack_.pop_back();
118       component.push_back(other_vertex);
119     } while (other_vertex != vertex && !stack_.empty());
120 
121     components_.emplace_back(component);
122   }
123 }
124 
125 template<class T>
Tarjan(Graph<T> & graph,SCCList<T> & out)126 void Tarjan(Graph<T>& graph, SCCList<T>& out) {
127   TarjanAlgorithm<T> tarjan{graph.get_allocator()};
128   tarjan.Execute(graph, out);
129 }
130 
131 #endif // LIBMEMUNREACHABLE_TARJAN_H_
132