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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 <assert.h>
     23 #include <algorithm>
     24 
     25 #include "Allocator.h"
     26 
     27 template<class T>
     28 class Node {
     29  public:
     30   allocator::set<Node<T>*> references_in;
     31   allocator::set<Node<T>*> references_out;
     32   size_t index;
     33   size_t lowlink;
     34 
     35   T* ptr;
     36 
     37   Node(T* ptr, Allocator<Node> allocator) : references_in(allocator), references_out(allocator),
     38       ptr(ptr) {};
     39   Node(Node&& rhs) = default;
     40   void Edge(Node<T>* ref) {
     41     references_out.emplace(ref);
     42     ref->references_in.emplace(this);
     43   }
     44   template<class F>
     45   void Foreach(F&& f) {
     46     for (auto& node: references_out) {
     47       f(node->ptr);
     48     }
     49   }
     50  private:
     51   DISALLOW_COPY_AND_ASSIGN(Node<T>);
     52 };
     53 
     54 template<class T>
     55 using Graph = allocator::vector<Node<T>*>;
     56 
     57 template<class T>
     58 using SCC = allocator::vector<Node<T>*>;
     59 
     60 template<class T>
     61 using SCCList = allocator::vector<SCC<T>>;
     62 
     63 template<class T>
     64 class TarjanAlgorithm {
     65  public:
     66   explicit TarjanAlgorithm(Allocator<void> allocator) : index_(0),
     67     stack_(allocator), components_(allocator) {}
     68 
     69   void Execute(Graph<T>& graph, SCCList<T>& out);
     70  private:
     71   static constexpr size_t UNDEFINED_INDEX = static_cast<size_t>(-1);
     72   void Tarjan(Node<T>* vertex, Graph<T>& graph);
     73 
     74   size_t index_;
     75   allocator::vector<Node<T>*> stack_;
     76   SCCList<T> components_;
     77 };
     78 
     79 template<class T>
     80 void TarjanAlgorithm<T>::Execute(Graph<T>& graph, SCCList<T>& out) {
     81   stack_.clear();
     82   components_.clear();
     83   index_ = 0;
     84   for (auto& it: graph) {
     85     it->index = UNDEFINED_INDEX;
     86     it->lowlink = UNDEFINED_INDEX;
     87   }
     88 
     89   for (auto& it: graph) {
     90     if (it->index == UNDEFINED_INDEX) {
     91       Tarjan(it, graph);
     92     }
     93   }
     94   out.swap(components_);
     95 }
     96 
     97 template<class T>
     98 void TarjanAlgorithm<T>::Tarjan(Node<T>* vertex, Graph<T>& graph) {
     99   assert(vertex->index == UNDEFINED_INDEX);
    100   vertex->index = index_;
    101   vertex->lowlink = index_;
    102   index_++;
    103   stack_.push_back(vertex);
    104   for (auto& it: vertex->references_out) {
    105     Node<T>* vertex_next = it;
    106     if (vertex_next->index == UNDEFINED_INDEX) {
    107       Tarjan(vertex_next, graph);
    108       vertex->lowlink = std::min(vertex->lowlink, vertex_next->lowlink);
    109     } else if (std::find(stack_.begin(), stack_.end(), vertex_next) != stack_.end()) {
    110       vertex->lowlink = std::min(vertex->lowlink, vertex_next->index);
    111     }
    112   }
    113   if (vertex->lowlink == vertex->index) {
    114     SCC<T> component{components_.get_allocator()};
    115     Node<T>* other_vertex;
    116     do {
    117       other_vertex = stack_.back();
    118       stack_.pop_back();
    119       component.push_back(other_vertex);
    120     } while (other_vertex != vertex && !stack_.empty());
    121 
    122     components_.emplace_back(component);
    123   }
    124 }
    125 
    126 template<class T>
    127 void Tarjan(Graph<T>& graph, SCCList<T>& out) {
    128   TarjanAlgorithm<T> tarjan{graph.get_allocator()};
    129   tarjan.Execute(graph, out);
    130 }
    131 
    132 #endif // LIBMEMUNREACHABLE_TARJAN_H_
    133