net/base/linked_hash_map.h

   1 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4 //
   5 // This is a simplistic insertion-ordered map.  It behaves similarly to an STL
   6 // map, but only implements a small subset of the map's methods.  Internally, we
   7 // just keep a map and a list going in parallel.
   8 //
   9 // This class provides no thread safety guarantees, beyond what you would
  10 // normally see with std::list.
  11 //
  12 // Iterators should be stable in the face of mutations, except for an
  13 // iterator pointing to an element that was just deleted.
  14
  15 #ifndef UTIL_GTL_LINKED_HASH_MAP_H_
  16 #define UTIL_GTL_LINKED_HASH_MAP_H_
  17
  18 #include <list>
  19 #include <utility>
  20
  21 #include "base/containers/hash_tables.h"
  22 #include "base/logging.h"
  23
  24 // This holds a list of pair<Key, Value> items.  This list is what gets
  25 // traversed, and it's iterators from this list that we return from
  26 // begin/end/find.
  27 //
  28 // We also keep a map<Key, list::iterator> for find.  Since std::list is a
  29 // doubly-linked list, the iterators should remain stable.
  30 template<class Key, class Value>
  31 class linked_hash_map {
  32  private:
  33   typedef std::list<std::pair<Key, Value> > ListType;
  34   typedef base::hash_map<Key, typename ListType::iterator> MapType;
  35
  36  public:
  37   typedef typename ListType::iterator iterator;
  38   typedef typename ListType::reverse_iterator reverse_iterator;
  39   typedef typename ListType::const_iterator const_iterator;
  40   typedef typename ListType::const_reverse_iterator const_reverse_iterator;
  41   typedef typename MapType::key_type key_type;
  42   typedef typename ListType::value_type value_type;
  43   typedef typename ListType::size_type size_type;
  44
  45   linked_hash_map() : map_(), list_() {
  46   }
  47
  48   // Returns an iterator to the first (insertion-ordered) element.  Like a map,
  49   // this can be dereferenced to a pair<Key, Value>.
  50   iterator begin() {
  51     return list_.begin();
  52   }
  53   const_iterator begin() const {
  54     return list_.begin();
  55   }
  56
  57   // Returns an iterator beyond the last element.
  58   iterator end() {
  59     return list_.end();
  60   }
  61   const_iterator end() const {
  62     return list_.end();
  63   }
  64
  65   // Returns an iterator to the last (insertion-ordered) element.  Like a map,
  66   // this can be dereferenced to a pair<Key, Value>.
  67   reverse_iterator rbegin() {
  68     return list_.rbegin();
  69   }
  70   const_reverse_iterator rbegin() const {
  71     return list_.rbegin();
  72   }
  73
  74   // Returns an iterator beyond the first element.
  75   reverse_iterator rend() {
  76     return list_.rend();
  77   }
  78   const_reverse_iterator rend() const {
  79     return list_.rend();
  80   }
  81
  82   // Clears the map of all values.
  83   void clear() {
  84     map_.clear();
  85     list_.clear();
  86   }
  87
  88   // Returns true iff the map is empty.
  89   bool empty() const {
  90     return list_.empty();
  91   }
  92
  93   // Erases values with the provided key.  Returns the number of elements
  94   // erased.  In this implementation, this will be 0 or 1.
  95   size_type erase(const Key& key) {
  96     typename MapType::iterator found = map_.find(key);
  97     if (found == map_.end()) return 0;
  98
  99     list_.erase(found->second);
 100     map_.erase(found);
 101
 102     return 1;
 103   }
 104
 105   // Erases values with the provided iterator. If the provided iterator is
 106   // invalid or there is inconsistency between the map and list, a CHECK() error
 107   // will occur.
 108   void erase(iterator position) {
 109     typename MapType::iterator found = map_.find(position->first);
 110     CHECK(found->second == position)
 111         << "Inconsisent iterator for map and list, or the iterator is invalid.";
 112
 113     list_.erase(position);
 114     map_.erase(found);
 115   }
 116
 117   // Erases values between first and last, not including last.
 118   void erase(iterator first, iterator last) {
 119     while (first != last && first != end()) {
 120       erase(first++);
 121     }
 122   }
 123
 124   // Finds the element with the given key.  Returns an iterator to the
 125   // value found, or to end() if the value was not found.  Like a map, this
 126   // iterator points to a pair<Key, Value>.
 127   iterator find(const Key& key) {
 128     typename MapType::iterator found = map_.find(key);
 129     if (found == map_.end()) {
 130       return end();
 131     }
 132     return found->second;
 133   }
 134
 135   const_iterator find(const Key& key) const {
 136     typename MapType::const_iterator found = map_.find(key);
 137     if (found == map_.end()) {
 138       return end();
 139     }
 140     return found->second;
 141   }
 142
 143   // Returns the bounds of a range that includes all the elements in the
 144   // container with a key that compares equal to x.
 145   std::pair<iterator, iterator> equal_range(const key_type& key) {
 146     std::pair<typename MapType::iterator, typename MapType::iterator> eq_range =
 147         map_.equal_range(key);
 148
 149     return std::make_pair(eq_range.first->second, eq_range.second->second);
 150   }
 151
 152   std::pair<const_iterator, const_iterator> equal_range(
 153       const key_type& key) const {
 154     std::pair<typename MapType::const_iterator,
 155         typename MapType::const_iterator> eq_range =
 156         map_.equal_range(key);
 157     const const_iterator& start_iter = eq_range.first != map_.end() ?
 158         eq_range.first->second : end();
 159     const const_iterator& end_iter = eq_range.second != map_.end() ?
 160         eq_range.second->second : end();
 161
 162     return std::make_pair(start_iter, end_iter);
 163   }
 164
 165   // Returns the value mapped to key, or an inserted iterator to that position
 166   // in the map.
 167   Value& operator[](const key_type& key) {
 168     return (*((this->insert(std::make_pair(key, Value()))).first)).second;
 169   }
 170
 171   // Inserts an element into the map
 172   std::pair<iterator, bool> insert(const std::pair<Key, Value>& pair) {
 173     // First make sure the map doesn't have a key with this value.  If it does,
 174     // return a pair with an iterator to it, and false indicating that we
 175     // didn't insert anything.
 176     typename MapType::iterator found = map_.find(pair.first);
 177     if (found != map_.end()) return std::make_pair(found->second, false);
 178
 179     // Otherwise, insert into the list first.
 180     list_.push_back(pair);
 181
 182     // Obtain an iterator to the newly added element.  We do -- instead of -
 183     // since list::iterator doesn't implement operator-().
 184     typename ListType::iterator last = list_.end();
 185     --last;
 186
 187     CHECK(map_.insert(std::make_pair(pair.first, last)).second)
 188         << "Map and list are inconsistent";
 189
 190     return std::make_pair(last, true);
 191   }
 192
 193   size_type size() const {
 194     return list_.size();
 195   }
 196
 197   void swap(linked_hash_map& other) {
 198     map_.swap(other.map_);
 199     list_.swap(other.list_);
 200   }
 201
 202  private:
 203   // The map component, used for speedy lookups
 204   MapType map_;
 205
 206   // The list component, used for maintaining insertion order
 207   ListType list_;
 208 };
 209
 210 #endif  // UTIL_GTL_LINKED_HASH_MAP_H_