include/o3tl/lru_map.hxx

   1 /* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
   2 /*
   3  * This file is part of the LibreOffice project.
   4  *
   5  * This Source Code Form is subject to the terms of the Mozilla Public
   6  * License, v. 2.0. If a copy of the MPL was not distributed with this
   7  * file, You can obtain one at http://mozilla.org/MPL/2.0/.
   8  *
   9  */
  10
  11 #ifndef INCLUDED_O3TL_LRU_MAP_HXX
  12 #define INCLUDED_O3TL_LRU_MAP_HXX
  13
  14 #include <cassert>
  15 #include <list>
  16 #include <unordered_map>
  17 #include <cstddef>
  18
  19 namespace o3tl
  20 {
  21 /** LRU map
  22  *
  23  * Similar to unordered_map (it actually uses it) with additionally functionality
  24  * which removes the entries that have been "least recently used" when the size
  25  * hits the specified capacity.
  26  *
  27  * It only implements the minimal methods needed and the implementation is NOT
  28  * thread safe.
  29  *
  30  * The implementation is as simple as possible but it still uses O(1) complexity
  31  * for most of the operations with a combination unordered map and linked list.
  32  *
  33  **/
  34 template <typename Key, typename Value, class KeyHash = std::hash<Key>,
  35           class KeyEqual = std::equal_to<Key>>
  36 class lru_map final
  37 {
  38 public:
  39     typedef typename std::pair<Key, Value> key_value_pair_t;
  40
  41 private:
  42     typedef std::list<key_value_pair_t> list_t;
  43     typedef typename list_t::iterator list_iterator_t;
  44     typedef typename list_t::const_iterator list_const_iterator_t;
  45
  46     typedef std::unordered_map<Key, list_iterator_t, KeyHash, KeyEqual> map_t;
  47     typedef typename map_t::iterator map_iterator_t;
  48     typedef typename map_t::const_iterator map_const_iterator_t;
  49
  50     list_t mLruList;
  51     map_t mLruMap;
  52     const size_t mMaxSize;
  53
  54     void checkLRU()
  55     {
  56         if (mLruMap.size() > mMaxSize)
  57         {
  58             // remove from map
  59             mLruMap.erase(mLruList.back().first);
  60             // remove from list
  61             mLruList.pop_back();
  62         }
  63     }
  64
  65 public:
  66     typedef list_iterator_t iterator;
  67     typedef list_const_iterator_t const_iterator;
  68
  69     // a size of 0 effectively disables the LRU cleanup code
  70     lru_map(size_t nMaxSize)
  71         : mMaxSize(nMaxSize ? nMaxSize : std::min(mLruMap.max_size(), mLruList.max_size()))
  72     {
  73     }
  74     ~lru_map()
  75     {
  76         // Some code .e.g. SalBitmap likes to remove itself from a cache during it's destructor, which means we
  77         // get calls into lru_map while we are in destruction, so use the swap-and-clear idiom to avoid those problems.
  78         mLruMap.clear();
  79         list_t aLruListTemp;
  80         aLruListTemp.swap(mLruList);
  81     }
  82
  83     void insert(key_value_pair_t& rPair)
  84     {
  85         map_iterator_t i = mLruMap.find(rPair.first);
  86
  87         if (i == mLruMap.end()) // doesn't exist -> add to queue and map
  88         {
  89             // add to front of the list
  90             mLruList.push_front(rPair);
  91             // add the list position (iterator) to the map
  92             auto it = mLruList.begin();
  93             mLruMap[it->first] = it;
  94             checkLRU();
  95         }
  96         else // already exists -> replace value
  97         {
  98             // replace value
  99             i->second->second = rPair.second;
 100             // bring to front of the lru list
 101             mLruList.splice(mLruList.begin(), mLruList, i->second);
 102         }
 103     }
 104
 105     void insert(key_value_pair_t&& rPair)
 106     {
 107         map_iterator_t i = mLruMap.find(rPair.first);
 108
 109         if (i == mLruMap.end()) // doesn't exist -> add to list and map
 110         {
 111             // add to front of the list
 112             mLruList.push_front(std::move(rPair));
 113             // add the list position (iterator) to the map
 114             auto it = mLruList.begin();
 115             mLruMap[it->first] = it;
 116             checkLRU();
 117         }
 118         else // already exists -> replace value
 119         {
 120             // replace value
 121             i->second->second = std::move(rPair.second);
 122             // push to back of the lru list
 123             mLruList.splice(mLruList.begin(), mLruList, i->second);
 124         }
 125     }
 126
 127     list_const_iterator_t find(const Key& key)
 128     {
 129         const map_iterator_t i = mLruMap.find(key);
 130         if (i == mLruMap.cend()) // can't find entry for the key
 131         {
 132             // return empty iterator
 133             return mLruList.cend();
 134         }
 135         else
 136         {
 137             // push to back of the lru list
 138             mLruList.splice(mLruList.begin(), mLruList, i->second);
 139             return i->second;
 140         }
 141     }
 142
 143     // reverse-iterates the list removing all items matching the predicate
 144     template <class UnaryPredicate> void remove_if(UnaryPredicate pred)
 145     {
 146         auto it = mLruList.rbegin();
 147         while (it != mLruList.rend())
 148         {
 149             if (pred(*it))
 150             {
 151                 mLruMap.erase(it->first);
 152                 it = decltype(it){ mLruList.erase(std::next(it).base()) };
 153             }
 154             else
 155                 ++it;
 156         }
 157     }
 158
 159     list_const_iterator_t begin() const { return mLruList.cbegin(); }
 160
 161     list_const_iterator_t end() const { return mLruList.cend(); }
 162
 163     size_t size() const
 164     {
 165         assert(mLruMap.size() == mLruList.size());
 166         return mLruMap.size();
 167     }
 168
 169     void clear()
 170     {
 171         mLruMap.clear();
 172         mLruList.clear();
 173     }
 174 };
 175 }
 176
 177 #endif /* INCLUDED_O3TL_LRU_MAP_HXX */
 178
 179 /* vim:set shiftwidth=4 softtabstop=4 expandtab: */