arangodb/lib/Basics/LruCache.h

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//////////////////////////////////////////////////////////////////////////////////

#ifndef ARANGODB_LRUCACHE_H
#define ARANGODB_LRUCACHE_H 1

#include <unordered_map>
#include <list>
#include <cstddef>
#include <stdexcept>

namespace arangodb {
namespace basics {

template<typename key_t, typename value_t>
class LruCache {
  public:
    typedef typename std::pair<key_t, value_t> key_value_pair_t;
    typedef typename std::list<key_value_pair_t>::iterator list_iterator_t;

    LruCache(size_t max_size) :
      _max_size(max_size) {
      }

    void put(const key_t& key, const value_t& value) {
      auto it = _cache_items_map.find(key);
      _cache_items_list.push_front(key_value_pair_t(key, value));
      if (it != _cache_items_map.end()) {
        _cache_items_list.erase(it->second);
        _cache_items_map.erase(it);
      }
      _cache_items_map[key] = _cache_items_list.begin();

      if (_cache_items_map.size() > _max_size) {
        auto last = _cache_items_list.end();
        last--;
        _cache_items_map.erase(last->first);
        _cache_items_list.pop_back();
      }
    }

    value_t const* get(const key_t& key) {
      auto it = _cache_items_map.find(key);
      if (it == _cache_items_map.end()) {
        return nullptr;
      } else {
        _cache_items_list.splice(_cache_items_list.begin(), _cache_items_list, it->second);
        return &it->second->second;
      }
    }

    bool remove(key_t const& key) {
      auto it = _cache_items_map.find(key);
      if (it == _cache_items_map.end()) {
        return false;
      } else {
        _cache_items_list.erase(it->second);
        _cache_items_map.erase(it);
        return true;
      }
    }

    void clear() {
      _cache_items_map.clear();
      _cache_items_list.clear();
    }

    bool exists(const key_t& key) const {
      return _cache_items_map.find(key) != _cache_items_map.end();
    }

    size_t size() const {
      return _cache_items_map.size();
    }

  private:
    std::list<key_value_pair_t> _cache_items_list;
    std::unordered_map<key_t, list_iterator_t> _cache_items_map;
    size_t _max_size;
};

}
}

#endif