lrucache.hpp

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/*
 * This file is part of OpenTTD.
 * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
 * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
 */
 
#ifndef LRUCACHE_HPP
#define LRUCACHE_HPP
 
#include <utility>
#include <list>
#include <functional>
#include <unordered_map>
#include <stdexcept>
 
template <class Tkey, class Tdata>
class LRUCache {
private:
  typedef std::pair<Tkey, Tdata *> Tpair;
  typedef typename std::list<Tpair>::iterator Titer;
 
  std::list<Tpair> data;                         
  std::unordered_map<Tkey, Titer> lookup;  
 
  const size_t capacity; 
 
public:
  LRUCache(size_t max_items) : capacity(max_items) {}
 
  inline bool Contains(const Tkey key)
  {
    return this->lookup.find(key) != this->lookup.end();
  }
 
  Tdata *Insert(const Tkey key, Tdata *item)
  {
    Tdata *old = nullptr;
 
    if (this->Contains(key)) {
      /* Replace old value. */
      old = this->lookup[key]->second;
      this->lookup[key]->second = item;
    } else {
      /* Delete least used item if maximum items cached. */
      if (this->data.size() >= this->capacity) {
        Tpair last = data.back();
        this->lookup.erase(last.first);
        this->data.pop_back();
 
        old = last.second;
      }
 
      /* Insert new item. */
      this->data.push_front(std::make_pair(key, item));
      this->lookup.emplace(key, this->data.begin());
    }
 
    return old;
  }
 
  inline Tdata *Pop()
  {
    if (this->data.empty()) return nullptr;
 
    Tdata *value = this->data.back().second;
    this->lookup.erase(this->data.back().first);
    this->data.pop_back();
    return value;
  }
 
  inline Tdata *Get(const Tkey key)
  {
    if (this->lookup.find(key) == this->lookup.end()) throw std::out_of_range("item not found");
    /* Move to front if needed. */
    this->data.splice(this->data.begin(), this->data, this->lookup[key]);
 
    return this->data.front().second;
  }
};
 
#endif /* LRUCACHE_HPP */