Cache.h

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/*

             *
            ***
           *****
   *********************       Mercenaries Engineering SARL
     *****************              Copyright (C) 2024
       *************
         *********        http://www.mercenaries-engineering.com
        ***********
       ****     ****
      **           **

*/

#pragma once

#include <memory>
#include <mutex>
#include <unordered_map>

// An object cache
/* This cache associates a key to a value using a weak pointer.
 * Once the last shared pointer on the value has been dereferenced, the value is destroyed.
 * Removal of internal null weak pointers is done when the cache size doubles.
 * The key must have a hash function available.
 */
template<typename Key, typename Value>
class Cache
{
public:
  std::shared_ptr<Value> query(const Key& key) const;


  template<typename Func>
  std::shared_ptr<Value> test_insert(const Key& key, Func craft);
private:

  void _house_keeping();  // Remove null entries

  mutable std::mutex _mutex;
  int _count_to_house_keeping = 2;
  std::unordered_map<Key, std::weak_ptr<Value>> _map;
};

template<typename Key, typename Value>
void Cache<Key, Value>::_house_keeping()
{
  _count_to_house_keeping = 0;
  for (auto it = _map.begin(); it != _map.end();)
  {
    if (it->second.expired())
      it = _map.erase(it);
    else
    {
      ++_count_to_house_keeping;
      ++it;
    }
  }
}

template<typename Key, typename Value>
std::shared_ptr<Value> Cache<Key, Value>::query(const Key& key) const
{
  std::lock_guard<std::mutex> lock(_mutex);
  auto ite = _map.find(key);
  if (ite != _map.end())
  {
    std::shared_ptr<Value> v = ite->second.lock();
    if (v)
      return v;
  }
  return {};
}

template<typename Key, typename Value>
template<typename Func>
std::shared_ptr<Value> Cache<Key, Value>::test_insert(const Key& key, Func craft)
{
  std::lock_guard<std::mutex> lock(_mutex);

  // Try insert first
  auto result = _map.emplace(key, std::shared_ptr<Value>());
  std::shared_ptr<Value> v = result.first->second.lock();
  if (v == nullptr)
  {
    v = craft();
    result.first->second = v;

    --_count_to_house_keeping;
    if (_count_to_house_keeping <= 0)
      _house_keeping();
  }

  return v;
}