lru_memory_cache.h

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/*  Copyright (c) SILVERCODERS Ltd, http://silvercoders.com                                                                                  */
/*  Project homepage: https://github.com/docwire/docwire                                                                                     */
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/*  SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-DocWire-Commercial                                                                  */
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#ifndef DOCWIRE_LRU_MEMORY_CACHE_H
#define DOCWIRE_LRU_MEMORY_CACHE_H
 
#include <functional>
#include <limits>
#include <list>
#include <unordered_map>
 
namespace docwire
{
 
template<typename Key, typename Value>
class lru_memory_cache
{
public:
    explicit lru_memory_cache(size_t max_size = std::numeric_limits<size_t>::max())
        : m_max_size(max_size)
    {}
 
    Value& get_or_create(const Key& key, const std::function<Value(const Key&)>& producer)
    {
        auto it = m_entry_list_iter_map.find(key);
        if (it != m_entry_list_iter_map.end()) {
            m_entry_list.splice(m_entry_list.begin(), m_entry_list, it->second);
        } else {
            Value value = producer(key);
            m_entry_list.emplace_front(key, value);
            m_entry_list_iter_map.emplace(key, m_entry_list.begin());
            if (m_entry_list.size() > m_max_size) {
                m_entry_list_iter_map.erase(std::prev(m_entry_list.end())->key);
                m_entry_list.pop_back();
            }
        }
        return m_entry_list.begin()->value;
    }
 
private:
    size_t m_max_size;
    struct entry
    {
        Key key;
        Value value;
        entry(const Key& key, const Value& value) : key(key), value(value) {}
    };
    std::list<entry> m_entry_list;
    std::unordered_map<Key, typename std::list<entry>::iterator> m_entry_list_iter_map;
};
 
} // namespace docwire
 
#endif // DOCWIRE_LRU_MEMORY_CACHE_H