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#pragma once
#include "base/assert.hpp"
#include <cstddef>
#include <functional>
#include <unordered_map>
#include <boost/circular_buffer.hpp>
template<class Key, class Value>
class FifoCache
{
template <typename K, typename V> friend class FifoCacheTest;
public:
using Loader = std::function<void(Key const & key, Value & value)>;
/// \param capacity maximum size of the cache in number of items.
/// \param loader Function which is called if it's necessary to load a new item for the cache.
FifoCache(size_t capacity, Loader const & loader)
: m_fifo(capacity), m_capacity(capacity), m_loader(loader)
{
}
/// \brief Loads value, if it's necessary, by |key| with |m_loader|, puts it to cache and
/// returns the reference to the value to |m_map|.
Value const & GetValue(Key const & key)
{
auto const it = m_map.find(key);
if (it != m_map.cend())
return it->second;
if (Size() >= m_capacity)
{
CHECK(!m_fifo.empty(), ());
m_map.erase(m_fifo.back());
}
m_fifo.push_front(key);
auto & v = m_map[key];
m_loader(key, v);
return v;
}
private:
size_t Size() const { return m_map.size(); }
std::unordered_map<Key, Value> m_map;
boost::circular_buffer<Key> m_fifo;
size_t m_capacity;
Loader m_loader;
};
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