Repo created

libs/base/fifo_cache.hpp

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+#pragma once
+
+#include "base/assert.hpp"
+
+#include <cstddef>
+#include <functional>
+#include <unordered_map>
+
+#ifdef __clang__
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wdeprecated-copy"
+#endif
+#include <boost/circular_buffer.hpp>
+#ifdef __clang__
+#pragma clang diagnostic pop
+#endif
+
+template <class Key, class Value, class HashContainer = std::unordered_map<Key, 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(); }
+
+  HashContainer m_map;
+  boost::circular_buffer<Key> m_fifo;
+  size_t m_capacity;
+  Loader m_loader;
+};