implemented the first version of statement_cache

include/matador/sql/statement_cache.hpp

@@ -0,0 +1,38 @@
+#ifndef STATEMENT_CACHE_HPP
+#define STATEMENT_CACHE_HPP
+
+#include "matador/sql/executor.hpp"
+
+#include "matador/sql/interface/statement_proxy.hpp"
+
+#include <list>
+#include <mutex>
+#include <unordered_map>
+
+namespace matador::sql {
+
+class connection_pool;
+
+class statement_cache final {
+public:
+  explicit statement_cache(connection_pool &pool, size_t max_size = 50);
+
+  [[nodiscard]] utils::result<statement, utils::error> acquire(const query_context &ctx);
+
+  [[nodiscard]] size_t size() const;
+  [[nodiscard]] size_t capacity() const;
+  [[nodiscard]] bool empty() const;
+
+private:
+  using list_iterator = std::list<size_t>::iterator;
+
+  size_t max_size_{};
+  std::list<size_t> usage_list_;  // LRU: front = most recent, back = least recent
+  std::unordered_map<size_t, std::pair<statement, list_iterator>> cache_map_;
+  std::mutex mutex_;
+
+  connection_pool &pool_;
+  const sql::dialect &dialect_;
+};
+}
+#endif //STATEMENT_CACHE_HPP