finished statement cache class

include/matador/sql/statement_cache.hpp

@@ -2,8 +2,7 @@
 #define STATEMENT_CACHE_HPP
 
 #include "matador/sql/executor.hpp"
-
-#include "matador/sql/interface/statement_proxy.hpp"
+#include "matador/sql/statement.hpp"
 
 #include <list>
 #include <mutex>
@@ -13,9 +12,36 @@ namespace matador::sql {
 
 class connection_pool;
 
+struct statement_cache_event {
+  enum class Type { Accessed, Added, Evicted };
+  Type type;
+  std::string sql;
+  std::chrono::steady_clock::time_point timestamp;
+};
+
+class statement_cache_observer_interface {
+public:
+  virtual void on_event(const statement_cache_event&) = 0;
+  virtual ~statement_cache_observer_interface() = default;
+};
+
 class statement_cache final {
+private:
+  using list_iterator = std::list<size_t>::iterator;
+
+  struct cache_entry {
+    statement stmt;
+    std::chrono::steady_clock::time_point last_access;
+    list_iterator position;
+  };
+
 public:
   explicit statement_cache(connection_pool &pool, size_t max_size = 50);
+  statement_cache(const statement_cache &) = delete;
+  statement_cache &operator=(const statement_cache &) = delete;
+  statement_cache(statement_cache &&) = delete;
+  statement_cache &operator=(statement_cache &&) = delete;
+  ~statement_cache() = default;
 
   [[nodiscard]] utils::result<statement, utils::error> acquire(const query_context &ctx);
 
@@ -23,16 +49,22 @@ public:
   [[nodiscard]] size_t capacity() const;
   [[nodiscard]] bool empty() const;
 
+  void subscribe(statement_cache_observer_interface &observer);
+
+private:
+  void push(statement_cache_event::Type type, const std::string& sql) 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::unordered_map<size_t, cache_entry> cache_map_;
   std::mutex mutex_;
 
   connection_pool &pool_;
   const sql::dialect &dialect_;
+
+  std::vector<statement_cache_observer_interface*> observers_;
 };
 }
 #endif //STATEMENT_CACHE_HPP