-module(timed_cache).
-behavior(gen_server).

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%% TYPES %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%% EXPORTS %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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%%%% 'gen_server' Exports
-export
(
   [
      init/1,
      handle_cast/2,
      handle_call/3, %% No reply will ever be given.
      terminate/2,
      code_change/3,
      format_status/2,
      handle_info/2
   ]
).

%%%% Actual Interface
-export
(
   [
      fetch/3,
      invalidate/3
   ]
).

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% LOCAL FUNCTIONS %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
add_to_cache (DB, Owner, ObjectID) ->
   {ok, TimerPID} = gen_server:start(?MODULE, {DB, {Owner, ObjectID}}, []),
   {ok, Data} = database_shim:fetch(DB, ObjectID),
   ets:insert(DB, {{Owner, ObjectID}, TimerPID, Data}),
   Data.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% EXPORTED FUNCTIONS %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%% 'gen_server' functions
init ({DB, ObjectID}) ->
   {ok, {DB, ObjectID}}.

handle_call (invalidate, _, State) ->
   {stop, normal, State};
handle_call (ping, _, State) ->
   {noreply, State, timed_caches_manager:get_timeout()}.

handle_cast (invalidate, State) ->
   {stop, normal, State};
handle_cast (ping, State) ->
   {noreply, State, timed_caches_manager:get_timeout()}.

terminate (_, {DB, ObjectID}) ->
   io:format
   (
      "~nCache entry timed out or was invalidated: ~p.~n",
      [{DB, ObjectID}]
   ),
   ets:delete(DB, ObjectID).

code_change (_, State, _) ->
   {ok, State}.

format_status (_, [_, State]) ->
   [{data, [{"State", State}]}].

handle_info(timeout, State) ->
   {stop, normal, State};
handle_info(_, {DB, ObjectID}) ->
   {noreply, {DB, ObjectID}, timed_caches_manager:get_timeout()}.

%%%% Interface Functions
fetch (DB, Owner, ObjectID) ->
   io:format("~nfetch from cache: ~p.~n", [{DB, {Owner, ObjectID}}]),
   case ets:lookup(DB, {Owner, ObjectID}) of
      [] -> add_to_cache(DB, Owner, ObjectID);

      [{_, TimerPID, Data}] ->
         gen_server:cast(TimerPID, ping),
         Data
   end.

invalidate (DB, Owner, ObjectID) ->
   case ets:lookup(DB, {Owner, ObjectID}) of
      [] -> ok;

      [{_, TimerPID, _}] ->
         gen_server:stop(TimerPID)
   end.

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%% Notes %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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