%% @doc Individual worker process managing a Lexbor C port.
%%
%% Each worker manages an independent C port process that communicates with
%% the Lexbor library. Workers are:
%% <ul>
%%   <li>Single-threaded: Process one request at a time</li>
%%   <li>Isolated: Have their own document registry</li>
%%   <li>Supervised: Restarted independently if they crash</li>
%%   <li>Registered: Can be discovered by name</li>
%% </ul>
%%
%% Workers register themselves with a unique name based on their worker ID,
%% allowing the pool to discover them dynamically and enabling independent
%% supervision.
%%
%% @end
-module(lexbor_erl_worker).

-behaviour(gen_server).

-export([start_link/1, call/3, worker_name/1]).
-export([init/1, handle_call/3, handle_cast/2, handle_info/2, terminate/2,
         code_change/3]).

-record(state, {port :: port(), worker_id :: pos_integer()}).

%% ===================================================================
%% Public API
%% ===================================================================

%% @doc Start a worker and register it with a unique name.
%%
%% The worker opens a port to the Lexbor C executable and registers itself
%% as `lexbor_erl_worker_N' where N is the WorkerId.
%%
%% @param WorkerId Unique identifier for this worker (1..PoolSize)
%% @returns `{ok, Pid}' on success, `{error, Reason}' on failure
-spec start_link(pos_integer()) -> {ok, pid()} | {error, term()}.
start_link(WorkerId) ->
    gen_server:start_link({local, worker_name(WorkerId)}, ?MODULE, [WorkerId], []).

%% @doc Generate the registered name for a worker.
%%
%% @param WorkerId Worker identifier
%% @returns Atom name like `lexbor_erl_worker_1'
worker_name(WorkerId) ->
    list_to_atom("lexbor_erl_worker_" ++ integer_to_list(WorkerId)).

%% @doc Send a command to a worker and wait for reply.
%%
%% @param Worker Worker process PID
%% @param CmdTag Command tag (e.g., "PARSE_DOC")
%% @param Payload Binary payload for the command
%% @returns `{ok, Binary}' with reply data, or `{error, Reason}'
-spec call(pid(), binary(), binary()) -> {ok, binary()} | {error, term()}.
call(Worker, CmdTag, Payload) ->
    gen_server:call(Worker, {call, CmdTag, Payload}, infinity).

%% ===================================================================
%% gen_server callbacks
%% ===================================================================

init([WorkerId]) ->
    process_flag(trap_exit, true),

    % Get the port command path
    PortCmd =
        case application:get_env(lexbor_erl, port_cmd) of
            {ok, Cmd} ->
                Cmd;
            undefined ->
                % Find the priv directory for this application
                case code:priv_dir(lexbor_erl) of
                    {error, bad_name} ->
                        % Fallback for development/test scenarios
                        BeamDir =
                            filename:dirname(
                                code:which(?MODULE)),
                        filename:join([BeamDir, "..", "priv", "lexbor_port"]);
                    PrivDir ->
                        filename:join(PrivDir, "lexbor_port")
                end
        end,

    % Verify the port executable exists before trying to open it
    case filelib:is_file(PortCmd) of
        false ->
            error_logger:error_msg("Port executable not found at: ~p~n", [PortCmd]),
            {stop, {port_not_found, PortCmd}};
        true ->
            Port =
                open_port({spawn_executable, PortCmd},
                          [{packet, 4}, binary, exit_status, use_stdio, stderr_to_stdout]),
            {ok, #state{port = Port, worker_id = WorkerId}}
    end.

handle_call({call, CmdTag, Payload}, _From, #state{port = Port} = S) ->
    %% Wire format:
    %% <<1-byte version, 16-byte CmdTag, payload-binary>>
    Version = <<1>>,
    Tag16 = pad16(CmdTag),
    true = port_command(Port, <<Version/binary, Tag16/binary, Payload/binary>>),
    receive
        {Port, {data, Reply}} ->
            %% Reply is binary: <<1, Tag16, Data/binary>> (same shape)
            case Reply of
                <<1, _Tag:16/binary, Data/binary>> ->
                    {reply, {ok, Data}, S};
                _ ->
                    {reply, {error, bad_reply}, S}
            end;
        {'EXIT', Port, Reason} ->
            {reply, {error, {port_exit, Reason}}, S}
    after application:get_env(lexbor_erl, op_timeout_ms, 3000) ->
        {reply, {error, timeout}, S}
    end.

handle_cast(_Msg, S) ->
    {noreply, S}.

handle_info({'EXIT', Port, Status}, #state{port = Port, worker_id = WorkerId} = S) ->
    error_logger:error_msg("lexbor_erl worker ~p port exited: ~p~n", [WorkerId, Status]),
    {stop, port_died, S};
handle_info(_Msg, S) ->
    {noreply, S}.

terminate(_Reason, #state{port = Port}) ->
    catch port_close(Port),
    ok.

code_change(_OldVsn, State, _Extra) ->
    {ok, State}.

%% ===================================================================
%% Internal functions
%% ===================================================================

pad16(Bin) when is_binary(Bin) ->
    <<(binary:part(Bin, 0, min(byte_size(Bin), 16)))/binary,
      (pad_zeros(16 - min(byte_size(Bin), 16)))/binary>>.

pad_zeros(N) when N =< 0 ->
    <<>>;
pad_zeros(N) ->
    <<0:8, (pad_zeros(N - 1))/binary>>.