-module(porttest).

-behavior(gen_server).

-export([
  hello/0,
  factorial/1
]).

-export([
  start_link/0,
  stop/0
]).

-export([
  init/1,
  handle_call/3,
  handle_cast/2,
  terminate/2
]).

-record(state, {port}).


start_link() ->
  gen_server:start_link({local, ?MODULE}, ?MODULE, [], []).


stop() ->
  gen_server:call(?MODULE, stop).



hello() ->
  gen_server:call(?MODULE, hello).


factorial(N) when is_integer(N), N >= 0 ->
  case gen_server:call(?MODULE, {factorial, N}) of
    {ok, Result} when is_integer(Result) -> Result;
    {error, _} = Error -> Error
  end;

factorial(_) ->
  {error, badarg}.


%% helper
parse_result("error:" ++ Rest) ->
  {error, list_to_binary(Rest)};
parse_result(Str) ->
  case string:to_integer(Str) of
    {Int, []} -> {ok, Int};
     _ -> {error, badarg}
  end.



init([]) ->
  Path = "./porttest", %% compiled zig program
  Port = open_port({spawn, Path}, [binary, exit_status]),
  {ok, #state{port = Port}}.


handle_call(hello, _From, State) ->
  Port = State#state.port,
  Cmd = <<"hello">>,
  port_command(Port, <<(byte_size(Cmd)):16, Cmd/binary>>),
  receive
    {Port, {data, <<_Len:16, Resp/binary>>}} ->
      {reply, binary_to_list(Resp), State}
  after 5000 ->
    {reply, {error, timeout}, State}
  end;


handle_call({factorial, N}, _From, State) ->
  Port = State#state.port,
  BinCmd = <<"factorial ", (integer_to_binary(N))/binary>>,
  port_command(Port, <<(byte_size(BinCmd)):16, BinCmd/binary>>),
  receive
    {Port, {data, <<_Len:16, Resp/binary>>}} ->
      Reply = parse_result( binary_to_list(Resp) ),
      {reply, Reply, State}
    after 5000 ->
      {reply, {error, timeout}, State}
    end;


handle_call(stop, _From, State) ->
  Port = State#state.port,
  port_close(Port),
  {stop, normal, ok, State}.


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


terminate(_Reason, _State) ->
  ok.