defmodule P2pChat.Transport.GenTun do
  require Logger
  use Rustler, otp_app: :p2p_chat, crate: "p2pchat_transport_gen_tun"

  @behaviour GenServer

  defstruct [:tun_handle]

  #
  # Server API
  # 
  @impl true
  def init(args) do
    {:ok, tun_handle} = make_tun_device()
    state = %__MODULE__{tun_handle: tun_handle}
    { :ok, state, { :continue, :recv } }
  end

  @impl true
  def handle_continue(:recv, state) do
    case tun_recv(state.tun_handle) do
       {:ok, buf} ->
         Logger.debug("Received #{byte_size(buf)} bytes")
       {:error, :would_block} -> {}
       {:error, e} ->
         Logger.error("Error during receive: #{e}")
    end
    
    {:noreply, state, { :continue, :recv }}
  end

  @impl true
  def handle_call({:recv}, _from, state) do
    data = tun_recv(state.tun_handle)
    { :reply, data, state }
  end

  #
  # Client API
  # 
  def open() do
    GenServer.start_link(__MODULE__, nil)
  end  

  #
  # NIFs
  # 

  defp make_tun_device(), do: :erlang.nif_error(:nif_not_loaded)
  defp tun_recv(_handle, _bufsize \\ 2*16), do: :erlang.nif_error(:nif_not_loaded)
end