defmodule A2S.Statem do
  @moduledoc false
  # A state machine process responsible for handling all A2S queries to a game server running at the given address.
  # Queries must be performed sequentially per address as A2S provides no way to associate what replies associate to what responses.
  # Each instance should exit normally after a certain interval of inactivity (currently hard-coded to 2 minutes).

  @behaviour :gen_statem

  require Logger

  @impl :gen_statem
  def callback_mode, do: :handle_event_function

  defmodule Data do
    @moduledoc false
    defstruct [:address, :caller, :query, :total, :parts, :socket]
  end

  ## Initialization

  @type init_args() :: {{:inet.ip_address(), :inet.port_number()}, term()}

  @spec child_spec(init_args) :: Supervisor.child_spec()
  def child_spec({address, client}) do
    # need to revisit this child specification
    %{
      id: __MODULE__,
      start: {__MODULE__, :start_link, [{address, client}]},
      restart: :transient
    }
  end

  @spec start_link(init_args) :: :ignore | {:error, any} | {:ok, pid}
  def start_link({address, client}) do
    registry = A2S.Client.registry_name(client)
    socket = A2S.Client.socket_name(client)

    :gen_statem.start_link(via_registry(registry, address), __MODULE__, {address, socket}, [])
  end

  @impl :gen_statem
  def init({address, socket}) do
    {:ok, :idle, %Data{address: address, socket: socket}, idle_timeout()}
  end

  defp via_registry(registry, address), do: {:via, Registry, {registry, address}}

  ## State Machine Callbacks

  # Received a query to perform

  @impl :gen_statem
  def handle_event({:call, from}, query, :idle, data) do
    %Data{address: address, socket: socket} = data

    :ok = GenServer.call(socket, {address, A2S.challenge_request(query)})

    {
      :next_state,
      :await_challenge,
      %Data{data | caller: from, query: query},
      recv_timeout()
    }
  end

  # A2S queries don't have IDs so they must be processed sequentially to avoid ambiguity.
  @impl :gen_statem
  def handle_event({:call, _from}, _query_type, _state, _data) do
    {:keep_state_and_data, :postpone}
  end

  # Received a packet

  @impl :gen_statem
  def handle_event(:cast, packet, :await_challenge, data) do
    %Data{
      address: address,
      query: query,
      socket: socket
    } = data

    case A2S.parse_challenge(packet) do
      {:immediate, msg} ->
        reply_and_next(msg, data)

      {:challenge, challenge} ->
        :ok = GenServer.call(socket, {address, A2S.sign_challenge(query, challenge)})
        {:next_state, :await_response, data, recv_timeout()}

      {:multipacket, {header, _body} = part} ->
        {
          :next_state,
          :collect_multipacket,
          %Data{data | total: header.total, parts: [part]},
          recv_timeout()
        }

      {:error, reason} ->
        reply_and_next({:error, reason}, data)
    end
  end

  @impl :gen_statem
  def handle_event(:cast, packet, :await_response, data) do
    case A2S.parse_response(packet) do
      {:multipacket, {header, _body} = part} ->
        {
          :next_state,
          :collect_multipacket,
          %Data{data | total: header.total, parts: [part]},
          recv_timeout()
        }

      # reply with whatever the result is.
      msg ->
        reply_and_next(msg, data)
    end
  end

  @impl :gen_statem
  def handle_event(:cast, packet, :collect_multipacket, data) do
    %Data{
      total: total,
      parts: parts
    } = data

    {:multipacket, part} = A2S.parse_response(packet)
    parts = [part | parts]

    if Enum.count(parts) === total do
      A2S.parse_multipacket_response(parts)
      |> reply_and_next(data)
    else
      {
        :next_state,
        :collect_multipacket,
        %Data{data | parts: parts},
        recv_timeout()
      }
    end
  end

  # Received a timeout

  @impl :gen_statem
  def handle_event(:state_timeout, :idle_timeout, :idle, _data) do
    {:stop, :normal}
  end

  @impl :gen_statem
  def handle_event(:state_timeout, :recv_timeout, _state, data) do
    reply_and_next({:error, :recv_timeout}, data)
  end

  defp reply_and_next(msg, %Data{address: address, caller: caller, socket: socket}) do
    :gen_statem.reply(caller, msg)

    {
      :next_state,
      :idle,
      %Data{address: address, socket: socket}
    }
  end

  ## Timeout Definitions

  defp idle_timeout() do
    timeout = :persistent_term.get({__MODULE__, :idle_timeout})
    {:state_timeout, timeout, :idle_timeout}
  end

  defp recv_timeout() do
    timeout = :persistent_term.get({__MODULE__, :recv_timeout})
    {:state_timeout, timeout, :recv_timeout}
  end
end