defmodule Extreme.Subscription do
  use GenServer
  require Logger
  alias Extreme.Msg, as: ExMsg

  def start_link(connection, subscriber, read_params) do
    GenServer.start_link(__MODULE__, {connection, subscriber, read_params})
  end

  def start_link(connection, subscriber, stream, resolve_link_tos) do
    GenServer.start_link(__MODULE__, {connection, subscriber, stream, resolve_link_tos})
  end

  def init(
        {connection, subscriber,
         {stream, from_event_number, per_page, resolve_link_tos, require_master}}
      ) do
    read_params = %{
      stream: stream,
      from_event_number: from_event_number,
      per_page: per_page,
      resolve_link_tos: resolve_link_tos,
      require_master: require_master
    }

    GenServer.cast(self(), :read_and_stay_subscribed)

    {:ok,
     %{
       subscriber: subscriber,
       connection: connection,
       read_params: read_params,
       status: :initialized,
       buffered_messages: [],
       read_until: -1
     }}
  end

  def init({connection, subscriber, stream, resolve_link_tos}) do
    read_params = %{stream: stream, resolve_link_tos: resolve_link_tos}
    GenServer.cast(self(), :subscribe)

    {:ok,
     %{
       subscriber: subscriber,
       connection: connection,
       read_params: read_params,
       status: :initialized,
       buffered_messages: [],
       read_until: -1
     }}
  end

  def handle_cast(:read_and_stay_subscribed, state) do
    {:ok, subscription_confirmation} =
      GenServer.call(state.connection, {:subscribe, self(), subscribe(state.read_params)})

    Logger.debug("Successfully subscribed to stream #{inspect(subscription_confirmation)}")
    GenServer.cast(self(), :read_events)
    read_until = subscription_confirmation.last_event_number + 1
    {:noreply, %{state | read_until: read_until, status: :reading_events}}
  end

  def handle_cast(:subscribe, state) do
    {:ok, subscription_confirmation} =
      GenServer.call(state.connection, {:subscribe, self(), subscribe(state.read_params)})

    Logger.debug("Successfully subscribed to stream #{inspect(subscription_confirmation)}")
    {:noreply, %{state | status: :subscribed}}
  end

  def handle_cast(
        :read_events,
        %{read_params: %{from_event_number: from}, read_until: from} = state
      ) do
    GenServer.cast(self(), :push_buffered_messages)
    {:noreply, %{state | status: :pushing_buffered}}
  end

  def handle_cast(:read_events, state) do
    {read_events, keep_reading} = read_events(state.read_params, state.read_until)

    state =
      case keep_reading do
        true -> state
        false -> %{state | status: :pushing_buffered}
      end

    state =
      Extreme.execute(state.connection, read_events)
      |> process_response(state)

    {:noreply, state}
  end

  def handle_cast(:push_buffered_messages, state) do
    state.buffered_messages |> Enum.each(fn e -> send(state.subscriber, {:on_event, e}) end)
    send(state.subscriber, :caught_up)
    {:noreply, %{state | status: :subscribed, buffered_messages: []}}
  end

  def handle_cast({:ok, %ExMsg.StreamEventAppeared{} = e}, %{status: :subscribed} = state) do
    send(state.subscriber, {:on_event, e.event})
    {:noreply, state}
  end

  def handle_cast({:ok, %ExMsg.StreamEventAppeared{} = e}, state) do
    buffered_messages =
      state.buffered_messages
      |> List.insert_at(-1, e.event)

    {:noreply, %{state | buffered_messages: buffered_messages}}
  end

  def process_response({:ok, %ExMsg.ReadStreamEventsCompleted{} = response}, state) do
    Logger.debug("Last read event: #{inspect(response.next_event_number - 1)}")
    push_events({:ok, response}, state.subscriber)
    send_next_request(response, state)
  end

  def process_response(
        {:error, :StreamDeleted, %ExMsg.ReadStreamEventsCompleted{} = response},
        state
      ) do
    Logger.error("Stream is HARD deleted")

    push_events(
      {:extreme, :error, :stream_hard_deleted, state.read_params.stream},
      state.subscriber
    )

    send_next_request(response, state)
  end

  def process_response({:error, :NoStream, %ExMsg.ReadStreamEventsCompleted{} = response}, state) do
    Logger.warn("Stream doesn't exist yet")
    push_events({:extreme, :warn, :no_stream, state.read_params.stream}, state.subscriber)
    send_next_request(response, state)
  end

  defp push_events({:ok, %ExMsg.ReadStreamEventsCompleted{} = response}, subscriber) do
    response.events |> Enum.each(fn e -> send(subscriber, {:on_event, e}) end)
  end

  defp push_events({:extreme, _, _, _} = msg, subscriber), do: send(subscriber, msg)

  defp send_next_request(_, %{status: :pushing_buffered} = state) do
    GenServer.cast(self(), :push_buffered_messages)
    state
  end

  defp send_next_request(%{next_event_number: next_event_number}, state) do
    GenServer.cast(self(), :read_events)
    %{state | read_params: %{state.read_params | from_event_number: next_event_number}}
  end

  defp read_events(%{from_event_number: from, per_page: per_page} = params, read_until)
       when from + per_page < read_until do
    result =
      ExMsg.ReadStreamEvents.new(
        event_stream_id: params.stream,
        from_event_number: from,
        max_count: per_page,
        resolve_link_tos: params.resolve_link_tos,
        require_master: params.require_master
      )

    {result, true}
  end

  defp read_events(params, read_until) do
    result =
      ExMsg.ReadStreamEvents.new(
        event_stream_id: params.stream,
        from_event_number: params.from_event_number,
        max_count: read_until - params.from_event_number,
        resolve_link_tos: params.resolve_link_tos,
        require_master: params.require_master
      )

    {result, false}
  end

  defp subscribe(params) do
    ExMsg.SubscribeToStream.new(
      event_stream_id: params.stream,
      resolve_link_tos: params.resolve_link_tos
    )
  end
end