defmodule Pleroma.Gun.ConnectionPool.Worker do
  alias Pleroma.Gun
  use GenServer, restart: :temporary

  @registry Pleroma.Gun.ConnectionPool

  def start_link([key | _] = opts) do
    GenServer.start_link(__MODULE__, opts, name: {:via, Registry, {@registry, key}})
  end

  @impl true
  def init([_key, _uri, _opts, _client_pid] = opts) do
    {:ok, nil, {:continue, {:connect, opts}}}
  end

  @impl true
  def handle_continue({:connect, [key, uri, opts, client_pid]}, _) do
    with {:ok, conn_pid} <- Gun.Conn.open(uri, opts),
         Process.link(conn_pid) do
      time = :erlang.monotonic_time(:millisecond)

      {_, _} =
        Registry.update_value(@registry, key, fn _ ->
          {conn_pid, [client_pid], 1, time}
        end)

      send(client_pid, {:conn_pid, conn_pid})

      {:noreply,
       %{key: key, timer: nil, client_monitors: %{client_pid => Process.monitor(client_pid)}},
       :hibernate}
    else
      err ->
        {:stop, {:shutdown, err}, nil}
    end
  end

  @impl true
  def handle_cast({:add_client, client_pid}, state) do
    case handle_call(:add_client, {client_pid, nil}, state) do
      {:reply, conn_pid, state, :hibernate} ->
        send(client_pid, {:conn_pid, conn_pid})
        {:noreply, state, :hibernate}
    end
  end

  @impl true
  def handle_cast({:remove_client, client_pid}, state) do
    case handle_call(:remove_client, {client_pid, nil}, state) do
      {:reply, _, state, :hibernate} ->
        {:noreply, state, :hibernate}
    end
  end

  @impl true
  def handle_call(:add_client, {client_pid, _}, %{key: key} = state) do
    time = :erlang.monotonic_time(:millisecond)

    {{conn_pid, _, _, _}, _} =
      Registry.update_value(@registry, key, fn {conn_pid, used_by, crf, last_reference} ->
        {conn_pid, [client_pid | used_by], crf(time - last_reference, crf), time}
      end)

    state =
      if state.timer != nil do
        Process.cancel_timer(state[:timer])
        %{state | timer: nil}
      else
        state
      end

    ref = Process.monitor(client_pid)

    state = put_in(state.client_monitors[client_pid], ref)
    {:reply, conn_pid, state, :hibernate}
  end

  @impl true
  def handle_call(:remove_client, {client_pid, _}, %{key: key} = state) do
    {{_conn_pid, used_by, _crf, _last_reference}, _} =
      Registry.update_value(@registry, key, fn {conn_pid, used_by, crf, last_reference} ->
        {conn_pid, List.delete(used_by, client_pid), crf, last_reference}
      end)

    {ref, state} = pop_in(state.client_monitors[client_pid])
    # DOWN message can receive right after `remove_client` call and cause worker to terminate
    state =
      if is_nil(ref) do
        state
      else
        Process.demonitor(ref)

        timer =
          if used_by == [] do
            max_idle = Pleroma.Config.get([:connections_pool, :max_idle_time], 30_000)
            Process.send_after(self(), :idle_close, max_idle)
          else
            nil
          end

        %{state | timer: timer}
      end

    {:reply, :ok, state, :hibernate}
  end

  @impl true
  def handle_info(:idle_close, state) do
    # Gun monitors the owner process, and will close the connection automatically
    # when it's terminated
    {:stop, :normal, state}
  end

  @impl true
  def handle_info({:gun_up, _pid, _protocol}, state) do
    {:noreply, state, :hibernate}
  end

  # Gracefully shutdown if the connection got closed without any streams left
  @impl true
  def handle_info({:gun_down, _pid, _protocol, _reason, []}, state) do
    {:stop, :normal, state}
  end

  # Otherwise, wait for retry
  @impl true
  def handle_info({:gun_down, _pid, _protocol, _reason, _killed_streams}, state) do
    {:noreply, state, :hibernate}
  end

  @impl true
  def handle_info({:DOWN, _ref, :process, pid, reason}, state) do
    :telemetry.execute(
      [:pleroma, :connection_pool, :client_death],
      %{client_pid: pid, reason: reason},
      %{key: state.key}
    )

    handle_cast({:remove_client, pid}, state)
  end

  # LRFU policy: https://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.55.1478
  defp crf(time_delta, prev_crf) do
    1 + :math.pow(0.5, 0.0001 * time_delta) * prev_crf
  end
end