defmodule EventStore.Notifications.PostgrexNotifications do
  @moduledoc """
  This is a copy of the new Postgrex.Notifications module that contains the timeout mechanism.

  We copied here so we don't need to wait for the next stable postgrex release to have this feature
  without using the postgrex master branch.
  """

  use Connection
  require Logger

  alias Postgrex.Protocol

  @timeout 5000

  defstruct idle_timeout: 5000,
            protocol: nil,
            parameters: nil,
            listeners: Map.new(),
            listener_channels: Map.new()

  ## PUBLIC API ##

  @type server :: GenServer.server()

  @doc """
  Start the notification connection process and connect to postgres.

  The option that this function accepts are exactly the same accepted by
  `Postgrex.start_link/1`. Note `:sync_connect` defaults to `true`.
  """
  @spec start_link(Keyword.t()) :: {:ok, pid} | {:error, Postgrex.Error.t() | term}
  def start_link(opts) do
    {server_opts, opts} = Keyword.split(opts, [:name])
    opts = Keyword.put_new(opts, :sync_connect, true)
    connection_opts = Postgrex.Utils.default_opts(opts)
    Connection.start_link(__MODULE__, connection_opts, server_opts)
  end

  @doc """
  Listens to an asynchronous notification channel using the `LISTEN` command.
  A message `{:notification, connection_pid, ref, channel, payload}` will be
  sent to the calling process when a notification is received.

  ## Options

    * `:timeout` - Call timeout (default: `#{@timeout}`)
  """
  @spec listen(server, String.t(), Keyword.t()) :: {:ok, reference}
  def listen(pid, channel, opts \\ []) do
    message = {:listen, channel}
    timeout = opts[:timeout] || @timeout
    Connection.call(pid, message, timeout)
  end

  @doc """
  Listens to an asynchronous notification channel `channel`. See `listen/2`.
  """
  @spec listen!(server, String.t(), Keyword.t()) :: reference
  def listen!(pid, channel, opts \\ []) do
    {:ok, ref} = listen(pid, channel, opts)
    ref
  end

  @doc """
  Stops listening on the given channel by passing the reference returned from
  `listen/2`.

  ## Options

    * `:timeout` - Call timeout (default: `#{@timeout}`)
  """
  @spec unlisten(server, reference, Keyword.t()) :: :ok
  def unlisten(pid, ref, opts \\ []) do
    message = {:unlisten, ref}
    timeout = opts[:timeout] || @timeout

    case Connection.call(pid, message, timeout) do
      :ok -> :ok
      {:error, %ArgumentError{} = err} -> raise err
    end
  end

  @doc """
  Stops listening on the given channel by passing the reference returned from
  `listen/2`.
  """
  @spec unlisten!(server, reference, Keyword.t()) :: :ok
  def unlisten!(pid, ref, opts \\ []) do
    unlisten(pid, ref, opts)
  end

  ## CALLBACKS ##

  def init(opts) do
    if opts[:sync_connect] do
      case connect(:init, opts) do
        {:ok, _, _} = ok -> ok
        {:stop, reason, _} -> {:stop, reason}
      end
    else
      {:connect, :init, opts}
    end
  end

  def connect(_, opts) do
    case Protocol.connect([types: nil] ++ opts) do
      {:ok, protocol} ->
        idle_timeout = Keyword.get(opts, :idle_timeout, 5000)
        {:ok, %__MODULE__{idle_timeout: idle_timeout, protocol: protocol}, idle_timeout}

      {:error, reason} ->
        {:stop, reason, opts}
    end
  end

  def handle_call({:listen, channel}, {pid, _} = from, s) do
    ref = Process.monitor(pid)

    s = put_in(s.listeners[ref], {channel, pid})
    s = update_in(s.listener_channels[channel], &((&1 || Map.new()) |> Map.put(ref, pid)))

    # If this is the first listener for the given channel,
    # we need to actually issue the LISTEN query.
    if Map.size(s.listener_channels[channel]) == 1 do
      listener_query("LISTEN \"#{channel}\"", {:ok, ref}, from, s)
    else
      {:reply, {:ok, ref}, s, s.idle_timeout}
    end
  end

  def handle_call({:unlisten, ref}, from, s) do
    case Map.fetch(s.listeners, ref) do
      :error ->
        {:reply, {:error, %ArgumentError{}}, s, s.idle_timeout}

      {:ok, {channel, _pid}} ->
        Process.demonitor(ref, [:flush])
        s = remove_monitored_listener(s, ref, channel)

        if Map.size(s.listener_channels[channel]) == 0 do
          s = update_in(s.listener_channels, &Map.delete(&1, channel))
          listener_query("UNLISTEN \"#{channel}\"", :ok, from, s)
        else
          {:reply, :ok, s, s.idle_timeout}
        end
    end
  end

  def handle_info({:DOWN, ref, :process, _, _}, s) do
    case Map.fetch(s.listeners, ref) do
      :error ->
        {:noreply, s, s.idle_timeout}

      {:ok, {channel, _pid}} ->
        s = remove_monitored_listener(s, ref, channel)

        if Map.size(s.listener_channels[channel]) == 0 do
          s = update_in(s.listener_channels, &Map.delete(&1, channel))
          listener_query("UNLISTEN \"#{channel}\"", :ok, nil, s)
        else
          {:noreply, s, s.idle_timeout}
        end
    end
  end

  def handle_info(:timeout, %{protocol: protocol} = state) do
    case Protocol.ping(protocol) do
      {:ok, protocol} ->
        {:noreply, %{state | protocol: protocol}, state.idle_timeout}

      {error, reason, protocol} when error in [:error, :disconnect] ->
        {:stop, reason, %{state | protocol: protocol}}
    end
  end

  def handle_info(msg, s) do
    %{protocol: protocol, listener_channels: channels, listeners: listeners} = s
    opts = [notify: &notify_listeners(channels, listeners, &1, &2)]

    case Protocol.handle_info(msg, opts, protocol) do
      {:ok, protocol} ->
        {:noreply, %{s | protocol: protocol}, s.idle_timeout}

      {error, reason, protocol} when error in [:error, :disconnect] ->
        {:stop, reason, %{s | protocol: protocol}}
    end
  end

  defp listener_query(statement, result, from, s) do
    %{protocol: protocol, listener_channels: channels, listeners: listeners} = s
    opts = [notify: &notify_listeners(channels, listeners, &1, &2)]

    case Protocol.handle_listener(statement, opts, protocol) do
      {:ok, %Postgrex.Result{}, protocol} ->
        if from, do: Connection.reply(from, result)
        checkin(protocol, s)

      {error, reason, protocol} when error in [:error, :disconnect] ->
        {:stop, reason, %{s | protocol: protocol}}
    end
  end

  defp notify_listeners(channels, listeners, channel, payload) do
    Enum.each(Map.get(channels, channel) || [], fn {ref, _pid} ->
      {_, pid} = Map.fetch!(listeners, ref)
      send(pid, {:notification, self(), ref, channel, payload})
    end)
  end

  defp checkin(protocol, s) do
    case Protocol.checkin(protocol) do
      {:ok, protocol} ->
        {:noreply, %{s | protocol: protocol}, s.idle_timeout}

      {error, reason, protocol} when error in [:error, :disconnect] ->
        {:stop, reason, %{s | protocol: protocol}}
    end
  end

  defp remove_monitored_listener(s, ref, channel) do
    s = update_in(s.listeners, &Map.delete(&1, ref))
    update_in(s.listener_channels[channel], &Map.delete(&1, ref))
  end
end