defmodule Drab do
  @moduledoc """
  Drab allows to query and manipulate the User Interface directly from the Phoenix server backend.

  All Drab functions (callbacks and event handlers) should be placed in a module called
  a 'commander'. It is very similar to controller, but it does not render any pages - it works
  with the live page instead. For example, pages generated by `DrabExample.PageController` are
  handled by commander with the corresponding name, in this case `DrabExample.PageCommander`:

      defmodule DrabExample.PageCommander do
        use Drab.Commander

        onload :page_loaded

        # Drab Callbacks
        def page_loaded(socket) do
          set_prop socket, "div.jumbotron h2", innerHTML: "This page has been DRABBED"
        end

        # Drab Events
        defhandler button_clicked(socket, sender) do
          set_prop socket, this(sender), innerText: "already clicked"
        end
      end

  More on commander is in `Drab.Commander` documentation.

  Drab handler are launched from the client (browser) side by running javascript function
  [`Drab.exec_elixir()`](Drab.Client.html#module-drab-exec_elixir-elixir_function_name-argument),
  or defining the handler function name directly in the html:

      <button drab-click="button_clicked">Clickety click</button>

  More on event handlers in `Drab.Core` documentation.

  ## Debugging Drab in IEx

  When started with iex (`iex -S mix phx.server`) Drab shows the helpful message on how to
  debug its functions:

          Started Drab for /drab/docs, handling events in DrabPoc.DocsCommander
          You may debug Drab functions in IEx by copy/paste the following:
      import Drab.{Core, Query, Modal, Waiter}
      socket = Drab.get_socket(pid("0.443.0"))

          Examples:
      socket |> select(:htmls, from: "h4")
      socket |> exec_js("alert('hello from IEx!')")
      socket |> alert("Title", "Sure?", buttons: [ok: "Azaliż", cancel: "Poniechaj"])

  All you need to do is to copy/paste the line with `socket = ...` and now you can run Drab function
  directly from IEx, observing the results on the running browser in the realtime.

  ## Handling Exceptions

  Drab intercepts all exceptions from event handler function and let it die, but before it presents
  the error message in the logs and an alert for a user on the page.

  By default it is just an `alert()`, but you can easly override it by creating the template in the
  `priv/templates/drab/drab.error_handler.js` folder with your own javascript presenting the
  message. You may use the local variable `message` there to get the exception description, like:

      alert(<%= message %>);

  ## Modules

  Drab is modular. You may choose which modules to use in the specific Commander by using `:module`
  option in `use Drab.Commander` directive. By default, `Drab.Live` and `Drab.Element` are loaded,
  but you may override it using  `modules` option with `use Drab.Commander` directive.

  Every module must have the corresponding javascript template, which is added to the client code
  in case the module is loaded.

  `Drab.Core` module is always loaded.

  ## Drab in production and behind a proxy

  When using in production, an app is often behind an apache/nginx server for domain virtualization
  or balancing, so the external port (80) is different from the actual app port (i.e. 4000).
  The necessary mapping between the two ports is usually done by configuring a proxy,
  but _a particularly care have to be taken to correctly handle_ `websocket` _calls_,
  as they are at the core of Drab mechanism to communicate between the client browser and the backend server.

  You can find more information and examples to how to configure your nxinx or apache environments
  on the Drab wiki page at https://github.com/grych/drab/wiki/Drab-in-production-and-behind-a-proxy

  ## Learnig Drab

  There is a [tutorial/demo page](https://tg.pl/drab).

  The point to start reading docs should be `Drab.Core`.
  """

  require Logger
  use GenServer

  @type t :: %Drab{
          store: map,
          session: map,
          commander: atom,
          commanders: list,
          controller: atom,
          socket: Phoenix.Socket.t(),
          priv: map
        }

  defstruct store: %{},
            session: %{},
            commander: nil,
            commanders: [],
            controller: nil,
            socket: nil,
            priv: %{}

  @doc false
  @spec start_link(Phoenix.Socket.t()) :: GenServer.on_start()
  def start_link(socket) do
    GenServer.start_link(__MODULE__, %Drab{
      commander: Drab.get_commander(socket),
      commanders: Drab.get_commanders(socket),
      controller: Drab.get_controller(socket)
    })
  end

  @doc false
  @spec init(t) :: {:ok, t}
  def init(state) do
    Process.flag(:trap_exit, true)
    {:ok, state}
  end

  @doc false
  @spec terminate(any, t) :: {:noreply, t}
  def terminate(_reason, %Drab{store: store, session: session, commander: commander} = state) do
    if commander.__drab__().ondisconnect do
      # TODO: timeout
      :ok = apply(commander, commander_config(commander).ondisconnect, [store, session])
    end

    {:noreply, state}
  end

  @doc false
  @spec handle_info(tuple, t) :: {:noreply, t}
  def handle_info({:EXIT, pid, :normal}, state) when pid != self() do
    # ignore exits of the subprocesses
    {:noreply, state}
  end

  @doc false
  def handle_info({:EXIT, pid, :killed}, state) when pid != self() do
    failed(state.socket, %RuntimeError{message: "Drab Process #{inspect(pid)} has been killed."})
    {:noreply, state}
  end

  @doc false
  def handle_info({:EXIT, pid, {reason, stack}}, state) when pid != self() do
    # subprocess died
    Logger.error("""
    Drab Process #{inspect(pid)} died because of #{inspect(reason)}
    #{Exception.format_stacktrace(stack)}
    """)

    {:noreply, state}
  end

  @doc false
  @spec handle_cast(tuple, t) :: {:noreply, t}
  def handle_cast({:onconnect, socket, payload}, %Drab{commander: commander} = state) do
    socket = transform_socket(payload["payload"], socket, state)

    Drab.Core.save_session(
      socket,
      Drab.Core.detokenize_store(socket, payload["drab_session_token"])
    )

    Drab.Core.save_store(socket, Drab.Core.detokenize_store(socket, payload["drab_store_token"]))
    Drab.Core.save_socket(socket)

    onconnect = commander_config(commander).onconnect
    handle_callback(socket, commander, onconnect)

    for shared_commander <- state.commanders do
      onconnect = commander_config(shared_commander).onconnect
      handle_callback(socket, shared_commander, onconnect)
    end

    {:noreply, state}
  end

  @doc false
  def handle_cast({:onload, socket, payload}, %Drab{commander: commander} = state) do
    socket = transform_socket(payload["payload"], socket, state)

    onload = commander_config(commander).onload
    handle_callback(socket, commander, onload)

    for shared_commander <- state.commanders do
      onload = commander_config(shared_commander).onload
      handle_callback(socket, shared_commander, onload)
    end

    {:noreply, state}
  end

  # casts for update values from the state
  Enum.each([:store, :session, :socket, :priv], fn name ->
    msg_name = String.to_atom("set_#{name}")
    @doc false
    def handle_cast({unquote(msg_name), value}, state) do
      new_state = Map.put(state, unquote(name), value)
      {:noreply, new_state}
    end
  end)

  @doc false
  # any other cast is an event handler
  def handle_cast({event_name, socket, payload, event_handler_function, reply_to}, state) do
    handle_event(socket, event_name, event_handler_function, payload, reply_to, state)
  end

  # calls for get values from the state
  Enum.each([:store, :session, :socket, :priv], fn name ->
    msg_name = String.to_atom("get_#{name}")
    @doc false
    def handle_call(unquote(msg_name), _from, state) do
      value = Map.get(state, unquote(name))
      {:reply, value, state}
    end
  end)

  @spec handle_callback(Phoenix.Socket.t(), atom, atom) :: Phoenix.Socket.t()
  defp handle_callback(socket, commander, callback) do
    if callback do
      # TODO: rethink the subprocess strategies - now it is just spawn_link
      spawn_link(fn ->
        try do
          Process.put(:__drab_event_handler_or_callback, true)
          apply(commander, callback, [socket])
        rescue
          e ->
            failed(socket, e)
        end
      end)
    end

    socket
  end

  @spec transform_payload(map, t) :: map
  defp transform_payload(payload, state) do
    all_modules = DrabModule.all_modules_for(state.commander.__drab__().modules)

    # transform payload via callbacks in DrabModules
    Enum.reduce(all_modules, payload, fn m, p ->
      m.transform_payload(p, state)
    end)
  end

  @spec transform_socket(map, Phoenix.Socket.t(), t) :: Phoenix.Socket.t()
  defp transform_socket(payload, socket, state) do
    all_modules = DrabModule.all_modules_for(state.commander.__drab__().modules)

    # transform socket via callbacks
    Enum.reduce(all_modules, socket, fn m, s ->
      m.transform_socket(s, payload, state)
    end)
  end

  @spec handle_event(Phoenix.Socket.t(), any, atom, map, atom, t) :: {:noreply, t}
  defp handle_event(
         socket,
         _event_name,
         event_handler_function,
         payload,
         reply_to,
         %Drab{commander: commander_module, controller: controller_module} = state
       ) do
    spawn_link(fn ->
      try do
        Process.put(:__drab_event_handler_or_callback, true)
        argument = payload["__additional_argument"]
        payload = Map.delete(payload, "__additional_argument")
        arity = if argument, do: 3, else: 2

        {commander_module, event_handler} =
          case event_handler(event_handler_function) do
            {nil, function} -> raise_if_handler_not_exists(commander_module, function, arity)
            {module, function} -> raise_if_handler_is_not_public(module, function)
          end

        # Warn if handler is not public
        # TODO: in v0.8.0, all handler must be public
        unless is_public?(commander_module, event_handler) do
          Logger.warn("""
          handler #{commander_module}.#{event_handler} must be declared as public in the commander.

          Please use Drab.Commander.public or Drab.Commander.defhandler macro as following:
              defhandler #{event_handler}(socket, sender) do
                ...
              end

          This warning will become an error in v0.8.0.
          """)
        end

        # Warn if shared commander is not declared
        # TODO: in v0.8.0, all shared commanders must be declared
        unless is_commander_declared?(commander_module, controller_module) do
          Logger.warn("""
          shared commander #{commander_module} is not declared in #{controller_module}.

          Please whitelist all shared commanders in the controller:
              use Drab.Controller, commanders: [#{commander_module}]

          This warning will become an error in v0.8.0.
          """)
        end

        payload = transform_payload(payload, state)
        socket = transform_socket(payload, socket, state)

        commander_cfg = commander_config(commander_module)

        # run before_handlers first
        returns_from_befores =
          Enum.map(
            callbacks_for(event_handler, commander_cfg.before_handler),
            fn callback_handler ->
              apply(commander_module, callback_handler, [socket, payload])
            end
          )

        # if ANY of them fail (return false or nil), do not proceed
        unless Enum.any?(returns_from_befores, &(!&1)) do
          # run actuall event handler
          arguments =
            if argument,
              do: [socket, payload, argument],
              else: [socket, payload]

          returned_from_handler = apply(commander_module, event_handler, arguments)

          Enum.map(
            callbacks_for(event_handler, commander_cfg.after_handler),
            fn callback_handler ->
              apply(commander_module, callback_handler, [socket, payload, returned_from_handler])
            end
          )
        end
      rescue
        e ->
          failed(socket, e)
      after
        # push reply to the browser, to re-enable controls
        push_reply(socket, reply_to, commander_module, event_handler_function)
      end
    end)

    {:noreply, state}
  end

  @spec event_handler(String.t()) :: {atom | nil, atom}
  defp event_handler(function_name) do
    case String.split(function_name, ".") do
      [function] ->
        {nil, String.to_existing_atom(function)}

      module_and_function ->
        module = module_and_function |> List.delete_at(-1) |> Module.safe_concat()

        unless Code.ensure_loaded?(module) do
          raise """
          module #{inspect(module)} does not exist.
          """
        end

        function = module_and_function |> List.last() |> String.to_existing_atom()
        {module, function}
    end
  end

  @spec raise_if_handler_not_exists(atom, atom, integer) :: {atom, atom} | no_return
  defp raise_if_handler_not_exists(module, function, arity) do
    if !({function, arity} in apply(module, :__info__, [:functions])) ||
         is_callback?(module, function) do
      raise """
      handler `#{function}/#{arity}` does not exist.
      """
    end

    {module, function}
  end

  @spec is_callback?(atom, atom) :: boolean
  defp is_callback?(module, function) do
    options = apply(module, :__drab__, [])
    # TODO: group callbacks in compile time
    callbacks = Map.get(options, :before_handler, []) ++ Map.get(options, :after_handler, [])
    function in callbacks
  end

  @spec raise_if_handler_is_not_public(atom, atom) :: {atom, atom} | no_return
  defp raise_if_handler_is_not_public(module, function) do
    unless is_public?(module, function) do
      raise """
      handler #{module}.#{function} is not public.

      Use `Drab.Commander.public/1` macro to make it executable from any page.
      """
    end

    {module, function}
  end

  @spec is_public?(atom, atom) :: boolean | no_return
  defp is_public?(module, function) do
    if {:__drab__, 0} in apply(module, :__info__, [:functions]) do
      options = apply(module, :__drab__, [])
      function in Map.get(options, :public_handlers, [])
    else
      raise """
      #{module} is not a Drab module.
      """
    end
  end

  @spec is_commander_declared?(atom, atom) :: boolean
  defp is_commander_declared?(commander, controller) do
    cond do
      commander in Drab.Config.drab_internal_commanders() ->
        true

      commander.__drab__().controller == controller ->
        true

      {:__drab__, 0} in apply(controller, :__info__, [:functions]) ->
        commander in controller.__drab__().commanders

      true ->
        false
    end
  end

  @spec failed(Phoenix.Socket.t(), Exception.t()) :: :ok
  defp failed(socket, e) do
    error = """
    Drab Handler failed with the following exception:
    #{Exception.format_banner(:error, e)}
    #{Exception.format_stacktrace(System.stacktrace())}
    """

    Logger.error(error)

    if socket do
      js =
        Drab.Template.render_template(
          "drab.error_handler.js",
          message: Drab.Core.encode_js(error)
        )

      {:ok, _} = Drab.Core.exec_js(socket, js)
    end

    :ok
  end

  @spec push_reply(Phoenix.Socket.t(), atom, any, any) :: :ok
  defp push_reply(socket, reply_to, _, _) do
    Phoenix.Channel.push(socket, "event", %{
      finished: reply_to
    })
  end

  @doc false
  # Returns the list of callbacks (before_handler, after_handler) defined in handler_config
  @spec callbacks_for(atom, list) :: list
  def callbacks_for(_, []) do
    []
  end

  @doc false
  def callbacks_for(event_handler_function, handler_config) do
    Enum.map(handler_config, fn {callback_name, callback_filter} ->
      case callback_filter do
        [] ->
          callback_name

        [only: handlers] ->
          if event_handler_function in handlers, do: callback_name, else: false

        [except: handlers] ->
          if event_handler_function in handlers, do: false, else: callback_name

        _ ->
          false
      end
    end)
    |> Enum.filter(& &1)
  end

  # setter and getter functions
  Enum.each([:store, :session, :socket, :priv], fn name ->
    get_name = String.to_atom("get_#{name}")
    update_name = String.to_atom("set_#{name}")

    @doc false
    def unquote(get_name)(pid) do
      GenServer.call(pid, unquote(get_name))
    end

    @doc false
    def unquote(update_name)(pid, new_value) do
      GenServer.cast(pid, {unquote(update_name), new_value})
    end
  end)

  @doc false
  @spec push_and_wait_for_response(Phoenix.Socket.t(), pid, String.t(), Keyword.t(), Keyword.t()) ::
          Drab.Core.result()
  def push_and_wait_for_response(socket, pid, message, payload \\ [], options \\ []) do
    if Process.alive?(Drab.pid(socket)) do
      ref = make_ref()
      push(socket, pid, ref, message, payload)
      timeout = options[:timeout] || Drab.Config.get(:browser_response_timeout)
      receive do
        {:got_results_from_client, status, ^ref, reply} ->
          {status, reply}
      after
        timeout ->
          # TODO: message is still in a queue
          {:timeout, "timed out after #{timeout} ms."}
      end
    else
      {:error, :disconnected}
    end
  end

  @doc false
  @spec push_and_wait_forever(Phoenix.Socket.t(), pid, String.t(), Keyword.t()) ::
          Drab.Core.result()
  def push_and_wait_forever(socket, pid, message, payload \\ []) do
    push(socket, pid, nil, message, payload)

    receive do
      {:got_results_from_client, status, _, reply} ->
        {status, reply}
    end
  end

  @doc false
  @spec push(Phoenix.Socket.t(), pid, reference | nil, String.t(), Keyword.t()) :: :ok | no_return
  def push(socket, pid, ref, message, payload \\ []) do
    do_push_or_broadcast(socket, pid, ref, message, payload, &Phoenix.Channel.push/3)
  end

  @doc false
  @spec broadcast(Drab.Core.subject(), pid | nil, String.t(), Keyword.t()) :: :ok | no_return
  def broadcast(subject, pid, message, payload \\ [])

  def broadcast(%Phoenix.Socket{} = socket, pid, message, payload) do
    do_push_or_broadcast(socket, pid, nil, message, payload, &Phoenix.Channel.broadcast/3)
  end

  def broadcast(subject, _pid, message, payload) when is_binary(subject) do
    Phoenix.Channel.Server.broadcast(
      Drab.Config.pubsub(),
      "__drab:#{subject}",
      message,
      Map.new(payload)
    )
  end

  @doc false
  def broadcast(topics, _pid, message, payload) when is_list(topics) do
    for topic <- topics do
      broadcast(topic, nil, message, payload)
    end

    :ok
  end

  @spec do_push_or_broadcast(
          Phoenix.Socket.t() | Drab.Core.subject(),
          pid | nil,
          reference | nil,
          String.t(),
          Keyword.t(),
          function
        ) :: :ok | no_return
  defp do_push_or_broadcast(socket, pid, ref, message, payload, function) do
    m = payload |> Enum.into(%{}) |> Map.merge(%{sender: tokenize(socket, {pid, ref})})
    function.(socket, message, m)
  end

  @doc false
  @spec tokenize(Phoenix.Socket.t() | Plug.Conn.t(), term(), String.t()) :: String.t()
  def tokenize(socket, what, salt \\ "drab token") do
    Phoenix.Token.sign(socket, salt, what)
  end

  @doc false
  @spec detokenize(Phoenix.Socket.t() | Plug.Conn.t(), String.t(), String.t()) ::
          term() | no_return
  def detokenize(socket, token, salt \\ "drab token") do
    case Phoenix.Token.verify(socket, salt, token, max_age: 86_400) do
      {:ok, detokenized} ->
        detokenized

      {:error, reason} ->
        # let it die
        raise "Can't verify the token `#{salt}`: #{inspect(reason)}"
    end
  end

  # returns the commander name for the given controller (assigned in socket)
  @doc false
  @spec get_commander(Phoenix.Socket.t()) :: atom
  def get_commander(socket) do
    socket.assigns.__commander
  end

  # returns the list of shared commanders name for the given controller (assigned in socket)
  @doc false
  @spec get_commanders(Phoenix.Socket.t()) :: list
  def get_commanders(socket) do
    controller = get_controller(socket)

    if controller && {:__drab__, 0} in apply(controller, :__info__, [:functions]) do
      controller.__drab__().commanders
    else
      []
    end

    # socket.assigns.__commanders
  end

  # returns the controller name used with the socket
  @doc false
  @spec get_controller(Phoenix.Socket.t()) :: atom
  def get_controller(socket) do
    socket.assigns.__controller
  end

  # returns the view name used with the socket
  @doc false
  @spec get_view(Phoenix.Socket.t()) :: atom
  def get_view(socket) do
    socket.assigns.__view
  end

  # returns the drab_pid from socket
  @doc "Extract Drab PID from the socket"
  @spec pid(Phoenix.Socket.t()) :: pid
  def pid(socket) do
    socket.assigns.__drab_pid
  end

  # if module is commander or controller with drab enabled,
  # it has __drab__/0 function with Drab configuration
  @spec commander_config(atom) :: map
  defp commander_config(module) do
    module.__drab__()
  end
end