defmodule GenMQTT do
  @moduledoc ~S"""
  A behaviour module for implementing MMQT client processes.

  ## Example

  This example assumes an MQTT server running on localhost on port 1883.

      defmodule TemperatureLogger do
        use GenMQTT

        def start_link do
          GenMQTT.start_link(__MODULE__, nil)
        end

        def on_connect(state) do
          :ok = GenMQTT.subscribe(self, "room/+/temp", 0)
          {:ok, state}
        end

        def on_publish(["room", location, "temp"], message, state) do
          IO.puts "It is #{message} degrees in #{location}"
          {:ok, state}
        end
      end

  This will log to the console every time a sensor posts a temperature
  to the broker.

  ## Callbacks

  GenMQTT defines 12 callbacks, all of them are automatically defined
  when you use GenMQTT in your module, letting you define the callbacks
  you want to customize. Six of the callbacks are similar to the ones
  you know from GenServer, and the GenServer documentation should be
  consulted for info on these. They are: `init/1`, `handle_call/3`,
  `handle_cast/2`, `handle_info/2`, `terminate/2`, and `code_change/3`.

  The remaining six are specific to GenMQTT and deal with various
  events in a MQTT life cycle:

    * `on_connect/1` is run when the client connects or reconnects with
      the broker.

    * `on_connect_error/2` is triggered if the connection fails for
      whatever reason.

    * `on_disconnect/1` is run when the client disconnects from the MQTT
      broker.

    * `on_subscribe/2` run when the client subscribes to a topic.

    * `on_unsubscribe/2` run when the client stops subscribing to a
      topic.

    * `on_publish/3` triggered everytime something is published to the
      broker.

  All callbacks are optional. A macro will define a default function for
  undefined callbacks, so you only need to implement `on_publish/3` if
  that is what you need.

  ## Name Registration

  A GenMQTT is bound to the same name registration rules as GenServers.
  Read more about it in the Elixir `GenServer` docs.
  """

  # gen_server ---------------------------------------------------------
  @doc """
  Invoked when the server is started. `start_link/3` and `start/3` will
  block until it returns. `state` is the second term passed into either
  of the two start functions.

  When this function returns `{:ok, state}` it will enter its loop and
  will start receiving messages from the broker, or send messages to it
  as soon as it has entered the connected state.

  Returning `{:stop, reason}` will cause the start function to return
  `{:error, reason}`, and the process will exit with `reason` without
  entering its loop or calling `terminate/2`.
  """
  @callback init(state) ::
    {:ok, state} |
    {:ok, state, timeout | :hibernate} |
    :ignore |
    {:stop, reason :: any} when state: any

  @type from :: {pid, tag :: term}
  @callback handle_call(request :: term, from, state) ::
    {:reply, reply, new_state} |
    {:reply, reply, new_state, timeout | :hibernate} |
    {:noreply, new_state} |
    {:noreply, new_state, timeout | :hibernate} |
    {:stop, reason, reply, new_state} |
    {:stop, reason, new_state} when reply: term, state: term, new_state: term, reason: term

  @callback handle_cast(request :: term, state) ::
    {:noreply, new_state} |
    {:noreply, new_state, timeout | :hibernate} |
    {:stop, reason :: term, new_state} when state: term, new_state: term

  @callback handle_info(msg :: :timeout | term, state) ::
    {:noreply, new_state} |
    {:noreply, new_state, timeout | :hibernate} |
    {:stop, reason :: term, new_state} when state: term, new_state: term

  @callback terminate(reason, state) ::
    term when state: term, reason: :normal | :shutdown | {:shutdown, term} | term

  @callback code_change(old_vsn, state :: term, extra :: term) ::
    {:ok, new_state :: term} |
    {:error, reason :: term} when old_vsn: term | {:down, term}

  # gen_emqtt ----------------------------------------------------------
  @type topic :: [binary] | binary
  @type qos :: 0 | 1 | 2

  @doc """
  Triggered when the client successfully establish a connection to the
  broker. It will get run again if the client should disconnect from
  the broker, i.e. it temporarily becomes unavailable for whatever
  reason, if some numeral value has been set to the start option
  `reconnect_timeout`.

  ## Examples

  Subscribe to a topic as soon as a connection has been made to the
  broker:

      def on_connect(state) do
        :ok = GenMQTT.subscribe(self, "room/living-room/temp", 0)
        {:ok, state}
      end
  """
  @callback on_connect(state) ::
    {:ok, state} when state: term

  @doc """
  Callback triggered if there was a problem connecting to the broker.
  The `reason` is given as the first argument as an atom, making
  it possible to pattern match and react. The second argument is the
  process state.
  """
  @callback on_connect_error(reason, state) ::
    {:ok, state} when [state: term,
                       reason: :server_not_found |
                               :server_not_available |
                               :wrong_protocol_version |
                               :invalid_id |
                               :invalid_credentials |
                               :not_authorized]

  @doc """
  Callback triggered when the client disconnects from the broker for
  whatever reason.
  """
  @callback on_disconnect(state) ::
    {:ok, state} when state: term

  @doc """
  Callback triggered when the client successfully subscribes to one or
  more topics.

  The subscriptions are given in tuples containing the topic name and
  its quality of service.
  """
  @callback on_subscribe([{topic, qos}], state) ::
    {:ok, state} when state: term

  @doc """
  Callback triggered when the client successfully unsubscribes from one
  or more subscriptions. It will receive the unsubscribed
  subscriptions as a list of binaries as the first argument, and the
  process state as the second.
  """
  @callback on_unsubscribe(topic, state) ::
    {:ok, state} when state: term

  @doc ~S"""
  Callback triggered when a message has been published to a topic the
  client subscribes to.

  ## Examples

  The following will print the messages sent to the topic `room/+/temp`.

      def on_publish(["room", room, "temp"], temperature, state) do
        IO.puts "It is #{temperature} degrees in #{room}"
        {:ok, state}
      end
  """
  @callback on_publish(topic, payload :: binary, state) ::
    {:ok, state} when state: term

  defmacro __using__(_) do
    quote location: :keep do
      @behaviour :gen_emqtt

      @doc false
      def init(state) do
        {:ok, state}
      end

      @doc false
      def on_connect(state) do
        {:ok, state}
      end

      @doc false
      def on_connect_error(reason, state) do
        {:ok, state}
      end

      @doc false
      def on_disconnect(state) do
        {:ok, state}
      end

      @doc false
      def on_subscribe([{_topic, _qos}]=subscription, state) do
        {:ok, state}
      end

      @doc false
      def on_unsubscribe([_topic], state) do
        {:ok, state}
      end

      @doc false
      def on_publish(_topic, _msg, state) do
        {:ok, state}
      end

      @doc false
      def handle_call(msg, _from, state) do
        # We do this to trick Dialyzer to not complain about non-local returns.
        reason = {:bad_call, msg}
        case :erlang.phash2(1, 1) do
          0 -> exit(reason)
          1 -> {:stop, reason, state}
        end
      end

      @doc false
      def handle_cast(msg, state) do
        # We do this to trick Dialyzer to not complain about non-local returns.
        reason = {:bad_cast, msg}
        case :erlang.phash2(1, 1) do
          0 -> exit(reason)
          1 -> {:stop, reason, state}
        end
      end

      @doc false
      def handle_info(_msg, state) do
        {:noreply, state}
      end

      @doc false
      def terminate(_reason, _state) do
        :ok
      end

      @doc false
      def code_change(_old_version, state, _extra) do
        {:ok, state}
      end

      defoverridable [
        init: 1,

        on_connect: 1, on_connect_error: 2, on_disconnect: 1,
        on_subscribe: 2, on_unsubscribe: 2,
        on_publish: 3,

        handle_call: 3, handle_cast: 2, handle_info: 2,
        terminate: 2, code_change: 3
      ]
    end
  end

  @typedoc "Return values of `start*` functions"
  @type on_start ::
    {:ok, pid} |
    :ignore |
    {:error, {:already_started, pid} | term}

  @typedoc "Debug options supported by the `start*` functions"
  @type debug :: [:trace | :log | :statistics | {:log_to_file, Path.t}]

  @typedoc "The GenMQTT process name"
  @type name :: atom | {:global, term} | {:via, module, term}

  @type info_action :: :connack_in |
                       :connect_out |
                       :puback_in | :puback_out |
                       :pubcomp_in | :pubcomp_out |
                       :publish_in | :publish_out |
                       :pubrec_in | :pubrec_out |
                       :pubrel_in | :pubrel_out |
                       :reconnect |
                       :suback | :subscribe_out |
                       :unsuback | :unsubscribe_out
  @type info_fun :: {
    ({info_action, message_id :: char_list}, state :: term -> new_state :: term),
    initial_state :: term
  }

  @typedoc "Option values used by the `start*` functions"
  @type option :: {:debug, debug} |
                  {:name, name} |
                  {:timeout, timeout} |
                  {:spawn_opt, Process.spawn_opt} |
                  {:host, :inet.ip_address() | binary} |
                  {:port, :inet.port_number()} |
                  {:username, username :: binary | :undefined} |
                  {:password, password :: binary | :undefined} |
                  {:client, client_id :: binary} |
                  {:clean_session, boolean} |
                  {:last_will_topic, topic :: char_list | binary | :undefined} |
                  {:last_will_msg, payload :: char_list | binary | :undefined} |
                  {:last_will_qos, qos} |
                  {:reconnect_timeout, pos_integer | :undefined} |
                  {:keepalive_interval, pos_integer} |
                  {:retry_interval, pos_integer} |
                  {:proto_version, version :: pos_integer} |
                  {:info_fun, info_fun} |
                  {:transport, {:gen_tcp, config :: list} | {:ssl, config :: list}}

  @type options :: [option]

  @doc """
  Start a linked connection to a MQTT broker

  ## Options

    * `:name` the name given to the process.

    * `:host` the host name or ip address of the MQTT broker.

    * `:port` the port number the MQTT broker is running on given as
      an integer. This will default to `1883`.

    * `:username` the name of the user on the MQTT broker, defaults to
      undefined and is not needed if it connect to a broker that
      allows anonymous connections.

    * `:password` the password for the user on the MQTT broker. This
      can be omitted if the broker accepts anonymous connections.

    * `:client` the client ID. A randomly generated client ID will be
      used if this is option is not supplied. Notice that all
      connected clients should have a unique client id. Should you
      choose to generate your own client ID it should be no longer
      than 23 characters, unless the broker supports longer client
      ids. The requirements for a client id is described in the MQTT
      specifications:

        - http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Toc385349242

    * `:clean_session` boolean value, defaults to `true`.

    * `:last_will_topic` topic to send message to if the MQTT client
      disappears from the broker.

    * `:last_will_msg` the message that will get sent to
      `last_will_topic` if the client disappears from the broker.

    * `:last_will_qos` the quality of service the last will message
      should get sent with. This should be specified as an integer
      value between 0 and 2. It defaults to `0`.

    * `:reconnect_timeout` the number of seconds the client will wait
      for a connection when attempting to reconnect to a broker.

    * `:keepalive_interval` the number of seconds between keep alives.

    * `:retry_interval` the number of seconds between reconnection
      attempts if the client disconnects from the broker.

    * `:proto_version` which MQTT protocol version to use, defaults to
      version `3`.

    * `:transport` the network transport the client should use to
      communicate with the broker and its respective options.  The
      default transport is `{:gen_tcp, []}`. For basic SSL support use
      `{:ssl, ssl_options}`, which can be configured according to the
      Erlang documentation on the `:ssl` module:
      http://erlang.org/documentation/doc-1/man/ssl.html

    * `info_fun` a function that can be passed in for logging,
      benchmarks, debugging, etc. It should not be used in production.
      Please refer to the unit tests of this project for a simple
      example of usage.
  """
  @spec start_link(module, any, options) :: on_start
  def start_link(module, args, options \\ []) when is_atom(module) and is_list(options) do
    do_start(options, :start_link, module, args)
  end

  @doc """
  Starts a `GenMQTT` process without links (outside of a supervision tree).

  See `start_link/3` for more information.
  """
  @spec start(module, any, options) :: on_start
  def start(module, args, options \\ []) when is_atom(module) and is_list(options) do
    do_start(options, :start, module, args)
  end

  defp do_start(options, mode, module, args) do
    options =
      options
      |> normalize_options
      |> generate_client_id
      |> validate_options

    case Keyword.pop(options, :name) do
      {nil, opts} ->
        apply(:gen_emqtt, mode, [module, args, opts])
      {name, opts} when is_atom(name) ->
        apply(:gen_emqtt, mode, [{:local, name}, module, args, opts])
      {other, opts} when is_tuple(other) ->
        apply(:gen_emqtt, mode, [other, module, args, opts])
    end
  end

  @cast_to_char_list [:host, :client, :last_will_topic, :last_will_msg]
  defp normalize_options(opts) do
    Enum.map(opts, fn
      {key, val} when is_binary(val) and key in @cast_to_char_list ->
        {key, String.to_char_list(val)}

      option ->
        option
    end)
  end

  # If no client ID is set we will provide a randomly generated one.
  # Notice that we need to keep the client name below 23 chars
  # according to the MQTT specs:
  #
  # http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Toc385349242
  #
  # Notice that we will not attempt any validation on the length of
  # user generated client ids because some MQTT servers may allow for
  # longer names.
  defp generate_client_id(opts) do
    case opts[:client] do
      nil ->
        client_name =
          10 |> :crypto.strong_rand_bytes |> Base.encode16
        Keyword.put(opts, :client, client_name)

      _ ->
        opts
    end
  end

  defp validate_options(opts) do
    case {opts[:username], opts[:password]} do
      {nil, nil} ->
        # anonymous login
        opts

      {_username, nil} ->
        raise ArgumentError, message: "`username` set but no `password` was given"

      {nil, _passwd} ->
        raise ArgumentError, message: "`password` set but no `username` was given"

      {_username, _passwd} ->
        opts
    end
  end

  @doc """
  Disconnect from the MQTT broker and stop the process.

  `on_disconnect/1` will not be triggered, if something needs to be
  cleaned up it can be done in the `terminate/2` callback, and the
  shutdown reason will be `:normal`
  """
  @spec disconnect(pid) :: :ok
  defdelegate disconnect(pid), to: :gen_emqtt

  @doc """
  Subscribe to one or multiple topics given a list of tuples containing
  the topic name and its quality of service `[{"topic", 0}, ..]`
  """
  @spec subscribe(pid, [{topic :: binary, qos}]) :: :ok
  defdelegate subscribe(pid, topics), to: :gen_emqtt

  @doc """
  Subscribe to `topic` with quality of service set to `qos`
  """
  @spec subscribe(pid, topic, qos) :: :ok
  defdelegate subscribe(pid, topic, qos), to: :gen_emqtt

  @doc """
  Unsubscribe from one or more `topic`
  """
  @spec unsubscribe(pid, topic) :: :ok
  defdelegate unsubscribe(pid, topic), to: :gen_emqtt

  @doc """
  Publish `payload` to `topic` with quality of service set to `qos`
  """
  @spec publish(pid, topic, payload :: binary, qos) :: :ok
  def publish(pid, topic, payload, qos) when is_list(topic) do
    :gen_emqtt.publish(pid, topic, payload, qos)
  end
  def publish(pid, topic, payload, qos) when is_binary(topic) do
    publish(pid, [topic], payload, qos)
  end

  @doc """
  Make a call to the underlying state machine
  """
  @spec call(pid, request :: term) :: term
  defdelegate call(pid, request), to: :gen_emqtt

  @doc """
  Make a cast to the underlying state machine
  """
  @spec cast(pid, request :: term) :: :ok
  defdelegate cast(pid, request), to: :gen_emqtt
end