defmodule Supervisor.Spec do
  @moduledoc """
  Convenience functions for defining a supervision specification.

  ## Example

  By using the functions in this module one can define a supervisor
  and start it with `Supervisor.start_link/2`:

      import Supervisor.Spec

      children = [
        worker(MyWorker, [arg1, arg2, arg3]),
        supervisor(MySupervisor, [arg1])
      ]

      Supervisor.start_link(children, strategy: :one_for_one)

  In many situations, it may be handy to define supervisors backed
  by a module:

      defmodule MySupervisor do
        use Supervisor

        def start_link(arg) do
          Supervisor.start_link(__MODULE__, arg)
        end

        def init(arg) do
          children = [
            worker(MyWorker, [arg], restart: :temporary)
          ]

          supervise(children, strategy: :simple_one_for_one)
        end
      end

  Notice in this case we don't have to explicitly import
  `Supervisor.Spec` as `use Supervisor` automatically does so.

  Explicit supervisors as above are required when there is a need to:

    1. Partially change the supervision tree during hot-code swaps.

    2. Define supervisors inside other supervisors.

    3. Perform actions inside the supervision `init/1` callback.

       For example, you may want to start an ETS table that is linked to
       the supervisor (i.e. if the supervision tree needs to be restarted,
       the ETS table must be restarted too).

  ## Supervisor and worker options

  In the example above, we have defined workers and supervisors
  and each accepts the following options:

    * `:id` - a name used to identify the child specification
      internally by the supervisor; defaults to the given module
      name

    * `:function` - the function to invoke on the child to start it

    * `:restart` - defines when the child process should restart

    * `:shutdown` - defines how a child process should be terminated

    * `:modules` - it should be a list with one element `[module]`,
      where module is the name of the callback module only if the
      child process is a `Supervisor` or `GenServer`; if the child
      process is a `GenEvent`, modules should be `:dynamic`

  ### Restart values (:restart)

  The following restart values are supported:

    * `:permanent` - the child process is always restarted

    * `:temporary` - the child process is never restarted (not even
      when the supervisor's strategy is `:rest_for_one` or `:one_for_all`)

    * `:transient` - the child process is restarted only if it
      terminates abnormally, i.e. with another exit reason than
      `:normal`, `:shutdown` or `{:shutdown, term}`

  ### Shutdown values (:shutdown)

  The following shutdown values are supported:

    * `:brutal_kill` - the child process is unconditionally terminated
      using `exit(child, :kill)`.

    * `:infinity` - if the child process is a supervisor, it is a mechanism
      to give the subtree enough time to shutdown. It can also be used with
      workers with care.

    * Finally, it can also be any integer meaning that the supervisor tells
      the child process to terminate by calling `Process.exit(child, :shutdown)`
      and then waits for an exit signal back. If no exit signal is received
      within the specified time (in milliseconds), the child process is
      unconditionally terminated using `Process.exit(child, :kill)`.
  """

  @typedoc "Supported strategies"
  @type strategy :: :simple_one_for_one | :one_for_one | :one_for_all | :rest_for_one

  @typedoc "Supported restart values"
  @type restart :: :permanent | :transient | :temporary

  @typedoc "Supported shutdown values"
  @type shutdown :: :brutal_kill | :infinity | non_neg_integer

  @typedoc "Supported worker values"
  @type worker :: :worker | :supervisor

  @typedoc "Supported module values"
  @type modules :: :dynamic | [module]

  @typedoc "Supported id values"
  @type child_id :: term

  @typedoc "The supervisor specification"
  @type spec :: {child_id,
                  start_fun :: {module, atom, [term]},
                  restart,
                  shutdown,
                  worker,
                  modules}

  @doc """
  Receives a list of children (workers or supervisors) to
  supervise and a set of options.

  Returns a tuple containing the supervisor specification.

  ## Examples

      supervise children, strategy: :one_for_one

  ## Options

    * `:strategy` - the restart strategy option. It can be either
      `:one_for_one`, `:rest_for_one`, `:one_for_all`, or
      `:simple_one_for_one`. You can learn more about strategies
      in the `Supervisor` module docs.

    * `:max_restarts` - the maximum amount of restarts allowed in
      a time frame. Defaults to 3.

    * `:max_seconds` - the time frame in which `:max_restarts` applies.
      Defaults to 5.

  The `:strategy` option is required and by default maximum 3 restarts
  are allowed within 5 seconds. Please check the `Supervisor` module for
  a complete description of the available strategies.
  """
  @spec supervise([spec], strategy: strategy,
                          max_restarts: non_neg_integer,
                          max_seconds: non_neg_integer) :: {:ok, tuple}
  def supervise(children, options) do
    unless strategy = options[:strategy] do
      raise ArgumentError, "expected :strategy option to be given"
    end

    maxR = Keyword.get(options, :max_restarts, 3)
    maxS = Keyword.get(options, :max_seconds, 5)

    assert_unique_ids(Enum.map(children, &elem(&1, 0)))
    {:ok, {{strategy, maxR, maxS}, children}}
  end

  defp assert_unique_ids([id|rest]) do
    if id in rest do
      raise ArgumentError,
        "duplicated id #{inspect id} found in the supervisor specification, " <>
        "please explicitly pass the :id option when defining this worker/supervisor"
    else
      assert_unique_ids(rest)
    end
  end

  defp assert_unique_ids([]) do
    :ok
  end

  @doc """
  Defines the given `module` as a worker which will be started
  with the given arguments.

      worker ExUnit.Runner, [], restart: :permanent

  By default, the function `start_link` is invoked on the given
  module. Overall, the default values for the options are:

      [id: module,
       function: :start_link,
       restart: :permanent,
       shutdown: 5000,
       modules: [module]]

  Check `Supervisor.Spec` module docs for more information on
  the options.
  """
  @spec worker(module, [term], [restart: restart, shutdown: shutdown,
                                id: term, function: atom, modules: modules]) :: spec
  def worker(module, args, options \\ []) do
    child(:worker, module, args, options)
  end

  @doc """
  Defines the given `module` as a supervisor which will be started
  with the given arguments.

      supervisor ExUnit.Runner, [], restart: :permanent

  By default, the function `start_link` is invoked on the given
  module. Overall, the default values for the options are:

      [id: module,
       function: :start_link,
       restart: :permanent,
       shutdown: :infinity,
       modules: [module]]

  Check `Supervisor.Spec` module docs for more information on
  the options.
  """
  @spec supervisor(module, [term], [restart: restart, shutdown: shutdown,
                                    id: term, function: atom, modules: modules]) :: spec
  def supervisor(module, args, options \\ []) do
    options = Keyword.put_new(options, :shutdown, :infinity)
    child(:supervisor, module, args, options)
  end

  defp child(type, module, args, options) do
    id       = Keyword.get(options, :id, module)
    modules  = Keyword.get(options, :modules, modules(module))
    function = Keyword.get(options, :function, :start_link)
    restart  = Keyword.get(options, :restart, :permanent)
    shutdown = Keyword.get(options, :shutdown, 5000)

    {id, {module, function, args},
      restart, shutdown, type, modules}
  end

  defp modules(GenEvent), do: :dynamic
  defp modules(module),   do: [module]
end
