defmodule AutoProcessRegistry do
  @moduledoc """
  AutoProcessRegistry is a process registry that is setup with a
  starter function to start new processes on demand.

  Processes aren't supervised. The idea is that if they die, for
  whichever reason, they can be restarted on demand when necessary. It
  is not hard, though, to make the starter function register a process
  under a supervisor.

  The registry implements the `whereis` message that is needed to
  make it usable for via naming lookups in the style of
  `{:via, YourRegistryModule, key}`.

  Here is a quick example using agents, lifted from the test suite:

      defmodule SampleModule do
        use AutoProcessRegistry

        def start_new(config, key) do
          Agent.start_link(fn -> config * key end)
        end
      end

      SampleModule.start_link(10)

  With this module defined, it's now possible to launch processes (in
  this case, simple agents, but it should be hard to see how to extend
  this for `GenServer`, etcetera:

      Agent.get({:via, SampleModule, 42}, fn(key) -> key end) # --> 420
      Agent.get({:via, SampleModule, 666}, fn(key) -> key end) # --> 6660

  A potential use case for this library is for example genservers that have
  their state persisted in a database. `start_new` can be setup to read
  data for a primary key, start a process with the state from the database,
  and from then reading from the database basically is just done on the fly
  by starting to talk to an instance with a certain primary key. What would
  be neat in that case, of course, is to extend this with an LRU cache and a
  maximum number of live processes, maybe for a later iteration.
  """

  @doc """
  Start the GenServer process implementing the registry. The GenServer
  is registered with the module name, so keeping the pid around is not
  necessary. The config argument is passed on to `start_new`
  """
  @callback start_link(any) :: GenServer.on_start

  @doc """
  Start a new process with the config argument as the first argument
  and the key as the second argument.
  """
  @callback start_new(any, any) :: GenServer.on_start

  @doc false
  defmacro __using__(_) do
    quote do
      @behaviour AutoProcessRegistry

      use GenServer

      # Client API

      @doc false
      def start_link(config) do
        GenServer.start_link(__MODULE__, config, name: __MODULE__)
      end

      def send(key, msg) do
        case whereis_name(key) do
          pid when is_pid(pid) ->
            Kernel.send(pid, msg)

          :undefined ->
            {:badarg, {key, msg}}
        end
      end

      def whereis_name(key) do
        GenServer.call(__MODULE__, {:whereis_name, key})
      end

      # Server API #

      def init(config) do
        {:ok, {config, %{}}}
      end

      def handle_call({:whereis_name, key}, _from, {config, registry}) do
        case Map.get(registry, key, nil) do
          nil ->
            {:ok, pid} = start_new(config, key)
            Process.monitor(pid)
            registry = Map.put(registry, key, pid)
            {:reply, pid, {config, registry}}

          pid ->
            {:reply, pid, {config, registry}}
        end
      end

      def handle_info({:DOWN, _ref, :process, pid, _reason}, {config, registry}) do
        {:noreply, {config, deregister(registry, pid)}}
      end

      def handle_info(_info, state), do: {:noreply, state}

      # Helper Functions #
      defp deregister(registry, pid) when is_pid(pid) do
        case Enum.find(registry, nil, fn({_key, cur_pid}) -> cur_pid == pid end) do
          nil -> registry
          {key, _pid} -> deregister(registry, key)
        end
      end

      defp deregister(registry, key) do
        Map.delete(registry, key)
      end
    end
  end
end