meta_pid

defmodule MetaPid do
  defmacro __using__(into: into, name: name) do
    quote bind_quoted: [ into: into, name: name ] do
      @moduledoc """
      A simple KV store for aggregating meta data about processes.

      ## Example Usage

        ```elixir

        # 1) Register a pid

        pid = self()
        #{__MODULE__ |> Atom.to_string}.register_pid(pid)


        # 2) Register a pid with some data

        pid = self()
        #{__MODULE__ |> Atom.to_string}.register_pid(pid, %#{into |> Atom.to_string}{})


        # 3) Update a pid's meta data

        #{__MODULE__ |> Atom.to_string}.update_pid(pid, %#{into |> Atom.to_string}{})


        # 4) Remove a pid from the registry

        #{__MODULE__ |> Atom.to_string}.unregister_pid(pid)
        ```
      """

      use GenServer

      @type registry :: %{pid => unquote(into).t}

      @server_name name

      # CLIENT INTERFACE

      @spec start_link([any()]) :: {:ok, pid()} | {:error, any()}
      def start_link(options \\ []) do
        GenServer.start_link(__MODULE__, nil, name: @server_name)
      end

      def stop() do
        GenServer.stop(@server_name)
      end

      @spec register_pid(pid()) :: atom()
      def register_pid(pid) do
        data = struct(unquote(into))
        GenServer.call(@server_name, {:register_pid, pid, data})
      end

      @spec register_pid(pid(), unquote(into).t) :: atom()
      def register_pid(pid, data) do
        GenServer.call(@server_name, {:register_pid, pid, data})
      end

      @spec put_pid(pid(), unquote(into).t) :: atom()
      def put_pid(pid, data) do
        GenServer.call(@server_name, {:put_pid, pid, data})
      end

      @spec fetch_pid(pid()) :: {:ok, unquote(into).t} | :error
      def fetch_pid(pid) do
        GenServer.call(@server_name, {:fetch_pid, pid})
      end

      @spec unregister_pid(pid()) :: atom()
      def unregister_pid(pid) do
        GenServer.call(@server_name, {:unregister_pid, pid})
      end

      @spec get_registry() :: registry()
      def get_registry() do
        GenServer.call(@server_name, :get_registry)
      end

      @spec transform_pid(pid(), (unquote(into).t -> unquote(into).t)) :: atom()
      def transform_pid(pid, transform_fn) do
        GenServer.call(@server_name, {:transform_pid, pid, transform_fn})
      end

      # SERVER INTERFACE

      @spec init([any()]) :: {:ok, registry()}
      def init(options \\ []) do
        {:ok, %{}}
      end

      @spec handle_call(arg, {pid(), any()}, registry()) :: {:reply, atom(), registry()} when arg: {atom(), pid(), unquote(into).t}
      @spec handle_call(arg, {pid(), any()}, registry()) :: {:reply, atom(), registry()} when arg: {:unregister_pid, pid()}
      @spec handle_call(arg, {pid(), any()}, registry()) :: {:reply, :error | {:ok, unquote(into).t}, map()} when arg: {:fetch_pid, pid()}
      @spec handle_call(arg, {pid(), any()}, registry()) :: {:reply, registry(), registry()} when arg: :get_registry
      @spec handle_call(arg, {pid(), any()}, registry()) :: {:reply, atom(), registry()} when arg: {atom(), pid(), (unquote(into).t -> unquote(into).t)}

      def handle_call({:register_pid, pid, data}, _from, registry) do
        exit_callback(pid)
        {:reply, :ok, Map.put(registry, pid, data)}
      end

      def handle_call({:put_pid, pid, data}, _from, registry) do
        {:reply, :ok, Map.put(registry, pid, data)}
      end

      def handle_call({:unregister_pid, pid}, _from, registry) do
        {:reply, :ok, Map.delete(registry, pid)}
      end

      def handle_call({:fetch_pid, pid}, _from, registry) do
        {:reply, Map.fetch(registry, pid), registry}
      end

      def handle_call(:get_registry, _from, registry) do
        {:reply, registry, registry}
      end

      def handle_call({:transform_pid, pid, transform_fn}, _from, registry) do
        case Map.fetch(registry, pid) do
          {:ok, data} -> {:reply, :ok, Map.put(registry, pid, transform_fn.(data))}
          _ -> {:reply, :error, registry}
        end
      end

      @spec exit_callback(pid) :: pid
      defp exit_callback(pid) do
        spawn(fn ->
          Process.monitor(pid)

          receive do
            {:DOWN, _, _, _, _} -> __MODULE__.unregister_pid(pid)
          end
        end)
      end
    end
  end
end