defmodule TODO do

  @moduledoc File.read!(__DIR__ <> "/../README.md")

  def config(key, default \\ nil) do
    case Application.get_env(:todo, key) do
      nil -> default
      x -> x
    end
  end

  defmacro __using__(opts) do
    print_conf = config(:print, :overdue)
    persist = config(:persist, false)
    quote do
      persist = unquote(persist) || unquote(opts[:persist])
      Module.register_attribute(__MODULE__, :todo, accumulate: true, persist: persist)
      @before_compile unquote(__MODULE__)
      @todo_version Mix.Project.config[:version]
      @todo_print_conf (case {Keyword.get(unquote(opts), :print), unquote(print_conf)} do
        {:all,_} -> :all
        {_,:all} -> :all
        _ -> :overdue
      end)

      defmacrop todo(items) do
        # TODO.print_todos(__MODULE__, items, @todo_version, @todo_print_conf)
        Module.put_attribute(__MODULE__, :todo, items)
      end
    end
  end

  defmacro __before_compile__(env) do
    app_version = Module.get_attribute(env.module, :todo_version)
    print_conf = Module.get_attribute(env.module, :todo_print_conf)
    Module.get_attribute(env.module, :todo)
    |> output_todos(env.module, app_version, print_conf)
  end

  def output_todos(todos, module, app_version, print_conf) do
    case todos do
      [] -> nil
      nil -> nil
      items ->
        [items]
        |> List.flatten
        |> Enum.map(&wrap_unversionned/1)
        |> Enum.sort(&sort_todos/2)
        |> Enum.reduce([], &group_todos/2)
        |> Enum.map(fn({version, ts}) -> {version, Enum.reverse ts} end)
        |> Enum.reverse
        |> format_todos(app_version, print_conf)
        |> (fn(x) -> [format_module(module), x] end).()
        |> IO.puts
    end
  end

  # put unversionned last
  def wrap_unversionned(x={_version, _message}), do: x
  def wrap_unversionned(message), do: {:any, message}

  def sort_todos({_,_}, {:any, _}), do: true
  def sort_todos({:any, _}, {_,_}), do: false
  def sort_todos({v1,_}, {v2,_}), do: v1 < v2

  def group_todos({same_version, t}, [{same_version,ts}|rest]) do
    result = [{same_version,[t|ts]}|rest]
    result
  end
  def group_todos(item, {version,t}) do
    # first accumulator is not a list, it's a todo item, wrap and redo
    group_todos(item, [{version, [t]}])
  end
  def group_todos({version, t}, rest) do
    [{version,[t]}|rest]
  end

  def format_todos([], _, _) do
    []
  end
  def format_todos([{version, ts}|rest], app_version, print_conf) do
    current = case display_mode(version, app_version, print_conf) do
      :ignore -> []
      :info -> [format_version(version), format_messages(ts)]
      :warn -> IO.ANSI.format(yellow([format_version(version), Enum.map(ts, &format_message/1)]))
    end
    [current|format_todos(rest, app_version, print_conf)]
  end

  def format_module(atom) do
    ["@todos found in module ", to_string atom]
  end

  def format_version(:any) do
    format_version("Other")
  end
  def format_version(version) do
    ["\n * ", to_string(version), " :"]
  end

  def format_messages([message|[]]) do
    # A single message => same line
    [" ", message]
  end
  def format_messages(messages) do
    Enum.map(messages, &(["\n     - ", &1]))
  end

  def format_message(message) do

  end

  def display_mode(:any, _, _) do
    :info
  end
  def display_mode(version, app_version, print_conf) do
    version = to_string version
    case {Version.compare(version, app_version), print_conf} do
      {:gt, :all} ->
        # Version when the feature is required is greater than the current
        # version, it's not overdue, but we're asked to print all.
        :info
      {:gt, :overdue} ->
        # Same case, but we must print only overdue items, so no-print
        :ignore
      _ ->
        # App version is higher than feature version. The feature is due for a
        # past version. This is not good.
        :warn
    end
  end

  defp yellow(message) do
    [:yellow, message]
  end

end