defprotocol ICalendar.Value do
  @fallback_to_any true
  def to_ics(data)
end

alias ICalendar.Value
alias ICalendar.Util.RRULE, as: Util

defimpl Value, for: ICalendar.RRULE do
  @doc """
  This function converts RRULE structs into an RRULE string
  """
  def to_ics(rrule = %ICalendar.RRULE{}) do
    rrule
    |> Map.from_struct
    |> Map.keys
    |> order_conventionally
    |> Enum.map(&(Util.serialize(rrule, &1)))
    |> Enum.reject(&(&1 == nil))
    |> Enum.join(";")
  end

  # some clients rely on FREQ coming first, COUNT or UNTIL next, then others
  # even though against spec (http://www.kanzaki.com/docs/ical/recur.html#descr)
  # Apple Calendar, I'm looking at you
  defp order_conventionally(keys) do
    first_keys = [:frequency, :count, :until, :interval]
    last_keys = keys -- first_keys

    first_keys ++ last_keys
  end
end

defimpl Value, for: BitString do
  def to_ics(x) do
    x
    |> String.replace(~S"\n", ~S"\\n")
    |> String.replace("\n", ~S"\n")
  end
end

defimpl Value, for: Tuple do
  defmacro elem2(x, i1, i2) do
    quote do
      unquote(x) |> elem(unquote(i1)) |> elem(unquote(i2))
    end
  end

  @doc """
  This macro is used to establish whether a tuple is in the Erlang Timestamp
  format (`{{year, month, day}, {hour, minute, second}}`).
  """
  defmacro is_datetime_tuple(x) do
    quote do
      # Year
      ( unquote(x) |> elem2(0, 0)  |> is_integer) and
      # Month
      ( unquote(x) |> elem2(0, 1)  |> is_integer) and
      ((unquote(x) |> elem2(0, 1)) <= 12) and
      ((unquote(x) |> elem2(0, 1)) >= 1) and
      # Day
      ( unquote(x) |> elem2(0, 2)  |> is_integer) and
      ((unquote(x) |> elem2(0, 2)) <= 31) and
      ((unquote(x) |> elem2(0, 2)) >= 1) and
      # Hour
      ( unquote(x) |> elem2(1, 0)  |> is_integer) and
      ((unquote(x) |> elem2(1, 0)) <= 23) and
      ((unquote(x) |> elem2(1, 0)) >= 0) and
      # Minute
      ( unquote(x) |> elem2(1, 1)  |> is_integer) and
      ((unquote(x) |> elem2(1, 1)) <= 59) and
      ((unquote(x) |> elem2(1, 1)) >= 0) and
      # Second
      ( unquote(x) |> elem2(1, 2)  |> is_integer) and
      ((unquote(x) |> elem2(1, 2)) <= 60) and
      ((unquote(x) |> elem2(1, 2)) >= 0)
    end
  end

  @doc """
  This function converts Erlang timestamp tuples into DateTimes.
  """
  def to_ics(timestamp) when is_datetime_tuple(timestamp) do
    timestamp
    |> Timex.to_datetime
    |> Value.to_ics
  end

  def to_ics(x), do: x
end

defimpl Value, for: DateTime do
  use Timex

  @doc """
  This function converts DateTimes to UTC timezone and then into Strings in the
  iCal format
  """
  def to_ics(%DateTime{} = timestamp) do
    format_string = "{YYYY}{0M}{0D}T{h24}{m}{s}"

    {:ok, result} =
      timestamp
      |> Timex.format(format_string)
    result
  end
end

defimpl Value, for: Date do
  use Timex

  @doc """
  This function converts DateTimes to UTC timezone and then into Strings in the
  iCal format
  """
  def to_ics(%Date{} = timestamp) do
    format_string = "{YYYY}{0M}{0D}"

    {:ok, result} =
      timestamp
      |> Timex.format(format_string)
    result
  end
end

defimpl Value, for: Any do
  def to_ics(x), do: x
end