defmodule RNS.HDLC do
  @moduledoc """
  HDLC framing.
  This seems to be based on [HDLC](https://en.wikipedia.org/wiki/HDLC) but does not have addresses or anything, just the flag and escaping...
  """

  @doc """
  Frame raw data into HDLC.
  Escapes the data and puts it between two HDLC flags.
  The code might be a bit ugly, so let me explain it:
  It returns a binary, and you can see that it ends and starts with `0x7E`, which is the flag that show the start and end of the HDLC data.
  The complicated part is with the `String.replace/4`, it replaces the characters `0x7E`(the flag) and `0x7D`(the escape character) by `0x7D`(the escape character)
  and the character that is getting replaced xored with `0x20`. There is also `String.to_charlist/1` and `hd/1`, that's just to convert the binary(`String`) to an `Integer`.
  Basically, `<<0x7E>>` is replaced by `<<0x7D, 0x5E>>` and `<<0x7D>>` by `<<0x7D, 0x5D>>`.
  """
  @spec frame(binary()) :: binary()
  def frame(raw) do
    <<0x7E,
      String.replace(raw, [<<0x7E>>, <<0x7D>>], fn char ->
        int =
          char
          |> String.to_charlist()
          |> hd

        <<0x7D, Bitwise.bxor(int, 0x20)>>
      end)::binary, 0x7E>>
  end

  @doc """
  Unframes HDLC data.
  Removes HDLC flags or gives an error if they are not present, and unescapes the characters.
  """
  @spec unframe(binary()) :: {:ok, binary()} | {:error, :no_hldc_flags}
  def unframe(raw) do
    case raw do
      <<0x7E, unframed::binary-size(byte_size(raw) - 2), 0x7E>> ->
        escaped =
          String.replace(unframed, [<<0x7D, 0x5E>>, <<0x7D, 0x5D>>], fn string ->
            int =
              String.at(string, 1)
              |> String.to_charlist()
              |> hd

            Bitwise.bxor(int, 0x20)
          end)

        {:ok, escaped}

      _ ->
        {:error, :no_hdlc_flags}
    end
  end
end