defmodule Pico.Protocol.Decoder do
  @type message :: {String.t, map}

  @moduledoc """
    Decode Pico messages
  """

  @doc """
    Given an encoded Pico message, decode it (using the associated key and iv,
    if encrypted)
  """
  @spec decode(binary, binary, binary) :: message
  def decode(msg, secret \\ nil, iv \\ nil)

  def decode(<<"PICO", major_b::bytes-size(1), minor_b::bytes-size(1), ciphertag::bytes-size(16), body::binary>>, key, iv) do
    <<major::integer-8-unsigned>> = major_b
    <<minor::integer-8-unsigned>> = minor_b

    # {_version, _} = Float.parse("#{major}.#{minor}")

    body =
      if key do
        :crypto.block_decrypt(:aes_gcm, key, iv, {<<major, minor>>, body, ciphertag})
      else
        body
      end

    [opname, data] = String.split(body, "|", [parts: 2])

    data =
      case data do
        <<0>> -> nil
        <<0, _::binary>> -> nil
        _ -> :erlang.binary_to_term(data)
      end

    {opname, data}
  end

  def decode(_, _, _), do: {:error, :protocol_mismatch}

end