defmodule P2pChat.TransportProto do
  defmodule Messages do
    defmodule Hello do
      defstruct [:netid]
    end
    
    defmodule InformNeighbor do
      defstruct [:netid, :addr, :port]
    end

    defmodule Goodbye do
      defstruct []
    end
  end

  #
  # Public API
  # 

  def encode_message(%Messages.Hello{} = msg) do
    [encode_version(), <<0x01::8>>, encode_string(msg.netid)]
  end
  
  def encode_message(%Messages.InformNeighbor{} = msg) do
    [encode_version(), <<0x02::8>>, encode_string(msg.netid), encode_ip_addr(msg.addr)]
  end

  def encode_message(%Messages.Goodbye{} = _msg) do
    [encode_version(), <<0x03::8>>]
  end

  def decode_message(<<0x01::8, 0x01::8, payload::binary>>) do
    <<netid_len::16, netid::size(netid_len)-binary>> = payload
    %Messages.Hello{netid: netid}
  end

  def decode_message(<<0x01::8, 0x02::8, payload::binary>>) do
    <<netid_len::16, netid::size(netid_len)-binary, addr_len::16, addr::size(addr_len)-binary>> = payload
    %Messages.InformNeighbor{netid: netid, addr: addr}
  end

  def decode_message(<<0x01::8, 0x03::8>>) do
    %Messages.Goodbye{}
  end

  def decode_message(<<0x01::8, _::binary>>), do: :error
  def decode_message(<<_::binary>>), do: :error

  #
  # Private Utilities
  # 

  defp encode_version() do
    <<0x01::8>>
  end

  defp encode_string(str) do
    [<<byte_size(str)::16>>, str]
  end

  defp encode_ip_addr({a1, a2, a3, a4, a5, a6, a7, a8}) do
    <<6::8, a1::8, a2::8, a3::8, a4::8, a5::8, a6::8, a7::8, a8::8>>
  end
  defp encode_ip_addr({a1, a2, a3, a4}) do
    <<4::8, a1::8, a2::8, a3::8, a4::8>>
  end
end