Grab a buffer, make sure it's at least as long as BHS, shove into BHS, ok, make sure it's at least as long as what the BHS requires...

Pattern matching is the hidden power of Elixir that's probably the hardest to get fluent with, it's IMHO harder than Rust's equivalent (because of gradual typing!) - but when it works it blows Pythons' match syntax way, way out.

  # Example: Login Request with immediate = 0x03 | 0x40 = 0x43.
  defp parse_lengths(<<
         _reserved::1, _i_bit::1, opcode::6,
         _flags::8,
         _specific1::16,
         ahs_length::8,
         data_length::24,
         _rest::binary
       >>) do
    {:ok, opcode, ahs_length * 4, data_length}
  end

  defp parse_lengths(_), do: {:error, :invalid_bhs}