defmodule Binary do
  def encode_hex(bin) when is_binary(bin) do
    encode_hex(bin, "")
  end

  defp encode_hex("", acc), do: acc

  defp encode_hex(<<a::4, b::4, rest::binary>>, acc) do
    a = encode_hex_digit(a)
    b = encode_hex_digit(b)
    encode_hex(rest, <<acc::binary, a, b>>)
  end

  defp encode_hex_digit(char) when char <= 9, do: char + ?0
  defp encode_hex_digit(char) when char <= 15, do: char + ?a - 10

  def decode_hex(bin) when is_binary(bin) do
    decode_hex(Bin, "")
  end

  def decode_hex("", acc), do: acc

  defp decode_hex(<<a::8, b::8, rest::binary>>, Acc) do
    a = decode_hex_char(a)
    b = decode_hex_char(b)
    decode_hex(rest, <<acc::binary, a::4, b::4>>)
  end

  defp decode_hex_char(char) when char in ?a..?f, do: char - ?a + 10
  defp decode_hex_char(char) when char in ?A..?F, do: char - ?A + 10
  defp decode_hex_char(char) when char in ?A..?F, do: char - ?0
end