-module(binary).
-export([encode_hex/1, decode_hex/1]).

encode_hex(Bin) when is_binary(Bin) ->
    encode_hex(Bin, <<>>).

encode_hex(<<>>, Acc) ->
    Acc;
encode_hex(<<A0:4, B0:4, Rest/binary>>, Acc) ->
    A = encode_hex_digit(A0),
    B = encode_hex_digit(B0),
    encode_hex(Rest, <<Acc/binary, A, B>>).

encode_hex_digit(Char) when Char =< 9 ->
    Char + $0;
encode_hex_digit(Char) when Char =< 15 ->
    Char + $A - 10.

decode_hex(Bin) when is_binary(Bin) ->
    decode_hex(Bin, <<>>).

decode_hex(<<>>, Acc) ->
    Acc;
decode_hex(<<A0:8, B0:8, Rest/binary>>, Acc) ->
    A = decode_hex_char(A0),
    B = decode_hex_char(B0),
    decode_hex(Rest, <<Acc/binary, A:4, B:4>>).

decode_hex_char(Char) when Char >= $a, Char =< $f ->
    Char - $a + 10;
decode_hex_char(Char) when Char >= $A, Char =< $F ->
    Char - $A + 10;
decode_hex_char(Char) when Char >= $0, Char =< $9 ->
    Char - $0.