Encode/Decode problem on Erlang.

codec.erl

-module(codec).

-export([encode/1, decode/1]).


encode(Ss) ->
    encode(Ss, "").

encode([], Enc) ->
    lists:reverse(Enc);   
encode([ S | Ss ], Enc) ->
    case S of
        "" when Ss /= [] -> encode(Ss, ":" ++ Enc);
        ""               -> encode([], Enc);
        "!" ++ S1        -> encode([ S1 | Ss ], "!!" ++ Enc);
        ":" ++ S1        -> encode([ S1 | Ss ], ":!" ++ Enc);
        [C|S1]           -> encode([ S1 | Ss ], [C|Enc])
    end.

decode(S) ->    
      decode(S, [""]).

decode(Str, [S | Ss] = Acc) ->
    case Str of
        "!!" ++ Tail -> decode(Tail, [ "!" ++ S | Ss ]);
        "!:" ++ Tail -> decode(Tail, [ ":" ++ S | Ss ]);
        ":" ++ Tail  -> decode(Tail, [ "",  lists:reverse(S) | Ss ]);
        [X|Tail]     -> decode(Tail, [ [X|S] | Ss ]);
        ""           -> lists:reverse(Acc)
    end.

Algorithm is:
encode

1. perform substitution '!' -> "!!"
2. perform substitution ':' -> "!:"
3. join strings with ':'

decode
scan string from left to right

1. "!!" -> "!"
2. "!:" -> ":"
3. ":" -> substring finished
4. goto 1.