encode words

encode_decode.py

class InvalidException(Exception):
    pass


def encode1(words):
    res = ""
    for word in words:
        res += str(len(word)) + ':' + word
    return res


def decode1(s):
    n, i, words = len(s), 0, []
    while i < len(s):
        idx = s.find(':', i)
        if idx == -1:
            raise InvalidException
        try:
            m = int(s[i:idx])
            if m+idx+1 > n:
                raise InvalidException
            words.append(s[idx+1:idx+m+1])
        except ValueError:
            raise InvalidException
        i = idx+m+1
    return words


def encode2(words):
    res = ""
    for word in words:
        for c in word:
            if c != '\t':
                res += c
            else:
                res += '\t\t'
        res += '\t\n'
    return res


def decode2(s):
    i, n, word, words = 0, len(s), "", []
    while i < n:
        if s[i] != '\t':
            word += s[i]
        else:
            if i+1 < n and s[i+1] == '\t':
                word += '\t'
            elif i+1 < n and s[i+1] == '\n':
                words.append(word)
                word = ""
            else:
                raise InvalidException
            i += 1
        i += 1
    return words


words = ['abc', 'a', 't\t\t']
s = encode2(words)
print s
print decode2(s)