base91.py

import struct


base91_alphabet = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M',
                   'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z',
                   'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm',
                   'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z',
                   '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '!', '#', '$',
                   '%', '&', '(', ')', '*', '+', ',', '.', '/', ':', ';', '<', '=',
                   '>', '?', '@', '[', ']', '^', '_', '`', '{', '|', '}', '~', '"']

decode_table = dict((v, k) for k, v in enumerate(base91_alphabet))


def decode(encoded_str):
    ''' Decode Base91 string to a bytearray '''
    v = -1
    b = 0
    n = 0
    out = bytearray()
    for strletter in encoded_str:
        if not strletter in decode_table:
            continue
        c = decode_table[strletter]
        if (v < 0):
            v = c
        else:
            v += c * 91
            b |= v << n
            n += 13 if (v & 8191) > 88 else 14
            while True:
                out += struct.pack('B', b & 255)
                b >>= 8
                n -= 8
                if not n > 7:
                    break
            v = -1
    if v + 1:
        out += struct.pack('B', (b | v << n) & 255)
    return out


def encode(bindata):
    ''' Encode a bytearray to a Base91 string '''
    b = 0
    n = 0
    out = ''
    for count in range(len(bindata)):
        byte = bindata[count:count + 1]
        b |= struct.unpack('B', byte)[0] << n
        n += 8
        if n > 13:
            v = b & 8191
            if v > 88:
                b >>= 13
                n -= 13
            else:
                v = b & 16383
                b >>= 14
                n -= 14
            out += base91_alphabet[v % 91] + base91_alphabet[v // 91]
    if n:
        out += base91_alphabet[b % 91]
        if n > 7 or b > 90:
            out += base91_alphabet[b // 91]
    return out