def a64l(s):
"""
An implementation of a64l as from the c stdlib.
Convert between a radix-64 ASCII string and a 32-bit integer.
'.' (dot) for 0, '/' for 1, '0' through '9' for [2,11],
'A' through 'Z' for [12,37], and 'a' through 'z' for [38,63].
TODO:
do some implementations use '' instead of '.' for 0?
>>> a64l('.')
0
>>> a64l('ZZZZZZ')
40359057765L
#only the first 6 chars are significant
>>> a64l('ZZZZZZ.')
40359057765L
>>> a64l('A')
12
>>> a64l('Chris')
951810894
"""
MASK = 0xffffffff
BITSPERCHAR = 6
orda, ordZ, ordA, ord9, ord0 = ord('a'), ord('Z'), ord('A'), ord('9'), ord('0')
r = 0
for shift, c in enumerate(s[:6]):
c = ord(c)
if c > ordZ:
c -= orda - ordZ - 1
if c > ord9:
c -= ordA - ord9 - 1
r = (r | ((c - (ord0 - 2)) << (shift * BITSPERCHAR))) & MASK
return r
