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#!/usr/bin/python
# sdnv_decode() takes a string argument s, which is assumed to be an
# SDNV. The function returns a pair of the non-negative integer n
# that is the numeric value encoded in the SDNV, and and integer l
# that is the distance parsed into the input string. If the slen
# argument is not given (or is not a non-zero number) then, s is
# parsed up to the first byte whose high-order bit is 0 -- the
# length of the SDNV portion of s does not have to be pre-computed
# by calling code. If the slen argument is given as a non-zero
# value, then slen bytes of s are parsed. The value for n of -1 is
# returned for any type of parsing error.
#
# NOTE: In python, integers can be of arbitrary size. In other
# languages, such as C, SDNV-parsing routines should take
# precautions to avoid overflow (e.g. by using the Gnu MP library,
# or similar).
#
def sdnv_decode(s, slen=0):
n = long(0)
for i in range(0, len(s)):
v = ord(s[i])
n = n<<7
n = n + (v & 0x7F)
if v>>7 == 0:
slen = i+1
break
elif i == len(s)-1 or (slen != 0 and i > slen):
n = -1 # reached end of input without seeing end of SDNV
return (n, slen)
# sdnv_encode() returns the SDNV-encoded string that represents n.
# An empty string is returned if n is not a non-negative integer
def sdnv_encode(n):
r = ""
# validate input
if n >= 0 and (type(n) in [type(int(1)), type(long(1))]):
flag = 0
done = False
while not done:
# encode lowest 7 bits from n
newbits = n & 0x7F
n = n>>7
r = chr(newbits + flag) + r
if flag == 0:
flag = 0x80
if n == 0:
done = True
return r
# test cases from LTP and BP internet-drafts, only print failures
if __name__ == '__main__':
tests = [(0xABC, chr(0x95) + chr(0x3C)),
(0x1234, chr(0xA4) + chr (0x34)),
(0x4234, chr(0x81) + chr(0x84) + chr(0x34)),
(0x7F, chr(0x7F))]
for tp in tests:
# test encoding function
if sdnv_encode(tp[0]) != tp[1]:
print "sdnv_encode fails on input %s" % hex(tp[0])
# test decoding function
if sdnv_decode(tp[1])[0] != tp[0]:
print "sdnv_decode fails on input %s, giving %s" % \
(hex(tp[0]), sdnv_decode(tp[1]))
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