Tuvas wrote:
> Anyone know a module that does CRC16 for Python? I have an aplication
> that I need to run it, and am not having alot of sucess. I have a
> program in C that uses a CRC16 according to CCITT standards, but need
> to get a program that tests it with python as well. Thanks!
I'll include one below. There's a tricky bit in that most standards
assume a serial bit stream, not byte data, leading to the relection
issue discussed in the reference in the doc. Let me know if gets
the same answers as your C version.
--
--Bryan
===============================================================
#!/usr/bin/env python
# crc16.py by Bryan G. Olson, 2005
# This module is free software and may be used and
# distributed under the same terms as Python itself.
"""
CRC-16 in Python, as standard as possible. This is
the 'reflected' version, which is usually what people
want. See Ross N. Williams' /A Painless Guide to
CRC error detection algorithms/.
"""
from array import array
def crc16(string, value=0):
""" Single-function interface, like gzip module's crc32
"""
for ch in string:
value = table[ord(ch) ^ (value & 0xff)] ^ (value >> 8)
return value
class CRC16(object):
""" Class interface, like the Python library's cryptographic
hash functions (which CRC's are definitely not.)
"""
def __init__(self, string=''):
self.val = 0
if string:
self.update(string)
def update(self, string):
self.val = crc16(string, self.val)
def checksum(self):
return chr(self.val >> 8) + chr(self.val & 0xff)
def hexchecksum(self):
return '%04x' % self.val
def copy(self):
clone = CRC16()
clone.val = self.val
return clone
# CRC-16 poly: p(x) = x**16 + x**15 + x**2 + 1
# top bit implicit, reflected
poly = 0xa001
table = array('H')
for byte in range(256):
crc = 0
for bit in range(8):
if (byte ^ crc) & 1:
crc = (crc >> 1) ^ poly
else:
crc >>= 1
byte >>= 1
table.append(crc)
crc = CRC16()
crc.update("123456789")
assert crc.checksum() == '\xbb\x3d'
--
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