Generating CRC32 in Python
Filed under Python on oct 15, 2009
Unfortunately, Python does not know about signedness and assumes all integers are signed. I wanted to get a CRC32 hexadecimal checksum for some files.
Oddly, the mechanisms for the CRC32 checksumming are in the zlib package. There is also a version in binascii, but this algorithm looks slower (might depend on Python version and CPU architecture). Why doesn’t this algorithm reside in the hashlib module?
This is how you generate a CRC32 checksum (to check .sfv files for example):
>>> import zlib
>>> hash = zlib.crc32('hi there!')
>>> hash = zlib.crc32('more data', hash)
>>> print '%08X' % (hash & 0xFFFFFFFF,)
31A783A9
Speed comparison:
>>> import timeit
>>> f = ''\
... hash = crc32('')
... for x in xrange(0, 10):
... hash = crc32('%04d' % (x,), hash)
... '''
>>> t = timeit.Timer(stmt=f, setup='from binascii import crc32')
>>> print "%.2f usec/pass" % (1000000 * t.timeit(number=100000)/100000)
19.12 usec/pass
>>> t = timeit.Timer(stmt=f, setup='from zlib import crc32')
>>> print "%.2f usec/pass" % (1000000 * t.timeit(number=100000)/100000)
17.23 usec/pass
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