Gonzalo wrote: """ I missed the point to add the *and* to workaround the long result issue! I think I understand it now.
I am timing the code once translated, so here are the results for the crc calculation function. """ Yes, and both of us were missing the point that the "16" means 16 bits wide!! It is carrying an extra 16 redundant bits that don't contribute to the checking ability. If your software *needs* to generate/check the exact same 32-bit-wide CRC as calculated by that C routine, then what I gave you should be OK. Do you have any more test values? In particular test an empty string and say "\x00" * 8 -- sometimes a CRC specification will do something special with short strings e.g. pad them put with null bytes. That C routine is actually, on further reading, mind-boggling!! It stops on a null byte, rather than working on a given length. A CRC is normally expected to work on any data (including binary data) whether it contains \x00 or not. What are you using it for?? If you don't have to interface with other software, AND a 16-bit check is adequate, then at the end of this message is a 16-bit version which will use only ints and therefore may run faster. However you may wish to use a CRC32 which is supplied with Python (in the binascii module). A further note: what we have so far seems not to be the CCITT aka X.25 standard CRC-16 , but a reflection. See http://www.joegeluso.com/software/articles/ccitt.htm and also if you have the time and the determination, the article by Ross Williams that it refers to. Cheers, John === 16-bit version === gCRC16Table = [] def InitCRC16(): global gCRC16Table for i in xrange(0,256): crc = i << 8 for j in xrange(0,8): if (crc & 0x8000) != 0: tmp = 0x1021 else: tmp = 0 crc = (crc << 1) ^ tmp crc &= 0xFFFF gCRC16Table.append(crc) def CalcCRC16(astr): crc = 0xFFFF for c in astr: crc = gCRC16Table[((crc >> 8) & 255)] ^ ((crc & 0xFF) << 8) ^ ord(c) assert 0 <= crc <= 0xffff # remove when doing timings :-) return crc test = "123456asdfg12345123" InitCRC16() result = CalcCRC16(test) print result, hex(result) ====== -- http://mail.python.org/mailman/listinfo/python-list
