Issue 29004: binascii.crc_hqx() implements CRC-CCITT (original) (raw)

Created on 2016-12-18 10:52 by martin.panter, last changed 2022-04-11 14:58 by admin. This issue is now closed.

Messages (6)
msg283548 - (view)	Author: Martin Panter (martin.panter) *	Date: 2016-12-18 10:52
If I had known this it would have saved me getting a separate implementation working. >>> hex(binascii.crc_hqx(b"\x01", 0)) '0x1021' https://files.stairways.com/other/binhex-40-specs-info.txt Documenting this might helped many other people. Top Google hits seem oblivious to crc_hqx(), using other CRC implementations, and even other helper functions from the binascii module: https://pypi.python.org/pypi/crc16 http://stackoverflow.com/questions/26204060/calculate-crc-ccitt-0xffff-for-hex-string
msg283550 - (view)	Author: Serhiy Storchaka (serhiy.storchaka) *	Date: 2016-12-18 11:13
If document the polynomial for crc_hqx, maybe document it for crc32 and adler32? Wouldn't be better to write the formula as *x*\ :sup:`16` + *x*\ :sup:`12 + *x*\ :sup:`5` + 1 ?
msg283552 - (view)	Author: Martin Panter (martin.panter) *	Date: 2016-12-18 11:47
It seems I can write it without the escaped spaces. Is there a problem with this: *x*:sup:`16` + *x*:sup:`12` + *x*:sup:`5` + 1 I’m happy to add the CRC-32 polynomial if you think it would be useful, although it is a lot longer (fifteen terms instead of four). And this CRC is already easily identified by the CRC-32 name. As well as the polynomial, there are other details that identify a CRC. The bits in CRC-32 are reversed and inverted compared to CRC-CCITT. >>> hex(crc32(b"\x80", 0xFFFFFFFF) ^ 0xFFFFFFFF) '0xedb88320' # 0xEDB88320 is the reversed polynomial representation; the x^0 term corresponds to bit 31 Adler32 is not a CRC, and I don’t think there are multiple versions of the algorithm, so I don’t think it would need any special explanation.
msg283553 - (view)	Author: Serhiy Storchaka (serhiy.storchaka) *	Date: 2016-12-18 11:53
Okay, then the patch LGTM. Use the form of the polynomial that you prefer.
msg283925 - (view)	Author: Roundup Robot (python-dev)	Date: 2016-12-24 08:18
New changeset a33472f8a2c6 by Martin Panter in branch '3.5': Issue #29004: Document binascii.crc_hqx() implements CRC-CCITT https://hg.python.org/cpython/rev/a33472f8a2c6 New changeset 52db2072e88b by Martin Panter in branch '3.6': Issue #29004: Merge crc_hqx() doc from 3.5 https://hg.python.org/cpython/rev/52db2072e88b New changeset 3af3702b2f0a by Martin Panter in branch 'default': Issue #29004: Merge crc_hqx() doc from 3.6 https://hg.python.org/cpython/rev/3af3702b2f0a New changeset 5ae6102270df by Martin Panter in branch '2.7': Issue #29004: Document binascii.crc_hqx() implements CRC-CCITT https://hg.python.org/cpython/rev/5ae6102270df
msg283933 - (view)	Author: Martin Panter (martin.panter) *	Date: 2016-12-24 09:51
Thanks for the help Serhiy