gh-91349: Expose the crc32 function from the lzma library

[morotti]

Hello,

This is a simple PR to expose the crc32 function from the lzma library.
The code and tests are very similar to the crc in binascii and zlib. I don't think it's too controversial.

I came across this while looking at compression libraries for #91349
crc32 is always available from the lzma library (xz-utils/liblzma.so). I think it should be exposed.
there are more hashing functions available (namely crc64) but they need compilation/runtime checks to verify whether they are available.

Regards.

• Issue: Use zlib-ng (fast!) rather than mainline stale zlib in binary releases #91349

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📚 Documentation preview 📚: https://cpython-previews--131721.org.readthedocs.build/

[morotti]

sorry for delay,
@merwok@AA-Turner I started over from the main branch and made all the adjustments.

all the docstring/comments were copy pasted from the zlib function. I guess the old function had an old style that is no longer wanted.

for the return type in the docstring, we can settle with "positive integer"

https://docs.python.org/3/library/zlib.html#zlib.crc32
the docstring on zlib.crc32 says "The result is an unsigned 32-bit integer." but there is no concept of unsigned or 32 bits for python int.

[AA-Turner]

Thanks!

[morotti]

@AA-Turner , I merged all your suggestions.

for the versionadded: next, I thought changes on main would ship in 3.14? I am too late for 3.14 release?

[AA-Turner]

'next' is updated by the Release Manager for each version, it's a convenience tool that we use internally. We can still get this in for 3.14, yes.

A

[morotti]

regarding performance, don't mind that for now, I will come back later.

in principle

• zlib is abandoned and the slowest.
• liblzma is 2 or 3 times faster.
• zlib-ng is two orders of magnitude faster. it's using hardware instructions.

liblzma had code to start using hardware crc instructions soon ... and I see they just made a new stable release few days ago https://github.com/tukaani-project/xz/releases
I will check what's better and come back with more PR to use the best available crc function.

[morotti]

it appears the Ubuntu (free-threading) / build and test (ubuntu-24.04-arm is flaky

[AA-Turner]

I'll re-run it.

[morotti]

build passed, ready to merge

[AA-Turner]

Do you want to include your man.com email in the commit? Currently GH lists it as a co-authored-by, but the commit itself will be under your GMail address.

[morotti]

Please don't mind the email.
My work and my personal emails can both show up depending how the commits and PRs are opened. It doesn't matter which one shows up. My organization signed the CLA for the cpython interpreter, I have permissions to contribute.

[morotti]

ping, can this be merged?

[morotti]

@merwok@AA-Turner are you able to review?

I've rebased on main and redid the chance, since it's been 3 months and main had changed.

[morotti]

@gpshead you've replied multiple times to my questions on improving compression code with "PR welcome". maybe you would be interested in reviewing this PR?