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bpo-37295: More direct computation of power-of-two factor in math.comb #30313

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merged 5 commits into from
Dec 31, 2021
Merged

bpo-37295: More direct computation of power-of-two factor in math.comb #30313

merged 5 commits into from
Dec 31, 2021

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mdickinson
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@mdickinson mdickinson commented Dec 31, 2021 •
edited by bedevere-bot
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This PR replaces the current popcount-based code for computing the largest power of two dividing math.comb(n, k) (for small n) with a more direct method based on counting trailing zeros of the factorials involved. See the issue for further discussion.

https://bugs.python.org/issue37295

serhiy-storchaka
serhiy-storchaka reviewed Dec 31, 2021
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Can it be used in count_set_bits()?

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Can it be used in count_set_bits()?

I'm not sure how. Can you elaborate?

We could use a lookup table for small values in the popcount implementation, but I'm not sure it's worth changing.

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We could use the same lookup table in the popcount implementation for factorial() and comb()/perm(). But it may be a separate issue.

For #30305 I will need to extend the table to 127.

serhiy-storchaka
serhiy-storchaka reviewed Dec 31, 2021
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serhiy-storchaka approved these changes Dec 31, 2021
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LGTM besides few comments.

mdickinson and others added 3 commits December 31, 2021 19:23
@mdickinson mdickinson merged commit 0b58bac into python:main Dec 31, 2021
@mdickinson mdickinson deleted the math-comb-zero-counts branch December 31, 2021 19:52
@rhettinger rhettinger mentioned this pull request Oct 6, 2023
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@mdickinson @serhiy-storchaka @the-knights-who-say-ni @bedevere-bot