GeezerVison is showing github.com — Add an efficient popcount method for integers · Issue #74068 · python/cpython
Add an efficient popcount method for integers #74068Description
| BPO |
29882 |
| Nosy |
@tim-one, @rhettinger, @mdickinson, @vstinner, @njsmith, @markshannon, @serhiy-storchaka, @vedgar, @DimitrisJim, @niklasf, @gbtami |
| PRs |
bpo-29882: Add an efficient popcount method for integers #771bpo-29882: Add _Py_popcount32() function #20518bpo-29882: _Py_popcount32() doesn't need 64x64 multiply #30774bpo-29882: Fix portability bug introduced in GH-30774 #30794 |
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GitHub fields:
assignee = None
closed_at = <Date 2020-05-29.16:28:39.179>
created_at = <Date 2017-03-22.17:30:42.549>
labels = ['interpreter-core', 'type-feature', '3.10']
title = 'Add an efficient popcount method for integers'
updated_at = <Date 2022-01-23.09:59:43.073>
user = 'https://github.com/niklasf' bugs.python.org fields:
activity = <Date 2022-01-23.09:59:43.073>
actor = 'mark.dickinson'
assignee = 'none'
closed = True
closed_date = <Date 2020-05-29.16:28:39.179>
closer = 'mark.dickinson'
components = ['Interpreter Core']
creation = <Date 2017-03-22.17:30:42.549>
creator = 'niklasf'
dependencies = []
files = []
hgrepos = []
issue_num = 29882
keywords = []
message_count = 26.0
messages = ['290003', '290013', '290014', '290015', '290016', '290017', '290073', '344215', '344216', '344223', '344224', '344225', '369860', '369878', '369879', '369881', '369887', '369966', '370323', '370423', '370447', '370456', '370987', '372497', '411211', '411357']
nosy_count = 12.0
nosy_names = ['tim.peters', 'rhettinger', 'mark.dickinson', 'vstinner', 'casevh', 'njs', 'Mark.Shannon', 'serhiy.storchaka', 'veky', 'Jim Fasarakis-Hilliard', 'niklasf', 'gbtami']
pr_nums = ['771', '20518', '30774', '30794']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue29882'
versions = ['Python 3.10'] Activityniklasf commented on Mar 22, 2017 MannequinAuthor More actions An efficient popcount (something equivalent to bin(a).count("1")) would
be useful for numerics, parsing binary formats, scientific applications
and others.
DESIGN DECISIONS
- Is a popcount method useful enough?
- How to handle negative values?
- How should the method be named?
SURVEY
gmpy calls the operation popcount and returns -1/None for negative values:
>>> import gmpy2
>>> gmpy2.popcount(-10)
-1
>> import gmpy
>> gmpy.popcount(-10)
From the documentation [1]:
If x < 0, the number of bits with value 1 is infinite
so -1 is returned in that case.
(I am not a fan of the arbitrary return value).
The bitarray module has a count(value=True) method:
>>> from bitarray import bitarray
>>> bitarray(bin(123456789).strip("0b")).count()
16Bitsets [2] exposes __len__.
There is an SSE4 POPCNT instruction. C compilers call the corresponding
intrinsic popcnt or popcount (with some prefix and suffix) and they
accept unsigned arguments.
Rust calls the operation count_ones [3]. Ones are counted in the binary
representation of the *absolute* value. (I propose to do the same).
Introducing popcount was previously considered here but closed for lack
of a PEP or patch: http://bugs.python.org/issue722647
Sensible names could be bit_count along the lines of the existing
bit_length or popcount for gmpy compability and to distinguish it more.
PERFORMANCE
$ ./python -m timeit "bin(123456789).count('1')" # equivalent
1000000 loops, best of 5: 286 nsec per loop
$ ./python -m timeit "(123456789).bit_count()" # fallback
5000000 loops, best of 5: 46.3 nsec per loop[1] https://gmpy2.readthedocs.io/en/latest/mpz.html#mpz-functions
[2] https://pypi.python.org/pypi/bitsets/0.7.9
[3] https://doc.rust-lang.org/std/primitive.i64.html#method.count_ones mdickinson commented on Mar 22, 2017 Can you give some examples of concrete use-cases? I've spent the last six years or so writing scientific applications and parsing all sorts of odd binary formats, and haven't needed or wanted a popcount yet.
(I am not a fan of the arbitrary return value).
Agreed: if this were implemented, I think raising ValueError would be the most appropriate thing to do for negative inputs. niklasf commented on Mar 22, 2017 MannequinAuthor More actions mdickinson commented on Mar 22, 2017 Many of those applications are really for bitstrings (chess bitboards, hamming distance), which aren't really the same thing as integers.
Nice find for the mathmodule.c case. I'd forgotten about that one (though according to git blame, apparently I'm responsible for checking it in). It's a fairly obscure corner case, though.
Overall, I'm -1 on adding this: I don't think it meets the bar of being useful enough to justify the extra method. I'd suggest that people needing this kind of efficient bitstring operation use a 3rd-party bitstring library instead. serhiy-storchaka commented on Mar 22, 2017 I think that adding bitarray or bitset (or both) in the stdlib would better satisfy the needs. There are open issues for adding ability to read or set selected bits or range of bits in int or for bitwise operations on bytes. I think that bitarray and bitset would provide better interface for these operations. tim-one commented on Mar 22, 2017 See also:
https://en.wikipedia.org/wiki/Hamming_weight
As that says, there are a number of languages and processors with first class support for a popcount function. I've frequently implemented it in Python when using integers as integer bitsets (i is in the set if and only if bit 2**i is set in the integer), which often - except for finding the cardinality - runs much faster than using general Python sets. casevh commented on Mar 24, 2017 I like the name bit_count and I'll gladly add it to gmpy2 with the appropriate changes to exceptions, etc. rhettinger commented on Jun 1, 2019 Is everyone comfortable with how negative numbers are handled by this patch? It might be better to limit the domain and raise a ValueError rather than make a presumption about what the user intends. serhiy-storchaka commented on Jun 1, 2019 I am going to add the imath module. If we decide to add popcount(), it may be better to add it in this module instead of int class. mdickinson commented on Jun 1, 2019
Is everyone comfortable with how negative numbers are handled by this patch?
Not entirely, but it's not terribly wrong and it's consistent with how int.bit_length works. (I'm also a bit uncomfortable about int.bit_lengths behaviour on negative numbers, but it is the way it is.) tim-one commented on Jun 1, 2019 I prefer that a negative int raise ValueError, but am OK with it using the absolute value instead (i.e., what it does now). mdickinson commented on Jun 1, 2019
I am going to add the imath module.
Aimed at 3.8, or 3.9? This seems somewhat rushed for 3.8. mdickinson commented on May 25, 2020
For example, should numpy.int64 get this method as well?
That's for the NumPy folks to decide (and I've added Nathaniel Smith to the nosy in case he wants to comment), but I don't see any particularly strong reason that NumPy would need to add it. It looks as though the NumPy integer types have survived happily without a bit_length method, for example - I don't even see any issues in the NumPy tracker suggesting that anyone missed it. (Though perhaps that's because in the case of a NumPy int one always has at least an upper bound on the bit_length available.)
What is the effect on https://docs.python.org/3.9/library/numbers.html?
No effect, just as int.bit_length has no effect.
Does it make sense to call (True).popcount()?
It would be spelled True.bit_count() if the PR goes in as-is, but sure, why not. :-) mdickinson commented on May 26, 2020 PR is updated and mergeable. mdickinson commented on May 29, 2020 markshannon commented on May 31, 2020 Why are calling a population count method "bit_count()"?
That seems likely to cause confusion with "bit_length()".
I might reasonable expect that 0b1000.bit_count() be 4, not 1. It does have 4 bits.
Whereas 0b1000.population_count() is clearly 1.
I have no objection to adding this method, just the choice of name. mdickinson commented on May 31, 2020
Why are calling a population count method "bit_count()"?
Naming things is hard, but I don't think this is an unreasonable name, and it's not without precedent. Java similarly has Integer.bitCount and BigInteger.bitCount. MySQL has BIT_COUNT. mdickinson commented on May 31, 2020 A couple of other data points:
@mark Shannon: what name would you suggest, and why? The term "population count" feels too non-obvious and specialist to me, and anything involving "Hamming" likewise.
"count_ones" isn't obviously a bit operation.
"count_set_bits"? vstinner commented on Jun 8, 2020 vedgar commented on Jun 28, 2020 Well, bit_sum is what it really is. But I agree it's a terrible name. :-) vstinner commented on Jan 21, 2022 mdickinson commented on Jan 23, 2022 donhuvy commented on Nov 13, 2022
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