From: "tenderlovemaking (Aaron Patterson) via ruby-core" <ruby-core@...>
Date: 2025-08-29T17:43:59+00:00
Subject: [ruby-core:123123] [Ruby Feature#20163] Introduce #bit_count method on Integer

Issue #20163 has been updated by tenderlovemaking (Aaron Patterson).


mame (Yusuke Endoh) wrote in #note-10:
> What are the intended use cases for this proposal?
> 
> My experience (in other languages) involves two use cases of popcount:
> 
> * Bitboards for game AI (like Reversi) to count pieces.
> * Succinct data structures (like LOUDS Tries) for rank operations.
> 
> In both scenarios, integers are treated as unsigned bitsets.

My experience is similar.  I've used it for sets (like I linked above) as well as modeling [undirected graphs](https://tenderlovemaking.com/2023/03/19/bitmap-matrix-and-undirected-graphs-in-ruby/) (a bit matrix, but I omitted popcount from the blog post).  I've only used unsigned integers, and I think it would be a bug in my code if the integers were signed.

> Does anyone have a use case where popcount on a negative number is necessary?
> If not, I guess raising an exception would be the best behavior.

I agree, and I think it should raise an exception.

ahorek (Pavel Rosick��) wrote in #note-12:
> That said, Ruby isn���t the ideal choice for implementing memory allocators, SIMD masks, parity checks, GCD calculations, UTF parsers, or prime sieving���

Not yet!  But hopefully someday! ����



----------------------------------------
Feature #20163: Introduce #bit_count method on Integer
https://bugs.ruby-lang.org/issues/20163#change-114450

* Author: garrison (Garrison Jensen)
* Status: Open
----------------------------------------
This feature request is to implement a method called #bit_count on Integer that returns the number of ones in the binary representation of the absolute value of the integer.
```
n = 19
n.bit_count #=> 3
(-n).bit_count #=> 3
```
This is often useful when you use an integer as a bitmask and want to count how many bits are set. 

This would be equivalent to
```
n.to_s(2).count("1")
```

However, this can be outperformed by
```
def bit_count(n)
  count = 0
  while n > 0
    n &= n - 1 # Flip the least significant 1 bit to 0
    count += 1
  end
  count
end
```

I think this would be a useful addition because it would fit alongside the other bit-related methods defined on integer: `#bit_length,` `#allbits?`, `#anybits?`, `#nobits?`. Also, when working with bitmasks, a minor upgrade to performance often results in a significant improvement. 

Similar methods from other languages:
https://docs.python.org/3/library/stdtypes.html#int.bit_count
https://doc.rust-lang.org/std/primitive.i32.html#method.count_ones



-- 
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