From: ahorek via ruby-core <ruby-core@...>
Date: 2025-08-29T13:12:35+00:00
Subject: [ruby-core:123120] [Ruby Feature#20163] Introduce #bit_count method on Integer

Issue #20163 has been updated by ahorek (Pavel Rosick��).


x64 and ARM have specialized CPU instructions
https://godbolt.org/z/xvGvzsvd9

and Ruby already uses it internally, for instance
https://github.com/ruby/ruby/blob/dc555a48e750b4d50eb7a7000ca1bfb927fa9459/string.c#L2209

That said, Ruby isn���t the ideal choice for implementing memory allocators, SIMD masks, parity checks, GCD calculations, UTF parsers, or prime sieving��� but since many other languages, even Python, provide support for popcount, why not?

----------------------------------------
Feature #20163: Introduce #bit_count method on Integer
https://bugs.ruby-lang.org/issues/20163#change-114448

* 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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