[ruby-core:123329] [Ruby Feature#20163] Introduce #bit_count method on Integer

Issue #20163 has been updated by YO4 (Yoshinao Muramatsu).


We can use Integer#[] to negative numbers.
```
-1[0,8].popcount # => 8
-2[0,8].popcount # => 7
-3[0,8].popcount # => 7
```
In computers that use two's complement for negative numbers, I think this is expected behavior.

If generating sliced values becomes an overhead, we can add arguments to popcount.

----------------------------------------
Feature #20163: Introduce #bit_count method on Integer
https://bugs.ruby-lang.org/issues/20163#change-114697

* 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



-- 
https://bugs.ruby-lang.org/
______________________________________________
 ruby-core mailing list -- [e-mail]
 To unsubscribe send an email to [e-mail]
 ruby-core info -- https://ml.ruby-lang.org/mailman3/lists/ruby-core.ml.ruby-lang.org/