Issue #20163 has been updated by akr (Akira Tanaka). I prefer an exception for popcount to negative values. I think an array of Fixnums (63 bit signed integers) can be used for mutable bit array. (Ruby's Integer is immutable. So mutable bit array needs a mutable data structure.) In this scenario, we would like to count bits in a negative fixnum: `(n & 0x7ffffffff).popcount`. This is not `n.abs.popcount`. Also, this behavior is not suitable for Integer#popcount because it ignores bits higher than 63th bit. So, I prefer an exception for negative values. ---------------------------------------- Feature #20163: Introduce #bit_count method on Integer https://bugs.ruby-lang.org/issues/20163#change-114487 * 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/