Issue #20163 has been updated by slewsys (Andrew Moore). garrison (Garrison Jensen) wrote in #note-6:
``` unsigned int pop_count(long value) { #ifdef __GNUC__ // Use GCC built-in return __builtin_popcountl(value); #else // Fallback method for compilers without the built-in unsigned int count = 0; while (value) { count += value & 1; value >>= 1; } return count; #endif }
// Wrapper function for Ruby VALUE rb_pop_count(VALUE self) { long value = NUM2LONG(self); unsigned int count = pop_count(labs(value)); return UINT2NUM(count); } ```
Section 1.8 of *Matters Computational* by Jörg Arndt offers efficient algorithms for popcount, including for arrays. There is also an x86 instruction POPCNT. Matters Computational: https://www.jjj.de/fxt/fxtbook.pdf E.g., ``` unsigned long popcount (unsigned long value) { value -= (value >> 1) & 0x5555555555555555UL; value = ((value >> 2) & 0x3333333333333333UL) + (value & 0x3333333333333333UL); value = ((value >> 4) + value) & 0x0f0f0f0f0f0f0f0fUL; value *= 0x0101010101010101UL; return value >> 56; } ``` By the way, "popcount" is short for "population count", whereas "pop_count" suggests (to me) a stack operation - a bit (hm) confusing. ---------------------------------------- Feature #20163: Introduce #bit_count method on Integer https://bugs.ruby-lang.org/issues/20163#change-114663 * 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/