binary - Efficiently count the number of bits in an integer in JavaScript

Question

Let's say I have an integer I and want to get the count of 1s in its binary form.

I am currently using the following code.

Number(i.toString(2).split("").sort().join("")).toString().length;

Is there a faster way to do this? I am thinking about using bitwise operators. Any thoughts?

NOTE: i is within the 32-bit limitation.

Answer 1

You can use a strategy from this collection of Bit Twiddling Hacks:

function bitCount (n) {
  n = n - ((n >> 1) & 0x55555555)
  n = (n & 0x33333333) + ((n >> 2) & 0x33333333)
  return ((n + (n >> 4) & 0xF0F0F0F) * 0x1010101) >> 24
}

console.log(bitCount(0xFF)) //=> 8