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Comment by omegalulw | Hacker News Reader

For those wondering how this works, n & n-1 zeros out the least significant 1. Simply by doing this repeatedly you count the number of 1s.

Right. And the reason that zeroes out the least significant bit is because subtracting one just borrows up to it. E.g., for n = 464:

    n       = 464d 111010000b
    n-1     = 463d 111001111b
    n&(n-1) = 448d 111000000b