There is a divide and conquer strategy, that will come in advanced bithacks article.

I agree 1-5 are basic. That's why I say at #6 "Now it finally gets more interesting!!! Bit hacks #1 - #5 were kind of boring to be honest." I included them for completeness.

Here is method for counting one bits:

    int one_bits(unsigned int x) {
       x = x - ((x >> 1) & 0x55555555); 
       x = (x & 0x33333333) + ((x >> 2) & 0x33333333); 
       x = (x + (x >> 4)) & 0x0F0F0F0F; 
       x = x + (x >> 8); 
       x = x + (x >> 16); 
       return x & 0x0000003F; 
    }