exprF

    int count_bits(uint32_t x)
    {
    	static const uint32_t mask[] = {
    		0x55555555,
    		0x33333333,
    		0x0f0f0f0f,
    		0x00ff00ff,
    		0x0000ffff
    	};
    	
    	for (int i = 0, shift = 1; i < 5; ++i, shift *= 2)
    		x = (x & mask[i]) + ((x >> shift) & mask[i]);
    	return x;
    }