What is the most hard to understand piece of C++ code you know? [closed]

Answer 1

unsigned int reverse(register unsigned int x)
{
 x = (((x & 0xaaaaaaaa) >> 1) | ((x & 0x55555555) << 1));
 x = (((x & 0xcccccccc) >> 2) | ((x & 0x33333333) << 2));
 x = (((x & 0xf0f0f0f0) >> 4) | ((x & 0x0f0f0f0f) << 4));
 x = (((x & 0xff00ff00) >> 8) | ((x & 0x00ff00ff) << 8));
 return((x >> 16) | (x << 16));
}

Reverses the order of the bits in an int.