gistfile1.txt

/*
 * If you think you are smart than the compiler , then you can optimize this code.
 * otherwise, just leave it untouched.
 *
 * the base idea this code is that, simplify the md5 digest Algorithm to calculate only 4byte.
 * thus, only 4 basic md5 round is needed.
 * after 4 basic md5 round, shift the result and xor itself.
 * the the lower 16bits is the peerhash.
 *
 * have fun reading this crap. ^_^
 */
boost::uint16_t peerid_hash(boost::uint32_t peerid)
{
	unsigned int a = 0x67452301;

	{
		a += ((((0xefcdab89)) & ((0x98badcfe))) | ((~(0xefcdab89)) & ((0x10325476)))) + (peerid)
			+ (unsigned int) (3614090360);
		a = (((a) << ((7))) | ((a) >> (32 - ((7)))));
		a += (0xefcdab89);
	}
	{
		a += ((((0xefcdab89)) & ((0x10325476))) | (((0x98badcfe)) & (~(0x10325476)))) + (peerid)
			+ (unsigned int) (4129170786);
		a = (((a) << ((5))) | ((a) >> (32 - ((5)))));
		a += (0xefcdab89);
	}
	{
		a += (((0xefcdab89)) ^ ((0x98badcfe)) ^ ((0x10325476))) + (peerid) + (unsigned int) (4294588738);
		a = (((a) << ((4))) | ((a) >> (32 - ((4)))));
		a += (0xefcdab89);
	}
	{
		a += (((0x98badcfe)) ^ (((0xefcdab89)) | (~(0x10325476)))) + (peerid) + (unsigned int) (4096336452);
		a = (((a) << ((6))) | ((a) >> (32 - ((6)))));
		a += (0xefcdab89);
	}

	return (a ^ (a >> 16)) & 0xFFFF;
}