Function DifferentialChecksum

freebsd/netinet/libalias/alias_util.c:153–177

#endif	/* not _KERNEL */

void
DifferentialChecksum(u_short * cksum, void *newp, void *oldp, int n)
{
	int i;
	int accumulate;
	u_short *new = newp;
	u_short *old = oldp;

	accumulate = *cksum;
	for (i = 0; i < n; i++) {
		accumulate -= *new++;
		accumulate += *old++;
	}

	if (accumulate < 0) {
		accumulate = -accumulate;
		accumulate = (accumulate >> 16) + (accumulate & 0xffff);
		accumulate += accumulate >> 16;
		*cksum = (u_short) ~ accumulate;
	} else {
		accumulate = (accumulate >> 16) + (accumulate & 0xffff);
		accumulate += accumulate >> 16;
		*cksum = (u_short) accumulate;
	}
}