/*-
 * Copyright (c) 1990 The Regents of the University of California.
 * All rights reserved.
 *
 * %sccs.include.redist.c%
 *
 * from tahoe:	in_cksum.c	1.2	86/01/05
 *	@(#)in_cksum.c	1.3 (Berkeley) 01/19/91
 */

#include "param.h"
#include "sys/mbuf.h"
#include "netinet/in.h"
#include "netinet/in_systm.h"

/*
 * Checksum routine for Internet Protocol family headers.
 *
 * This routine is very heavily used in the network
 * code and should be modified for each CPU to be as fast as possible.
 * 
 * This implementation is 386 version.
 */

#undef	ADDCARRY
#define ADDCARRY(sum)  {				\
			if (sum & 0xffff0000) {		\
				sum &= 0xffff;		\
				sum++;			\
			}				\
		}
in_cksum(m, len)
	register struct mbuf *m;
	register int len;
{
	union word {
		char	c[2];
		u_short	s;
	} u;
	register u_short *w;
	register int sum = 0;
	register int mlen = 0;

	for (;m && len; m = m->m_next) {
		if (m->m_len == 0)
			continue;
		w = mtod(m, u_short *);
		if (mlen == -1) {
			/*
			 * The first byte of this mbuf is the continuation
			 * of a word spanning between this mbuf and the
			 * last mbuf.
			 */

			/* u.c[0] is already saved when scanning previous 
			 * mbuf.
			 */
			u.c[1] = *(u_char *)w;
			sum += u.s;
			ADDCARRY(sum);
			w = (u_short *)((char *)w + 1);
			mlen = m->m_len - 1;
			len--;
		} else
			mlen = m->m_len;

		if (len < mlen)
			mlen = len;
		len -= mlen;

		/*
		 * add by words.
		 */
		while ((mlen -= 2) >= 0) {
			if ((int)w & 0x1) {
				/* word is not aligned */
				u.c[0] = *(char *)w;
				u.c[1] = *((char *)w+1);
				sum += u.s;
				w++;
			} else
				sum += *w++;
			ADDCARRY(sum);
		}
		if (mlen == -1)
			/*
			 * This mbuf has odd number of bytes. 
			 * There could be a word split betwen
			 * this mbuf and the next mbuf.
			 * Save the last byte (to prepend to next mbuf).
			 */
			u.c[0] = *(u_char *)w;
	}
	if (len)
		printf("cksum: out of data\n");
	if (mlen == -1) {
		/* The last mbuf has odd # of bytes. Follow the
		   standard (the odd byte is shifted left by 8 bits) */
		u.c[1] = 0;
		sum += u.s;
		ADDCARRY(sum);
	}
	return (~sum & 0xffff);
}