source: trunk/minix/lib/ip/oneC_sum.c@ 21

/*      oneC_sum() - One complement's checksum          Author: Kees J. Bot
 *                                                              8 May 1995
 * See RFC 1071, "Computing the Internet checksum"
 */

#include <sys/types.h>
#include <net/gen/oneCsum.h>

u16_t oneC_sum(U16_t prev, void *data, size_t size)
{
        u8_t *dptr;
        size_t n;
        u16_t word;
        u32_t sum;
        int swap= 0;

        sum= prev;
        dptr= data;
        n= size;

        swap= ((size_t) dptr & 1);
        if (swap) {
                sum= ((sum & 0xFF) << 8) | ((sum & 0xFF00) >> 8);
                if (n > 0) {
                        ((u8_t *) &word)[0]= 0;
                        ((u8_t *) &word)[1]= dptr[0];
                        sum+= (u32_t) word;
                        dptr+= 1;
                        n-= 1;
                }
        }

        while (n >= 8) {
                sum+= (u32_t) ((u16_t *) dptr)[0]
                    + (u32_t) ((u16_t *) dptr)[1]
                    + (u32_t) ((u16_t *) dptr)[2]
                    + (u32_t) ((u16_t *) dptr)[3];
                dptr+= 8;
                n-= 8;
        }

        while (n >= 2) {
                sum+= (u32_t) ((u16_t *) dptr)[0];
                dptr+= 2;
                n-= 2;
        }

        if (n > 0) {
                ((u8_t *) &word)[0]= dptr[0];
                ((u8_t *) &word)[1]= 0;
                sum+= (u32_t) word;
        }

        sum= (sum & 0xFFFF) + (sum >> 16);
        if (sum > 0xFFFF) sum++;

        if (swap) {
                sum= ((sum & 0xFF) << 8) | ((sum & 0xFF00) >> 8);
        }
        return sum;
}