lib/libminlib/oneC_sum.c

   1 /*      oneC_sum() - One complement's checksum          Author: Kees J. Bot
   2  *                                                              8 May 1995
   3  * See RFC 1071, "Computing the Internet checksum"
   4  */
   5
   6 #include <sys/types.h>
   7 #include <net/gen/oneCsum.h>
   8
   9 u16_t oneC_sum(u16_t prev, void *data, size_t size)
  10 {
  11         u8_t *dptr;
  12         size_t n;
  13         u16_t word;
  14         u32_t sum;
  15         int swap= 0;
  16
  17         sum= prev;
  18         dptr= data;
  19         n= size;
  20
  21         swap= ((size_t) dptr & 1);
  22         if (swap) {
  23                 sum= ((sum & 0xFF) << 8) | ((sum & 0xFF00) >> 8);
  24                 if (n > 0) {
  25                         ((u8_t *) &word)[0]= 0;
  26                         ((u8_t *) &word)[1]= dptr[0];
  27                         sum+= (u32_t) word;
  28                         dptr+= 1;
  29                         n-= 1;
  30                 }
  31         }
  32
  33         while (n >= 8) {
  34                 sum+= (u32_t) ((u16_t *) dptr)[0]
  35                     + (u32_t) ((u16_t *) dptr)[1]
  36                     + (u32_t) ((u16_t *) dptr)[2]
  37                     + (u32_t) ((u16_t *) dptr)[3];
  38                 dptr+= 8;
  39                 n-= 8;
  40         }
  41
  42         while (n >= 2) {
  43                 sum+= (u32_t) ((u16_t *) dptr)[0];
  44                 dptr+= 2;
  45                 n-= 2;
  46         }
  47
  48         if (n > 0) {
  49                 ((u8_t *) &word)[0]= dptr[0];
  50                 ((u8_t *) &word)[1]= 0;
  51                 sum+= (u32_t) word;
  52         }
  53
  54         sum= (sum & 0xFFFF) + (sum >> 16);
  55         if (sum > 0xFFFF) sum++;
  56
  57         if (swap) {
  58                 sum= ((sum & 0xFF) << 8) | ((sum & 0xFF00) >> 8);
  59         }
  60         return sum;
  61 }