release/src/router/busybox/libbb/inet_cksum.c

   1 /*
   2  * Checksum routine for Internet Protocol family headers (C Version)
   3  *
   4  * Licensed under GPLv2, see file LICENSE in this source tree.
   5  */
   6
   7 #include "libbb.h"
   8
   9 uint16_t FAST_FUNC inet_cksum(uint16_t *addr, int nleft)
  10 {
  11         /*
  12          * Our algorithm is simple, using a 32 bit accumulator,
  13          * we add sequential 16 bit words to it, and at the end, fold
  14          * back all the carry bits from the top 16 bits into the lower
  15          * 16 bits.
  16          */
  17         unsigned sum = 0;
  18         while (nleft > 1) {
  19                 sum += *addr++;
  20                 nleft -= 2;
  21         }
  22
  23         /* Mop up an odd byte, if necessary */
  24         if (nleft == 1) {
  25                 if (BB_LITTLE_ENDIAN)
  26                         sum += *(uint8_t*)addr;
  27                 else
  28                         sum += *(uint8_t*)addr << 8;
  29         }
  30
  31         /* Add back carry outs from top 16 bits to low 16 bits */
  32         sum = (sum >> 16) + (sum & 0xffff);     /* add hi 16 to low 16 */
  33         sum += (sum >> 16);                     /* add carry */
  34
  35         return (uint16_t)~sum;
  36 }