blob: 3d5dc3adf14c40f29b2ce02f51b1abf276847ca4
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 | } |
37 |