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1 //-----------------------------------------------------------------------------
2 // This code is licensed to you under the terms of the GNU GPL, version 2 or,
3 // at your option, any later version. See the LICENSE.txt file for the text of
4 // the license.
5 //-----------------------------------------------------------------------------
6 // CRC16
7 //-----------------------------------------------------------------------------
8
9 #include "crc16.h"
10
11 unsigned short update_crc16( unsigned short crc, unsigned char c )
12 {
13 unsigned short i, v, tcrc = 0;
14
15 v = (crc ^ c) & 0xff;
16 for (i = 0; i < 8; i++) {
17 tcrc = ( (tcrc ^ v) & 1 ) ? ( tcrc >> 1 ) ^ 0x8408 : tcrc >> 1;
18 v >>= 1;
19 }
20
21 return ((crc >> 8) ^ tcrc)&0xffff;
22 }
23
24 uint16_t crc16(uint8_t const *message, int length, uint16_t remainder, uint16_t polynomial) {
25
26 if (length == 0) return (~remainder);
27
28 for (int byte = 0; byte < length; ++byte) {
29 remainder ^= (message[byte] << 8);
30 for (uint8_t bit = 8; bit > 0; --bit) {
31 if (remainder & 0x8000) {
32 remainder = (remainder << 1) ^ polynomial;
33 } else {
34 remainder = (remainder << 1);
35 }
36 }
37 }
38 return remainder;
39 }
40
41 uint16_t crc16_ccitt(uint8_t const *message, int length) {
42 return crc16(message, length, 0xffff, 0x1021);
43 }
44
45 uint16_t crc16_ccitt_kermit(uint8_t const *message, int length) {
46 return bit_reverse_uint16(crc16(message, length, 0x0000, 0x1021));
47 }
48
49 uint16_t bit_reverse_uint16 (uint16_t value) {
50 const uint16_t mask0 = 0x5555;
51 const uint16_t mask1 = 0x3333;
52 const uint16_t mask2 = 0x0F0F;
53 const uint16_t mask3 = 0x00FF;
54
55 value = (((~mask0) & value) >> 1) | ((mask0 & value) << 1);
56 value = (((~mask1) & value) >> 2) | ((mask1 & value) << 2);
57 value = (((~mask2) & value) >> 4) | ((mask2 & value) << 4);
58 value = (((~mask3) & value) >> 8) | ((mask3 & value) << 8);
59
60 return value;
61 }
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