git.zerfleddert.de Git - proxmark3-svn/blob - common/crc16.c

   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 }