cvs.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 #define CRC16_MASK_CCITT 0x1021
  11
  12 unsigned short update_crc16( unsigned short crc, unsigned char c )
  13 {
  14   unsigned short i, v, tcrc = 0;
  15
  16   v = (crc ^ c) & 0xff;
  17   for (i = 0; i < 8; i++) {
  18       tcrc = ( (tcrc ^ v) & 1 ) ? ( tcrc >> 1 ) ^ 0x8408 : tcrc >> 1;
  19       v >>= 1;
  20   }
  21
  22   return ((crc >> 8) ^ tcrc)&0xffff;
  23 }
  24
  25 uint16_t crc16(uint8_t const *message, int length, uint16_t remainder, uint16_t polynomial) {
  26
  27         if (length == 0)
  28         return (~remainder);
  29
  30     for (int byte = 0; byte < length; ++byte) {
  31         remainder ^= (message[byte] << 8);
  32         for (uint8_t bit = 8; bit > 0; --bit) {
  33             if (remainder & 0x8000) {
  34                 remainder = (remainder << 1) ^ polynomial;
  35             } else {
  36                 remainder = (remainder << 1);
  37             }
  38         }
  39     }
  40     return remainder;
  41 }
  42
  43 uint16_t crc16_ccitt(uint8_t const *message, int length) {
  44     return crc16(message, length, 0xffff, CRC16_MASK_CCITT);
  45 }
  46
  47 uint16_t crc16_ccitt_kermit(uint8_t const *message, int length) {
  48     return bit_reverse_uint16(crc16(message, length, 0x0000, CRC16_MASK_CCITT));
  49 }
  50 uint16_t bit_reverse_uint16 (uint16_t value) {
  51         const uint16_t mask0 = 0x5555;
  52         const uint16_t mask1 = 0x3333;
  53         const uint16_t mask2 = 0x0F0F;
  54         const uint16_t mask3 = 0x00FF;
  55
  56         value = (((~mask0) & value) >> 1) | ((mask0 & value) << 1);
  57         value = (((~mask1) & value) >> 2) | ((mask1 & value) << 2);
  58         value = (((~mask2) & value) >> 4) | ((mask2 & value) << 4);
  59         value = (((~mask3) & value) >> 8) | ((mask3 & value) << 8);
  60
  61         return value;
  62 }