CRC.cpp

00001 #include "CRC.h"
00002 
00003 const unsigned short CRCPOLYNOM = 0x8005;
00004 
00005 // The last two bytes of data corresponds to the remainder. If they are of 0,0, this function will return the remainder to put into them.
00006 // If the two last bytes contains the crc received, the return should be of 0
00007 int crc16Remainder(std::vector<unsigned char> data, unsigned short generator)
00008 {   
00009     // shift generator
00010     unsigned int polynom = generator <<  16;
00011     
00012     if (data.empty())
00013     {
00014         return -1;
00015     }
00016 
00017     unsigned int remainder = data[0] << 24 | data[1] << 16;
00018 
00019     for (int i = 2; i < data.size(); i++)
00020     {
00021         remainder = remainder | data[i] << 8;
00022         for (int j = 0; j < 8; j++)
00023         {
00024             // if msb is 1, shift and xor
00025             if (remainder >> 31 ==1)
00026             {
00027                 remainder = (remainder << 1) ^ polynom;
00028             }
00029             else
00030             {
00031                 remainder = remainder << 1;
00032             }
00033         }
00034     }
00035     // Calculates remainder of last 16 bits 
00036     for (int i = 0; i < 16; i++)
00037     {
00038         // if msb is 1, shift and xor
00039         if (remainder >> 31 == 1)
00040         {
00041             remainder = (remainder << 1) ^ polynom;
00042         }
00043         else
00044         {
00045             remainder = remainder << 1;
00046         }
00047     }
00048     return remainder >> 16;
00049 
00050 }