CRC-16/CRC-32 Program code

CRC-16 is used to write a program not long ago, but looking for to find only in Uddf found a Delphi CRC-32 program code, but also use look-up table method, although the speed of look-up table method, but 256 32-bit data I suspect there may be input errors, people are not so relieved, and I do not know this How the table was calculated. Then I found a section about CRC in a notebook two years ago, also do not know where to copy from, fortunately there is a section of the program code, is CRC-16, this procedure is to produce CRC table, but this is just a few lines of the program (basically with the following BuilderTable16 function is the same) see my confused, until these two days to understand, and the introduction of the CRC-32 algorithm, now all the procedures listed below, and make some explanations to help understand, not only to know it, but also to know why:

//Note: Because the highest position must be "1", so omit


Const unsigned short cncrc_16 = 0x8005;


//CRC-16 = X16 + X15 + X2 + X0


Const unsigned short Cncrc_ccitt = 0x1021;


//Crc-ccitt = X16 + X12 + X5 + X0, it is said that this 16-bit CRC polynomial is better than the previous


Const unsigned long cncrc_32 = 0X04C10DB7;


//CRC-32 = X32 + X26 + X23 + X22 + X16 + X11 + X10 + X8 + X7 + X5 + X4 + X2 + X1 + X0


unsigned long table_crc[256]; CRC Table


//Constructed 16-bit CRC table


void BuildTable16 (unsigned short apoly)


{


unsigned short i, J;


unsigned short ndata;


unsigned short naccum;


for (i = 0; i < 256; i++)


{


ndata = (unsigned short) (I << 8);


naccum = 0;


for (j = 0; J < 8; j + +)


{


if ((ndata ^ naccum) & 0x8000)


naccum = (naccum << 1) ^ apoly;


Else


naccum <<= 1;


ndata <<= 1;


}


Table_crc[i] = (unsigned long) naccum;


}


}


//Calculates 16-bit CRC value, CRC-16 or Crc-ccitt


unsigned short crc_16 (unsigned char * adata, unsigned long asize)


{


unsigned long i;


unsigned short naccum = 0;


BuildTable16 (CNCRC_16); or Cncrc_ccitt


for (i = 0; i < asize; i++)


Naccum = (naccum << 8) ^ (unsigned short) table_crc[(naccum >> 8) ^ *adata++];


return naccum;


}


//Constructed 32-bit CRC table


void BuildTable32 (unsigned long apoly)


{


unsigned long I, J;


unsigned long ndata;


unsigned long naccum;


for (i = 0; i < 256; i++)


{


ndata = (unsigned long) (I << 24);


naccum = 0;


for (j = 0; J < 8; j + +)


{


if ((ndata ^ naccum) & 0x80000000)


naccum = (naccum << 1) ^ apoly;


Else


naccum <<= 1;


ndata <<= 1;


}


Table_crc[i] = naccum;


}


}


//Calculate 32-bit CRC-32 value


unsigned long crc_32 (unsigned char * adata, unsigned long asize)


{


unsigned long i;


unsigned long naccum = 0;


BuildTable32 (CNCRC_32);


for (i = 0; i < asize; i++)


naccum = (naccum << 8) ^ table_crc[(naccum >>) ^ *adata++];


return naccum;


}