UPD> Unique ID: String->Integer

[Jim Sommer]

I had been just assigning consecutive integer values to each unique ID. 
This worked fine but I ran into some situations (installing/uninstalling 
options) where the integer ID changed for a specific unique ID string. This 
meant that any integer ID that I had saved in the driver was no longer valid.

I was looking for an algorithm to convert the unique ID strings into unique 
ID integers. I don't really need all ID integers to be unique, just all 
those in the same list (MediaSizeList, MediaTypeList, etc) need to be 
unique. I also need them to be consistent even if new ID's are added or 
some disappear.

Norbert suggested the algorithm used to generate the LPTENUM checksum. I 
hunted down the algorithm and found that it generates unique ID integers 
for all the ID strings in the sample implementation. I would be a little 
happier if it generated 32-bit ID integers, but so far it works fine.

Here's the code:

WORD wCRC16a[16]={
     0000000,    0140301,    0140601,    0000500,
     0141401,    0001700,    0001200,    0141101,
     0143001,    0003300,    0003600,    0143501,
     0002400,    0142701,    0142201,    0002100,
};
WORD wCRC16b[16]={
     0000000,    0146001,    0154001,    0012000,
     0170001,    0036000,    0024000,    0162001,
     0120001,    0066000,    0074000,    0132001,
     0050000,    0116001,    0104001,    0043000,
};

short Get_CRC_CheckSum(void *pBuffer, ULONG ulSize)
{
   BYTE   *pb;
   BYTE    bTmp;
   ULONG   ulSeed=0;

   for (pb=(BYTE *)pBuffer; ulSize; ulSize--, pb++)
   {
     bTmp=(BYTE)(((WORD)*pb)^((WORD)ulSeed));    // Xor CRC with new char
     ulSeed=((ulSeed)>>8) ^ wCRC16a[bTmp&0x0F] ^ wCRC16b[bTmp>>4];
   }
   return (short)ulSeed;
}

The seed is set to 0 each time so that the ID will always be the same for a 
given string.

If anyone has any comments or suggestions for a 32-bit CRC, I'd be happy hear.

Jim

mailto:sommer at granitesystems.com
978-486-3068