Juha, There are indeed many different CRC algorithms. The division polynomial typically determines the function - but there are other tweaks including whether you are dealing with big endian or little endian numbers. The division polynomial is chosen such that it never divides exactly into the predicted error pattern (i.e. leaves a zero remainder) and as such is tuned to a specific communications medium.
The super fast function you have found is almost certainly table driven as these algorithms are much faster than direct computation. In the 'C' world, you can generate your own CRC generator and checker for any given polynomial using the generator at http://www.tty1.net/pycrc/ I have used this successfully on 'C' projects. I suggest that you use this to generate your CRC checker and either link it directly into your Pascal program or translate the result to Pascal. Regards Tony Whyman MWA On 25/08/14 09:46, Juha Manninen wrote: > We have an old function for calculating CRC like: > > function CalcCRC(const Str: String): Cardinal; > var > Len, i, CRCVal: Cardinal; > Begin > Len := Length(Str); > CRCVal := $FFFFFFFF; > if Len > 0 then > for i := 1 to Len do > CRCVal := CRCTbl[Byte(CRCVal xor Cardinal(Str[i]))] xor ((CRCVal > shr 8) and $00FFFFFF); > Result := not(CRCVal); > end; > > "CRCTbl" is a table with hex numbers. > > Then I tested a new super-fast function crc32c() from Synopse mORMot > lib that uses Intel SSE instructions: > > > http://blog.synopse.info/post/2014/05/25/New-crc32c%28%29-function-using-optimized-asm-and-SSE-4.2-instruction > > Wow, with long input data strings it is 10 * faster! > But, bummer, it returns a different value. :( > Our CRCs are stored around in DBs and we need the same value. > > I thought CRC algorith is a standard but apparently not. > It says at the Synopse blog-page: > --- > In fact, most popular file formats and protocols (Ethernet, MPEG-2, > ZIP, RAR, 7-Zip, GZip, and PNG) use the polynomial $04C11DB7, while > Intel's hardware implementation is based on another polynomial, > $1EDC6F41 (used in iSCSI and Btrfs). > So you would not use this new crc32c() function to replace the zlib's > crc32() function, but as a convenient very fast hashing function at > application level. > For instance, our TDynArray wrapper will use it for fast items hashing. > --- > > I don't know what the "polynomial" does here. Does it mean Intel CPUs > have built-in support for specific CRC calculations? > > Does anybody know of other optimized CRC functions? > We would love the speedup. > > Regards, > Juha > _______________________________________________ > fpc-pascal maillist - fpc-pascal@lists.freepascal.org > http://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-pascal _______________________________________________ fpc-pascal maillist - fpc-pascal@lists.freepascal.org http://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-pascal
