"Mikheev, Vadim" <vmikheev@SECTORBASE.COM> writes:
> This may be implemented very fast (if someone points me where
> I can find CRC func).
Lifted from the PNG spec (RFC 2083):
15. Appendix: Sample CRC Code
The following sample code represents a practical implementation of the CRC (Cyclic Redundancy Check) employed in PNG
chunks. (See also ISO 3309 [ISO-3309] or ITU-T V.42 [ITU-V42] for a formal specification.)
/* Make the table for a fast CRC. */ void make_crc_table(void) { unsigned long c; int n, k;
for (n = 0; n < 256; n++) { c = (unsigned long) n; for (k = 0; k < 8; k++) { if (c & 1)
c = 0xedb88320L ^ (c >> 1); else c = c >> 1; } crc_table[n] = c; }
crc_table_computed = 1; }
/* Update a running CRC with the bytes buf[0..len-1]--the CRC should be initialized to all 1's, and the
transmittedvalue is the 1's complement of the final running CRC (see the crc() routine below)). */
unsigned long update_crc(unsigned long crc, unsigned char *buf, int len) {
unsignedlong c = crc; int n;
if (!crc_table_computed) make_crc_table(); for (n = 0; n < len; n++) { c = crc_table[(c ^
buf[n])& 0xff] ^ (c >> 8); } return c; }
/* Return the CRC of the bytes buf[0..len-1]. */ unsigned long crc(unsigned char *buf, int len) {
returnupdate_crc(0xffffffffL, buf, len) ^ 0xffffffffL; }
regards, tom lane
