Tuukka Toivonen wrote:
> Download GZIP and deflate compression specifications. They explain CRC
> (32-bit if my memory serves) and sample implementation. It was very easy to
> understand, even I did.
I don't find it so easy ... :
Spending 5 minutes reading the crc code, all i understood
is that gzip first compute a table containing the first
256 values of the polynom
x^32+x^26+x^23+x^22+x^16+x^12+x^11+x^10+x^8+x^7+x^5+x^4+x^2+x+1
( I don't even undestand how those values are computed
( in sample/makecrc.c))
This table (crc_32_tab) is then used in util.c/updcrc :
ulg updcrc(s, n)
uch *s; /* pointer to bytes to pump through */
unsigned n; /* number of bytes in s[] */
{
register ulg c; /* temporary variable */
static ulg crc = (ulg)0xffffffffL; /* shift register contents */
if (s == NULL) {
c = 0xffffffffL;
} else {
c = crc;
if (n) do {
c = crc_32_tab[((int)c ^ (*s++)) & 0xff] ^ (c >> 8);
} while (--n);
}
crc = c;
return c ^ 0xffffffffL; /* (instead of ~c for 64-bit machines) */
}
I think I'll use the weighted checksum sugested by glynn Clemens
which is a lot simpler to me.
david
