On Wed, Mar 08, 2006 at 03:04:14AM -0800, Junio C Hamano wrote:
> Sergey Vlasov <vsu@xxxxxxxxxxx> writes:
> > However, a straight reuse still will not be possible, because
> > sha1write_compressed() uses deflateInit(&stream, Z_DEFAULT_COMPRESSION),
> > which writes zlib headers around the deflate stream, and the zlib footer
> > contains adler32 checksum. So, as a minimum, you will need to
> > decompress the object data, calculate its adler32 checksum and write the
> > zlib header yourself.
>
> Hmph. Thanks for helping, but it sounds like my original plan
> was not useful at all. Probably inflating would be still
> cheaper than inflating and then deflating, but it would not be
> as cool as a straight copy. Sigh...
Actually you can calculate adler32 checksum of object data from
adler32(header+data) (available at the end of the loose object file),
adler32(header) (which you will need to calculate) and len(data)
(which is available in the header):
#define ADLER32_BASE 65521UL
unsigned int adler32_split(unsigned int adler_full, unsigned int adler_1,
unsigned long len_2)
{
unsigned long s1_1 = adler_1 & 0xffff;
unsigned long s1_2 = (adler_1 >> 16) & 0xffff;
unsigned long rem = len_2 % ADLER32_BASE;
unsigned long s_1_offset = (s1_1 + ADLER32_BASE - 1) % ADLER32_BASE;
unsigned long s_2_offset = (s1_2 + s_1_offset*rem) % ADLER32_BASE;
unsigned long sf_1 = adler_full & 0xffff;
unsigned long sf_2 = (adler_full >> 16) & 0xffff;
unsigned long s2_1 = (sf_1 + ADLER32_BASE - s_1_offset) % ADLER32_BASE;
unsigned long s2_2 = (sf_2 + ADLER32_BASE - s_2_offset) % ADLER32_BASE;
return (s2_2 << 16) | s2_1;
}
However, the resulting code probably won't be pretty...
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