Re: [PATCH] write_sha1_file(): Perform Z_FULL_FLUSH between header and data

On Wed, Mar 08, 2006 at 03:04:14AM -0800, Junio C Hamano wrote:
> Sergey Vlasov <vsu@altlinux.ru> 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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