Re: Debdiff project announcement.

[Tom Lees <tal26@cam.ac.uk>]

On Sun, Feb 06, 2000 at 09:48:01AM -0500, Tom Rothamel wrote:
> On Sat, Feb 05, 2000 at 11:17:16AM -0500, Joey Hess wrote:
> > Tom Rothamel wrote:
> > > Information on the order of elements within control.tar and data.tar
> > > will be preserved in the debdiff. This should lead to identical output
> > > to the original when these files are compressed with gzip.
> > 
> > It won't, because .gz files have timestamps in them that change when they
> > are repacked. Try and see..
> 
> Grr... you're right, of course. I'll need to figure out a way around
> this one. (Probably originally by coming out with a tool that can
> checksum the individual members of a deb, ignoring differences in the
> gzip compression.)

Sorry about the late-ish reply, but you can fix this problem by just copying
the gzip header across (its just really a few bytes).

The attached small C prog calculates the length of a gzip header and the
compression factor used (roughly). This can be used to reconstruct a gzip
with the same md5sum by copying the header across.

-- 
Tom <tom@debian.org>

This restaurant was advertising breakfast any time. So I ordered
french toast in the renaissance.
- Steven Wright, comedian

#include <stdio.h>

int hdr_len = 0;

int byte (void)
{
	hdr_len++;
	return getchar ();
}

main()
{
	int flags;
	int i;
	
	// Reads the GZIP file on stdin and returns the length of the header
	i = byte(); i |= (byte () << 8);
	if (i != 0x8b1f) { printf ("0\n"); exit (1); }
	byte();	// Skip method
	flags = byte();
	byte();byte();byte();byte();	// Skip timestamp
	i = byte();	// Extra flags
	switch (i)
	{
		case 0:
		printf ("-6\n");
		break;
		
		case 2:
		printf ("-9\n");
		break;
		
		case 4:
		printf ("-1\n");
		break;
		
		default:
		printf ("%d\n", i);
	}
	byte();	// Skip OS type
	if (flags & 0x04)
	{
		i = byte(); i |= (byte()<<8);
		for (;i>0;i--)
			byte();
	}		// Skip extra fields
	if (flags & 0x08)
		while (byte() != 0) ;	// Skip orig name
	if (flags & 0x10)
		while (byte() != 0) ;	// Skip comment
	byte();byte();byte();byte();	// CRC
	byte();byte();			// Size
	printf ("%d\n", hdr_len);
	exit (0);
}