From: Philip Oakley <philipoakley@iee.org>
Date: Mon, 20 Feb 2012 09:05:43 GMT
Subject: Re: Manually decoding a git object
Message-ID: <5D290031F60941368FB1A36464201DCF@PhilipOakley>
In-Reply-To: <87mx8dj4at.fsf@thomas.inf.ethz.ch>

From: "Thomas Rast" <trast@inf.ethz.ch> Sent: Monday, February 20, 2012 8:29 
AM
> "Philip Oakley" <philipoakley@iee.org> writes:
>
>> If I have a renamed file which is a git object, such a "Git_Object", was
>> 8c-something-or-other, what is the easiest way of examining / decoding /
>> recreating the original file (either as its sha1, or a cat-file).
>>
>> I don't appear to be able to unzip the file in its raw format...  I'm 
>> using
>> Msysgit on windows XP.

Correction to reply to  xu's message: and I have /NOT/ located an
unzip programme that will take the plain git object and decode it.

>
> The SHA1 is over the decompressed object contents.  The file simply
> holds a zlib-compressed stream of those contents.  (It's pretty much
> like gzip without the file header.)
>
> You can use any bindings to zlib and something that does sha1, e.g. in
> python:
>
>  $ cd g/.git/objects/aa/  # my git.git
>  $ ls
>  592bda986a8380b64acd8cbb3d5bdfcbc0834d 
> 6322a757bee31919f54edcc127608a3d724c99
>  $ python
>  Python 2.7.2 (default, Aug 19 2011, 20:41:43) [GCC] on linux2
>  Type "help", "copyright", "credits" or "license" for more information.
>  >>> import hashlib
>  >>> 
> hashlib.sha1(open('592bda986a8380b64acd8cbb3d5bdfcbc0834d').read().decode('zlib')).digest().encode('hex')
>  'aa592bda986a8380b64acd8cbb3d5bdfcbc0834d'
>
> Notice that the first byte of the hash goes into the directory name.
>

At the moment I'm in a Catch 22 situation where I can't make the first step 
of examining the deflated contents, so I can't do all those next steps to 
get the sha1 etc.. Have I misunderstood your suggestions?

Philip