From: Bo Yang <struggleyb.nku@gmail.com>
Date: Fri, 09 Apr 2010 05:11:55 GMT
Subject: Re: [PATCH 1/2] Add a basic idea section for git-blame.
Message-ID: <x2z41f08ee11004082211g6ae76f38ic1ee65e508b16343@mail.gmail.com>
In-Reply-To: <7vmxxe74s2.fsf@alter.siamese.dyndns.org>

Hi Junio,
On Thu, Apr 8, 2010 at 1:00 AM, Junio C Hamano <gitster@pobox.com> wrote:
> The name of the game is not "take blame by looking at the diff to see what
> we introduced"; it is "avoid taking blame at all cost, by looking at the
> diff to see what we might have inherited from our parents".
>
> A description closer to the truth would be:
>
>  - Look at output of "git diff <rev>^$n <rev>" for each parent ($n runs
>   from 1 to number of parents);
>
>  - Ignore all +/- lines.  The context ' ' lines and lines outside of diff
>   hunks are known to have been inherited from $n-th parent and they are
>   not our fault.  We can happily pass blame for these lines to our
>   parent.
>
>  - Do the above for all the parents.  We (grudgingly) take blame for lines
>   that we failed to find corresponding lines in our parents.
>
> The -M option affects what happens between the second and the third step.
> We try to see if the lines that we did not pass blame to <rev>^$n might
> have come from <rev>^$n by running diff between the remainder and the blob
> in <rev>^$n *again*.  This lets us catch code movement within the blob,
> hence the name of the option -M.

I think here, not only the code movement, but also the copies will be found too.
Please correct me if I am wrong here. :-)

> The -C option affects the choice of the blob in <rev>^$n.  Usually, we
> internally run an equivalent of "git diff -M <rev>^$n <rev>" to notice
> that the file F that we are analyzing used to be called F' in $n-th
> parent, and run the diff against it, but -C allows us to check with paths
> other than that, and additional -C enlarges the search space.  This is to
> notice code movement (with a single -C) or copies (with more -Cs) across
> paths.
>
> For illustrated description, read the classic:
>
>    http://thread.gmane.org/gmane.comp.version-control.git/28826

Thanks a lot for providing this reference. It really helps me too much
to understand how blame works. I will try to re-read the whole code
according the description here. ;-)

Regards!
Bo