Re: [PATCH v6] Add new git-related helper to contrib

From: Junio C Hamano <gitster@pobox.com>
Date: May 23, 2013, 23:47 UTC
Message-ID: <7vvc69b6nl.fsf@alter.siamese.dyndns.org>
In-Reply-To: <CAMP44s14Gdadmgy2zi6dRnKLYdZ=HO85OiPSVxGZ8Hquv_58=g@mail.gmail.com>

Felipe Contreras <felipe.contreras@gmail.com> writes:

Show 6 quoted lines

>> If the change in HEAD^ in the above example were to copy the whole A
>> and C from a different file, then your !found_guilty logic would
>> kick in and say all lines of A where copied from elsewhere in HEAD^,
>> but again we would not learn the same information for C.
>
> We would, when it's the turn for C, which is not guilty at this point.

In _this_ round of the while(1) loop, pass_blame_to_parent() gets the scoreboard and two origins (HEAD^ that we are looking at and HEAD^^ that is its parent); it does not even know what blame entry this request came from.

It runs a single diff using diff_hunks(), and asks blame_chunk() to split all the blame entries in the scoreboard that suspect it is looking at may be guilty for. Blame entry for A and C are both processed exactly the same way when HEAD^ is given to pass_blame() for the first time, which is when assign_blame() decided to call it with HEAD^ because it happened to have seen A before seeing C. At that point, both A and C are processed, and the post-processing loop "Take responsibility for the remaining" will clean up remnants from both A and C. After this round ends, the suspect for A and C are both set to HEAD^^.

In the next round of the while(1) loop, C already forgot that its line movement happened in HEAD^. Its suspect is now HEAD^^. When "it's the turn for C" [*1*], you can say "These lines originate in that different path in HEAD^^", but it is too late to say "But the first time they appeared in the original file was HEAD^" (which is when they were moved from the different path in HEAD^^), isn't it?

[Footnote]

*1* It does not make much sense to say "turn for C", by the way.
    pass_blame() work per suspect (not per blame entry), so what you
    are talking about really is "when it is turn for HEAD^^", at
    which point we will discover that these lines in C originate in
    that different path in the commit.  And in the scenario we are
    discussing, we will decide to check HEAD^^ because the blame ent
    for A is also the same origin as C's suspect in HEAD^^, so
    technically it is still "turn for A" ;-).

    The sample would be like

	(cat A; cat C) >file0; git commit file0	 ; HEAD^^
	(cat A; cat C) >file1; git commit file1	 ; HEAD^
        (cat A; cat B; cat C) >file1; git commit file1 ; HEAD
        git blame -C -C -C file1 HEAD