Re: [PATCH 1/2] xdiff: implement a zealous diff3, or "zdiff3"

From: Junio C Hamano <gitster@pobox.com>
Date: Jun 15, 2021, 06:13 UTC
Message-ID: <xmqqv96fven9.fsf@gitster.g>
In-Reply-To: <b7561a67c192d4bdede47fee5b7b1cb30c44b785.1623734171.git.gitgitgadget@gmail.com>

"Elijah Newren via GitGitGadget" <gitgitgadget@gmail.com> writes:

[An example to reduce
    1234<ABCDE|56=AXCYE>789
to
    1234A<BCD|56=XCY>E789
elided]

Show 6 quoted lines

> Note that the common lines, 'A', and 'E' were moved outside the
> conflict.  Unlike with the two-way conflicts from the 'merge'
> conflictStyle, the zdiff3 conflict is NOT split into multiple conflict
> regions to allow the common 'C' lines to be shown outside a conflict,
> because zdiff3 shows the base version too and the base version cannot be
> reasonably split.

Another thing that was brought up during the original discussion is that zdiff3 uses different meaning of "base version" from diff3. In diff3 output, if you remove everything enclosed in <<< and |||, and everything enclosed in === and >>>, i.e. 123456789 in the original example above, you will get "the common ancestor" version, which is what is shown as the "base". After zdiff3 munges the lines, that is not the case, 1234A56E789 never appeared in anybody's version. It is a "starting from the common ancestor version, both sides agreed to make the same uncontroversial changes, which is already included in this base version" and zdiff3 inserts the conflict markers and material unique to each side into it. Being able to recover the common ancestor version is not always necessary and that is what makes zdiff3 a plausible solution, but to some workflows and tools it may matter, so it would be helpful to mention it in the documentation.

Thanks.