Re: [PATCH v2] xdiff: re-diff shifted change groups when using histogram algorithm

>
> On 13/03/2026 07:07, Junio C Hamano wrote:
> > "Yee Cheng Chin via GitGitGadget" <gitgitgadget@gmail.com> writes:
> >
> >> From: Yee Cheng Chin <ychin.git@gmail.com>
> >>
> >> After a diff algorithm has been run, the compaction phase
> >> (xdl_change_compact()) shifts and merges change groups to produce a
> >> cleaner output. However, this shifting could create a new matched group
> >> where both sides now have matching lines. This results in a
> >> wrong-looking diff output which contains redundant lines that are the
> >> same on both files.
> >>
> >> Fix this by detecting this situation, and re-diff the texts on each side
> >> to find similar lines, using the fall-back Myer's diff. Only do this for
> >> histogram diff as it's the only algorithm where this is relevant. Below
> >> contains an example, and more details.
> >> ...
> >> This issue is rare in a normal repository. Below is a table of
> >> repositories (`git log --no-merges -p --histogram -1000`), showing how
> >> many times a re-diff was done and how many times it resulted in finding
> >> matching lines (therefore addressing this issue) with the fix. In
> >> general it is fewer than 1% of diff's that exhibit this offending
> >> behavior:
> >>
> >> | Repo (1k commits)  | Re-diff | Found matching lines |
> >> |--------------------|---------|----------------------|
> >> | llvm-project       |  45     | 11                   |
> >> | vim                | 110     |  9                   |
> >> | git                |  18     |  2                   |
> >> | WebKit             | 168     |  1                   |
> >> | ripgrep            |  22     |  1                   |
> >> | cpython            |  32     |  0                   |
> >> | vscode             |  13     |  0                   |
> >>
> >> Signed-off-by: Yee Cheng Chin <ychin.git@gmail.com>
> >> ---
> >
> > Thanks for the updated patch, and sorry for nobody responding to the
> > patch for over a week.
>
> Yes, sorry for the slow response. I agree with Junio that this is
> explained well and looks good
>
> Thanks
>
> Phillip
>
> > The detailed explanation of the issue and the inclusion of the
> > repository analysis results are very helpful; they clearly show that
> > while this is a rare edge case, it significantly improves the
> > quality of histogram diffs when it does occur.
> >
> >   - The removal of go_orig is correct since g and go are kept in sync
> >     throughout the slide loops.
> >
> >   - Clearing the algorithm mask while preserving other flags ensures that
> >     user-provided options like --ignore-all-space are correctly applied
> >     during the re-diff.
> >
> >   - While ignore_regex and anchors are not passed to the sub-diff, they
> >     aren't currently available to xdl_change_compact anyway. Given that
> >     compaction happens before regex filtering in the main pipeline, this
> >     is OK, I guess.
> >
> > Let me mark the topic for 'next'.
> >
>