Re: Question: How range-diff lapjv algorithm work

From: ZheNing Hu <adlternative@gmail.com>
Date: Mar 9, 2023, 10:38 UTC
Message-ID: <CAOLTT8QAd_dzmDrkLpyVCZykb_sPzR0VzsPf_jtQqim_NpqZFg@mail.gmail.com>
In-Reply-To: <CAN0heSoVBmkuEOkW3wRNXRRjG7y8UqvORie6ocYnmQRz4jEXkg@mail.gmail.com>

Martin Ågren <martin.agren@gmail.com> 于2023年3月8日周三 15:47写道：

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>
> On Wed, 8 Mar 2023 at 07:50, ZheNing Hu <adlternative@gmail.com> wrote:
> >
> > My question is:
> > 1. In step 3, why is the matrix size (a.nr + b.br) * (a.nr + b.br)
> > instead of a.nr * b.nr?
>
> There's some explanation of that in the man page for `git range-diff`,
> under "ALGORITHM". Look for "To explain also new commits, we introduce
> dummy nodes on both sides:".
>

Thanks, I can understand why the length of the matrix is "a.nr + b.nr" now. Patches in one collection may have no matching patches in the other collection, this mismatch situation ("o--C" in the documentation) should also count the cost.

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> > 2. Why the cost(x,y) which satisfies "x ∈ [a.nr, a.nr + b.nr) y ∈ [0,
> > b.nr) || x ∈ [0, a.nr) y ∈ [b.nr, b. The cost of nr + a.nr)" is set to
> > "diffsize(a or b) * creation_factor / 100 : COST_MAX"? What is the
> > role of creation_factor? [1]
>
> The `--creation-factor` command line option is also described in the
> manpage.  There was a thread on the mailing list with various
> discussions around this creation factor a while back. Maybe there's
> something interesting there [1].
>

I understand it now. Because mismatch "o--C" "1--0" cost are generally greater than the cost of two completely different patches "1--C" "0--0". Use the creation-factor to reduce the cost of "0-C" "1--0" make "o--C", "1--0" as matching result.

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> > 3. How does LAPJV work here? [2]
> >
> > [1]: https://github.com/git/git/blob/725f57037d81e24eacfda6e59a19c60c0b4c8062/range-diff.c#L310
> > [2]: https://github.com/git/git/blob/725f57037d81e24eacfda6e59a19c60c0b4c8062/linear-assignment.c#L15
>
> This appears to be based on work by Jonker and Volgenant. Maybe
> searching for those names online could find something. Maybe not
> git-specific, but even if it's just the general algorithm as such, it
> might be possible to find different explanations of the algorithm.
>
> I haven't really studied this algorithm, but since it's faster than the
> Hungarian algorithm, I could imagine that either
>
>   * it's super-useful to first understand the Hungarian algorithm, then
>     understand why and how the Jonker-Volgenant algorithm does better,
>     or,
>
>   * it's completely useless to first understand the Hungarian algorithm,
>     since they're so different.
>
> :-)
>

Ah, I had a look at the Hungarian algorithm earlier, because it is the most typical algorithm in linear assignment problem, it can still be understood. I didn't read that paper by Jonker and Volgenant, but I should try to read it later.

> [1] https://lore.kernel.org/git/1196830250.20220726145447@yandex.ru/
>
> Martin

Thanks for the answer!

-- ZheNing Hu