Re: [PATCH] Documentation/diff-options: explain different diff algorithms

From: Stefan Beller <sbeller@google.com>
Date: Aug 9, 2018, 19:51 UTC
Message-ID: <CAGZ79kby35z6pQb9DN+SvF6jA1aPMKmxyrtHM7kiwGjwD3abPA@mail.gmail.com>
In-Reply-To: <20180806231843.GA4117@aiede.svl.corp.google.com>

On Mon, Aug 6, 2018 at 4:18 PM Jonathan Nieder <jrnieder@gmail.com> wrote:

> > +DIFF ALGORITHMS
> > +---------------
>
> Please add some introductory words about what the headings refer to.

ok.

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>
> > +the shortest output.
>
> Trivia: the `minimal` variant of Myers doesn't guarantee shortest
> output, either: what it minimizes is the number of lines marked as
> added or removed.  If you want to minimize context lines too, then
> that would be a new variant. ;-)

... and take line length into account. ;-)

It minimizes the edit distance in terms of lines, i.e. in a context-less diff we get the lowest number of lines possible.

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> > +This algorithm finds the longest common substring and recursively
> > +diffs the content before and after the longest common substring.
>
> optional: may be worth a short aside in the text about the distinction
> between LCS and LCS. ;-)
>
> It would be especially useful here, since the alphabet used in these
> strings is *lines* instead of characters, so the first-time reader
> could probably use some help in building their intuition.

That makes sense.

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>
> > +This is often the fastest, but in corner cases (when there are
> > +many common substrings of the same length) it produces bad
>
> Can you clarify what "bad" means?  E.g. would "unexpected", or "poorly
> aligned", match what you mean?

I'll just go with unexpected.

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> > +results as seen in:
> > +
> > +     seq 1 100 >one
> > +     echo 99 > two
> > +     seq 1 2 98 >>two
> > +     git diff --no-index --histogram one two