Re: [BUG-ish] diff compaction heuristic false positive

[Stefan Beller <sbeller@google.com>]

On Fri, Jun 10, 2016 at 8:56 AM, Junio C Hamano <gitster@pobox.com> wrote:
> Jeff King <peff@peff.net> writes:
>
>> On Fri, Jun 10, 2016 at 03:50:43AM -0400, Jeff King wrote:
>>
>>> I found a false positive with the new compaction heuristic in v2.9:
>>> [...]
>>
>> And by the way, this is less "hey neat, I found a case" and more "wow,
>> this is a lot worse than I thought".
>>
>> I diffed the old and new output for the top 10,000 commits in this
>> particular ruby code base. There were 45 commits with changed diffs.
>> Spot-checking them manually, a little over 1/3 of them featured this bad
>> pattern. The others looked like strict improvements.
>>
>> That's a lot worse than the outcomes we saw on other code bases earlier.
>> 1/3 bad is still a net improvement, so I dunno. Is this worth worrying
>> about? Should we bring back the documentation for the knob to disable
>> it? Should we consider making it tunable via gitattributes?
>>
>> I don't think that last one really helps; the good cases _and_ the bad
>> ones are both in ruby code (though certainly the C code we looked at
>> earlier was all good).
>>
>> It may also be possible to make it Just Work by using extra information
>> like indentation. I haven't thought hard enough about that to say.
>>
>> -Peff
>
> I recall saying "we'd end up being better in some and worse in
> others" at the very beginning.  How about toggling the default back
> for the upcoming release, keeping the experimentation knob in the
> code, and try different heuristics like the "indentation" during the
> next cycle?

Sure. I thought about for a while now and by now I agree with Junio.
No matter what kind of heuristic we can come up with it is easy to construct
a counter example.

That said, let's try the indentation thing, though I suspect
one of the early motivating examples (an excerpt from a  kernel config file)
would not do well with it, as it had not an indentation scheme as programming
languages do.

Thanks,
Stefan