Re: [PATCH 8/8] diff: improve positioning of add/delete blocks in diffs

[Michael Haggerty <mhagger@alum.mit.edu>]

On 08/04/2016 09:56 AM, Jeff King wrote:
> On Thu, Aug 04, 2016 at 12:00:36AM +0200, Michael Haggerty wrote:
> 
>> This table shows the number of diff slider groups that were positioned
>> differently than the human-generated values, for various repositories.
>> "default" is the default "git diff" algorithm. "compaction" is Git 2.9.0
>> with the `--compaction-heuristic` option "indent" is an earlier,
> 
> s/option/&./

Thanks, will change.

>>  static int diff_detect_rename_default;
>> +static int diff_indent_heuristic; /* experimental */
>>  static int diff_compaction_heuristic; /* experimental */
> 
> These two flags are mutually exclusive in the xdiff code, so we should
> probably handle that here.
> 
> TBH, I do not care that much what:
> 
>   [diff]
>   compactionHeuristic = true
>   indentHeuristic = true
> 
> does. But right now:
> 
>   git config diff.compactionHeuristic true
>   git show --indent-heuristic
> 
> still prefers the compaction heuristic, which I think is objectively
> wrong.

I wasn't worrying about that yet, given that these two features are both
still experimental. I also have a strong inkling that at most one of
them needs to be made permanent. I propose that I repair the semantics
in the simplest way possible for now while we decide on the long-term
plan, which might conceivably be:

* keep both options permanently
* keep only one option permanently
* choose one heuristic and use it always (i.e., make it part of the new
standard one-and-only diff algorithm)
* discard both heuristics (I hope not!)

After we've decided on that, *then* let's decide on a suitable UI and
implement it before we declare either feature non-experimental.

> [...]
> Speaking of absurd amounts of work, I was curious if there was a
> noticeable performance penalty for using this heuristic [...]

I included some performance numbers in my response to Junio [1].

>> +#define START_OF_FILE_BONUS 9
>> +#define END_OF_FILE_BONUS 46
>> +#define TOTAL_BLANK_WEIGHT 4
>> +#define PRE_BLANK_WEIGHT 16
>> +#define RELATIVE_INDENT_BONUS -1
>> +#define RELATIVE_INDENT_HAS_BLANK_BONUS 15
>> +#define RELATIVE_OUTDENT_BONUS -19
>> +#define RELATIVE_OUTDENT_HAS_BLANK_BONUS 2
>> +#define RELATIVE_DEDENT_BONUS -63
>> +#define RELATIVE_DEDENT_HAS_BLANK_BONUS 50
> 
> I see there is a comment below here mentioning that these are empirical
> voodoo, but it might be worth one at the top (or just moving these below
> the comment) because the comment looks like it's just associated with
> the function (and these are sufficiently bizarre that anybody reading is
> going to double-take on them).

Good idea.

>> +        return 10 * score - bonus;
> 
> I don't mind this not "10" not being a #define constant, but after
> reading the exchange between you and Stefan, I think it would be nice to
> describe what it is in a comment. The rest of the function is commented
> so nicely that this one left me thinking "huh?" upon seeing the "10".

Done. Thanks for your review.

Michael

[1]
http://public-inbox.org/git/5fe0edbc-3659-058f-3328-639d1343fa05@alum.mit.edu/