Re: [PATCH 2/3] xdiff: -W: include immediately preceding non-empty lines in context

[Vegard Nossum <vegard.nossum@oracle.com>]

On 13/01/2017 20:51, Junio C Hamano wrote:
> René Scharfe <l.s.r@web.de> writes:
>> That's true, but I'm not sure "non-empty line before function line" is
>> good enough a definition for desirable lines.  It wouldn't work for
>> people who don't believe in empty lines.  Or for those that put a
>> blank line between comment and function.  (I have an opinion on such
>> habits, but git diff should probably stay neutral.)  And that's just
>> for C code; I have no idea how this heuristic would hold up for other
>> file types like HTML.
>
> As you are, I am fairly negative on the heuristic based on the
> "non-blank" thing.  We tried once with compaction-heuristics already
> and it did not quite perform well.  Let's not hardcode another one.

The patch will work as intended and as expected for 95% of the users out
there (javadoc, Doxygen, kerneldoc, etc. all have the comment
immediately preceding the function) and fixes a very real problem for me
(and I expect many others) _today_; for the remaining 5% (who put a
blank line between their comment and the start of the function) it will
revert back to the current behaviour, so there should be no regression
for them.

For the 0% who don't put even a single blank line between their
functions, it will probably not work as expected, but then again I have
never seen such a coding style in any language, so I am doubtful if this
is something that needs to be taken into account in the first place.

>> We can identify function lines with arbitrary precision (with a
>> xfuncname regex, if needed), but there is no accurate way to classify
>> lines as comments, or as the end of functions.  Adding optional
>> regexes for single- and multi-line comments would help, at least for
>> C.
>
> The funcline regexp is used for two related but different purposes.
> It identifies a single line to be placed on @@ ... @@ line before a
> diff hunk.  This line however does not have to be at the beginning
> of a function.  It has to be the line that conveys the most
> significant information (e.g. the name of the function).
>
> The way "diff -W" codepath used it as if it were always the very
> first line of a function was bound to invite a patch like this, and
> if we want to be extra elaborate, I agree that an extra mechanism to
> say "the line the funcline regexp matches is not the beginning of a
> function, but the beginning is a line that matches this other regexp
> before that line" may help.
>
> Do we really want to be that elaborate, though?  I dunno.

Adding a regex instead of the simple "blank line" test doesn't seem very
difficult to do, but I am doubtful that it will make any difference in
practice. But that can be done incrementally as well by the people who
would actually need it (who I strongly suspect do not exist in the first
place).

> I wonder if it would be sufficient to make -W take an optional
> number, e.g. "git show -W4", to add extre context lines before the
> funcline.
>

I don't like specifying a fixed number, that negates almost all the
reason for using -W in the first place; I would much prefer adding
a config variable to control the -W behaviour (or a new diff flag).


Vegard