bug#46859: 28.0.50; [PATCH]: Add option to truncate long lines in xref.el

[Dmitry Gutov]

On 03.03.2021 19:42, Gregory Heytings wrote:
> > > I wrote too fast.  In fact you can get the column number with GNU 
> > > grep without parsing the original line:
> > >
> > > grep -nb -oE '.{0,100}PATTERN.{0,100}'
> >
> > This outputs byte offset from the beginning of the file, doesn't it?
>
> Yes.  You get, for each match: the line number (from the beginning of 
> the file), the byte offset (from the beginning of the file) of the first 
> displayed character, and the context of the match.

OK, so we get the byte offset, but not the length of the match (which 
we'll also need later, for purposes such as highlighting and 
replacement). And what happens if there are several matches on the same 
line? We need columns for all of them.

> > Which will require at least reading the file into memory to convert.
>
> I don't understand what you mean by that, but it seems to me that in any 
> case it's much more efficient than parsing the output of grep with Elisp.

We currently don't visit the file buffer if it's not already visited, 
parsing the line in a temp buffer instead. That approach resulted in a 
nice perf improvement.

> And you can easily get the byte offset of each beginning of line with 
> "grep -nbo '^.'", so calculating the byte offset from the beginning of 
> the line is easy.

Do you mean to suggest we call grep one more time for each matching line?