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Re: Native display of line numbers, improved

From: Alex
Subject: Re: Native display of line numbers, improved
Date: Mon, 26 Jun 2017 13:36:40 -0600
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.0.50 (gnu/linux)

Eli Zaretskii <address@hidden> writes:

>> From: Stefan Monnier <address@hidden>
>> Date: Mon, 26 Jun 2017 11:28:23 -0400
>> I don't understand your comment: Alex is referring here to the approach
>> used in nlinum, which I chose specifically to avoid having to scan the
>> whole buffer.
> I alluded to this:
>> that can
>> be worked around by setting 'display-line-number-width' to a
>> sufficiently large number, but I would like it to be no larger than what
>> it needs to be for the current text in the buffer.
> I provided 'display-line-number-width' to cater to the desire of not
> shrinking the width too much, and it seemed to me that if someone's
> ideal is not to change the width at all, as Alex said up-thread,
> counting the lines at the beginning will do that for him.

Counting at the beginning helps a lot, but it doesn't help for when the
buffer grows over an editing session.

>> Basically, start with a small value of display-line-number-width, and if
>> during display we find that this value is too small to fit the largest
>> displayed line-number, we increase it.
> I wanted to avoid using a buffer-local variable settable by the
> display engine.  (I cannot easily use display-line-number-width,
> because that's a user-settable option; I need another variable.)

What's the difference between the display engine setting a buffer-local
variable and Lisp libraries doing so?

I've included a diff that accomplishes what I want. Is there something
fundamentally wrong it?

PS: There's a bug with tab stops when display-line-number-width is a
small positive number. Look at around L20814 in xdisp.c with a
display-line-number-width being nil, 1, 2, and 3.

diff --git a/src/xdisp.c b/src/xdisp.c
index aa75fcaf77..442b09b10b 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -20810,8 +20810,21 @@ maybe_produce_line_number (struct it *it)
   /* Compute the required width if needed.  */
   if (!it->lnum_width)
-      if (NATNUMP (Vdisplay_line_number_width))
-       it->lnum_width = XFASTINT (Vdisplay_line_number_width);
+      if (display_line_numbers_grow_only && NATNUMP 
+       {
+         int min_lnum = XFASTINT (Vdisplay_line_number_width);
+         ptrdiff_t max_lnum;
+         max_lnum = this_line + it->w->desired_matrix->nrows - 1 - it->vpos;
+         min_lnum = max (log10 (max_lnum) + 1, min_lnum);
+         it->lnum_width = min_lnum;
+         Vdisplay_line_number_width = make_number (min_lnum);
+       }
+      else if (NATNUMP (Vdisplay_line_number_width))
+       {
+         it->lnum_width = XFASTINT (Vdisplay_line_number_width);
+       }
          /* Max line number to be displayed cannot be more than
@@ -32495,6 +32508,14 @@ even if the actual number needs less space.
 The default value of nil means compute the space dynamically.
 Any other value is treated as nil.  */);
   Vdisplay_line_number_width = Qnil;
+  DEFSYM (Qdisplay_line_number_width, "display-line-number-width");
+  Fmake_variable_buffer_local (Qdisplay_line_number_width);
+  DEFVAR_BOOL ("display-line-numbers-grow-only", 
+              doc: /* Non-nil means only dynamically grow the
+display, and never shrink. This variable is only taken into account
+when `display-line-number-width' is a positive number.*/);
+  display_line_numbers_grow_only = false;
   DEFVAR_BOOL ("inhibit-eval-during-redisplay", inhibit_eval_during_redisplay,
     doc: /* Non-nil means don't eval Lisp during redisplay.  */);

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