Re: Need `truncate-string-to-pixel-width` and `glyph-pixel-width` functions in C

[Eli Zaretskii]

> From: Jimmy Yuen Ho Wong <wyuenho@gmail.com>
> Date: Wed, 23 Oct 2024 21:03:55 +0100
> Cc: emacs-devel@gnu.org
> 
> Each completion in a list of completion candidates is a triple of (candidate 
> string, prefix, annotation), all in
> varying widths. Aligning these 3 columns requires calculating paddings. Since 
> there are paddings, the
> padding between the candidate string and the annotation can also be 
> arbitrarily large. Naive truncation that
> truncates the entire concatenated line against the width of the pop up box, 
> which is actually the window width
> in a child frame, may leave some arbitrarily long empty space in the end. In 
> addition, naive truncation does
> not replace extra characters in the relevant columns with ellipses to 
> indicate truncation happened. Please look
> at the link in my first email for further details.
>  
>  So the issue is not "pixel-wise truncation", the issue is to take
>  variable-width display elements into account when truncating, is that
>  right?
> 
> To be precise, by " truncate strings correctly and preferably in pixel 
> precision", I mean truncation that respects
> pixel width constraints (and grapheme clusters, as clarified by you). 
> Truncating variable-width display
> elements on GUIs necessitate the ability to calculate the widths of the 
> individual elements, I call this the
> hypothetical `glyph-pixel-width`. Whether it is exposed to Elisp is less 
> important to a hypothetical
> `truncate-string-to-pixel-width` for my use case.

OK, thanks.

So let me see if I understand the issue.  You want to create in a
child frame a display such as below:

  <icon1><candidate1>                      <Type1>
  <icon2><longer-name-candidate>     <AnotherType>
  ...

And you want to do it in a way that all the "Type"s are aligned to the
right (i.e. end at the right edge of the frame), no matter what is
shown in the <icon>s and in the <candidate>s, which could be of
different width and could include characters of different fonts and
even images/emoji.  Is that correct, or is there something else?

If the above is what you want, and since the popup is actually a
frame, my first suggestion would be to use :align-to display property
on the whitespace between the <candidate>s and the <Type>s.  Did you
try that?

The advantage of :align-to is that you don't have to measure the
pixel-width of the candidate names, only the pixel-width of each
<Type> string (to correctly set up the offset from the right edge of
the text area) -- the rest will be done by the display engine.  And
the display engine always does everything in pixel resolution.