bug#47712: 27.1; Provide `string-display-width` function, which takes properties into account, `substring-width`

[Daniel Mendler <mail@daniel-mendler.de>]

On 4/13/21 2:00 PM, Eli Zaretskii wrote:
>> From: Lars Ingebrigtsen <larsi@gnus.org>
>>> Note that I would like to have an implementation of
>>> `substring-width`/`string-display-width`/`substring-display-width`
>>> which does not allocate, since this reduces the GC pressure when
>>> formatting many items. How do you think about this?
>>
>> Sure; I think a non-allocating function to count monospaced string width
>> would be useful.
> 
> I won't mount the barricades over this, but please keep in mind that
> even with fixed-pitch fonts this will not always produce correct
> results, because on GUI frames the "double-width" characters not
> always take exactly two columns (it depends on the font).  So this
> function will only work reliably on text-mode (a.k.a. TTY) frames and
> for Latin characters on GUI frames.
> 
> If you still think we should have such a semi-broken function, at
> least include some warning in its doc string, so that users wouldn't
> be unpleasantly surprised.

Yes, `string-width` is broken for face-dependent purposes. However there 
are still these handful of use cases which will probably not go away 
(org-mode table, formatting monospaced text, ...). In those existing 
cases the `string-width` function is often used in combination with 
`substring`, i.e., `(string-width (substring str beg end)`.

Therefore I would be happy with the following resolution:

1. Add two arguments begin and end to `string-width` to improve the 
current uses of `string-width`.

2. Document the caveats of `string-width` in the docstring (works only 
reliable in text mode for multi-width chars) and maybe mention 
`window-text-pixel-size` or the `string-pixel-width` function by Martin.

----

(defun string-pixel-width (string)
  (let ((buffer (get-buffer-create " *string-pixel-width*"))
        (old-buffer (window-buffer)))
    (unwind-protect
        (with-current-buffer buffer
          (erase-buffer)
          (insert string)
          (set-window-buffer (selected-window) buffer)
          (car (window-text-pixel-size nil (point-min) (point-max))))
      (set-window-buffer (selected-window) old-buffer))))

(I don't think it is needed to add the `string-pixel-width` function to 
the core. The function or some variant of it is easy enough for packages 
to include and it cannot be used in this form since it leads to these 
temporary buffers lying around.)