bug#68215: Bug in what-page

[Eli Zaretskii]

> From: Lars Brinkhoff <lars.brinkhoff@gmail.com>
> Date: Tue, 2 Jan 2024 11:39:22 +0100
> 
> The Emacs manual says:
>   "M-x what-page
>    Display the page number of point, and the line number within that page."
> 
> I would argue that this is the intended function, since that is what
> GNU Emacs did before 2010, and it is the same as "What Page" in the
> original TECO Emacs.
> 
> Bug https://debbugs.gnu.org/cgi/bugreport.cgi?bug=6825 noted that the
> line number was wrong if the cursor was not at the beginning of a
> line.  There was a fix for this, but the fix only works for the first
> page.  In subsequent pages, the line number is no longer relative to
> the current page.

Right, thanks for finding this bug.

> I'm attaching a patch which restores the original function and also
> fixes the bug.  The updated test checks this.  I also updated the
> what-page doc string to more clearly explain what line numbers are
> reported.

Using line-number-at-pos in the fix for bug#6825 had one more adverse
consequence: the line counts returned by what-page no longer honor
invisible lines and selective-display, which count-lines did.  So I'd
prefer to fix both bugs at once, by going back to count-lines, but in
a way that would avoid the off-by-one error fixed in bug#6825.  AFAIU,
the off-by-one error is due to the fact that count-lines treats buffer
position at BOL specially, e.g.:

            (t
             (goto-char (point-max))
             (if (bolp)
                 (1- (line-number-at-pos))
               (line-number-at-pos)))))))

It should be a simple matter to countermand this special-casing in
what-page, while still using count-lines.  Or am I missing something?

>  (defun page--what-page ()
> -  "Return a list of the page and line number of point."
> +  "Return a list of the page number of point, and the line number
> +within that page."

Our convention is to have the first line of a doc string be a single
complete sentence.

>  (defun what-page ()
> -  "Print page and line number of point."
> +  "Display the page number of point, and the line number within
> +that page."

Likewise here.

Thanks.