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Re: [Request for Mentor] subst-char-in-region

From: Stefan Monnier
Subject: Re: [Request for Mentor] subst-char-in-region
Date: Mon, 15 Dec 2014 09:29:07 -0500
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.0.50 (gnu/linux)

> Initially, because I wasn't sure that the a-c-f gave me all the
> information that I needed. I thought it might do, but I was confused by
> the third argument which is "length in bytes of the pre-change text". Is
> "length in bytes" the same as "char position". I presumed note.

Ha, nicely spotted, that's a bug in the docstring.  It passes (or
should anyway) the length in characters.

> The difficulty is that in some cases the conversion function uses the
> contents of this buffer to work out the equivalent location in that
> buffer.  When the b-c-f is called the two buffers are in sync, so this
> conversion works.  But when the a-c-f is called, the two buffers are not
> in sync because we haven't percolated the change yet.

So, rather than the length of the previous text, you'd need to actually
know the previous text?  Of course, you can use b-c-f to stash the
"previous text", but indeed it won't necessarily always be exactly
right, in the sense that the previous text stashed in b-c-f may not have
the exact same boundaries, so you'll need to massage it a bit.

The way I see it, you'll need to do something like

   (defvar-local my-before-change-text nil)

   (defun my-bcf (beg end)
     (setq my-before-change-text
           (if (null my-before-change-text)
               (cons beg (buffer-substring beg end))
             (let* ((oldbeg (car my-before-change-text))
                    (oldtext (cdr my-before-change-text))
                    (oldend (+ oldbeg (length oldtext)))
                    (newbeg (min beg oldbeg))
                    (newend (max end oldend)))
               (cl-assert (equal oldtext (buffer-substring oldbeg oldend)))
               (if (or (< newbeg oldbeg) (> newend oldend))
                   (cons newbeg (buffer-substring newbeg newend))
   (add-hook 'before-change-functions #'my-bcf)

And then in after-change-functions, you'll need something like

   (defun my-acf (beg end oldlen)
     (let ((oldtext (substring (cdr my-before-change-text)
                               (- beg (car my-before-change-text))
                               (+ oldlen (- beg (car my-before-change-text))))))
       (setq my-before-change-text nil)

-- Stefan

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