bug#46933: Possible bugs in filepos-to-bufferpos / bufferpos-to-filepos

[handa <handa@gnu.org>]

In article <83zgyif2aq.fsf@gnu.org>, Eli Zaretskii <eliz@gnu.org> writes:

> Leaving the :pre-write/:post-read-conversion use case aside, do we
> have some means of find where ISO-2022 shift-in/out sequence begins
> and ends, so that we never try to decode a partial sequence (and
> produce "characters" that are not really in the original buffer)?
> If not, where can I find the description of every kind of such
> sequences, i.e. sequences that modify the decoder state without
> producing any characters?

The official definition is in the standard ISO/IEC 2022, but
it seems that this wiki page:
  https://en.wikipedia.org/wiki/ISO/IEC_2022
is more concise.  Emacs implements all control sequences shown in the
sections: "Shift functions", "Character set designations", and
"Interaction with other coding systems".

> > By the way, what is the intention of filepos-to-bufferpos?  Why that
> > function was introduce?

> The original (and so far the only) use case was an Info manual
> separated into several files, where the tag table at the end of the
> main file specifies offsets in bytes.  See the function
> Info-find-node-2 in info.el.

As filepos-to-bufferpos accepts the optional arg CODING-SYSTEM,
I've thought BYTE arg is:
  a byte position in a file that will be created by encoding the current
  buffer by CODING-SYSTEM

But it seems that the usage in Info-find-node-2 is:
  a byte position in an existing file that may not be created by Emacs

There's a case that they are different.  The method I wrote in the
previous mail works only in the former case.   And it seems that the
current implementation of filepos-to-bufferpos is the same because it
tries to get byte sequence by encode-coding-region.

For the latter case, perhaps something like the following code works.

;; Return the buffer position correspoinding to the byte position
;; FILEPOS in FILE provided that FILE is decoded by CODING-SYSTEM.
(defun temp (file filepos coding-system)
  (with-temp-buffer
    (set-buffer-multibyte nil)
    (insert-file-contents-literally file)
    (let ((full (decode-coding-region 1 (point-max) coding-system t))
	  partial)
      (while (and (setq partial (decode-coding-region 1 (1+ filepos)
						      coding-system t))
		  (not (eq (compare-strings full 0 (length partial)
					    partial 0 (length partial))
			   t)))
	  (setq filepos (1+ filepos)))
      (1+ (length partial)))))

If it is too slow, there are a few ways to make it faster.

---
K. Handa
handa@gnu.org