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bug#46933: Possible bugs in filepos-to-bufferpos / bufferpos-to-filepos
From: |
handa |
Subject: |
bug#46933: Possible bugs in filepos-to-bufferpos / bufferpos-to-filepos |
Date: |
Sun, 04 Apr 2021 01:12:06 +0900 |
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