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bug#37580: 26.3; setting buffer as unibyte temporarily may change buffer
From: |
Eli Zaretskii |
Subject: |
bug#37580: 26.3; setting buffer as unibyte temporarily may change buffer contents |
Date: |
Sat, 05 Oct 2019 21:56:36 +0300 |
> From: ynyaaa@gmail.com
> Cc: 37580@debbugs.gnu.org
> Date: Sun, 06 Oct 2019 02:18:08 +0900
>
> Sometimes I find broken utf-8 texts on the Internet.
> Some characters are split into surrogate pairs, and each surrogate
> character is encoded as if it is a normal BMP character.
>
> utf-8 coding system does not decode such sequences.
> Changing multibyte-ness converts them to surrogate characters.
> And encode-decode process with utf-16be outputs the intended characeters.
>
> Suppose the character is #x10000,
> the correspoding pair is (#xD800 #xDC00).
> The miss-encoded sequence is:
> (encode-coding-string "\xD800\xDC00" 'utf-8)
> => "\355\240\200\355\260\200"
>
> It is not decoded with utf-8.
> (decode-coding-string (encode-coding-string "\xD800\xDC00" 'utf-8)
> 'utf-8)
> => "\355\240\200\355\260\200"
>
> Changing multibyte-ness, the sequence is converted into surrogate
> characters.
> (with-temp-buffer
> (insert (encode-coding-string "\xD800\xDC00" 'utf-8))
> (set-buffer-multibyte nil)
> (set-buffer-multibyte t)
> (buffer-string))
> => "\xD800\xDC00"
>
> The surrogate pair can be converted into the original character.
> (decode-coding-string (encode-coding-string "\xD800\xDC00" 'utf-16be)
> 'utf-16be)
> => "\x10000"
So where's the problem in all this? AFAIU, you describe a sequence of
actions that successfully recovers text in an obscure situation.
I think the problem is that you enable undo. So in that case, just
don't do that.