Subject: Re: bug#48324: 27.2; hexl-mode duplicates the UTF-8 BOM

From: Lars Ingebrigtsen <[email-masked]>
Date: Sun, 03 Jul 2022 13:08:04 +0200

Eli Zaretskii <[email-masked]> writes:

> The problem is not just with BOM.  The problem will happen with any
> coding-system that produces prefix and/or suffix bytes when it encodes
> strings.  The FIXME I added mentions ISO-2022 7-bit encodings as
> another example.
>
> And then there are coding-system's with pre-write-conversion, and
> those can produce any additions they like.
>
>> If we had both, then we could strip the BOM from the individual chars,
>> and add one to the front.
>
> AFAIR, what we have now already handles BOM in coding-system's that
> are known to produce a BOM.  See encode-coding-char.

Ah, OK, it uses (coding-system-get coding-system :bom) and then
special-cases utf-8 and -16 to remove the BOM.

Hm...  I guess the only reliable solution across all coding systems is
(like your comment in the code says) to drop the encode-every-char and
try encoding strings, and then see whether the result is short enough.
That could be done somewhat efficiently using a binary search.  I'll
have a go at it...

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no