Re: UTF-32

[Eli Zaretskii]

> From: Ulrich Mueller <ulm@gentoo.org>
> Cc: emacs-devel@gnu.org
> Date: Fri, 07 Jul 2023 12:34:17 +0200
> 
> >>>>> On Fri, 07 Jul 2023, Eli Zaretskii wrote:
> 
> >> https://www.unicode.org/reports/tr19/tr19-9.html
> 
> > Yes, I know.  Not sure why you posted this, though.  If you are saying
> > that this somehow contradicts what I wrote above, please elaborate,
> > because I don't see the contradiction.
> 
> I don't understand how "codepoints that are wider than 32 bits"
> are related to UTF-32.

Because using UTF-32 for codepoints that fit in 32 bits makes very
little sense.  See, e.g., https://en.wikipedia.org/wiki/UTF-32.

> UTF-8, UTF-16, and UTF-32 all encode the same
> repertoire (U+0000 to U+10FFFF).

UTF-8 is identical with the codepoints as long as the codepoints are
plain-ASCII.  UTF-16 is identical with the codepoints as long as the
codepoints are inside the BMP.  UTF-32 is identical with the
codepoints as long as the codepoints don't exceed 32 bits.  Since
Unicode doesn't exceed 32 bits, and Emacs extensions of the Unicode
codepoint space also don't exceed 32 bits, Emacs doesn't need to use
UTF-32.

> Emacs knows about UTF-8 and UTF-16 but not about UTF-32. Is it an
> unreasonable question to ask why that is so? (Just out of interest,
> I do not challenge it, and I have no need for UTF-32.)

The question is fine, and I think I answered it.  Did I miss some
aspects of the question?