Re: UTF-32

[Ulrich Mueller]

>>>>> On Fri, 07 Jul 2023, Eli Zaretskii wrote:

>> 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.

Sure, it is a wasteful encoding, and it has issues with byte ordering
(but the same is true for UTF-16).

>> 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?

The previous discussion was in the context of _file_ coding systems.
Emacs cannot read or write files encoded in UTF-32, correct?

So probably such files just don't exist, or somebody would have
implemented it in the meantime? (OTOH, GNU Recode knows about UTF-32,
UTF-32BE, and UTF-32LE. No UTF-32NUXI, though. :)