bug#58168: string-lessp glitches and inconsistencies

[Eli Zaretskii]

> From: Mattias Engdegård <mattias.engdegard@gmail.com>
> Date: Thu, 29 Sep 2022 18:24:04 +0200
> 
> We really want string< to be consistent with string= and itself since this is 
> fundamental for string ordering in searching and sorting applications.
> This means that for any pair of strings A and B, we should either have A<B, 
> B<A or A=B.
> 
> Unfortunately:
> 
>   (let* ((a "ü")
>          (b "\xfc"))
>     (list (string= a b)
>           (string< a b)
>           (string< b a)))
> => (nil nil nil)
> 
> because string< considers the unibyte raw byte 0xFC and the multibyte char 
> U+00FC to be the same, but string= thinks they are different.

Why do we care?  Unibyte strings should never be compared with
multibyte, unless they are both pure-ASCII.

> So, what can be done? The current string< implementation uses the character 
> order
> 
>  ASCII < ub raw 80..FF = mb U+0080..U+00FF < U+0100..10FFFF < mb raw 80..FF
> 
> in conflict with string= which unifies unibyte and multibyte ASCII but not 
> raw bytes and Latin-1.

It would be unimaginable to unify raw bytes with Latin-1.  Raw bytes
are not Latin-1 characters, they can stand for any characters, or for
no characters at all.

> It suggests the following alternative collation orders:
> 
> A. ASCII < ub raw 80..FF < mb U+0080..10FFFF < mb raw 80..FF
> 
> which puts all non-ASCII multibyte chars after unibyte.
> 
> B. ASCII < ub raw 80..FF < mb raw 80..FF < mb U+0080..10FFFF
> 
> which inserts multibyte raw bytes after the unibyte ones, permitting any 
> ub-ub and mb-mb comparisons to be made using memcmp, and a slow decoding loop 
> only required for unibyte against non-ASCII multibyte strings.
> 
> C. ASCII < mb U+0080..10FFFF < mb raw 80..FF < ub raw 80..FF

Neither, IMNSHO.  Unibyte characters don't belong to this order.  They
should be converted to multibyte representation to be sensibly
comparable.

> Otherwise, I'll go with B or C, depending on what the resulting code looks 
> like.

Please don't.  Let's first decide that we want to change this, and
what are the reasons for that.  Theoretical "impurity" doesn't count,
IMO.