bug#58168: string-lessp glitches and inconsistencies

[Lars Ingebrigtsen <larsi@gnus.org>]

Mattias Engdegård <mattias.engdegard@gmail.com> writes:

> 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.

You also have

(string 4194176)
=> "\200"
"\x80"
=> "\200"

which are kinda equal in some ways, and not in other ways.

> 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
>
> which instead moves unibyte raw bytes to after the multibyte raw range. This has the same memcmp benefit as alternative B, but may be slightly faster for ub-mb comparisons since only unibyte 80..FF need to be remapped.

I think A makes the most intuitive sense, somehow.  But perhaps my
intuition is off.