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bug#58168: string-lessp glitches and inconsistencies
From: |
Lars Ingebrigtsen |
Subject: |
bug#58168: string-lessp glitches and inconsistencies |
Date: |
Fri, 30 Sep 2022 15:52:12 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/29.0.50 (gnu/linux) |
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.