Re: [idn] Re: stringprep and unassigned code points

[David Hopwood <david.hopwood@zetnet.co.uk>]

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Yves Arrouye wrote:
> Honestly, I do not see any other way than to upgrade everybody to a newer
> Nameprep when the mapping tables change for new code points.

New unicameral scripts (without case distinctions), that have no NFC/NFKC
mappings can be supported without any changes to nameprep. That leaves
bicameral scripts, and scripts that have NFC mappings, i.e. they have
both precomposed and decomposed characters (compatibility mappings for
new scripts are extremely unlikely). In order for those scripts to
be used at all, the client's operating system will need to be updated to
support a new version of Unicode. If the OS provides case folding and
normalisation functions [*], then old applications calling the new
functions will work fine. Even if the OS doesn't provide all of the
needed functionality, it could do in new versions, and an IDN library
could use the OS whereever possible, falling back to its own
implementation otherwise.

Of course this requires that nameprep is defined strictly in terms of
standard Unicode mappings and properties. That is possible if we use NFC,
and define the disallowed characters in terms of Unicode character
properties (note that no "additional mappings" are needed to make
casefold o NFC idempotent, I think; I've recently asked on the Unicode
mailing list to confirm that).

[*] Win32 does for NFC normalisation, I'm not sure about case folding.

- -- 
David Hopwood <david.hopwood@zetnet.co.uk>

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