Re: [I18nrp] Mappings for IDNA2008 ?

Andrew Sullivan <ajs@anvilwalrusden.com> Wed, 13 February 2019 10:56 UTC

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Date: Wed, 13 Feb 2019 05:56:15 -0500
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Subject: Re: [I18nrp] Mappings for IDNA2008 ?
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Surely the assumption fails because case folding and mapping of characters 
I  IDNA2003 is not always idempotent -- the very thing 2008 set out to fix?

A
--
Andrew Sullivan
Please excuse my clumbsy thums.

On February 13, 2019 05:31:25 "Martin J. Dürst" <duerst@it.aoyama.ac.jp>; wrote:

> Hello Patrik, others,
>
> On 2019/02/13 17:25, Patrik Fältström wrote:
>>
>>
>>> On 13 Feb 2019, at 09:19, Asmus Freytag (c) <asmusf@ix.netcom.com>; wrote:
>>>
>>>> On 2/12/2019 11:46 PM, Patrik Fältström wrote:
>>>>
>>>>> On 13 Feb 2019, at 08:18, Martin J. Dürst <duerst@it.aoyama.ac.jp>; wrote:
>>>>>
>>>>> If f(x) is the generic mapping, and f'(x) is the mapping of the
>>>>> exceptions, then f(f'(x)), which applies f' *before* f, should do the job.
>>>> This works in some cases and not in other cases.
>>> Examples?
>>
>> This is math. :-)
>>
>> If f(x)=x’ and in the locale used f(x) should be x and not x’, you have 
>> dependencies between f(x) and f’(x) if you want f(f’(x)) to be x’ and 
>> f(f”(x)) to be x.
>
> Well, I have to admit that I didn't explicitly talk about that case in
> my 'proof'. For functions in general (e.g. a function such as f(x) =
> (x+7) mod 15), you are right.
>
> But f(x) (the general mapping function) is idempotent (i.e. f(x) =
> f(f(x)), which means that you can map as many times as you want, you
> always get the same result). This is also true for any locale-dependent
> mapping function. If it were not the case, human users would be very
> surprised: a maps to A, but then A maps to a: That wouldn't make any sense.
>
> Also, and even more important, for the general mapping function, if x is
> allowed in IDNs, then f(x) = x. This may not be true for a
> locale-dependent mapping function (i.e. it would be possible for a
> German mapping function to map (Hungarian) ȁ to ä, although that's not
> needed because German keyboards won't produce ȁ in the first place).
>
> So if f(x)=x', that would mean that x is not allowed in IDNs. That would
> mean that it is impossible that in a specific locale, the desired result
> of f(f'(x)) is x. It may be different from x', i.e. y, but we can easily
> get this by defining that f'(x) = y.
>
> So applying the (in most if not all cases very small) locale-specific
> mapping before the general mapping always will work.
>
> Regards,   Martin.
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