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From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: Micro Python -- a lean and efficient implementation of Python 3
Date: Wed, 04 Jun 2014 03:00:05 -0400
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On 6/4/2014 1:55 AM, Ian Kelly wrote:
>
> On Jun 3, 2014 11:27 PM, "Steven D'Aprano" <steve@pearwood.info
> <mailto:steve@pearwood.info>> wrote:
>  > For technical reasons which I don't fully understand, Unicode only
>  > uses 21 of those 32 bits, giving a total of 1114112 available code
>  > points.
>
> I think mainly it's to accommodate UTF-16. The surrogate pair scheme is
> sufficient to encode up to 16 supplementary planes, so if Unicode were
> allowed to grow any larger than that, UTF-16 would no longer be able to
> encode all codepoints.

I believe the original utf-8 used up to 6 bytes per char to encode 2**32 
potential chars. Just 4 bytes limits to 2**21 and for whatever reason 
(easier decoding?), utf-8 was revised down (unusual ;-).

-- 
Terry Jan Reedy