On 6/4/2014 1:55 AM, Ian Kelly wrote:

On Jun 3, 2014 11:27 PM, "Steven D'Aprano" <st...@pearwood.info
<mailto:st...@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


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