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   Re: Character Range: surrogate blocks

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  • From: John Cowan <cowan@locke.ccil.org>
  • To: xml-dev@ic.ac.uk
  • Date: Sat, 17 Oct 1998 18:57:26 -0400 (EDT)

Richard Emberson scripsit:

> To extend the available characters in Unicode one
> can use to 16 bit characters with surrogate blocks.

Well, no.  One uses the 16-bit *codes* in the surrogate blocks.
They aren't *characters* (what in Unicode is called *abstract characters*)
at all.

> Now in production rule #2 titled Character Range 
> surrogate blocks are explicitly excluded (along 
> with FFFF and FFFE). 

Correct, because these codes do not represent characters.

> Does that mean that if one were reading a character
> stream that included characters not in the basic 
> set of Unicode characters (those not using surrogate
> blocks) that it would be a wellformedness violation?

I don't understand this question.  Is there an extra "not" somewhere?

> There are the extra, beyond 16-bit, characters specified
> by the spec in production rule #2 as "[x10000-#x10FFFF]".
> Is this how Unicode characters that use the surrogate
> blocks get represented in an XML document?

In UTF-16 (= Unicode) representation, yes.  In UTF-8 representation
they are represented as the appropriate 4-byte sequences.

> Is there
> an algorithm for the convertions defined somewhere? 

Same as Unicode or ISO 10646 UTF-16, namely: a codepoint in the
range D800-DBFF followed by one in the range DC00-DFFF represents
the character whose code is (first-D800)*400+(second-DC00)+10000
(hex arithmetic).

> Short of getting a copy of the Unicode 2.0 spec, is there 
> anywhere where the conversion algorithm is documented?


> Why was it decided to exclude the uses of surrogate 
> block-base Unicode characters within XML documents?

What is excluded is surrogate characters appearing in unpaired form.
These could be generated, e.g. by UTF-8 ED A0 80 = U+D800.

John Cowan					cowan@ccil.org
		e'osai ko sarji la lojban.

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