Jianping wrote:
> One thing needs to clarify here is that there is no four byte encoding in
> UTF-8S proposal and four byte encoding is illegal but not irregular. As
> everything in UTF-8S is perfect match to UTF-16, any blame to this proposal
> also applies to UTF-16 encoding form.
Well after a couple months arguing about this, it is nice to have
this little detail drop into place. Perhaps in another couple
of months we could have a complete specification, and then
restart the argument.
[BTW, as Peter has recently noted: 4 lines of new content, quoting my
234 lines of content, with no commentary interspersed. This rampant
failure to edit reply-to's is threatening to bring the wrath of
Sarasvati back down on the list, folks.]
So, given the new information that the four byte form is illegal, not irregular,
in UTF-8s, here is a revised summary of UTF-8s:
===========================================================
Case III. Code points U-0000D800..U-0000DFFF included
in the UTF's, using UTF-8s "The vision provided
by the Oracle."
code point UTF-8s UTF-16 UTF-32
a. 00000000 <=> 00 0000 00000000
b. 0000D700 <=> ED 9F BF D7FF 0000D7FF
g. 0000E000 <=> EE 80 80 E000 0000E000
h. 0000FFFF <=> EF BF BF FFFF 0000FFFF
i. 00010000 <=> ED A0 80 ED B0 80 D800 DC00 00010000
j. 0010FFFF <=> ED AF BF ED BF BF DBFF DFFF 0010FFFF
Round-tripping isolated surrogate code points:
c. 0000D800 <=> ED A0 80 D800 0000D800
d. 0000DBFF <=> ED AF BF DBFF 0000DBFF
e. 0000DC00 <=> ED B0 80 DC00 0000DC00
f. 0000DFFF <=> EF BF BF DFFF 0000DFFF
Code point sequences that do not round-trip from all UTF code
unit sequences. (Could be termed "irregular code point
sequences" --Ken):
k. 0000D800 0000DC00 => ED A0 80 ED B0 80 D800 DC00 0000D800 0000DC00
l. 0000DBFF 0000DFFF => ED AF BF ED BF BF DBFF DFFF 0000DBFF 0000DFFF
=============================================================
What Jianping is saying now is that F0..F4 are illegal as
initiators in UTF-8s. (They are legal initiators in UTF-8.)
Also, judging from his statement that "everything in UTF-8S is
perfect match to UTF-16", it is quite clear that UTF-8s does
*not* meet the Unicode Standard's definition of a UTF. To be
a UTF, it has to be a reversible transform of code points (or
Unicode scalar values -- there is some argument about which).
But UTF-8s is designed and conceived as a CODE UNIT TRANSFORM
of UTF-16. (A "CUT", not a "UTF".)
Basically, instead of starting with the code points, and deriving
the three UTF's, for UTF-8s you start with UTF-16 and derive
UTF-8s directly from it. (This is why I have been pounding on
the point that in order to understand the Oracle proposal, you
have to think in terms of the UTF-16 <==> UTF-8 convertors,
rather than in terms of the definitional UTF's.)
In other words, while others are seeing:
U-00010000 ==> ED A0 80 ED B0 80 in UTF-8s
==> D800 DC00 in UTF-16
Oracle is seeing:
(D800)(DC00) <==> (ED A0 80)(ED B0 80)
and pointing out the tremendous simplicity of the fact that
a code point, err... code unit in UTF-16 always corresponds
*exactly* to a code point, errr... well a 1-, 2-, or 3- code
unit sequence in UTF-8s that always corresponds to a, umm..
character, well, sort of.
Now, perhaps Jianping will care to step in an clarify how UTF-32
fits in this picture. How, for example, are the irregular UTF-32
sequences in k and l above to be treated? As I have indicated?
(in which case, as Peter points out, there is an ambiguity in
the interpretation of any 6-byte UTF-8s representation) Or in
some other manner? And if so, how so?
--Ken
This archive was generated by hypermail 2.1.2 : Fri Jul 06 2001 - 00:17:18 EDT