| Revision as of 17:45, 11 August 2013 edit82.113.121.184 (talk) →Technical details: avoid using ambiguous term "characters" where "code points" would be more applicable← Previous edit |
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#REDIRECT ] |
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{{original research|date=February 2013}} |
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{{Refimprove|date=July 2010}} |
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'''UTF-9''' and '''UTF-18''' (9- and 18-] ], respectively) were two ] joke specifications for encoding Unicode on systems where the ] (nine bit group) is a better fit for the native word size than the ], such as the 36-bit ]. Both encodings were specified in RFC 4042, written by ] (inventor of ]) and released on April 1, 2005. The encodings suffer from a number of flaws and it is confirmed by their author that they were intended as a joke.<ref>{{cite web|url=http://panda.com/mrc/|title=Mark Crispin's Web Page|accessdate=2006-09-17}} Points out ] for two of his RFCs.</ref> |
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{{Rcat shell| |
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However, unlike some of the "specifications" given in other April 1 ], they are actually technically possible to implement, and have in fact been implemented in ] assembly language. They are ''not'' endorsed by the ]. |
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{{R to related topic}} |
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{{Rwh}} |
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==Technical details== |
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}} |
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Like the 8-bit code commonly called ], UTF-9 uses a system of putting an octet in the low 8 ]s of each nonet and using the high bit to indicate continuation. This means that ] and ] characters take one nonet each, the rest of the ] characters take two nonets each and non-BMP code points take three. Code points that require multiple nonets are stored starting with the most significant non-zero nonet. |
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UTF-18 is a fixed length encoding using an 18 bit integer per code point. This allows representation of 4 planes, which are mapped to the 4 planes currently used by ] (planes 0–2 and 14). This means that the two private use planes (15 and 16) and the currently unused planes (3–13) are not supported. The UTF-18 specification does not say why they did not allow surrogates to be used for these code points, though when talking about UTF-16 earlier in the RFC, it says "This transformation format requires complex surrogates to represent code points outside the BMP". After complaining about their complexity, it would have looked a bit hypocritical to use surrogates in their new standard. It is unlikely that planes 3–13 will be assigned by ] any time in the foreseeable future. Thus, UTF-18, like ] and ], guarantees a fixed width for all code points (although not for all glyphs). |
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==Problems== |
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Both specifications suffer from the problem that standard communication protocols are built around octets rather than nonets, and so it would not be possible to exchange text in these formats without further encoding or specially designed protocols (except possibly with protocols based on 1-bit streams). This alone would probably be sufficient reason to consider their use impractical in most cases. However, this would be less of a problem with pure bit-stream communication protocols<!-- do such protocols actually exist? yes but only within low-level framing protocols, mixed with special control bits for synchronization and clock alignments or for error recovery, but not in actual transport protocols or storage formats-->. |
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Furthermore, both UTF-9 and UTF-18 have specific problems of their own: |
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* UTF-9 requires special care when searching, as a shorter sequence can be found at the end of a longer sequence. This means that it is necessary to search backwards before the start of the sequence in order to find the actual start of the sequence, because only the highest bit of each nonet indicates continuation when it is set, but not the start of the sequence (this problem does not occur with UTF-8 where you can safely determine the start of the sequence from a random position without having to scan before the start position). |
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* UTF-18 cannot represent all Unicode code points (although unlike UCS-2 it can represent all the planes that '''currently''' have non-private use code point assignments, i.e. characters in the 4 planes 0, 1, 2, and 14, but not planes 3 though 13, which are '''currently''' unused, nor planes 15 or 16, which are for private use) making it a bad choice for a system that may need to support new languages (or rare ] ]s that are added after the ] fills up) in the future: plane 3 will very likely be used for newer CJK extensions, and other planes may be used other ideographic scripts or pictographic sets still not encoded, so UTF-18 would not support these characters (UTF-18 provides no surrogates mechanism like UTF-16: it not only prohibits the use of the range U+D800–U+DBFF not just for encoding the supported supplementary planes 1, 2, and 14, but also prohibits using them for all other standard planes 3 through 13, and supplementary private use planes 15 and 16). |
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== See also == |
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* ] |
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* ] |
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* ] |
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==External links== |
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* : UTF-9 and UTF-18 Efficient Transformation Formats of Unicode |
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==Notes== |
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<references/> |
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{{character encoding}} |
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{{DEFAULTSORT:Utf-09 And Utf-18}} |
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{{DEFAULTSORT:Utf-09 And Utf-18}} |
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] |
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] |
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] |
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