UTF-1
{{Short description|Obsolete multibyte encoding for Unicode}}
{{technical|date=September 2024}}
{{no footnotes|date=September 2024}}
{{Infobox character encoding
| name = UTF-1
| mime = ISO-10646-UTF-1
| alias =
| image =
| caption =
| standard =
| lang = International
| status = Obscure, of mainly historical interest.
| classification = Unicode Transformation Format, extended ASCII, variable-width encoding
| encodes = ISO/IEC 10646 (Unicode)
| extends = US-ASCII
| prev =
| next = UTF-8
}}
UTF-1 is an obsolete method of transforming ISO/IEC 10646/Unicode into a stream of bytes. Its design does not provide self-synchronization, which makes searching for substrings and error recovery difficult. It reuses the ASCII printing characters for multi-byte encodings, making it unsuited for some uses (for instance Unix filenames cannot contain the byte value used for forward slash). UTF-1 is also slow to encode or decode due to its use of division and multiplication by a number which is not a power of 2. Due to these issues, it did not gain acceptance and was quickly replaced by UTF-8.
Design
Similar to UTF-8, UTF-1 is a variable-width encoding that is backwards-compatible with ASCII. Every Unicode code point is represented by either a single byte, or a sequence of two, three, or five bytes. All ASCII code points are a single byte (the code points {{mono|U+0080}} through {{mono|U+009F}} are also single bytes).
UTF-1 does not use the C0 and C1 control codes or the space character in multi-byte encodings: a byte in the range 0–0x20 or 0x7F–0x9F always stands for the corresponding code point. This design with 66 protected characters tried to be ISO/IEC 2022 compatible.
UTF-1 uses "modulo 190" arithmetic (256 − 66 = 190). For comparison, UTF-8 protects all 128 ASCII characters and needs one bit for this, and a second bit to make it self-synchronizing, resulting in "modulo 64" arithmetic ({{nowrap|1=8 − 2 = 6}}; {{nowrap|1=26 = 64}}). BOCU-1 protects only the minimal set required for MIME-compatibility (0x00, 0x07–0x0F, 0x1A–0x1B, and 0x20), resulting in "modulo 243" arithmetic (256 − 13 = 243).
| border="1" cellspacing="3" cellpadding="3" class="wikitable" style="font-family: monospace, monospace"
! code point !! UTF-8 !! UTF-1 | ||
| U+007F | 7F | 7F |
| U+0080 | C2 80 | 80 |
| U+009F | C2 9F | 9F |
| U+00A0 | C2 A0 | A0 A0 |
| U+00BF | C2 BF | A0 BF |
| U+00C0 | C3 80 | A0 C0 |
| U+00FF | C3 BF | A0 FF |
| U+0100 | C4 80 | A1 21 |
| U+015D | C5 9D | A1 7E |
| U+015E | C5 9E | A1 A0 |
| U+01BD | C6 BD | A1 FF |
| U+01BE | C6 BE | A2 21 |
| U+07FF | DF BF | AA 72 |
| U+0800 | E0 A0 80 | AA 73 |
| U+0FFF | E0 BF BF | B5 48 |
| U+1000 | E1 80 80 | B5 49 |
| U+4015 | E4 80 95 | F5 FF |
| U+4016 | E4 80 96 | F6 21 21 |
| U+D7FF | ED 9F BF | F7 2F C3 |
| U+E000 | EE 80 80 | F7 3A 79 |
| U+F8FF | EF A3 BF | F7 5C 3C |
| U+FDD0 | EF B7 90 | F7 62 BA |
| U+FDEF | EF B7 AF | F7 62 D9 |
| U+FEFF | EF BB BF | F7 64 4C |
| U+FFFD | EF BF BD | F7 65 AD |
| U+FFFE | EF BF BE | F7 65 AE |
| U+FFFF | EF BF BF | F7 65 AF |
| U+10000 | F0 90 80 80 | F7 65 B0 |
| U+38E2D | F0 B8 B8 AD | FB FF FF |
| U+38E2E | F0 B8 B8 AE | FC 21 21 21 21 |
| U+FFFFF | F3 BF BF BF | FC 21 37 B2 7A |
| U+100000 | F4 80 80 80 | FC 21 37 B2 7B |
| U+10FFFF | F4 8F BF BF | FC 21 39 6E 6C |
| U+7FFFFFFF | FD BF BF BF BF BF | FD BD 2B B9 40 |
Although modern Unicode ends at U+10FFFF, both UTF-1 and UTF-8 were designed to encode the complete 31 bits of the original Universal Character Set (UCS-4), and the last entry in this table shows this original final code point.
See also
References
- {{cite web |title=The Unicode Standard: Appendix F FSS-UTF |version=Version 1.1 |institution=Unicode, Inc. |url=https://www.unicode.org/versions/Unicode1.1.0/appF.pdf |type=PDF, 768 KiB}}
- {{cite web |title=ISO IR 178: UCS Transformation Format One (UTF-1) |author=ISO/IEC JTC 1/SC2/WG2 |author-link=ISO/IEC JTC 1/SC2/WG2 |date=1993-01-21 |edition=1 |id=Registration number 178 |url=http://kikaku.itscj.ipsj.or.jp/ISO-IR/178.pdf |archive-url=https://web.archive.org/web/20150318032101/http://kikaku.itscj.ipsj.or.jp/ISO-IR/178.pdf |archive-date=2015-03-18 |type=PDF, 256 KiB}}
- {{cite web |author-first=Roman |author-last=Czyborra |title=Unicode Transformation Formats: UTF-8 & Co. |date=1998-11-30 |url=http://czyborra.com/utf/#UTF-1 |access-date=2016-06-07 |url-status=live |archive-url=https://web.archive.org/web/20160607111732/http://czyborra.com/utf/#UTF-1 |archive-date=2016-06-07}}
- {{Cite IETF |rfc=3629 |std=63 |title=UTF-8, a transformation format of ISO 10646 |first1=F. |last1=Yergeau |publisher=IETF |date=November 2003 |doi=10.17487/RFC3629}}
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