UTF-1
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UTF-1 is a way of transforming ISO 10646/Unicode into a stream of bytes. Due to the design it is not possible to resynchronise if decoding starts in the middle of a character (this makes truncation hard among other things) and simple byte-oriented search routines cannot be reliably used with it. UTF-1 is also fairly slow due to its use of division. Due to these issues, UTF-1 never gained wide acceptance and has been almost totally replaced by UTF-8.
[edit] Design
UTF-1 is a multi-byte encoding like UTF-8, a single Unicode code point can be encoded in one, two, three, or five octets. While the ASCII range is encoded as one octet as in UTF-8 the ASCII octets 0x21 - 0x7E (decimal 33 - 126) are also used in UTF-1 multi-byte encodings, therefore UTF-1 is unsuited for many Internet protocols including MIME.
UTF-1 does not use the C0 and C1 control codes in other encodings, any 0x00 - 0x20 (decimal 0 - 32) and any 0x7F - 0x9F (decimal 127 - 159) octet stands for the corresponding code point u+0000 - u+0020 and u+007F - u+009F, respectively. This design with 66 protected octets tried to be ISO 2022 compatible.
The UTF-1 encoding scheme uses "modulo 190" arithmetics (256 − 66 = 190), it was designed to encode the complete 31 bits of the original Universal Character Set (UCS-4). For comparison, UTF-8 protects all 128 ASCII octets, and needs two bits in trail bytes of multi-byte encodings for this purpose, resulting in "modulo 64" (base 64) arithmetics (8 − 2 = 6, 26 = 64). BOCU-1 protects only the 13 octets 0x00, 0x07 - 0x0F, 0x1A - 0x1B, and 0x20 (space), covering the minimal set required for MIME-compatibility, resulting in "modulo 243" arithmetics (256 − 13 = 243).
| codepoint | UTF-16BE | UTF-16LE | UTF-8 | UTF-1 |
|---|---|---|---|---|
| U+007F | 007F | 7F00 | 7F | 7F |
| U+0080 | 0080 | 8000 | C280 | 80 |
| U+009F | 009F | 9F00 | C29F | 9F |
| U+00A0 | 00A0 | A000 | C2A0 | A0A0 |
| U+00BF | 00BF | BF00 | C2BF | A0BF |
| U+00C0 | 00C0 | C000 | C380 | A0C0 |
| U+00FF | 00FF | FF00 | C3BF | A0FF |
| U+0100 | 0100 | 0001 | C480 | A121 |
| U+015D | 015D | 5D01 | C59D | A17E |
| U+015E | 015E | 5E01 | C59E | A1A0 |
| U+01BD | 01BD | BD01 | C6BD | A1FF |
| U+01BE | 01BE | BE01 | C6BE | A221 |
| U+07FF | 07FF | FF07 | DFBF | AA72 |
| U+0800 | 0800 | 0008 | E0A080 | AA73 |
| U+0FFF | 0FFF | FF0F | E0BFBF | B548 |
| U+1000 | 1000 | 0010 | E18080 | B549 |
| U+4015 | 4015 | 1540 | E48095 | F5FF |
| U+4016 | 4016 | 1640 | E48096 | F62121 |
| U+D7FF | D7FF | FFD7 | ED9FBF | F72FC3 |
| U+E000 | E000 | 00E0 | EE8080 | F73A79 |
| U+F8FF | F8FF | FFF8 | EFA3BF | F75C3C |
| U+FDD0 | FDD0 | D0FD | EFB790 | F762BA |
| U+FDEF | FDEF | EFFD | EFB7AF | F762D9 |
| U+FEFF | FEFF | FFFE | EFBBBF | F7644C |
| U+FFFD | FFFD | FDFF | EFBFBD | F765AD |
| U+FFFE | FFFE | FEFF | EFBFBE | F765AE |
| U+FFFF | FFFF | FFFF | EFBFBF | F765AF |
| U+10000 | D800DC00 | 00D800DC | F0908080 | F765B0 |
| U+38E2D | D8A3DE2D | A3D82DDE | F0B8B8AD | FBFFFF |
| U+38E2E | D8A3DE2E | A3D82EDE | F0B8B8AE | FC21212121 |
| U+FFFFF | DBBFDFFF | BFDBFFDF | F3BFBFBF | FC2137B27A |
| U+100000 | DBC0DC00 | C0DB00DC | F4808080 | FC2137B27B |
| U+10FFFF | DBFFDFFF | FFDBFFDF | F48FBFBF | FC21396E6C |
[edit] See also
[edit] References
- ISO IR 178 (PDF, 256 KB, the retired UTF-1 specification)
