UTF-1


UTF-1 is a method of transforming ISO 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. 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. ASCII is supported via the single-byte encodings, which, unlike those of UTF-8, also include the non-ASCII code points U+0080 through U+009F.
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 2022 compatible.
UTF-1 uses "modulo 190" arithmetic. 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. BOCU-1 protects only the minimal set required for MIME-compatibility, resulting in "modulo 243" arithmetic.
code pointUTF-8UTF-1
U+007F7F7F
U+0080C2 8080
U+009FC2 9F9F
U+00A0C2 A0A0 A0
U+00BFC2 BFA0 BF
U+00C0C3 80A0 C0
U+00FFC3 BFA0 FF
U+0100C4 80A1 21
U+015DC5 9DA1 7E
U+015EC5 9EA1 A0
U+01BDC6 BDA1 FF
U+01BEC6 BEA2 21
U+07FFDF BFAA 72
U+0800E0 A0 80AA 73
U+0FFFE0 BF BFB5 48
U+1000E1 80 80B5 49
U+4015E4 80 95F5 FF
U+4016E4 80 96F6 21 21
U+D7FFED 9F BFF7 2F C3
U+E000EE 80 80F7 3A 79
U+F8FFEF A3 BFF7 5C 3C
U+FDD0EF B7 90F7 62 BA
U+FDEFEF B7 AFF7 62 D9
U+FEFFEF BB BFF7 64 4C
U+FFFDEF BF BDF7 65 AD
U+FFFEEF BF BEF7 65 AE
U+FFFFEF BF BFF7 65 AF
U+10000F0 90 80 80F7 65 B0
U+38E2DF0 B8 B8 ADFB FF FF
U+38E2EF0 B8 B8 AEFC 21 21 21 21
U+FFFFFF3 BF BF BFFC 21 37 B2 7A
U+100000F4 80 80 80FC 21 37 B2 7B
U+10FFFFF4 8F BF BFFC 21 39 6E 6C
U+7FFFFFFFFD BF BF BF BF BFFD 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, and the last entry in this table shows this original final code point.