The Compatibility Encoding Scheme for UTF-16: 8-Bit (CESU-8) is a variant of UTF-8 that is described in Unicode Technical Report #26 . A Unicode code point from the Basic Multilingual Plane (BMP), i.e. a code point in the range U+0000 to U+FFFF, is encoded in the same way as in UTF-8. A Unicode supplementary character, i.e. a code point in the range U+10000 to U+10FFFF, is first represented as a surrogate pair, like in UTF-16, and then each surrogate code point is encoded in UTF-8. Therefore, CESU-8 needs six bytes (3 bytes per surrogate) for each Unicode supplementary character while UTF-8 needs only four. Each CESU-8 character code (1, 2, or 3 bytes) can be converted to exactly one UTF-16 code unit (2 bytes).
The encoding of Unicode supplementary characters works out to
11101101 1010yyyy 10xxxxxx 11101101 1011xxxx 10xxxxxx (yyyy represents the top five bits of the character minus one i.e. U+10**** becomes 1111, U+01**** becomes 0000, x represents the remaining bits of the character).
CESU-8 is not an official part of the Unicode Standard, because Unicode Technical Reports are informative documents only. It should be used exclusively for internal processing and never for external data exchange.
CESU-8 is similar to Java's Modified UTF-8 but does not have the special encoding of the NUL character (U+0000).
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