Standard Compression Scheme for Unicode

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The Standard Compression Scheme for Unicode (SCSU)^[1] is a Unicode Technical Standard for reducing the number of bytes needed to represent Unicode text, especially if that text uses mostly characters from one or a small number of per-language character blocks. It does so by dynamically mapping values in the range 128–255 to offsets within particular blocks of 128 characters. The initial conditions of the encoder mean that existing strings in ASCII and ISO-8859-1 that do not contain C0 control codes other than NULL TAB CR and LF can be treated as SCSU strings. Since most alphabets do reside in blocks of contiguous Unicode codepoints, texts that use small alphabets and either ASCII punctuation or punctuation that fits within the window for the main alphabet can be encoded at one byte per character (plus setup overhead, which for common languages is often only 1 byte), most other punctuation can be encoded at 2 bytes per symbol through non-locking shifts. SCSU can also switch to UTF-16 internally to handle non-alphabetic languages.