003

witness{
 📎 3. Basic Concepts

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    001 EXI achieves broad generality, flexibility, and performance, by unifying
    002 concepts form formal language theory and information theory into a single, 
    003 relatively simple algorithm. The algorithm uses a grammar to determine what
    004 is likely to occur at any given point in an XML document and encodes the
    005 most likely alternatives in fewer bits. The fully generalized algorithm
    006 works for any language that can be described by a grammar (e.g. XML, JAVA,
    007 HTTP, etc.); however, EXI is optimized specifically for XML languages.
    008
    009 The built-in EXI grammars accept any XML document or fragment and may be
    010 augmented with productions derived from schemas or other sources of
    011 information about what is likely to occur in a set of XML documents.
    012 When schemas are used, EXI also supports a user-customizable set of
    013 Datatype Representations for efficiently representing typed values.
    014 Though use of any schema languages including XML Schemas [XML Schema
    015 Structures] [XML Schema Datatypes], RELAX NG schemas [ISO/IEC 19757-
    016 2-2008], DTDs [XML 1.0] [XML 1.1] is permitted, EXI grammars and 
    017 datatype representations need to be given bindings for each schema 
    018 language used. This specification only defines how EXI grammars
    019 and datatype representations relate to XNL Schema.
    020
    021 The EXI stream encoder uses the grammar to map a stream of XNL information
    022 items onto a smaller, lower entropy, stream of events.
    023
    024 The EXI stream encoder then represents the stream of events using a set of
    025 simple variable length codes called event codes. Event codes are similar
    026 to Huffman codes [Huffman Coding], but are much simpler to compute and 
    027 maintain. They are encoded directly as a sequence of values, or if 
    028 additional compression is desired, they are passed to the EXI compression
    029 algorithm, which replaces frequently occuring event patterns to further
    030 reduce size.

}