Xqueeze: Compact XML Alternative


About Xqueeze

The Concept

Feature Details




Xqueeze Specifications


  1. About this document
    1. Copying
    2. Availability
    3. Terms of Use

  2. xqML language
    1. xqML Symbols
    2. Formal xqML (0.2) Grammar in EBNF notation

  3. Xqueeze Association
    1. Xqueeze Association algorithm (0.1.0)
    2. Xqueeze Association format (0.1.0)

  4. Changes
    1. xqML
    2. Xqueeze Association algorithm
    3. Xqueeze Association format

  5. GNU Free Documentation License

About this document


© 2003 Xqueeze Developers
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in section 5 entitled ``GNU Free Documentation License''.


The latest version of this document can be downloaded from WWW by pointing your browser to:

This document is also available in HTML format at:

Terms of Use

Usage of the Specification in this document for developing an Implementation is subject to the following terms and conditions:

  1. An Implementation advertizing itself to be based on the Specification should support the provisions of the Specification in whole or partially, provided it also fulfils condition 4.
  2. An Implementation advertizing itself to be conforming to the Specification should put into effect the provisions of the Specification in whole.
  3. The Specifications may be modified and re-distributed according to the provisions of GNU FDL (§5).
  4. An Implementation must not advertize itself to be based on or conforming to the Specification if it is based on a modification of the Specification unless such modifications are also distributed under the terms of GNU FDL (§ 5) or compatible license.
  5. Items 1 through 4 of these terms of use only apply to specifications and implementations that are distributed commercially or non-commercially to the public.


The document or documents as designated by the Original Contributor that defines the form, interface and semantics to the technology covered by the contents of the Specification.
An implementation of the specification is a program or set of programs that puts into effect the form, interface and semantics defined by the Specification.
Any deletion, addition or alteration to the form, interface or semantics defined by the Specification.
Original Contributor
The initiator of the Specification.

xqML language

xqML is the binary markup language that is used by Xqueeze to achieve compact document sizes as compared to XML documents. xqML is structurally very similar to XML. The greatest contributors to xqML's compact nature are the elimination of redundant information and representation of XML identifiers (NMTOKENs) whose definitions are available in the DTD/Schema with binary xqML Symbols.

xqML Symbols

xqML Symbols are octect sequences that represent unsigned integers written in MSB (Most Significant Byte first) order. Additionally, the least significant bit of each octet of a symbol, except the last one, should be one. For example, the integer 256 can be a valid xqML Symbol since when written in MSB order, it's bit pattern is [00000001 00000000]. Thus the least significant bit of each octet in the symbol acts as a continuation flag. A 1 indicates that the next octet is a part of the symbol, a 0 indicates the end.

It is trivial that all xqML Symbols must be even numbers. Additionally, one bit of each octet is rendered unusable since it acts as a continuation flag. 16 bit xqML Symbols can represent 16384 different identifiers while 32 bit ones can represent over 268 million different identifiers. A conforming implementation is required to support atleast 16 bit long symbols.

xqML Symbols start from 0x02 (decimal 2) and symbols up to 0xFE (decimal 254) are reserved for special purposes. Higher values are available for generating associations using the Xqueeze Association algorithm. While associating a symbol with an identifier, its type is also stored. Thus, a symbol represents not only the string literal, but also the type of an identifier.

Reserved Symbols

Symbol Value Purpose
Hex Dec
0x04004xqA EL Section
0x06006xqA EE Section
0x08008xqA AT Section
0x0A010xqA AP Section
0x0C012xqA VA Section
0x0E014xqA EN Section
0x10016xqA end marker
0x12018Doctype declaration marker
0x14020CDSection delimiter
0x16022Attribute end marker
0x1E030Markup flag
0x22034Comment start
0x26038Character reference
0x3E062End tag marker
The remaining symbols are unutilized so far

Formal xqML (0.2) Grammar in EBNF notation

   document ::= prolog element Misc* 
     prolog ::= xqMLDecl? Misc* (doctypedecl Misc*)? 
   xqMLDecl ::= '{0x1E}{0x02}xqml-' Num '.' Num ('.' Num)?
       Misc ::= Comment 
doctypedecl ::= '{0x1E}{0x12}' DoctypeName | xqA
DoctypeName ::= Char*
    element ::= (EE_STag | STag) content ETag? 
    EE_STag ::= '{0x1E}' EESymbol (attribute)* 
       STag ::= '{0x1E}' ELSymbol (attribute)* 
    content ::= Char* ((element | Reference | CDSect | Comment) Char*)* 
  attribute ::= ATAttribute | APAttribute
ATAttribute ::= '{0x1E}' ATSymbol Char* (Reference Char*)* '{0x1E}{0x16}'
APAttribute ::= '{0x1E}' APSymbol VASymbol 
  Reference ::= EntityRef | CharRef 
  EntityRef ::= '{0x1E}' ENSymbol 
    CharRef ::= '{0x1E}{0x26}' Num '{0x1E}'
     CDSect ::= CDDelim Char* CDDelim 
    CDDelim ::= '{0x1E}{0x14}' 
    Comment ::= '{0x1E}{0x22}' Char* '{0x1E}'
       ETag ::= '{0x1E}{0x3E}' ElementsToClose
       Char ::= [^RS].
        Num ::= [0-9]+ 

Note: Comments are now deprecated and may be removed from future versions.


Figures enclosed within braces ({})
are hex codes for the value of an xqML Symbol that should occur within.
is a string literal identifying a document type
is the ASCII control character 0x1E (Record Separator). Thus, Char matches any character that is not 0x1E
is the Xqueeze Association (without preamble)
is an xqML Symbol of the type ``element''
is an xqML Symbol of the type ``empty element''
is an xqML Symbol of the type ``attribute''
is an xqML Symbol of the type ``attribute with predefined value''
is an xqML Symbol of the type ``attribute value''
is an xqML Symbol of the type ``entity reference''
is an octet to be interpreted as an unsigned integer.

Xqueeze Association

Xqueeze uses an association between symbols and their corresponding XML identifiers and types as defined in a specification (DTD/Schema). This enables representation of known identifiers in the markup with symbols. Associating the type of an identifier along with it's name also makes it easy to various structural units of the document without having to use too many special characters and character-combinations.

Xqueeze Association algorithm (0.1)

This is the allgorithm that is used to map the identifiers found in a DTD/Schema to xqML Symbols. The steps of the algorithm are:
  1. collect all Element identifiers together, discarding duplicates
  2. collect all Empty Element identifiers together, discarding duplicates
  3. collect all Attribute identifiers together, discarding duplicates
  4. collect all identifiers together for Attributes that have predefined values, discarding duplicates
  5. collect all Attribute Value identifiers together, discarding duplicates
  6. collect all Entity References together, discarding duplicates
  7. sort the above collections alphabetically and incrementally assign them symbols in the order enumerated in steps 1 to 6

This simple algorithm assures that the assignments would remain the same even if a particular specification (DTD/Schema) has slight variations in the way it's written in the generator's and consumer's copies, as long as both define the same things.

Xqueeze Association format (0.1)

Xqueeze associations are represented in a format that itself is quite compact and uses xqML Symbols themselves. The specification begins with an optional prolog that begins with the sequence <{0x02}. The prolog continues upto the next occurence of <. It is followed by sections that are denoted by <, followed by an xqML Symbol. The symbols for various sections are:
Empty Elements
Attributes with predefined values
Attribute values
Entity References

Within each section, individual entries are listed as <, followed by the a symbol, followed by the string that the symbol corresponds to. Thus the type of a symbol is determined by the section it is present in. A single association specification may have multiple sections of the same type and the ordering is not restricted. However, sections can't be nested.

The end of specifications is denoted by the sequence <{0x10}. This structure enables inline specification of the symbols associations, if required by a document.



Version 0.2

  • xqML Symbol '{0x1E}' replaces '<' for the latter's role in xqML markup
  • Attribute is split into ATAttribute and APAttribute, together referred as attribute.
  • ATAttribute can contain Reference.
  • ATAttribute is terminated by `{0x1E}{0x16}' instead of `<'
  • CharRef starts with `{0x1E}{0x26}' instead of `&{0x26}'
  • CharRef ends with `{0x1E}'. Earlier there was no end-marker
  • Comment ends with `{0x1E}' instead of ETag?
  • Comment is deprecated
  • Char is a terminal that matches any printable character
  • Num does not contain `.'

Version 0.1

First Release

Xqueeze Association algorithm

Version 0.1

First Release

Xqueeze Association algorithm

Version 0.1

First Release

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