18.5.10.Adding New Languages And Encodings Into Virtuoso
There are too many languages to be able to support them all by default so Virtuoso is user extensible in this respect. The built-in 'x-any' language supports most languages to a degree, but it is not the optimum solution for some specific languages or if you want to perform a words' normalization to make text search more effective. To make Virtuoso extensible, language-specific functions are organized into language handlers , and handlers are organized in hierarchical trees. Every handler contains pointers to such functions as "count words in given string", "call given callback once for every word in the string" etc.
XML documents and SQL procedures may identify languages by their
names, for example by value of xml:lang attribute,
content_language argument of built-in
functions, or by
option etc... Every language handler defines up to two names of the
language it supports, one matching ISO 639 regulations (e.g. 'en'),
and one matching RFC 1766 (e.g. 'en-UK'). When Virtuoso finds a
match to the language name specified, it searches through the an
internal hash-table. If the name is unknown, the 'x-any' handler
will be returned as a default.
Custom language handlers should contain a pointer to a more generic handler, e.g. to the handler, Handler may have NULLs stored instead of pointers to required functions, these NULLs will be replaced with pointers to generic handler's functions automatically when the custom handler will be activated.
There are two trees of language handlers in current version of Virtuoso. "Main" tree starts from 'x-any' root and contains handlers of languages used in documents, another tree starts from 'x-ftq-x-any' root contains handlers of Free Text query ('ftq') languages. The difference is in handling of wildcard characters: query string 'hello, wo*ld' consists of two "words", 'hello' and 'wo*ld', and 'x-ftq-x-any' will properly locate them, but 'x-any' handler will report three words -- 'hello', 'wo' and 'ld', because it knows nothing about special meaning of '*' in query strings. That is why every handler may contain a pointer to a handler of its own query language.
In addition to plain language handlers, it's possible to add handlers of "encoded language" They are useful if you have large number of documents in some particular encoding and speed of free text indexing is critical for your applications. While usual handlers deal with Unicode data, and it requires data to be decoded before processing, functions of "encoded language" handler may accept buffers of encoded text, eliminating decoding.
The OpenLink Virtuoso Server distribution contains sources of sample language handler for 'en-UK' language. The difference between 'x-any' handler and this one is in handling of abbreviations and numbers. 'en-UK' handler will read phrase '$3.54 per sq.inch.' as the sequence of words '3.54', 'per' and 'sq.inch', instead of sequence '54', 'per', 'sq' and 'inch' that 'x-any' will read. The generic 'x-any' handler has no specific rules for dealing with the "decimal point" because in many scripts "decimal comma" is used, thus '3.54' will be processed as pair of words '3' and '54', but '3' will be ignored in many cases as noise word due to its 1-character length.
In addition to the language extension interface, Virtuoso provides an eh_load_handler function to add new encodings, but it should be used solely for multi-character encodings which cannot be supported through the usual Virtuoso International Character Support . If an encoding was created by the CHARSET_DEFINE function, Virtuoso can build special lookup tables for very fast text translation from Unicode to the encoding, thus you are not likely to gain in performance by writing your own C code, but some applications will know nothing about your encoding because they will check only the SYS_CHARSETS system table.