Re: I have implemented SAX based XPath Engine
Hi, I have implemented few years ago such a tool, the global design is described here: http://reflex.gforge.inria.fr/saxPatterns.html It is designed to filter SAX streams with XPath-based patterns, but has also useful filters to process text (non-XML) inputs: http://reflex.gforge.inria.fr/tutorial-pipelinesAndFilters.html XPath is rather well supported: position() and last() are supported but it doesn't support preceding:: and preceding-sibling:: axis except in very few circumstances, but I also propose a workaround when filtering a SAX stream (juggling with a local DOM subtree when necessary). To answer to Michael about how predicates are evaluated when reading forward is required, the engine uses a lookahead buffer and goes on reading until the actual predicate becomes evaluable; for that purpose, as explained in the article, the engine uses coroutines that are implemented using threads (one to evaluate the predicate, the other to hold the position in the call stack of the current startElement() event); I think that a finite state machine based on a pull parser would be much more efficient: although the stuff works somewhat well, I have noticed that it runs slowly when I use lots of XPath patterns in a pipeline made of lots of filters, and it can be an issue when reading GB of XML. I also know that things here and there have to be optimized, for example instead of evaluating the entire set of XPath patterns on each event, I could recognize that a subset is irrelevant for a given branch and I should discard them in that branch (but currently it doesn't work like that); there are also things to do better about partial evaluation specifically when comparison operators are involved, for example [count(foo)>9] should exit when 10 <foo>s are met rather than when the 1000000 specimen are read. I have imagined a strategy where the count() function should return something different than a number, a numeric object evaluable several times by the operator that could fetch more data on demand, until the NumberThatIsAtLeast object reach (or not) the expected value. Lots of work in sigth. Of course I will have a look at Santhosh's work :) -- Cordialement, /// (. .) --------ooO--(_)--Ooo-------- | Philippe Poulard | ----------------------------- http://reflex.gforge.inria.fr/ Have the RefleX !
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