Re: [Libguestfs] [PATCH v11 5/6] mllib: add XPath helper xpath_get_nodes()

The subject says ‘xpath_get_nodes()‘, but this function doesn't actually take a unit parameter, so it's better to drop ‘()’. On Thu, Oct 05, 2017 at 04:58:29PM +0200, Cédric Bosdonnat wrote: > + > +let xpath_get_nodes xpathctx expr = > + let obj = Xml.xpath_eval_expression xpathctx expr in > + let nodes = ref [] in > + for i = 0 to Xml.xpathobj_nr_nodes obj - 1 do > + let node = Xml.xpathobj_node obj i in > + push_back nodes node > + done; > + !nodes ‘push_back’ is unfortunately O(n) and no tail recursive, and so the whole loop is O(n²). It's going to be much more efficient therefore to build the list up in reverse and reverse it at the end: for ... ... push_front node nodes done; List.rev !nodes Despite that problem this seems like quite a useful function, it would be nice to extend this commit so it changes existing code to use the function. Grepping the code for ‘Xml.xpathobj_nr_nodes.*- 1’ shows some candidates. In v2v/output_libvirt.ml we presently have: (* Get guest/features/* nodes. *) let obj = Xml.xpath_eval_expression xpathctx "features/*" in let features = ref [] in for i = 0 to Xml.xpathobj_nr_nodes obj - 1 do let feature_node = Xml.xpathobj_node obj i in let feature_name = Xml.node_name feature_node in push_front feature_name features done; !features I think this can be rewritten as: (* Get guest/features/* nodes. *) let features = xpath_get_nodes xpathctx "features/*" in let features = List.map Xml.node_name features in