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
Rich.
--
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