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

[Richard W.M. Jones]

On Sun, Oct 08, 2017 at 08:15:51PM +0100, Richard W.M. Jones wrote:
> 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

This patch can be pushed separately if we sort this out.

Rich.

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