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

[Richard W.M. Jones]

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.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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