Add randomize(nodeset) and randomize(nodeset, seed) functions

[lognaturel]

See getodk/xforms-spec#51 (comment)

• Use Fisher-Yates for shuffling
• If a seed is provided, that seed is passed when building a Random object. nextInt is used to get random values
• Tests and expected values should be shared on the XForms spec issue so other implementations can verify their outcomes are the same

[lognaturel]

I started exploring this a bit and it's trickier than I originally imagined. ItemsetBinding is built on a path expression, not an arbitrary expression containing function calls.

[lognaturel]

Here's a quick and dirty first stab at the function itself: https://github.com/lognaturel/javarosa/tree/randomize To be clear, the real challenge comes in actually using it.

The intended usage is something like

<select1 ref="/form/state">
    <itemset nodeset="randomize(instance('choices')/states/state)">
        <label ref="name"/>
        <value ref="value"/>
    </itemset>
</select1>

randomize.xml.txt

parseItemset in XFormParser currently fails because there are places where it expects XPath paths but is getting XPath expressions. Overall, ItemsetBinding is very much built on the assumption the nodeset attribute is populated by a reference to a nodeset, not something that evaluates to a nodeset.

[lognaturel]

Interestingly, someone just asked a question about using the result of two repeats as the options for a select -- https://forum.opendatakit.org/t/two-repeating-group-and-filtering-specific-field-data/12452. This should be possible by setting the nodeset to something like "/my-form/people1 | /my-form/people2" (| is the union operator) but we run into the same limitation.