Custom Validation


I am an experienced programmer but relatively new to ODK and Android
development. I am hoping I can get some guidance from the ODK developer

I need to add some custom validation that is not possible with XPath, to
validate participant ids and to calculate z-scores. The calculations for
both of these in Java is relatively simple however I cannot figure out how
to incorporate them into the software in such a way that they can be called
from the form.

I would welcome any advice/guidance and if possible a short code snippet to
help me understand how best to incorporate this functionality. Ideally, I
would create a separate package of custom functions that could be called
from various places in the form, passing in existing values as parameters.

Any help would be greatly appreciated.

With best wishes,