op:numeric-mod returns incorrect remainders

[aj-stein-nist]

Describe the bug

While testing example Metaschema modules for usnistgov/metaschema#530, I was unable to correctly use mod to compute the remainder of integer division between two numeric values. Debugging indicates there is an issue with the implementation of op:numeric-mod (aliased to mod like in XPath 3.0, this holds for Metapath), potentially in the context that handles decimal precision (review the straightforward implementation code for the function, it is unclear how else there can be a bug, but further investigation needed).

Who is the bug affecting?

Developers of Metaschema modules or Metaschema-enabled tools who need functioning numeric operations (including mod) for advanced constraint usage.

What is affected by this bug?

Any field or flag element in core Metaschema syntax where a module developer can use mod to constrain model requirements (currently, that is most impactful for @target and @test on relevant constraint types).

When does this occur?

Consistently.

How do we replicate the issue?

{What are the steps to reproduce the behavior?

1. Run the following tests.
2. Review unexpected errors because the tool's code is performing normal integer division, not modulo division.

Expected behavior (i.e. solution)

Modulo division would successfully divide [the official examples test vectors for Metapath matching the op:mod-numeric examples inputs and function outputs.

Other Comments

N/A

[aj-stein-nist]

Paired with @david-waltermire and it turns out it is mapping the mod expression to the correct function from the core parser handler driven by the ANTLR grammar (in BuildCSTVisitor). I will update the issue with the broken tests to explain the bug and then make it a fix branch once I am done with the small, tactical PR. 😄