1) Yep - that's a scalar UDF alright 😀

2) At least it doesn't have ancillary data access!

3) Looks like some kind of funky iterative logic that might be able to take advantage of a Numbers table

4) How is it used in the query? If in the WHERE clause you are pretty much guaranteed to be hosed.

5) I wonder if a persisted calculated column would be workable here...