[Rd] as.roman upper limit

[Jonathan Carroll]

I was recently participating in a coding challenge which involved
converting integers to Roman numerals. I knew R offers this
functionality already via as.roman() and hoped to leverage that for a
quick solution, but was surprised that one of the challenge's tests
failed; conversion of the number 3999, which should result in
"MMMCMXCIX". In R, that produces NA.

I looked into the source and documentation and it's clear that an
upper limit of 3899 is enforced in several places, and documented as
such. I detailed some of these explorations on my blog [0]. Ben Bolker
traced the first implementation (or at the least the creation of
src/library/utils/R/roman.R) in the (GitHub cloned) source [1] dating
back to 2006 and this uses the 3899 hard limit.

Wikipedia [2] claims the largest uniquely representable Roman numeral is 3999.

Other languages appear to use 3999 as the largest input value, e.g.
the python-cookbook [3].

Common lisp's "~@r" format errors with values larger than 3999

```
(print (format nil "~@r" 4000))
*** - The ~@R format directive requires an integer in the range 1 -
3999, not 4000
```

Is 3899 enforced as the largest valid input in R for some other
reason, or is this a long-standing oversight?

Perhaps tangentially... while exploring I did notice the unexported
utils:::.as.roman() takes a check.range argument that is not available
from as.roman (which is simply an exported wrapper without that
argument) but setting this to FALSE does not enable circumvention of
enforcement of the upper limit (which appears to occur during the
setting of the "roman" class), it simply performs an earlier
conversion to NA when set (potentially for simplification when used in
Ops dispatch). Enforcement of the limit of 3899 happens deeper within
the code, e.g. within the unexported utils:::.numeric2roman which
implements the conversion, but without a way to avoid setting the
result to NA for larger values. Given that other languages strictly
limit the input to an upper bound, perhaps this is not unexpected
behaviour.

Regards,

- Jonathan.

[0]: https://jcarroll.xyz/2024/02/10/friends-romans-countrymen.html
[1]: https://github.com/r-devel/r-svn/commit/ba30f3dc716effe22489bf88511bd1d60272f6de
[2]: https://en.wikipedia.org/wiki/Roman_numerals
[3]: https://www.oreilly.com/library/view/python-cookbook/0596001673/ch03s24.html