use units::ud_convert directly in PEcAn.utils::ud_convert?

[infotroph]

Noted by @dlebauer in #3925 (review):

there are a lot of calls in the PEcAn version of ud_convert (ud_are_convertible, as_units, set_units, drop_units) while the units package directly calls a C function. So I checked out the penalty, and found that the units implementation is about 20x faster and uses infinitely less memory.

bench::mark(
  pecan = PEcAn.utils::ud_convert(42, "m", "km"),
  units = units::ud_convert(42, "m", "km"),
  iterations = 500
)
#> # A tibble: 2 × 6
#>   expression      min   median `itr/sec` mem_alloc `gc/sec`
#>   <bch:expr> <bch:tm> <bch:tm>     <dbl> <bch:byt>    <dbl>
#> 1 pecan      142.35µs 151.56µs     6445.    6.44MB     52.0
#> 2 units        6.56µs   7.58µs   114353.        0B    229.

Consider replacing the as_units/set_units dance inside PEcAn.utils::ud_convert with a single call to units::ud_convert? This would (probably) capture the speed gains without requiring all other packages to add a direct {units} dependency.

One piece to consider: PEcAn.utils::ud_convert has a special case for difftime object that I don't think udunits::ud_convert knows how to handle. My impression is that's a little-used feature that could be retained without losing the everyday speedup, but I haven't checked in detail.

[anshul23102]

Hi @infotroph
I'd love to work on this if it's still open!

The fix seems straightforward: replace the as_units/set_units path with a direct call to units::ud_convert for the common case, while keeping the existing difftime handling as a fallback. Something like:

if (inherits(x, "difftime")) {
  # existing difftime logic
} else {
  units::ud_convert(x, from, to)
}

Before I start, a couple of quick questions:

1. Is units::ud_convert already a declared dependency of PEcAn.utils, or would this require adding it?
2. Are there any existing tests for the difftime path I should make sure stay green?

Happy to proceed once I hear from you!

[infotroph]

Both of those questions are good ones to answer for yourself in the process of planning your approach! R package dependencies are declared in DESCRIPTION, tests are in the tests/ folder and organized by the usual testthat convention that tests for R/[filename].R live in tests/testthat/test-[filename].R.

[anshul23102]

Thanks for the pointers!

I checked both:

1. units is already listed under Imports in base/utils/DESCRIPTION - no new dependency needed.
2. There are existing difftime tests in tests/testthat/test-ud_convert.R - I'll make sure that path stays green.

I'll open a draft PR with the fix shortly.