Implement leap_year() in C

[mikmart]

I noticed the fastymd package hitting CRAN recently. One of its features is fast leap year calculations, which made me wonder how lubridate stacks up.

Currently leap_year() is pure R and is relatively speaking quite slow. With the lubridate main branch:

library(microbenchmark)

set.seed(42)
years <- sample(4000, 1e4, replace = TRUE)

microbenchmark(
  fastymd::is_leap(years),
  lubridate::leap_year(years),
  times = 1e4
)
#> Unit: microseconds
#>                         expr   min    lq       mean median     uq      max neval cld
#>      fastymd::is_leap(years)  29.4  47.8   65.64309   52.6   64.2  11538.7 10000  a 
#>  lubridate::leap_year(years) 792.3 986.5 1203.30433 1020.2 1139.0 298644.7 10000   b

^{Created on 2025-05-01 with reprex v2.1.1}

This PR moves the leap year calculation to C, simply wrapping the IS_LEAP macro already used elsewhere in the codebase. With it I see a roughly 30-fold performance increase on my machine:

library(microbenchmark)

set.seed(42)
years <- sample(4000, 1e4, replace = TRUE)

microbenchmark(
  fastymd::is_leap(years),
  lubridate::leap_year(years),
  times = 1e4
)
#> Unit: microseconds
#>                         expr  min   lq     mean median   uq    max neval cld
#>      fastymd::is_leap(years) 29.2 44.1 64.90436   48.4 55.8  20797 10000   a
#>  lubridate::leap_year(years) 15.6 30.4 72.39227   34.6 40.2 310873 10000   a

^{Created on 2025-05-01 with reprex v2.1.1}

[vspinu]

Thanks @mikmart! This is nice.