isoweek()/isoyear(): use convertDate() instead of format()

NEWS.md

@@ -50,6 +50,8 @@
 
 14. `melt()` is faster on large inputs when `measure.vars` is a list with `variable.factor=FALSE` (e.g. 3x) or uses `measure()` (e.g. 2x). Thanks @m-muecke for the PR.
 
+15. `isoweek()` and `isoyear()` are much faster (e.g. 25x and 45x) by computing the result with date arithmetic instead of `format()`. Thanks @m-muecke for the PR.
+
 ### BUG FIXES
 
 1. `fread()` with `skip=0` and `(header=TRUE|FALSE)` no longer skips the first row when it has fewer fields than subsequent rows, [#7463](https://github.com/Rdatatable/data.table/issues/7463). Thanks @emayerhofer for the report and @ben-schwen for the fix.

R/IDateTime.R

@@ -393,20 +393,15 @@ yday    = function(x) convertDate(as.IDate(x), "yday")
 wday    = function(x) convertDate(as.IDate(x), "wday")
 mday    = function(x) convertDate(as.IDate(x), "mday")
 week    = function(x) convertDate(as.IDate(x), "week")
-# TODO(#3279): Investigate if improved as.IDate() makes our below implementation faster than this
-isoweek = function(x) as.integer(format(as.IDate(x), "%V"))
-  # ISO 8601-conformant week, as described at
-  #   https://en.wikipedia.org/wiki/ISO_week_date
-  # Approach:
-  # * Find nearest Thursday to each element of x
-  # * Find the number of weeks having passed between
-  #   January 1st of the year of the nearest Thursdays and x
-
-#  x = as.IDate(x)   # number of days since 1 Jan 1970 (a Thurs)
-#  nearest_thurs = as.IDate(7L * (as.integer(x + 3L) %/% 7L))
-#  year_start = as.IDate(format(nearest_thurs, '%Y-01-01'))
-#  1L + (nearest_thurs - year_start) %/% 7L
-isoyear = function(x) as.integer(format(as.IDate(x), "%G"))
+# ISO 8601-conformant week, as described at
+#   https://en.wikipedia.org/wiki/ISO_week_date
+# Approach:
+# * Find nearest Thursday to each element of x
+# * Find the number of weeks having passed between
+#   January 1st of the year of the nearest Thursdays and x
+nearest_thurs = function(x) 7L * ((as.integer(as.IDate(x)) + 3L) %/% 7L)
+isoweek = function(x) (convertDate(nearest_thurs(x), "yday") - 1L) %/% 7L + 1L
+isoyear = function(x) convertDate(nearest_thurs(x), "year")
 
 month   = function(x) convertDate(as.IDate(x), "month")
 quarter = function(x) convertDate(as.IDate(x), "quarter")

inst/tests/tests.Rraw

@@ -22011,3 +22011,8 @@ test(2388.21, nafill(c(1, NA, NA, 5), type="nocb", limit=1, nan=NaN), c(1, NA, 5
 test(2388.22, nafill(c(1L, NA, NA, 5L), type="nocb", limit=1), c(1L, NA, 5L, 5L))
 test(2388.23, nafill(c("a", NA, NA), type="const", fill="x", limit=1), c("a", "x", NA))
 test(2388.24, nafill(c("a", NA, NA, "z"), type="nocb", limit=1), c("a", NA, "z", "z"))
+
+# isoweek() and isoyear() use date arithmetic instead of format()
+x = seq(as.IDate("1999-01-01"), as.IDate("2027-12-31"), by="day")
+test(2389.1, isoweek(x), as.integer(format(x, "%V")))
+test(2389.2, isoyear(x), as.integer(format(x, "%G")))