Add unit_time_delta to make time_delta_to_n_steps make sense

Author: brookslogan

NAMESPACE

@@ -98,6 +98,7 @@ export(slice)
 export(sum_groups_epi_df)
 export(time_column_names)
 export(ungroup)
+export(unit_time_delta)
 export(unnest)
 export(validate_epi_archive)
 export(version_column_names)

R/utils.R

@@ -1122,7 +1122,7 @@ validate_slide_window_arg <- function(arg, time_type, lower = 1, allow_inf = TRU
 #' @param time_type as in `validate_slide_window_arg`
 #' @return [bare integerish][rlang::is_integerish] vector (with possible
 #'   infinite values) that produces the same result as `time_delta` when
-#'   multiplied by the "natural" "unit time delta" (not yet implemented) for
+#'   multiplied by the natural [`unit_time_delta`] for
 #'   that time type and added to time values of time type `time_type`. If the
 #'   given time type does not support infinite values, then it should produce
 #'   +Inf or -Inf for analogous entries of `time_delta`, and match the addition
@@ -1157,6 +1157,26 @@ time_delta_to_n_steps <- function(time_delta, time_type) {
   }
 }
 
+#' Object that, added to time_values of time_type, advances by one time step/interval
+#'
+#' @param time_type string; `epi_df`'s or `epi_archive`'s `time_type`
+#' @return an object `u` such that `time_values + u` represents advancing by one
+#'   time step / moving to the subsequent time interval for any `time_values`
+#'   object of time type `time_type`, and such that `time_values + k * u` for
+#'   integerish vector `k` advances by `k` steps (with vectorization,
+#'   recycling).
+#'
+#' @export
+unit_time_delta <- function(time_type) {
+  switch(time_type,
+    day = as.difftime(1, units = "days"),
+    week = as.difftime(1, units = "weeks"),
+    yearmonth = 1,
+    integer = 1L,
+    cli_abort("Unsupported time_type: {time_type}")
+  )
+}
+
 # Using these unit abbreviations happens to make our automatic slide output
 # naming look like taking ISO-8601 duration designations, removing the P, and
 # lowercasing any characters. Fortnightly or sub-daily time types would need an

man/time_delta_to_n_steps.Rd

@@ -362,3 +362,59 @@ test_that("validate_slide_window_arg works", {
     class = "epiprocess__validate_slide_window_arg"
   )
 })
+
+test_that("unit_time_delta works", {
+  expect_equal(
+    as.Date("2020-01-01") + 5 * unit_time_delta("day"),
+    as.Date("2020-01-06")
+  )
+  expect_equal(
+    as.Date("2020-01-01") + 2 * unit_time_delta("week"),
+    as.Date("2020-01-15")
+  )
+  expect_equal(
+    tsibble::make_yearmonth(2000, 1) + 5 * unit_time_delta("yearmonth"),
+    tsibble::make_yearmonth(2000, 6)
+  )
+  expect_equal(
+    1L + 5L * unit_time_delta("integer"),
+    6L
+  )
+  #
+  expect_equal(
+    as.Date("2020-01-01") +
+      time_delta_to_n_steps(as.Date("2020-01-06") - as.Date("2020-01-01"), "day") *
+        unit_time_delta("day"),
+    as.Date("2020-01-06")
+  )
+  expect_equal(
+    as.Date("2020-01-01") +
+      time_delta_to_n_steps(as.integer(as.Date("2020-01-06") - as.Date("2020-01-01")), "day") *
+        unit_time_delta("day"),
+    as.Date("2020-01-06")
+  )
+  expect_equal(
+    as.Date("2020-01-01") +
+      time_delta_to_n_steps(as.Date("2020-01-15") - as.Date("2020-01-01"), "week") *
+        unit_time_delta("week"),
+    as.Date("2020-01-15")
+  )
+  expect_equal(
+    as.Date("2020-01-01") +
+      time_delta_to_n_steps(as.difftime(2, units = "weeks"), "week") *
+        unit_time_delta("week"),
+    as.Date("2020-01-15")
+  )
+  expect_equal(
+    tsibble::make_yearmonth(2000, 1) +
+      time_delta_to_n_steps(5, "yearmonth") *
+        unit_time_delta("yearmonth"),
+    tsibble::make_yearmonth(2000, 6)
+  )
+  expect_equal(
+    1L +
+      time_delta_to_n_steps(5, "integer") *
+        unit_time_delta("integer"),
+    6L
+  )
+})