[GitHub] [arrow] jonkeane commented on a change in pull request #12154: ARROW-14821: [R] Implement bindings for lubridate's floor_date, ceiling_date, and round_date

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jonkeane commented on a change in pull request #12154:
URL: https://github.com/apache/arrow/pull/12154#discussion_r786790257



##########
File path: r/R/util.R
##########
@@ -209,3 +209,74 @@ handle_csv_read_error <- function(e, schema) {
 
   abort(e)
 }
+
+
+parse_period_unit <- function(x) {
+
+  # the regexp matches against fractional units, but per lubridate
+  # supports integer multiples of a known unit only
+  match_info <- regexpr(
+    pattern = " *(?<multiple>[0-9.,]+)? *(?<unit>[^ \t\n]+)",
+    text = x[[1]],
+    perl = TRUE
+  )
+
+  capture_start <- attr(match_info, "capture.start")
+  capture_length <- attr(match_info, "capture.length")
+  capture_end <- capture_start + capture_length - 1L
+
+  str_unit <- substr(x, capture_start[[2]], capture_end[[2]])
+  str_multiple <- substr(x, capture_start[[1]], capture_end[[1]])
+
+  known_units <- c("nanosecond", "microsecond", "millisecond", "second",
+                   "minute", "hour", "day", "week", "month", "quarter", "year")
+
+  # match the period unit
+  str_unit_start <- substr(str_unit, 1, 3)
+  unit <- as.integer(pmatch(str_unit_start, known_units)) - 1L
+
+  if(any(is.na(unit))) {
+    abort(sprintf("Unknown unit '%s'", str_unit))
+  }
+
+  # empty string in multiple interpreted as 1
+  if(capture_length[[1]] == 0) {
+    multiple <- 1L
+
+  } else {
+
+    # special cases: interpret fractions of 1 second as integer
+    # multiples of nanoseconds, microseconds, or milliseconds
+    # to mirror lubridate syntax
+    multiple <- as.numeric(str_multiple)
+
+    if(unit == 3L & multiple < 10^-6) {
+      unit <- 0L
+      multiple <- 10^9 * multiple
+    }
+    if(unit == 3L & multiple < 10^-3) {
+      unit <- 1L
+      multiple <- 10^6 * multiple
+    }
+    if(unit == 3L & multiple < 1) {
+      unit <- 2L
+      multiple <- 10^3 * multiple
+    }
+
+    multiple <- as.integer(multiple)
+  }
+
+  # more special cases: lubridate imposes sensible maximum
+  # values on the number of seconds, minutes and hours
+  if(unit == 3L & multiple > 60) {
+    abort("Rounding with second > 60 is not supported")
+  }

Review comment:
       What y'all have already said is great, and totally agree with this being an edge case for if extending just a bit is fine or if we should error in the same way.
   
   How easy/hard would it be to construct the `arrow_round_temporal()` call if someone did want to do that?




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