cloud-fan commented on a change in pull request #28067: [WIP][SPARK-31297][SQL]
Speed up dates rebasing
URL: https://github.com/apache/spark/pull/28067#discussion_r400179977
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File path:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/DateTimeUtils.scala
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@@ -1033,6 +1033,40 @@ object DateTimeUtils {
instantToMicros(localDateTime.atZone(ZoneId.systemDefault).toInstant)
}
+ /**
+ * Rebases days since the epoch from an original to an target calendar, from
instance
+ * from a hybrid (Julian + Gregorian) to Proleptic Gregorian calendar.
+ *
+ * It finds the latest switch day which is less than `days`, and adds the
difference
+ * in days associated with the switch day to the given `days`. The function
is based
+ * on linear search which starts from the most recent switch days. This
allows to perform
+ * less comparisons for modern dates.
+ *
+ * @param switchDays The days when difference in days between original and
target
+ * calendar was changed.
+ * @param diffs The differences in days between calendars.
+ * @param days The number of days since the epoch 1970-01-01 to be rebased
to the
+ * target calendar.
+ * @return The rebased day
+ */
+ private def rebaseDays(switchDays: Array[Int], diffs: Array[Int], days:
Int): Int = {
+ var i = switchDays.length - 1
+ while (i >= 0 && days < switchDays(i)) {
+ i -= 1
+ }
+ val rebased = days + diffs(if (i < 0) 0 else i)
+ rebased
+ }
+
+ // The differences in days between Julian and Proleptic Gregorian dates.
+ // The diff at the index `i` is applicable for all days in the date interval:
+ // [julianGregDiffSwitchDay(i), julianGregDiffSwitchDay(i+1))
Review comment:
let's document it clearly.
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