Date intervals keeping state - scala

Question

My task is to write function, that takes 2 LocalDateTime instances and generate list of intervals, splitted by 15 minutes
case class Interval(start:LocalDateTime, end:LocalDateTime)
so, for example if startDate = 2 Aug 2017 14:30:15 and endDate is 2 Aug 2017 15:00:00, intervals should be
List(Interval(2 Aug 2017 14:30:15, 2 Aug 2017 14:45:15), Interval(2 Aug 2017 14:45:15, 2 Aug 2017 15:00:00))
Here, is 2 main complications(at least for me)
1) If end date is less then prevDate + 15 min, then we need take min from(prevdate + 15 min, endDate)
2) We need somehow keep the state, because start of each interval is end of previous interval.
I am able to create imperative version, but I want to do it in functional style, please, help!)
x => {
var start = x.startTime
var end = min(findRightBorder(start), x.endTime)
var result = ListBuffer(Interval(start, end))
while (end.isBefore(x.endTime)) {
start = end
end = min(start.plusMinutes(QUARTER_MINUTES), x.endTime)
result += Interval(start, end)
}
result.toList
}
private def findRightBorder(dt: LocalDateTime): LocalDateTime = {
val minute = dt.getMinute
if (minute >= 0 && minute < 15) dt.withMinute(15)
else if (minute >= 15 && minute < 30) dt.withMinute(30)
else if (minute >= 30 && minute < 45) dt.withMinute(45)
else if (minute >= 45 && minute < 60) dt.withMinute(0).plusHours(1)
else dt
}
private def min(dt1: LocalDateTime, dt2: LocalDateTime): LocalDateTime = {
if (dt1.compareTo(dt2) < 0) dt1 else dt2
}

Answer 1

Another solution with streams:
x => {
Stream
.from(0)
.map(i => x.startTime.plusMinutes(i * QUARTER_MINUTES))
.takeWhile(_.isBefore(x.endTime))
.map(s => Interval(s, min(s.plusMinutes(QUARTER_MINUTES), x.endTime)))
.toList
}

Answer 2

Here's one way to go about this.
import java.time.LocalDateTime
case class Interval(start:LocalDateTime, end:LocalDateTime)
val dt1: LocalDateTime = ??? //some start DateTime
val dt2: LocalDateTime = ??? //some end DateTime
// a (potentially) infinite Stream of dates at 15 minute intervals
// starting at dt1 but ending before dt2
val dates = Stream.iterate(dt1)(_.plusMinutes(15L))
.takeWhile(_.isBefore(dt2))
val result = dates.sliding(2) //pair the dates together
.toSeq //a Seq is easier to append to
.map{case Seq(from,to) => Interval(from,to)} //make Intervals
.:+(Interval(dates.last,dt2)) //append the final Interval
.toList //if you need a List result