How to calculate 'n' days interval date in functional style? - scala

Question

I am trying to calculae the interval of 'n' days from start date to end date.Function signature would have start_date,end_date, interval as argument which return a map with list of start, end days of given intervals.
Example: start_date:2018-01-01 , End_date : 2018-02-20 interval: 20
Expected Output:
2018-01-01 , 2018-01-20 (20 days)
2018-01-21 , 2018-02-09 (20 days)
2018-02-09 , 2018-01-20 (remaining)
I tried to write in scala but i dont feel it's a proper functional style of writing.
case class DateContainer(period: String, range: (LocalDate, LocalDate))
def generateDates(startDate: String, endDate: String,interval:Int): Unit = {
import java.time._
var lstDDateContainer = List[DateContainer]()
var start = LocalDate.parse(startDate)
val end = LocalDate.parse(endDate)
import java.time.temporal._
var futureMonth = ChronoUnit.DAYS.addTo(start, interval)
var i = 1
while (end.isAfter(futureMonth)) {
lstDDateContainer = DateContainer("P" + i, (start, futureMonth)):: lstDDateContainer
start=futureMonth
futureMonth = ChronoUnit.DAYS.addTo(futureMonth, interval)
i += 1
}
lstDDateContainer= DateContainer("P" + i, (start, end))::lstDDateContainer
lstDDateContainer.foreach(println)
}
generateDates("2018-01-01", "2018-02-20",20)
Could anyone help me to write in a functional style.

Answer 1

I offer a solution that produces a slightly different result than given in the question but could be easily modified to get the desired answer:
//Preliminaries
val fmt = DateTimeFormatter.ofPattern("yyyy-MM-dd")
val startDate ="2018-01-01"
val endDate = "2018-02-21"
val interval = 20L
val d1 = LocalDate.parse(startDate, fmt)
val d2 = LocalDate.parse(endDate, fmt)
//The main code
Stream.continually(interval)
.scanLeft((d1, d1.minusDays(1), interval)) ((x,y) => {
val finDate = x._2.plusDays(y)
if(finDate.isAfter(d2))
(x._2.plusDays(1), d2, ChronoUnit.DAYS.between(x._2, d2))
else
(x._2.plusDays(1), x._2.plusDays(y), y)
}).takeWhile(d => d._3 > 0).drop(1).toList
Result:
(2018-01-01,2018-01-20,20)
(2018-01-21,2018-02-09,20)
(2018-02-10,2018-02-21,12)
The idea is to scan a 3-tuple through a stream of interval and stop when no more days are remaining.

Answer 2

Use the java.time library to generate the dates and Stream.iterate() to generate the sequence of intervals.
import java.time.LocalDate
def generateDates( startDate :LocalDate
, endDate :LocalDate
, dayInterval :Int ) :Unit = {
val intervals =
Stream.iterate((startDate, startDate plusDays dayInterval-1)){
case (_,lastDate) =>
val nextDate = lastDate plusDays dayInterval
(lastDate plusDays 1, if (nextDate isAfter endDate) endDate
else nextDate)
}.takeWhile(_._1 isBefore endDate)
println(intervals.mkString("\n"))
}
usage:
generateDates(LocalDate.parse("2018-01-01"), LocalDate.parse("2018-02-20"), 20)
// (2018-01-01,2018-01-20)
// (2018-01-21,2018-02-09)
// (2018-02-10,2018-02-20)

Answer 3

Something like (Untested):
def dates(startDate: LocalDate, endDate: LocalDate, dayInterval: Int): List[(LocalDate, LocalDate, Int)] = {
if(startDate.isAfter(endDate)) {
Nil
}
else {
val nextStart = startDate.plusDays(dayInterval)
if(nextStart.isAfter(startDate)) {
List((startDate, endDate, ChronoUnit.DAYS.between(startDate, endDate)))
}
else {
(startDate, nextStart, dayInterval) :: dates(nextStart, endDate, dayInterval)
}
}
}

Answer 4

If your'e open to using Joda for date-time manipulations, here's what I use
import org.joda.time.{DateTime, Days}
// given from & to dates, find no of days elapsed in between (integer)
def getDaysInBetween(from: DateTime, to: DateTime): Int = Days.daysBetween(from, to).getDays
def getDateSegments(from: DateTime, to: DateTime, interval: Int): Seq[(DateTime, DateTime)] = {
// no of days between from & to dates
val days: Int = DateTimeUtils.getDaysInBetween(from, to) + 1
// no of segments (date ranges) between to & from dates
val segments: Int = days / interval
// (remaining) no of days in last range
val remainder: Int = days % interval
// last date-range
val remainderRanges: Seq[(DateTime, DateTime)] =
if (remainder != 0) from -> from.plusDays(remainder - 1) :: Nil
else Nil
// all (remaining) date-ranges + last date-range
(0 until segments).map { segment: Int =>
to.minusDays(segment * interval + interval - 1) -> to.minusDays(segment * interval)
} ++ remainderRanges
}