Is there a Round Robin/Circular Queue available in Scala Collections

Question

Is there a Round Robin Queue available in Scala Collections?

I need to repeatedly iterate a list that circles through itself

val x = new CircularList(1,2,3,4) x.next (returns 1) x.next (returns 2) x.next (returns 3) x.next (returns 4) x.next (returns 1) x.next (returns 2) x.next (returns 3) 

... and so on

Answer 1

It's pretty easy to roll your own with continually and flatten:

scala> val circular = Iterator.continually(List(1, 2, 3, 4)).flatten circular: Iterator[Int] = non-empty iterator  scala> circular.take(17).mkString(" ") res0: String = 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 

There's also a continually method on Stream—just be careful not to hold onto a reference to the head of the stream if you're going to be generating lots of elements.

Answer 2

You can very easily create a circular list using a Stream.

scala> val l = List(1, 2, 3, 4).toStream l: scala.collection.immutable.Stream[Int] = Stream(1, ?)  scala> def b: Stream[Int] = l #::: b b: Stream[Int]  scala> b.take(20).toList res2: List[Int] = List(1, 2, 3, 4, 1, 2, 3, 4, 1, 2, 3, 4, 1, 2, 3, 4, 1, 2, 3, 4) 

Edit: you want to make sure to define the repeated part beforehand, once and only once, to avoid blowing the heap (structural sharing in Stream). As in:

def circular[A](a: Seq[A]): Stream[A] = {   val repeat = a.toStream   def b: Stream[A] = repeat #::: b   b }