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

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
}

Answer 3

Version more concentrated on getting new element on every execution.
val getNext: () => Int = {
def b: Stream[Int] = List(1, 2, 3, 4).toStream #::: b
var cyclicIterator: Stream[Int] = b
() => {
val tail = cyclicIterator.tail
val result = tail.head
cyclicIterator = tail
result
}
} // could be written more sexy?
In your problem you can use it like:
for(i <- 1 to 10) yield getNext()

Answer 4

This is ugly in having an external mutable index, but it does do what's requested:
scala> var i = 0
scala> val ic4 = Iterator.continually { val next = IndexedSeq(1, 2, 3, 4)(i % 4); i += 1; next }
i: Int = 0
ic4: Iterator[Int] = non-empty iterator
scala> ic4 take 10 foreach { i => printf("ic4.next=%d%n", i) }
ic4.next=1
ic4.next=2
ic4.next=3
ic4.next=4
ic4.next=1
ic4.next=2
ic4.next=3
ic4.next=4
ic4.next=1
ic4.next=2
At least it illustrates Iterator.continually. There is also Stream.continually, which has the same signature.