How to globally order multiple ordered observables in Monix

Question

Suppose I have multiple iterators that are ordered. If I wanted to merge these iterators while globally ordering them (eg [(1,3,4), (2,4,5)] -> [1,2,3,4,4,5] ) using monix how would I do it?

Answer 1

Doesn't use Monix, but I'm not sure if that's relevant

import scala.collection.BufferedIterator


def merge[A:Ordering](xs: Seq[Iterator[A]]) = 
  new Iterator[A] {
    val its = xs.map(_.buffered)
    def hasNext = its.exists(_.hasNext)
    def next = its.filter{ _.hasNext}
                  .minBy(_.head)
                  .next
  }


val ys = merge(Seq(List(1,3,5).toIterator, List(2,4,6).toIterator, List(10,11).toIterator))

ys.toList  //> res0: List[Int] = List(1, 2, 3, 4, 5, 6, 10, 11)

Answer 2

Since observable is a stream of items, it can be generalized as two types:

• Finite streams
• Infinite streams

Note that, in order to sort correctly, you'll need all the items. So, there's no easy way to do this.

For finite streams , you'll have to accumulate all the items and then sort. You can turn this back into an observable with Observable.fromIterable .

val items = List((1,3,4), (2,4,5))

val sortedList = Observable
  .fromIterable(items)
  .flatMap(item => Observable.fromIterable(List(item._1, item._2, item._3)))
  .toListL // Flatten to an Observable[Int]
  .map(_.sorted) 

For infinite streams, the only thing you can do is to buffer the items up to a certain time or size. I don't see any way around since you don't know when the stream will end.

For example,

val itemsStream: Observable[(Int, Int, Int)] = ???

itemsStream
  .bufferIntrospective(10)
  .flatMap((itemList: List[(Int, Int, Int)]) => // You'll have to sort this
     ???
  )