Akka streams — filtering by the number of elements in stream

I'm writing an app in Scala and I'm using Akka streams.

At one point, I need to filter out streams that have less than N elements, with N given. So, for example, with N=5:

Source(List(1,2,3)).via(myFilter)       // => List()
Source(List(1,2,3,4)).via(myFilter)     // => List()

will become empty streams, and

Source(List(1,2,3,4,5)).via(myFilter)   // => List(1,2,3,4,5)
Source(List(1,2,3,4,5,6)).via(myFilter) // => List(1,2,3,4,5,6)

will be unchanged.

Of course, we can't know the number of elements in the stream until it's over, and waiting till the end before pushing it through might not be the best idea.

So, instead, I've thought about the following algorithm:

1. for the first N-1 elements, just buffer them, without passing further;
2. if the input stream finishes before reaching the Nth element, output an empty stream;
3. if the input stream reaches Nth element, output the buffered N-1 elements, then output the Nth element, then pass all the following elements that come.

However, I have no idea how to build a Flow element implementing it. Are there some built-in Akka elements I could use?

Edit:

Okay, so I played with it yesterday and I came up with something like that:

Flow[Int].
  prefixAndTail(N).
  flatMapConcat {
    case (prefix, tail) if prefix.length == N =>
      Source(prefix).concat(tail)
    case _ =>
      Source.empty[Int]
  }

Will it do what I want?