Rewriting a sequence by partitioning and collapsin

Question

what is the most elegant and simple algorithm to map a sequential collection such that contiguous elements that satisfy some predicate are collapsed into another element, and those that do not satisfy the predicate are mapped 1:1 into another element?

here is an example:

sealed trait A  // say the input elements are of this type
sealed trait B  // say the output elements are of this type
case class C(i: Int) extends A // these are the input elements satisfying the predicate
case class D(s: C*) extends B // they should be collapsed into this
case class E(i: Int) extends A with B // these are input elems that are left as such

given this input sequence:

val input  = Seq(C(1), C(2), C(3), E(4), E(5), C(6), E(7), C(8), C(9))

the expected output is:

val output = Seq(D(C(1), C(2), C(3)), E(4), E(5), D(C(6)), E(7), D(C(8), C(9)))
//                ---------------       -    -      -       -      --------
// the dashes indicate how the sequence is regrouped (collapsed)

here is one way of doing it, but i'm not sure this is particularly elegant:

def split(xs: Seq[A]): Seq[B] = split1(Seq.empty[B], true, xs)
@annotation.tailrec def split1(done: Seq[B], test: Boolean, rem: Seq[A]) : Seq[B] = {
    val (pre, post) = rem.span { case _: C => test; case _ => !test }
    val add = if(test) {
        D(pre.collect({ case x: C => x }): _*) :: Nil
    } else {
        pre.collect({ case x: E => x })
    }
    val done2 = done ++ add
    if(post.isEmpty) done2 else split1(done2, !test, post)
}

verify:

val output2 = split(input)
output2 == output  // ok

Answer 1

I would add a convenience method to D so you can "add" another C and get a new D back. Then it would be easy to use a simple foldLeft or so to build a new Seq.

Answer 2

@Landei yes, indeed, that looks like a good approach!

val output2 = input.foldLeft(Seq.empty[B]) {
  case (res, c: C) => res.lastOption match {
    case Some(D(x @ _*)) => res.dropRight(1) :+ D((x :+ c): _*)
    case _ => res :+ D(c)
  }
  case (res, e: E) => res :+ e
}

output2 == output // ok

(Of course, an IndexedSeq is better for lastOption, dropRight and append.)