groupAdjacent

gistfile1.scala

  @tailrec
  final def groupAdjacent[A,B](
    items: Seq[(A,B)],
    acc: Seq[(A, Seq[B])] = Vector.empty
  ): Seq[(A, Seq[B])] = {
    items match {
      case Seq() => acc
      case Seq(a, _*) =>
        val (same, remaining) = items.span { case (e1, _) => a._1 == e1 }
        val updated = acc :+ (a._1, same.map { _._2 })
        groupAdjacent(remaining, updated)
    }
  }
}

// Usage:

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

scala> groupAdjacent(items)
res2: Seq[(Int, Seq[Int])] = Vector((1,List(1, 2)), (2,List(3)), (3,List(4, 5)))