gistfile1.txt

scala> val hist = List(3,2,2).groupBy(identity).mapValues(_.length)
hist: scala.collection.immutable.Map[Int,Int] = Map(2 -> 2, 3 -> 1)

scala> ((hist, List[Int]()) /: List(4, 3, 2, 2, 2, 1)) { 
     |   case ((h, as), a) => h.get(a).map {
     |     case 0 => (h, a :: as)
     |     case n => (h + (a -> (n - 1)), as)
     |   }.getOrElse((h, a :: as)) }
res25: (scala.collection.immutable.Map[Int,Int], List[Int]) = (Map(2 -> 0, 3 -> 0),List(1, 2, 4))

scala> 

scala> ((hist.withDefaultValue(0), List[Int]()) /: List(4, 3, 2, 2, 2, 1)) { 
     |   case ((h, as), a) => h(a) match {
     |     case 0 => (h, a :: as)
     |     case n => (h + (a -> (n - 1)), as)
     |   }}
res26: (scala.collection.immutable.Map[Int,Int], List[Int]) = (Map(2 -> 0, 3 -> 0),List(1, 2, 4))