MultiMap

MultiMap.scala

type ListMultiMap[A, B] = Map[A, List[B]]

implicit class ListMultiMapOps[A, B](val map: ListMultiMap[A, B]) extends AnyVal {
  /** May be overridden */
  def makeList: List[B] = Nil
  
  def addBinding(key: A, value: B): ListMultiMap[A, B] = map + (key -> { value :: map.getOrElse(key, makeList) })

  def removeBinding(key: A, value: B): ListMultiMap[A, B] = map.get(key) match {
    case None => map
    case Some(List(value)) => map - key 
    case Some(list) => map + (key -> list.diff(List(value)))
  }
}
  
def toListMultiMap[A,B](seq: Seq[(A, B)]) = seq.foldLeft(Map[A, List[B]]())((acc, t) => acc.addBinding(t._1, t._2))