Elegant way to invert a map in Scala

Question

Learning Scala currently and needed to invert a Map to do some inverted value->key lookups. I was looking for a simple way to do this, but came up with only:

Answer 1

OK, so this is a very old question with many good answers, but I've built the ultimate, be-all-and-end-all, Swiss-Army-knife, Map inverter and this is the place to post it.

It's actually two inverters. One for individual value elements...

//from Map[K,V] to Map[V,Set[K]], traverse the input only once
implicit class MapInverterA[K,V](m :Map[K,V]) {
  def invert :Map[V,Set[K]] =
    m.foldLeft(Map.empty[V, Set[K]]) {
      case (acc,(k, v)) => acc + (v -> (acc.getOrElse(v,Set()) + k))
    }
}

...and another, quite similar, for value collections.

import scala.collection.generic.CanBuildFrom
import scala.collection.mutable.Builder
import scala.language.higherKinds

//from Map[K,C[V]] to Map[V,C[K]], traverse the input only once
implicit class MapInverterB[K,V,C[_]](m :Map[K,C[V]]
                                     )(implicit ev :C[V] => TraversableOnce[V]) {
  def invert(implicit bf :CanBuildFrom[Nothing,K,C[K]]) :Map[V,C[K]] =
    m.foldLeft(Map.empty[V, Builder[K,C[K]]]) {
      case (acc, (k, vs)) =>
        vs.foldLeft(acc) {
          case (a, v) => a + (v -> (a.getOrElse(v,bf()) += k))
        }
    }.mapValues(_.result())
}

usage:

Map(2 -> Array('g','h'), 5 -> Array('g','y')).invert
//res0: Map(g -> Array(2, 5), h -> Array(2), y -> Array(5))

Map('q' -> 1.1F, 'b' -> 2.1F, 'c' -> 1.1F, 'g' -> 3F).invert
//res1: Map(1.1 -> Set(q, c), 2.1 -> Set(b), 3.0 -> Set(g))

Map(9 -> "this", 8 -> "that", 3 -> "thus", 2 -> "thus").invert
//res2: Map(this -> Set(9), that -> Set(8), thus -> Set(3, 2))

Map(1L -> Iterator(3,2), 5L -> Iterator(7,8,3)).invert
//res3: Map(3 -> Iterator(1, 5), 2 -> Iterator(1), 7 -> Iterator(5), 8 -> Iterator(5))

Map.empty[Unit,Boolean].invert
//res4: Map[Boolean,Set[Unit]] = Map()

I would prefer to have both methods in the same implicit class but the more time I spent looking into it the more problematic it appeared.