Reverse / transpose a one-to-many map in Scala

What is the best way to turn a Map[A, Set[B]] into a Map[B, Set[A]]?

For example, 开发者_StackOverflow社区how do I turn a

Map(1 -> Set("a", "b"),
    2 -> Set("b", "c"),
    3 -> Set("c", "d"))

into a

Map("a" -> Set(1),
    "b" -> Set(1, 2),
    "c" -> Set(2, 3),
    "d" -> Set(3))

(I'm using immutable collections only here. And my real problem has nothing to do with strings or integers. :)

with help from aioobe and Moritz:

def reverse[A, B](m: Map[A, Set[B]]) =
  m.values.toSet.flatten.map(v => (v, m.keys.filter(m(_)(v)))).toMap

It's a bit more readable if you explicitly call contains:

def reverse[A, B](m: Map[A, Set[B]]) =
  m.values.toSet.flatten.map(v => (v, m.keys.filter(m(_).contains(v)))).toMap

Best I've come up with so far is

val intToStrs = Map(1 -> Set("a", "b"),
                    2 -> Set("b", "c"),
                    3 -> Set("c", "d"))

def mappingFor(key: String) =
    intToStrs.keys.filter(intToStrs(_) contains key).toSet

val newKeys = intToStrs.values.flatten
val inverseMap = newKeys.map(newKey => (newKey -> mappingFor(newKey))).toMap

Or another one using folds:

  def reverse2[A,B](m:Map[A,Set[B]])=
      m.foldLeft(Map[B,Set[A]]()){case (r,(k,s)) =>
         s.foldLeft(r){case (r,e)=>
            r + (e -> (r.getOrElse(e, Set()) + k))
         }
      }

Here's a one statement solution

 orginalMap
 .map{case (k, v)=>value.map{v2=>(v2,k)}}
 .flatten
 .groupBy{_._1}
 .transform {(k, v)=>v.unzip._2.toSet}

This bit rather neatly (*) produces the tuples needed to construct the reverse map

Map(1 -> Set("a", "b"),
    2 -> Set("b", "c"),
    3 -> Set("c", "d"))
.map{case (k, v)=>v.map{v2=>(v2,k)}}.flatten

produces

 List((a,1), (b,1), (b,2), (c,2), (c,3), (d,3))

Converting it directly to a map overwrites the values corresponding to duplicate keys though

Adding .groupBy{_._1} gets this

 Map(c -> List((c,2), (c,3)),
     a -> List((a,1)),
     d -> List((d,3)), 
     b -> List((b,1), (b,2)))

which is closer. To turn those lists into Sets of the second half of the pairs.

  .transform {(k, v)=>v.unzip._2.toSet}

gives

  Map(c -> Set(2, 3), a -> Set(1), d -> Set(3), b -> Set(1, 2))

QED :)

(*) YMMV

A simple, but maybe not super-elegant solution:

  def reverse[A,B](m:Map[A,Set[B]])={
      var r = Map[B,Set[A]]()
      m.keySet foreach { k=>
          m(k) foreach { e =>
            r = r + (e -> (r.getOrElse(e, Set()) + k))
          }
      }
      r
  }

The easiest way I can think of is:

// unfold values to tuples (v,k)
// for all values v in the Set referenced by key k
def vk = for {
  (k,vs) <- m.iterator
  v <- vs.iterator
} yield (v -> k)

// fold iterator back into a map
(Map[String,Set[Int]]() /: vk) {
// alternative syntax: vk.foldLeft(Map[String,Set[Int]]()) {
  case (m,(k,v)) if m contains k =>
    // Map already contains a Set, so just add the value
    m updated (k, m(k) + v)
  case (m,(k,v)) =>
    // key not in the map - wrap value in a Set and return updated map
    m updated (k, Set(v))
}