Inverting a Map[Long, Set[Long]) to a Map[Long, Long] - scala

Question

Trying to convert a Map[Long, Set[Long]] to a Map[Long, Long].
I tried this but having compile issues:
m.map(_.swap).map(k => k._1.map((_, k._2)))
Example:
Map(10 -> Set(1,2,3), 11 -> Set(4,5))
Should become:
Map(1 -> 10,
2 -> 10,
3 -> 10,
4 -> 11,
5 -> 11)

Answer 1

flatMap on Map[A,B] will "just work" with collections of tuples:
m.flatMap {case (k,v) => v.map(_ -> k)} // Map[Long,Long]
going from a Map[Long,Set[Long]] to a series of Set[(Long,Long)] that gets flattened to a Map[Long,Long].

Answer 2

Assuming in is your Map[Long, Set[Long]]:
in./:(Map.empty[Long, Long]) { case (acc, (key, values)) => acc ++ values.map(_ -> key) }

Answer 3

To clarify, seem like you have this:
Map(10 -> Set(1,2,3), 11 -> Set(4,5))
And you want to convert this map in another map, but with something like this:
Map(1 -> 10,
2 -> 10,
3 -> 10,
4 -> 11,
5 -> 11)
As you can see if the sets are not disjoint, some keys in the resulted map with be missing:
Having this in consideration, the code will look like this:
val m: Map[Long, Set[Long]] = Map(10l -> Set(1l,2l,3l), 11l -> Set(4l,5l))
m.map(_.swap).map(k => k._1.map((_, k._2)))
val foo: Iterable[(Long, Long)] = m.flatMap { t =>
val (key, value) = t
value.map(_ -> key)
}
val result: Map[Long, Long] = foo.toMap

Answer 4

This will invert your Map m from Map[Long, Set[Long]] to Map[Long, List[Long]].
m flatten {case(k, vs) => vs.map((_, k))} groupBy (_._1) mapValues {_.map(_._2)}
You haven't specified what should happen when different Set values contains some of the same Longs (i.e. Map(8 -> Set(1,2), 9 -> Set(2,3))). If you're sure that won't happen you can use the following adjustment.
m flatten {case(k, vs) => vs.map((_, k))} groupBy (_._1) mapValues {_.head._2}
Or even more simply:
m.flatten {case(k, vs) => vs.map((_, k))}.toMap