How to reverse a Map in Kotlin?

If you need to reverse a multimap like m: Map<K, List<V>> to a Map<V, List<K>> you can do

m
  .flatMap { it.value.map { oneValue -> oneValue to it.key } }
  .groupBy({ it.first }, { it.second })
  .toMap()

In sequence,

• mapOf('a' to listOf('b', 'c'), 'd' to listOf('b')) gets flat mapped to a sequence like
• listOf('b' to 'a', 'c' to 'a', 'b' to 'd') which gets grouped to
• listOf('b' to listOf('a', 'd'), 'c' to listOf('a')) which then gets converted to a map.

This probably creates intermediate objects.

Here is a simple extension function that reverse a map - without generating unneeded garbage (like pairs, intermediate data structures and unnecessary closures )

fun <K, V> Map<K, V>.reversed() = HashMap<V, K>().also { newMap ->
    entries.forEach { newMap.put(it.value, it.key) }
}

note that apply is inlined, and entries.forEach is also inlined (which is not the same for Map::forEach)

Since the Map consists of Entrys and it is not Iterable you can use Map#entries instead. It will be mapped to Map#entrySet to create a backed view of Set<Entry>, for example:

val reversed = map.entries.associateBy({ it.value }) { it.key }

OR use Iterable#associate, which will create additional Pairs.

val reversed = map.entries.associate{(k,v)-> v to k}

OR using Map#forEach:

val reversed = mutableMapOf<Int, String>().also {
    //     v-- use `forEach` here     
    map.forEach { (k, v) -> it.put(v, k) } 
}.toMap()
// ^--- you can add `toMap()` to create an immutable Map.

In case your map is not a 1-1 mapping and you want the inversion to be a list of values:

mapOf(1 to "AAA", 2 to "BBB", 3 to "BBB").toList()
        .groupBy { pair -> pair.second } // Pair<Int, String>
        .mapValues { entry -> 
          entry.value.map { it.first } // Entry<String, List<Pair<Int, String>>
        }