Is there an idiomatic way to go from `Map<Key,`...

# stdlib

[Gianfranco]

Is there an idiomatic way to go from Map<Key, Set<Value>> to Map<Value, Key> ?

[araqnid]

I don’t know a provided way to do it, but seems like you just need to do

fun <Key, Value> Map<Key, Set<Value>>.invert(): Map<Value, Key> =
    entries.flatMap { (k, vs) -> vs.map { it to k } }.toMap()

[Gianfranco]

Thank you. That looks much better than my current code 😅

[Klitos Kyriacou]

Beware of the possibility of duplicates though. (i.e. a value being in more than one set).

[ephemient]

I'd define

fun <K, V> Map<K, Iterable<V>>.invert(): Map<V, K> = buildMap {
    for ((k, vs) in this@invert) {
        for (v in vs) put(v, k)
    }
}

just for fewer intermediary structures, but yes, this will result in losing duplicates

[ephemient]

you can keep all the duplicates with things like

fun <K, V> Map<K, Iterable<V>>.invert(): Map<V, Set<K>> = entries.asSequence()
    .flatMap { (k, vs) -> vs.asSequence().map(k::to) }
    .groupingBy { it.second }
    .fold(
        initialValueSelector = { _, (k, _) -> mutableSetOf(k) },
        operation = { _, set, (k, _) -> set.apply { add(k) } },
    )

of course