Mapping set of keys to a matching list of lists - kotlin

Question

What is an idiomatic way to map keys to a matching list of lists? An example - given:
val s = listOf(1, 9)
val u = listOf(listOf(1, 2, 3), listOf(1, 4, 7), listOf(1, 5, 9))
I would like to have a Map<Int, List<List<Int>>> such that every key in s is mapped to a list of lists containing that key:
{1=[ [1, 2, 3], [1, 4, 7], [1, 5, 9] ], 9=[ [1, 5, 9] ]}
The following:
s.groupBy({ it }, { x -> u.filter { it.contains(x) } })
produces:
{1=[[[1, 2, 3], [1, 4, 7], [1, 5, 9]]], 9=[[[1, 5, 9]]]}
which is not quite right and it isn't clear how to flatten the result to the expected shape.

Answer 1

I would recommend associateWith and use it like this:
s.associateWith { num -> u.filter { list -> num in list } }
Output:
{1=[[1, 2, 3], [1, 4, 7], [1, 5, 9]], 9=[[1, 5, 9]]}
I recommended associate at first, but you can shorten the code even further if you use associateWith. Thanks to Abhay Agarwal who recommended it.

Answer 2

Update
You just need to flatten the values of the result Map.
val w = s.groupBy({ it }, { x -> u.filter { it.contains(x) } })
.mapValues { it.value.flatten() }
My solution map the first collection to pairs from each element to the list where it appears, and then groupBy the result list.
Example
val w = s.map { elem -> Pair(elem, u.filter { list -> elem in list }) }
.groupBy ({ it.first }, { it.second })
.mapValues { it.value.flatten() }
check(w[1] == listOf(listOf(1, 2, 3), listOf(1, 4, 7), listOf(1, 5, 9)))
check(w[9] == listOf(listOf(1, 5, 9)))
println(w)
Output
{1=[[1, 2, 3], [1, 4, 7], [1, 5, 9]], 9=[[1, 5, 9]]}

Answer 3

Idiomatic to me would be s.groupBy(....) The answer by #Omar Mainegra - s.groupBy(...).mapValues( flatten ) absolutely works but it looks like a hack where the initial result needs some extra massaging.
The issue is with the implementation of groupBy and more specifically with groupByTo:
public inline fun <T, K, V, M : MutableMap<in K, MutableList<V>>> Iterable<T>.groupByTo(destination: M, keySelector: (T) -> K, valueTransform: (T) -> V): M {
for (element in this) {
val key = keySelector(element)
val list = destination.getOrPut(key) { ArrayList<V>() }
list.add(valueTransform(element))
}
return destination
}
The implementation wraps the values associated with a key in a list because in general multiple values can be associated with a key which is not the case here
where values in s are unique which means that groupBy is the wrong function to use. The right function is associateWith:
s.associateWith { x -> u.filter { it.contains(x) } }
produces:
{1=[[1, 2, 3], [1, 4, 7], [1, 5, 9]], 9=[[1, 5, 9]]}