Is it possible to extract the substream key in akkastreams?

I can't seem to find any documentation on this but I know that AkkaStreams stores the keys used to group a stream into substreams when calling groupBy in memory. Is it possible to extract those keys from the substream? Say I create a bunch of substreams from my main stream, pass those through a fold that counts the objects in each substream and then store the count in a class. Can I get the key of the substream to also pass to that class? Or is there a better way of doing this? I need to count each element per substream but I also need to store which group the count belongs to.

Solution

A nice example is shown in the stream-cookbook:

val counts: Source[(String, Int), NotUsed] = words
// split the words into separate streams first
  .groupBy(MaximumDistinctWords, identity)
  //transform each element to pair with number of words in it
  .map(_ -> 1)
  // add counting logic to the streams
  .reduce((l, r) => (l._1, l._2 + r._2))
  // get a stream of word counts
  .mergeSubstreams

Then the following:

val words = Source(List("Hello", "world", "let's", "say", "again", "Hello", "world"))
counts.runWith(Sink.foreach(println))

Will print:

(world,2)
(Hello,2)
(let's,1)
(again,1)
(say,1)

Another example I thought of, was counting numbers by their remainders. So the following, as example:

Source(0 to 101)
  .groupBy(10, x => x % 4)
  .map(e => e % 4 -> 1)
  .reduce((l, r) => (l._1, l._2 + r._2))
  .mergeSubstreams.to(Sink.foreach(println)).run()

will print:

(0,26)
(1,26)
(2,25)
(3,25)