How to change my helper function so that is collects the results of the parallel processing tasks - kotlin

Question

I wrote this helper function, so that I can easily process a list in parallel and only continue code execution when all the work is done. It works nicely when you don't need to return a result.
(I know it isn't the best practice to create new pools every time, it can be easily moved out, but I wanted to keep the examples simple.)
fun recursiveAction(action: () -> Unit): RecursiveAction {
return object : RecursiveAction() {
override fun compute() {
action()
}
}
}
fun <T> List<T>.parallelForEach(parallelSize: Int, action: (T) -> Unit) {
ForkJoinPool(parallelSize).invoke(recursiveAction {
this.parallelStream().forEach { action(it) }
})
}
Example use:
val myList: List<SomeClass> [...]
val parallelSize: Int = 8
myList.parallelForEach(parallelSize) { listElement ->
//Some task here
}
Is there any way to make a similar helper construct for when you want to collect the results back into a list?
I know I have to use a RecursiveTask instead of the RecursiveAction, but I couldn't manage to write a helper function like I had above to wrap it.
I'd like to use it like this:
val myList: List<SomeClass> [...]
val parallelSize: Int = 8
val result: List<SomeClass> = myList.parallelForEach(parallelSize) { listElement ->
//Some task here
}
Alternatively, is there a simpler way to do this alltogether?

Answer 1

Answered by JeffMurdock over on Reddit
fun <T> recursiveTask(action: () -> T): RecursiveTask<T> {
return object : RecursiveTask<T>() {
override fun compute(): T {
return action()
}
}
}
fun <T, E> List<T>.parallelForEach(parallelSize: Int, action: (T) -> E): List<E> {
val pool = ForkJoinPool(parallelSize)
val result = mutableListOf<ForkJoinTask<E>>()
for (item in this) {
result.add(pool.submit(recursiveTask {
action(item)
}))
}
return result.map { it.join() }
}
fun main(args: Array<String>) {
val list = listOf(1, 2, 3)
list.parallelForEach(3) { it + 2 }.forEach { println(it) }
}