Batch unordered

[ahadadi]

This version of batchUnordered treats the list of elements as a balanced binary tree, by leveraging the heap-on-array structure, aiming to make the recursion depth O(log(n)) instead of O(n).
It uses an AtomicIntegerArray to maintain the set of pending nodes, and uses an auxiliary AtomicLong to get the index of the pending node which is left-most and highest in the tree.
The algorithm does a Pre-order traversal of the tree. When it encounters a node processed by another thread, it hops to the left-most, highest node in the tree which is not yet processed. This increases the chance that the recursion will be shallow, and reduces the number of times an already processed node is encountered.

[oshai]

How about doing it without recursion to prevent stackoverflow?

[ahadadi]

I was not able to implement it without recursion due to the nature of ComposableFuture, i.e. without using anything but map / flatMap on the produced future.
Can you write down a non recursive solution?