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Optimise immutable.Queue for head/tail #11396

NthPortal opened this Issue Feb 9, 2019 · 0 comments


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NthPortal commented Feb 9, 2019

Optimise immutable.Queue so that head/tail usage pattern does not cause double traversal of in.

Take the following usage pattern:

var queue: Queue[Int] = ???
while (queue.nonEmpty) {
  val elem = queue.head
  queue = queue.tail
  // do something with `elem` here

If out is empty and in is not, val elem = queue.head will require traversing in to find its last element, and queue = queue.tail will require reversing in (which requires traversal).

immutable.Queue's methods should be optimised so that when dequeueing elements, it does not leave out empty (unless in is also empty), and when enqueueing elements, it puts at least one element in out if both in and out are empty (although once you're checking, might as well put everything in out).

This change should be binary compatible, as it only requires changes to class internals.

@NthPortal NthPortal added this to the 2.13.1 milestone Feb 9, 2019

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