Optimize list subtraction (A -- B) and make it yield on large inputs #1998
@kvakvs had to pause his work on #1993 so this PR takes over where he left off. I've refactored it to make state handling easier to follow, and started using a tree instead of an array for the removal set, decreasing run-time complexity from
Other than that it's pretty much the same, and it should still be relatively easy to backport.
The removal set now uses a red-black tree instead of an array on large inputs, decreasing runtime complexity from `n*n` to `n*log(n)`. It will also exit early when there are no more items left in the removal set, drastically improving performance and memory use when the items to be removed are present near the head of the list. This got a lot more complicated than before as the overhead of always using a red-black tree was unacceptable when either of the inputs were small, but this compromise has okay-to-decent performance regardless of input size. Co-authored-by: Dmytro Lytovchenko <email@example.com>