Imported From: https://issues.scala-lang.org/browse/SI-9954?orig=1
Reporter: @Atry
Affected Versions: 2.11.8

Can I pick this one? I need some guidance on how should i approach it.
I've just started to contribute

This change is most likely not binary compatible (I haven't checked), so it has to target 2.13. In this case, it does not make sense to fix it in the current source code under scala/scala, as the collections library is being reworked at https://github.com/scala/collection-strawman. You could check in the new implementation if the issue is already fixed (or maybe @szeiger knows).

the 2.13 collections are now far enough along that now would be a good time for someone to check if this is resolved there yet or not.

It does not seem to be resolved yet - VectorIterator inherits all of the specified methods from Iterator. The implementation of take in Iterator seems to be constant time (I'm not sure why it wouldn't be previously), but the others appear to be linear time.

It might be worth looking to solve this in general for Iterators over IndexedSeqs, rather than just for VectorIterator.

The more general optimizations could be added to IndexedSeqView.IndexedSeqViewIterator. I'd go a step further and generalized IndexedSeqViewIterator to an IndexedIterator that can be used with any Iterable so you don't need to create the intermediate View. Both should be relatively simple to implement.

This won't be any use for Vector though because it uses its own, even more optimized VectorIterator. You would have to implement another Vector-specifc version of the optimized operations which looks much harder (because you have to know how Vector works internally).

Sequential indexing can often be optimized surprisingly well, but the custom iterator shaves off another 15% of the runtime. Also, it can't always be optimized that well (if the loop is more complex), and then there's a major penalty (~10x, if you have to dig all the way down the 32-wide tree each time).

VectorIterator.drop exists in 2.13 but still no slice or take

drop exists but turns out to be broken as I discovered while trying to implement take and slice:

scala> Vector.from(0 until 20).iterator.drop(19).knownSize
res7: Int = 1

scala> Vector.from(0 until 20).iterator.drop(20).knownSize
res8: Int = 20