Use NavigableSet for representing version sets, rather than List

[thecoop]

We can use NavigableSet to represent version sets to show that they are sorted and unique, rather than lists that don't enforce this.

[elasticsearchmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[ldematte]

I skimmed though this and I thing it's a good idea; the only drawbacks I can think of are related to performance (e.g. if you need to traverse all versions), but it seems to me that most of the operations are equivalent (e.g. first, last). Wdyt?

[thecoop]

Yes, the main change is use of randomFrom implementation using an iterator. This will not be iterating over a lot of items each time, and it's only use is in tests, so I don't think this will have an measurable impact.

We could use lists in some places in test code if you think this will matter, or define some kind of immutable sorted set with a backing list?

[ldematte]

define some kind of immutable sorted set with a backing list?

That's a cool idea, but I don't think it's necessary; like you said, the iteration is limited and performed only by test code

[rjernst]

I think we only need it to be navigable in tests, right? Could we maintain the sorted List in production code, and have the version util for tests build a TreeSet from that?

[thecoop]

I've kept a List on the production classes. I've also introduced a logarithmic O(nlogn) algo for randomFrom in navigable sets, which eliminates the sequential iteration over the sets

[ldematte]

LGTM, I like the new random utility!

[elasticsearchmachine]

💚 Backport successful

Status	Branch	Result
✅	9.0