RFR: 8345314: Add a red–black tree as a utility data structure [v3]

[Afshin Zafari]

On Tue, 3 Dec 2024 12:51:18 GMT, Casper Norrbin <cnorrbin at openjdk.org> wrote:

>> Hi everyone,
>> 
>> This effort began as an exploration of replacing the current NMT treap with a red-black tree. Along the way, I discovered that others were also interested in having a general-purpose tree structure available within HotSpot.
>> 
>> The red-black tree is designed to serve as a drop-in replacement for the existing NMT treap, keeping a nearly identical interface. However, I’ve also added a few additional requested features, such as an iterator.
>> 
>> Testing builds off the treap tests, adding a few extra that inserts/removes and checks that the tree is correct. Testing uses the function `verify_self`, which iterates over the tree and checks that all red-black tree properties hold. Additionally, the tree has been tested in vmatree instead of the treap without any errors.
>> 
>> For those who may want to revisit the fundamentals of red-black trees, [Wikipedia](https://en.wikipedia.org/wiki/Red%E2%80%93black_tree) offers a great summary with tables covering the various balancing cases. Alternatively, your favorite data structure book could provide even more insight.
>
> Casper Norrbin has updated the pull request incrementally with one additional commit since the last revision:
> 
>   fixed nmt tag

Thank you for the nice implementation. The code is so neat and clean.
One point I want to bring here is testing the RBTree if it is really balanced or not. This is the main characteristic of the RBTree. It can be done, for example, by a new API as `RBTree.get_max_depth()` which is expected to be $\approx log_2(n)$ if the tree has $n$ nodes.

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PR Review: https://git.openjdk.org/jdk/pull/22360#pullrequestreview-2482515114