Issue #16989 has been updated by Eregon (Benoit Daloze).


Also, does moving Set to core mean rewriting it to C?
I think that would be suboptimal, because every implementation would have to maintain its own native implementation then (or reuse the C extension).
It would also hurt readability.
Much of Ruby's performance can be achieved by JIT'ing Ruby code, and Hash is well optimized, so I'm not sure it's a performance gain either.

BTW SortedSet has this `rbtree` usage, that seems problematic to move in core (core should not depend on RubyGems):
https://github.com/ruby/ruby/blob/eada6350332155972f19bad52bd8621f607520a2/lib/set.rb#L708
Only moving Set but not SortedSet in core would avoid that issue though, but be somewhat inconsistent.

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Feature #16989: Sets: need 
https://bugs.ruby-lang.org/issues/16989#change-87381

* Author: marcandre (Marc-Andre Lafortune)
* Status: Assigned
* Priority: Normal
* Assignee: knu (Akinori MUSHA)
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I am opening a series of feature requests on `Set`, all of them based on this usecase.

The main usecase I have in mind is my recent experience with `RuboCop`. I noticed a big number of frozen arrays being used only to later call `include?` on them. This is `O(n)` instead of `O(1)`.

Trying to convert them to `Set`s causes major compatibility issues, as well as very frustrating situations and some cases that would make them much less efficient.

Because of these incompatibilities, `RuboCop` is in the process of using a custom class based on `Array` with optimized `include?` and `===`. `RuboCop` runs multiple checks on Ruby code. Those checks are called cops. `RuboCop` performance is (IMO) pretty bad and some cops  currently are in `O(n^2)` where n is the size of the code being inspected. Even given these extremely inefficient cops, optimizing the 100+ such arrays (most of which are quite small btw) gave a 5% speed boost.

RuboCop PRs for reference: https://github.com/rubocop-hq/rubocop-ast/pull/29
https://github.com/rubocop-hq/rubocop/pull/8133

My experience tells me that there are many other opportunities to use `Set`s that are missed because `Set`s are not builtin, not known enough and have no shorthand notation.

In this issue I'd like to concentrate the discussion on the following request: `Set`s should be core objects, in the same way that `Complex` were not and are now. Some of the upcoming feature requests would be easier (or only possible) to implement were `Set`s builtin.



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