rust-lang / rust-lang/hashbrown

`foldhash::fast::RandomState` is very bad

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Dominant language
Rust
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Description

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=8d9004344a0ddd7799b90e24f5866e75

This counts collisions. The number fluctuates wildly from 1.5% to 7% and mostly depends on global seed (if one measurement is bad then all of them are). Both foldhash::quality::RandomState and ahash::RandomState remain at around 2.9% and would be acceptable as default.

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First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start with the linked Rust Playground reproduction and compare the collision counts for foldhash::fast::RandomState, foldhash::quality::RandomState, and ahash::RandomState across global seeds. Done means the collision behavior is understood and the issue's reported instability has a justified resolution.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
performance
Issue type
Bug
Difficulty
5/5
Estimated time
Over a week
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
25/100

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