Doc change: Remove mention of fnv in HashMap

[tkaitchuck]

Disclaimer: I am the author of aHash.

This changes the Rustdoc in HashMap from mentioning the fnv crate to mentioning the aHash crate, as an alternative Hasher implementation.

Why

Fnv has poor hash quality, is slow for larger keys, and does not provide dos resistance, because it is unkeyed (this can also cause other problems).

Fnv has acceptable performance for integers and has very poor performance with keys >32 bytes. This is the reason it was removed from the standard library in #37229 .

Because regardless of which dimension you value, there are better alternatives, it does not make sense for anyone to consider using fnv.

The text mentioning fnv in the standard library continues to create confusion: rust-lang/hashbrown#153 rust-lang/hashbrown#9 . There are also a number of crates using it a great many of which are hashing strings (Which is when Fnv is the worst, possible, choice.)

I think aHash makes the most sense to mention as an alternative because it is the most credible option (in my obviously biased opinion). It offers good performance on numbers and strings, is of high quality, and provides dos resistance. It is popular (see stats) and is the default hasher for hashbrown and dashmap which are the most popular alternative hashmaps. Finally it does not have any of the gotcha cases that FxHash suffers from. (Which is the other popular hashing option when DOS attacks are not a concern)

Signed-off-by: Tom Kaitchuck tom.kaitchuck@emc.com

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @LukasKalbertodt (or someone else) soon.

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[jonas-schievink]

I'd rather not use libstd to promote hash functions that haven't seen much review, especially since aHash makes claims about DOS-resistance.

[Mark-Simulacrum]

cc @rust-lang/libs

Historically we've tried to not mention external crates in std docs; I think it may make sense to drop fnv here. One of the options might be to point at some summary post, though I worry that it will be an endless treadmill of updating it. I guess one option is something like https://en.wikipedia.org/wiki/List_of_hash_functions.

[sfackler]

I would also prefer to link to either hash functions in general or a widely used and understood one.

[jyn514]

@tkaitchuck FYI I tried changing rustc_data_structures to ahash just now and it gave

error[E0277]: the trait bound `K: Hash` is not satisfied
  --> compiler/rustc_data_structures/src/sso/map.rs:89:29
   |
89 |             SsoHashMap::Map(FxHashMap::with_capacity_and_hasher(cap, Default::default()))
   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Hash` is not implemented for `K`
   |
   = note: required by `AHashMap::<K, V, S>::with_capacity_and_hasher`

Consider relaxing that bound, I think you shouldn't need it for constructing the hashmap, only for operations involving the elements (get/set/entry).

[jyn514]

Same for various other functions, new(), capacity(), len(), etc.

[tkaitchuck]

@jyn514

@tkaitchuck FYI I tried changing rustc_data_structures to ahash just now and it gave

The error message there indicates the code is using an FxHashmap which is by definition using FxHash. If you want to switch that code, you will need to update it. (If there are suggestions please open an issue on the repo rather than adding more here.)

[tkaitchuck]

@Mark-Simulacrum

I guess one option is something like https://en.wikipedia.org/wiki/List_of_hash_functions.

Or perhaps an external list of Rust crates that implement the interface?

[LukasKalbertodt]

I agree with the previous comments: I'd rather not mention aHash in these docs. However, removing the mention of fnv makes sense to me. I'm fine with either of those:

• Just removing the mention of fnv.
• Linking Wikipedia.
• Linking to the "hash" keyword on crates.io (this includes lots of hashing libraries that cannot be used with the std hashmap; so maybe that would confuse readers.)

[tkaitchuck]

@LukasKalbertodt I have linked to the "hasher" keyword on crates.io. This avoids the problem of "hash" which contains lots of crates that don't implement the interface. Currently there are only 6 crates which have this keyword all but 1 provides an implementation of the interface. Of course many (most) of the popular alternatives do not have this keyword tagged to them, but that is easy to fix on the individual crates.

[LukasKalbertodt]

The hasher keyword indeed seems to be closer to what we want. However, until the popular crates add this keyword too, this is also not optimal.

@jonas-schievink @Mark-Simulacrum @sfackler What do you think about linking to this keyword?

[Mark-Simulacrum]

hasher keyword seems to contain hashers, but it's... very short (6 crates) and neither ahash, fnv, or any of the other "immediate" candidates I'd reach for are in there, despite being available in general. I think the hash keyword is closer to the right one right now, but maybe the Wikipedia link is preferable.

Edit: I also think that we can link to "hasher" but we should update crates first, and it creates a sort of "first mover" problem in some sense (e.g., if ahash ends up being first and only when this is merged). But maybe that's not too big a concern.

[tkaitchuck]

@Mark-Simulacrum I have opened several PRs.

[m-ou-se]

Thanks!

@bors r+ rollup

[bors]

📌 Commit 4e58483 has been approved by m-ou-se