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Rollup merge of #77996 - tkaitchuck:master, r=m-ou-se

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Doc change: Remove mention of `fnv` in HashMap

Disclaimer: I am the author of [aHash](https://github.com/tkaitchuck/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](https://github.com/rurban/smhasher), is [slow for larger keys](https://github.com/tkaitchuck/aHash/blob/master/compare/readme.md#speed), and does not provide dos resistance, because it is unkeyed (this can also cause [other problems](https://accidentallyquadratic.tumblr.com/post/153545455987/rust-hash-iteration-reinsertion)).

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 https://github.com/rust-lang/rust/pull/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: https://github.com/rust-lang/hashbrown/issues/153  https://github.com/rust-lang/hashbrown/issues/9 . There are also a number of [crates using it](https://crates.io/crates/fnv/reverse_dependencies) a great many of which are hashing strings (Which is when Fnv is the [worst](https://github.com/cbreeden/fxhash#benchmarks), [possible](https://github.com/tkaitchuck/aHash#speed), [choice](http://cglab.ca/~abeinges/blah/hash-rs/).)

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](https://github.com/tkaitchuck/aHash/blob/master/compare/readme.md#speed), is [of high quality](https://github.com/tkaitchuck/aHash#hash-quality), and [provides dos resistance](https://github.com/tkaitchuck/aHash/wiki/How-aHash-is-resists-DOS-attacks). It is popular (see [stats](https://crates.io/crates/ahash)) and is the default hasher for [hashbrown](https://crates.io/crates/hashbrown) and [dashmap](https://crates.io/crates/dashmap) which are the most popular alternative hashmaps. Finally it does not have any of the [`gotcha` cases](https://github.com/tkaitchuck/aHash#fxhash) 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>
This commit is contained in:
Guillaume Gomez 2020-11-13 15:26:10 +01:00 committed by GitHub
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6
library/std/src/collections/hash/map.rs
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@ -34,8 +34,8 @@ use crate::sys;
/// attacks such as HashDoS. /// attacks such as HashDoS.
/// ///
/// The hashing algorithm can be replaced on a per-`HashMap` basis using the /// The hashing algorithm can be replaced on a per-`HashMap` basis using the
/// [`default`], [`with_hasher`], and [`with_capacity_and_hasher`] methods. Many /// [`default`], [`with_hasher`], and [`with_capacity_and_hasher`] methods.
/// alternative algorithms are available on crates.io, such as the [`fnv`] crate. /// There are many alternative [hashing algorithms available on crates.io].
/// ///
/// It is required that the keys implement the [`Eq`] and [`Hash`] traits, although /// It is required that the keys implement the [`Eq`] and [`Hash`] traits, although
/// this can frequently be achieved by using `#[derive(PartialEq, Eq, Hash)]`. /// this can frequently be achieved by using `#[derive(PartialEq, Eq, Hash)]`.
@ -57,6 +57,7 @@ use crate::sys;
/// The original C++ version of SwissTable can be found [here], and this /// The original C++ version of SwissTable can be found [here], and this
/// [CppCon talk] gives an overview of how the algorithm works. /// [CppCon talk] gives an overview of how the algorithm works.
/// ///
/// [hashing algorithms available on crates.io]: https://crates.io/keywords/hasher
/// [SwissTable]: https://abseil.io/blog/20180927-swisstables /// [SwissTable]: https://abseil.io/blog/20180927-swisstables
/// [here]: https://github.com/abseil/abseil-cpp/blob/master/absl/container/internal/raw_hash_set.h /// [here]: https://github.com/abseil/abseil-cpp/blob/master/absl/container/internal/raw_hash_set.h
/// [CppCon talk]: https://www.youtube.com/watch?v=ncHmEUmJZf4 /// [CppCon talk]: https://www.youtube.com/watch?v=ncHmEUmJZf4
@ -154,7 +155,6 @@ use crate::sys;
/// [`default`]: Default::default /// [`default`]: Default::default
/// [`with_hasher`]: Self::with_hasher /// [`with_hasher`]: Self::with_hasher
/// [`with_capacity_and_hasher`]: Self::with_capacity_and_hasher /// [`with_capacity_and_hasher`]: Self::with_capacity_and_hasher
/// [`fnv`]: https://crates.io/crates/fnv
/// ///
/// ``` /// ```
/// use std::collections::HashMap; /// use std::collections::HashMap;