gecko-dev/third_party/rust/ahash

aHash

AHash is the fastest, DOS resistant hash currently available in Rust. AHash is intended exclusively for use in in-memory hashmaps.

AHash's output is of high quality but aHash is not a cryptographically secure hash.

Design

Because AHash is a keyed hash, each map will produce completely different hashes, which cannot be predicted without knowing the keys. This prevents DOS attacks where an attacker sends a large number of items whose hashes collide that get used as keys in a hashmap.

This also avoids accidentally quadratic behavior by reading from one map and writing to another.

Goals and Non-Goals

AHash does not have a fixed standard for its output. This allows it to improve over time. For example, if any faster algorithm is found, aHash will be updated to incorporate the technique. Similarly, should any flaw in aHash's DOS resistance be found, aHash will be changed to correct the flaw.

Because it does not have a fixed standard, different computers or computers on different versions of the code will observe different hash values. As such, aHash is not recommended for use other than in-memory maps. Specifically, aHash is not intended for network use or in applications which persist hashed values. (In these cases HighwayHash would be a better choice)

Additionally, aHash is not intended to be cryptographically secure and should not be used as a MAC, or anywhere which requires a cryptographically secure hash. (In these cases SHA-3 would be a better choice)

Usage

AHash is a drop in replacement for the default implementation of the Hasher trait. To construct a HashMap using aHash as its hasher do the following:

use ahash::{AHasher, RandomState};
use std::collections::HashMap;

let mut map: HashMap<i32, i32, RandomState> = HashMap::default();
map.insert(12, 34);

For convenience, wrappers called AHashMap and AHashSet are also provided. These do the same thing with slightly less typing.

use ahash::AHashMap;

let mut map: AHashMap<i32, i32> = AHashMap::new();
map.insert(12, 34);
map.insert(56, 78);

Flags

The aHash package has the following flags:

• std: This enables features which require the standard library. (On by default) This includes providing the utility classes AHashMap and AHashSet.
• serde: Enables serde support for the utility classes AHashMap and AHashSet.
• compile-time-rng: Whenever possible aHash will seed hashers with random numbers using the getrandom crate. This is possible for OS targets which provide a source of randomness. (see the full list.) For OS targets without access to a random number generator, compile-time-rng provides an alternative. If getrandom is unavailable and compile-time-rng is enabled, aHash will generate random numbers at compile time and embed them in the binary. This allows for DOS resistance even if there is no random number generator available at runtime (assuming the compiled binary is not public). This makes the binary non-deterministic, unless getrandom is available for the target in which case the flag does nothing. (If non-determinism is a problem see constrandom's documentation)

NOTE: If getrandom is unavailable and compile-time-rng is disabled aHash will fall back on using the numeric value of memory addresses as a source of randomness. This is somewhat strong if ALSR is turned on (it is by default) but for embedded platforms this will result in weak keys. As a result, it is recommended to use compile-time-rng anytime random numbers will not be available at runtime.

Comparison with other hashers

A full comparison with other hashing algorithms can be found here

For a more representative performance comparison which includes the overhead of using a HashMap, see HashBrown's benchmarks as HashBrown now uses aHash as its hasher by default.

Hash quality

AHash passes the full SMHasher test suite.

The code to reproduce the result, and the full output are checked into the repo.

Additional FAQ

A separate FAQ document is maintained here. If you have questions not covered there, open an issue here.

License

Licensed under either of:

• Apache License, Version 2.0, (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.