| # rustc-hash |
| |
| [](https://crates.io/crates/rustc-hash) |
| [](https://docs.rs/rustc-hash) |
| |
| A speedy, non-cryptographic hashing algorithm used by `rustc`. |
| The [hash map in `std`](https://doc.rust-lang.org/std/collections/struct.HashMap.html) uses SipHash by default, which provides resistance against DOS attacks. |
| These attacks aren't a concern in the compiler so we prefer to use a quicker, |
| non-cryptographic hash algorithm. |
| |
| The original hash algorithm provided by this crate was one taken from Firefox, |
| hence the hasher it provides is called FxHasher. This name is kept for backwards |
| compatibility, but the underlying hash has since been replaced. The current |
| design for the hasher is a polynomial hash finished with a single bit rotation, |
| together with a wyhash-inspired compression function for strings/slices, both |
| designed by Orson Peters. |
| |
| For `rustc` we have tried many different hashing algorithms. Hashing speed is |
| critical, especially for single integers. Spending more CPU cycles on a higher |
| quality hash does not reduce hash collisions enough to make the compiler faster |
| on real-world benchmarks. |
| |
| ## Usage |
| |
| This crate provides `FxHashMap` and `FxHashSet` as collections. |
| They are simply type aliases for their `std::collection` counterparts using the Fx hasher. |
| |
| ```rust |
| use rustc_hash::FxHashMap; |
| |
| let mut map: FxHashMap<u32, u32> = FxHashMap::default(); |
| map.insert(22, 44); |
| ``` |
| |
| ### `no_std` |
| |
| The `std` feature is on by default to enable collections. |
| It can be turned off in `Cargo.toml` like so: |
| |
| ```toml |
| rustc-hash = { version = "2.0", default-features = false } |
| ``` |
