hashbrown

Fork of https://github.com/rust-lang/hashbrown with added COW support.

HashMap implementation that will copy the data when-ever it modification is made to either the entry list or the value in the map itself.

The original crate was a Rust port of Google's high-performance SwissTable hash map, adapted to make it a drop-in replacement for Rust's standard HashMap and HashSet types.

The original C++ version of SwissTable can be found here, and this CppCon talk gives an overview of how the algorithm works.

Since Rust 1.36, this is now the HashMap implementation for the Rust standard library. However you may still want to use this crate instead since it works in environments without std, such as embedded systems and kernels.

Change log

Features

• Drop-in replacement for the standard library HashMap and HashSet types.
• Uses AHash as the default hasher, which is much faster than SipHash. However, AHash does not provide the same level of HashDoS resistance as SipHash, so if that is important to you, you might want to consider using a different hasher.
• Around 2x faster than the previous standard library HashMap.
• Lower memory usage: only 1 byte of overhead per entry instead of 8.
• Compatible with #[no_std] (but requires a global allocator with the alloc crate).
• Empty hash maps do not allocate any memory.
• SIMD lookups to scan multiple hash entries in parallel.

Performance

Compared to the previous implementation of std::collections::HashMap (Rust 1.35).

With the hashbrown default AHash hasher:

With the libstd default SipHash hasher:

Usage

Add this to your Cargo.toml:

[dependencies]
hashbrown = "0.14"

Then:

use cow_hashbrown::HashMap;

let mut map = HashMap::new();
map.insert(1, "one");

Flags

This crate has the following Cargo features:

• nightly: Enables nightly-only features including: #[may_dangle].
• serde: Enables serde serialization support.
• borsh: Enables borsh serialization support.
• rkyv: Enables rkyv serialization support.
• rayon: Enables rayon parallel iterator support.
• equivalent: Allows comparisons to be customized with the Equivalent trait.
• raw: Enables access to the experimental and unsafe RawTable API.
• inline-more: Adds inline hints to most functions, improving run-time performance at the cost of compilation time. (enabled by default)
• default-hasher: Compiles with ahash as default hasher. (enabled by default)
• allocator-api2: Enables support for allocators that support allocator-api2. (enabled by default)

License

Licensed under either of:

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