rust-hashbrown/README.md

hashbrown

This crate is 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:

name	oldstdhash ns/iter	hashbrown ns/iter	diff ns/iter	diff %	speedup
insert_ahash_highbits	18,865	8,020	-10,845	-57.49%	x 2.35
insert_ahash_random	19,711	8,019	-11,692	-59.32%	x 2.46
insert_ahash_serial	19,365	6,463	-12,902	-66.63%	x 3.00
insert_erase_ahash_highbits	51,136	17,916	-33,220	-64.96%	x 2.85
insert_erase_ahash_random	51,157	17,688	-33,469	-65.42%	x 2.89
insert_erase_ahash_serial	45,479	14,895	-30,584	-67.25%	x 3.05
iter_ahash_highbits	1,399	1,092	-307	-21.94%	x 1.28
iter_ahash_random	1,586	1,059	-527	-33.23%	x 1.50
iter_ahash_serial	3,168	1,079	-2,089	-65.94%	x 2.94
lookup_ahash_highbits	32,351	4,792	-27,559	-85.19%	x 6.75
lookup_ahash_random	17,419	4,817	-12,602	-72.35%	x 3.62
lookup_ahash_serial	15,254	3,606	-11,648	-76.36%	x 4.23
lookup_fail_ahash_highbits	21,187	4,369	-16,818	-79.38%	x 4.85
lookup_fail_ahash_random	21,550	4,395	-17,155	-79.61%	x 4.90
lookup_fail_ahash_serial	19,450	3,176	-16,274	-83.67%	x 6.12

With the libstd default SipHash hasher:

name	oldstdhash ns/iter	hashbrown ns/iter	diff ns/iter	diff %	speedup
insert_std_highbits	19,216	16,885	-2,331	-12.13%	x 1.14
insert_std_random	19,179	17,034	-2,145	-11.18%	x 1.13
insert_std_serial	19,462	17,493	-1,969	-10.12%	x 1.11
insert_erase_std_highbits	50,825	35,847	-14,978	-29.47%	x 1.42
insert_erase_std_random	51,448	35,392	-16,056	-31.21%	x 1.45
insert_erase_std_serial	87,711	38,091	-49,620	-56.57%	x 2.30
iter_std_highbits	1,378	1,159	-219	-15.89%	x 1.19
iter_std_random	1,395	1,132	-263	-18.85%	x 1.23
iter_std_serial	1,704	1,105	-599	-35.15%	x 1.54
lookup_std_highbits	17,195	13,642	-3,553	-20.66%	x 1.26
lookup_std_random	17,181	13,773	-3,408	-19.84%	x 1.25
lookup_std_serial	15,483	13,651	-1,832	-11.83%	x 1.13
lookup_fail_std_highbits	20,926	13,474	-7,452	-35.61%	x 1.55
lookup_fail_std_random	21,766	13,505	-8,261	-37.95%	x 1.61
lookup_fail_std_serial	19,336	13,519	-5,817	-30.08%	x 1.43

Usage

Add this to your Cargo.toml:

[dependencies]
hashbrown = "0.12"

Then:

use 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.
• rayon: Enables rayon parallel iterator support.
• 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)
• bumpalo: Provides a BumpWrapper type which allows bumpalo to be used for memory allocation.
• ahash: Compiles with ahash as default hasher. (enabled by default)
• ahash-compile-time-rng: Activates the compile-time-rng feature of ahash. For targets with no random number generator this pre-generates seeds at compile time and embeds them as constants. See aHash's documentation (disabled 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.