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.
• 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 (not HashDoS-resistant):

 name                       oldstdhash ns/iter  hashbrown ns/iter  diff ns/iter   diff %  speedup 
 insert_ahash_highbits        20,846              7,397                   -13,449  -64.52%   x 2.82 
 insert_ahash_random          20,515              7,796                   -12,719  -62.00%   x 2.63 
 insert_ahash_serial          21,668              7,264                   -14,404  -66.48%   x 2.98 
 insert_erase_ahash_highbits  29,570              17,498                  -12,072  -40.83%   x 1.69 
 insert_erase_ahash_random    39,569              17,474                  -22,095  -55.84%   x 2.26 
 insert_erase_ahash_serial    32,073              17,332                  -14,741  -45.96%   x 1.85 
 iter_ahash_highbits          1,572               2,087                       515   32.76%   x 0.75 
 iter_ahash_random            1,609               2,074                       465   28.90%   x 0.78 
 iter_ahash_serial            2,293               2,120                      -173   -7.54%   x 1.08 
 lookup_ahash_highbits        3,460               4,403                       943   27.25%   x 0.79 
 lookup_ahash_random          6,377               3,911                    -2,466  -38.67%   x 1.63 
 lookup_ahash_serial          3,629               3,586                       -43   -1.18%   x 1.01 
 lookup_fail_ahash_highbits   5,286               3,411                    -1,875  -35.47%   x 1.55 
 lookup_fail_ahash_random     12,365              4,171                    -8,194  -66.27%   x 2.96 
 lookup_fail_ahash_serial     4,902               3,240                    -1,662  -33.90%   x 1.51 

With the libstd default SipHash hasher (HashDoS-resistant):

 name                       oldstdhash ns/iter  hashbrown ns/iter  diff ns/iter   diff %  speedup 
 insert_std_highbits        32,598              20,199                  -12,399  -38.04%   x 1.61 
 insert_std_random          29,824              20,760                   -9,064  -30.39%   x 1.44 
 insert_std_serial          33,151              17,256                  -15,895  -47.95%   x 1.92 
 insert_erase_std_highbits  74,731              48,735                  -25,996  -34.79%   x 1.53 
 insert_erase_std_random    73,828              47,649                  -26,179  -35.46%   x 1.55 
 insert_erase_std_serial    73,864              40,147                  -33,717  -45.65%   x 1.84 
 iter_std_highbits          1,518               2,264                       746   49.14%   x 0.67 
 iter_std_random            1,502               2,414                       912   60.72%   x 0.62 
 iter_std_serial            6,361               2,118                    -4,243  -66.70%   x 3.00 
 lookup_std_highbits        21,705              16,962                   -4,743  -21.85%   x 1.28 
 lookup_std_random          21,654              17,158                   -4,496  -20.76%   x 1.26 
 lookup_std_serial          18,726              14,509                   -4,217  -22.52%   x 1.29 
 lookup_fail_std_highbits   25,852              17,323                   -8,529  -32.99%   x 1.49 
 lookup_fail_std_random     25,913              17,760                   -8,153  -31.46%   x 1.46 
 lookup_fail_std_serial     22,648              14,839                   -7,809  -34.48%   x 1.53 

Usage

Add this to your Cargo.toml:

[dependencies]
hashbrown = "0.6"

Then:

use hashbrown::HashMap;

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

This crate has the following Cargo features:

• nightly: Enables nightly-only features: #[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)
• ahash: Compiles with ahash as default hasher. (enabled by default)
• ahash-compile-time-rng: Activates the compile-time-rng feature of ahash, to increase the DOS-resistance, but can result in issues for no_std builds. More details in issue#124. (enabled by default)

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.