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.

Documentation

Change log

Features

• Drop-in replacement for the standard library HashMap and HashSet types.
• Uses FxHash as the default hasher, which is much faster than SipHash.
• Around 2x faster than FxHashMap and 8x faster than the standard HashMap.
• Lower memory usage: only 1 byte of overhead per entry instead of 8.
• Compatible with #[no_std] (currently requires nightly for the alloc crate).
• Empty hash maps do not allocate any memory.
• SIMD lookups to scan multiple hash entries in parallel.

Performance

Compared to std::collections::HashMap:

 name               stdhash ns/iter  hashbrown ns/iter  diff ns/iter    diff %  speedup
 find_existing      23,831           2,935                   -20,896   -87.68%   x 8.12
 find_nonexisting   25,326           2,283                   -23,043   -90.99%  x 11.09
 get_remove_insert  124              25                          -99   -79.84%   x 4.96
 grow_by_insertion  197              177                         -20   -10.15%   x 1.11
 hashmap_as_queue   72               18                          -54   -75.00%   x 4.00
 new_drop           14               0                           -14  -100.00%    x inf
 new_insert_drop    78               55                          -23   -29.49%   x 1.42

Compared to rustc_hash::FxHashMap (standard HashMap using FxHash instead of SipHash):

 name               fxhash ns/iter  hashbrown ns/iter  diff ns/iter    diff %  speedup
 find_existing      5,951           2,935                    -3,016   -50.68%   x 2.03
 find_nonexisting   4,637           2,283                    -2,354   -50.77%   x 2.03
 get_remove_insert  29              25                           -4   -13.79%   x 1.16
 grow_by_insertion  160             177                          17    10.62%   x 0.90
 hashmap_as_queue   22              18                           -4   -18.18%   x 1.22
 new_drop           9               0                            -9  -100.00%    x inf
 new_insert_drop    64              55                           -9   -14.06%   x 1.16

Usage

Add this to your Cargo.toml:

[dependencies]
hashbrown = "0.2"

This crate has the following Cargo features:

• nightly: Enables nightly-only features: no_std support and #[may_dangle].
• serde: Enables serde serialization support.
• rayon: Enables rayon parallel iterator support.

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.