Rust port of Google's SwissTable hash map
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Latest commit 4496e06 Nov 8, 2018

README.md

hashbrown

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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 = "0.1"

and this to your crate root:

extern crate hashbrown;

This crate has the following Cargo features:

  • nightly: Enables nightly-only features: no_std support and ~10% speedup from branch hint intrinsics.

License

Licensed under either of:

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.