README.md
1hashbrown
2=========
3
4[](https://github.com/rust-lang/hashbrown/actions)
5[](https://crates.io/crates/hashbrown)
6[](https://docs.rs/hashbrown)
7[](https://github.com/rust-lang/hashbrown)
8
9This crate is a Rust port of Google's high-performance [SwissTable] hash
10map, adapted to make it a drop-in replacement for Rust's standard `HashMap`
11and `HashSet` types.
12
13The original C++ version of SwissTable can be found [here], and this
14[CppCon talk] gives an overview of how the algorithm works.
15
16Since Rust 1.36, this is now the `HashMap` implementation for the Rust standard
17library. However you may still want to use this crate instead since it works
18in environments without `std`, such as embedded systems and kernels.
19
20[SwissTable]: https://abseil.io/blog/20180927-swisstables
21[here]: https://github.com/abseil/abseil-cpp/blob/master/absl/container/internal/raw_hash_set.h
22[CppCon talk]: https://www.youtube.com/watch?v=ncHmEUmJZf4
23
24## [Change log](CHANGELOG.md)
25
26## Features
27
28- Drop-in replacement for the standard library `HashMap` and `HashSet` types.
29- Uses [AHash](https://github.com/tkaitchuck/aHash) as the default hasher, which is much faster than SipHash.
30 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.
31- Around 2x faster than the previous standard library `HashMap`.
32- Lower memory usage: only 1 byte of overhead per entry instead of 8.
33- Compatible with `#[no_std]` (but requires a global allocator with the `alloc` crate).
34- Empty hash maps do not allocate any memory.
35- SIMD lookups to scan multiple hash entries in parallel.
36
37## Performance
38
39Compared to the previous implementation of `std::collections::HashMap` (Rust 1.35).
40
41With the hashbrown default AHash hasher:
42
43| name | oldstdhash ns/iter | hashbrown ns/iter | diff ns/iter | diff % | speedup |
44| :-------------------------- | :----------------: | ----------------: | :----------: | ------: | ------- |
45| insert_ahash_highbits | 18,865 | 8,020 | -10,845 | -57.49% | x 2.35 |
46| insert_ahash_random | 19,711 | 8,019 | -11,692 | -59.32% | x 2.46 |
47| insert_ahash_serial | 19,365 | 6,463 | -12,902 | -66.63% | x 3.00 |
48| insert_erase_ahash_highbits | 51,136 | 17,916 | -33,220 | -64.96% | x 2.85 |
49| insert_erase_ahash_random | 51,157 | 17,688 | -33,469 | -65.42% | x 2.89 |
50| insert_erase_ahash_serial | 45,479 | 14,895 | -30,584 | -67.25% | x 3.05 |
51| iter_ahash_highbits | 1,399 | 1,092 | -307 | -21.94% | x 1.28 |
52| iter_ahash_random | 1,586 | 1,059 | -527 | -33.23% | x 1.50 |
53| iter_ahash_serial | 3,168 | 1,079 | -2,089 | -65.94% | x 2.94 |
54| lookup_ahash_highbits | 32,351 | 4,792 | -27,559 | -85.19% | x 6.75 |
55| lookup_ahash_random | 17,419 | 4,817 | -12,602 | -72.35% | x 3.62 |
56| lookup_ahash_serial | 15,254 | 3,606 | -11,648 | -76.36% | x 4.23 |
57| lookup_fail_ahash_highbits | 21,187 | 4,369 | -16,818 | -79.38% | x 4.85 |
58| lookup_fail_ahash_random | 21,550 | 4,395 | -17,155 | -79.61% | x 4.90 |
59| lookup_fail_ahash_serial | 19,450 | 3,176 | -16,274 | -83.67% | x 6.12 |
60
61
62With the libstd default SipHash hasher:
63
64| name | oldstdhash ns/iter | hashbrown ns/iter | diff ns/iter | diff % | speedup |
65| :------------------------ | :----------------: | ----------------: | :----------: | ------: | ------- |
66| insert_std_highbits | 19,216 | 16,885 | -2,331 | -12.13% | x 1.14 |
67| insert_std_random | 19,179 | 17,034 | -2,145 | -11.18% | x 1.13 |
68| insert_std_serial | 19,462 | 17,493 | -1,969 | -10.12% | x 1.11 |
69| insert_erase_std_highbits | 50,825 | 35,847 | -14,978 | -29.47% | x 1.42 |
70| insert_erase_std_random | 51,448 | 35,392 | -16,056 | -31.21% | x 1.45 |
71| insert_erase_std_serial | 87,711 | 38,091 | -49,620 | -56.57% | x 2.30 |
72| iter_std_highbits | 1,378 | 1,159 | -219 | -15.89% | x 1.19 |
73| iter_std_random | 1,395 | 1,132 | -263 | -18.85% | x 1.23 |
74| iter_std_serial | 1,704 | 1,105 | -599 | -35.15% | x 1.54 |
75| lookup_std_highbits | 17,195 | 13,642 | -3,553 | -20.66% | x 1.26 |
76| lookup_std_random | 17,181 | 13,773 | -3,408 | -19.84% | x 1.25 |
77| lookup_std_serial | 15,483 | 13,651 | -1,832 | -11.83% | x 1.13 |
78| lookup_fail_std_highbits | 20,926 | 13,474 | -7,452 | -35.61% | x 1.55 |
79| lookup_fail_std_random | 21,766 | 13,505 | -8,261 | -37.95% | x 1.61 |
80| lookup_fail_std_serial | 19,336 | 13,519 | -5,817 | -30.08% | x 1.43 |
81
82## Usage
83
84Add this to your `Cargo.toml`:
85
86```toml
87[dependencies]
88hashbrown = "0.14"
89```
90
91Then:
92
93```rust
94use hashbrown::HashMap;
95
96let mut map = HashMap::new();
97map.insert(1, "one");
98```
99## Flags
100This crate has the following Cargo features:
101
102- `nightly`: Enables nightly-only features including: `#[may_dangle]`.
103- `serde`: Enables serde serialization support.
104- `rkyv`: Enables rkyv serialization support.
105- `rayon`: Enables rayon parallel iterator support.
106- `raw`: Enables access to the experimental and unsafe `RawTable` API.
107- `inline-more`: Adds inline hints to most functions, improving run-time performance at the cost
108 of compilation time. (enabled by default)
109- `ahash`: Compiles with ahash as default hasher. (enabled by default)
110- `allocator-api2`: Enables support for allocators that support `allocator-api2`. (enabled by default)
111
112## License
113
114Licensed under either of:
115
116 * Apache License, Version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or https://www.apache.org/licenses/LICENSE-2.0)
117 * MIT license ([LICENSE-MIT](LICENSE-MIT) or https://opensource.org/licenses/MIT)
118
119at your option.
120
121### Contribution
122
123Unless you explicitly state otherwise, any contribution intentionally submitted
124for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any
125additional terms or conditions.
126
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