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hashbrown
=========
[](https://github.com/rust-lang/hashbrown/actions)
[](https://crates.io/crates/hashbrown)
[](https://docs.rs/hashbrown)
[](https://github.com/rust-lang/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.
[SwissTable]: https://abseil.io/blog/20180927-swisstables
[here]: https://github.com/abseil/abseil-cpp/blob/master/absl/container/internal/raw_hash_set.h
[CppCon talk]: https://www.youtube.com/watch?v=ncHmEUmJZf4
## [Change log](CHANGELOG.md)
## Features
- Drop-in replacement for the standard library `HashMap` and `HashSet` types.
- Uses [AHash](https://github.com/tkaitchuck/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`:
```toml
[dependencies]
hashbrown = "0.12"
```
Then:
```rust
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](https://github.com/tkaitchuck/aHash#flags) (disabled by default)
## License
Licensed under either of:
* Apache License, Version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or https://www.apache.org/licenses/LICENSE-2.0)
* MIT license ([LICENSE-MIT](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.
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