hashbrown
=========
[](https://travis-ci.com/rust-lang/hashbrown)
[](https://crates.io/crates/hashbrown)
[](https://docs.rs/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.
[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 `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`:
```toml
[dependencies]
hashbrown = "0.3"
```
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](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
* MIT license ([LICENSE-MIT](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.