vendor/hashbrown/README.md

   1 hashbrown
   2 =========
   3
   4 [![Build Status](https://travis-ci.com/rust-lang/hashbrown.svg?branch=master)](https://travis-ci.com/rust-lang/hashbrown)
   5 [![Crates.io](https://img.shields.io/crates/v/hashbrown.svg)](https://crates.io/crates/hashbrown)
   6 [![Documentation](https://docs.rs/hashbrown/badge.svg)](https://docs.rs/hashbrown)
   7 [![Rust](https://img.shields.io/badge/rust-1.49.0%2B-blue.svg?maxAge=3600)](https://github.com/rust-lang/hashbrown)
   8
   9 This crate is a Rust port of Google's high-performance [SwissTable] hash
  10 map, adapted to make it a drop-in replacement for Rust's standard `HashMap`
  11 and `HashSet` types.
  12
  13 The original C++ version of SwissTable can be found [here], and this
  14 [CppCon talk] gives an overview of how the algorithm works.
  15
  16 Since Rust 1.36, this is now the `HashMap` implementation for the Rust standard
  17 library. However you may still want to use this crate instead since it works
  18 in 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
  39 Compared to the previous implementation of `std::collections::HashMap` (Rust 1.35).
  40
  41 With 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
  62 With 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
  84 Add this to your `Cargo.toml`:
  85
  86 ```toml
  87 [dependencies]
  88 hashbrown = "0.11"
  89 ```
  90
  91 Then:
  92
  93 ```rust
  94 use hashbrown::HashMap;
  95
  96 let mut map = HashMap::new();
  97 map.insert(1, "one");
  98 ```
  99 ## Flags
 100 This crate has the following Cargo features:
 101
 102 - `nightly`: Enables nightly-only features including: `#[may_dangle]`.
 103 - `serde`: Enables serde serialization support.
 104 - `rayon`: Enables rayon parallel iterator support.
 105 - `raw`: Enables access to the experimental and unsafe `RawTable` API.
 106 - `inline-more`: Adds inline hints to most functions, improving run-time performance at the cost
 107   of compilation time. (enabled by default)
 108 - `bumpalo`: Provides a `BumpWrapper` type which allows `bumpalo` to be used for memory allocation.
 109 - `ahash`: Compiles with ahash as default hasher. (enabled by default)
 110 - `ahash-compile-time-rng`: Activates the `compile-time-rng` feature of ahash. For targets with no random number generator
 111 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)
 112
 113 ## License
 114
 115 Licensed under either of:
 116
 117  * Apache License, Version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
 118  * MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
 119
 120 at your option.
 121
 122 ### Contribution
 123
 124 Unless you explicitly state otherwise, any contribution intentionally submitted
 125 for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any
 126 additional terms or conditions.