Vendor things

third-party/vendor/ahash/README.md

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+# aHash     ![Build Status](https://img.shields.io/github/actions/workflow/status/tkaitchuck/aHash/rust.yml?branch=master) ![Licence](https://img.shields.io/crates/l/ahash) ![Downloads](https://img.shields.io/crates/d/ahash) 
+
+AHash is the [fastest](https://github.com/tkaitchuck/aHash/blob/master/compare/readme.md#Speed), 
+[DOS resistant hash](https://github.com/tkaitchuck/aHash/wiki/How-aHash-is-resists-DOS-attacks) currently available in Rust.
+AHash is intended *exclusively* for use in in-memory hashmaps. 
+
+AHash's output is of [high quality](https://github.com/tkaitchuck/aHash/blob/master/compare/readme.md#Quality) but aHash is **not** a cryptographically secure hash.
+
+## Design
+
+Because AHash is a keyed hash, each map will produce completely different hashes, which cannot be predicted without knowing the keys.
+[This prevents DOS attacks where an attacker sends a large number of items whose hashes collide that get used as keys in a hashmap.](https://github.com/tkaitchuck/aHash/wiki/How-aHash-is-resists-DOS-attacks)
+
+This also avoids [accidentally quadratic behavior by reading from one map and writing to another.](https://accidentallyquadratic.tumblr.com/post/153545455987/rust-hash-iteration-reinsertion)
+
+## Goals and Non-Goals
+
+AHash does *not* have a fixed standard for its output. This allows it to improve over time. For example,
+if any faster algorithm is found, aHash will be updated to incorporate the technique.
+Similarly, should any flaw in aHash's DOS resistance be found, aHash will be changed to correct the flaw.
+
+Because it does not have a fixed standard, different computers or computers on different versions of the code will observe different hash values.
+As such, aHash is not recommended for use other than in-memory maps. Specifically, aHash is not intended for network use or in applications which persist hashed values.
+(In these cases `HighwayHash` would be a better choice)
+
+Additionally, aHash is not intended to be cryptographically secure and should not be used as a MAC, or anywhere which requires a cryptographically secure hash.
+(In these cases `SHA-3` would be a better choice)
+
+## Usage
+
+AHash is a drop in replacement for the default implementation of the `Hasher` trait. To construct a `HashMap` using aHash 
+as its hasher do the following:
+
+```rust
+use ahash::{AHasher, RandomState};
+use std::collections::HashMap;
+
+let mut map: HashMap<i32, i32, RandomState> = HashMap::default();
+map.insert(12, 34);
+```
+For convenience, wrappers called `AHashMap` and `AHashSet` are also provided.
+These do the same thing with slightly less typing.
+```rust
+use ahash::AHashMap;
+
+let mut map: AHashMap<i32, i32> = AHashMap::new();
+map.insert(12, 34);
+map.insert(56, 78);
+```
+
+## Flags
+
+The aHash package has the following flags:
+* `std`: This enables features which require the standard library. (On by default) This includes providing the utility classes `AHashMap` and `AHashSet`.
+* `serde`: Enables `serde` support for the utility classes `AHashMap` and `AHashSet`.
+* `runtime-rng`: To obtain a seed for Hashers will obtain randomness from the operating system. (On by default)
+This is done using the [getrandom](https://github.com/rust-random/getrandom) crate.
+* `compile-time-rng`: For OS targets without access to a random number generator, `compile-time-rng` provides an alternative.
+If `getrandom` is unavailable and `compile-time-rng` is enabled, aHash will generate random numbers at compile time and embed them in the binary.
+* `nightly-arm-aes`: To use AES instructions on 32-bit ARM, which requires nightly. This is not needed on AArch64.
+This allows for DOS resistance even if there is no random number generator available at runtime (assuming the compiled binary is not public).
+This makes the binary non-deterministic. (If non-determinism is a problem see [constrandom's documentation](https://github.com/tkaitchuck/constrandom#deterministic-builds))
+
+If both `runtime-rng` and `compile-time-rng` are enabled the `runtime-rng` will take precedence and `compile-time-rng` will do nothing.
+If neither flag is set, seeds can be supplied by the application. [Multiple apis](https://docs.rs/ahash/latest/ahash/random_state/struct.RandomState.html)
+are available to do this.
+
+## Comparison with other hashers
+
+A full comparison with other hashing algorithms can be found [here](https://github.com/tkaitchuck/aHash/blob/master/compare/readme.md)
+
+![Hasher performance](https://docs.google.com/spreadsheets/d/e/2PACX-1vSK7Li2nS-Bur9arAYF9IfT37MP-ohAe1v19lZu5fd9MajI1fSveLAQZyEie4Ea9k5-SWHTff7nL2DW/pubchart?oid=1323618938&format=image)
+
+For a more representative performance comparison which includes the overhead of using a HashMap, see [HashBrown's benchmarks](https://github.com/rust-lang/hashbrown#performance)
+as HashBrown now uses aHash as its hasher by default.
+
+## Hash quality
+
+AHash passes the full [SMHasher test suite](https://github.com/rurban/smhasher). 
+
+The code to reproduce the result, and the full output [are checked into the repo](https://github.com/tkaitchuck/aHash/tree/master/smhasher).
+
+## Additional FAQ
+
+A separate FAQ document is maintained [here](https://github.com/tkaitchuck/aHash/blob/master/FAQ.md). 
+If you have questions not covered there, open an issue [here](https://github.com/tkaitchuck/aHash/issues).
+
+## 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.
+
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