# xxhash-rust

![Rust](https://github.com/DoumanAsh/xxhash-rust/workflows/Rust/badge.svg?branch=master)
[![Crates.io](https://img.shields.io/crates/v/xxhash-rust.svg)](https://crates.io/crates/xxhash-rust)
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Implementation of [xxHash](https://github.com/Cyan4973/xxHash) in Rust

Each algorithm is implemented via feature, allowing precise control over code size.

## Example

- Cargo.toml

```toml
[dependencies.xxhash-rust]
version = "0.8.5"
features = ["xxh3", "const_xxh3"]
```

- main.rs

```rust
use xxhash_rust::const_xxh3::xxh3_64 as const_xxh3;
use xxhash_rust::xxh3::xxh3_64;

const TEST: u64 = const_xxh3(b"TEST");

fn test_input(text: &str) -> bool {
    match xxh3_64(text.as_bytes()) {
        TEST => true,
        _ => false
    }
}

assert!(!test_input("tEST"));
assert!(test_input("TEST"));
```

## Features:

By default all features are off.

- `std` - Enables `std::io::Write` trait implementation
- `xxh32` - Enables 32bit algorithm. Suitable for x86 targets
- `const_xxh32` - `const fn` version of `xxh32` algorithm
- `xxh64` - Enables 64 algorithm. Suitable for x86_64 targets
- `const_xxh64` - `const fn` version of `xxh64` algorithm
- `xxh3` - Enables `xxh3` family of algorithms, superior to `xxh32` and `xxh64` in terms of performance.
- `const_xxh3` - `const fn` version of `xxh3` algorithm

## HW acceleration

Similar to reference implementation, crate implements various SIMDs in `xxh3` depending on provided flags.
All checks are performed only at compile time, hence user is encouraged to enable these accelerations (for example via `-C target_cpu=native`)

Used SIMD acceleration:

- SSE2 - widely available, can be safely enabled in 99% of cases. Enabled by default in `x86_64` targets.
- AVX2;
- Neon - Enabled by default on aarch64 targets (most likely)
- Wasm SIMD128 - Has to be enabled via rust flag: `-Ctarget-feature=+simd128`

## Streaming vs One-shot

For performance reasons one-shot version of algorithm does not re-use streaming version.
Unless needed, user is advised to use one-shot version which tends to be more optimal.

## `const fn` version

While `const fn` provides compile time implementation, it does so at performance cost.
Hence you should only use it at _compile_ time.

To guarantee that something is computed at compile time make sure to initialize hash output
as `const` or `static` variable, otherwise it is possible function is executed at runtime, which
would be worse than regular algorithm.

`const fn` is implemented in best possible way while conforming to limitations of Rust `const
fn`, but these limitations are quite strict making any high performance code impossible.

## Version note

- `0.8.*` corresponds to C's `0.8.*`

In order to  keep up with original implementation version I'm not planning to bump major/minor until C implementation does so.

## Comparison with twox-hash

Refer to my [comment](https://github.com/DoumanAsh/xxhash-rust/issues/10#issuecomment-980488647)

## aHash compares xxhash as slow

Stateful `Xxh3`, while not as efficient as one-shot implementation, is by no means slow.

`aHash` tests are constructed around std's inefficient `Hasher` interface that require to re-create hasher every time:
https://github.com/tkaitchuck/aHash/blob/d9b5c3ff8ce4acae3d2de0de53f5f023818b29c0/compare/tests/compare.rs#L116-L119

This is not intended way to use `Xxh3` hasher.

Regardless whether it is intentional or not, it is false statement and you should not take it at face value.

Using hasher correctly, or better oneshot version, will provide with results on par or even better with long inputs.

However, it is true that `aHash` performs very well with short inputs.
