Rapidhash Unity port

Ten years ago I was writing about various non-cryptographic hash functions. Back then xxHash was new (introduced in 2014)! However, quite some things have changed since then. xxHash itself got a new “XXH3” version (2020); “wyhash” appeared (2020+), and eventually evolved into “rapidhash” (2024+). Many others too, but this is about rapidhash.

It is small and beautiful. Yes, current (V3) version is over 500 lines of C code, but that is three hash function variants and several tweaking options.

I ported it to C# (Unity/Burst) and the full core rapidhash implementation is barely over 100 lines of code.

• Full repository: UnitySmolRapidhash on github (MIT license).
• The actual source file: SmolRapidhash3.cs
• It uses Unity’s Burst to get access to 128 bit multiply function, and the code itself has [BurstCompile] on it.
• API is similar to Unity.Collections.xxHash3 class, except it returns 64 bit value directly instead of an int2, and has helper entry points for hashing a single struct or various arrays:

  static ulong Hash64<T>(ref T key) where T : unmanaged;
  static ulong Hash64<T>(T[] key) where T : unmanaged;
  static ulong Hash64<T>(Span<T> key) where T : unmanaged;
  static ulong Hash64<T>(NativeArray<T> key) where T : unmanaged;
  static ulong Hash64(void* key, long length);
  

Performance

Burst approaches native (C) performance of rapidhash at larger input sizes, nice!

• The calling benchmark program is just a C# (not Burst) script tested in the editor; might be cause of some overhead for small input sizes.
• Curiously, C#/Burst port of XXH3 (as provided by Unity.Collections package) is 30-40% slower than native (C) implementation. This slowdown is not there for rapidhash.

Rapidhash is always faster than XXH3; the difference is more pronounced on arm64.

Ryzen 5950X / Windows / Visual Studio 2022 (17.14.23): rapidhash reaches 38GB/s. Native XXH3 version is similar for large input sizes (slower for small sizes). However the C#/Burst version of XXH3 only reaches 24GB/s.

Apple M4 Max / macOS / Xcode 16.1: rapidhash reaches 67GB/s. Native XXH3 version reaches 50GB/s, and C#/Burst version of XXH3 reaches only 30GB/s.

That’s it!