ChibiHash: Small, Fast 64 bit hash function

I started writing this because all the 64 bit hash functions I came across were either too slow (FNV-1a, one byte at a time processing), or too large spanning hundreds of lines of code, or non-portable due to using hardware specific instructions. Being small and portable, the goal is to be able to use ChibiHash as a good "default" for non-cryptographic 64-bit hashing needs.

Some key features:

• Small: ~65 loc in C
• Fast: See benchmark table below
• Portable: Doesn't use hardware specific instructions (e.g SSE)
• Good Quality: Passes smhasher and smhasher3, so should be good quality (I think)
• Unencumbered: Released into the public domain
• Free of undefined behavior and gives same result regardless of host system's endianness.
• Streaming API available at chibihash64-stream.h.
• Non-cryptographic

Here's some benchmark (made via smhasher3) against other similar themed hash functions:

1. Requires compiler/cpu support for retrieving the full 128 bit result of a 64x64 bit multiply.
2. Universal, but has a complicated seeding step.

When NOT to use

The introduction should make it clear on why you'd want to use this. Here are some reasons to avoid using this:

• For cryptographic purposes.
• For protecting against collision attacks (SipHash is the recommended one for this purpose).
• When you need very strong probability against collisions: ChibiHash does very minimal amount of mixing compared to other hashes (e.g xxhash64). And so chances of collision should in theory be higher.

Unofficial ports

A list of unofficial ports to other languages is maintained here.

Changelog

v2

• Faster performance on short string (42 cycles/hash vs 34 cycles/hash). The tail end handling has been reworked entirely with some inspiration from wyhash's short input reading.
• Better seeding. v1 seed only affected 64 bits of the initial state. v2 seed affects the full 256 bits. This allows it to pass smhasher3's SeedBlockLen and SeedBlockOffset tests.
• Slightly better mixing in bulk handling.
• Passes all 252 tests in smhasher3 (commit 34093a3), v1 failed 3.