feat: add xxhash64

[PsiACE]

I just found that we've implemented murmur3hash ourselves, while Java also implements xxhash.

• Ported xxhash64 (seeded) to match DataSketches Java behavior.

[Copilot]

Pull request overview

This PR adds an xxhash64 implementation to the hash module, matching the DataSketches Java behavior. The implementation provides a fast, non-cryptographic 64-bit hash function with seeded hashing support.

• Implements XxHash64 struct with the standard Hasher trait
• Provides both streaming API (write/finish64) and convenience method (hash_u64)
• Includes comprehensive test coverage with reference test vectors for verification

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/hash/xxhash.rs	New complete implementation of xxhash64 with constants, hasher struct, trait implementations, helper functions, and test suite
src/hash/mod.rs	Adds module declaration and public export for XxHash64 (with unused_imports attribute as hash module is currently internal-only)

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[notfilippo]

I just found that we've implemented murmur3hash ourselves, while Java also implements xxhash.

I think we should wait for sketches that need xxhash before shipping it ourselves. cc @tisonkun

[tisonkun]

I just found that we've implemented murmur3hash ourselves, while Java also implements xxhash.

I think we should wait for sketches that need xxhash before shipping it ourselves. cc @tisonkun

Agree. We can hold this PR for now.

Whether or not publish the hash function is still under discussion, somehow datasketches-java and dataskecthes-cpp use them internally and datasketches-go clearly define the hash function internally.

Seems mainly BloomFilter uses XxHash. @PsiACE you can take a look at #5 to see where we can have those sketches and xxhash as part of its implementation.

[notfilippo]

I am working on a implementation of bloom filters at the moment. I can rework it to use this PR.

[tisonkun]

I am working on a implementation of bloom filters at the moment. I can rework it to use this PR.

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