Switch to foldhash-portable and add generic BuildHasher support

[arthurprs]

Summary

• Replace xxhash-rust with foldhash-portable (portable feature)
• Add S: BuildHasher generic parameter to Filter, FilterRef, and Builder
• Preserve hasher across grow/shrink/merge via S: Clone
• Add _with_hasher constructors for Filter; Builder::new(filter) takes an empty Filter
• Add Filter::with_hasher() for swapping hasher (e.g. after deserialization)
• Add truncate_to_fingerprint() and compute_fingerprint_with_hasher() public API
• Remove compute_fingerprint() from public API (now test-only)
• Export StableBuildHasher for use with fingerprint APIs
• Remove StableHasher LE normalization (foldhash-portable handles it)
• Keep usize/isize → u64/i64 widening for 32-bit compatibility
• Add cross-platform hash stability test (verified on x86_64 LE + PowerPC 32-bit BE)
• Add cross-platform CI job (powerpc, i686, aarch64)
• Document T: Hash stability caveats and custom hasher usage

[Copilot]

Pull request overview

This PR updates qfilter’s hashing to use foldhash-portable (portable/stable across platforms) and extends the public API to support custom hashers via a generic S: BuildHasher on Filter and Builder.

Changes:

• Replace xxhash-rust with foldhash-portable and introduce StableBuildHasher as the default stable hasher.
• Add generic hasher support (S) to Filter/Builder, plus _with_hasher constructors and with_hasher() to swap hashers.
• Update docs, tests, and CI (including cross-target runs) to validate cross-platform stability.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/stable_hasher.rs	Switch stable hashing implementation to foldhash-portable and add StableBuildHasher.
src/lib.rs	Add S: BuildHasher generic support across Filter/Builder, hasher swapping, and updated hashing paths/tests.
README.md	Update documentation for new hashing backend and custom-hasher caveats.
Cargo.toml	Swap dependency from xxhash-rust to foldhash-portable with portable feature.
.github/workflows/ci.yml	Add cross-target test job to validate portability/stability.

Comments suppressed due to low confidence (2)

src/lib.rs:2269

• The merge/merge_sorted signatures use other: &Filter<impl AsRef<[u8]>, impl Sized>. impl Sized adds no constraint (all type params are Sized by default) and makes the API harder to read. Consider using explicit generic parameters instead (e.g., B2: AsRef<[u8]>, S2) so it's clear this accepts any buffer/hasher types.

            *self = self.merge_sorted(keep_duplicates, other)?;
        } else if other.fingerprint_size() >= self.fingerprint_size() {
            // Different fingerprint sizes: truncation changes sort order,
            // so fall back to one-by-one insertion.
            let max_count = if keep_duplicates { u64::MAX } else { 1 };

src/lib.rs:1284

• Builder<S> is already declared under impl<S: Default> Builder<S>, so the extra where S: Default on insert_fingerprint is redundant noise. Dropping the where clause will simplify the signature without changing bounds.

        match self.insert_impl(duplicate, hash) {
            Ok(inserted) => Ok(inserted),
            Err(Error::CapacityExceeded) => {
                *self = self.filter.rebuild_grown()?;

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[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 5 comments.

Comments suppressed due to low confidence (1)

src/lib.rs:2349

• rebuild_grown() returns Builder<S>, but the builder it constructs ultimately uses S::default() (via Builder::with_qr / Filter::with_qr_generic) instead of preserving self.build_hasher. For custom hashers this means an automatic grow will change the hasher/seed and invalidate future contains/insert/remove calls that accept T: Hash. Ensure rebuild uses the same build_hasher as self (e.g. clone/move it into the rebuilt filter).

    fn rebuild_grown(&self) -> Result<Builder<S>, Error> {
        let max = self.max_qbits.ok_or(Error::CapacityExceeded)?;
        if max <= self.qbits {
            return Err(Error::CapacityExceeded);
        }
        let qbits = self.qbits.checked_add(1).ok_or(Error::CapacityExceeded)?;
        let rbits = NonZeroU8::new(self.rbits.get() - 1).ok_or(Error::CapacityExceeded)?;
        let mut inserter = Builder::with_qr(qbits, rbits, self.max_qbits)?;
        // Use insert_impl directly: the new filter has 2x capacity so growth cannot happen.

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[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

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[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

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[Copilot]

Same issue as merge: impl Trait inside Filter<...> generic arguments (Filter<impl AsRef<[u8]>, impl Sized>) is not supported on stable Rust and will not compile. Prefer explicit generic parameters for other (e.g. B2, S2) and return Result<Self, Error> as you already do.

[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

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