Switch to foldhash-portable and add generic BuildHasher support

- Replace xxhash-rust with foldhash-portable (portable feature)
- Add S: BuildHasher generic parameter to Filter 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)
- 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

Author: arthurprs

.github/workflows/ci.yml

@@ -36,6 +36,23 @@ jobs:
       - run: cargo test --no-default-features
       - run: cargo test --all-features
 
+  cross-platform-test:
+    name: Cross-platform tests
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        target:
+          - powerpc-unknown-linux-gnu    # 32-bit big-endian
+          - i686-unknown-linux-gnu       # 32-bit little-endian
+          - aarch64-unknown-linux-gnu    # 64-bit little-endian (ARM)
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+      - uses: taiki-e/install-action@v2
+        with:
+          tool: cross
+      - run: cross test --features serde --target ${{ matrix.target }} --lib
+
   fuzz-tests:
     name: Fuzz tests
     runs-on: ubuntu-latest

Cargo.toml

@@ -19,7 +19,7 @@ jsonschema = ["schemars"]
 serde = ["dep:serde", "dep:serde_bytes"]
 
 [dependencies]
-xxhash-rust = { version = "0.8.12", features = ["xxh3"] }
+foldhash-portable = { version = "0.2.0", features = ["portable"] }
 serde = { optional = true, version = "1", features = ["derive"] }
 serde_bytes = { optional = true, version = "0.11" }
 schemars = { optional = true, version = "1" }

README.md

@@ -42,7 +42,9 @@ for i in 0..1000 {
 
 ### Hasher
 
-The hashing algorithm used is [xxhash3](https://crates.io/crates/xxhash-rust) which offers both high performance and stability across platforms.
+Methods accepting `T: Hash` are provided for convenience using [foldhash-portable](https://crates.io/crates/foldhash-portable), which offers high performance and stability across platforms. Note that a fixed seed is used (no DoS resistance) and `#[derive(Hash)]` output is [not guaranteed stable](https://github.com/hoxxep/portable-hash#whats-wrong-with-the-stdhash-traits) across Rust compiler versions.
+
+`Filter` supports a custom `BuildHasher` via its `S` type parameter (similar to `HashMap`). Use `Filter::new_with_hasher()` and related constructors. The hasher is not serialized; on deserialization it is reconstructed via `S::default()`. If that doesn't produce the correct hasher, use `Filter::with_hasher()` to restore it. Filters being merged must use the same hasher.
 
 ### Filter size