[myluma]

What

• rust/src/single_threaded.rs is a thread-free replacement for tantivy's IndexWriter, behind a new single-threaded Cargo feature. It is built from tantivy's public pieces (SegmentWriter, write_alive_bitset, merge_filtered_segments, LogMergePolicy) and keeps IndexWriter's behaviour:
  • the same opstamps from commit and rollback;
  • a delete affects only documents added before it;
  • delete_all and rollback work the same way;
  • the writer lock is held for the writer's lifetime;
  • segments flush when they outgrow the memory budget;
  • tantivy's default merge policy still applies.
• scripts/build-wasm.sh compiles the existing C ABI to wasm32-wasip1 with that feature, without mmap or zstd, as a cdylib. JavaScript calls the same tantivy_* functions Swift does. The only WASM-specific code is rust/src/wasm.rs, which adds tantivy_alloc / tantivy_dealloc.
• js/ is a TypeScript package, @carbon/tantivy, marked private: true for now so it can't be published by accident. It mirrors the Swift API: SchemaBuilder, Index, IndexWriter, Query, SearchHit and SearchCollection. The mapping table is in js/README.md.
  • It has no runtime dependencies, and the 8 WASI imports are shimmed in engine.ts.
  • Each index gets its own WebAssembly instance, so close() returns its memory and a panic retires only that index.
• CI:
  • The Swift job becomes a matrix that runs the full suite against both writers.
  • A new Ubuntu job builds the WASM module and runs the JS tests.
• scripts/build-host.sh forwards its arguments to cargo, e.g. --features single-threaded.

Why

We want to run TantivySwift's engine in the browser without SharedArrayBuffer or cross-origin-isolation headers, which the stock threaded build would need. Keeping the JS package on the same C ABI keeps its API in step with Swift.

Reviewer notes

• The Swift build is unchanged. The native default feature re-enables mmap and columnar-zstd-compression exactly as tantivy's defaults did, and IndexWriter is still used. The only other native change is that indexes are now created through Index::builder(), which is equivalent.
• Merges run during commit() in the single-threaded writer, instead of in the background.
• Why the merge copies files: merge_filtered_segments writes into its own index, so the merge output is copied back into the main index. IndexMerger isn't public.
• Size: the WASM module is 5.1 MB, 1.6 MB gzipped, with symbols stripped. opt-level=s saves about 280 KB gzipped but makes search about 1.6× slower, so the build stays at 3.

Testing

• swift test passes 420/420 with the threaded writer and 420/420 with --features single-threaded, including the on-disk tests.
• npm test passes 40/40 under Node 24.
• js/example/index.html ran in Chromium: it loaded, indexed and searched, with no console errors.
• cargo test passes under both feature sets.

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