index folding#124
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Pull request overview
This PR ports the “index folding” operation into the casefold crate, reusing the existing paged-bitmap/run-table approach and the same fast ASCII pre-pass to provide a compact 1-byte-per-character fold suitable for indexing.
Changes:
- Adds
index_fold(s: String) -> Vec<u8>that simple-folds and then collapses each character to a single byte (lossy, non-UTF-8) without allocating a second buffer. - Refactors
simple_foldinto its own module and shares atest_support::reference()parser across fold implementations. - Extends the generated table with a parallel
INDEX_DELTAside table and updates docs/benchmarks accordingly.
Show a summary per file
| File | Description |
|---|---|
| crates/casefold/src/simple_fold.rs | New module containing the optimized simple UTF-8 case-fold implementation and its tests. |
| crates/casefold/src/index_fold.rs | New index-fold implementation (1 byte per character) plus correctness tests against the reference map. |
| crates/casefold/src/lib.rs | Wires in the new modules/exports, adjusts bitmap popcount helper signature, and adds shared test support. |
| crates/casefold/build.rs | Emits the new INDEX_DELTA side table and updates size accounting. |
| crates/casefold/README.md | Documents index_fold and updates the table layout/size discussion. |
| crates/casefold/benchmarks/conversion.rs | Adds a Criterion benchmark variant for index_fold and updates benchmark descriptions. |
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| /// Like [`simple_fold`](crate::simple_fold), characters are never fully decoded | ||
| /// and the fold needs no UTF-8 reconstruction: the page coordinates come from | ||
| /// the lead/continuation bytes, and on a fold hit the folded low 7 bits are | ||
| /// `(cp & 0x7F)` plus the run's 7-bit `INDEX_DELTA`, masked back to 7 bits. |
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| #[test] | ||
| fn index_fold_matches_reference_map_exhaustive() { | ||
| // Drive every assigned code point through the byte-oriented index path | ||
| // and cross-check against the reference fold map. | ||
| let r = reference(); |
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| // collapses each code point to `cp & 0x7F`; by modular arithmetic the folded | ||
| // low-7-bit value is `((cp & 0x7F) + (delta & 0x7F)) & 0x7F`, so storing the | ||
| // code-point delta reduced mod 128 lets `index_fold` derive the folded index | ||
| // byte with one `wrapping_add` + mask — no UTF-8 reconstruction. The high | ||
| // bit is added unconditionally at write time, so only 7 bits are stored. |
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| // only folds that grow are U+023A/U+023E (2→3 bytes), so every 2 | ||
| // input bytes yield ≤3 output bytes; `+ 4` covers the 4-byte | ||
| // over-store of the final character. The non-zero capacity makes | ||
| // `out.as_mut_ptr()` non-null, so `dst` is non-null from here on. | ||
| out = Vec::with_capacity(bytes.len() + bytes.len() / 2 + 4); |
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Port our index fold operation as well using the same speedup tricks.