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normalizers: implement the Lowercase normalizer - #72
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`NormalizerConfig::Lowercase` is already parsed but has no runtime
implementation, so tokenizers declaring `{"type": "Lowercase"}` fail to load
with "unsupported normalizer type: Lowercase". Add the Lowercase normalizer
and wire it into `Normalizer`. This unlocks standalone `Lowercase` and the
CLIP / sentence-transformers shape `Sequence[NFC, Replace, Lowercase]`;
uncased BERT still needs `NFD` + `StripAccents`.
Lowercasing is done per character with `char::to_lowercase`, mirroring HF's
`NormalizedString::lowercase`. This is deliberately not `str::to_lowercase`:
the std method additionally applies the Greek final-sigma rule, rendering
`ὈΔΥΣΣΕΎΣ` as `ὀδυσσεύς` where HF produces `ὀδυσσεύσ`. U+03A3 is the only
character the two disagree on. The "would this character change?" predicate
compares against the mapping itself rather than `char::is_uppercase()`, which
is false for titlecase letters such as `Dž` U+01C5 and would produce wrong
output.
Normalization runs single-threaded over the whole document before rayon
parallelism starts at BPE, so both the scan and the transform take a SWAR fast
path over ASCII, mirroring `ascii_lower_run_end` in `pre_tokenizers::scan`. The
transform is branch-free: `0x80 >> 2 == 0x20` is exactly the ASCII case bit.
Measured against a straightforward `chars().flat_map(char::to_lowercase)` on
~100 KiB inputs: 68x faster on already-lowercase ASCII, 45x on mixed case and
on ALL CAPS, and 0.95x on CJK (both dominated by std's LOWERCASE_TABLE binary
search, which HF pays too).
Tests cover the Cow::Borrowed guarantee, a capital at every one of 28 byte
offsets to pin the SWAR lane indexing and window boundary, titlecase letters,
the 1->2 expansion of `İ` U+0130, and final sigma — the last asserting
inequality with `str::to_lowercase` so a refactor towards it fails loudly.
`local_tests::lowercase_normalizer_matches_huggingface` diffs against the
`tokenizers` crate directly.
Also re-exports `Prepend` at the crate root, which was missed when it landed
in edccd31 (`Nfc` and `Replace` are both there).
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Rebasing onto main brings in `Normalizer::is_identity_on` (crusoecloud#74), whose match did not cover the new `Lowercase` variant, so the crate no longer compiled. `Lowercase` is the identity exactly when `first_change` finds nothing to lower, which also holds for every substring, so expose that scan as `Lowercase::is_normalized` (as `Nfc::is_normalized`) and use it: already lowercase text keeps the in-place encode path instead of being copied through normalization. The HF parity test now also asserts that `is_identity_on` agrees with whether HF's Lowercase changed the text, since the encode path skips normalization when it says so. Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
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Summary
Adds the
Lowercasenormalizer. Tokenizers declaring{"type": "Lowercase"}previously failed to load with
unsupported normalizer type: Lowercase— theconfig was parsed but had no runtime implementation.
Notes
Parity. Lowercasing is per character via
char::to_lowercase, mirroring HF'sNormalizedString::lowercase. Deliberately notstr::to_lowercase, which alsoapplies the Greek final-sigma rule —
ὈΔΥΣΣΕΎΣ→ὀδυσσεύςwhere HF givesὀδυσσεύσ. U+03A3 is the only character the two disagree on.Testing
9 unit tests covering the
Cow::Borrowedguarantee, titlecase letters, the 1→2expansion of
İU+0130, final sigma, and a capital at every one of 28 byteoffsets to pin the SWAR lane indexing. Plus
local_tests::lowercase_normalizer_matches_huggingface, which diffs directlyagainst the
tokenizerscrate. Full suite passes;fmtandclippy -D warningsclean.
Also re-exports
Prependat the crate root, missed when it landed inedccd31.