⚡ Find the single-quoted scalar end with SIMD memchr

[frenck]

Breaking change

None. Pure internal optimization; parse results are byte-for-byte identical.

Proposed change

Follow-up to the quoted-scalar work (#224, #225, #226). The single-quoted content scan introduced in #226 walked the run one byte at a time, testing each byte against the closing quote, the line breaks, and the non-ASCII range.

This replaces that loop with a SIMD memchr3 search for the next stop byte (', \n, \r), so a long scalar (a description, an embedded script, a base64 blob) is skipped in a handful of vector loads instead of a comparison per byte. Column tracking stays exact: column is per character, so it advances by the run's byte length when the run is pure ASCII (the common case, checked with a SIMD is_ascii) and by an exact character count only when the run carries multi-byte content. memchr was already a transitive dependency (via regex), so promoting it to a direct dependency adds no build cost.

Single-quoted has exactly three stop bytes, a natural fit for memchr3; double-quoted has four (it also stops at \), so it keeps its byte loop.

Measured with callgrind on a multi-document single-quoted corpus: ~3% fewer instructions overall (its scalars are mostly short). The win scales with scalar length: on a stream of long single-quoted strings the scan is several times faster, since the per-byte loop is replaced by vector search. Behavior is unchanged: the full suite, the YAML compliance suite, all Rust unit tests, and round-trip plus parse fuzzing pass with no differences.

Type of change

• Dependency or tooling upgrade
• Bugfix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Deprecation (replaces or removes a feature, with a migration path)
• Breaking change (a fix or feature that changes existing behavior)
• Code quality, refactor, or test-only change
• Documentation only

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to: ⚡ Bulk-scan quoted-scalar content instead of walking char by char #226
• Link to a separate documentation pull request:

Checklist

• I have read the AI Policy, and this pull request was not created by an autonomous agent.
• I fully understand the code in this pull request and can explain every line, including any AI-assisted changes.
• The change is covered by tests, and uv run pytest passes locally. A pull request cannot be merged unless CI is green.
• uv run ruff check . and uv run ruff format --check . pass.
• cargo fmt --check and cargo clippy --all-targets -- -D warnings pass.
• Round-trip fidelity is preserved: an unmodified document still re-emits byte-for-byte.
• No commented-out or dead code is left in the pull request.

If the change is user-facing:

• Documentation under docs/ is added or updated, and docs/verify_examples.py still passes.

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Review details

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 339a93d3-a487-4fbe-bcd6-01d2da0f9c3f

📥 Commits

Reviewing files that changed from the base of the PR and between ab21d8a and e900f33.

📒 Files selected for processing (1)

• src/scanner/reader.rs

📝 Walkthrough

Walkthrough

Changes

A memchr = "2" dependency was added to Cargo.toml. The Reader::take_single_quoted_run function in src/scanner/reader.rs was reimplemented to use memchr::memchr3 to locate the next single quote, newline, or carriage return, replacing the previous byte-by-byte scan that also stopped on non-ASCII bytes. Column tracking now uses byte length for ASCII runs and UTF-8 character counting for non-ASCII runs, with pos advanced to the computed stop index.

Sequence Diagram(s)

Not applicable; the change is a scanning algorithm modification without cross-component interaction changes.

Related Issues: None referenced in the provided information.

Related PRs: None referenced in the provided information.

Suggested labels: performance, dependencies, scanner

Suggested reviewers: frenck

---

Poem
A rabbit twitched its whiskers keen,
found memchr's speed, both swift and clean,
through quotes and lines it darts ahead,
counting chars where UTF-8 led,
one hop, one scan, the scalar's read.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the SIMD memchr optimization for single-quoted scalar scanning.
Description check	✅ Passed	The description matches the changeset and accurately explains the performance-focused scanner update.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[codspeed-hq]

Merging this PR will not alter performance

✅ 16 untouched benchmarks

---

_{Comparing frenck/memchr-single-quoted (e900f33) with main (582cf73)}

[Copilot]

Pull request overview

Optimizes the YAML scanner’s single-quoted scalar fast path by replacing the per-byte stop-byte loop with a SIMD-accelerated memchr3 search, improving throughput on long single-quoted scalars while preserving identical parse results.

Changes:

• Replace the single-quoted scalar run scan with memchr::memchr3 and adjust column tracking to handle non-ASCII runs accurately.
• Promote memchr to a direct Rust dependency (already present transitively) and record it in the lockfile.

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/scanner/reader.rs	Reworks take_single_quoted_run to locate stop bytes via memchr3 and updates column advancement logic accordingly.
Cargo.toml	Adds memchr = "2" as a direct dependency with rationale comments.
Cargo.lock	Records memchr as a direct dependency of the crate.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.52%. Comparing base (4fd21ad) to head (e900f33).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #228      +/-   ##
==========================================
- Coverage   92.55%   92.52%   -0.03%     
==========================================
  Files          41       41              
  Lines       12782    12837      +55     
==========================================
+ Hits        11830    11878      +48     
- Misses        952      959       +7     

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