onebrc-probe: lane S — SWAR delimiter scan + branchless temp parse (+23% v3, +31% native)

[AdaWorldAPI]

Idea

Add the two 1BRC-frontier compute levers on top of lane F's flat open-addressed table (its group-by was already right — only scan/parse varies):

• (b) SWAR delimiter find — the haszero bit trick over a u64 (x = word ^ needle; (x - ONES) & !x & HIGH), 8 bytes/step with a scalar tail, replacing the byte-by-byte while data[i] != b';' loop.
• (b) branchless temp parse — -?\d?\d.\d → fixed-point tenths.
• (c) name compare — kept as &[u8] == &[u8], which LLVM lowers to a vectorized memcmp.

Reuses lane F's SoaTable / fnv1a64 / morton_slot / table_to_map verbatim and the same chunk_bounds/merge_maps threading. std-only, zero external deps.

Measured

10M-row corpus, 4 workers, n=11 median, throughput_mrows_s:

build	flat F (best)	SWAR S (best)	Δ
v3 (x86-64-v3)	84.6	103.9	+23%
native	74.0	96.9	+31%

Correctness

• Parity: lane_s_agrees_with_lane_a — SWAR lane produces aggregates identical to lane A on a generated corpus.
• The match_mask uses the strict (x - ONES) & !x & HIGH form; find returns the earliest match, so the residual borrow-propagation false-positive mode cannot mislocate a delimiter (independently verified by a brutally-honest pre-merge review: LAND verdict, all findings P2).

Scope note

throughput_mrows_s is compute-only (main.rs reads the file before Instant::now()), so (a) mmap — a wall-clock / 13 GB-allocation lever — is not measurable in this harness and is deliberately not implemented here (it would break the std-only, zero-dep contract).

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Generated by Claude Code

Summary by CodeRabbit

• New Features

  • Added a new s processing mode for the CLI.
  • Introduced a faster parsing path that improves record scanning and temperature handling.
  • Results are still merged consistently with existing output behavior.

• Tests

  • Added coverage for delimiter detection, temperature parsing, and end-to-end output consistency.

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: b88a2993-1c0e-4aa1-bcad-bb2d29c60945

📥 Commits

Reviewing files that changed from the base of the PR and between 8a14994 and 3b62b78.

📒 Files selected for processing (1)

• crates/onebrc-probe/src/main.rs

📝 Walkthrough

Walkthrough

Adds a new SWAR-accelerated probe lane (lane_s) to the onebrc-probe crate, implementing branchless delimiter search and temperature parsing, an entrypoint that splits work across threads and merges results, crate-level exports, and a new CLI dispatch option ("s") to invoke it.

Changes

Lane S SWAR Implementation

Layer / File(s)	Summary
SWAR delimiter search and temperature parsing crates/onebrc-probe/src/lane_s.rs	Implements match_mask, a SWAR-based find delimiter scanner with scalar tail handling, and parse_tenths for parsing fixed-format temperatures into tenths.
Record accumulation and threaded entrypoint crates/onebrc-probe/src/lane_s.rs	Adds accumulate_swar to parse records and populate SoaTable, and the public lane_s_swar function that chunks input, spawns worker threads, and merges per-chunk results.
Crate wiring, CLI dispatch, and tests crates/onebrc-probe/src/lib.rs, crates/onebrc-probe/src/main.rs	Declares and re-exports the lane_s module, adds a "s" lane option in the CLI, and includes tests verifying find, parse_tenths, and equality against lane_a_scalar.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Sequence Diagram(s)

sequenceDiagram
  participant CLI
  participant lane_s_swar
  participant WorkerThread
  participant accumulate_swar
  participant SoaTable

  CLI->>lane_s_swar: lane_s_swar(data, workers)
  lane_s_swar->>lane_s_swar: compute chunk_bounds
  loop per chunk
    lane_s_swar->>WorkerThread: spawn scoped thread(chunk)
    WorkerThread->>accumulate_swar: scan & parse chunk
    accumulate_swar->>SoaTable: observe(name, temp)
    WorkerThread-->>lane_s_swar: chunk map
  end
  lane_s_swar->>lane_s_swar: merge chunk maps
  lane_s_swar-->>CLI: BTreeMap<String, Stats>

Loading

Poem

A rabbit hops through bytes so fast,
SWAR words scanned, no cycles wasted,
Semicolons found, tenths well-tasted,
Threads race parallel, merged at last,
lane_s joins the field — hop hop, contrast! 🐇⚡

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the new lane S implementation and its main performance-oriented parsing changes.
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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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@crates/onebrc-probe/src/main.rs`:
- Line 135: The lane dispatch already handles "s" in the CLI, but the
user-facing usage/help text and unknown-lane error message are out of date.
Update the help/usage string in main and the unknown-lane branch that prints the
expected lanes so they include the newly supported lane "s" at minimum, and make
sure the listed options stay consistent with the dispatch arms in main.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: b9fb7888-3c1a-4295-97f7-4eb8e47fdfd3

📥 Commits

Reviewing files that changed from the base of the PR and between e4bea83 and 8a14994.

📒 Files selected for processing (3)

• crates/onebrc-probe/src/lane_s.rs
• crates/onebrc-probe/src/lib.rs
• crates/onebrc-probe/src/main.rs