perf: optimize hot paths and fix benchmark infrastructure

[matthew-dean]

Summary

• Performance: Replace closures and forEach/map with for loops in hot paths (findMatch, Selector.eval, Ruleset.eval), yielding ~8% improvement on the 104KB benchmark (median 38.6ms → 36.4ms)
• Benchmark fixes: Fix percentage() calls in benchmark .less files for v4's default parens-division math mode; add --math=always passthrough to benchmark runner for cross-version consistency
• Benchmark infrastructure: Handle all v3.12+/v4.x build scenarios in run-historical.sh (pnpm for workspace protocol, fallback tsc, separate runtime dep install for broken monorepo npm installs); use last patch of each minor version

Benchmark results (104KB file, Apple M4 Pro)

Version	Median (ms)	Notes
v3.9.0	40.5	pre-regression baseline
v3.10.0	140.5	3.5x regression (Object.defineProperty)
v4.5.1 (released)	40.5	regression recovered
This PR	36.6	10% faster than v3.9

Test plan

• All 205 tests pass (grunt test)
• Benchmarked across v2.0–v4.5.1 with run-historical.sh
• No behavioral changes — only constant-factor optimizations

Summary by CodeRabbit

Release Notes

• Bug Fixes

  • Enhanced Less percentage calculation parsing by clarifying division expressions in mathematical operations.

• Performance

  • Optimized internal evaluation and iteration patterns in selector and ruleset processing for improved efficiency.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 8bcc0971-2eaa-45cf-a9b2-0a2e46b049b9

📥 Commits

Reviewing files that changed from the base of the PR and between a1bdc97 and 749656d.

📒 Files selected for processing (5)

• packages/less/benchmark/results/.gitignore
• packages/less/benchmark/results/latest/macbook-pro_arm64.json
• packages/less/benchmark/results/runs/2026-03-09T21-34-11Z_macbook-pro_arm64.json
• packages/less/benchmark/results/runs/2026-03-09_macbook-pro_arm64.json
• packages/less/benchmark/run-historical.sh

📝 Walkthrough

Walkthrough

The pull request refines the Less.js benchmark infrastructure and compiler internals. Changes include syntactic adjustments to Less files wrapping division expressions in parentheses, enhancement of the benchmark runner to accept command-line options, substantial updates to the historical benchmark script with TypeScript fallback mechanisms and workspace handling, consolidation of benchmark results, and refactoring of internal compiler evaluation patterns in ruleset, selector, and extend-visitor modules for explicit iteration and context handling.

Changes

Cohort / File(s)	Summary
Benchmark Less Syntax Updates benchmark-import-reference-target.less, benchmark-v39.less, benchmark.less	Added parentheses around division expressions in percentage calculations (e.g., percentage((@i/@n))) to clarify parsing without altering behavior.
Benchmark Infrastructure benchmark-runner.js, run-historical.sh, results/latest/macbook-pro_arm64.json	Extended benchmark runner to parse command-line options (--key=value) and forward them to less.render; updated historical script with TypeScript fallback mechanism, workspace detection, expanded version list, and explicit --math=always flag; consolidated macbook-pro benchmark results JSON into single entry.
Compiler Evaluation Refactoring src/less/tree/ruleset.js, src/less/tree/selector.js, src/less/visitors/extend-visitor.js	Refactored registry discovery from IIFE to explicit loop; replaced map-based array evaluation with guarded for loops in selector evaluation; refactored extend-visitor nested loops with explicit variable scoping and local iteration, removing unused context references.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~35 minutes

Poem

🐰 With parentheses placed and options in hand,
The benchmark script runs throughout the land,
Old loops give way to explicit flows clear,
Compiler paths refined, optimization near!
✨ Registry discoveries, selectors align—
Less renders faster with each tweak so fine! 🏃‍♂️💨

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main changes: performance optimizations to hot code paths and fixes to the benchmark infrastructure, which align with the core changes across the entire PR.

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[coderabbitai]

🧹 Nitpick comments (1)

packages/less/benchmark/benchmark-runner.js (1)

13-19: Regex pattern only matches lowercase option names.

The pattern /^--([a-z-]+)=(.*)$/ won't match Less options with uppercase letters (e.g., --sourceMap=true, --strictMath=true). While this works for the current use case (--math=always), it may cause confusion if other options are needed later.

🔧 Suggested fix to support camelCase options

-  var optMatch = process.argv[ai].match(/^--([a-z-]+)=(.*)$/);
+  var optMatch = process.argv[ai].match(/^--([a-zA-Z-]+)=(.*)$/);

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In `@packages/less/benchmark/benchmark-runner.js` around lines 13 - 19, The
arg-parsing regex only allows lowercase option names, so update the parser in
the process.argv loop that builds extraOpts (the var extraOpts and optMatch
usage) to accept camelCase and digits; replace the pattern /^--([a-z-]+)=(.*)$/
with one that permits letters, digits, underscores and hyphens (e.g., use a
character class like [A-Za-z0-9_-] or \w plus hyphen) so options such as
--sourceMap=true and --strictMath=true are matched and stored in extraOpts.

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Nitpick comments:
In `@packages/less/benchmark/benchmark-runner.js`:
- Around line 13-19: The arg-parsing regex only allows lowercase option names,
so update the parser in the process.argv loop that builds extraOpts (the var
extraOpts and optMatch usage) to accept camelCase and digits; replace the
pattern /^--([a-z-]+)=(.*)$/ with one that permits letters, digits, underscores
and hyphens (e.g., use a character class like [A-Za-z0-9_-] or \w plus hyphen)
so options such as --sourceMap=true and --strictMath=true are matched and stored
in extraOpts.

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 9d2fe84f-66bf-4a45-aef0-fe3c2a8b0a7f

📥 Commits

Reviewing files that changed from the base of the PR and between 7e48ad9 and a1bdc97.

📒 Files selected for processing (9)

• packages/less/benchmark/benchmark-import-reference-target.less
• packages/less/benchmark/benchmark-runner.js
• packages/less/benchmark/benchmark-v39.less
• packages/less/benchmark/benchmark.less
• packages/less/benchmark/results/latest/macbook-pro_arm64.json
• packages/less/benchmark/run-historical.sh
• packages/less/src/less/tree/ruleset.js
• packages/less/src/less/tree/selector.js
• packages/less/src/less/visitors/extend-visitor.js