perf: cache C# ERROR/malformed-declaration reason to avoid duplicate regex scan

[github-actions]

Summary

An explore pass looking for performance opportunities beyond the already-completed node.children array-allocation fixes found a real duplicate-work pattern in csharpAnalyzer.ts.

Root cause

visit() calls getComplexityIncrement(node) and, if it returns >0, immediately calls getComplexityReason(node) for the same node. For C# ERROR nodes and malformed preprocessor declarations, both the increment path (getComplexityFromErrorNode / getComplexityFromMalformedDeclaration) and the reason path (getComplexityReasonFromErrorNode / getComplexityReasonFromMalformedDeclaration) independently:

1. Re-extract the exact same source substring (sourceText.substring(node.startIndex, node.endIndex))
2. Re-run the same ~7 regex tests (if/while/for/foreach/logical-operator/ternary/try/catch) in the same order, just to determine which one matched

ERROR nodes can span large fragmented preprocessor blocks, so this duplicated substring + regex work on every hit.

Fix

The increment functions now record which pattern matched first (the same first-match order the reason functions already used) into a small heuristicReasonCache field keyed by node reference. The reason functions check this cache first and only fall back to full recomputation as a defensive path if called independently of the increment function (now /* c8 ignore */d as unreachable via normal visit() flow, consistent with existing unreachable-fallback annotations in this file).

No behavior change - same increments and reason strings are produced, just computed once instead of twice per node.

Trade-offs

• Adds one small object field (heuristicReasonCache) to the analyzer instance, overwritten between nodes - negligible memory cost, no functional impact since it's only consulted by reference-equality against the node that populated it.

Test Status

• npm run compile: clean
• npm run lint: clean
• npm run test:unit: 227/227 passing
• Coverage: 98.68 stmts / 94.67 branch / 99.01 funcs / 98.68 lines (matches pre-change baseline of 98.69/94.47/99.01/98.69; branch coverage improved slightly)

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[codecov]

Codecov Report

❌ Patch coverage is 82.14286% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.95%. Comparing base (c036b79) to head (0e673c0).

Files with missing lines	Patch %	Lines
src/metricsAnalyzer/languages/csharpAnalyzer.ts	82.14%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #565      +/-   ##
==========================================
+ Coverage   97.90%   97.95%   +0.05%     
==========================================
  Files          10       10              
  Lines        3766     3815      +49     
  Branches      455      446       -9     
==========================================
+ Hits         3687     3737      +50     
+ Misses         79       78       -1     

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[Copilot]

🟡 Changes recommended

Cached syntax nodes can unnecessarily retain entire parse trees when analyzer instances are reused.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Optimizes C# malformed-syntax analysis by caching heuristic reason strings and avoiding duplicate regex scans.

Changes:

• Caches reasons for ERROR and malformed declaration nodes.
• Adds defensive fallback reason computation.

File summaries

File	Description
src/metricsAnalyzer/languages/csharpAnalyzer.ts	Adds heuristic reason caching for C# analysis.

Review details

Suppressed comments (1)

src/metricsAnalyzer/languages/csharpAnalyzer.ts:800

• The malformed-declaration fast path also keeps its SyntaxNode cached after the value is used, retaining the associated parse tree for the lifetime of a reused analyzer. Clear the cache while consuming the reason, as in the ERROR-node path.

    if (this.heuristicReasonCache?.node === declarationNode) {
      return this.heuristicReasonCache.reason;
    }

• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Balanced

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