Replace Hashtable/ArrayList with generic collections in formula eval hot path

[ken-swyfft]

Summary

• Replace Hashtable with Dictionary<K,V> and ArrayList with List<T> in the formula evaluation hot path, eliminating boxing overhead and double-lookup patterns
• Fix bug in FormulaCellCache.Remove() which was keying on the cell object instead of cell.IdentityKey, causing silent removal failures
• Fix Decimal overflow crash in MultiplyEval when operands exceed decimal range (~7.9e28), falling back to double arithmetic to match Excel behavior
• Pass args to BenchmarkSwitcher.Run() so --filter works from CLI

Files changed

• PlainCellCache.cs — Hashtable → Dictionary<Loc, PlainValueCellCacheEntry>
• FormulaCellCache.cs — Hashtable → Dictionary<object, FormulaCellCacheEntry>
• OperationEvaluatorFactory.cs — Hashtable → Dictionary<OperationPtg, Function>
• FormulaUsedBlankCellSet.cs — Hashtable → Dictionary<BookSheetKey, BlankCellSheetGroup>
• Ptg.cs — ArrayList → List<Ptg>
• FormulaParser.cs — ArrayList → List<ParseNode> / List<object>
• MultiplyEval.cs — catch OverflowException for large values
• Program.cs — pass args through to BenchmarkSwitcher

Benchmark results (EvaluateAll on 1.43M formulas, 17MB .xlsx)

Metric	master	this PR	Change
Mean time	16.19 s	11.00 s	-32%
Allocated	14.91 GB	14.92 GB	~same

Test plan

• All 4,616 tests pass on net8.0 (0 failures)
• All 4,619 tests pass on net472 (0 failures)
• Benchmark: LargeExcelFileBenchmark.Evaluate completes without crash
• Review with Tony for merge suitability

🤖 Generated with Claude Code

[ken-swyfft]

Code Review

Overall: Approve with minor suggestions. The collection replacements are mechanically correct, no public API signatures change, and no thread-safety regression.

Good catch: FormulaCellCache.Remove() bug fix

The old code was calling _formulaEntriesByCell.Remove(cell) instead of Remove(cell.IdentityKey) — meaning entries were never actually removed, causing a memory leak during cell mutations. The fix is correct.

Moderate concerns

1. Missing test coverage for the FormulaCellCache.Remove fix — This is a real behavioral change (fixing a memory leak). A regression test demonstrating that entries are actually evicted after NotifyDeleteCell would guard against this silently regressing.

2. Missing test coverage for the MultiplyEval overflow fix — Casting a double > ~7.9e28 to decimal throws OverflowException; the fallback to double arithmetic is correct and matches Excel. However, the existing test file (TestMultiplyEval.cs) doesn't cover this scenario. A test case like Confirm(new NumberEval(1e29), new NumberEval(1e29), 1e58) would lock this in.

Minor notes

• The 32% benchmark improvement is plausible — the double-lookup → TryGetValue in the innermost eval loop is likely the primary driver, and the Remove bug may have caused unbounded cache growth slowing hash operations. Profiler attribution would strengthen the claim.
• FormulaCellCache key type remains object (because IEvaluationCell.IdentityKey returns Object). Fine for this PR's scope, just noting it.
• Pre-existing Equals null-safety issues in Loc and BookSheetKey (direct cast without null/type check) — out of scope but worth a follow-up.
• Removal of unused System.Runtime.Serialization.Formatters.Binary using in Ptg.cs is correct cleanup.

[ken-swyfft]

The CI is failing above, but I think those are failures that will be fixed by this PR here: #1746. All tests pass clean on my local.

[ken-swyfft]

Thanks for the thorough review! Both moderate concerns were valid — addressed in 42364a3:

1. FormulaCellCache.Remove regression test — Added TestFormulaCellCacheRemoveActuallyEvicts in TestEvaluationCache.cs. Creates a FormulaCellCache directly, puts two entries, removes one, and verifies it's actually evicted (Get returns null) while the other entry is unaffected.

2. MultiplyEval overflow test — Added TestLargeValuesOverflowDecimal in TestMultiplyEval.cs. Multiplies values > 7.9×10²⁸ that would throw OverflowException on the decimal cast, verifying the try/catch fallback to double arithmetic works correctly.

Full suite green: 2,752 main + 1,787 OOXML = 4,539 passed across both net8.0 and net472.