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.

[tonyqus]

There is a logic change for XSSFRow.GetEnumerator in the recent PR #1753, which caused the failure of unit test TestCellIterationOrderWithSparseColumns.

After abandoning SortedDictionary, the cells returned from GetEnumerator will NOT be sorted by column index anymore. After this change, the performance is so good.

But this may cause some problems for the existing NPOI-based logic. Developers may assume that the cells returned by GetEnumerator should be sorted in the legacy NPOI package. How do you think?

[ken-swyfft]

Good catch — you're right that this is a concern. After SortedDictionary → Dictionary, GetEnumerator() and Cells return cells in arbitrary hash order instead of column-sorted order, which would break any existing code that does foreach (var cell in row) expecting ascending column order.

I've put up #1759 to fix this with a lazy-sorted cache approach:

• Keep Dictionary<int, ICell> for O(1) insert/lookup (your perf win)
• Add a List<ICell> _sortedCellCache that's built on first read and invalidated on mutation
• Since reads dominate writes (sheets built once, iterated many times), the sort cost is amortized across iterations

Applied to XSSFRow, HSSFRow, and SXSSFRow for consistency. All row tests pass on both net8.0 and net472.

🤖 This response was written with the help of Claude (AI assistant).

[tonyqus]

/oc please help review this PR. Remember you are doing pair programming with Claude Pro

[opencode-agent]

PR Review: Replace Hashtable/ArrayList with generic collections

Approve — clean mechanical refactoring with solid tests.

Changes Reviewed

File	Change	Status
PlainCellCache.cs	Hashtable → Dictionary<Loc, PlainValueCellCacheEntry>	✓
FormulaCellCache.cs	Hashtable → Dictionary + critical bug fix (Remove now uses IdentityKey)	✓
OperationEvaluatorFactory.cs	Hashtable → Dictionary	✓
FormulaUsedBlankCellSet.cs	Hashtable → Dictionary	✓
Ptg.cs	ArrayList → List	✓
FormulaParser.cs	ArrayList → List (Arguments, ParseArrayRow, ParseArray)	✓
MultiplyEval.cs	OverflowException catch → double fallback	✓
Program.cs	args passed to BenchmarkSwitcher	✓

Key Observations

1. FormulaCellCache.Remove bug fix (lines 68-73): The old code called Remove(cell) instead of Remove(cell.IdentityKey) — entries were never evicted, causing a memory leak. The fix is correct.

2. TryGetValue pattern is consistently applied across all four cache files, eliminating double-lookup overhead.

3. MultiplyEval overflow (lines 28-39): Fallback to double arithmetic on OverflowException matches Excel behavior for values exceeding ~7.9e28.

4. Tests added:

   • TestFormulaCellCacheRemoveActuallyEvicts — directly tests cache eviction
   • TestLargeValuesOverflowDecimal — covers decimal overflow path

5. Minor: BlankCellSheetGroup.ContainsCell (line 116) still has a redundant (BlankCellRectangleGroup) cast since List<BlankCellRectangleGroup> already provides strongly-typed access. This is harmless but could be simplified to return _rectangleGroups[i].ContainsCell(...).

All tests pass on both net8.0 and net472. No public API changes.

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[tonyqus]

LGTM