Add LargeStylesBenchmark with deterministic HugeStyles.xlsx generator

[Copilot]

Adds a BenchmarkDotNet benchmark class targeting large xl/styles.xml scenarios, plus a deterministic fixture generator. Provides a baseline for measuring the cost of styles parsing and the "copy-through styles.xml when not dirty" optimisation.

New files

ZipUtils.cs

Reads the uncompressed byte length of a named zip entry from an xlsx without extracting it.

GenerateHugeStylesWorkbook.cs

Step-wise fixture generator: starts at 10k styles, increments by 5k until xl/styles.xml uncompressed ≥ 20 MB (max 63,500 — just below NPOI's 64k hard cap).

Two constraints shaped the design:

• 250-format limit: the naïve approach of one unique numFmt per cell immediately crashes. Fixed by pre-building 20 fixed format IDs and cycling through them.
• 64k-style cap: NPOI (and the xlsx spec) cap cellXfs at ~64k entries; at a plain ~315 bytes/style that yields only ~20 MB max, making a 30 MB target impossible. The target is set to 20 MB, which the loop reliably hits at ~45k styles (~20.1 MB).

To reach ~469 bytes/style (vs the naïve ~315), each entry is made verbose:

• Font: underline, strikeout, charset, TypeOffset (<vertAlign>), family, scheme, and shadow/condense/extend via CT_Font accessors
• xf: alignment sub-element with 7 attributes + quotePrefix + protection (locked/hidden)

Font creation uses forceRegistration=true internally so generation is O(n), not O(n²).

LargeStylesBenchmark.cs

Three benchmarks; GlobalSetup calls EnsureExists once:

Benchmark	Measures
XSSFWorkbookLargeStylesOpenDispose	Open + dispose, no style API — baseline for lazy load
XSSFWorkbookLargeStylesForceLoad	CreateCellStyle() + CreateDataFormat() — cost of forced parse
XSSFWorkbookLargeStylesOpenWrite	Full write to MemoryStream — baseline for copy-through save

NPOI.Benchmarks.csproj

Conditional PreserveNewest copy rule for data\HugeStyles.xlsx (skipped when file doesn't exist, so clean builds don't fail).

Original prompt

Implement a new BenchmarkDotNet benchmark for large styles.xml scenarios.

Repository: nissl-lab/npoi (base branch: master)

Goals:

1. Add a new benchmark class LargeStylesBenchmark under benchmarks/NPOI.Benchmarks/ that measures:

   • Opening and disposing a workbook without touching style APIs.
   • Forcing styles usage (e.g., CreateCellStyle() and CreateDataFormat())
   • Writing the workbook to a stream.

2. Add a deterministic generator that creates an .xlsx file with a large xl/styles.xml by creating many unique styles and applying them to cells.

   • Generate a workbook with initially 10,000 unique styles applied to 10,000 cells.
   • Increase style count until xl/styles.xml uncompressed size reaches at least 30MB (so it should land in the 20–30MB+ range; small overshoot acceptable). Use step size 5,000 styles, max 150,000.
   • Store the file at benchmarks/NPOI.Benchmarks/data/HugeStyles.xlsx.
   • Generator should run in LargeStylesBenchmark.GlobalSetup() only if the file is missing or too small.

3. Add a small utility for reading the uncompressed size of a zip entry inside the .xlsx (ZipArchive) and specifically xl/styles.xml.

Constraints:

• Do not modify benchmarks/NPOI.Benchmarks/Program.cs unless required; it already uses BenchmarkSwitcher.FromAssembly(...).Run().
• Keep the new code in the NPOI.Benchmarks namespace.
• Use idiomatic C# with using var.

Acceptance criteria:

• dotnet run -c Release --project benchmarks/NPOI.Benchmarks discovers and can run LargeStylesBenchmark.
• On first run, benchmark generates the workbook if not present; subsequent runs reuse it if size >= 30MB uncompressed styles.xml.
• Code compiles.

The following is the prior conversation context from the user's chat exploration (may be truncated):

User: 1

Assistant: Proceed with (1) strict laziness: GetStylesSource() returns the StylesTable instance without forcing parse; the first real style-table usage triggers StylesTable.EnsureLoaded().

Concrete changes to implement

1) StylesTable: defer parsing + thread-safe EnsureLoaded()

• Add fields:

  • private PackagePart _stylesPart;
  • private bool _isLoaded;
  • private readonly object _loadLock = new object();

• Change internal StylesTable(PackagePart part) to store the part and not parse immediately.

• Add internal void EnsureLoaded() with double-checked locking, and move the existing parse logic into a private LoadFromPart() method that:

  • reads _stylesPart.GetInputStream()
  • ConvertStreamToXml(...)
  • ReadFrom(xmldoc)
  • sets _isLoaded = true

• Add EnsureLoaded() at the top of all relevant public/internal methods/properties on StylesTable that rely on parsed content (fonts/fills/borders/xfs/number formats/dxfs/table styles/colors, etc.). This is important because callers may obtain StylesTable and use it directly.

2) XSSFWorkbook: don’t force parse in GetStylesSource()

• Ensure GetStylesSource() (or equivalent accessor used by tests) does not call any load method.
• Style-related workbook APIs should still call stylesSource.EnsureLoaded() (or go through a local helper) before using the style table:
  • CreateCellStyle()
  • GetCellStyleAt(...)
  • CreateDataFormat()
  • font APIs (CreateFont, GetFontAt, FindFont, etc.)
  • any border/fill related workbook APIs

This meets your requirement: parse styles.xml only on first style-related API use.

3) Tests

Add a test proving strict laziness:

• Open a real .xlsx with styles (any existing test sample that has styles)
• Immediately after open, call wb.GetStylesSource() and then check (via internal visibility / reflection) that styles are not loaded yet.
• Trigger a style API (e.g., wb.CreateCellStyle() or wb.CreateDataFormat())
• Assert styles are now loaded.

Implementation detail: to avoid changing public API, we can expose internal bool IsLoadedForTests in StylesTable and use [assembly: InternalsVisibleTo("...TestCases...")] if that’s already how the test project works. Otherwise we can use reflection in the test.

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If you want me to actually implement this in the repo, tell me which workflow you prefer:

• A) you will code it and want me to provide patch guidance + list of methods to touch, or
• B) you want a PR opened against nissl-lab/npoi (you’d need to explicitly ask “create a PR”).

User: If styles are not changed, don't serialize or commit the styles, use existing styles.xml as output stream instead

Assistant: That’s doable, but it’s a separate (and very worthwhile) optimization: skip rewriting xl/styles.xml when it was never loaded/modified, and just copy the original part bytes through to the output package.

Key idea

For loaded workbooks:

• If styles.xml was never parsed and no style-related API was called that would mutate styles, then we can treat styles as unchanged.
• On save, instead of calling `StylesT...

This pull request was created from Copilot chat.

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