Fix styled stream pivot fields and formula XML

[zfuchen]

Summary

This fixes two regressions in the StreamWriter + PivotTable path after the large streamed data range fix in #183:

• Preserve styled source headers when seeding pivot source headers before AddPivotTable.
• Normalize formulas before passing them to excelize so worksheet XML does not contain formulas starting with =.

Root Cause

seedPivotSourceHeaders reads the source header row from ew.Content[sheet]["Data"] and seeds those values back into excelize so AddPivotTable can validate fields after streamed worksheets are flushed.

For styled StreamWriter sheets, performStreamWrite mutates each styled cell from the original []interface{}{value, style} into an excelize.Cell. The previous getCellScalarValue only unwrapped []interface{} and returned any other value as-is. As a result, styled header cells were written back as struct values such as:

{111 Category}
{112 42}

Those names no longer matched the PivotTable field definitions, so pivot cache fields could be malformed and pageFields, rowFields, or dataFields could be missing from the generated pivot table XML.

Separately, formulas were passed to excelize with the leading = included. Excelize expects formula text without that leading =, otherwise worksheet XML can contain invalid formulas like <f>=SUM(A2:A3)</f>.

Changes

• Unwrap excelize.Cell.Value in getCellScalarValue.
• Add normalizeFormula and strip one leading = before setting formulas in both StreamWriter and NormalWriter paths.
• Add regression coverage for:
  • excelize.Cell scalar extraction.
  • styled StreamWriter pivot cache field names and row/page/data fields.
  • StreamWriter and NormalWriter formula XML without leading =.

Validation

~/go/bin/go1.24.0 test ./pyfastexcel/core -run 'TestCreateCell|TestWriteExcelWritesFormulaXMLWithoutLeadingEquals|TestGetCellScalarValueFromExcelizeCell|TestWriteExcelCreatesPivotTableForStyledStreamedData|TestWriteExcelCreatesPivotTableForLargeStreamedDataRange' -count=1
~/go/bin/go1.24.0 test ./...

Both commands pass locally.

I also validated the generated shared library against a representative styled StreamWriter pivot workbook. Without any Python-side OOXML repair, the generated workbook now has:

• clean pivot cache field names, e.g. Category, Amount, Region
• present pageFields, rowFields, and dataFields
• worksheet formulas without leading = in <f> XML nodes

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 35 complexity · 14 duplication

Metric	Results
Complexity	35
Duplication	14

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.44%. Comparing base (1f20511) to head (8fe73a1).

Files with missing lines	Patch %	Lines
pyfastexcel/core/writer.go	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #184      +/-   ##
==========================================
+ Coverage   86.14%   86.44%   +0.30%     
==========================================
  Files           8        8              
  Lines         837      841       +4     
==========================================
+ Hits          721      727       +6     
+ Misses         78       75       -3     
- Partials       38       39       +1     

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