Cache the column mapping count in CellReferenceConverter

[virzak]

Fixes #1024

EnsureMappingsUpTo checked IntMappingToAlphabet.Count, and ConcurrentDictionary.Count takes every internal lock. Querying without a header row calls it for every cell. The count is now cached in a volatile field, which is safe because entries are only added, contiguously from 0.

QueryExcelBenchmark on net10.0:

Method	master	this PR
MiniExcel Query	3,878.82 ms	399.45 ms
MiniExcel Query with Mapping	331.35 ms	320.11 ms

Allocations are unchanged.

Summary by CodeRabbit

• Performance
  • Reduced repeated mapping-count lookups while preserving existing column limits and mapping behavior.

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ℹ️ Recent review info

⚙️ Run configuration

• Configuration used: defaults
• Review profile: CHILL
• Plan: Advanced
• Run ID: 0110db36-fcd7-4ae7-a308-f3d8a710190e

📥 Commits

Reviewing files that changed from the base of the PR and between 6175350 and 97785d7.

📒 Files selected for processing: 1

• src/MiniExcel.Core/Helpers/CellReferenceConverter.cs

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📝 Walkthrough

EnsureMappingsUpTo now checks a volatile cached mapping count instead of reading ConcurrentDictionary.Count on each call. After adding mappings, it refreshes the cached count. The existing column-limit check and mapping behavior remain unchanged.

Changes: Mapping count cache

Layer / File(s)	Summary
Cache and use mapping count src/MiniExcel.Core/Helpers/CellReferenceConverter.cs	A volatile field stores the mapping count. EnsureMappingsUpTo checks it before adding mappings and refreshes it afterward.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Medium

Merge Risk: ⚪ Minimal · up to 97785

No concrete correctness, availability, or compatibility risk is supported by the change. It appears mergeable subject to normal checks.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 1 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely identifies the main change: caching the column mapping count in CellReferenceConverter.
Linked Issues check		Issue #1024 requires reducing the per-cell cost of EnsureMappingsUpTo, which previously read ConcurrentDictionary.Count. The reviewed CellReferenceConverter adds volatile _mappedCount, uses it…
Out of Scope Changes check		The reviewed change is limited to the mapping-count cache in src/MiniExcel.Core/Helpers/CellReferenceConverter.cs. The cache supports the issue #1024 performance objective and does not introduce unr…

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