[matt-edmondson]

Fixes #66

What changed

NaturalStringComparer can now compare text ignoring case. Ordinal stays the default.

• There is a new NaturalStringComparer(bool ignoreCase) constructor, plus static Ordinal and OrdinalIgnoreCase instances. new NaturalStringComparer() behaves exactly as before.
• In ignore-case mode, text chunks are compared character by character after char.ToUpperInvariant, which is how StringComparison.OrdinalIgnoreCase folds. The comparison is still in place and allocates nothing.
• Every place that compares a text character goes through the same Fold, so the order stays a total order. That includes text against a number's first digit, in the main loop and in the "text runs on into the other side's number" case. In practice no character folds across the ASCII digit range, so the fold on the number-vs-text path never changes a result today. It is there so the two paths can't disagree.
• Strings that are equal ignoring case still tie-break with string.CompareOrdinal, as [patch] Break natural-order ties ordinally so distinct strings never compare equal #65 does for leading zeros. They sit next to each other and never compare equal. A SortedSet built with OrdinalIgnoreCase keeps both file1 and File1.
• The README has an "Ignoring case" section.

Tests

The new NaturalStringComparerIgnoreCaseTests covers:

• the repro from the issue, which sorts to build3.log, Build20.log, file1.txt, File2.txt, file10.txt, README.md
• the default instance, Ordinal, and new NaturalStringComparer() keeping today's order
• strings that differ only in case: ordinal ties, staying between their neighbours, and SortedSet keeping all three
• an antisymmetry, transitivity, and "only ordinally equal strings tie" fuzz with the option on. It mixes both cases with the punctuation that folding moves relative to letters (`_ ^ [ ``), several digit scripts, and numbers next to text
• allocation-free comparisons with the option on

The new tests fail without the fix:

• With the fix reverted, they don't compile, because the new API is missing.
• With a stub that keeps the API but ignores the flag, the repro and neighbour tests fail (2 of 35).

With the fix applied, all 35 tests pass. That count includes the existing Compare_AllocatesNothing and transitivity tests.

Notes

• The constructor parameter is a primary constructor, which the repository's analyzers require (IDE0290). It changes the class declaration line, so this PR and Implement the non-generic IComparer on NaturalStringComparer [minor] #75 (non-generic IComparer, NaturalStringComparer can't be passed where a non-generic IComparer is required (ArrayList.Sort, WPF CollectionView.CustomSort, WinForms ListView sorting) #74) will conflict on that one line. Whichever merges second needs a one-line rebase to public partial class NaturalStringComparer(bool ignoreCase) : IComparer<string?>, IComparer.
• dotnet pack package validation fails locally on main because of the Polyfill 11.4.5 drift tracked by No release since 1.0.49: pack fails package validation (CP0016/CP0001/CP0014…) after the Polyfill bump, so the #70 base-name ordering fix isn't on NuGet #72 and fixed by Regenerate CompatibilitySuppressions.xml after the Polyfill 11.4.5 bump [patch] #73. This change adds no new CP errors of its own.

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Generated by Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
3 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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