Improve PathSelector to use brace expansion for POSIX ** semantics instead of combinatorial approach

[gnodet]

Summary

The current PathSelector implementation in Maven 4 handles POSIX ** semantics (where ** matches 0 or more directories vs Java's 1 or more) by generating multiple patterns using the addPatternsWithOneDirRemoved method. This approach works but could be simplified and potentially optimized by leveraging Java's built-in brace expansion feature.

Current Implementation

The current approach in PathSelector.addPatternsWithOneDirRemoved() (lines 464+) generates multiple glob patterns by recursively removing ** patterns to simulate POSIX behavior:

// For pattern "src/**/test/**/*.java", generates:
// 1. "src/**/test/**/*.java" (original)
// 2. "src/**/test/*.java" (second ** removed)  
// 3. "src/test/**/*.java" (first ** removed)
// 4. "src/test/*.java" (both ** removed)

Proposed Improvement

Replace the combinatorial pattern generation with brace expansion:

// Transform: "src/**/test/**/*.java" 
// Into:      "src/{**/,}test/{**/,}*.java"
String posixPattern = globPart.replaceAll("\\*\\*/", "{**/,}");

Java's PathMatcher with glob syntax natively supports brace expansion and will automatically generate all the same combinations, but potentially more efficiently.

Benefits

1. Simpler Code: Reduces ~40 lines of complex recursive logic to ~3 lines
2. Better Performance: Leverages Java's optimized brace expansion instead of multiple matchers
3. More Maintainable: Easier to understand and debug
4. Leverages Platform: Uses built-in capabilities instead of reimplementing them

Compatibility

This change would be:

• ✅ Functionally identical: Same matching behavior
• ✅ Backward compatible: No API changes
• ✅ Performance neutral or better: Single optimized matcher vs multiple matchers

Implementation

The change would be in the PathSelector.addPatternsWithOneDirRemoved() method, replacing the recursive pattern generation with a simple regex replacement that transforms **/ into {**/,}.

Related Work

• This builds on the excellent PathSelector improvements in Port the bug fixes identified when using that class in Maven clean and compiler plugin. #10909 and Add PathMatcherFactory service with directory filtering optimization #10923
• Similar pattern transformation approaches are used successfully in other build tools
• The brace expansion feature is well-established in Java's glob implementation

Testing

The existing comprehensive test suite in PathSelectorTest would validate that the behavior remains identical while the implementation becomes simpler.

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This improvement would make Maven's PathSelector implementation more elegant while maintaining full compatibility and potentially improving performance.

[desruisseaux]

It looks a very good idea to me. There is maybe one additional simplification that may be possible. I wonder if **/*.java can be replaced by **.java (without generating multiple combinations). I do not remember having tested that possibility.