Optimize BitStringParser.getZeroBitStuffedString with caching to avoid redundant string operations

[Copilot]

• [x] Analyze the current implementation and performance issue
• [x] Cache the zero-bit-stuffed string to avoid recomputation
• [x] Add tests to verify the optimization doesn't break functionality
• [x] Run all tests to ensure backward compatibility (121 unit tests + 8 integration tests pass)
• [x] Verify minimal changes were made
• [x] Performance testing shows ~6x speedup
• [x] Simplify implementation based on feedback

Summary

Optimized BitStringParser.getZeroBitStuffedString() by introducing caching to address the issue where getZeroBitStuffedString was being called on every getBits(int, int) invocation.

Changes Made (Simplified)

1. Added single caching field to BitStringParser:

   • cachedZeroBitStuffedString: stores the padded string

2. Optimized getZeroBitStuffedString() method:

   • Fast path: returns original string when no padding needed
   • Cache reuse: returns cached padded string when endIndex <= cachedZeroBitStuffedString.length()
   • Cache update: only creates new padded string when needed

3. Simplified implementation:

   • Removed redundant cachedPaddedLength field - use cachedZeroBitStuffedString.length() directly
   • Simplified constructor - rely on default null initialization

4. Added comprehensive tests:

   • caching_multiplePaddedCallsReuseCache: verifies cache reuse behavior
   • caching_noPaddingNeededReturnsOriginal: verifies fast path

Performance Impact

Microbenchmark shows ~6x speedup for repeated calls with padding requirements. Real-world performance improvement depends on usage patterns, but eliminates unnecessary string concatenation and String.repeat() calls.

Test Results

• ✅ All 121 unit tests pass
• ✅ All 8 integration tests pass
• ✅ Full build (compile, test, javadoc) succeeds
• ✅ Backward compatible - no API changes
• ✅ Simplified based on maintainer feedback

Original prompt

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This section details on the original issue you should resolve

<issue_title>AISMessage: optimize getZeroBitStuffedString </issue_title> <issue_description>getZeroBitStuffedString should not be run every time we call getBits(int, int).</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes tbsalling/aismessages#26

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