Summary

This PR introduces significant performance and safety improvements to the jsoniter library, focusing on JSON path navigation, Unicode handling, and iterator bounds checking. The changes eliminate callback-based iteration in favor of direct iteration patterns and optimize Unicode hex digit parsing.

Key Changes

🚀 Performance Improvements

Path Navigation Refactoring

• Replaced callback-based iteration with direct iteration using ReadObjectRaw() and ReadArray()
• Eliminated intermediate byte array allocations by removing SkipAndReturnBytes() + ResetBytes() pattern
• Renamed functions for clarity: locateObjectField → findObjectField, locateArrayElement → findArrayElement
• Changed return types from []byte to bool for more efficient existence checking

Unicode Hex Parsing Optimization

• Simplified hex digit parsing using bitwise operations (c |= 0x20) to convert uppercase to lowercase
• Reduced code duplication across parseU4(), readU4(), and readAndFillU4() methods
• Improved performance by eliminating separate uppercase/lowercase handling branches

Iterator Bounds Checking

• Added bounds validation in isNextTokenBuffered(), trySkipNumber(), and skipString()
• Prevents buffer overruns with checks: iter.head < 0 || iter.head >= iter.tail || iter.tail > len(iter.buf)

Benefits

1. Performance: Direct iteration eliminates callback overhead and intermediate allocations
2. Memory Efficiency: Reduces temporary byte array creation during path navigation
3. Safety: Comprehensive bounds checking prevents potential buffer overruns
4. Maintainability: Cleaner, more consistent code patterns
5. Unicode Handling: More efficient and robust hex digit parsing

Testing

• Added comprehensive Unicode tests in iter_str_test.go covering:
  • Basic ASCII characters (\u0020)
  • Chinese characters with lowercase hex (\u4e2d)
  • Chinese characters with uppercase hex (\u4E2D)
  • Invalid hex sequences (\u658U)

Backward Compatibility

This refactoring maintains public API compatibility while changing internal implementation details. The path navigation behavior should remain functionally equivalent.