Add CLAUDE.md for AI-assisted development

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+MessagePack-Java is a binary serialization library that provides a fast and compact alternative to JSON. The project consists of two main modules:
+- **msgpack-core**: Standalone MessagePack implementation with no external dependencies
+- **msgpack-jackson**: Jackson integration for object mapping capabilities
+
+## Essential Development Commands
+
+### Build and Compile
+```bash
+./sbt compile              # Compile source code
+./sbt test:compile         # Compile source and test code
+./sbt package              # Create JAR files
+```
+
+### Testing
+```bash
+./sbt test                 # Run all tests
+./sbt ~test                # Run tests continuously on file changes
+./sbt testOnly *TestClass  # Run specific test class
+./sbt "testOnly *TestClass -- -z pattern"  # Run tests matching pattern
+
+# Test with universal buffer mode (for compatibility testing)
+./sbt test -J-Dmsgpack.universal-buffer=true
+```
+
+### Code Quality
+```bash
+./sbt jcheckStyle          # Run checkstyle (Facebook Presto style)
+./sbt scalafmtAll          # Format Scala test code
+```
+
+### Publishing
+```bash
+./sbt publishLocal         # Install to local .ivy2 repository
+./sbt publishM2            # Install to local .m2 Maven repository
+```
+
+## Architecture Overview
+
+### Core API Structure
+The main entry point is the `MessagePack` factory class which creates:
+- **MessagePacker**: Serializes objects to MessagePack binary format
+- **MessageUnpacker**: Deserializes MessagePack binary data
+
+Key locations:
+- Core interfaces: `msgpack-core/src/main/java/org/msgpack/core/`
+- Jackson integration: `msgpack-jackson/src/main/java/org/msgpack/jackson/dataformat/`
+
+### Buffer Management System
+MessagePack uses an efficient buffer abstraction layer:
+- **MessageBuffer**: Platform-optimized buffer implementations
+  - Uses `sun.misc.Unsafe` for performance when available
+  - Falls back to ByteBuffer on restricted platforms
+- **MessageBufferInput/Output**: Manages buffer sequences for streaming
+
+### Jackson Integration
+The msgpack-jackson module provides:
+- **MessagePackFactory**: Jackson JsonFactory implementation
+- **MessagePackMapper**: Pre-configured ObjectMapper for MessagePack
+- Support for field IDs (integer keys) for compact serialization
+- Extension type support including timestamps
+
+### Testing Structure
+- **msgpack-core tests**: Written in Scala using AirSpec framework
+  - Location: `msgpack-core/src/test/scala/`
+- **msgpack-jackson tests**: Written in Java using JUnit
+  - Location: `msgpack-jackson/src/test/java/`
+
+## Important JVM Options
+
+For JDK 17+ compatibility, these options are automatically added:
+```
+--add-opens=java.base/java.nio=ALL-UNNAMED
+--add-opens=java.base/sun.nio.ch=ALL-UNNAMED
+```
+
+## Code Style Requirements
+- Java code follows Facebook Presto style (enforced by checkstyle)
+- Scala test code uses Scalafmt with 180 character line limit
+- Checkstyle runs automatically during compilation
+- No external dependencies allowed in msgpack-core
+
+## Key Design Principles
+1. **Zero Dependencies**: msgpack-core has no external dependencies
+2. **Platform Optimization**: Uses platform-specific optimizations when available
+3. **Streaming Support**: Both streaming and object-based APIs
+4. **Type Safety**: Immutable Value hierarchy for type-safe data handling
+5. **Extension Support**: Extensible type system for custom data types