Refactor: Implement composable integration architecture with decorators

Co-authored-by: wenyin.wei.ww <[email protected]>

Author: cursoragent

IMPROVEMENTS_SUMMARY.md

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+# Improvements Summary
+
+## Architectural Transformation: From Combinatorial Explosion to Composable Design
+
+This document summarizes the major improvements made to the diffeq library to address architectural issues and implement the Sourcery bot's suggestions from the [GitHub PR](https://github.com/WenyinWei/diffeq/pull/3).
+
+## 🎯 Problem Solved: Combinatorial Explosion
+
+### The Issue
+The initial architecture was heading toward a combinatorial explosion problem:
+- `ParallelTimeoutIntegrator` tightly coupled parallel and timeout facilities
+- Adding N facilities would require 2^N classes (exponential growth)
+- Maintenance nightmare and inflexible design
+
+### The Solution: Composable Architecture
+Implemented **high cohesion, low coupling** design using the decorator pattern:
+
+```cpp
+// ❌ OLD: Tightly coupled, combinatorial explosion
+class ParallelTimeoutIntegrator { ... };
+class ParallelTimeoutAsyncIntegrator { ... };
+class ParallelTimeoutAsyncSignalIntegrator { ... };
+// ... 2^N classes needed
+
+// ✅ NEW: Composable, linear scaling
+auto integrator = make_builder(base_integrator)
+    .with_timeout()     // Independent facility
+    .with_parallel()    // Independent facility  
+    .with_async()       // Independent facility
+    .with_signals()     // Independent facility
+    .with_output()      // Independent facility
+    .build();           // N classes for N facilities
+```
+
+## 🏗️ Architecture Reorganization
+
+### File Structure Improvements
+**Before**: Monolithic files with tight coupling
+**After**: One class per file for maximum clarity
+
+```
+include/core/composable/
+├── integrator_decorator.hpp      # Base decorator pattern
+├── timeout_decorator.hpp         # Timeout protection only
+├── parallel_decorator.hpp        # Parallel execution only
+├── async_decorator.hpp          # Async execution only
+├── output_decorator.hpp         # Output handling only
+├── signal_decorator.hpp         # Signal processing only
+└── integrator_builder.hpp       # Composition builder
+```
+
+### Removed Files
+- ❌ `include/core/parallel_timeout_integrator.hpp` (flawed combinatorial approach)
+
+## 🔧 Individual Facility Improvements
+
+### 1. TimeoutDecorator Enhancements
+- **Configuration Validation**: Comprehensive parameter validation
+- **Progress Monitoring**: User-cancellable progress callbacks
+- **Robust Error Handling**: Detailed error reporting and status information
+- **Thread Safety**: Proper async execution with cleanup
+
+### 2. ParallelDecorator Features
+- **Load Balancing**: Automatic chunk size optimization
+- **Monte Carlo Support**: Parallel Monte Carlo simulations
+- **Hardware Detection**: Automatic thread count detection
+- **Graceful Fallback**: Falls back to sequential on errors
+
+### 3. AsyncDecorator Capabilities
+- **Operation Tracking**: Active operation counting and monitoring
+- **Cancellation Support**: Cooperative cancellation mechanism
+- **Progress Monitoring**: Optional progress monitoring with callbacks
+- **Resource Management**: RAII scope guards for proper cleanup
+
+### 4. OutputDecorator Features
+- **Multiple Modes**: Online, offline, and hybrid output
+- **File Output**: Optional file output with compression support
+- **Statistics**: Detailed performance monitoring and statistics
+- **Buffer Management**: Configurable buffering with overflow handling
+
+### 5. SignalDecorator Enhancements
+- **Multiple Processing Modes**: Synchronous, asynchronous, and batch
+- **Priority System**: Signal priority handling with queues
+- **Thread Safety**: Mutex protection for concurrent signal access
+- **Performance Statistics**: Signal processing metrics and monitoring
+
+## 🧪 Test Improvements (Addressing Sourcery Bot Suggestions)
+
+### Added Missing Test Cases
+
+#### 1. DOP853 Timeout Failure Test
+```cpp
+TEST_F(DOP853Test, TimeoutFailureHandling) {
+    // Test timeout expiration with DOP853 integrator
+    auto stiff_system = [](double t, const std::vector<double>& y, std::vector<double>& dydt) {
+        double lambda = -100000.0;  // Extremely stiff system
+        dydt[0] = lambda * y[0];
+        dydt[1] = -lambda * y[1];
+    };
+    
+    // Use very short timeout to force timeout condition
+    const std::chrono::milliseconds SHORT_TIMEOUT{10};
+    bool completed = diffeq::integrate_with_timeout(integrator, y, 1e-8, 1.0, SHORT_TIMEOUT);
+    
+    // Should have timed out
+    EXPECT_FALSE(completed);
+}
+```
+
+#### 2. Async Integration Timeout Failure Path
+```cpp
+bool test_async_timeout_failure() {
+    // Create artificially slow system to force timeout
+    auto slow_system = [](double t, const std::vector<double>& y, std::vector<double>& dydt) {
+        std::this_thread::sleep_for(std::chrono::milliseconds(100));  // Artificial delay
+        for (size_t i = 0; i < y.size(); ++i) {
+            dydt[i] = 1e-8 * y[i];  // Very slow dynamics
+        }
+    };
+    
+    auto timeout_future = async_integrator->integrate_async(state, 0.01, 10.0);
+    const std::chrono::milliseconds SHORT_TIMEOUT{50};
+    
+    // Should timeout
+    return (timeout_future.wait_for(SHORT_TIMEOUT) == std::future_status::timeout);
+}
+```
+
+### Test Improvements Made
+- ✅ Added timeout failure path test for DOP853 integrator
+- ✅ Added async integration timeout failure path test  
+- ✅ Improved test timing and duration validation
+- ✅ Enhanced error message clarity and debugging output
+- ✅ Added comprehensive edge case coverage
+
+## 🚀 Usage Examples
+
+### Real-World Scenarios
+
+#### Research Computing
+```cpp
+auto research_integrator = make_builder(base_integrator)
+    .with_timeout(TimeoutConfig{.timeout_duration = std::chrono::hours{24}})
+    .with_parallel(ParallelConfig{.max_threads = 16})
+    .with_output(OutputConfig{.mode = OutputMode::OFFLINE})
+    .build();
+```
+
+#### Real-time Control
+```cpp
+auto control_integrator = make_builder(base_integrator)
+    .with_timeout(TimeoutConfig{.timeout_duration = std::chrono::milliseconds{10}})
+    .with_async()
+    .with_signals()
+    .build();
+```
+
+#### Production Server
+```cpp
+auto server_integrator = make_builder(base_integrator)
+    .with_timeout(TimeoutConfig{.throw_on_timeout = false})  // Don't crash server
+    .with_output(OutputConfig{.mode = OutputMode::HYBRID})
+    .build();
+```
+
+#### Interactive Application
+```cpp
+auto interactive_integrator = make_builder(base_integrator)
+    .with_timeout(TimeoutConfig{
+        .enable_progress_callback = true,
+        .progress_callback = [](double current, double end, auto elapsed) {
+            update_progress_bar(current / end);
+            return !user_cancelled();  // Allow cancellation
+        }
+    })
+    .with_async().with_signals().with_output()
+    .build();
+```
+
+## 📊 Benefits Achieved
+
+### 1. Architectural Quality
+- ✅ **High Cohesion**: Each decorator focuses on single responsibility
+- ✅ **Low Coupling**: Decorators combine without dependencies
+- ✅ **Linear Scaling**: O(N) classes for N facilities vs O(2^N)
+- ✅ **Order Independence**: Facilities work in any composition order
+
+### 2. Extensibility
+- ✅ **Zero Modification**: New facilities add without changing existing code
+- ✅ **Unlimited Combinations**: Any mix of facilities possible
+- ✅ **Future-Proof**: Architecture scales indefinitely
+
+### 3. Performance
+- ✅ **Minimal Overhead**: Each decorator adds minimal indirection
+- ✅ **Pay-for-What-You-Use**: Only active decorators consume resources
+- ✅ **Compiler Optimization**: Virtual calls can be inlined
+
+### 4. Developer Experience
+- ✅ **Intuitive API**: Fluent builder interface
+- ✅ **Type Safety**: Compile-time checking
+- ✅ **Clear Documentation**: Comprehensive examples and guides
+
+## 🔮 Future Extensibility Examples
+
+The new architecture easily supports future facilities:
+
+```cpp
+// Future facilities can be added without any modification to existing code
+class NetworkDecorator : public IntegratorDecorator<S, T> { 
+    // Distributed integration across machines
+};
+
+class GPUDecorator : public IntegratorDecorator<S, T> { 
+    // GPU acceleration for compute-intensive tasks
+};
+
+class CompressionDecorator : public IntegratorDecorator<S, T> { 
+    // State compression for memory efficiency
+};
+
+// Automatically work with all existing facilities
+auto ultimate_integrator = make_builder(base)
+    .with_timeout()
+    .with_parallel()
+    .with_gpu()        // New facility
+    .with_network()    // New facility  
+    .with_compression() // New facility
+    .with_async()
+    .with_signals()
+    .with_output()
+    .build();
+```
+
+## 📝 Documentation Updates
+
+### Created Documentation
+1. **`COMPOSABLE_ARCHITECTURE.md`** - Technical architecture documentation
+2. **`ARCHITECTURE_TRANSFORMATION_SUMMARY.md`** - Complete evolution story
+3. **`examples/composable_facilities_demo.cpp`** - Comprehensive demonstration
+4. **Updated `examples/README.md`** - New usage patterns and best practices
+
+### Updated Integration
+- ✅ Updated main `include/diffeq.hpp` header  
+- ✅ Re-exported all composable types in `diffeq::` namespace
+- ✅ Removed references to deleted combinatorial classes
+- ✅ Added migration guidance for existing users
+
+## 🎉 Summary
+
+The transformation from a flawed combinatorial explosion approach to a clean composable architecture represents a textbook example of applying proper software engineering principles:
+
+**Before**: Heading toward 2^N classes with tight coupling
+**After**: Clean N classes with loose coupling and unlimited flexibility
+
+The improvements address all Sourcery bot suggestions while creating a robust, extensible, and maintainable codebase that will scale elegantly as the library grows.
+
+**Key Achievement**: 🎯 **Problem Solved** - No more combinatorial explosion! The library now embodies "Make simple things simple, and complex things possible" while maintaining clean architecture principles.