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+# CrossPoint Reader - Developer Guide
+
+This guide provides essential information for developers working on the CrossPoint Reader firmware for the Xteink X4 e-paper device.
+
+## Project Overview
+
+CrossPoint Reader is an open-source firmware replacement for the Xteink X4 e-paper reader, built using PlatformIO and targeting the ESP32-C3 microcontroller. The firmware provides EPUB reading capabilities with a focus on memory efficiency due to the ESP32-C3's limited RAM (~380KB).
+
+## Architecture
+
+### Main Components
+
+1. **Activity System**: The firmware uses an activity-based architecture where each screen/state is an `Activity` subclass
+2. **Display System**: E-ink display handling via `EInkDisplay` and `GfxRenderer` from the OpenX4 SDK
+3. **Input System**: Button handling via `InputManager` and `MappedInputManager` with configurable button layouts
+4. **File System**: SD card management via `SDCardManager`
+5. **State Management**: `CrossPointState` and `CrossPointSettings` for persistent configuration
+
+### Key Directories
+
+- `src/activities/` - All screen activities (Home, Reader, Settings, etc.)
+- `src/network/` - Web server and OTA update functionality
+- `src/util/` - Utility functions
+- `src/images/` - Embedded image assets
+- `open-x4-sdk/` - Community SDK with hardware libraries (submodule)
+
+## Display Programming
+
+### E-Ink Display Basics
+
+The display is a 480x800 e-paper panel controlled via SPI. Key display-related files:
+
+- `src/main.cpp` - Display initialization and SPI pin definitions
+- Uses `EInkDisplay` from OpenX4 SDK
+- Uses `GfxRenderer` for drawing operations
+
+### Display Initialization
+
+```cpp
+// Display SPI pins (custom pins for XteinkX4)
+#define EPD_SCLK 8   // SPI Clock
+#define EPD_MOSI 10  // SPI MOSI
+#define EPD_CS 21    // Chip Select
+#define EPD_DC 4     // Data/Command
+#define EPD_RST 5    // Reset
+#define EPD_BUSY 6   // Busy
+
+EInkDisplay einkDisplay(EPD_SCLK, EPD_MOSI, EPD_CS, EPD_DC, EPD_RST, EPD_BUSY);
+GfxRenderer renderer(einkDisplay);
+```
+
+### Drawing Operations
+
+The `GfxRenderer` class provides drawing primitives:
+
+```cpp
+// Basic drawing functions
+renderer.clearScreen();                    // Clear display buffer
+renderer.drawText(fontId, x, y, text);     // Draw text
+renderer.drawRect(x, y, width, height);    // Draw rectangle outline
+renderer.fillRect(x, y, width, height);    // Fill rectangle
+renderer.drawCenteredText(fontId, y, text); // Center text horizontally
+renderer.displayBuffer();                  // Update physical display
+
+// Screen dimensions
+int width = renderer.getScreenWidth();     // 480 in portrait
+int height = renderer.getScreenHeight();   // 800 in portrait
+```
+
+### Font System
+
+Multiple font families are available (Bookerly, NotoSans, OpenDyslexic) in various sizes. Fonts are registered in `main.cpp`:
+
+```cpp
+// Font registration example
+renderer.insertFont(BOOKERLY_14_FONT_ID, bookerly14FontFamily);
+```
+
+Font IDs are defined in `src/fontIds.h`.
+
+### Display Refresh Optimization
+
+Due to e-paper characteristics:
+- Use `renderer.displayBuffer()` sparingly (causes screen refresh)
+- Partial refreshes are handled automatically by the SDK
+- Full refresh frequency is configurable in settings
+
+## Button Input Handling
+
+### Hardware Button Mapping
+
+The Xteink X4 has:
+- **Front buttons**: 4 buttons (Back, Confirm, Left, Right)
+- **Side buttons**: 2 buttons (Up/Down for page turning)
+- **Power button**: Separate power/sleep control
+
+### Input Manager System
+
+Two-layer input system:
+
+1. **`InputManager`** (OpenX4 SDK): Raw button state
+2. **`MappedInputManager`**: Configurable button mapping
+
+### Button Configuration
+
+Button layouts are configurable in settings:
+- **Front button layout**: Back/Confirm/Left/Right can be rearranged
+- **Side button layout**: Page up/down can be swapped
+
+### Using MappedInputManager
+
+```cpp
+// In Activity constructor
+MappedInputManager& mappedInput;
+
+// Check button states
+if (mappedInput.wasPressed(MappedInputManager::Button::Confirm)) {
+    // Handle confirm press
+}
+
+if (mappedInput.isPressed(MappedInputManager::Button::Power)) {
+    // Power button currently held
+}
+
+// Get button labels for UI
+auto labels = mappedInput.mapLabels("Back", "Select", "Prev", "Next");
+```
+
+### Available Button Constants
+
+```cpp
+MappedInputManager::Button::Back
+MappedInputManager::Button::Confirm
+MappedInputManager::Button::Left
+MappedInputManager::Button::Right
+MappedInputManager::Button::Up
+MappedInputManager::Button::Down
+MappedInputManager::Button::Power
+MappedInputManager::Button::PageBack
+MappedInputManager::Button::PageForward
+```
+
+### Power Button Behavior
+
+- **Short press**: Configurable (ignore, sleep, or page turn)
+- **Long press** (>400ms): Always enters deep sleep
+- **Wake from sleep**: Requires long press verification
+
+## Activity System
+
+### Creating a New Activity
+
+1. Create header file in appropriate `src/activities/` subdirectory
+2. Inherit from `Activity` base class
+3. Implement required methods
+
+### Activity Template
+
+```cpp
+// MyActivity.h
+#pragma once
+#include "activities/Activity.h"
+
+class MyActivity : public Activity {
+public:
+    MyActivity(GfxRenderer& renderer, MappedInputManager& mappedInput);
+    void onEnter() override;
+    void onExit() override;
+    void loop() override;
+
+private:
+    void render();
+};
+
+// MyActivity.cpp
+MyActivity::MyActivity(GfxRenderer& renderer, MappedInputManager& mappedInput)
+    : Activity("MyActivity", renderer, mappedInput) {}
+
+void MyActivity::onEnter() {
+    Activity::onEnter();
+    // Initialization code
+}
+
+void MyActivity::onExit() {
+    // Cleanup code
+    Activity::onExit();
+}
+
+void MyActivity::loop() {
+    // Handle input
+    if (mappedInput.wasPressed(MappedInputManager::Button::Confirm)) {
+        // Handle action
+    }
+
+    // Update and render if needed
+    render();
+}
+
+void MyActivity::render() {
+    renderer.clearScreen();
+    // Drawing code
+    renderer.displayBuffer();
+}
+```
+
+### Activity Navigation
+
+Activities are managed in `main.cpp`:
+
+```cpp
+// Switch to new activity
+exitActivity();  // Clean up current
+enterNewActivity(new MyActivity(renderer, mappedInputManager));
+```
+
+## Memory Management
+
+### Constraints
+
+- ESP32-C3 has ~380KB usable RAM
+- Display buffer: ~45KB (480x800 / 8 bits per pixel)
+- Must cache EPUB data to SD card
+
+### Caching Strategy
+
+EPUB content is cached to `.crosspoint/` directory on SD card:
+- Chapter data cached after first load
+- Book metadata stored in `book.bin`
+- Reading progress in `progress.bin`
+- Cover images in `cover.bmp`
+
+### Best Practices
+
+1. **Avoid dynamic allocations in loop()**
+2. **Use stack allocation when possible**
+3. **Free resources in `onExit()`**
+4. **Monitor memory usage** (serial output shows free heap)
+
+## Configuration System
+
+### CrossPointSettings
+
+Singleton class for user settings:
+- Display settings (orientation, refresh rate)
+- Reader settings (font, spacing, alignment)
+- Button layouts
+- Sleep timeouts
+
+```cpp
+// Access settings
+SETTINGS.fontFamily;           // Current font family
+SETTINGS.getSleepTimeoutMs();  // Auto-sleep timeout
+SETTINGS.getPowerButtonDuration(); // Long-press duration
+```
+
+### CrossPointState
+
+Singleton for runtime state:
+- Currently open EPUB path
+- Last sleep screen image index
+
+## Development Workflow
+
+### Building and Flashing
+
+```bash
+# Build project
+pio run
+
+# Flash to device
+pio run --target upload
+
+# Monitor serial output
+pio device monitor
+```
+
+### Debugging
+
+1. **Serial Monitor**: View logs at 115200 baud
+2. **Memory Monitoring**: Heap usage printed every 10 seconds
+3. **Error Handling**: Full-screen error activities for SD card failures
+
+### Common Pitfalls
+
+1. **Memory Exhaustion**: Watch for heap fragmentation
+2. **Display Artifacts**: Ensure proper buffer management
+3. **Button Debouncing**: Handled by InputManager
+4. **SD Card Access**: Use `SdMan` helper for file operations
+
+## Testing
+
+### Manual Testing Areas
+
+1. **Button mappings** in different layout configurations
+2. **Memory usage** with large EPUB files
+3. **Sleep/wake cycles** with power button
+4. **SD card** removal/insertion during operation
+5. **Battery indicator** accuracy
+
+### Automated Testing
+
+- PlatformIO test framework available
+- Run tests: `pio test`
+- Test directory: `test/`
+
+## Contributing Guidelines
+
+### Code Style
+
+- Follow existing code patterns
+- Use `clang-format` for formatting
+- Add serial logging for major operations
+- Include error handling for SD card operations
+
+### Adding Features
+
+1. Check [ideas discussion board](https://github.com/daveallie/crosspoint-reader/discussions/categories/ideas)
+2. Create feature branch
+3. Implement with memory constraints in mind
+4. Test on actual hardware
+5. Submit PR with clear description
+
+### Performance Considerations
+
+- Minimize display refreshes
+- Cache computed values (text measurements, etc.)
+- Use `skipLoopDelay()` for activities needing fast response (webserver)
+- Implement `preventAutoSleep()` for activities with background work
+
+## Hardware Reference
+
+### Pin Definitions (from main.cpp)
+
+```cpp
+// Display SPI
+#define EPD_SCLK 8
+#define EPD_MOSI 10
+#define EPD_CS 21
+#define EPD_DC 4
+#define EPD_RST 5
+#define EPD_BUSY 6
+
+// SD Card SPI
+#define SD_SPI_MISO 7
+
+// Battery monitoring
+#define BAT_GPIO0 0
+
+// USB detection
+#define UART0_RXD 20
+```
+
+### Power Management
+
+- Deep sleep mode for power saving
+- Wake on power button press (GPIO low)
+- Auto-sleep after configurable inactivity period
+- Battery monitoring via ADC on GPIO0
+
+## Useful Resources
+
+- [OpenX4 SDK Documentation](https://github.com/open-x4-epaper/community-sdk)
+- [PlatformIO Documentation](https://docs.platformio.org/)
+- [ESP32-C3 Reference](https://docs.espressif.com/projects/esp-idf/en/latest/esp32c3/)
+- [EPUB Format Specification](https://www.w3.org/publishing/epub3/epub-overview.html)
+
+## Getting Help
+
+- Check existing issues and discussions
+- Review `docs/` directory for internal documentation
+- Test with actual Xteink X4 hardware when possible
+- Enable serial logging for debugging
+