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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
+
+Gym Tracker is a containerized web application for tracking gym sessions and muscle group training frequency. It displays a heatmap of workout sessions, tracks muscle group balance, and provides insights into training patterns.
+
+**Tech Stack:**
+- **Backend:** Node.js with Express (REST API)
+- **Database:** SQLite (file-based, for simplicity)
+- **Frontend:** Vanilla JavaScript (no build process)
+- **Deployment:** Docker container with Nginx reverse proxy
+
+## Commands
+
+### Backend Development
+```bash
+# Install dependencies
+cd backend && npm install
+
+# Start the backend server (port 3000)
+npm start
+```
+
+### Docker
+```bash
+# Build the Docker image
+docker build -t gym-tracker .
+
+# Run the container
+docker run -p 80:80 -p 3000:3000 -v $(pwd)/data:/app/data gym-tracker
+
+# Note: Volume mount for /app/data ensures SQLite database persists across restarts
+```
+
+## Architecture
+
+### Backend Structure
+- **`backend/server.js`**: Express server with REST API endpoints
+  - `GET /api/sessions` - Fetch all gym sessions
+  - `POST /api/sessions` - Create new session (requires date and muscle_groups array)
+  - `PUT /api/sessions/:id` - Update existing session
+  - `DELETE /api/sessions/:id` - Delete session
+- **`backend/database.js`**: SQLite database layer with schema and query functions
+- **Database schema**: Single `sessions` table with columns: `id` (TEXT), `date` (TEXT, ISO format), `muscle_groups` (TEXT, JSON array)
+
+### Frontend Structure
+- **`frontend/index.html`**: Main HTML structure
+- **`frontend/app.js`**: Core application logic and state management
+- **`frontend/api.js`**: API client for backend communication
+- **`frontend/styles.css`**: Application styling
+- **Frontend architecture**: Modular vanilla JS without build tools, served directly by Nginx
+
+### Muscle Groups
+The application tracks 6 muscle groups: Chest, Legs, Delts (shoulders), Lats (back), Triceps, Biceps
+
+### Balance Indicator Logic
+- **Happy**: All 6 muscle groups trained 2+ times in last 7 days
+- **Neutral**: All 6 muscle groups trained 1+ times in last 7 days
+- **Angry**: Any muscle group not trained in last 7 days
+
+### Docker Configuration
+- Single container runs both Nginx (frontend) and Express (backend)
+- Nginx serves static files and proxies `/api/*` requests to Express
+- SQLite database requires volume mount for persistence
+- Ports: 80 (Nginx/HTTP), 3000 (Express API)
+
+## Important Implementation Details
+
+### Date Format
+All dates are stored and transmitted in ISO 8601 format (YYYY-MM-DD) for consistency between frontend and backend.
+
+### Session IDs
+Sessions use UUID format for unique identification.
+
+### Muscle Groups Storage
+Muscle groups are stored as JSON-encoded arrays in the SQLite TEXT column, e.g., `["Chest", "Legs", "Delts"]`.
+
+### Heatmap Implementation
+Uses Cal-Heatmap library (loaded via CDN) to visualize workout frequency across the year.