Migration to Group2/addTransaction complete. All legacy Group/addExpense code has been removed.

A lightweight expense splitting application for tracking shared costs in groups. Create groups, add expenses, and automatically calculate who owes what—all while working completely offline.

• Group Management: Create expense groups with descriptions and participants (e.g., "Trip to Paris" with Alice, Bob, Charlie). Add participants during group creation with built-in validation to prevent duplicates and ensure proper formatting
• Add Expenses: Add expenses to groups with single or multiple payers that can be split equally among all participants, a selected subset, or with custom amounts for each participant. Supports decimal amounts with precise handling down to 0.01 currency units
• UI to Add Expenses: User-friendly interface to add expenses with form validation, including decimal amount input, description, payer selection, and participant selection for splitting
• Expenses Display: View all group expenses in a formatted table showing description, date, amount, payers, and participants for each expense
• Balance Calculations: Automatic calculation and display of outstanding debts and owed amounts for each participant, showing who owes money and who is owed money with precise balance tracking
• Participants Display: View all group participants with their current balance status in an organized table (desktop) and card layout (mobile)
• Local Persistence: All data is saved to browser local storage—no server required
• Group Overview: View a list of your groups and access their details
• Conflict Resolution: Built-in merge functionality to handle simultaneous edits to the same group
• Responsive Design: Mobile-first design built with Bulma.io framework for optimal viewing on all devices

• Add participants to existing groups (edit functionality)
• Share groups via links
• Full offline functionality

Tech Stack: Single Page Application built with modern web technologies

• Frontend: Preact with TypeScript for lightweight, React-compatible components
• Styling: Bulma.io CSS framework for responsive, mobile-first design
• Build Tool: Deno for dependency management, testing, and build scripts
• Routing: Wouter-preact with hash-based navigation for maximum compatibility
• Storage: Browser Local Storage (no external dependencies)
• Bundling: Deno's built-in bundler with esbuild integration

Key Design Principles:

• Offline-first: Works without internet connection
• No backend required: All data stays in your browser
• Privacy-focused: No data collection or external services
• Lightweight: Minimal dependencies and fast loading

Immutable Transaction Design: Cashsplitter follows an append-only data model where all transactions (expenses) are immutable once created. This ensures data integrity and enables reliable conflict resolution for concurrent edits.

Key Principles:

• No Editing: Transactions cannot be modified after creation
• No Direct Deletion: Transactions cannot be removed from the history
• Correction Pattern: To "edit" or "delete" an expense, use delete transactions and replacements:
  • Add a delete transaction that voids the existing transaction
  • If necessary, add a new transaction with the correct values (participants, amounts, shares, datetime)

Benefits:

• Audit Trail: Complete history of all financial changes
• Conflict Resolution: Concurrent edits can be merged safely
• Data Integrity: No risk of accidentally losing transaction history
• Rollback Capability: Can always trace back to any previous state

Implementation: When future edit/delete UI features are added, they will create appropriate corrective transactions behind the scenes while presenting a user-friendly editing interface.

• Payment processing: This is a tracking tool, not a payment system
• Cross-group transactions: Each group is independent
• Cloud sync: Data stays local to your device
• User accounts: No registration or login required

# Clone the repository
git clone https://github.com/user/cashsplitter.git
cd cashsplitter

# Build the application
deno task build

# Start development server
deno task serve

# Open http://localhost:8000 in your browser

deno task build       # Build production version to ./build/
deno task bundle      # Bundle TypeScript/JSX to single JS file
deno task serve       # Start development server on localhost:8000
deno task test        # Run all tests with required permissions

All tests are written using Deno's built-in testing framework:

deno task test       # Run all tests with required permissions
deno test --watch    # Run tests in watch mode

# Run specific test suites
deno test src/model/            # Business logic tests
deno test src/components/       # Component tests

Test Coverage:

• Business Logic: Complete test coverage for group creation, serialization, and data models
• Components: Basic test coverage for UI components with JSDOM simulation
  • Notification component: rendering, interaction, click handlers
  • Additional component tests in development

Testing Infrastructure:

• Custom test utilities for Preact component testing with JSDOM
• Mock utilities for localStorage and context providers
• Timer cleanup to prevent test leaks

Pre-commit Hooks: Automatic code quality checks are enforced via git pre-commit hooks:

# Pre-commit automatically runs:
deno fmt --check       # Verify code formatting
deno lint             # Check for linting issues

If checks fail, the commit is rejected. Fix issues with:

deno task fmt         # Auto-format code
deno task lint        # Show linting issues

Available Tasks:

deno task test        # Run all tests with required permissions
deno task fmt         # Format all code
deno task fmt:check   # Check formatting without changing files
deno task lint        # Run linter

├── src/
│   ├── components/     # Reusable UI components
│   ├── context/        # Preact context providers
│   ├── model/          # Business logic and data models
│   ├── routes/         # Page components
│   ├── storage/        # Local storage interface
│   ├── merge/          # Conflict resolution logic
│   └── utils/          # Helper utilities
├── scripts/            # Build and development scripts
├── static/             # Static assets (HTML, CSS)
└── build/              # Production build output

Automated testing and deployment via GitHub Actions:

• Testing: All commits trigger test suite
• Deployment: Successful builds on main deploy to GitHub Pages
• Environment: Uses Deno v2.4.2 for consistency

1. Check the TODO.md for current development priorities
2. Follow TDD practices: write tests first, see them fail, implement features
3. Keep changes focused and atomic
4. Update documentation as needed

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