🍰 CakePay

Split Bills Like Slicing Cake - Simple, Sweet, On-Chain

Built with ❤️ by Rather Labs

Live Demo • Documentation • Contributing

🎯 What is CakePay?

CakePay is a decentralized bill-splitting dApp that makes group expense management as easy as slicing cake. Whether you're splitting rent with roommates, managing team dinners, or tracking shared subscriptions, CakePay provides a transparent, blockchain-powered solution that works seamlessly across Web3 wallets, Lemon Cash, and Farcaster Mini Apps.

Why CakePay?

🎂 Create "Cakes": Group multiple people together for ongoing shared expenses
🥧 Add "Ingredients": Track expenses with custom splits and multiple payers
🔪 Cut the Cake: Batch submit expenses on-chain to update balances
💰 Settle Up: Pay what you owe or claim what you're owed with transparent on-chain settlement
📊 Track Everything: Real-time balance updates with complete transaction history
🔐 Trustless & Transparent: All financial logic lives on the blockchain

✨ Key Features

🏢 Multi-Platform Support

Standard Web3: Full Wagmi + RainbowKit integration
Lemon Cash: Native integration with Lemon Cash Mini App SDK
Farcaster: Seamless Warpcast Mini App experience
One codebase, three wallet experiences

💳 Smart Group Management

Create groups (cakes) with 2+ members
Weighted expense splitting (equal or custom ratios)
Multiple payers per expense
Configurable interest rates for late payments
Democratic voting to disable groups

⛓️ Blockchain-First Architecture

On-Chain: Core financial data, balances, and settlements
Off-Chain: Extended metadata, descriptions, and receipts
User ID system for gas-efficient storage
Batched transactions to minimize gas costs

🎨 Beautiful UX

Pixel-art inspired design
Real-time balance calculations
Pending vs settled expense tracking
Member leaderboards showing who's owed the most
Mobile-first responsive design

🏗️ Architecture Overview

CakePay follows a hybrid on-chain/off-chain architecture optimized for both security and user experience:

┌─────────────────────────────────────────────────────────────┐
│                     CakePay Architecture                     │
├─────────────────────────────────────────────────────────────┤
│                                                               │
│  ┌─────────────────────────────────────────────────────┐   │
│  │           Frontend Layer (Next.js 14)               │   │
│  │                                                       │   │
│  │  • Wagmi/RainbowKit (Standard Web3)                 │   │
│  │  • Lemon Cash Mini App SDK                          │   │
│  │  • Farcaster/Warpcast SDK                           │   │
│  │  • Server Components (Database APIs)                │   │
│  │  • Client Components (Wallet Interactions)          │   │
│  └────────────────────┬────────────────────────────────┘   │
│                       │                                      │
│  ┌────────────────────┴──────────────────────────────────┐ │
│  │         Smart Contract Layer (Ethereum/L2s)          │ │
│  │                                                        │ │
│  │  CakeFactory.sol                                      │ │
│  │  • User ID Mapping (address → uint64)                │ │
│  │  • Cake Management (groups, members, weights)        │ │
│  │  • Batched Ingredients (expenses with payers)        │ │
│  │  • Balance Tracking (int256[] per cake)              │ │
│  │  • Settlement System (pay/claim functions)           │ │
│  │  • Interest Accrual (configurable rates)             │ │
│  │  • Voting System (disable inactive cakes)            │ │
│  └────────────────────┬──────────────────────────────────┘ │
│                       │                                      │
│  ┌────────────────────┴──────────────────────────────────┐ │
│  │           Data Layer (Supabase)                       │ │
│  │                                                        │ │
│  │  • User Profiles (username, avatar)                  │ │
│  │  • Cake Metadata (name, description, icon)           │ │
│  │  • Ingredient Details (name, description, receipts)  │ │
│  │  • Status Tracking (pending, submitted, settled)     │ │
│  └────────────────────────────────────────────────────────┘ │
│                                                               │
└─────────────────────────────────────────────────────────────┘

Data Flow

Creating a Cake:

User fills form with group details → Frontend
Validates all members are registered on-chain → Smart Contract Query
Submits transaction via wallet → Smart Contract Write
Event emitted → Frontend listens
Metadata saved → Database

Adding Expenses:

User adds expense (off-chain) → Database (pending status)
Multiple expenses accumulate → Database
User clicks "Submit on-chain" → Smart Contract (batched)
Balances updated → Smart Contract
Status updated to "submitted" → Database

🚀 Quick Start

Prerequisites

Node.js 18+
npm or yarn
MetaMask or other Web3 wallet
(Optional) Lemon Cash app for mini-app testing
(Optional) Warpcast for Farcaster integration

Installation

# Clone the repository
git clone https://github.com/rather-labs/rather-cake-pay.git
cd rather-cake-pay

# Install smart contract dependencies
cd hardhat
npm install

# Install frontend dependencies
cd ../frontend
npm install

Smart Contract Deployment

cd hardhat

# Compile contracts
npm run compile

# Deploy to local network
npm run node          # Terminal 1
npm run deploy:local  # Terminal 2

# Deploy to Sepolia testnet
npm run deploy:sepolia

# Deploy to Base Sepolia
npm run deploy:base-sepolia

Frontend Setup

cd frontend

# Configure environment variables
cp .env.example .env.local
# Edit .env.local with your values

# Run development server
npm run dev

Visit http://localhost:3000 to see the app!

📦 Technology Stack

Smart Contracts

Solidity 0.8.24: Latest secure Solidity version
Hardhat: Development framework
Hardhat Ignition: Deployment system
Slither: Security analysis (mandatory in CI)

Frontend

Next.js 14: App Router with Server Components
React 18: Modern React with hooks
TypeScript: Full type safety
Wagmi: Ethereum interactions
RainbowKit: Wallet connection UI
Viem: Low-level Ethereum library
Lemon Cash SDK: Mini-app integration
Farcaster SDK: Warpcast mini-app support
Tailwind CSS: Utility-first styling

Infrastructure

Supabase: PostgreSQL database
Vercel: Frontend hosting (recommended)
Tenderly: RPC provider + debugging

🎮 User Flows

1️⃣ Registration Flow

User connects wallet
  → Checks if registered on-chain
  → If not: Transaction to register (gets user ID)
  → Saves profile to database
  → Redirects to dashboard

2️⃣ Creating a Cake (Group)

User clicks "Create Group"
  → Fills form (name, members, interest rate)
  → System validates all members registered on-chain
  → Transaction sent to create cake
  → On-chain user IDs used (not wallet addresses)
  → Cake metadata saved to database
  → User redirected to cake page

3️⃣ Adding Expenses

User clicks "Add Expense"
  → Fills form (name, amount, payers, split)
  → Expense saved to database (status: pending)
  → User can add multiple expenses
  → When ready: "Submit on-chain" button
  → Batches all pending expenses
  → Single transaction updates balances
  → Status changes to "submitted"

4️⃣ Settling Up

User views their balance
  → If positive (owed): Click "Claim"
  → If negative (owes): Click "Pay"
  → Transaction sent with exact amount
  → Balance updated on-chain
  → UI reflects new balance

🏛️ Smart Contract Details

CakeFactory.sol

The core contract managing all financial logic.

Key Data Structures

Cake (Group)

struct Cake {
    uint64 createdAt;
    uint64 lastCutAt;
    uint64 lastCutBatchedIngredientsId;
    uint16 interestRate;
    bool active;
    address token;              // 0x0 for ETH
    bool[] votesToDisable;
    uint64[] memberIds;         // User IDs (not addresses)
    uint16[] memberWeights;     // BPS (sum = 10000)
    int256[] currentBalances;   // Positive = owes, negative = owed
}

BatchedCakeIngredients (Expense)

struct BatchedCakeIngredients {
    uint64 createdAt;
    uint16[] weights;           // Per-member split
    uint64[] payerIds;          // Who paid
    uint256[] payedAmounts;     // How much each paid
}

Core Functions

Function	Description	Gas Optimization
`registerUser(address)`	Register wallet and get user ID	One-time per user
`createCake(...)`	Create new group	User IDs instead of addresses
`addBatchedCakeIngredients(...)`	Add expenses batch	Multiple expenses in one tx
`cutCake(uint128)`	Update balances	Processes all pending ingredients
`payCakeSlice(uint128)`	Pay what you owe	Direct balance update
`claimCakeSlice(uint128)`	Claim what you're owed	Transfers funds to caller

🔧 Configuration

Environment Variables

Frontend (frontend/.env.local)

# Contract Deployment
NEXT_PUBLIC_CONTRACT_ADDRESS_ETH_SEPOLIA=0x...
NEXT_PUBLIC_CHAIN_ID=11155111

# RPC
NEXT_PUBLIC_RPC_URL=https://sepolia.gateway.tenderly.co

# Database
NEXT_PUBLIC_SUPABASE_URL=https://...supabase.co
NEXT_PUBLIC_SUPABASE_ANON_KEY=eyJ...

# Wallets
NEXT_PUBLIC_WALLETCONNECT_PROJECT_ID=...

Contracts (hardhat/.env)

SEPOLIA_RPC_URL=https://sepolia.infura.io/v3/...
SEPOLIA_PRIVATE_KEY=0x...
ETHERSCAN_API_KEY=...

🧪 Testing

Smart Contracts

cd hardhat

# Run tests
npm test

# Run with coverage
npm run coverage

# Security analysis (mandatory)
slither . --exclude-dependencies

Frontend

cd frontend

# Type check
npx tsc --noEmit

# Lint
npm run lint

# Format check
npm run format:check

# Build test
npm run build

CI/CD

# Test everything locally before pushing
./scripts/test-ci.sh

# Test only contracts
./scripts/test-ci.sh contracts

# Test only frontend
./scripts/test-ci.sh frontend

🛡️ Security

Smart Contract Security

Slither Analysis: Mandatory in CI pipeline
OpenZeppelin Patterns: Used where applicable
Reentrancy Protection: All external calls protected
Integer Overflow: Built-in Solidity 0.8+ checks
Access Control: Proper member validation

Best Practices

Never commit private keys or secrets
All contract changes require security review
Slither must pass before merging
Use established patterns (OpenZeppelin)
Test edge cases thoroughly

🤝 Contributing

We welcome contributions! Please see CONTRIBUTING.md for detailed guidelines.

Development Workflow

Fork the repository
Create feature branch from develop
Make your changes
Run ./scripts/test-ci.sh locally
Submit PR to develop branch
Wait for review and CI to pass

Commit Convention

We follow Conventional Commits:

feat(contracts): add voting system
fix(frontend): correct balance calculation
docs(readme): update deployment steps

📚 Additional Resources

Smart Contracts: See hardhat/README.md
Contributing Guide: See CONTRIBUTING.md
Frontend API: See frontend/lib/api/
Database Schema: See Database Data Structures section

🗺️ Roadmap

📄 License

This project is licensed under the MIT License - see the LICENSE file for details.

🙏 Acknowledgments

Built by Rather Labs - A leading blockchain development studio specializing in Web3 applications, smart contracts, and decentralized systems.

Powered by:

Lemon Cash - Crypto banking made simple
Farcaster - Decentralized social protocol
Ethereum - World computer

Made with 🍰 by Rather Labs

Website • Twitter • LinkedIn

Name		Name	Last commit message	Last commit date
Latest commit History 46 Commits
.github		.github
foundry		foundry
frontend		frontend
hardhat		hardhat
scripts		scripts
.gitignore		.gitignore
CLAUDE.md		CLAUDE.md
CONTRIBUTING.md		CONTRIBUTING.md
README.md		README.md

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🍰 CakePay

Split Bills Like Slicing Cake - Simple, Sweet, On-Chain

🎯 What is CakePay?

Why CakePay?

✨ Key Features

🏢 Multi-Platform Support

💳 Smart Group Management

⛓️ Blockchain-First Architecture

🎨 Beautiful UX

🏗️ Architecture Overview

Data Flow

🚀 Quick Start

Prerequisites

Installation

Smart Contract Deployment

Frontend Setup

📦 Technology Stack

Smart Contracts

Frontend

Infrastructure

🎮 User Flows

1️⃣ Registration Flow

2️⃣ Creating a Cake (Group)

3️⃣ Adding Expenses

4️⃣ Settling Up

🏛️ Smart Contract Details

CakeFactory.sol

Key Data Structures

Core Functions

🔧 Configuration

Environment Variables

🧪 Testing

Smart Contracts

Frontend

CI/CD

🛡️ Security

Smart Contract Security

Best Practices

🤝 Contributing

Development Workflow

Commit Convention

📚 Additional Resources

🗺️ Roadmap

📄 License

🙏 Acknowledgments

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