OnChain Tic-Tac-Toe

A gas-optimized Tic-Tac-Toe implementation on Ethereum using bit manipulation and assembly code. README.md by Claude Code

This project explores smart contract optimization techniques by implementing a simple game with minimal gas consumption. The entire game state is packed into 24 bits, demonstrating efficient storage patterns for blockchain applications.

What Makes This Different?

Key Features

• 24 bits to store entire game state (vs. traditional 288+ bits)
• Significant gas savings compared to standard implementations
• Yul assembly for optimized operations
• Efficient storage using bit manipulation

Technical Approach

Standard Implementation

// Traditional approach
mapping(uint256 => uint8[9]) public boards;  // 288 bits per game
mapping(uint256 => address) public currentPlayer;

Optimized Implementation

// Bit-packed approach
struct Game {
    bytes3 board;        // 24 bits total
    address player1;
    address player2;
}

Bit Layout

The game state is packed into 24 bits:

Bit Layout (24 bits total):
├─ Bits 0-8:   Player 1 marks (9 positions)
├─ Bits 9-17:  Player 2 marks (9 positions)  
├─ Bit 18-22:  Unused (future expansion)
└─ Bit 23:     Turn indicator (0=P1, 1=P2)

Example Game State:

Player 1 plays positions [0,4,8]: 0b100010001 (bits 0-8)
Player 2 plays positions [1,5]:   0b000100010 (bits 9-17, shifted)
Current turn: Player 2             0b1         (bit 23)

Combined: 0b1000001000100100010001 (binary)
         = 0x808121 (hex)

Implementation Details

Optimization Techniques

1. Bit Manipulation

// Check if position is occupied by either player
if (iszero(iszero(and(shl(index, 513), board)))) {
    revert() // 513 = 0b1000000001 - checks both player bits
}

2. Assembly Optimization

Critical functions use Yul assembly for gas efficiency:

// Get player's board state in assembly
switch iszero(and(shl(23, 1), board))
case 0 {
    // Player 2's turn - extract bits 9-17
    playerBoard := shl(223, and(261632, board))
}
default {  
    // Player 1's turn - extract bits 0-8
    playerBoard := shl(232, and(511, board))
}

3. Pre-computed Winning Patterns

Winning combinations are pre-computed for efficiency:

winningMarks[bytes3(0x000007)] = true; // [0,1,2] - top row
winningMarks[bytes3(0x000111)] = true; // [0,4,8] - diagonal
// ... all 8 winning combinations

4. Event-Driven Architecture

event NewGame(address indexed player1, address indexed player2, uint256 gameId);
event Played(uint256 indexed gameId, bytes3 board);

Architecture Analysis

Advantages

1. Gas Efficiency

   • Reduced gas costs compared to standard implementations
   • Single SSTORE operation for moves
   • Bit-packed storage minimizes storage costs

2. Performance

   • Board state fits in CPU registers
   • Atomic operations prevent race conditions
   • Predictable gas costs

3. Scalability

   • Minimal storage per game
   • Supports multiple concurrent games
   • Suitable for high-frequency gaming

4. Code Quality

   • Pure functions where possible
   • Predictable execution paths
   • Documented assembly code

Trade-offs

1. Development Complexity

   • Requires understanding of bit manipulation
   • Assembly debugging is challenging
   • Higher learning curve for contributors

2. Code Readability

   • Assembly code is less readable than Solidity
   • Bit operations require careful documentation
   • More complex to understand and maintain

3. Flexibility Limitations

   • Difficult to modify game rules
   • Adding features requires careful planning
   • Less suitable for rapid prototyping

Testing Strategy

The project uses comprehensive fuzz testing to verify correctness:

Fuzz Testing Approach

function testFuzz_ValidMoves(uint8 position) public {
    vm.assume(position < 9);
    // Tests various game state combinations
}

Test Categories

• Valid Scenarios: Legal moves and game flows
• Invalid Positions: Out-of-bounds and occupied cells
• Wrong Players: Unauthorized and out-of-turn attempts
• Game Isolation: Multiple games don't interfere
• State Invariants: Game state consistency
• Event Emissions: Proper logging verification

Coverage

• 15+ fuzz test functions
• Extensive randomized test cases
• Critical path coverage
• Edge case detection

Getting Started

Prerequisites

• Foundry installed
• Basic understanding of Solidity and EVM

Build & Test

# Clone and build
git clone <repository>
cd tic-tac-toe

# Compile the contract
forge build

# Run the test suite
forge test

# Run with maximum verbosity for debugging
forge test -vvvv

# Generate coverage report
forge coverage

# Format code
forge fmt

Deploy to Testnet

# Deploy to your favorite testnet
forge script script/TicTacToe.s.sol --rpc-url $RPC_URL --private-key $PRIVATE_KEY --broadcast

🎮 How to Play

1. Create Game: Call newGame(player1, player2)
2. Make Moves: Call play(gameId, position) where position is 0-8
3. Check State: Call getBoard(gameId) to see current state
4. Find Winner: Call whoWon(gameId) after game ends

// Example game flow
uint256 gameId = ttt.newGame(alice, bob);
ttt.play(gameId, 4); // Alice plays center
ttt.play(gameId, 0); // Bob plays top-left
// ... continue until someone wins or draws

Technical Specifications

• Solidity Version: ^0.8.13
• Framework: Foundry
• Dependencies: forge-std, solady
• Gas Limit: ~50,000 per move
• Storage: 1 slot per game (32 bytes)
• Max Concurrent Games: Limited by Ethereum storage

Contributing

Contributions are welcome. When contributing:

1. Understand the bit layout before making changes
2. Test thoroughly - gas optimizations can be fragile
3. Document assembly code clearly
4. Benchmark changes to ensure performance is maintained

Future Improvements

• NFT Integration: Game states as collectibles

Learn More

• Solidity Assembly Documentation
• EVM Opcodes Reference
• Gas Optimization Guide
• Foundry Book

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README.md by Claude Code