Solidity Event Design Patterns

## ROLE
You are a smart contract developer focused on event-driven architectures and off-chain data indexing.

## CONTEXT
I need to design the event system for my smart contract.

## CONTRACT INFO
- Contract type: ${{TYPE}}
- Key state changes: ${{STATE_CHANGES}}
- Off-chain needs: ${{OFFCHAIN_NEEDS}}
- Indexing requirements: ${{INDEXING}}

## TASK
Design comprehensive event system:

**1. EVENT FUNDAMENTALS**
```solidity
// Basic event structure
event Transfer(
    address indexed from,    // Indexed = filterable
    address indexed to,      // Indexed = filterable
    uint256 amount           // Not indexed = cheaper
);

// Indexed vs non-indexed
// - Indexed: Can filter/search (3 max for non-anonymous)
// - Non-indexed: Cheaper, data only
```

**2. BEST PRACTICES**
```solidity
// DO: Clear, descriptive events
event TokensMinted(
    address indexed recipient,
    uint256 amount,
    uint256 timestamp
);

// DON'T: Generic, unclear events
event Action(address user, uint256 value);

// DO: Include all relevant data
event Swap(
    address indexed sender,
    address indexed tokenIn,
    address indexed tokenOut,
    uint256 amountIn,
    uint256 amountOut,
    uint256 fee
);

// DON'T: Require additional calls to understand
event Swap(uint256 indexed id);
```

**3. STATE CHANGE EVENTS**
```solidity
contract StateEvents {
    // Administrative changes
    event OwnershipTransferred(
        address indexed previousOwner,
        address indexed newOwner
    );

    event Paused(address account);
    event Unpaused(address account);

    // Configuration changes
    event FeeUpdated(uint256 oldFee, uint256 newFee);

    event WhitelistUpdated(
        address indexed account,
        bool indexed status
    );
}
```

**4. FINANCIAL EVENTS**
```solidity
contract DeFiEvents {
    event Deposit(
        address indexed user,
        address indexed token,
        uint256 amount,
        uint256 shares
    );

    event Withdraw(
        address indexed user,
        address indexed token,
        uint256 amount,
        uint256 shares
    );

    event Swap(
        address indexed user,
        address indexed tokenIn,
        address indexed tokenOut,
        uint256 amountIn,
        uint256 amountOut
    );

    event Liquidation(
        address indexed liquidator,
        address indexed borrower,
        uint256 repaidAmount,
        uint256 collateralSeized
    );
}
```

**5. NFT EVENTS**
```solidity
contract NFTEvents {
    // ERC721 standard
    event Transfer(
        address indexed from,
        address indexed to,
        uint256 indexed tokenId
    );

    // Custom events
    event TokenMinted(
        address indexed to,
        uint256 indexed tokenId,
        string metadataURI
    );

    event MetadataUpdated(
        uint256 indexed tokenId,
        string oldURI,
        string newURI
    );
}
```

**6. INDEXING CONSIDERATIONS**
```solidity
// For The Graph / Subgraphs
// Use indexed for fields you'll filter by
event Sale(
    uint256 indexed tokenId,      // Filter by token
    address indexed seller,        // Filter by seller
    address indexed buyer,         // Filter by buyer
    uint256 price,                 // Data only
    uint256 timestamp              // Data only
);

// Max 3 indexed for non-anonymous events
// Choose wisely based on query patterns
```

**7. GAS OPTIMIZATION**
```solidity
// Events are relatively cheap
// ~375 gas base + 375 per indexed topic + 8 per byte

// Use bytes32 instead of string for fixed data
event ConfigUpdated(bytes32 indexed key, bytes32 value);

// Pack multiple booleans
event FlagsUpdated(uint256 flags);
```

**8. EVENT DOCUMENTATION**
```solidity
/// @notice Emitted when tokens are transferred
/// @param from The sender address (zero for mints)
/// @param to The recipient address (zero for burns)
/// @param amount The number of tokens transferred
event Transfer(
    address indexed from,
    address indexed to,
    uint256 amount
);
```

**9. RECOMMENDED EVENTS BY CONTRACT TYPE**
| Contract Type | Essential Events |
|--------------|------------------|
| Token | Transfer, Approval |
| NFT | Transfer, Approval, MetadataUpdate |
| DeFi | Deposit, Withdraw, Swap, Liquidation |
| Governance | ProposalCreated, Voted, Executed |
| Staking | Staked, Withdrawn, RewardPaid |