// VULNERABLE TO REENTRANCY
contract VulnerableBank {
    mapping(address => uint256) public balances;
 
    function withdraw() public {
        uint256 amount = balances[msg.sender];
 
        // DANGER: External call before state update
        (bool success, ) = msg.sender.call{value: amount}("");
        require(success);
 
        balances[msg.sender] = 0;  // Too late!
    }
}

contract SecureBank {
    mapping(address => uint256) public balances;
 
    function withdraw() public {
        uint256 amount = balances[msg.sender];
        require(amount > 0, "Insufficient balance");
 
        // EFFECTS: Update state BEFORE external call
        balances[msg.sender] = 0;
 
        // INTERACTIONS: External call last
        (bool success, ) = msg.sender.call{value: amount}("");
        require(success, "Transfer failed");
    }
}

import "@openzeppelin/contracts/security/ReentrancyGuard.sol";
 
contract SecureBank is ReentrancyGuard {
    mapping(address => uint256) public balances;
 
    function withdraw() public nonReentrant {
        uint256 amount = balances[msg.sender];
        require(amount > 0, "Insufficient balance");
 
        balances[msg.sender] = 0;
 
        (bool success, ) = msg.sender.call{value: amount}("");
        require(success, "Transfer failed");
    }
}

// VULNERABLE
contract VulnerableToken {
    mapping(address => uint256) public balances;
 
    function transfer(address to, uint256 amount) public {
        // No overflow check - can wrap around
        balances[msg.sender] -= amount;  // Can underflow!
        balances[to] += amount;          // Can overflow!
    }
}

// Solidity 0.8+ has built-in overflow/underflow checks
contract SecureToken {
    mapping(address => uint256) public balances;
 
    function transfer(address to, uint256 amount) public {
        // Automatically reverts on overflow/underflow
        balances[msg.sender] -= amount;
        balances[to] += amount;
    }
}

import "@openzeppelin/contracts/utils/math/SafeMath.sol";
 
contract SecureToken {
    using SafeMath for uint256;
    mapping(address => uint256) public balances;
 
    function transfer(address to, uint256 amount) public {
        balances[msg.sender] = balances[msg.sender].sub(amount);
        balances[to] = balances[to].add(amount);
    }
}

// VULNERABLE: Anyone can call critical functions
contract VulnerableContract {
    address public owner;
 
    function withdraw(uint256 amount) public {
        // No access control!
        payable(msg.sender).transfer(amount);
    }
}

import "@openzeppelin/contracts/access/Ownable.sol";
 
contract SecureContract is Ownable {
    function withdraw(uint256 amount) public onlyOwner {
        payable(owner()).transfer(amount);
    }
}
 
// Or implement custom role-based access
contract RoleBasedContract {
    mapping(address => bool) public admins;
 
    modifier onlyAdmin() {
        require(admins[msg.sender], "Not an admin");
        _;
    }
 
    function criticalFunction() public onlyAdmin {
        // Protected function
    }
}

// VULNERABLE TO FRONT-RUNNING
contract VulnerableDEX {
    function swap(uint256 amount, uint256 minOutput) public {
        // Attacker sees this in mempool and front-runs
        uint256 output = calculateOutput(amount);
        require(output >= minOutput, "Slippage too high");
        // Perform swap
    }
}

contract SecureDEX {
    mapping(bytes32 => bool) public usedCommitments;
 
    // Step 1: Commit to trade
    function commitTrade(bytes32 commitment) public {
        usedCommitments[commitment] = true;
    }
 
    // Step 2: Reveal trade (next block)
    function revealTrade(
        uint256 amount,
        uint256 minOutput,
        bytes32 secret
    ) public {
        bytes32 commitment = keccak256(abi.encodePacked(
            msg.sender, amount, minOutput, secret
        ));
        require(usedCommitments[commitment], "Invalid commitment");
        // Perform swap
    }
}

contract SecurePattern {
    mapping(address => uint256) public balances;
 
    function withdraw(uint256 amount) public {
        // 1. CHECKS: Validate conditions
        require(amount <= balances[msg.sender], "Insufficient balance");
        require(amount > 0, "Amount must be positive");
 
        // 2. EFFECTS: Update state
        balances[msg.sender] -= amount;
 
        // 3. INTERACTIONS: External calls last
        (bool success, ) = msg.sender.call{value: amount}("");
        require(success, "Transfer failed");
    }
}

// Prefer this (pull)
contract SecurePayment {
    mapping(address => uint256) public pendingWithdrawals;
 
    function recordPayment(address recipient, uint256 amount) internal {
        pendingWithdrawals[recipient] += amount;
    }
 
    function withdraw() public {
        uint256 amount = pendingWithdrawals[msg.sender];
        require(amount > 0, "Nothing to withdraw");
 
        pendingWithdrawals[msg.sender] = 0;
        payable(msg.sender).transfer(amount);
    }
}
 
// Over this (push)
contract RiskyPayment {
    function distributePayments(address[] memory recipients, uint256[] memory amounts) public {
        for (uint i = 0; i < recipients.length; i++) {
            // If any transfer fails, entire batch fails
            payable(recipients[i]).transfer(amounts[i]);
        }
    }
}

contract SecureContract {
    function transfer(address to, uint256 amount) public {
        // Validate inputs
        require(to != address(0), "Invalid recipient");
        require(to != address(this), "Cannot send to contract");
        require(amount > 0, "Amount must be positive");
        require(amount <= balances[msg.sender], "Insufficient balance");
 
        // Proceed with transfer
        balances[msg.sender] -= amount;
        balances[to] += amount;
    }
}

import "@openzeppelin/contracts/security/Pausable.sol";
 
contract EmergencyStop is Pausable, Ownable {
    function criticalFunction() public whenNotPaused {
        // Function logic
    }
 
    function emergencyStop() public onlyOwner {
        _pause();
    }
 
    function resume() public onlyOwner {
        _unpause();
    }
}

// More gas efficient
contract GasEfficient {
    uint256 public value;  // Optimal
 
    function set(uint256 _value) public {
        value = _value;
    }
}
 
// Less efficient
contract GasInefficient {
    uint8 public value;  // Still uses 256-bit slot
 
    function set(uint8 _value) public {
        value = _value;  // Extra gas for type conversion
    }
}

// Gas efficient (3 variables in 1 slot)
contract PackedStorage {
    uint128 public a;  // Slot 0
    uint64 public b;   // Slot 0
    uint64 public c;   // Slot 0
    uint256 public d;  // Slot 1
}
 
// Gas inefficient (each variable in separate slot)
contract UnpackedStorage {
    uint256 public a;  // Slot 0
    uint256 public b;  // Slot 1
    uint256 public c;  // Slot 2
    uint256 public d;  // Slot 3
}

contract GasOptimized {
    // More gas efficient
    function processData(uint256[] calldata data) public pure returns (uint256) {
        return data[0];
    }
 
    // Less efficient
    function processDataMemory(uint256[] memory data) public pure returns (uint256) {
        return data[0];
    }
}

contract EventStorage {
    // Emitting events is cheaper than storage
    event DataStored(address indexed user, uint256 indexed id, bytes data);
 
    function storeData(uint256 id, bytes calldata data) public {
        emit DataStored(msg.sender, id, data);
        // Don't store in contract storage unless needed
    }
}

// Security Checklist Contract
contract SecurityChecklist {
    /**
     * [ ] Reentrancy protection (ReentrancyGuard or CEI pattern)
     * [ ] Integer overflow/underflow (Solidity 0.8+ or SafeMath)
     * [ ] Access control (Ownable, roles, modifiers)
     * [ ] Input validation (require statements)
     * [ ] Front-running mitigation (commit-reveal if applicable)
     * [ ] Gas optimization (packed storage, calldata)
     * [ ] Emergency stop mechanism (Pausable)
     * [ ] Pull over push pattern for payments
     * [ ] No delegatecall to untrusted contracts
     * [ ] No tx.origin for authentication (use msg.sender)
     * [ ] Proper event emission
     * [ ] External calls at end of function
     * [ ] Check return values of external calls
     * [ ] No hardcoded addresses
     * [ ] Upgrade mechanism (if proxy pattern)
     */
}

// Hardhat test example
const { expect } = require("chai");
const { ethers } = require("hardhat");
describe("Security Tests", function () {
    it("Should prevent reentrancy attack", async function () {
        const [attacker] = await ethers.getSigners();
        const VictimBank = await ethers.getContractFactory("SecureBank");
        const bank = await VictimBank.deploy();
        const Attacker = await ethers.getContractFactory("ReentrancyAttacker");
        const attackerContract = await Attacker.deploy(bank.address);
        // Deposit funds
        await bank.deposit({value: ethers.utils.parseEther("10")});
        // Attempt reentrancy attack
        await expect(
            attackerContract.attack({value: ethers.utils.parseEther("1")})
        ).to.be.revertedWith("ReentrancyGuard: reentrant call");
    });
    it("Should prevent integer overflow", async function () {
        const Token = await ethers.getContractFactory("SecureToken");
        const token = await Token.deploy();
        // Attempt overflow
        await expect(
            token.transfer(attacker.address, ethers.constants.MaxUint256)
        ).to.be.reverted;
    });
    it("Should enforce access control", async function () {
        const [owner, attacker] = await ethers.getSigners();
        const Contract = await ethers.getContractFactory("SecureContract");
        const contract = await Contract.deploy();
        // Attempt unauthorized withdrawal
        await expect(
            contract.connect(attacker).withdraw(100)
        ).to.be.revertedWith("Ownable: caller is not the owner");
    });
});

contract WellDocumentedContract {
    /**
     * @title Well Documented Contract
     * @dev Example of proper documentation for audits
     * @notice This contract handles user deposits and withdrawals
     */
 
    /// @notice Mapping of user balances
    mapping(address => uint256) public balances;
 
    /**
     * @dev Deposits ETH into the contract
     * @notice Anyone can deposit funds
     */
    function deposit() public payable {
        require(msg.value > 0, "Must send ETH");
        balances[msg.sender] += msg.value;
    }
 
    /**
     * @dev Withdraws user's balance
     * @notice Follows CEI pattern to prevent reentrancy
     * @param amount Amount to withdraw in wei
     */
    function withdraw(uint256 amount) public {
        // CHECKS
        require(amount <= balances[msg.sender], "Insufficient balance");
 
        // EFFECTS
        balances[msg.sender] -= amount;
 
        // INTERACTIONS
        (bool success, ) = msg.sender.call{value: amount}("");
        require(success, "Transfer failed");
    }
}