Gold 24K Reserve (GOLDX)

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

/// @title Gold 24K Reserve (GOLDX) - ERC20 compliant token representing physical gold
/// @notice 1 GOLDX = 1 onza troy (31.1035 g) de oro 24K (999.9), respaldado por bóvedas auditadas o activos institucionales como PAXG.
///         Standard ERC20 with admin mint/burn, address freeze, reentrancy guard, and 2-step ownership transfer
contract Gold24KReserve {
    string public constant name = "Gold 24K Reserve";
    string public constant symbol = "GOLDX";
    uint8 public constant decimals = 18;

    uint256 public totalSupply;
    uint256 public constant INITIAL_SUPPLY = 1_000_000_000 * 1e18;

    address public owner;
    address public pendingOwner;
    address public compliance;

    mapping(address => uint256) public balanceOf;
    mapping(address => mapping(address => uint256)) public allowance;
    mapping(address => bool) public frozen;

    // ReentrancyGuard
    uint256 private _entered;
    modifier nonReentrant() {
        require(_entered == 0, "REENTRANCY");
        _entered = 1;
        _;
        _entered = 0;
    }

    event Transfer(address indexed from, address indexed to, uint256 value);
    event Approval(address indexed ownerAddr, address indexed spender, uint256 value);
    event OwnershipTransferStarted(address indexed previousOwner, address indexed newOwner);
    event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);
    event ComplianceTransferred(address indexed previousCompliance, address indexed newCompliance);
    event AddressFrozen(address indexed account, bool frozen);
    event Mint(address indexed to, uint256 amount);
    event Burn(address indexed from, uint256 amount);

    error NotOwner();
    error NotCompliance();
    error FrozenAddress();
    error InsufficientBalance();
    error AllowanceExceeded();
    error ZeroAddress();

    constructor() {
        owner = msg.sender;
        compliance = msg.sender;
        totalSupply = INITIAL_SUPPLY;
        balanceOf[msg.sender] = INITIAL_SUPPLY;
        emit Transfer(address(0), msg.sender, INITIAL_SUPPLY);
    }

    modifier onlyOwner() {
        if (msg.sender != owner) revert NotOwner();
        _;
    }

    modifier onlyCompliance() {
        if (msg.sender != compliance) revert NotCompliance();
        _;
    }

    modifier notFrozen(address account) {
        if (frozen[account]) revert FrozenAddress();
        _;
    }

    function transfer(address to, uint256 value)
        external
        notFrozen(msg.sender)
        notFrozen(to)
        returns (bool)
    {
        require(to != address(0), "ZeroAddress");
        uint256 senderBalance = balanceOf[msg.sender];
        if (senderBalance < value) revert InsufficientBalance();
        unchecked {
            balanceOf[msg.sender] = senderBalance - value;
            balanceOf[to] += value;
        }
        emit Transfer(msg.sender, to, value);
        return true;
    }

    function approve(address spender, uint256 value)
        external
        notFrozen(msg.sender)
        notFrozen(spender)
        returns (bool)
    {
        require(spender != address(0), "ZeroAddress");
        allowance[msg.sender][spender] = value;
        emit Approval(msg.sender, spender, value);
        return true;
    }

    function transferFrom(address from, address to, uint256 value)
        external
        notFrozen(msg.sender)
        notFrozen(from)
        notFrozen(to)
        nonReentrant
        returns (bool)
    {
        require(to != address(0), "ZeroAddress");

        uint256 fromBalance = balanceOf[from];
        if (fromBalance < value) revert InsufficientBalance();

        uint256 allowed = allowance[from][msg.sender];
        if (allowed < value) revert AllowanceExceeded();

        if (allowed != type(uint256).max) {
            unchecked {
                allowance[from][msg.sender] = allowed - value;
            }
            emit Approval(from, msg.sender, allowance[from][msg.sender]);
        }

        unchecked {
            balanceOf[from] = fromBalance - value;
            balanceOf[to] += value;
        }
        emit Transfer(from, to, value);
        return true;
    }

    function increaseAllowance(address spender, uint256 addedValue)
        external
        notFrozen(msg.sender)
        notFrozen(spender)
        returns (bool)
    {
        require(spender != address(0), "ZeroAddress");
        uint256 newAllowance = allowance[msg.sender][spender] + addedValue;
        allowance[msg.sender][spender] = newAllowance;
        emit Approval(msg.sender, spender, newAllowance);
        return true;
    }

    function decreaseAllowance(address spender, uint256 subtractedValue)
        external
        notFrozen(msg.sender)
        notFrozen(spender)
        returns (bool)
    {
        require(spender != address(0), "ZeroAddress");
        uint256 current = allowance[msg.sender][spender];
        uint256 newAllowance = (subtractedValue >= current) ? 0 : current - subtractedValue;
        allowance[msg.sender][spender] = newAllowance;
        emit Approval(msg.sender, spender, newAllowance);
        return true;
    }

    function mint(address to, uint256 amount)
        external
        onlyOwner
        nonReentrant
    {
        require(to != address(0), "ZeroAddress");
        totalSupply += amount;
        balanceOf[to] += amount;
        emit Mint(to, amount);
        emit Transfer(address(0), to, amount);
    }

    function burn(address from, uint256 amount)
        external
        onlyOwner
        nonReentrant
    {
        require(from != address(0), "ZeroAddress");
        uint256 bal = balanceOf[from];
        if (bal < amount) revert InsufficientBalance();
        unchecked {
            balanceOf[from] = bal - amount;
            totalSupply -= amount;
        }
        emit Burn(from, amount);
        emit Transfer(from, address(0), amount);
    }

    function burnFrom(address from, uint256 amount)
        external
        notFrozen(msg.sender)
        notFrozen(from)
        nonReentrant
    {
        require(from != address(0), "ZeroAddress");
        uint256 allowed = allowance[from][msg.sender];
        if (allowed < amount) revert AllowanceExceeded();

        if (allowed != type(uint256).max) {
            unchecked {
                allowance[from][msg.sender] = allowed - amount;
            }
            emit Approval(from, msg.sender, allowance[from][msg.sender]);
        }

        uint256 bal = balanceOf[from];
        if (bal < amount) revert InsufficientBalance();
        unchecked {
            balanceOf[from] = bal - amount;
            totalSupply -= amount;
        }
        emit Burn(from, amount);
        emit Transfer(from, address(0), amount);
    }

    function setCompliance(address newCompliance) external onlyOwner {
        require(newCompliance != address(0), "ZeroAddress");
        emit ComplianceTransferred(compliance, newCompliance);
        compliance = newCompliance;
    }

    // Ownership transfer in 2 steps
    function transferOwnership(address newOwner) external onlyOwner {
        require(newOwner != address(0), "ZeroAddress");
        pendingOwner = newOwner;
        emit OwnershipTransferStarted(owner, newOwner);
    }

    function acceptOwnership() external {
        require(msg.sender == pendingOwner, "Not pending owner");
        emit OwnershipTransferred(owner, pendingOwner);
        owner = pendingOwner;
        pendingOwner = address(0);
    }

    function setFrozen(address account, bool isFrozen) external onlyCompliance {
        frozen[account] = isFrozen;
        emit AddressFrozen(account, isFrozen);
    }
}