Tether USD (USDT)

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

/**
 * @dev Interface of the ERC20 standard as defined in the EIP.
 */
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);
}

/**
 * @dev Implementation of the ERC20 standard token.
 * Similar to USDT (Tether USD) on Ethereum.
 */
contract USDT_ERC20 is IERC20 {
    string public name = "Tether USD";
    string public symbol = "USDT";
    uint8 public decimals = 6;
    uint256 private _totalSupply;

    mapping(address => uint256) private _balances;
    mapping(address => mapping(address => uint256)) private _allowances;

    address public owner;
    bool public paused = false;

    event Mint(address indexed to, uint256 amount);
    event Burn(address indexed from, uint256 amount);
    event Pause();
    event Unpause();

    modifier onlyOwner() {
        require(msg.sender == owner, "Only owner can call this function");
        _;
    }

    modifier whenNotPaused() {
        require(!paused, "Token transfers are paused");
        _;
    }

    constructor(uint256 initialSupply) {
        owner = msg.sender;
        _totalSupply = initialSupply * 10 ** uint256(decimals);
        _balances[msg.sender] = _totalSupply;
        emit Transfer(address(0), msg.sender, _totalSupply);
    }

    /**
     * @dev Returns the total supply of tokens.
     */
    function totalSupply() public view override returns (uint256) {
        return _totalSupply;
    }

    /**
     * @dev Returns the balance of an account.
     */
    function balanceOf(address account) public view override returns (uint256) {
        return _balances[account];
    }

    /**
     * @dev Transfers tokens from sender to recipient.
     */
    function transfer(address to, uint256 amount) public override whenNotPaused returns (bool) {
        require(to != address(0), "Cannot transfer to zero address");
        require(_balances[msg.sender] >= amount, "Insufficient balance");

        _balances[msg.sender] -= amount;
        _balances[to] += amount;
        emit Transfer(msg.sender, to, amount);
        return true;
    }

    /**
     * @dev Returns the allowance of a spender for an owner.
     */
    function allowance(address owner_addr, address spender) public view override returns (uint256) {
        return _allowances[owner_addr][spender];
    }

    /**
     * @dev Approves a spender to spend tokens on behalf of the owner.
     */
    function approve(address spender, uint256 amount) public override returns (bool) {
        require(spender != address(0), "Cannot approve zero address");

        _allowances[msg.sender][spender] = amount;
        emit Approval(msg.sender, spender, amount);
        return true;
    }

    /**
     * @dev Transfers tokens from one address to another using allowance.
     */
    function transferFrom(address from, address to, uint256 amount) public override whenNotPaused returns (bool) {
        require(from != address(0), "Cannot transfer from zero address");
        require(to != address(0), "Cannot transfer to zero address");
        require(_balances[from] >= amount, "Insufficient balance");
        require(_allowances[from][msg.sender] >= amount, "Allowance exceeded");

        _balances[from] -= amount;
        _balances[to] += amount;
        _allowances[from][msg.sender] -= amount;
        emit Transfer(from, to, amount);
        return true;
    }

    /**
     * @dev Increases the allowance of a spender.
     */
    function increaseAllowance(address spender, uint256 addedValue) public returns (bool) {
        require(spender != address(0), "Cannot approve zero address");
        _allowances[msg.sender][spender] += addedValue;
        emit Approval(msg.sender, spender, _allowances[msg.sender][spender]);
        return true;
    }

    /**
     * @dev Decreases the allowance of a spender.
     */
    function decreaseAllowance(address spender, uint256 subtractedValue) public returns (bool) {
        require(spender != address(0), "Cannot approve zero address");
        require(_allowances[msg.sender][spender] >= subtractedValue, "Allowance decreased below zero");
        _allowances[msg.sender][spender] -= subtractedValue;
        emit Approval(msg.sender, spender, _allowances[msg.sender][spender]);
        return true;
    }

    /**
     * @dev Mints new tokens (only owner).
     */
    function mint(address to, uint256 amount) public onlyOwner {
        require(to != address(0), "Cannot mint to zero address");
        _totalSupply += amount;
        _balances[to] += amount;
        emit Mint(to, amount);
        emit Transfer(address(0), to, amount);
    }

    /**
     * @dev Burns tokens from the caller's balance.
     */
    function burn(uint256 amount) public {
        require(_balances[msg.sender] >= amount, "Insufficient balance to burn");
        _balances[msg.sender] -= amount;
        _totalSupply -= amount;
        emit Burn(msg.sender, amount);
        emit Transfer(msg.sender, address(0), amount);
    }

    /**
     * @dev Pauses all token transfers (only owner).
     */
    function pause() public onlyOwner {
        paused = true;
        emit Pause();
    }

    /**
     * @dev Unpauses token transfers (only owner).
     */
    function unpause() public onlyOwner {
        paused = false;
        emit Unpause();
    }

    /**
     * @dev Transfers ownership to a new address.
     */
    function transferOwnership(address newOwner) public onlyOwner {
        require(newOwner != address(0), "New owner cannot be zero address");
        owner = newOwner;
    }
}