pragma solidity 0.5.12;

library SafeMath {
    function add(uint a, uint b) internal pure returns (uint) {
        uint c = a + b;
        require(c >= a, "SafeMath: addition overflow");

        return c;
    }
    function sub(uint a, uint b) internal pure returns (uint) {
        return sub(a, b, "SafeMath: subtraction overflow");
    }
    function sub(uint a, uint b, string memory errorMessage) internal pure returns (uint) {
        require(b <= a, errorMessage);
        uint c = a - b;

        return c;
    }
    function mul(uint a, uint b) internal pure returns (uint) {
        if (a == 0) {
            return 0;
        }

        uint c = a * b;
        require(c / a == b, "SafeMath: multiplication overflow");

        return c;
    }
    function div(uint a, uint b) internal pure returns (uint) {
        return div(a, b, "SafeMath: division by zero");
    }
    function div(uint a, uint b, string memory errorMessage) internal pure returns (uint) {
        // Solidity only automatically asserts when dividing by 0
        require(b > 0, errorMessage);
        uint c = a / b;

        return c;
    }
}

library SafeERC20 {
    using SafeMath for uint;

    function isContract(address account) internal view returns (bool) {
        bytes32 codehash;
        bytes32 accountHash = 0xc5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470;
        // solhint-disable-next-line no-inline-assembly
        assembly { codehash := extcodehash(account) }
        return (codehash != 0x0 && codehash != accountHash);
    }

    function safeTransfer(IERC20 token, address to, uint value) internal {
        callOptionalReturn(token, abi.encodeWithSelector(token.transfer.selector, to, value));
    }

    function safeTransferFrom(IERC20 token, address from, address to, uint value) internal {
        callOptionalReturn(token, abi.encodeWithSelector(token.transferFrom.selector, from, to, value));
    }

    function safeApprove(IERC20 token, address spender, uint value) internal {
        require((value == 0) || (token.allowance(address(this), spender) == 0),
            "SafeERC20: approve from non-zero to non-zero allowance"
        );
        callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, value));
    }
    function callOptionalReturn(IERC20 token, bytes memory data) private {
        require(isContract(address(token)), "SafeERC20: call to non-contract");

        // solhint-disable-next-line avoid-low-level-calls
        (bool success, bytes memory returndata) = address(token).call(data);
        require(success, "SafeERC20: low-level call failed");

        if (returndata.length > 0) { // Return data is optional
            // solhint-disable-next-line max-line-length
            require(abi.decode(returndata, (bool)), "SafeERC20: ERC20 operation did not succeed");
        }
    }
}

/**
 * @title Roles
 * @dev Library for managing addresses assigned to a Role.
 */
library Roles {
    struct Role {
        mapping (address => bool) bearer;
    }

    function add(Role storage role, address account) internal {
        require(!has(role, account), "Roles: account already has role");
        role.bearer[account] = true;
    }

    function remove(Role storage role, address account) internal {
        require(has(role, account), "Roles: account does not have role");
        role.bearer[account] = false;
    }

    function has(Role storage role, address account) internal view returns (bool) {
        require(account != address(0), "Roles: account is the zero address");
        return role.bearer[account];
    }
}

/**
 * @title Ownable
 * @dev The Ownable contract has an owner address, and provides basic authorization control
 * functions, this simplifies the implementation of "user permissions".
 */
contract Ownable {
    address internal _owner;

    event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);

    constructor(address initialOwner) internal {
        require(initialOwner != address(0), "Ownable: initial owner is the zero address");
        _owner = initialOwner;
        emit OwnershipTransferred(address(0), _owner);
    }

    function owner() public view returns (address) {
        return _owner;
    }

    modifier onlyOwner() {
        require(_isOwner(msg.sender), "Ownable: caller is not the owner");
        _;
    }

    function _isOwner(address account) internal view returns (bool) {
        return account == _owner;
    }

    function transferOwnership(address newOwner) public onlyOwner {
        require(newOwner != address(0), "Ownable: new owner is the zero address");
        emit OwnershipTransferred(_owner, newOwner);
        _owner = newOwner;
    }
}

/**
 * @title MinterRole
 */
contract MinterRole is Ownable {
    using Roles for Roles.Role;

    event MinterAdded(address indexed account);
    event MinterRemoved(address indexed account);

    Roles.Role private _minters;

    modifier onlyMinter() {
        require(isMinter(msg.sender), "Caller has no permission");
        _;
    }

    function isMinter(address account) public view returns (bool) {
        return(_minters.has(account) || _isOwner(account));
    }

    function addMinter(address account) public onlyOwner {
        _minters.add(account);
        emit MinterAdded(account);
    }

    function removeMinter(address account) public onlyOwner {
        _minters.remove(account);
        emit MinterRemoved(account);
    }
}

interface IERC20 {
    function totalSupply() external view returns (uint);
    function balanceOf(address account) external view returns (uint);
    function transfer(address recipient, uint amount) external returns (bool);
    function allowance(address owner, address spender) external view returns (uint);
    function approve(address spender, uint amount) external returns (bool);
    function transferFrom(address sender, address recipient, uint amount) external returns (bool);
    event Transfer(address indexed from, address indexed to, uint value);
    event Approval(address indexed owner, address indexed spender, uint value);
}

contract Context {
    constructor () internal { }
    // solhint-disable-previous-line no-empty-blocks

    function _msgSender() internal view returns (address payable) {
        return msg.sender;
    }
}

contract ERC20 is Context, IERC20 {
    using SafeMath for uint;

    mapping (address => uint) private _balances;

    mapping (address => mapping (address => uint)) private _allowances;

    uint private _totalSupply;
    function totalSupply() public view returns (uint) {
        return _totalSupply;
    }
    function balanceOf(address account) public view returns (uint) {
        return _balances[account];
    }
    function transfer(address recipient, uint amount) public returns (bool) {
        _transfer(_msgSender(), recipient, amount);
        return true;
    }
    function allowance(address owner, address spender) public view returns (uint) {
        return _allowances[owner][spender];
    }
    function approve(address spender, uint amount) public returns (bool) {
        _approve(_msgSender(), spender, amount);
        return true;
    }
    function transferFrom(address sender, address recipient, uint amount) public returns (bool) {
        _transfer(sender, recipient, amount);
        _approve(sender, _msgSender(), _allowances[sender][_msgSender()].sub(amount, "ERC20: transfer amount exceeds allowance"));
        return true;
    }
    function increaseAllowance(address spender, uint addedValue) public returns (bool) {
        _approve(_msgSender(), spender, _allowances[_msgSender()][spender].add(addedValue));
        return true;
    }
    function decreaseAllowance(address spender, uint subtractedValue) public returns (bool) {
        _approve(_msgSender(), spender, _allowances[_msgSender()][spender].sub(subtractedValue, "ERC20: decreased allowance below zero"));
        return true;
    }
    function _transfer(address sender, address recipient, uint amount) internal {
        require(sender != address(0), "ERC20: transfer from the zero address");
        require(recipient != address(0), "ERC20: transfer to the zero address");

        _balances[sender] = _balances[sender].sub(amount, "ERC20: transfer amount exceeds balance");
        _balances[recipient] = _balances[recipient].add(amount);
        emit Transfer(sender, recipient, amount);
    }
    function _mint(address account, uint amount) internal {
        require(account != address(0), "ERC20: mint to the zero address");

        _totalSupply = _totalSupply.add(amount);
        _balances[account] = _balances[account].add(amount);
        emit Transfer(address(0), account, amount);
    }
    function _burn(address account, uint amount) internal {
        require(account != address(0), "ERC20: burn from the zero address");

        _balances[account] = _balances[account].sub(amount, "ERC20: burn amount exceeds balance");
        _totalSupply = _totalSupply.sub(amount);
        emit Transfer(account, address(0), amount);
    }
    function _burnFrom(address account, uint256 amount) internal {
        _burn(account, amount);
        _approve(account, msg.sender, _allowances[account][msg.sender].sub(amount));
    }
    function _approve(address owner, address spender, uint amount) internal {
        require(owner != address(0), "ERC20: approve from the zero address");
        require(spender != address(0), "ERC20: approve to the zero address");

        _allowances[owner][spender] = amount;
        emit Approval(owner, spender, amount);
    }
}

contract ERC20Detailed is IERC20 {
    string private _name;
    string private _symbol;
    uint8 private _decimals;

    constructor (string memory name, string memory symbol, uint8 decimals) public {
        _name = name;
        _symbol = symbol;
        _decimals = decimals;
    }
    function name() public view returns (string memory) {
        return _name;
    }
    function symbol() public view returns (string memory) {
        return _symbol;
    }
    function decimals() public view returns (uint8) {
        return _decimals;
    }
}

/**
 * @dev Extension of `ERC20` that allows token holders to destroy both their own
 * tokens and those that they have an allowance for, in a way that can be
 * recognized off-chain (via event analysis).
 */
contract ERC20Burnable is ERC20 {
    /**
     * @dev Destroys `amount` tokens from msg.sender's balance.
     */
    function burn(uint256 amount) public {
        _burn(msg.sender, amount);
    }

    /**
     * @dev Destroys `amount` tokens from `account`.`amount` is then deducted
     * from the caller's allowance.
     */
    function burnFrom(address account, uint256 amount) public {
        _burnFrom(account, amount);
    }
}

/**
 * @dev Extension of {ERC20} that adds a set of accounts with the {MinterRole},
 * which have permission to mint (create) new tokens as they see fit.
 */
contract ERC20Mintable is ERC20Burnable, MinterRole {

    // if additional minting of tokens is impossible
    bool public mintingFinished;

    // prevent minting of tokens when it is finished.
    // prevent total supply to exceed the limit of emission.
    modifier canMint(uint256 amount) {
        require(amount > 0, "Minting zero amount");
        require(!mintingFinished, "Minting is finished");
        _;
    }

    /**
     * @dev Stop any additional minting of tokens forever.
     * Available only to the owner.
     */
    function finishMinting() external onlyOwner {
        mintingFinished = true;
    }

    /**
     * @dev Minting of new tokens.
     * @param to The address to mint to.
     * @param value The amount to be minted.
     */
    function mint(address to, uint256 value) public onlyMinter canMint(value) returns (bool) {
        _mint(to, value);
        return true;
    }

}

/**
 * @title ApproveAndCall Interface.
 * @dev ApproveAndCall system allows to communicate with smart-contracts.
 */
contract ApproveAndCallFallBack {
    function receiveApproval(address from, uint256 amount, address token, bytes calldata extraData) external;
}

/**
 * @title The main project contract.
 */
contract SXTToken is ERC20Mintable, ERC20Detailed {
    using SafeERC20 for IERC20;
    using SafeMath for uint256;

    // registered contracts (to prevent loss of token via transfer function)
    mapping (address => bool) private _contracts;

    constructor(address initialOwner, address recipient) public ERC20Detailed("Sixteen", "SXT", 4) Ownable(initialOwner) {
        // creating of inital supply
        uint256 INITIAL_SUPPLY = 9999999e4;
        _mint(recipient, INITIAL_SUPPLY);
    }

    /**
     * @dev modified transfer function that allows to safely send tokens to smart-contract.
     * @param to The address to transfer to.
     * @param value The amount to be transferred.
     */
    function transfer(address to, uint256 value) public returns (bool) {

        if (_contracts[to]) {
            approveAndCall(to, value, new bytes(0));
        } else {
            super.transfer(to, value);
        }

        return true;

    }

    /**
    * @dev Allows to send tokens (via Approve and TransferFrom) to other smart-contract.
    * @param spender Address of smart contracts to work with.
    * @param amount Amount of tokens to send.
    * @param extraData Any extra data.
    */
    function approveAndCall(address spender, uint256 amount, bytes memory extraData) public returns (bool) {
        require(approve(spender, amount));

        ApproveAndCallFallBack(spender).receiveApproval(msg.sender, amount, address(this), extraData);

        return true;
    }

    /**
     * @dev Allows to register other smart-contracts (to prevent loss of tokens via transfer function).
     * @param account Address of smart contracts to work with.
     */
    function registerContract(address account) external onlyOwner {
        require(_isContract(account), "Token: account is not a smart contract");
        _contracts[account] = true;
    }

    /**
     * @dev Allows to unregister registered smart-contracts.
     * @param account Address of smart contracts to work with.
     */
    function unregisterContract(address account) external onlyOwner {
        require(isRegistered(account), "Token: account is not registered yet");
        _contracts[account] = false;
    }

    /**
    * @dev Allows to any owner of the contract withdraw needed ERC20 token from this contract (for example promo or bounties).
    * @param ERC20Token Address of ERC20 token.
    * @param recipient Account to receive tokens.
    */
    function withdrawERC20(address ERC20Token, address recipient) external onlyOwner {

        uint256 amount = IERC20(ERC20Token).balanceOf(address(this));
        IERC20(ERC20Token).safeTransfer(recipient, amount);

    }

    /**
     * @return true if the address is registered as contract
     * @param account Address to be checked.
     */
    function isRegistered(address account) public view returns (bool) {
        return _contracts[account];
    }

    /**
     * @return true if `account` is a contract.
     * @param account Address to be checked.
     */
    function _isContract(address account) internal view returns (bool) {
        uint256 size;
        assembly { size := extcodesize(account) }
        return size > 0;
    }

}