// File: contracts/sale/PrivateSale.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.7.6;

import {IERC20} from "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import {Ownable} from "@openzeppelin/contracts/access/Ownable.sol";
import {SafeERC20} from "@openzeppelin/contracts/token/ERC20/SafeERC20.sol";
import {
    ReentrancyGuard
} from "@openzeppelin/contracts/utils/ReentrancyGuard.sol";
import "@openzeppelin/contracts/math/SafeMath.sol";
import "../interfaces/IWTON.sol";

contract PrivateSale is Ownable, ReentrancyGuard {
    using SafeERC20 for IERC20;
    using SafeMath for uint256;

    
    struct UserInfoAmount {
        uint256 inputamount;
        uint256 totaloutputamount;
        uint256 inputTime;
        uint256 monthlyReward;
        uint256 firstReward;
    }

    struct UserInfoClaim {
        uint256 claimTime;
        uint256 claimAmount;
        uint256 firstClaimAmount;
        uint256 firstClaimTime;
        bool first;
    }

    struct WhiteList {
        uint256 amount;
    }

    event addList(
        address account,
        uint256 amount
    );

    event delList(
        address account,
        uint256 amount
    );

    event Buyinfo(
        address user,
        uint256 inputAmount,
        uint256 totalOutPutamount,
        uint256 inputTime,
        uint256 monthlyReward,
        uint256 firstReward
    );

    event FirstClaiminfo(
        address user,
        uint256 claimAmount,
        uint256 claimTime
    );

    event Claiminfo(
        address user,
        uint256 claimAmount,
        uint256 claimTime
    );

    event Withdrawinfo(
        address user,
        uint256 withdrawAmount
    );
    
    address public getTokenOwner;       //받은 ton을 받을 주소
    uint256 public totalGetAmount;      //총 TON받은양
    uint256 public totalSaleAmount;     //총 판매토큰

    uint256 public saleStartTime;           //sale시작 시간
    uint256 public saleEndTime;             //sale끝 시간

    uint256 public firstClaimTime;           //초기 claim 시간

    uint256 public claimStartTime;  //6개월 뒤 claim시작 시간
    uint256 public claimEndTime;    //claim시작시간 + 1년

    uint256 public saleTokenPrice;  //판매토큰가격
    uint256 public getTokenPrice;   //받는토큰가격(TON)

    IERC20 public saleToken;        //판매할 token주소
    IERC20 public getToken;         //TON 주소

    address public wton;             //WTON 주소

    mapping (address => UserInfoAmount) public usersAmount;
    mapping (address => UserInfoClaim) public usersClaim;
    mapping (address => WhiteList) public usersWhite;


    /// @dev basic setting
    /// @param _wton wtonAddress
    constructor(address _wton) {
        wton = _wton;
    }

    /// @dev calculator the SaleAmount(input TON how many get the anotherToken)
    /// @param _amount input the TON amount
    function calculSaleToken(uint256 _amount)
        public
        view
        returns (uint256)
    {
        uint256 tokenSaleAmount = _amount.mul(getTokenPrice).div(saleTokenPrice);
        return tokenSaleAmount;
    }

    /// @dev calculator the getAmount(want to get _amount how many input the TON?)
    /// @param _amount input the anotherTokenAmount
    function calculGetToken(uint256 _amount)
        public
        view
        returns (uint256)
    {
        uint256 tokenGetAmount = _amount.mul(saleTokenPrice).div(getTokenPrice);
        return tokenGetAmount;
    }

    /// @dev address setting
    /// @param _saleToken saleTokenAddress (contract have token)
    /// @param _getToken getTokenAddress (TON)
    /// @param _ownerToken get TON transfer to wallet
    function addressSetting(
        address _saleToken,
        address _getToken,
        address _ownerToken
    ) external onlyOwner {
        changeTokenAddress(_saleToken,_getToken);
        changeGetAddress(_ownerToken);
    }

    function changeWTONAddress(address _wton) external onlyOwner {
        wton = _wton;
    }

    function changeTokenAddress(address _saleToken, address _getToken) public onlyOwner {
        saleToken = IERC20(_saleToken);
        getToken = IERC20(_getToken);
    }

    function changeGetAddress(address _address) public onlyOwner {
        getTokenOwner = _address;
    }

    function settingAll(
        uint256[4] calldata _time,
        uint256 _saleTokenPrice,
        uint256 _getTokenPrice
    ) external onlyOwner {
        settingPrivateTime(_time[0],_time[1],_time[2],_time[3]);
        setTokenPrice(_saleTokenPrice,_getTokenPrice);
    }

    function settingPrivateTime(
        uint256 _startTime,
        uint256 _endTime,
        uint256 _firstTime,
        uint256 _claimTime
    ) public onlyOwner {
        settingSaleTime(_startTime,_endTime);
        settingFirstClaimTime(_firstTime);
        settingClaimTime(_claimTime);
    }

    function settingSaleTime(uint256 _startTime,uint256 _endTime) public onlyOwner {
        saleStartTime = _startTime;
        saleEndTime = _endTime;
    }

    function settingFirstClaimTime(uint256 _claimTime) public onlyOwner {
        firstClaimTime = _claimTime;
    }

    function settingClaimTime(uint256 _time) public onlyOwner {
        claimStartTime = _time;
        claimEndTime = _time.add(360 days);
    }

    function setTokenPrice(uint256 _saleTokenPrice, uint256 _getTokenPrice)
        public
        onlyOwner
    {
        saleTokenPrice = _saleTokenPrice;
        getTokenPrice = _getTokenPrice;
    }

    function claimAmount(
        address _account
    ) external view returns (uint256) {
        UserInfoAmount memory user = usersAmount[_account];

        require(user.inputamount > 0, "user isn't buy");
        require(block.timestamp > claimStartTime, "need to time for claim");
        
        UserInfoClaim memory userclaim = usersClaim[msg.sender];

        uint difftime = block.timestamp.sub(claimStartTime);
        uint monthTime = 30 days;

        if (difftime < monthTime) {
            uint period = 1;
            uint256 reward = (user.monthlyReward.mul(period)).sub(userclaim.claimAmount);
            return reward;
        } else {
            uint period = (difftime.div(monthTime)).add(1);
            if (period >= 12) {
                uint256 reward = user.totaloutputamount.sub(userclaim.claimAmount).sub(userclaim.firstClaimAmount);
                return reward; 
            } else {
                uint256 reward = (user.monthlyReward.mul(period)).sub(userclaim.claimAmount);
                return reward;
            }
        }
    }
    
    function calculClaimAmount(
        uint256 _nowtime, 
        uint256 _preclaimamount,
        uint256 _monthlyReward,
        uint256 _usertotaloutput,
        uint256 _firstReward
    ) internal view returns (uint256) {
        uint difftime = _nowtime.sub(claimStartTime);
        uint monthTime = 30 days;

        if (difftime < monthTime) {
            uint period = 1;
            uint256 reward = (_monthlyReward.mul(period)).sub(_preclaimamount);
            return reward;
        } else {
            uint period = (difftime.div(monthTime)).add(1);
            if (period >= 12) {
                uint256 reward = _usertotaloutput.sub(_preclaimamount).sub(_firstReward);
                return reward; 
            } else {
                uint256 reward = (_monthlyReward.mul(period)).sub(_preclaimamount);
                return reward;
            }
        }
    }

    function _toRAY(uint256 v) internal pure returns (uint256) {
        return v * 10 ** 9;
    }
    
    function addWhiteList(address _account,uint256 _amount) external onlyOwner {
        WhiteList storage userwhite = usersWhite[_account];
        userwhite.amount = userwhite.amount.add(_amount);

        emit addList(_account, _amount);
    }

    function addWhiteListArray(address[] calldata _account, uint256[] calldata _amount) external onlyOwner {
        for(uint i = 0; i < _account.length; i++) {
            WhiteList storage userwhite = usersWhite[_account[i]];
            userwhite.amount = userwhite.amount.add(_amount[i]);

            emit addList(_account[i], _amount[i]);
        }
    }

    function delWhiteList(address _account, uint256 _amount) external onlyOwner {
        WhiteList storage userwhite = usersWhite[_account];
        userwhite.amount = userwhite.amount.sub(_amount);

        emit delList(_account, _amount);
    }

    function buy(
        uint256 _amount
    ) external {
        require(saleStartTime != 0 && saleEndTime != 0, "need to setting saleTime");
        require(block.timestamp >= saleStartTime && block.timestamp <= saleEndTime, "privaSale period end");
        WhiteList storage userwhite = usersWhite[msg.sender];
        require(userwhite.amount >= _amount, "need to add whiteList amount");
        _buy(_amount);
        userwhite.amount = userwhite.amount.sub(_amount);
    }

    function _buy(
        uint256 _amount
    )
        internal
    {
        UserInfoAmount storage user = usersAmount[msg.sender];

        uint256 tokenSaleAmount = calculSaleToken(_amount);
        uint256 Saledtoken = totalSaleAmount.add(tokenSaleAmount);
        uint256 tokenBalance = saleToken.balanceOf(address(this));

        require(
            tokenBalance >= Saledtoken,
            "don't have token amount"
        );

        uint256 tonAllowance = getToken.allowance(msg.sender, address(this));
        uint256 tonBalance = getToken.balanceOf(msg.sender);

        if(tonBalance < _amount) {
            uint256 needUserWton;
            uint256 needWton = _amount.sub(tonBalance);
            needUserWton = _toRAY(needWton);
            require(IWTON(wton).allowance(msg.sender, address(this)) >= needUserWton, "privateSale: wton amount exceeds allowance");
            require(IWTON(wton).balanceOf(msg.sender) >= needUserWton, "need more wton");
            IERC20(wton).safeTransferFrom(msg.sender,address(this),needUserWton);
            IWTON(wton).swapToTON(needUserWton);
            require(tonAllowance >= _amount.sub(needWton), "privateSale: ton amount exceeds allowance");
            if(_amount.sub(needWton) > 0) {
                getToken.safeTransferFrom(msg.sender, address(this), _amount.sub(needWton));   
            }
            getToken.safeTransfer(getTokenOwner, _amount);
        } else {
            require(tonAllowance >= _amount, "privateSale: ton amount exceeds allowance");

            getToken.safeTransferFrom(msg.sender, address(this), _amount);
            getToken.safeTransfer(getTokenOwner, _amount);
        }

        user.inputamount = user.inputamount.add(_amount);
        user.totaloutputamount = user.totaloutputamount.add(tokenSaleAmount);
        user.firstReward = user.totaloutputamount.mul(5).div(100);
        user.monthlyReward = (user.totaloutputamount.sub(user.firstReward)).div(12);
        user.inputTime = block.timestamp;

        totalGetAmount = totalGetAmount.add(_amount);
        totalSaleAmount = totalSaleAmount.add(tokenSaleAmount);

        emit Buyinfo(
            msg.sender, 
            user.inputamount, 
            user.totaloutputamount,
            user.inputTime,
            user.monthlyReward,
            user.firstReward
        );
    }

    function claim() external {
        require(firstClaimTime != 0 && saleEndTime != 0, "need to setting Time");
        require(block.timestamp > saleEndTime && block.timestamp > firstClaimTime, "need the fisrClaimtime");
        if(block.timestamp < claimStartTime) {
            firstClaim();
        } else if(claimStartTime < block.timestamp){
            _claim();
        }
    }


    function firstClaim() public {
        UserInfoAmount storage user = usersAmount[msg.sender];
        UserInfoClaim storage userclaim = usersClaim[msg.sender];

        require(user.inputamount > 0, "need to buy the token");
        require(userclaim.firstClaimAmount == 0, "already getFirstreward");

        userclaim.firstClaimAmount = userclaim.firstClaimAmount.add(user.firstReward);
        userclaim.firstClaimTime = block.timestamp;

        saleToken.safeTransfer(msg.sender, user.firstReward);

        emit FirstClaiminfo(msg.sender, userclaim.firstClaimAmount, userclaim.firstClaimTime);
    }

    function _claim() public {
        require(block.timestamp >= claimStartTime, "need the time for claim");

        UserInfoAmount storage user = usersAmount[msg.sender];
        UserInfoClaim storage userclaim = usersClaim[msg.sender];

        require(user.inputamount > 0, "need to buy the token");
        require(!(user.totaloutputamount == (userclaim.claimAmount.add(userclaim.firstClaimAmount))), "already getAllreward");

        if(userclaim.firstClaimAmount == 0) {
            firstClaim();
        }

        uint256 giveTokenAmount = calculClaimAmount(block.timestamp, userclaim.claimAmount, user.monthlyReward, user.totaloutputamount, userclaim.firstClaimAmount);
    
        require(user.totaloutputamount.sub(userclaim.claimAmount) >= giveTokenAmount, "user is already getAllreward");
        require(saleToken.balanceOf(address(this)) >= giveTokenAmount, "dont have saleToken in pool");

        userclaim.claimAmount = userclaim.claimAmount.add(giveTokenAmount);
        userclaim.claimTime = block.timestamp;

        saleToken.safeTransfer(msg.sender, giveTokenAmount);

        emit Claiminfo(msg.sender, userclaim.claimAmount, userclaim.claimTime);
    }


    function withdraw(uint256 _amount) external onlyOwner {
        require(
            saleToken.balanceOf(address(this)) >= _amount,
            "dont have token amount"
        );
        saleToken.safeTransfer(msg.sender, _amount);

        emit Withdrawinfo(msg.sender, _amount);
    }

}

// File: @openzeppelin/contracts/token/ERC20/IERC20.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

/**
 * @dev Interface of the ERC20 standard as defined in the EIP.
 */
interface IERC20 {
    /**
     * @dev Returns the amount of tokens in existence.
     */
    function totalSupply() external view returns (uint256);

    /**
     * @dev Returns the amount of tokens owned by `account`.
     */
    function balanceOf(address account) external view returns (uint256);

    /**
     * @dev Moves `amount` tokens from the caller's account to `recipient`.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * Emits a {Transfer} event.
     */
    function transfer(address recipient, uint256 amount) external returns (bool);

    /**
     * @dev Returns the remaining number of tokens that `spender` will be
     * allowed to spend on behalf of `owner` through {transferFrom}. This is
     * zero by default.
     *
     * This value changes when {approve} or {transferFrom} are called.
     */
    function allowance(address owner, address spender) external view returns (uint256);

    /**
     * @dev Sets `amount` as the allowance of `spender` over the caller's tokens.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * IMPORTANT: Beware that changing an allowance with this method brings the risk
     * that someone may use both the old and the new allowance by unfortunate
     * transaction ordering. One possible solution to mitigate this race
     * condition is to first reduce the spender's allowance to 0 and set the
     * desired value afterwards:
     * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
     *
     * Emits an {Approval} event.
     */
    function approve(address spender, uint256 amount) external returns (bool);

    /**
     * @dev Moves `amount` tokens from `sender` to `recipient` using the
     * allowance mechanism. `amount` is then deducted from the caller's
     * allowance.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * Emits a {Transfer} event.
     */
    function transferFrom(address sender, address recipient, uint256 amount) external returns (bool);

    /**
     * @dev Emitted when `value` tokens are moved from one account (`from`) to
     * another (`to`).
     *
     * Note that `value` may be zero.
     */
    event Transfer(address indexed from, address indexed to, uint256 value);

    /**
     * @dev Emitted when the allowance of a `spender` for an `owner` is set by
     * a call to {approve}. `value` is the new allowance.
     */
    event Approval(address indexed owner, address indexed spender, uint256 value);
}

// File: @openzeppelin/contracts/access/Ownable.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

import "../GSN/Context.sol";
/**
 * @dev Contract module which provides a basic access control mechanism, where
 * there is an account (an owner) that can be granted exclusive access to
 * specific functions.
 *
 * By default, the owner account will be the one that deploys the contract. This
 * can later be changed with {transferOwnership}.
 *
 * This module is used through inheritance. It will make available the modifier
 * `onlyOwner`, which can be applied to your functions to restrict their use to
 * the owner.
 */
abstract contract Ownable is Context {
    address private _owner;

    event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);

    /**
     * @dev Initializes the contract setting the deployer as the initial owner.
     */
    constructor () internal {
        address msgSender = _msgSender();
        _owner = msgSender;
        emit OwnershipTransferred(address(0), msgSender);
    }

    /**
     * @dev Returns the address of the current owner.
     */
    function owner() public view returns (address) {
        return _owner;
    }

    /**
     * @dev Throws if called by any account other than the owner.
     */
    modifier onlyOwner() {
        require(_owner == _msgSender(), "Ownable: caller is not the owner");
        _;
    }

    /**
     * @dev Leaves the contract without owner. It will not be possible to call
     * `onlyOwner` functions anymore. Can only be called by the current owner.
     *
     * NOTE: Renouncing ownership will leave the contract without an owner,
     * thereby removing any functionality that is only available to the owner.
     */
    function renounceOwnership() public virtual onlyOwner {
        emit OwnershipTransferred(_owner, address(0));
        _owner = address(0);
    }

    /**
     * @dev Transfers ownership of the contract to a new account (`newOwner`).
     * Can only be called by the current owner.
     */
    function transferOwnership(address newOwner) public virtual onlyOwner {
        require(newOwner != address(0), "Ownable: new owner is the zero address");
        emit OwnershipTransferred(_owner, newOwner);
        _owner = newOwner;
    }
}

// File: @openzeppelin/contracts/token/ERC20/SafeERC20.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

import "./IERC20.sol";
import "../../math/SafeMath.sol";
import "../../utils/Address.sol";

/**
 * @title SafeERC20
 * @dev Wrappers around ERC20 operations that throw on failure (when the token
 * contract returns false). Tokens that return no value (and instead revert or
 * throw on failure) are also supported, non-reverting calls are assumed to be
 * successful.
 * To use this library you can add a `using SafeERC20 for IERC20;` statement to your contract,
 * which allows you to call the safe operations as `token.safeTransfer(...)`, etc.
 */
library SafeERC20 {
    using SafeMath for uint256;
    using Address for address;

    function safeTransfer(IERC20 token, address to, uint256 value) internal {
        _callOptionalReturn(token, abi.encodeWithSelector(token.transfer.selector, to, value));
    }

    function safeTransferFrom(IERC20 token, address from, address to, uint256 value) internal {
        _callOptionalReturn(token, abi.encodeWithSelector(token.transferFrom.selector, from, to, value));
    }

    /**
     * @dev Deprecated. This function has issues similar to the ones found in
     * {IERC20-approve}, and its usage is discouraged.
     *
     * Whenever possible, use {safeIncreaseAllowance} and
     * {safeDecreaseAllowance} instead.
     */
    function safeApprove(IERC20 token, address spender, uint256 value) internal {
        // safeApprove should only be called when setting an initial allowance,
        // or when resetting it to zero. To increase and decrease it, use
        // 'safeIncreaseAllowance' and 'safeDecreaseAllowance'
        // solhint-disable-next-line max-line-length
        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 safeIncreaseAllowance(IERC20 token, address spender, uint256 value) internal {
        uint256 newAllowance = token.allowance(address(this), spender).add(value);
        _callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, newAllowance));
    }

    function safeDecreaseAllowance(IERC20 token, address spender, uint256 value) internal {
        uint256 newAllowance = token.allowance(address(this), spender).sub(value, "SafeERC20: decreased allowance below zero");
        _callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, newAllowance));
    }

    /**
     * @dev Imitates a Solidity high-level call (i.e. a regular function call to a contract), relaxing the requirement
     * on the return value: the return value is optional (but if data is returned, it must not be false).
     * @param token The token targeted by the call.
     * @param data The call data (encoded using abi.encode or one of its variants).
     */
    function _callOptionalReturn(IERC20 token, bytes memory data) private {
        // We need to perform a low level call here, to bypass Solidity's return data size checking mechanism, since
        // we're implementing it ourselves. We use {Address.functionCall} to perform this call, which verifies that
        // the target address contains contract code and also asserts for success in the low-level call.

        bytes memory returndata = address(token).functionCall(data, "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");
        }
    }
}

// File: @openzeppelin/contracts/utils/ReentrancyGuard.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

/**
 * @dev Contract module that helps prevent reentrant calls to a function.
 *
 * Inheriting from `ReentrancyGuard` will make the {nonReentrant} modifier
 * available, which can be applied to functions to make sure there are no nested
 * (reentrant) calls to them.
 *
 * Note that because there is a single `nonReentrant` guard, functions marked as
 * `nonReentrant` may not call one another. This can be worked around by making
 * those functions `private`, and then adding `external` `nonReentrant` entry
 * points to them.
 *
 * TIP: If you would like to learn more about reentrancy and alternative ways
 * to protect against it, check out our blog post
 * https://blog.openzeppelin.com/reentrancy-after-istanbul/[Reentrancy After Istanbul].
 */
abstract contract ReentrancyGuard {
    // Booleans are more expensive than uint256 or any type that takes up a full
    // word because each write operation emits an extra SLOAD to first read the
    // slot's contents, replace the bits taken up by the boolean, and then write
    // back. This is the compiler's defense against contract upgrades and
    // pointer aliasing, and it cannot be disabled.

    // The values being non-zero value makes deployment a bit more expensive,
    // but in exchange the refund on every call to nonReentrant will be lower in
    // amount. Since refunds are capped to a percentage of the total
    // transaction's gas, it is best to keep them low in cases like this one, to
    // increase the likelihood of the full refund coming into effect.
    uint256 private constant _NOT_ENTERED = 1;
    uint256 private constant _ENTERED = 2;

    uint256 private _status;

    constructor () internal {
        _status = _NOT_ENTERED;
    }

    /**
     * @dev Prevents a contract from calling itself, directly or indirectly.
     * Calling a `nonReentrant` function from another `nonReentrant`
     * function is not supported. It is possible to prevent this from happening
     * by making the `nonReentrant` function external, and make it call a
     * `private` function that does the actual work.
     */
    modifier nonReentrant() {
        // On the first call to nonReentrant, _notEntered will be true
        require(_status != _ENTERED, "ReentrancyGuard: reentrant call");

        // Any calls to nonReentrant after this point will fail
        _status = _ENTERED;

        _;

        // By storing the original value once again, a refund is triggered (see
        // https://eips.ethereum.org/EIPS/eip-2200)
        _status = _NOT_ENTERED;
    }
}

// File: @openzeppelin/contracts/math/SafeMath.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

/**
 * @dev Wrappers over Solidity's arithmetic operations with added overflow
 * checks.
 *
 * Arithmetic operations in Solidity wrap on overflow. This can easily result
 * in bugs, because programmers usually assume that an overflow raises an
 * error, which is the standard behavior in high level programming languages.
 * `SafeMath` restores this intuition by reverting the transaction when an
 * operation overflows.
 *
 * Using this library instead of the unchecked operations eliminates an entire
 * class of bugs, so it's recommended to use it always.
 */
library SafeMath {
    /**
     * @dev Returns the addition of two unsigned integers, reverting on
     * overflow.
     *
     * Counterpart to Solidity's `+` operator.
     *
     * Requirements:
     *
     * - Addition cannot overflow.
     */
    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        require(c >= a, "SafeMath: addition overflow");

        return c;
    }

    /**
     * @dev Returns the subtraction of two unsigned integers, reverting on
     * overflow (when the result is negative).
     *
     * Counterpart to Solidity's `-` operator.
     *
     * Requirements:
     *
     * - Subtraction cannot overflow.
     */
    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        return sub(a, b, "SafeMath: subtraction overflow");
    }

    /**
     * @dev Returns the subtraction of two unsigned integers, reverting with custom message on
     * overflow (when the result is negative).
     *
     * Counterpart to Solidity's `-` operator.
     *
     * Requirements:
     *
     * - Subtraction cannot overflow.
     */
    function sub(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b <= a, errorMessage);
        uint256 c = a - b;

        return c;
    }

    /**
     * @dev Returns the multiplication of two unsigned integers, reverting on
     * overflow.
     *
     * Counterpart to Solidity's `*` operator.
     *
     * Requirements:
     *
     * - Multiplication cannot overflow.
     */
    function mul(uint256 a, uint256 b) internal pure returns (uint256) {
        // Gas optimization: this is cheaper than requiring 'a' not being zero, but the
        // benefit is lost if 'b' is also tested.
        // See: https://github.com/OpenZeppelin/openzeppelin-contracts/pull/522
        if (a == 0) {
            return 0;
        }

        uint256 c = a * b;
        require(c / a == b, "SafeMath: multiplication overflow");

        return c;
    }

    /**
     * @dev Returns the integer division of two unsigned integers. Reverts on
     * division by zero. The result is rounded towards zero.
     *
     * Counterpart to Solidity's `/` operator. Note: this function uses a
     * `revert` opcode (which leaves remaining gas untouched) while Solidity
     * uses an invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        return div(a, b, "SafeMath: division by zero");
    }

    /**
     * @dev Returns the integer division of two unsigned integers. Reverts with custom message on
     * division by zero. The result is rounded towards zero.
     *
     * Counterpart to Solidity's `/` operator. Note: this function uses a
     * `revert` opcode (which leaves remaining gas untouched) while Solidity
     * uses an invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function div(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b > 0, errorMessage);
        uint256 c = a / b;
        // assert(a == b * c + a % b); // There is no case in which this doesn't hold

        return c;
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * Reverts when dividing by zero.
     *
     * Counterpart to Solidity's `%` operator. This function uses a `revert`
     * opcode (which leaves remaining gas untouched) while Solidity uses an
     * invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function mod(uint256 a, uint256 b) internal pure returns (uint256) {
        return mod(a, b, "SafeMath: modulo by zero");
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * Reverts with custom message when dividing by zero.
     *
     * Counterpart to Solidity's `%` operator. This function uses a `revert`
     * opcode (which leaves remaining gas untouched) while Solidity uses an
     * invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function mod(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b != 0, errorMessage);
        return a % b;
    }
}

// File: contracts/interfaces/IWTON.sol



// SPDX-License-Identifier: MIT
pragma solidity ^0.7.6;

interface IWTON {
    function balanceOf(address account) external view returns (uint256);

    function transfer(address recipient, uint256 amount) external returns (bool);

    function transferFrom(address sender, address recipient, uint256 amount) external returns (bool);

    function onApprove(
        address owner,
        address spender,
        uint256 tonAmount,
        bytes calldata data
    ) external returns (bool);

    function burnFrom(address account, uint256 amount) external;

    function swapToTON(uint256 wtonAmount) external returns (bool);

    function swapFromTON(uint256 tonAmount) external returns (bool);

    function swapToTONAndTransfer(address to, uint256 wtonAmount)
        external
        returns (bool);

    function swapFromTONAndTransfer(address to, uint256 tonAmount)
        external
        returns (bool);

    function renounceTonMinter() external;

    function approve(address spender, uint256 amount) external returns (bool);

    function allowance(address sender, address recipient) external returns (uint256);
}

// File: @openzeppelin/contracts/GSN/Context.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

/*
 * @dev Provides information about the current execution context, including the
 * sender of the transaction and its data. While these are generally available
 * via msg.sender and msg.data, they should not be accessed in such a direct
 * manner, since when dealing with GSN meta-transactions the account sending and
 * paying for execution may not be the actual sender (as far as an application
 * is concerned).
 *
 * This contract is only required for intermediate, library-like contracts.
 */
abstract contract Context {
    function _msgSender() internal view virtual returns (address payable) {
        return msg.sender;
    }

    function _msgData() internal view virtual returns (bytes memory) {
        this; // silence state mutability warning without generating bytecode - see https://github.com/ethereum/solidity/issues/2691
        return msg.data;
    }
}

// File: @openzeppelin/contracts/utils/Address.sol



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.2 <0.8.0;

/**
 * @dev Collection of functions related to the address type
 */
library Address {
    /**
     * @dev Returns true if `account` is a contract.
     *
     * [IMPORTANT]
     * ====
     * It is unsafe to assume that an address for which this function returns
     * false is an externally-owned account (EOA) and not a contract.
     *
     * Among others, `isContract` will return false for the following
     * types of addresses:
     *
     *  - an externally-owned account
     *  - a contract in construction
     *  - an address where a contract will be created
     *  - an address where a contract lived, but was destroyed
     * ====
     */
    function isContract(address account) internal view returns (bool) {
        // This method relies on extcodesize, which returns 0 for contracts in
        // construction, since the code is only stored at the end of the
        // constructor execution.

        uint256 size;
        // solhint-disable-next-line no-inline-assembly
        assembly { size := extcodesize(account) }
        return size > 0;
    }

    /**
     * @dev Replacement for Solidity's `transfer`: sends `amount` wei to
     * `recipient`, forwarding all available gas and reverting on errors.
     *
     * https://eips.ethereum.org/EIPS/eip-1884[EIP1884] increases the gas cost
     * of certain opcodes, possibly making contracts go over the 2300 gas limit
     * imposed by `transfer`, making them unable to receive funds via
     * `transfer`. {sendValue} removes this limitation.
     *
     * https://diligence.consensys.net/posts/2019/09/stop-using-soliditys-transfer-now/[Learn more].
     *
     * IMPORTANT: because control is transferred to `recipient`, care must be
     * taken to not create reentrancy vulnerabilities. Consider using
     * {ReentrancyGuard} or the
     * https://solidity.readthedocs.io/en/v0.5.11/security-considerations.html#use-the-checks-effects-interactions-pattern[checks-effects-interactions pattern].
     */
    function sendValue(address payable recipient, uint256 amount) internal {
        require(address(this).balance >= amount, "Address: insufficient balance");

        // solhint-disable-next-line avoid-low-level-calls, avoid-call-value
        (bool success, ) = recipient.call{ value: amount }("");
        require(success, "Address: unable to send value, recipient may have reverted");
    }

    /**
     * @dev Performs a Solidity function call using a low level `call`. A
     * plain`call` is an unsafe replacement for a function call: use this
     * function instead.
     *
     * If `target` reverts with a revert reason, it is bubbled up by this
     * function (like regular Solidity function calls).
     *
     * Returns the raw returned data. To convert to the expected return value,
     * use https://solidity.readthedocs.io/en/latest/units-and-global-variables.html?highlight=abi.decode#abi-encoding-and-decoding-functions[`abi.decode`].
     *
     * Requirements:
     *
     * - `target` must be a contract.
     * - calling `target` with `data` must not revert.
     *
     * _Available since v3.1._
     */
    function functionCall(address target, bytes memory data) internal returns (bytes memory) {
      return functionCall(target, data, "Address: low-level call failed");
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`], but with
     * `errorMessage` as a fallback revert reason when `target` reverts.
     *
     * _Available since v3.1._
     */
    function functionCall(address target, bytes memory data, string memory errorMessage) internal returns (bytes memory) {
        return functionCallWithValue(target, data, 0, errorMessage);
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
     * but also transferring `value` wei to `target`.
     *
     * Requirements:
     *
     * - the calling contract must have an ETH balance of at least `value`.
     * - the called Solidity function must be `payable`.
     *
     * _Available since v3.1._
     */
    function functionCallWithValue(address target, bytes memory data, uint256 value) internal returns (bytes memory) {
        return functionCallWithValue(target, data, value, "Address: low-level call with value failed");
    }

    /**
     * @dev Same as {xref-Address-functionCallWithValue-address-bytes-uint256-}[`functionCallWithValue`], but
     * with `errorMessage` as a fallback revert reason when `target` reverts.
     *
     * _Available since v3.1._
     */
    function functionCallWithValue(address target, bytes memory data, uint256 value, string memory errorMessage) internal returns (bytes memory) {
        require(address(this).balance >= value, "Address: insufficient balance for call");
        require(isContract(target), "Address: call to non-contract");

        // solhint-disable-next-line avoid-low-level-calls
        (bool success, bytes memory returndata) = target.call{ value: value }(data);
        return _verifyCallResult(success, returndata, errorMessage);
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
     * but performing a static call.
     *
     * _Available since v3.3._
     */
    function functionStaticCall(address target, bytes memory data) internal view returns (bytes memory) {
        return functionStaticCall(target, data, "Address: low-level static call failed");
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-string-}[`functionCall`],
     * but performing a static call.
     *
     * _Available since v3.3._
     */
    function functionStaticCall(address target, bytes memory data, string memory errorMessage) internal view returns (bytes memory) {
        require(isContract(target), "Address: static call to non-contract");

        // solhint-disable-next-line avoid-low-level-calls
        (bool success, bytes memory returndata) = target.staticcall(data);
        return _verifyCallResult(success, returndata, errorMessage);
    }

    function _verifyCallResult(bool success, bytes memory returndata, string memory errorMessage) private pure returns(bytes memory) {
        if (success) {
            return returndata;
        } else {
            // Look for revert reason and bubble it up if present
            if (returndata.length > 0) {
                // The easiest way to bubble the revert reason is using memory via assembly

                // solhint-disable-next-line no-inline-assembly
                assembly {
                    let returndata_size := mload(returndata)
                    revert(add(32, returndata), returndata_size)
                }
            } else {
                revert(errorMessage);
            }
        }
    }
}