// File: solidity-common/contracts/interface/IERC20.sol

pragma solidity >=0.5.0 <0.7.0;


/**
 * https://github.com/ethereum/EIPs/blob/master/EIPS/eip-20.md
 */
interface IERC20 {
    /**
    * 可选方法
    */
    function name() external view returns (string memory);
    function symbol() external view returns (string memory);
    function decimals() external view returns (uint8);

    /**
     * 必须方法
     */
    function totalSupply() external view returns (uint256);
    function balanceOf(address account) external view returns (uint256);
    function transfer(address recipient, 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 sender, address recipient, uint256 amount) external returns (bool);

    /**
     * 事件类型
     */
    event Transfer(address indexed from, address indexed to, uint256 value);
    event Approval(address indexed owner, address indexed spender, uint256 value);
}

// File: contracts/library/UQ112x112.sol

pragma solidity >=0.5.0 <0.7.0;


// a library for handling binary fixed point numbers (https://en.wikipedia.org/wiki/Q_(number_format))

// range: [0, 2**112 - 1]
// resolution: 1 / 2**112


library UQ112x112 {
    uint224 constant Q112 = 2 ** 112;

    // encode a uint112 as a UQ112x112
    function encode(uint112 y) internal pure returns (uint224 z) {
        z = uint224(y) * Q112;
        // never overflows
    }

    // divide a UQ112x112 by a uint112, returning a UQ112x112
    function uqdiv(uint224 x, uint112 y) internal pure returns (uint224 z) {
        z = x / uint224(y);
    }

}

// File: contracts/interface/IBtswapCallee.sol

pragma solidity >=0.5.0 <0.7.0;


interface IBtswapCallee {
    function bitswapCall(address sender, uint256 amount0, uint256 amount1, bytes calldata data) external;

}

// File: contracts/interface/IBtswapFactory.sol

pragma solidity >=0.5.0 <0.7.0;


interface IBtswapFactory {
    function FEE_RATE_DENOMINATOR() external view returns (uint256);

    function feeTo() external view returns (address);

    function feeToSetter() external view returns (address);

    function feeRateNumerator() external view returns (uint256);

    function initCodeHash() external view returns (bytes32);

    function getPair(address tokenA, address tokenB) external view returns (address pair);

    function allPairs(uint256) external view returns (address pair);

    function allPairsLength() external view returns (uint256);

    function createPair(address tokenA, address tokenB) external returns (address pair);

    function setRouter(address) external;

    function setFeeTo(address) external;

    function setFeeToSetter(address) external;

    function setFeeRateNumerator(uint256) external;

    function setInitCodeHash(bytes32) external;

    function sortTokens(address tokenA, address tokenB) external pure returns (address token0, address token1);

    function pairFor(address factory, address tokenA, address tokenB) external view returns (address pair);

    function getReserves(address factory, address tokenA, address tokenB) external view returns (uint256 reserveA, uint256 reserveB);

    function quote(uint256 amountA, uint256 reserveA, uint256 reserveB) external pure returns (uint256 amountB);

    function getAmountOut(uint256 amountIn, uint256 reserveIn, uint256 reserveOut) external view returns (uint256 amountOut);

    function getAmountIn(uint256 amountOut, uint256 reserveIn, uint256 reserveOut) external view returns (uint256 amountIn);

    function getAmountsOut(address factory, uint256 amountIn, address[] calldata path) external view returns (uint256[] memory amounts);

    function getAmountsIn(address factory, uint256 amountOut, address[] calldata path) external view returns (uint256[] memory amounts);


    event PairCreated(address indexed token0, address indexed token1, address pair, uint256);

}

// File: contracts/interface/IBtswapPairToken.sol

pragma solidity >=0.5.0 <0.7.0;


interface IBtswapPairToken {
    function name() external pure returns (string memory);

    function symbol() external pure returns (string memory);

    function decimals() external pure returns (uint8);

    function totalSupply() external view returns (uint256);

    function balanceOf(address owner) external view returns (uint256);

    function allowance(address owner, address spender) external view returns (uint256);

    function approve(address spender, uint256 value) external returns (bool);

    function transfer(address to, uint256 value) external returns (bool);

    function transferFrom(address from, address to, uint256 value) external returns (bool);

    function DOMAIN_SEPARATOR() external view returns (bytes32);

    function PERMIT_TYPEHASH() external pure returns (bytes32);

    function nonces(address owner) external view returns (uint256);

    function permit(address owner, address spender, uint256 value, uint256 deadline, uint8 v, bytes32 r, bytes32 s) external;

    function MINIMUM_LIQUIDITY() external pure returns (uint256);

    function router() external view returns (address);

    function factory() external view returns (address);

    function token0() external view returns (address);

    function token1() external view returns (address);

    function getReserves() external view returns (uint112 reserve0, uint112 reserve1, uint32 blockTimestampLast);

    function price0CumulativeLast() external view returns (uint256);

    function price1CumulativeLast() external view returns (uint256);

    function kLast() external view returns (uint256);

    function mint(address to) external returns (uint256 liquidity);

    function burn(address to) external returns (uint256 amount0, uint256 amount1);

    function swap(uint256 amount0Out, uint256 amount1Out, address to, bytes calldata data) external;

    function skim(address to) external;

    function sync() external;

    function initialize(address, address, address) external;

    function price(address token) external view returns (uint256);


    event Approval(address indexed owner, address indexed spender, uint256 value);
    event Transfer(address indexed from, address indexed to, uint256 value);
    event Mint(address indexed sender, uint256 amount0, uint256 amount1);
    event Burn(address indexed sender, uint256 amount0, uint256 amount1, address indexed to);
    event Swap(address indexed sender, uint256 amount0In, uint256 amount1In, uint256 amount0Out, uint256 amount1Out, address indexed to);
    event Sync(uint112 reserve0, uint112 reserve1);

}

// File: contracts/interface/IBtswapRouter02.sol

pragma solidity >=0.5.0 <0.7.0;


interface IBtswapRouter02 {
    function factory() external pure returns (address);

    function WETH() external pure returns (address);

    function BT() external pure returns (address);

    function addLiquidity(address tokenA, address tokenB, uint256 amountADesired, uint256 amountBDesired, uint256 amountAMin, uint256 amountBMin, address to, uint256 deadline) external returns (uint256 amountA, uint256 amountB, uint256 liquidity);

    function addLiquidityETH(address token, uint256 amountTokenDesired, uint256 amountTokenMin, uint256 amountETHMin, address to, uint256 deadline) external payable returns (uint256 amountToken, uint256 amountETH, uint256 liquidity);

    function removeLiquidity(address tokenA, address tokenB, uint256 liquidity, uint256 amountAMin, uint256 amountBMin, address to, uint256 deadline) external returns (uint256 amountA, uint256 amountB);

    function removeLiquidityETH(address token, uint256 liquidity, uint256 amountTokenMin, uint256 amountETHMin, address to, uint256 deadline) external returns (uint256 amountToken, uint256 amountETH);

    function removeLiquidityWithPermit(address tokenA, address tokenB, uint256 liquidity, uint256 amountAMin, uint256 amountBMin, address to, uint256 deadline, bool approveMax, uint8 v, bytes32 r, bytes32 s) external returns (uint256 amountA, uint256 amountB);

    function removeLiquidityETHWithPermit(address token, uint256 liquidity, uint256 amountTokenMin, uint256 amountETHMin, address to, uint256 deadline, bool approveMax, uint8 v, bytes32 r, bytes32 s) external returns (uint256 amountToken, uint256 amountETH);

    function swapExactTokensForTokens(uint256 amountIn, uint256 amountOutMin, address[] calldata path, address to, uint256 deadline) external returns (uint256[] memory amounts);

    function swapTokensForExactTokens(uint256 amountOut, uint256 amountInMax, address[] calldata path, address to, uint256 deadline) external returns (uint256[] memory amounts);

    function swapExactETHForTokens(uint256 amountOutMin, address[] calldata path, address to, uint256 deadline) external payable returns (uint256[] memory amounts);

    function swapTokensForExactETH(uint256 amountOut, uint256 amountInMax, address[] calldata path, address to, uint256 deadline) external returns (uint256[] memory amounts);

    function swapExactTokensForETH(uint256 amountIn, uint256 amountOutMin, address[] calldata path, address to, uint256 deadline) external returns (uint256[] memory amounts);

    function swapETHForExactTokens(uint256 amountOut, address[] calldata path, address to, uint256 deadline) external payable returns (uint256[] memory amounts);

    function quote(uint256 amountA, uint256 reserveA, uint256 reserveB) external view returns (uint256 amountB);

    function getAmountOut(uint256 amountIn, uint256 reserveIn, uint256 reserveOut) external view returns (uint256 amountOut);

    function getAmountIn(uint256 amountOut, uint256 reserveIn, uint256 reserveOut) external view returns (uint256 amountIn);

    function getAmountsOut(uint256 amountIn, address[] calldata path) external view returns (uint256[] memory amounts);

    function getAmountsIn(uint256 amountOut, address[] calldata path) external view returns (uint256[] memory amounts);

    function removeLiquidityETHSupportingFeeOnTransferTokens(address token, uint256 liquidity, uint256 amountTokenMin, uint256 amountETHMin, address to, uint256 deadline) external returns (uint256 amountETH);

    function removeLiquidityETHWithPermitSupportingFeeOnTransferTokens(address token, uint256 liquidity, uint256 amountTokenMin, uint256 amountETHMin, address to, uint256 deadline, bool approveMax, uint8 v, bytes32 r, bytes32 s) external returns (uint256 amountETH);

    function swapExactTokensForTokensSupportingFeeOnTransferTokens(uint256 amountIn, uint256 amountOutMin, address[] calldata path, address to, uint256 deadline) external;

    function swapExactETHForTokensSupportingFeeOnTransferTokens(uint256 amountOutMin, address[] calldata path, address to, uint256 deadline) external payable;

    function swapExactTokensForETHSupportingFeeOnTransferTokens(uint256 amountIn, uint256 amountOutMin, address[] calldata path, address to, uint256 deadline) external;

    function weth(address token) external view returns (uint256);

    function onTransfer(address sender, address recipient) external returns (bool);

}

// File: solidity-common/contracts/library/SafeMath.sol

pragma solidity >=0.5.0 <0.7.0;


/**
 * 算术操作
 */
library SafeMath {
    uint256 constant WAD = 10 ** 18;
    uint256 constant RAY = 10 ** 27;

    function wad() public pure returns (uint256) {
        return WAD;
    }

    function ray() public pure returns (uint256) {
        return RAY;
    }

    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        require(c >= a, "SafeMath: addition overflow");

        return c;
    }

    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        return sub(a, b, "SafeMath: subtraction overflow");
    }

    function sub(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b <= a, errorMessage);
        uint256 c = a - b;

        return c;
    }

    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;
    }

    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        return div(a, b, "SafeMath: division by zero");
    }

    function div(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        // Solidity only automatically asserts when dividing by 0
        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;
    }

    function mod(uint256 a, uint256 b) internal pure returns (uint256) {
        return mod(a, b, "SafeMath: modulo by zero");
    }

    function mod(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b != 0, errorMessage);
        return a % b;
    }

    function min(uint256 a, uint256 b) internal pure returns (uint256) {
        return a <= b ? a : b;
    }

    function max(uint256 a, uint256 b) internal pure returns (uint256) {
        return a >= b ? a : b;
    }

    function sqrt(uint256 a) internal pure returns (uint256 b) {
        if (a > 3) {
            b = a;
            uint256 x = a / 2 + 1;
            while (x < b) {
                b = x;
                x = (a / x + x) / 2;
            }
        } else if (a != 0) {
            b = 1;
        }
    }

    function wmul(uint256 a, uint256 b) internal pure returns (uint256) {
        return mul(a, b) / WAD;
    }

    function wmulRound(uint256 a, uint256 b) internal pure returns (uint256) {
        return add(mul(a, b), WAD / 2) / WAD;
    }

    function rmul(uint256 a, uint256 b) internal pure returns (uint256) {
        return mul(a, b) / RAY;
    }

    function rmulRound(uint256 a, uint256 b) internal pure returns (uint256) {
        return add(mul(a, b), RAY / 2) / RAY;
    }

    function wdiv(uint256 a, uint256 b) internal pure returns (uint256) {
        return div(mul(a, WAD), b);
    }

    function wdivRound(uint256 a, uint256 b) internal pure returns (uint256) {
        return add(mul(a, WAD), b / 2) / b;
    }

    function rdiv(uint256 a, uint256 b) internal pure returns (uint256) {
        return div(mul(a, RAY), b);
    }

    function rdivRound(uint256 a, uint256 b) internal pure returns (uint256) {
        return add(mul(a, RAY), b / 2) / b;
    }

    function wpow(uint256 x, uint256 n) internal pure returns (uint256) {
        uint256 result = WAD;
        while (n > 0) {
            if (n % 2 != 0) {
                result = wmul(result, x);
            }
            x = wmul(x, x);
            n /= 2;
        }
        return result;
    }

    function rpow(uint256 x, uint256 n) internal pure returns (uint256) {
        uint256 result = RAY;
        while (n > 0) {
            if (n % 2 != 0) {
                result = rmul(result, x);
            }
            x = rmul(x, x);
            n /= 2;
        }
        return result;
    }
}

// File: contracts/interface/IBtswapERC20.sol

pragma solidity >=0.5.0 <0.7.0;


interface IBtswapERC20 {
    function name() external pure returns (string memory);

    function symbol() external pure returns (string memory);

    function decimals() external pure returns (uint8);

    function totalSupply() external view returns (uint256);

    function balanceOf(address owner) external view returns (uint256);

    function allowance(address owner, address spender) external view returns (uint256);

    function approve(address spender, uint256 value) external returns (bool);

    function transfer(address to, uint256 value) external returns (bool);

    function transferFrom(address from, address to, uint256 value) external returns (bool);

    function DOMAIN_SEPARATOR() external view returns (bytes32);

    function PERMIT_TYPEHASH() external pure returns (bytes32);

    function nonces(address owner) external view returns (uint256);

    function permit(address owner, address spender, uint256 value, uint256 deadline, uint8 v, bytes32 r, bytes32 s) external;


    event Approval(address indexed owner, address indexed spender, uint256 value);
    event Transfer(address indexed from, address indexed to, uint256 value);

}

// File: contracts/biz/BtswapERC20.sol

pragma solidity >=0.5.0 <0.7.0;




contract BtswapERC20 is IBtswapERC20 {
    using SafeMath for uint256;

    string public constant name = "Btswap Pair Token";
    string public constant symbol = "BPT";
    uint8 public constant decimals = 18;

    uint256  public totalSupply;
    mapping(address => uint256) public balanceOf;
    mapping(address => mapping(address => uint256)) public allowance;
    mapping(address => uint256) public nonces;

    bytes32 public DOMAIN_SEPARATOR;
    bytes32 public constant PERMIT_TYPEHASH = 0x6e71edae12b1b97f4d1f60370fef10105fa2faae0126114a169c64845d6126c9;

    constructor() public {
        uint256 chainId;
        assembly {
            chainId := chainid
        }
        DOMAIN_SEPARATOR = keccak256(
            abi.encode(
                keccak256("EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)"),
                keccak256(bytes(name)),
                keccak256(bytes("1")),
                chainId,
                address(this)
            )
        );
    }

    function _mint(address to, uint256 value) internal {
        totalSupply = totalSupply.add(value);
        balanceOf[to] = balanceOf[to].add(value);
        emit Transfer(address(0), to, value);
    }

    function _burn(address from, uint256 value) internal {
        balanceOf[from] = balanceOf[from].sub(value);
        totalSupply = totalSupply.sub(value);
        emit Transfer(from, address(0), value);
    }

    function _approve(address owner, address spender, uint256 value) private {
        allowance[owner][spender] = value;
        emit Approval(owner, spender, value);
    }

    function _transfer(address from, address to, uint256 value) private {
        balanceOf[from] = balanceOf[from].sub(value);
        balanceOf[to] = balanceOf[to].add(value);
        emit Transfer(from, to, value);
    }

    function approve(address spender, uint256 value) external returns (bool) {
        _approve(msg.sender, spender, value);
        return true;
    }

    function transfer(address to, uint256 value) public returns (bool) {
        _transfer(msg.sender, to, value);
        return true;
    }

    function transferFrom(address from, address to, uint256 value) public returns (bool) {
        if (allowance[from][msg.sender] != uint256(- 1)) {
            allowance[from][msg.sender] = allowance[from][msg.sender].sub(value);
        }
        _transfer(from, to, value);
        return true;
    }

    function permit(address owner, address spender, uint256 value, uint256 deadline, uint8 v, bytes32 r, bytes32 s) external {
        require(deadline >= block.timestamp, "BtswapPairToken: EXPIRED");
        bytes32 digest = keccak256(
            abi.encodePacked(
                "\x19\x01",
                DOMAIN_SEPARATOR,
                keccak256(abi.encode(PERMIT_TYPEHASH, owner, spender, value, nonces[owner]++, deadline))
            )
        );
        address recoveredAddress = ecrecover(digest, v, r, s);
        require(recoveredAddress != address(0) && recoveredAddress == owner, "BtswapPairToken: INVALID_SIGNATURE");
        _approve(owner, spender, value);
    }


    event Approval(address indexed owner, address indexed spender, uint256 value);
    event Transfer(address indexed from, address indexed to, uint256 value);

}

// File: contracts/biz/BtswapPairToken.sol

pragma solidity >=0.5.0 <0.7.0;









contract BtswapPairToken is IBtswapPairToken, BtswapERC20 {
    using SafeMath  for uint256;
    using UQ112x112 for uint224;

    uint256 public constant MINIMUM_LIQUIDITY = 10 ** 3;
    bytes4 private constant SELECTOR = bytes4(keccak256(bytes("transfer(address,uint256)")));

    address public router;
    address public factory;
    address public token0;
    address public token1;

    uint256 private unlocked = 1;
    uint112 private reserve0;           // uses single storage slot, accessible via getReserves
    uint112 private reserve1;           // uses single storage slot, accessible via getReserves
    uint32  private blockTimestampLast; // uses single storage slot, accessible via getReserves

    uint256 public price0CumulativeLast;
    uint256 public price1CumulativeLast;
    uint256 public kLast; // reserve0 * reserve1, as of immediately after the most recent liquidity event

    constructor() public {
        factory = msg.sender;
    }

    // called once by the factory at time of deployment
    function initialize(address _router, address _token0, address _token1) external {
        // sufficient check
        require(msg.sender == factory, "BtswapPairToken: FORBIDDEN");
        router = _router;
        token0 = _token0;
        token1 = _token1;
    }

    // update reserves and, on the first call per block, price accumulators
    function _update(uint256 balance0, uint256 balance1, uint112 _reserve0, uint112 _reserve1) private {
        require(balance0 <= uint112(- 1) && balance1 <= uint112(- 1), "BtswapPairToken: OVERFLOW");
        uint32 blockTimestamp = uint32(block.timestamp % 2 ** 32);
        // overflow is desired
        uint32 timeElapsed = blockTimestamp - blockTimestampLast;
        if (timeElapsed > 0 && _reserve0 != 0 && _reserve1 != 0) {
            // * never overflows, and + overflow is desired
            price0CumulativeLast += uint256(UQ112x112.encode(_reserve1).uqdiv(_reserve0)) * timeElapsed;
            price1CumulativeLast += uint256(UQ112x112.encode(_reserve0).uqdiv(_reserve1)) * timeElapsed;
        }
        reserve0 = uint112(balance0);
        reserve1 = uint112(balance1);
        blockTimestampLast = blockTimestamp;
        emit Sync(reserve0, reserve1);
    }

    // if fee is on, mint liquidity equivalent to 1/5th of the growth in sqrt(k)
    function _mintFee(uint112 _reserve0, uint112 _reserve1) private returns (bool feeOn) {
        address feeTo = IBtswapFactory(factory).feeTo();
        feeOn = feeTo != address(0);
        // gas savings
        uint256 _kLast = kLast;
        if (feeOn) {
            if (_kLast != 0) {
                uint256 rootK = SafeMath.sqrt(uint256(_reserve0).mul(_reserve1));
                uint256 rootKLast = SafeMath.sqrt(_kLast);
                if (rootK > rootKLast) {
                    uint256 numerator = totalSupply.mul(rootK.sub(rootKLast));
                    uint256 denominator = rootK.mul(4).add(rootKLast);
                    uint256 liquidity = numerator / denominator;
                    if (liquidity > 0) _mint(feeTo, liquidity);
                }
            }
        } else if (_kLast != 0) {
            kLast = 0;
        }
    }

    // this low-level function should be called from a contract which performs important safety checks
    function mint(address to) external lock returns (uint256 liquidity) {
        require(msg.sender == router, "BtswapPairToken: FORBIDDEN");

        // gas savings
        (uint112 _reserve0, uint112 _reserve1,) = getReserves();
        uint256 balance0 = IERC20(token0).balanceOf(address(this));
        uint256 balance1 = IERC20(token1).balanceOf(address(this));
        uint256 amount0 = balance0.sub(_reserve0);
        uint256 amount1 = balance1.sub(_reserve1);

        bool feeOn = _mintFee(_reserve0, _reserve1);
        // gas savings, must be defined here since totalSupply can update in _mintFee
        uint256 _totalSupply = totalSupply;
        if (_totalSupply == 0) {
            liquidity = SafeMath.sqrt(amount0.mul(amount1)).sub(MINIMUM_LIQUIDITY);
            // permanently lock the first MINIMUM_LIQUIDITY tokens
            _mint(address(0), MINIMUM_LIQUIDITY);
        } else {
            liquidity = SafeMath.min(amount0.mul(_totalSupply) / _reserve0, amount1.mul(_totalSupply) / _reserve1);
        }
        require(liquidity > 0, "BtswapPairToken: INSUFFICIENT_LIQUIDITY_MINTED");
        _mint(to, liquidity);

        _update(balance0, balance1, _reserve0, _reserve1);
        // reserve0 and reserve1 are up-to-date
        if (feeOn) kLast = uint256(reserve0).mul(reserve1);
        emit Mint(msg.sender, amount0, amount1);
    }

    // this low-level function should be called from a contract which performs important safety checks
    function burn(address to) external lock returns (uint256 amount0, uint256 amount1) {
        require(msg.sender == router, "BtswapPairToken: FORBIDDEN");

        // gas savings
        (uint112 _reserve0, uint112 _reserve1,) = getReserves();
        // gas savings
        address _token0 = token0;
        // gas savings
        address _token1 = token1;
        uint256 balance0 = IERC20(_token0).balanceOf(address(this));
        uint256 balance1 = IERC20(_token1).balanceOf(address(this));
        uint256 liquidity = balanceOf[address(this)];

        bool feeOn = _mintFee(_reserve0, _reserve1);
        // gas savings, must be defined here since totalSupply can update in _mintFee
        uint256 _totalSupply = totalSupply;
        // using balances ensures pro-rata distribution
        amount0 = liquidity.mul(balance0) / _totalSupply;
        // using balances ensures pro-rata distribution
        amount1 = liquidity.mul(balance1) / _totalSupply;
        require(amount0 > 0 && amount1 > 0, "BtswapPairToken: INSUFFICIENT_LIQUIDITY_BURNED");
        _burn(address(this), liquidity);
        _safeTransfer(_token0, to, amount0);
        _safeTransfer(_token1, to, amount1);
        balance0 = IERC20(_token0).balanceOf(address(this));
        balance1 = IERC20(_token1).balanceOf(address(this));

        _update(balance0, balance1, _reserve0, _reserve1);
        // reserve0 and reserve1 are up-to-date
        if (feeOn) kLast = uint256(reserve0).mul(reserve1);
        emit Burn(msg.sender, amount0, amount1, to);
    }

    // this low-level function should be called from a contract which performs important safety checks
    function swap(uint256 amount0Out, uint256 amount1Out, address to, bytes calldata data) external lock {
        require(msg.sender == router, "BtswapPairToken: FORBIDDEN");
        require(amount0Out > 0 || amount1Out > 0, "BtswapPairToken: INSUFFICIENT_OUTPUT_AMOUNT");
        // gas savings
        (uint112 _reserve0, uint112 _reserve1,) = getReserves();
        require(amount0Out < _reserve0 && amount1Out < _reserve1, "BtswapPairToken: INSUFFICIENT_LIQUIDITY");

        uint256 balance0;
        uint256 balance1;
        {// scope for _token{0,1}, avoids stack too deep errors
            address _token0 = token0;
            address _token1 = token1;
            require(to != _token0 && to != _token1, "BtswapPairToken: INVALID_TO");
            // optimistically transfer tokens
            if (amount0Out > 0) _safeTransfer(_token0, to, amount0Out);
            // optimistically transfer tokens
            if (amount1Out > 0) _safeTransfer(_token1, to, amount1Out);
            if (data.length > 0) IBtswapCallee(to).bitswapCall(msg.sender, amount0Out, amount1Out, data);
            balance0 = IERC20(_token0).balanceOf(address(this));
            balance1 = IERC20(_token1).balanceOf(address(this));
        }
        uint256 amount0In = balance0 > _reserve0 - amount0Out ? balance0 - (_reserve0 - amount0Out) : 0;
        uint256 amount1In = balance1 > _reserve1 - amount1Out ? balance1 - (_reserve1 - amount1Out) : 0;
        require(amount0In > 0 || amount1In > 0, "BtswapPairToken: INSUFFICIENT_INPUT_AMOUNT");
        {// scope for reserve{0,1}Adjusted, avoids stack too deep errors
            uint256 balance0Adjusted = balance0.mul(1e4).sub(amount0In.mul(IBtswapFactory(factory).feeRateNumerator()));
            uint256 balance1Adjusted = balance1.mul(1e4).sub(amount1In.mul(IBtswapFactory(factory).feeRateNumerator()));
            require(balance0Adjusted.mul(balance1Adjusted) >= uint256(_reserve0).mul(_reserve1).mul(1e8), "BtswapPairToken: K");
        }

        _update(balance0, balance1, _reserve0, _reserve1);
        emit Swap(msg.sender, amount0In, amount1In, amount0Out, amount1Out, to);
    }

    // force balances to match reserves
    function skim(address to) external lock {
        // gas savings
        address _token0 = token0;
        // gas savings
        address _token1 = token1;
        _safeTransfer(_token0, to, IERC20(_token0).balanceOf(address(this)).sub(reserve0));
        _safeTransfer(_token1, to, IERC20(_token1).balanceOf(address(this)).sub(reserve1));
    }

    // force reserves to match balances
    function sync() external lock {
        _update(IERC20(token0).balanceOf(address(this)), IERC20(token1).balanceOf(address(this)), reserve0, reserve1);
    }

    function getReserves() public view returns (uint112 _reserve0, uint112 _reserve1, uint32 _blockTimestampLast) {
        _reserve0 = reserve0;
        _reserve1 = reserve1;
        _blockTimestampLast = blockTimestampLast;
    }

    function _safeTransfer(address token, address to, uint256 value) private {
        (bool success, bytes memory data) = token.call(abi.encodeWithSelector(SELECTOR, to, value));
        require(success && (data.length == 0 || abi.decode(data, (bool))), "BtswapPairToken: TRANSFER_FAILED");
    }

    function price(address token) public view returns (uint256) {
        if ((token0 != token && token1 != token) || 0 == reserve0 || 0 == reserve1) {
            return 0;
        }

        if (token0 == token) {
            return SafeMath.wad().mul(reserve1).div(reserve0);
        } else {
            return SafeMath.wad().mul(reserve0).div(reserve1);
        }
    }

    function transfer(address to, uint256 value) public returns (bool) {
        super.transfer(to, value);
        return IBtswapRouter02(router).onTransfer(msg.sender, to);
    }

    function transferFrom(address from, address to, uint256 value) public returns (bool) {
        super.transferFrom(from, to, value);
        if (msg.sender == router) {
            return true;
        }
        return IBtswapRouter02(router).onTransfer(from, to);
    }


    modifier lock() {
        require(unlocked == 1, "BtswapPairToken: LOCKED");
        unlocked = 0;
        _;
        unlocked = 1;
    }


    event Mint(address indexed sender, uint256 amount0, uint256 amount1);
    event Burn(address indexed sender, uint256 amount0, uint256 amount1, address indexed to);
    event Swap(address indexed sender, uint256 amount0In, uint256 amount1In, uint256 amount0Out, uint256 amount1Out, address indexed to);
    event Sync(uint112 reserve0, uint112 reserve1);

}