pragma solidity >=0.5.0;

interface IERC20 {
    event Approval(address indexed owner, address indexed spender, uint value);
    event Transfer(address indexed from, address indexed to, uint value);

    function name() external view returns (string memory);
    function symbol() external view returns (string memory);
    function decimals() external view returns (uint8);
    function totalSupply() external view returns (uint);
    function balanceOf(address owner) external view returns (uint);
    function allowance(address owner, address spender) external view returns (uint);

    function approve(address spender, uint value) external returns (bool);
    function transfer(address to, uint value) external returns (bool);
    function transferFrom(address from, address to, uint value) external returns (bool);
}

pragma solidity >=0.5.0;

interface IFactory {
    event PairCreated(address indexed token0, address indexed token1, address pair, uint256);

    function owner() external view returns (address);
    function feePercentOwner() external view returns (address);
    function setStableOwner() external view returns (address);
    function feeTo() external view returns (address);

    function ownerFeeShare() external view returns (uint256);
    function referrersFeeShare(address) external view returns (uint256);

    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 setFeeTo(address) external;
    function feeInfo() external view returns (uint _ownerFeeShare, address _feeTo);
}

pragma solidity >=0.5.0;

interface IPair {
    event Approval(address indexed owner, address indexed spender, uint value);
    event Transfer(address indexed from, address indexed to, uint value);

    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 (uint);
    function balanceOf(address owner) external view returns (uint);
    function allowance(address owner, address spender) external view returns (uint);

    function approve(address spender, uint value) external returns (bool);
    function transfer(address to, uint value) external returns (bool);
    function transferFrom(address from, address to, uint 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 (uint);

    function permit(address owner, address spender, uint value, uint deadline, uint8 v, bytes32 r, bytes32 s) external;

    event Mint(address indexed sender, uint amount0, uint amount1);
    event Burn(address indexed sender, uint amount0, uint amount1, address indexed to);
    event Swap(
        address indexed sender,
        uint amount0In,
        uint amount1In,
        uint amount0Out,
        uint amount1Out,
        address indexed to
    );
    event Sync(uint112 reserve0, uint112 reserve1);

    function MINIMUM_LIQUIDITY() external pure returns (uint);
    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, uint16 token0feePercent, uint16 token1FeePercent);
    function getAmountOut(uint amountIn, address tokenIn) external view returns (uint);
    function kLast() external view returns (uint);

    function setFeePercent(uint16 token0FeePercent, uint16 token1FeePercent) external;
    function mint(address to) external returns (uint liquidity);
    function burn(address to) external returns (uint amount0, uint amount1);
    function swap(uint amount0Out, uint amount1Out, address to, bytes calldata data) external;
    function swap(uint amount0Out, uint amount1Out, address to, bytes calldata data, address referrer) external;
    function skim(address to) external;
    function sync() external;

    function initialize(address, address) external;
}

pragma solidity >=0.5.0;

interface IUniswapV2Callee {
    function uniswapV2Call(address sender, uint amount0, uint amount1, bytes calldata data) external;
}

pragma solidity >=0.5.0;

interface IUniswapV2ERC20 {
    event Approval(address indexed owner, address indexed spender, uint value);
    event Transfer(address indexed from, address indexed to, uint value);

    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 (uint);
    function balanceOf(address owner) external view returns (uint);
    function allowance(address owner, address spender) external view returns (uint);

    function approve(address spender, uint value) external returns (bool);
    function transfer(address to, uint value) external returns (bool);
    function transferFrom(address from, address to, uint 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 (uint);

    function permit(address owner, address spender, uint value, uint deadline, uint8 v, bytes32 r, bytes32 s) external;
}

pragma solidity =0.5.16;

// a library for performing various math operations

library Math {
    function min(uint x, uint y) internal pure returns (uint z) {
        z = x < y ? x : y;
    }

    // babylonian method (https://en.wikipedia.org/wiki/Methods_of_computing_square_roots#Babylonian_method)
    function sqrt(uint y) internal pure returns (uint z) {
        if (y > 3) {
            z = y;
            uint x = y / 2 + 1;
            while (x < z) {
                z = x;
                x = (y / x + x) / 2;
            }
        } else if (y != 0) {
            z = 1;
        }
    }
}

pragma solidity >=0.5.16;

// a library for performing overflow-safe math, courtesy of DappHub (https://github.com/dapphub/ds-math)

library SafeMath {
    function add(uint x, uint y) internal pure returns (uint z) {
        require((z = x + y) >= x, 'ds-math-add-overflow');
    }

    function sub(uint x, uint y) internal pure returns (uint z) {
        require((z = x - y) <= x, 'ds-math-sub-underflow');
    }

    function mul(uint x, uint y) internal pure returns (uint z) {
        require(y == 0 || (z = x * y) / y == x, 'ds-math-mul-overflow');
    }
}

pragma solidity =0.5.16;

import './interfaces/IUniswapV2ERC20.sol';
import './libraries/SafeMath.sol';

contract UniswapV2ERC20 is IUniswapV2ERC20 {
    using SafeMath for uint;

    string public constant name = 'Citadel LP';
    string public constant symbol = 'FORT-LP';
    uint8 public constant decimals = 18;
    uint  public totalSupply;
    mapping(address => uint) public balanceOf;
    mapping(address => mapping(address => uint)) public allowance;

    bytes32 public DOMAIN_SEPARATOR;
    // keccak256("Permit(address owner,address spender,uint256 value,uint256 nonce,uint256 deadline)");
    bytes32 public constant PERMIT_TYPEHASH = 0x6e71edae12b1b97f4d1f60370fef10105fa2faae0126114a169c64845d6126c9;
    mapping(address => uint) public nonces;

    event Approval(address indexed owner, address indexed spender, uint value);
    event Transfer(address indexed from, address indexed to, uint value);

    constructor() public {
        uint 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, uint value) internal {
        totalSupply = totalSupply.add(value);
        balanceOf[to] = balanceOf[to].add(value);
        emit Transfer(address(0), to, value);
    }

    function _burn(address from, uint value) internal {
        balanceOf[from] = balanceOf[from].sub(value);
        totalSupply = totalSupply.sub(value);
        emit Transfer(from, address(0), value);
    }

    function _approve(address owner, address spender, uint value) private {
        allowance[owner][spender] = value;
        emit Approval(owner, spender, value);
    }

    function _transfer(address from, address to, uint value) private {
        balanceOf[from] = balanceOf[from].sub(value);
        balanceOf[to] = balanceOf[to].add(value);
        emit Transfer(from, to, value);
    }

    function approve(address spender, uint value) external returns (bool) {
        _approve(msg.sender, spender, value);
        return true;
    }

    function transfer(address to, uint value) external returns (bool) {
        _transfer(msg.sender, to, value);
        return true;
    }

    function transferFrom(address from, address to, uint value) external returns (bool) {
        if (allowance[from][msg.sender] != uint(-1)) {
            uint remaining = allowance[from][msg.sender].sub(value);
            allowance[from][msg.sender] = remaining;
            emit Approval(from, msg.sender, remaining);
        }
        _transfer(from, to, value);
        return true;
    }

    function permit(address owner, address spender, uint value, uint deadline, uint8 v, bytes32 r, bytes32 s) external {
        require(deadline >= block.timestamp, 'UniswapV2: 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, 'UniswapV2: INVALID_SIGNATURE');
        _approve(owner, spender, value);
    }
}

pragma solidity =0.5.16;

import './interfaces/IPair.sol';
import './UniswapV2ERC20.sol';
import './libraries/Math.sol';
import './interfaces/IERC20.sol';
import './interfaces/IFactory.sol';
import './interfaces/IUniswapV2Callee.sol';

contract CitadelPair is IPair, UniswapV2ERC20 {
  using SafeMath  for uint;

  uint public constant MINIMUM_LIQUIDITY = 10 ** 3;
  bytes4 private constant SELECTOR = bytes4(keccak256(bytes('transfer(address,uint256)')));

  address public factory;
  address public token0;
  address public token1;

  bool public initialized;

  uint public constant FEE_DENOMINATOR = 100000;
  uint public constant MAX_FEE_PERCENT = 2000; // = 2%

  uint112 private reserve0;           // uses single storage slot, accessible via getReserves
  uint112 private reserve1;           // uses single storage slot, accessible via getReserves
  uint16 public token0FeePercent = 250; // default = 0.25%  // uses single storage slot, accessible via getReserves
  uint16 public token1FeePercent = 250; // default = 0.25%  // uses single storage slot, accessible via getReserves

  uint public precisionMultiplier0;
  uint public precisionMultiplier1;

  uint public kLast; // reserve0 * reserve1, as of immediately after the most recent liquidity event

  bool public stableSwap; // if set to true, defines pair type as stable
  bool public pairTypeImmutable; // if set to true, stableSwap states cannot be updated anymore

  uint private unlocked = 1;
  modifier lock() {
    require(unlocked == 1, 'CitadelPair: LOCKED');
    unlocked = 0;
    _;
    unlocked = 1;
  }

  function getReserves() public view returns (uint112 _reserve0, uint112 _reserve1, uint16 _token0FeePercent, uint16 _token1FeePercent) {
    _reserve0 = reserve0;
    _reserve1 = reserve1;
    _token0FeePercent = token0FeePercent;
    _token1FeePercent = token1FeePercent;
  }

  function _safeTransfer(address token, address to, uint value) private {
    (bool success, bytes memory data) = token.call(abi.encodeWithSelector(SELECTOR, to, value));
    require(success && (data.length == 0 || abi.decode(data, (bool))), 'CitadelPair: TRANSFER_FAILED');
  }

  event DrainWrongToken(address indexed token, address to);
  event FeePercentUpdated(uint16 token0FeePercent, uint16 token1FeePercent);
  event SetStableSwap(bool prevStableSwap, bool stableSwap);
  event SetPairTypeImmutable();
  event Mint(address indexed sender, uint amount0, uint amount1);
  event Burn(address indexed sender, uint amount0, uint amount1, address indexed to);
  event Swap(
    address indexed sender,
    uint amount0In,
    uint amount1In,
    uint amount0Out,
    uint amount1Out,
    address indexed to
  );
  event Sync(uint112 reserve0, uint112 reserve1);
  event Skim();

  constructor() public {
    factory = msg.sender;
  }

  // called once by the factory at time of deployment
  function initialize(address _token0, address _token1) external {
    require(msg.sender == factory && !initialized, 'CitadelPair: FORBIDDEN');
    // sufficient check
    token0 = _token0;
    token1 = _token1;

    precisionMultiplier0 = 10 ** uint(IERC20(_token0).decimals());
    precisionMultiplier1 = 10 ** uint(IERC20(_token1).decimals());

    initialized = true;
  }

  /**
  * @dev Updates the swap fees percent
  *
  * Can only be called by the factory's feeAmountOwner
  */
  function setFeePercent(uint16 newToken0FeePercent, uint16 newToken1FeePercent) external lock {
    require(msg.sender == IFactory(factory).feePercentOwner(), "CitadelPair: only factory's feeAmountOwner");
    require(newToken0FeePercent <= MAX_FEE_PERCENT && newToken1FeePercent <= MAX_FEE_PERCENT, "CitadelPair: feePercent mustn't exceed the maximum");
    require(newToken0FeePercent > 0 && newToken1FeePercent > 0, "CitadelPair: feePercent mustn't exceed the minimum");
    token0FeePercent = newToken0FeePercent;
    token1FeePercent = newToken1FeePercent;
    emit FeePercentUpdated(newToken0FeePercent, newToken1FeePercent);
  }

  function setStableSwap(bool stable, uint112 expectedReserve0, uint112 expectedReserve1) external lock {
    require(msg.sender == IFactory(factory).setStableOwner(), "CitadelPair: only factory's setStableOwner");
    require(!pairTypeImmutable, "CitadelPair: immutable");

    require(stable != stableSwap, "CitadelPair: no update");
    require(expectedReserve0 == reserve0 && expectedReserve1 == reserve1, "CitadelPair: failed");

    bool feeOn = _mintFee(reserve0, reserve1);

    emit SetStableSwap(stableSwap, stable);
    stableSwap = stable;
    kLast = (stable && feeOn) ? _k(uint(reserve0), uint(reserve1)) : 0;
  }

  function setPairTypeImmutable() external lock {
    require(msg.sender == IFactory(factory).owner(), "CitadelPair: only factory's owner");
    require(!pairTypeImmutable, "CitadelPair: already immutable");

    pairTypeImmutable = true;
    emit SetPairTypeImmutable();
  }

  // update reserves
  function _update(uint balance0, uint balance1) private {
    require(balance0 <= uint112(- 1) && balance1 <= uint112(- 1), 'CitadelPair: OVERFLOW');

    reserve0 = uint112(balance0);
    reserve1 = uint112(balance1);
    emit Sync(uint112(balance0), uint112(balance1));
  }

  // if fee is on, mint liquidity equivalent to "factory.ownerFeeShare()" of the growth in sqrt(k)
  // only for uni configuration
  function _mintFee(uint112 _reserve0, uint112 _reserve1) private returns (bool feeOn) {
    if(stableSwap) return false;

    (uint ownerFeeShare, address feeTo) = IFactory(factory).feeInfo();
    feeOn = feeTo != address(0);
    uint _kLast = kLast;
    // gas savings
    if (feeOn) {
      if (_kLast != 0) {
        uint rootK = Math.sqrt(_k(uint(_reserve0), uint(_reserve1)));
        uint rootKLast = Math.sqrt(_kLast);
        if (rootK > rootKLast) {
          uint d = (FEE_DENOMINATOR.mul(100) / ownerFeeShare).sub(100);
          uint numerator = totalSupply.mul(rootK.sub(rootKLast)).mul(100);
          uint denominator = rootK.mul(d).add(rootKLast.mul(100));
          uint 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 (uint liquidity) {
    (uint112 _reserve0, uint112 _reserve1,,) = getReserves();
    // gas savings
    uint balance0 = IERC20(token0).balanceOf(address(this));
    uint balance1 = IERC20(token1).balanceOf(address(this));
    uint amount0 = balance0.sub(_reserve0);
    uint amount1 = balance1.sub(_reserve1);

    bool feeOn = _mintFee(_reserve0, _reserve1);
    uint _totalSupply = totalSupply;
    // gas savings, must be defined here since totalSupply can update in _mintFee
    if (_totalSupply == 0) {
      liquidity = Math.sqrt(amount0.mul(amount1)).sub(MINIMUM_LIQUIDITY);
      _mint(address(0), MINIMUM_LIQUIDITY);
      // permanently lock the first MINIMUM_LIQUIDITY tokens
    } else {
      liquidity = Math.min(amount0.mul(_totalSupply) / _reserve0, amount1.mul(_totalSupply) / _reserve1);
    }
    require(liquidity > 0, 'CitadelPair: INSUFFICIENT_LIQUIDITY_MINTED');
    _mint(to, liquidity);

    _update(balance0, balance1);
    if (feeOn) kLast = _k(uint(reserve0), uint(reserve1));
    // reserve0 and reserve1 are up-to-date
    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 (uint amount0, uint amount1) {
    (uint112 _reserve0, uint112 _reserve1,,) = getReserves(); // gas savings
    address _token0 = token0; // gas savings
    address _token1 = token1; // gas savings
    uint balance0 = IERC20(_token0).balanceOf(address(this));
    uint balance1 = IERC20(_token1).balanceOf(address(this));
    uint liquidity = balanceOf[address(this)];

    bool feeOn = _mintFee(_reserve0, _reserve1);
    uint _totalSupply = totalSupply; // gas savings, must be defined here since totalSupply can update in _mintFee
    amount0 = liquidity.mul(balance0) / _totalSupply; // using balances ensures pro-rata distribution
    amount1 = liquidity.mul(balance1) / _totalSupply; // using balances ensures pro-rata distribution
    require(amount0 > 0 && amount1 > 0, 'CitadelPair: 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);
    if (feeOn) kLast = _k(uint(reserve0), uint(reserve1)); // reserve0 and reserve1 are up-to-date
    emit Burn(msg.sender, amount0, amount1, to);
  }

  struct TokensData {
    address token0;
    address token1;
    uint amount0Out;
    uint amount1Out;
    uint balance0;
    uint balance1;
    uint remainingFee0;
    uint remainingFee1;
  }

  // this low-level function should be called from a contract which performs important safety checks
  function swap(uint amount0Out, uint amount1Out, address to, bytes calldata data) external {
    TokensData memory tokensData = TokensData({
      token0: token0,
      token1: token1,
      amount0Out: amount0Out,
      amount1Out: amount1Out,
      balance0: 0,
      balance1: 0,
      remainingFee0: 0,
      remainingFee1: 0
    });
    _swap(tokensData, to, data, address(0));
  }

  // this low-level function should be called from a contract which performs important safety checks
  function swap(uint amount0Out, uint amount1Out, address to, bytes calldata data, address referrer) external {
    TokensData memory tokensData = TokensData({
      token0: token0,
      token1: token1,
      amount0Out: amount0Out,
      amount1Out: amount1Out,
      balance0: 0,
      balance1: 0,
      remainingFee0: 0,
      remainingFee1: 0
    });
    _swap(tokensData, to, data, referrer);
  }


  function _swap(TokensData memory tokensData, address to, bytes memory data, address referrer) internal lock {
    require(tokensData.amount0Out > 0 || tokensData.amount1Out > 0, 'CitadelPair: INSUFFICIENT_OUTPUT_AMOUNT');

    (uint112 _reserve0, uint112 _reserve1, uint16 _token0FeePercent, uint16 _token1FeePercent) = getReserves();
    require(tokensData.amount0Out < _reserve0 && tokensData.amount1Out < _reserve1, 'CitadelPair: INSUFFICIENT_LIQUIDITY');


    {
      require(to != tokensData.token0 && to != tokensData.token1, 'CitadelPair: INVALID_TO');
      // optimistically transfer tokens
      if (tokensData.amount0Out > 0) _safeTransfer(tokensData.token0, to, tokensData.amount0Out);
      // optimistically transfer tokens
      if (tokensData.amount1Out > 0) _safeTransfer(tokensData.token1, to, tokensData.amount1Out);
      if (data.length > 0) IUniswapV2Callee(to).uniswapV2Call(msg.sender, tokensData.amount0Out, tokensData.amount1Out, data);
      tokensData.balance0 = IERC20(tokensData.token0).balanceOf(address(this));
      tokensData.balance1 = IERC20(tokensData.token1).balanceOf(address(this));
    }

    uint amount0In = tokensData.balance0 > _reserve0 - tokensData.amount0Out ? tokensData.balance0 - (_reserve0 - tokensData.amount0Out) : 0;
    uint amount1In = tokensData.balance1 > _reserve1 - tokensData.amount1Out ? tokensData.balance1 - (_reserve1 - tokensData.amount1Out) : 0;
    require(amount0In > 0 || amount1In > 0, 'CitadelPair: INSUFFICIENT_INPUT_AMOUNT');

    tokensData.remainingFee0 = amount0In.mul(_token0FeePercent) / FEE_DENOMINATOR;
    tokensData.remainingFee1 = amount1In.mul(_token1FeePercent) / FEE_DENOMINATOR;

    {// scope for referer/stable fees management
      uint fee = 0;

      uint referrerInputFeeShare = referrer != address(0) ? IFactory(factory).referrersFeeShare(referrer) : 0;
      if (referrerInputFeeShare > 0) {
        if (amount0In > 0) {
          fee = amount0In.mul(referrerInputFeeShare).mul(_token0FeePercent) / (FEE_DENOMINATOR ** 2);
          tokensData.remainingFee0 = tokensData.remainingFee0.sub(fee);
          _safeTransfer(tokensData.token0, referrer, fee);
        }
        if (amount1In > 0) {
          fee = amount1In.mul(referrerInputFeeShare).mul(_token1FeePercent) / (FEE_DENOMINATOR ** 2);
          tokensData.remainingFee1 = tokensData.remainingFee1.sub(fee);
          _safeTransfer(tokensData.token1, referrer, fee);
        }
      }

      if(stableSwap){
        (uint ownerFeeShare, address feeTo) = IFactory(factory).feeInfo();
        if(feeTo != address(0)) {
          ownerFeeShare = FEE_DENOMINATOR.sub(referrerInputFeeShare).mul(ownerFeeShare);
          if (amount0In > 0) {
            fee = amount0In.mul(ownerFeeShare).mul(_token0FeePercent) / (FEE_DENOMINATOR ** 3);
            tokensData.remainingFee0 = tokensData.remainingFee0.sub(fee);
            _safeTransfer(tokensData.token0, feeTo, fee);
          }
          if (amount1In > 0) {
            fee = amount1In.mul(ownerFeeShare).mul(_token1FeePercent) / (FEE_DENOMINATOR ** 3);
            tokensData.remainingFee1 = tokensData.remainingFee1.sub(fee);
            _safeTransfer(tokensData.token1, feeTo, fee);
          }
        }
      }
      // readjust tokens balance
      if (amount0In > 0) tokensData.balance0 = IERC20(tokensData.token0).balanceOf(address(this));
      if (amount1In > 0) tokensData.balance1 = IERC20(tokensData.token1).balanceOf(address(this));
    }
    {// scope for reserve{0,1}Adjusted, avoids stack too deep errors
      uint balance0Adjusted = tokensData.balance0.sub(tokensData.remainingFee0);
      uint balance1Adjusted = tokensData.balance1.sub(tokensData.remainingFee1);
      require(_k(balance0Adjusted, balance1Adjusted) >= _k(uint(_reserve0), uint(_reserve1)), 'CitadelPair: K');
    }
    _update(tokensData.balance0, tokensData.balance1);
    emit Swap(msg.sender, amount0In, amount1In, tokensData.amount0Out, tokensData.amount1Out, to);
  }

  function _k(uint balance0, uint balance1) internal view returns (uint) {
    if (stableSwap) {
      uint _x = balance0.mul(1e18) / precisionMultiplier0;
      uint _y = balance1.mul(1e18) / precisionMultiplier1;
      uint _a = (_x.mul(_y)) / 1e18;
      uint _b = (_x.mul(_x) / 1e18).add(_y.mul(_y) / 1e18);
      return  _a.mul(_b) / 1e18; // x3y+y3x >= k
    }
    return balance0.mul(balance1);
  }

  function _get_y(uint x0, uint xy, uint y) internal pure returns (uint) {
    for (uint i = 0; i < 255; i++) {
      uint y_prev = y;
      uint k = _f(x0, y);
      if (k < xy) {
        uint dy = (xy - k) * 1e18 / _d(x0, y);
        y = y + dy;
      } else {
        uint dy = (k - xy) * 1e18 / _d(x0, y);
        y = y - dy;
      }
      if (y > y_prev) {
        if (y - y_prev <= 1) {
          return y;
        }
      } else {
        if (y_prev - y <= 1) {
          return y;
        }
      }
    }
    return y;
  }

  function _f(uint x0, uint y) internal pure returns (uint) {
    return x0 * (y * y / 1e18 * y / 1e18) / 1e18 + (x0 * x0 / 1e18 * x0 / 1e18) * y / 1e18;
  }

  function _d(uint x0, uint y) internal pure returns (uint) {
    return 3 * x0 * (y * y / 1e18) / 1e18 + (x0 * x0 / 1e18 * x0 / 1e18);
  }

  function getAmountOut(uint amountIn, address tokenIn) external view returns (uint) {
    uint16 feePercent = tokenIn == token0 ? token0FeePercent : token1FeePercent;
    return _getAmountOut(amountIn, tokenIn, uint(reserve0), uint(reserve1), feePercent);
  }

  function _getAmountOut(uint amountIn, address tokenIn, uint _reserve0, uint _reserve1, uint feePercent) internal view returns (uint) {
    if (stableSwap) {
      amountIn = amountIn.sub(amountIn.mul(feePercent) / FEE_DENOMINATOR); // remove fee from amount received
      uint xy = _k(_reserve0, _reserve1);
      _reserve0 = _reserve0 * 1e18 / precisionMultiplier0;
      _reserve1 = _reserve1 * 1e18 / precisionMultiplier1;

      (uint reserveA, uint reserveB) = tokenIn == token0 ? (_reserve0, _reserve1) : (_reserve1, _reserve0);
      amountIn = tokenIn == token0 ? amountIn * 1e18 / precisionMultiplier0 : amountIn * 1e18 / precisionMultiplier1;
      uint y = reserveB - _get_y(amountIn + reserveA, xy, reserveB);
      return y * (tokenIn == token0 ? precisionMultiplier1 : precisionMultiplier0) / 1e18;

    } else {
      (uint reserveA, uint reserveB) = tokenIn == token0 ? (_reserve0, _reserve1) : (_reserve1, _reserve0);
      amountIn = amountIn.mul(FEE_DENOMINATOR.sub(feePercent));
      return (amountIn.mul(reserveB)) / (reserveA.mul(FEE_DENOMINATOR).add(amountIn));
    }
  }

  // force balances to match reserves
  function skim(address to) external lock {
    address _token0 = token0;
    // gas savings
    address _token1 = token1;
    // gas savings
    _safeTransfer(_token0, to, IERC20(_token0).balanceOf(address(this)).sub(reserve0));
    _safeTransfer(_token1, to, IERC20(_token1).balanceOf(address(this)).sub(reserve1));
    emit Skim();
  }

  // force reserves to match balances
  function sync() external lock {
    uint token0Balance = IERC20(token0).balanceOf(address(this));
    uint token1Balance = IERC20(token1).balanceOf(address(this));
    require(token0Balance != 0 && token1Balance != 0, "CitadelPair: liquidity ratio not initialized");
    _update(token0Balance, token1Balance);
  }

  /**
  * @dev Allow to recover token sent here by mistake
  *
  * Can only be called by factory's owner
  */
  function drainWrongToken(address token, address to) external lock {
    require(msg.sender == IFactory(factory).owner(), "CitadelPair: only factory's owner");
    require(token != token0 && token != token1, "CitadelPair: invalid token");
    _safeTransfer(token, to, IERC20(token).balanceOf(address(this)));
    emit DrainWrongToken(token, to);
  }
}

{
  "compilationTarget": {
    "contracts/pair.sol": "CitadelPair"
  },
  "evmVersion": "istanbul",
  "libraries": {},
  "optimizer": {
    "enabled": true,
    "runs": 200
  },
  "remappings": []
}