pragma solidity 0.6.8;

library SafeMath {
  /**
  * @dev Multiplies two unsigned integers, reverts on 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-solidity/pull/522
    if (a == 0) {
        return 0;
    }

    uint256 c = a * b;
    require(c / a == b);

    return c;
  }

  /**
  * @dev Integer division of two unsigned integers truncating the quotient, reverts on division by zero.
  */
  function div(uint256 a, uint256 b) internal pure returns (uint256) {
    // Solidity only automatically asserts when dividing by 0
    require(b > 0);
    uint256 c = a / b;
    // assert(a == b * c + a % b); // There is no case in which this doesn't hold

    return c;
  }

  /**
  * @dev Subtracts two unsigned integers, reverts on overflow (i.e. if subtrahend is greater than minuend).
  */
  function sub(uint256 a, uint256 b) internal pure returns (uint256) {
    require(b <= a);
    uint256 c = a - b;

    return c;
  }

  /**
  * @dev Adds two unsigned integers, reverts on overflow.
  */
  function add(uint256 a, uint256 b) internal pure returns (uint256) {
    uint256 c = a + b;
    require(c >= a);

    return c;
  }

  /**
  * @dev Divides two unsigned integers and returns the remainder (unsigned integer modulo),
  * reverts when dividing by zero.
  */
  function mod(uint256 a, uint256 b) internal pure returns (uint256) {
    require(b != 0);
    return a % b;
  }
}

interface ERC20 {
  function balanceOf(address who) external view returns (uint256);
  function transfer(address to, uint value) external  returns (bool success);
  function transferFrom(address from, address to, uint256 value) external returns (bool success);
  function approve(address spender, uint value) external returns (bool success);
}

contract YFMSTokenSwap {
  using SafeMath for uint256;

  ERC20 public YFMSToken;
  ERC20 public LUCRToken;

  address public owner;

  constructor(address yfms, address lucr) public {
    owner = msg.sender;
    YFMSToken = ERC20(yfms);
    LUCRToken = ERC20(lucr);
  }

  function swap () public {
    uint256 balance = YFMSToken.balanceOf(msg.sender);
    require(balance > 0, "balance must be greater than 0");
    require(YFMSToken.transferFrom(msg.sender, address(this), balance), "YFMS transfer failed");
    require(LUCRToken.transferFrom(owner, msg.sender, balance), "LUCR transfer failed");
  }

  function withdrawYFMS () public {
    require(msg.sender == owner);
    YFMSToken.transfer(owner, YFMSToken.balanceOf(address(this)));
  }
}