pragma solidity ^0.5.8;

/**
 * @title ERC20 interface
 * @dev see https://github.com/ethereum/EIPs/issues/20
 */
interface IERC20 {
    function transfer(address to, uint256 value) external returns (bool);

    function approve(address spender, uint256 value) external returns (bool);

    function transferFrom(address from, address to, uint256 value) external returns (bool);

    function totalSupply() external view returns (uint256);

    function balanceOf(address who) external view returns (uint256);

    function allowance(address owner, address spender) external view returns (uint256);

    event Transfer(address indexed from, address indexed to, uint256 value);

    event Approval(address indexed owner, address indexed spender, uint256 value);
}


/**
 * @title SafeMath
 * @dev Unsigned math operations with safety checks that revert on error
 */
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;
    }
}


/**
 * @title Standard ERC20 token
 *
 * @dev Implementation of the basic standard token.
 * https://github.com/ethereum/EIPs/blob/master/EIPS/eip-20.md
 * Originally based on code by FirstBlood:
 * https://github.com/Firstbloodio/token/blob/master/smart_contract/FirstBloodToken.sol
 *
 * This implementation emits additional Approval events, allowing applications to reconstruct the allowance status for
 * all accounts just by listening to said events. Note that this isn't required by the specification, and other
 * compliant implementations may not do it.
 */
contract ERC20 is IERC20 {
    using SafeMath for uint256;

    mapping (address => uint256) private _balances;

    mapping (address => mapping (address => uint256)) private _allowed;

    uint256 private _totalSupply;

    /**
    * @dev Total number of tokens in existence
    */
    function totalSupply() public view returns (uint256) {
        return _totalSupply;
    }

    /**
    * @dev Gets the balance of the specified address.
    * @param owner The address to query the balance of.
    * @return An uint256 representing the amount owned by the passed address.
    */
    function balanceOf(address owner) public view returns (uint256) {
        return _balances[owner];
    }

    /**
     * @dev Function to check the amount of tokens that an owner allowed to a spender.
     * @param owner address The address which owns the funds.
     * @param spender address The address which will spend the funds.
     * @return A uint256 specifying the amount of tokens still available for the spender.
     */
    function allowance(address owner, address spender) public view returns (uint256) {
        return _allowed[owner][spender];
    }

    /**
    * @dev Transfer token for a specified address
    * @param to The address to transfer to.
    * @param value The amount to be transferred.
    */
    function transfer(address to, uint256 value) public returns (bool) {
        _transfer(msg.sender, to, value);
        return true;
    }

    /**
     * @dev Approve the passed address to spend the specified amount of tokens on behalf of msg.sender.
     * 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
     * @param spender The address which will spend the funds.
     * @param value The amount of tokens to be spent.
     */
    function approve(address spender, uint256 value) public returns (bool) {
        require(spender != address(0));

        _allowed[msg.sender][spender] = value;
        emit Approval(msg.sender, spender, value);
        return true;
    }

    /**
     * @dev Transfer tokens from one address to another.
     * Note that while this function emits an Approval event, this is not required as per the specification,
     * and other compliant implementations may not emit the event.
     * @param from address The address which you want to send tokens from
     * @param to address The address which you want to transfer to
     * @param value uint256 the amount of tokens to be transferred
     */
    function transferFrom(address from, address to, uint256 value) public returns (bool) {
        _allowed[from][msg.sender] = _allowed[from][msg.sender].sub(value);
        _transfer(from, to, value);
        emit Approval(from, msg.sender, _allowed[from][msg.sender]);
        return true;
    }

    /**
     * @dev Increase the amount of tokens that an owner allowed to a spender.
     * approve should be called when allowed_[_spender] == 0. To increment
     * allowed value is better to use this function to avoid 2 calls (and wait until
     * the first transaction is mined)
     * From MonolithDAO Token.sol
     * Emits an Approval event.
     * @param spender The address which will spend the funds.
     * @param addedValue The amount of tokens to increase the allowance by.
     */
    function increaseAllowance(address spender, uint256 addedValue) public returns (bool) {
        require(spender != address(0));

        _allowed[msg.sender][spender] = _allowed[msg.sender][spender].add(addedValue);
        emit Approval(msg.sender, spender, _allowed[msg.sender][spender]);
        return true;
    }

    /**
     * @dev Decrease the amount of tokens that an owner allowed to a spender.
     * approve should be called when allowed_[_spender] == 0. To decrement
     * allowed value is better to use this function to avoid 2 calls (and wait until
     * the first transaction is mined)
     * From MonolithDAO Token.sol
     * Emits an Approval event.
     * @param spender The address which will spend the funds.
     * @param subtractedValue The amount of tokens to decrease the allowance by.
     */
    function decreaseAllowance(address spender, uint256 subtractedValue) public returns (bool) {
        require(spender != address(0));

        _allowed[msg.sender][spender] = _allowed[msg.sender][spender].sub(subtractedValue);
        emit Approval(msg.sender, spender, _allowed[msg.sender][spender]);
        return true;
    }

    /**
    * @dev Transfer token for a specified addresses
    * @param from The address to transfer from.
    * @param to The address to transfer to.
    * @param value The amount to be transferred.
    */
    function _transfer(address from, address to, uint256 value) internal {
        require(to != address(0));

        _balances[from] = _balances[from].sub(value);
        _balances[to] = _balances[to].add(value);
        emit Transfer(from, to, value);
    }

    /**
     * @dev Internal function that mints an amount of the token and assigns it to
     * an account. This encapsulates the modification of balances such that the
     * proper events are emitted.
     * @param account The account that will receive the created tokens.
     * @param value The amount that will be created.
     */
    function _mint(address account, uint256 value) internal {
        require(account != address(0));

        _totalSupply = _totalSupply.add(value);
        _balances[account] = _balances[account].add(value);
        emit Transfer(address(0), account, value);
    }

    /**
     * @dev Internal function that burns an amount of the token of a given
     * account.
     * @param account The account whose tokens will be burnt.
     * @param value The amount that will be burnt.
     */
    function _burn(address account, uint256 value) internal {
        require(account != address(0));

        _totalSupply = _totalSupply.sub(value);
        _balances[account] = _balances[account].sub(value);
        emit Transfer(account, address(0), value);
    }

    /**
     * @dev Internal function that burns an amount of the token of a given
     * account, deducting from the sender's allowance for said account. Uses the
     * internal burn function.
     * Emits an Approval event (reflecting the reduced allowance).
     * @param account The account whose tokens will be burnt.
     * @param value The amount that will be burnt.
     */
    function _burnFrom(address account, uint256 value) internal {
        _allowed[account][msg.sender] = _allowed[account][msg.sender].sub(value);
        _burn(account, value);
        emit Approval(account, msg.sender, _allowed[account][msg.sender]);
    }
}


/**
 * @title Helps contracts guard against reentrancy attacks.
 * @author Remco Bloemen <remco@2π.com>, Eenae <alexey@mixbytes.io>
 * @dev If you mark a function `nonReentrant`, you should also
 * mark it `external`.
 */
contract ReentrancyGuard {
    /// @dev counter to allow mutex lock with only one SSTORE operation
    uint256 private _guardCounter;

    constructor() public {
        // The counter starts at one to prevent changing it from zero to a non-zero
        // value, which is a more expensive operation.
        _guardCounter = 1;
    }

    /**
     * @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() {
        _guardCounter += 1;
        uint256 localCounter = _guardCounter;
        _;
        require(localCounter == _guardCounter);
    }
}


/// @title Main contract for WrappedCK. This contract converts Cryptokitties between the ERC721 standard and the 
///  ERC20 standard by locking cryptokitties into the contract and minting 1:1 backed ERC20 tokens, that
///  can then be redeemed for cryptokitties when desired.
/// @notice When wrapping a cryptokitty, you get a generic WCK token. Since the WCK token is generic, it has no
///  no information about what cryptokitty you submitted, so you will most likely not receive the same kitty
///  back when redeeming the token. The token only entitles you to receive *a* cryptokitty in return, not 
///  necessarily the *same* cryptokitty in return. This is due to the very nature of the ERC20 standard being
///  fungible, and the ERC721 standard being nonfungible.
contract WrappedCK is ERC20, ReentrancyGuard {

    // OpenZeppelin's SafeMath library is used for all arithmetic operations to avoid overflows/underflows.
    using SafeMath for uint256;

    /* ****** */
    /* EVENTS */
    /* ****** */

    /// @dev This event is fired when a user deposits cryptokitties into the contract in exchange
    ///  for an equal number of WCK ERC20 tokens.
    /// @param kittyId  The cryptokitty id of the kitty that was deposited into the contract.
    /// @param tokensMinted  The number of WCK ERC20 tokens that were minted (measured in 10^18 times
    ///  the number of tokens minted, due to 18 decimal places).
    event DepositKittyAndMintToken(
        uint256 kittyId,
        uint256 tokensMinted
    );

    /// @dev This event is fired when a user deposits WCK ERC20 tokens into the contract in exchange
    ///  for an equal number of locked cryptokitties.
    /// @param kittyId  The cryptokitty id of the kitty that was withdrawn from the contract.
    /// @param tokensBurned  The number of WCK ERC20 tokens that were burned (measured in 10^18 times
    ///  the number of tokens minted, due to 18 decimal places).
    event BurnTokenAndWithdrawKitty(
        uint256 kittyId,
        uint256 tokensBurned
    );

    /* ******* */
    /* STORAGE */
    /* ******* */

    /// @dev A queue containing all of the cryptokitties that are locked in the contract, backing
    ///  WCK ERC20 tokens 1:1
    /// @notice We use a queue rather than a stack since many users in the community requested the
    ///  ability to deposit and withdraw a kitty in order to "reroll" its appearance.
    uint256[] private depositedKittiesQueue;
    uint256 private queueStartIndex;
    uint256 private queueEndIndex;
    
    /* ********* */
    /* CONSTANTS */
    /* ********* */

    /// @dev The metadata details about the "Wrapped CryptoKitties" WCK ERC20 token.
    uint8 constant public decimals = 18;
    string constant public name = "Wrapped CryptoKitties";
    string constant public symbol = "WCK";

    /// @dev The address of official CryptoKitties contract that stores the metadata about each cat.
    /// @notice The owner is not capable of changing the address of the CryptoKitties Core contract 
    ///  once the contract has been deployed.
    address public kittyCoreAddress = 0x06012c8cf97BEaD5deAe237070F9587f8E7A266d;
    KittyCore kittyCore;

    /* ********* */
    /* FUNCTIONS */
    /* ********* */

    /// @notice Allows a user to lock one cryptokitty in the contract in exchange for one WCK ERC20 
    ///  token.
    /// @param _kittyId  The id of the cryptokitty that will be locked into the contract.
    /// @notice The user must first call approve() in the Cryptokitties Core contract before calling 
    ///  depositAndMint(). There is no danger of this contract overreaching its approval, since the 
    ///  CryptoKitties Core contract's approve() function only approves this contract for a single 
    ///  Cryptokitty. Calling approve() allows this contract to transfer the specified kitty in the 
    ///  depositAndMint() function.
    function depositKittyAndMintToken(uint256 _kittyId) external nonReentrant {
        require(msg.sender == kittyCore.ownerOf(_kittyId), 'you do not own this cat');
        require(kittyCore.kittyIndexToApproved(_kittyId) == address(this), 'you must approve() this contract to give it permission to withdraw this cat before you can deposit a cat');
        kittyCore.transferFrom(msg.sender, address(this), _kittyId);
        _enqueueKitty(_kittyId);
        _mint(msg.sender, 10**18);
        emit DepositKittyAndMintToken(_kittyId, 10**18);
    }

    /// @notice Convenience function for calling depositAndMint() multiple times in a single transaction.
    /// @param _kittyIds  The ids of the cryptokitties that will be locked into the contract.
    function multiDepositKittyAndMintToken(uint256[] calldata _kittyIds) external nonReentrant {
        for(uint i = 0; i < _kittyIds.length; i++){
            uint256 kittyToDeposit = _kittyIds[i];
            require(msg.sender == kittyCore.ownerOf(kittyToDeposit), 'you do not own this cat');
            require(kittyCore.kittyIndexToApproved(kittyToDeposit) == address(this), 'you must approve() this contract to give it permission to withdraw this cat before you can deposit a cat');
            kittyCore.transferFrom(msg.sender, address(this), kittyToDeposit);
            _enqueueKitty(kittyToDeposit);
            emit DepositKittyAndMintToken(kittyToDeposit, 10**18);
        }
        _mint(msg.sender, (_kittyIds.length).mul(10**18));
    }

    /// @notice Allows a user to burn one WCK ERC20 token in exchange for one locked cryptokitty.
    /// @notice Due to the nature of WCK ERC20 being fungible and interchangeable, the contract is
    ///  not able to coordinate that you receive the same crypokitty that you originally locked into
    ///  the contract.
    function burnTokenAndWithdrawKitty() external nonReentrant {
        require(balanceOf(msg.sender) >= 10**18, 'you do not own enough tokens to withdraw an ERC721 cat');
        uint256 kittyId = _dequeueKitty();
        _burn(msg.sender, 10**18);
        kittyCore.transferFrom(address(this), msg.sender, kittyId);
        emit BurnTokenAndWithdrawKitty(kittyId, 10**18);
    }

    /// @notice Convenience function for calling burnAndWithdraw() multiple times in a single transaction.
    /// @param _numTokens  The number of WCK ERC20 tokens that will be burned in exchange for cryptokitties.
    function multiBurnTokenAndWithdrawKitty(uint256 _numTokens) external nonReentrant {
        require(balanceOf(msg.sender) >= _numTokens.mul(10**18), 'you do not own enough tokens to withdraw this many ERC721 cats');
        _burn(msg.sender, _numTokens.mul(10**18));
        for(uint i = 0; i < _numTokens; i++){
            uint256 kittyToWithdraw = _dequeueKitty();
            kittyCore.transfer(msg.sender, kittyToWithdraw);
            emit BurnTokenAndWithdrawKitty(kittyToWithdraw, 10**18);
        }
    }

    /// @notice Allows a user to lock one cryptokitty in the contract in exchange for unlocking a 
    ///  different cryptokitty that was previously locked in the contract.
    /// @param _kittyId  The id of the cryptokitty that will be locked into the contract.
    /// @notice The user must first call approve() in the Cryptokitties Core contract before calling 
    ///  depositAndMint(). There is no danger of this contract overreaching its approval, since the 
    ///  CryptoKitties Core contract's approve() function only approves this contract for a single 
    ///  Cryptokitty. Calling approve() allows this contract to transfer the specified kitty in the 
    ///  depositAndMint() function.
    /// @notice This is a convenience function so that users do not need to call both depositAndMint()
    ///  and burnAndWithdraw() in succession. Many users in the community requested the ability to 
    ///  deposit and withdraw a kitty in order to "reroll" its appearance, which this function 
    ///  accomplishes.
    function depositKittyAndWithdrawDifferentKitty(uint256 _kittyId) external nonReentrant {
        require(msg.sender == kittyCore.ownerOf(_kittyId), 'you do not own this cat');
        require(kittyCore.kittyIndexToApproved(_kittyId) == address(this), 'you must approve() this contract to give it permission to withdraw this cat before you can deposit a cat');
        kittyCore.transferFrom(msg.sender, address(this), _kittyId);
        _enqueueKitty(_kittyId);
        uint256 kittyToWithdraw = _dequeueKitty();
        kittyCore.transferFrom(address(this), msg.sender, kittyToWithdraw);
        emit DepositKittyAndMintToken(_kittyId, 10**18);
        emit BurnTokenAndWithdrawKitty(kittyToWithdraw, 10**18);
    }

    /// @notice Convenience function for calling depositAndWithdraw() multiple times in a single transaction.
    /// @param _kittyIds  The ids of the cryptokitties that will be locked into the contract.
    function multiDepositKittyAndWithdrawDifferentKitty(uint256[] calldata _kittyIds) external nonReentrant {
        for(uint i = 0; i < _kittyIds.length; i++){
            uint256 kittyToDeposit = _kittyIds[i];
            require(msg.sender == kittyCore.ownerOf(kittyToDeposit), 'you do not own this cat');
            require(kittyCore.kittyIndexToApproved(kittyToDeposit) == address(this), 'you must approve() this contract to give it permission to withdraw this cat before you can deposit a cat');
            kittyCore.transferFrom(msg.sender, address(this), kittyToDeposit);
            _enqueueKitty(kittyToDeposit);
            uint256 kittyToWithdraw = _dequeueKitty();
            kittyCore.transferFrom(address(this), msg.sender, kittyToWithdraw);
            emit DepositKittyAndMintToken(kittyToDeposit, 10**18);
            emit BurnTokenAndWithdrawKitty(kittyToWithdraw, 10**18);
        }
    }
    
    /// @notice Adds a locked cryptokitty to the end of the queue
    /// @param _kittyId  The id of the cryptokitty that will be locked into the contract.
    function _enqueueKitty(uint256 _kittyId) internal {
        depositedKittiesQueue.push(_kittyId);
        queueEndIndex = queueEndIndex.add(1);
    }

    /// @notice Adds a locked cryptokitty to the end of the queue
    /// @return  The id of the cryptokitty that will be unlocked from the contract.
    function _dequeueKitty() internal returns(uint256){
        require(queueStartIndex < queueEndIndex, 'there are no cats in the queue');
        uint256 kittyId = depositedKittiesQueue[queueStartIndex];
        queueStartIndex = queueStartIndex.add(1);
        return kittyId;
    }

    /// @return The number of cryptokitties locked in the contract that back outstanding
    ///  WCK tokens.
    function totalCatsLockedInContract() public view returns(uint256){
        return queueEndIndex.sub(queueStartIndex);
    }

    /// @notice The owner is not capable of changing the address of the CryptoKitties Core 
    ///  contract once the contract has been deployed.
    constructor() public {
        kittyCore = KittyCore(kittyCoreAddress);
    }

    /// @dev We leave the fallback function payable in case the current State Rent proposals require
    ///  us to send funds to this contract to keep it alive on mainnet.
    /// @notice There is no function that allows the contract creator to withdraw any funds sent
    ///  to this contract, so any funds sent directly to the fallback fucntion that are not used for 
    ///  State Rent are lost forever.
    function() external payable {}
}

/// @title Interface for interacting with the CryptoKitties Core contract created by Dapper Labs Inc.
contract KittyCore {
    function ownerOf(uint256 _tokenId) public view returns (address owner);
    function transferFrom(address _from, address _to, uint256 _tokenId) external;
    function transfer(address _to, uint256 _tokenId) external;
    mapping (uint256 => address) public kittyIndexToApproved;
}