// Sources flattened with hardhat v2.8.0 https://hardhat.org

// File @openzeppelin/contracts/utils/Context.sol@v4.4.1

// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts v4.4.1 (utils/Context.sol)

pragma solidity ^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 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) {
        return msg.sender;
    }

    function _msgData() internal view virtual returns (bytes calldata) {
        return msg.data;
    }
}


// File @openzeppelin/contracts/access/Ownable.sol@v4.4.1


// OpenZeppelin Contracts v4.4.1 (access/Ownable.sol)

pragma solidity ^0.8.0;

/**
 * @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() {
        _transferOwnership(_msgSender());
    }

    /**
     * @dev Returns the address of the current owner.
     */
    function owner() public view virtual 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 {
        _transferOwnership(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");
        _transferOwnership(newOwner);
    }

    /**
     * @dev Transfers ownership of the contract to a new account (`newOwner`).
     * Internal function without access restriction.
     */
    function _transferOwnership(address newOwner) internal virtual {
        address oldOwner = _owner;
        _owner = newOwner;
        emit OwnershipTransferred(oldOwner, newOwner);
    }
}


// File @openzeppelin/contracts/security/ReentrancyGuard.sol@v4.4.1


// OpenZeppelin Contracts v4.4.1 (security/ReentrancyGuard.sol)

pragma solidity ^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() {
        _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 making 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 contracts/ETCBitPixel.sol



pragma solidity ^0.8.4;


library SafeMath {
    /**
     * @dev Returns the addition of two unsigned integers, with an overflow flag.
     *
     * _Available since v3.4._
     */
    function tryAdd(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        uint256 c = a + b;
        if (c < a) return (false, 0);
        return (true, c);
    }

    /**
     * @dev Returns the substraction of two unsigned integers, with an overflow flag.
     *
     * _Available since v3.4._
     */
    function trySub(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        if (b > a) return (false, 0);
        return (true, a - b);
    }

    /**
     * @dev Returns the multiplication of two unsigned integers, with an overflow flag.
     *
     * _Available since v3.4._
     */
    function tryMul(uint256 a, uint256 b) internal pure returns (bool, 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 (true, 0);
        uint256 c = a * b;
        if (c / a != b) return (false, 0);
        return (true, c);
    }

    /**
     * @dev Returns the division of two unsigned integers, with a division by zero flag.
     *
     * _Available since v3.4._
     */
    function tryDiv(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        if (b == 0) return (false, 0);
        return (true, a / b);
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers, with a division by zero flag.
     *
     * _Available since v3.4._
     */
    function tryMod(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        if (b == 0) return (false, 0);
        return (true, a % b);
    }

    /**
     * @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) {
        require(b <= a, "SafeMath: subtraction overflow");
        return a - b;
    }

    /**
     * @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) {
        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, reverting 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) {
        require(b > 0, "SafeMath: division by zero");
        return a / b;
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * reverting 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) {
        require(b > 0, "SafeMath: modulo by zero");
        return a % b;
    }

    /**
     * @dev Returns the subtraction of two unsigned integers, reverting with custom message on
     * overflow (when the result is negative).
     *
     * CAUTION: This function is deprecated because it requires allocating memory for the error
     * message unnecessarily. For custom revert reasons use {trySub}.
     *
     * 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);
        return a - b;
    }

    /**
     * @dev Returns the integer division of two unsigned integers, reverting with custom message on
     * division by zero. The result is rounded towards zero.
     *
     * CAUTION: This function is deprecated because it requires allocating memory for the error
     * message unnecessarily. For custom revert reasons use {tryDiv}.
     *
     * 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);
        return a / b;
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * reverting with custom message when dividing by zero.
     *
     * CAUTION: This function is deprecated because it requires allocating memory for the error
     * message unnecessarily. For custom revert reasons use {tryMod}.
     *
     * 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;
    }
}


library EnumerableSet {
    struct UintSet {
        uint256[] _values;
        mapping (uint256 => uint256) _indexes;
    }

    function add(UintSet storage set, uint256 value) internal returns (bool) {
        if (contains(set, value)) {
            return false;
        }

        set._values.push(value);
        set._indexes[value] = set._values.length;
        return true;
    }


    function remove(UintSet storage set, uint256 value) internal returns (bool) {
        uint256 valueIndex = set._indexes[value];

        if (valueIndex != 0) { 
            uint256 toDeleteIndex = valueIndex - 1;
            uint256 lastIndex = set._values.length - 1;

            uint256 lastvalue = set._values[lastIndex];

            set._values[toDeleteIndex] = lastvalue;
            set._indexes[lastvalue] = toDeleteIndex + 1; // All indexes are 1-based
            set._values.pop();
            delete set._indexes[value];

            return true;
        } 
        else {
            return false;
        }
    }

    function contains(UintSet storage set, uint256 value) internal view returns (bool) {
        return set._indexes[value] != 0;
    }

    function length(UintSet storage set) internal view returns (uint256) {
        return set._values.length;
    }

    function at(UintSet storage set, uint256 index) internal view returns (uint256) {
        require(set._values.length < index, "EnumerableSet: index out of bounds");
        return set._values[index];
    }

    function values(UintSet storage set) internal view returns (uint256[] memory _vals) {
        return set._values;
    }
}


library Utils {
    function uintToString(uint _i) internal pure returns (string memory _uintAsString) {
        if (_i == 0) {
            return "0";
        }
        uint j = _i;
        uint len;
        while (j != 0) {
            len++;
            j /= 10;
        }
        bytes memory bstr = new bytes(len);
        uint k = len;
        while (_i != 0) {
            k = k-1;
            uint8 temp = (48 + uint8(_i - _i / 10 * 10));
            bytes1 b1 = bytes1(temp);
            bstr[k] = b1;
            _i /= 10;
        }
        return string(bstr);
    }
}


interface IERC165 {
    /// @notice Query if a contract implements an interface
    /// @param interfaceID The interface identifier, as specified in ERC-165
    /// @dev Interface identification is specified in ERC-165. This function
    ///  uses less than 30,000 energy.
    /// @return `true` if the contract implements `interfaceID` and
    ///  `interfaceID` is not 0xffffffff, `false` otherwise
    function supportsInterface(bytes4 interfaceID) external view returns (bool);

}

interface IERC721 {
    function balanceOf(address _owner) external view returns (uint256);
    function ownerOf(uint256 _tokenId) external view returns (address);

    function safeTransferFrom(address _from, address _to, uint256 _tokenId, bytes calldata _data) external payable;
    function safeTransferFrom(address _from, address _to, uint256 _tokenId) external payable;
    function transferFrom(address _from, address _to, uint256 _tokenId) external payable;

    function approve(address _approved, uint256 _tokenId) external payable;
    function getApproved(uint256 _tokenId) external view returns (address);
    function setApprovalForAll(address _operator, bool _approved) external;
    function isApprovedForAll(address _owner, address _operator) external view returns (bool);


    event Transfer(address indexed from, address indexed to, uint256 indexed tokenId);
    event Approval(address indexed owner, address indexed approved, uint256 indexed tokenId);
    event ApprovalForAll(address indexed owner, address indexed operator, bool approved);
}

/**
 * Interface for verifying ownership during Community Grant.
 */
interface IERC721TokenReceiver {
    function onERC721Received(address _operator, address _from, uint256 _tokenId, bytes calldata _data) external returns(bytes4);
}

interface IERC721Metadata {
    function name() external view returns (string memory _name);

    function symbol() external view returns (string memory _symbol);

    //Returns the URI of the external file corresponding to ‘_tokenId’. External resource files need to include names, descriptions and pictures. 
    function tokenURI(uint256 _tokenId) external view returns (string memory);
}


interface IERC721Enumerable {
    //Return the total supply of NFT
    function totalSupply() external view returns (uint256);

    //Return the corresponding ‘tokenId’ through ‘_index’
    function tokenByIndex(uint256 _index) external view returns (uint256);

     //Return the ‘tokenId’ corresponding to the index in the NFT list owned by the ‘_owner'
    function tokenOfOwnerByIndex(address _owner, uint256 _index) external view returns (uint256);
}


contract ETCBitPixel is Ownable, ReentrancyGuard, IERC721, IERC165, IERC721Metadata, IERC721Enumerable {
    using SafeMath for uint256;
    using EnumerableSet for EnumerableSet.UintSet;

    string private constant _name = "ETCBitPixel";
    string private constant _symbol = "EBP";
    string private _baseURI;

    uint256 internal totalMinted = 0;

    uint256 public constant MINTING_LIMIT = 10000;

    uint256 public mintingPrice = 3 ether;

    // marketer comission
    uint256 private immutable _marketerComission;
    address payable private immutable _marketer;

    //Service:
    bool internal isReentrancyLock = false;
    address internal addressZero = address(0);

    // Equals to `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`
    // which can be also obtained as `IERC721Receiver(0).onERC721Received.selector`
    bytes4 internal constant MAGIC_ON_ERC721_RECEIVED = 0x150b7a02;

    // Random
    uint256 internal nonce = 0;
    uint256[MINTING_LIMIT] internal indices;

    // Supported interfaces
    mapping(bytes4 => bool) internal supportedInterfaces;

    // storage
    mapping (uint256 => address) internal tokenToOwner;
    mapping (address => EnumerableSet.UintSet) internal ownerToTokensSet;
    mapping (uint256 => address) internal tokenToApproval;

    mapping (address => mapping (address => bool)) internal ownerToOperators;

    bool public isMintingEnabled = false;

    constructor(string memory baseURI_, address marketer, uint256 marketerComission) {
        supportedInterfaces[0x01ffc9a7] = true; // ERC165
        supportedInterfaces[0x80ac58cd] = true; // ERC721
        supportedInterfaces[0x780e9d63] = true; // ERC721 Enumerable
        supportedInterfaces[0x5b5e139f] = true; // ERC721 Metadata

        _setBaseURI(baseURI_);

        _marketer = payable(marketer);
        _marketerComission = marketerComission;
    }




    /*************************************************************************** */
    //                             IERC721Metadata: 

    function name() external pure override returns (string memory) {
        return _name;
    }

    function symbol() external pure override returns (string memory) {
        return _symbol;
    }

    function tokenURI(uint256 _tokenId) external view override returns (string memory) {
        return string(abi.encodePacked(_baseURI, Utils.uintToString(_tokenId), '.json'));
    }

    /*************************************************************************** */
    //                             Enumerable: 

    function totalSupply() public view override returns(uint256) {
        return totalMinted;
    }

    function tokenByIndex(uint256 _index) external pure override returns (uint256) {
        require(_index >= 0 && _index < MINTING_LIMIT);
        return _index;
    }

    function tokenOfOwnerByIndex(address _owner, uint256 _index) external view override returns (uint256 _tokenId) {
        require(_index < ownerToTokensSet[_owner].values().length);
        return ownerToTokensSet[_owner].values()[_index];
    }

    function getNotMintedAmount() external view returns(uint256) {
        return MINTING_LIMIT - totalMinted;
    }

    /*************************************************************************** */
    //                             IERC165: 
    function supportsInterface(bytes4 _interfaceID) external view override returns (bool) {
        return supportedInterfaces[_interfaceID];
    }


    /*************************************************************************** */
    //                             ERC-721: 

    function balanceOf(address _owner) external view override returns (uint256) {
        return ownerToTokensSet[_owner].values().length;
    }

    function ownerOf(uint256 _tokenId) external view override returns (address) {
        return tokenToOwner[_tokenId];
    }

    function safeTransferFrom(address _from, address _to, uint256 _tokenId, bytes calldata _data) 
        external override payable 
    {
        _safeTransferFrom(_from, _to, _tokenId, _data);
    }

    function safeTransferFrom(address _from, address _to, uint256 _tokenId) 
        external payable override
    {
        _safeTransferFrom(_from, _to, _tokenId, "");
    }

    function transferFrom(address _from, address _to, uint256 _tokenId) external payable override
        transferApproved(_tokenId)
        validTokenId(_tokenId) 
        notZeroAddress(_to)
    {
        _transfer(_from, _to, _tokenId);
    }

    function approve(address _approved, uint256 _tokenId) external override payable
        validTokenId(_tokenId)
        canOperate(_tokenId)
    {
        address tokenOwner = tokenToOwner[_tokenId];
        if (_approved != tokenOwner) {
            tokenToApproval[_tokenId] = _approved;
            emit Approval(tokenOwner, _approved, _tokenId);
        }
    }

    function setApprovalForAll(address _operator, bool _approved) external override {
        ownerToOperators[msg.sender][_operator] = _approved;
        emit ApprovalForAll(msg.sender, _operator, _approved);
    }

    function getApproved(uint256 _tokenId) external view override validTokenId(_tokenId)
        returns (address) 
    {
        return tokenToApproval[_tokenId];
    }

    function isApprovedForAll(address _owner, address _operator) external view override returns (bool) {
        return ownerToOperators[_owner][_operator];
    }


    /*************************************************************************** */

    function getUserTokens(address _user) external view returns (uint256[] memory) {
        return ownerToTokensSet[_user].values();
    }

    /*************************************************************************** */
    //                             Mint:

    function getNFTPrice(uint256 amount) public view returns (uint256) {
        require(totalSupply() < MINTING_LIMIT, "Sale has already ended.");

        return amount.mul(mintingPrice);
    }

    function mint(uint256 amount) external payable nonReentrant mintingEnabled {

        uint256 nftPrice = getNFTPrice(amount);

        require(msg.value >= nftPrice, "notEnough");

        require(amount >= 1 && amount <= 20, "amountLimit");
        require((totalMinted + amount) <= MINTING_LIMIT, "exceedsMaxSupply");

        if (msg.value > nftPrice) {
            (bool send, ) = msg.sender.call{ value: msg.value - nftPrice }("");
            require(send, "refund");
        }

        uint256 marketerComission = nftPrice * _marketerComission / 100;
        (bool sent, ) = _marketer.call{ value: marketerComission }("");
        require(sent, "marketerComission");

        for (uint i = 0; i < amount; i++) {
            _mint(msg.sender);
        }
    }


    function changeBaseURI(string memory baseURI) onlyOwner external {
       _setBaseURI(baseURI);
    }

    /*************************************************************************** */


    /*************************************************************************** */
    //                             Internal functions:

    function _mint(address _to) internal notZeroAddress(_to) returns (uint256 _mintedTokenId) {
        require(totalMinted < MINTING_LIMIT, "allMinted");

        uint randomId = _tryGenerateRandomId();
        totalMinted++;

        if (randomId == 0) {
            randomId = _tryGenerateRandomId();
            totalMinted++;
        }

        if (randomId != 0) {
            _addToken(_to, randomId);

            emit Mint(randomId, msg.sender, _to);
            emit Transfer(addressZero, _to, randomId);
        }

        return randomId;
    }
    

    function _addToken(address _to, uint256 _tokenId) private notZeroAddress(_to) {
        tokenToOwner[_tokenId] = _to;
        ownerToTokensSet[_to].add(_tokenId);
    }
    

    function _removeToken(address _from, uint256 _tokenId) private {
        if (tokenToOwner[_tokenId] != _from)
            return;
        
        if (tokenToApproval[_tokenId] != addressZero)
            delete tokenToApproval[_tokenId];
        
        delete tokenToOwner[_tokenId];
        ownerToTokensSet[_from].remove(_tokenId);
    }


    function _transfer(address _from, address _to, uint256 _tokenId) private {
        require(tokenToOwner[_tokenId] == _from, "wrongFrom");

        _removeToken(_from, _tokenId);
        _addToken(_to, _tokenId);

        emit Transfer(_from, _to, _tokenId);
    }

    function _safeTransferFrom(address _from,  address _to,  uint256 _tokenId,  bytes memory _data) private 
        transferApproved(_tokenId) 
        validTokenId(_tokenId) 
        notZeroAddress(_to)
    {
        _transfer(_from, _to, _tokenId);

        if (isContract(_to)) {
            bytes4 retval = IERC721TokenReceiver(_to).onERC721Received(msg.sender, _from, _tokenId, _data);
            require(retval == MAGIC_ON_ERC721_RECEIVED);
        }
    }

    function _setBaseURI(string memory baseURI_) private {
        _baseURI = baseURI_;
    }

    /*************************************************************************** */



    /*************************************************************************** */
    //                             Admin functions: 

    function _enableMinting() external onlyOwner {
        if (!isMintingEnabled) {
            isMintingEnabled = true;
            emit MintingEnabled();
        }
    }

    function _disableMinting() external onlyOwner {
        if (isMintingEnabled) {
            isMintingEnabled = false;
            emit MintingDisabled();
        }
    }

    function _setMintingPrice(uint256 newPrice) external onlyOwner {
        mintingPrice = newPrice;
    }

    function _claimETC(uint256 amount) external onlyOwner {
        payable(msg.sender).transfer(amount);
    }

    function withdraw() external onlyOwner {
        uint balance = address(this).balance;
        payable(msg.sender).transfer(balance);
    }
    
    /*************************************************************************** */




    /*************************************************************************** */
    //                             Service:
    function isContract(address _addr) internal view returns (bool addressCheck) {
        uint256 size;
        assembly { size := extcodesize(_addr) } // solhint-disable-line
        addressCheck = size > 0;
    }


    function _tryGenerateRandomId() private returns (uint256) {
        uint256 randId = _generateRandomId();
        
        if (tokenToOwner[randId] == addressZero) {
            return randId;
        } else {
            return 0;
        }
    }


    function _generateRandomId() private returns (uint256) {
        uint256 totalSize = MINTING_LIMIT - totalMinted;
        uint256 index = uint256(keccak256(abi.encodePacked(nonce, msg.sender, block.difficulty, block.timestamp))) % totalSize;
        uint256 value = 0;

        if (indices[index] != 0) {
            value = indices[index];
        } else {
            value = index;
        }

        // Move last value to selected position
        if (indices[totalSize - 1] == 0) {
            indices[index] = totalSize - 1;    // Array position not initialized, so use position
        } else { 
            indices[index] = indices[totalSize - 1];   // Array position holds a value so use that
        }
        nonce++;
        // Don't allow a zero index, start counting at 1
        return value + 1;
    }

    /*************************************************************************** */



    /*************************************************************************** */
    //                             Modifiers: 

    modifier validTokenId(uint256 _tokenId) {
        require(tokenToOwner[_tokenId] != addressZero, "invalidTokenId");
        _;
    }

    modifier mintingEnabled() {
        require(isMintingEnabled, "mintDisabled");
        _;
    }

    modifier transferApproved(uint256 _tokenId) {
        address tokenOwner = tokenToOwner[_tokenId];

        require(
            tokenOwner == msg.sender  || 
            tokenToApproval[_tokenId] == msg.sender || 
            (ownerToOperators[tokenOwner][msg.sender] && tokenOwner != addressZero)
            , "notApproved"
        );

        _;
    }

    modifier canOperate(uint256 _tokenId) {
        address tokenOwner = tokenToOwner[_tokenId];

        require(
            tokenOwner == msg.sender || 
            (ownerToOperators[tokenOwner][msg.sender] && tokenOwner != addressZero)
            ,"operationNotApproved"
        );

        _;
    }

    modifier notZeroAddress(address _addr) {
        require(_addr != addressZero, "zeroAddress");
        _;
    }

    /*************************************************************************** */



    /*************************************************************************** */
    //                             Events: 

    // NFT minted
    event Mint(uint indexed tokenId, address indexed mintedBy, address indexed mintedTo);
    // ETC is deposited into the contract.
    event Deposit(address indexed account, uint amount);
    // ETC is withdrawn from the contract.
    event Withdraw(address indexed account, uint amount);

    event MintingEnabled();
    event MintingDisabled();
}

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