// File: contracts/PBAB/bright-moments/GenArt721CoreV2_BrightMoments.sol



// SPDX-License-Identifier: LGPL-3.0-only
// Creatd By: Art Blocks Inc.

pragma solidity ^0.5.0;

import "../../libs/0.5.x/CustomERC721Metadata.sol";
import "../../libs/0.5.x/Strings.sol";

interface IRandomizer {
    function returnValue() external view returns (bytes32);
}

contract GenArt721CoreV2_BrightMoments is CustomERC721Metadata {
    using SafeMath for uint256;

    event Mint(
        address indexed _to,
        uint256 indexed _tokenId,
        uint256 indexed _projectId
    );

    IRandomizer public randomizerContract;

    struct Project {
        string name;
        string artist;
        string description;
        string website;
        string license;
        string projectBaseURI;
        uint256 invocations;
        uint256 maxInvocations;
        string scriptJSON;
        mapping(uint256 => string) scripts;
        uint256 scriptCount;
        string ipfsHash;
        bool active;
        bool locked;
        bool paused;
    }

    uint256 constant ONE_MILLION = 1_000_000;
    mapping(uint256 => Project) projects;

    //All financial functions are stripped from struct for visibility
    mapping(uint256 => address) public projectIdToArtistAddress;
    mapping(uint256 => string) public projectIdToCurrencySymbol;
    mapping(uint256 => address) public projectIdToCurrencyAddress;
    mapping(uint256 => uint256) public projectIdToPricePerTokenInWei;
    mapping(uint256 => address) public projectIdToAdditionalPayee;
    mapping(uint256 => uint256) public projectIdToAdditionalPayeePercentage;
    mapping(uint256 => uint256)
        public projectIdToSecondaryMarketRoyaltyPercentage;

    address public renderProviderAddress;
    uint256 public renderProviderPercentage = 10;

    mapping(uint256 => uint256) public tokenIdToProjectId;
    mapping(uint256 => bytes32) public tokenIdToHash;
    mapping(bytes32 => uint256) public hashToTokenId;

    address public admin;
    mapping(address => bool) public isWhitelisted;
    mapping(address => bool) public isMintWhitelisted;

    uint256 public nextProjectId = 0;

    modifier onlyValidTokenId(uint256 _tokenId) {
        require(_exists(_tokenId), "Token ID does not exist");
        _;
    }

    modifier onlyUnlocked(uint256 _projectId) {
        require(!projects[_projectId].locked, "Only if unlocked");
        _;
    }

    modifier onlyArtist(uint256 _projectId) {
        require(
            msg.sender == projectIdToArtistAddress[_projectId],
            "Only artist"
        );
        _;
    }

    modifier onlyAdmin() {
        require(msg.sender == admin, "Only admin");
        _;
    }

    modifier onlyWhitelisted() {
        require(isWhitelisted[msg.sender], "Only whitelisted");
        _;
    }

    modifier onlyArtistOrWhitelisted(uint256 _projectId) {
        require(
            isWhitelisted[msg.sender] ||
                msg.sender == projectIdToArtistAddress[_projectId],
            "Only artist or whitelisted"
        );
        _;
    }

    constructor(
        string memory _tokenName,
        string memory _tokenSymbol,
        address _randomizerContract
    ) public CustomERC721Metadata(_tokenName, _tokenSymbol) {
        admin = msg.sender;
        isWhitelisted[msg.sender] = true;
        renderProviderAddress = msg.sender;
        randomizerContract = IRandomizer(_randomizerContract);
    }

    function mint(
        address _to,
        uint256 _projectId,
        address _by
    ) external returns (uint256 _tokenId) {
        require(
            isMintWhitelisted[msg.sender],
            "Must mint from whitelisted minter contract."
        );
        require(
            projects[_projectId].invocations.add(1) <=
                projects[_projectId].maxInvocations,
            "Must not exceed max invocations"
        );
        require(
            projects[_projectId].active ||
                _by == projectIdToArtistAddress[_projectId],
            "Project must exist and be active"
        );
        require(
            !projects[_projectId].paused ||
                _by == projectIdToArtistAddress[_projectId],
            "Purchases are paused."
        );

        uint256 tokenId = _mintToken(_to, _projectId);

        return tokenId;
    }

    function _mintToken(address _to, uint256 _projectId)
        internal
        returns (uint256 _tokenId)
    {
        uint256 tokenIdToBe = (_projectId * ONE_MILLION) +
            projects[_projectId].invocations;

        projects[_projectId].invocations = projects[_projectId].invocations.add(
            1
        );

        bytes32 hash = keccak256(
            abi.encodePacked(
                projects[_projectId].invocations,
                block.number,
                blockhash(block.number - 1),
                randomizerContract.returnValue()
            )
        );
        tokenIdToHash[tokenIdToBe] = hash;
        hashToTokenId[hash] = tokenIdToBe;

        _mint(_to, tokenIdToBe);

        tokenIdToProjectId[tokenIdToBe] = _projectId;

        emit Mint(_to, tokenIdToBe, _projectId);

        return tokenIdToBe;
    }

    function updateAdmin(address _adminAddress) public onlyAdmin {
        admin = _adminAddress;
    }

    function updateRenderProviderAddress(address _renderProviderAddress)
        public
        onlyAdmin
    {
        renderProviderAddress = _renderProviderAddress;
    }

    function updateRenderProviderPercentage(uint256 _renderProviderPercentage)
        public
        onlyAdmin
    {
        require(_renderProviderPercentage <= 25, "Max of 25%");
        renderProviderPercentage = _renderProviderPercentage;
    }

    function addWhitelisted(address _address) public onlyAdmin {
        isWhitelisted[_address] = true;
    }

    function removeWhitelisted(address _address) public onlyAdmin {
        isWhitelisted[_address] = false;
    }

    function addMintWhitelisted(address _address) public onlyAdmin {
        isMintWhitelisted[_address] = true;
    }

    function removeMintWhitelisted(address _address) public onlyAdmin {
        isMintWhitelisted[_address] = false;
    }

    function updateRandomizerAddress(address _randomizerAddress)
        public
        onlyWhitelisted
    {
        randomizerContract = IRandomizer(_randomizerAddress);
    }

    function toggleProjectIsLocked(uint256 _projectId)
        public
        onlyWhitelisted
        onlyUnlocked(_projectId)
    {
        projects[_projectId].locked = true;
    }

    function toggleProjectIsActive(uint256 _projectId) public onlyWhitelisted {
        projects[_projectId].active = !projects[_projectId].active;
    }

    function updateProjectArtistAddress(
        uint256 _projectId,
        address _artistAddress
    ) public onlyArtistOrWhitelisted(_projectId) {
        projectIdToArtistAddress[_projectId] = _artistAddress;
    }

    function toggleProjectIsPaused(uint256 _projectId)
        public
        onlyArtist(_projectId)
    {
        projects[_projectId].paused = !projects[_projectId].paused;
    }

    function addProject(
        string memory _projectName,
        address _artistAddress,
        uint256 _pricePerTokenInWei
    ) public onlyWhitelisted {
        uint256 projectId = nextProjectId;
        projectIdToArtistAddress[projectId] = _artistAddress;
        projects[projectId].name = _projectName;
        projectIdToCurrencySymbol[projectId] = "ETH";
        projectIdToPricePerTokenInWei[projectId] = _pricePerTokenInWei;
        projects[projectId].paused = true;
        projects[projectId].maxInvocations = ONE_MILLION;
        nextProjectId = nextProjectId.add(1);
    }

    function updateProjectCurrencyInfo(
        uint256 _projectId,
        string memory _currencySymbol,
        address _currencyAddress
    ) public onlyArtist(_projectId) {
        projectIdToCurrencySymbol[_projectId] = _currencySymbol;
        projectIdToCurrencyAddress[_projectId] = _currencyAddress;
    }

    function updateProjectPricePerTokenInWei(
        uint256 _projectId,
        uint256 _pricePerTokenInWei
    ) public onlyArtist(_projectId) {
        projectIdToPricePerTokenInWei[_projectId] = _pricePerTokenInWei;
    }

    function updateProjectName(uint256 _projectId, string memory _projectName)
        public
        onlyUnlocked(_projectId)
        onlyArtistOrWhitelisted(_projectId)
    {
        projects[_projectId].name = _projectName;
    }

    function updateProjectArtistName(
        uint256 _projectId,
        string memory _projectArtistName
    ) public onlyUnlocked(_projectId) onlyArtistOrWhitelisted(_projectId) {
        projects[_projectId].artist = _projectArtistName;
    }

    function updateProjectAdditionalPayeeInfo(
        uint256 _projectId,
        address _additionalPayee,
        uint256 _additionalPayeePercentage
    ) public onlyArtist(_projectId) {
        require(_additionalPayeePercentage <= 100, "Max of 100%");
        projectIdToAdditionalPayee[_projectId] = _additionalPayee;
        projectIdToAdditionalPayeePercentage[
            _projectId
        ] = _additionalPayeePercentage;
    }

    function updateProjectSecondaryMarketRoyaltyPercentage(
        uint256 _projectId,
        uint256 _secondMarketRoyalty
    ) public onlyArtist(_projectId) {
        require(_secondMarketRoyalty <= 100, "Max of 100%");
        projectIdToSecondaryMarketRoyaltyPercentage[
            _projectId
        ] = _secondMarketRoyalty;
    }

    function updateProjectDescription(
        uint256 _projectId,
        string memory _projectDescription
    ) public onlyArtist(_projectId) {
        projects[_projectId].description = _projectDescription;
    }

    function updateProjectWebsite(
        uint256 _projectId,
        string memory _projectWebsite
    ) public onlyArtist(_projectId) {
        projects[_projectId].website = _projectWebsite;
    }

    function updateProjectLicense(
        uint256 _projectId,
        string memory _projectLicense
    ) public onlyUnlocked(_projectId) onlyArtistOrWhitelisted(_projectId) {
        projects[_projectId].license = _projectLicense;
    }

    function updateProjectMaxInvocations(
        uint256 _projectId,
        uint256 _maxInvocations
    ) public onlyArtist(_projectId) {
        require(
            (!projects[_projectId].locked ||
                _maxInvocations < projects[_projectId].maxInvocations),
            "Only if unlocked"
        );
        require(
            _maxInvocations > projects[_projectId].invocations,
            "You must set max invocations greater than current invocations"
        );
        require(_maxInvocations <= ONE_MILLION, "Cannot exceed 1000000");
        projects[_projectId].maxInvocations = _maxInvocations;
    }

    function addProjectScript(uint256 _projectId, string memory _script)
        public
        onlyUnlocked(_projectId)
        onlyArtistOrWhitelisted(_projectId)
    {
        projects[_projectId].scripts[
            projects[_projectId].scriptCount
        ] = _script;
        projects[_projectId].scriptCount = projects[_projectId].scriptCount.add(
            1
        );
    }

    function updateProjectScript(
        uint256 _projectId,
        uint256 _scriptId,
        string memory _script
    ) public onlyUnlocked(_projectId) onlyArtistOrWhitelisted(_projectId) {
        require(
            _scriptId < projects[_projectId].scriptCount,
            "scriptId out of range"
        );
        projects[_projectId].scripts[_scriptId] = _script;
    }

    function removeProjectLastScript(uint256 _projectId)
        public
        onlyUnlocked(_projectId)
        onlyArtistOrWhitelisted(_projectId)
    {
        require(
            projects[_projectId].scriptCount > 0,
            "there are no scripts to remove"
        );
        delete projects[_projectId].scripts[
            projects[_projectId].scriptCount - 1
        ];
        projects[_projectId].scriptCount = projects[_projectId].scriptCount.sub(
            1
        );
    }

    function updateProjectScriptJSON(
        uint256 _projectId,
        string memory _projectScriptJSON
    ) public onlyUnlocked(_projectId) onlyArtistOrWhitelisted(_projectId) {
        projects[_projectId].scriptJSON = _projectScriptJSON;
    }

    function updateProjectIpfsHash(uint256 _projectId, string memory _ipfsHash)
        public
        onlyUnlocked(_projectId)
        onlyArtistOrWhitelisted(_projectId)
    {
        projects[_projectId].ipfsHash = _ipfsHash;
    }

    function updateProjectBaseURI(uint256 _projectId, string memory _newBaseURI)
        public
        onlyArtist(_projectId)
    {
        projects[_projectId].projectBaseURI = _newBaseURI;
    }

    function projectDetails(uint256 _projectId)
        public
        view
        returns (
            string memory projectName,
            string memory artist,
            string memory description,
            string memory website,
            string memory license
        )
    {
        projectName = projects[_projectId].name;
        artist = projects[_projectId].artist;
        description = projects[_projectId].description;
        website = projects[_projectId].website;
        license = projects[_projectId].license;
    }

    function projectTokenInfo(uint256 _projectId)
        public
        view
        returns (
            address artistAddress,
            uint256 pricePerTokenInWei,
            uint256 invocations,
            uint256 maxInvocations,
            bool active,
            address additionalPayee,
            uint256 additionalPayeePercentage,
            string memory currency,
            address currencyAddress
        )
    {
        artistAddress = projectIdToArtistAddress[_projectId];
        pricePerTokenInWei = projectIdToPricePerTokenInWei[_projectId];
        invocations = projects[_projectId].invocations;
        maxInvocations = projects[_projectId].maxInvocations;
        active = projects[_projectId].active;
        additionalPayee = projectIdToAdditionalPayee[_projectId];
        additionalPayeePercentage = projectIdToAdditionalPayeePercentage[
            _projectId
        ];
        currency = projectIdToCurrencySymbol[_projectId];
        currencyAddress = projectIdToCurrencyAddress[_projectId];
    }

    function projectScriptInfo(uint256 _projectId)
        public
        view
        returns (
            string memory scriptJSON,
            uint256 scriptCount,
            string memory ipfsHash,
            bool locked,
            bool paused
        )
    {
        scriptJSON = projects[_projectId].scriptJSON;
        scriptCount = projects[_projectId].scriptCount;
        ipfsHash = projects[_projectId].ipfsHash;
        locked = projects[_projectId].locked;
        paused = projects[_projectId].paused;
    }

    function projectScriptByIndex(uint256 _projectId, uint256 _index)
        public
        view
        returns (string memory)
    {
        return projects[_projectId].scripts[_index];
    }

    function projectURIInfo(uint256 _projectId)
        public
        view
        returns (string memory projectBaseURI)
    {
        projectBaseURI = projects[_projectId].projectBaseURI;
    }

    function tokensOfOwner(address owner)
        external
        view
        returns (uint256[] memory)
    {
        return _tokensOfOwner(owner);
    }

    function getRoyaltyData(uint256 _tokenId)
        public
        view
        returns (
            address artistAddress,
            address additionalPayee,
            uint256 additionalPayeePercentage,
            uint256 royaltyFeeByID
        )
    {
        artistAddress = projectIdToArtistAddress[tokenIdToProjectId[_tokenId]];
        additionalPayee = projectIdToAdditionalPayee[
            tokenIdToProjectId[_tokenId]
        ];
        additionalPayeePercentage = projectIdToAdditionalPayeePercentage[
            tokenIdToProjectId[_tokenId]
        ];
        royaltyFeeByID = projectIdToSecondaryMarketRoyaltyPercentage[
            tokenIdToProjectId[_tokenId]
        ];
    }

    function tokenURI(uint256 _tokenId)
        external
        view
        onlyValidTokenId(_tokenId)
        returns (string memory)
    {
        return
            Strings.strConcat(
                projects[tokenIdToProjectId[_tokenId]].projectBaseURI,
                Strings.uint2str(_tokenId)
            );
    }
}

// File: contracts/libs/0.5.x/CustomERC721Metadata.sol



// SPDX-License-Identifier: LGPL-3.0-only
// Created By: Art Blocks Inc.

// **Please Note**: This library is not necessary for ^0.8.0 contracts and thus
//                  only exists for posterity reasons for compatibility with
//                  existing already-deployed ^0.5.0 PBAB contracts.

pragma solidity ^0.5.0;

import "@openzeppelin-0.5/contracts/introspection/ERC165.sol";
import "@openzeppelin-0.5/contracts/token/ERC721/ERC721.sol";
import "@openzeppelin-0.5/contracts/token/ERC721/ERC721Enumerable.sol";

/**
 * ERC721 base contract without the concept of tokenUri as this is managed by the parent
 */
contract CustomERC721Metadata is ERC165, ERC721, ERC721Enumerable {
    // Token name
    string private _name;

    // Token symbol
    string private _symbol;

    bytes4 private constant _INTERFACE_ID_ERC721_METADATA = 0x5b5e139f;

    /**
     * @dev Constructor function
     */
    constructor(string memory name, string memory symbol) public {
        _name = name;
        _symbol = symbol;

        // register the supported interfaces to conform to ERC721 via ERC165
        _registerInterface(_INTERFACE_ID_ERC721_METADATA);
    }

    /**
     * @dev Gets the token name
     * @return string representing the token name
     */
    function name() external view returns (string memory) {
        return _name;
    }

    /**
     * @dev Gets the token symbol
     * @return string representing the token symbol
     */
    function symbol() external view returns (string memory) {
        return _symbol;
    }
}

// File: contracts/libs/0.5.x/Strings.sol



// SPDX-License-Identifier: MIT
// File: contracts/Strings.sol
// Source: https://github.com/oraclize/ethereum-api/blob/master/oraclizeAPI_0.5.sol

// **Please Note**: This library is not necessary for ^0.8.0 contracts and thus
//                  only exists for posterity reasons for compatibility with
//                  existing already-deployed ^0.5.0 PBAB contracts.

pragma solidity ^0.5.0;

library Strings {
    function strConcat(string memory _a, string memory _b)
        internal
        pure
        returns (string memory _concatenatedString)
    {
        return strConcat(_a, _b, "", "", "");
    }

    function strConcat(
        string memory _a,
        string memory _b,
        string memory _c
    ) internal pure returns (string memory _concatenatedString) {
        return strConcat(_a, _b, _c, "", "");
    }

    function strConcat(
        string memory _a,
        string memory _b,
        string memory _c,
        string memory _d
    ) internal pure returns (string memory _concatenatedString) {
        return strConcat(_a, _b, _c, _d, "");
    }

    function strConcat(
        string memory _a,
        string memory _b,
        string memory _c,
        string memory _d,
        string memory _e
    ) internal pure returns (string memory _concatenatedString) {
        bytes memory _ba = bytes(_a);
        bytes memory _bb = bytes(_b);
        bytes memory _bc = bytes(_c);
        bytes memory _bd = bytes(_d);
        bytes memory _be = bytes(_e);
        string memory abcde = new string(
            _ba.length + _bb.length + _bc.length + _bd.length + _be.length
        );
        bytes memory babcde = bytes(abcde);
        uint256 k = 0;
        uint256 i = 0;
        for (i = 0; i < _ba.length; i++) {
            babcde[k++] = _ba[i];
        }
        for (i = 0; i < _bb.length; i++) {
            babcde[k++] = _bb[i];
        }
        for (i = 0; i < _bc.length; i++) {
            babcde[k++] = _bc[i];
        }
        for (i = 0; i < _bd.length; i++) {
            babcde[k++] = _bd[i];
        }
        for (i = 0; i < _be.length; i++) {
            babcde[k++] = _be[i];
        }
        return string(babcde);
    }

    function uint2str(uint256 _i)
        internal
        pure
        returns (string memory _uintAsString)
    {
        if (_i == 0) {
            return "0";
        }
        uint256 j = _i;
        uint256 len;
        while (j != 0) {
            len++;
            j /= 10;
        }
        bytes memory bstr = new bytes(len);
        uint256 k = len - 1;
        while (_i != 0) {
            bstr[k--] = bytes1(uint8(48 + (_i % 10)));
            _i /= 10;
        }
        return string(bstr);
    }
}

// File: @openzeppelin-0.5/contracts/introspection/ERC165.sol



pragma solidity ^0.5.0;

import "./IERC165.sol";

/**
 * @dev Implementation of the {IERC165} interface.
 *
 * Contracts may inherit from this and call {_registerInterface} to declare
 * their support of an interface.
 */
contract ERC165 is IERC165 {
    /*
     * bytes4(keccak256('supportsInterface(bytes4)')) == 0x01ffc9a7
     */
    bytes4 private constant _INTERFACE_ID_ERC165 = 0x01ffc9a7;

    /**
     * @dev Mapping of interface ids to whether or not it's supported.
     */
    mapping(bytes4 => bool) private _supportedInterfaces;

    constructor () internal {
        // Derived contracts need only register support for their own interfaces,
        // we register support for ERC165 itself here
        _registerInterface(_INTERFACE_ID_ERC165);
    }

    /**
     * @dev See {IERC165-supportsInterface}.
     *
     * Time complexity O(1), guaranteed to always use less than 30 000 gas.
     */
    function supportsInterface(bytes4 interfaceId) external view returns (bool) {
        return _supportedInterfaces[interfaceId];
    }

    /**
     * @dev Registers the contract as an implementer of the interface defined by
     * `interfaceId`. Support of the actual ERC165 interface is automatic and
     * registering its interface id is not required.
     *
     * See {IERC165-supportsInterface}.
     *
     * Requirements:
     *
     * - `interfaceId` cannot be the ERC165 invalid interface (`0xffffffff`).
     */
    function _registerInterface(bytes4 interfaceId) internal {
        require(interfaceId != 0xffffffff, "ERC165: invalid interface id");
        _supportedInterfaces[interfaceId] = true;
    }
}

// File: @openzeppelin-0.5/contracts/token/ERC721/ERC721.sol



pragma solidity ^0.5.0;

import "../../GSN/Context.sol";
import "./IERC721.sol";
import "./IERC721Receiver.sol";
import "../../math/SafeMath.sol";
import "../../utils/Address.sol";
import "../../drafts/Counters.sol";
import "../../introspection/ERC165.sol";

/**
 * @title ERC721 Non-Fungible Token Standard basic implementation
 * @dev see https://eips.ethereum.org/EIPS/eip-721
 */
contract ERC721 is Context, ERC165, IERC721 {
    using SafeMath for uint256;
    using Address for address;
    using Counters for Counters.Counter;

    // Equals to `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`
    // which can be also obtained as `IERC721Receiver(0).onERC721Received.selector`
    bytes4 private constant _ERC721_RECEIVED = 0x150b7a02;

    // Mapping from token ID to owner
    mapping (uint256 => address) private _tokenOwner;

    // Mapping from token ID to approved address
    mapping (uint256 => address) private _tokenApprovals;

    // Mapping from owner to number of owned token
    mapping (address => Counters.Counter) private _ownedTokensCount;

    // Mapping from owner to operator approvals
    mapping (address => mapping (address => bool)) private _operatorApprovals;

    /*
     *     bytes4(keccak256('balanceOf(address)')) == 0x70a08231
     *     bytes4(keccak256('ownerOf(uint256)')) == 0x6352211e
     *     bytes4(keccak256('approve(address,uint256)')) == 0x095ea7b3
     *     bytes4(keccak256('getApproved(uint256)')) == 0x081812fc
     *     bytes4(keccak256('setApprovalForAll(address,bool)')) == 0xa22cb465
     *     bytes4(keccak256('isApprovedForAll(address,address)')) == 0xe985e9c5
     *     bytes4(keccak256('transferFrom(address,address,uint256)')) == 0x23b872dd
     *     bytes4(keccak256('safeTransferFrom(address,address,uint256)')) == 0x42842e0e
     *     bytes4(keccak256('safeTransferFrom(address,address,uint256,bytes)')) == 0xb88d4fde
     *
     *     => 0x70a08231 ^ 0x6352211e ^ 0x095ea7b3 ^ 0x081812fc ^
     *        0xa22cb465 ^ 0xe985e9c ^ 0x23b872dd ^ 0x42842e0e ^ 0xb88d4fde == 0x80ac58cd
     */
    bytes4 private constant _INTERFACE_ID_ERC721 = 0x80ac58cd;

    constructor () public {
        // register the supported interfaces to conform to ERC721 via ERC165
        _registerInterface(_INTERFACE_ID_ERC721);
    }

    /**
     * @dev Gets the balance of the specified address.
     * @param owner address to query the balance of
     * @return uint256 representing the amount owned by the passed address
     */
    function balanceOf(address owner) public view returns (uint256) {
        require(owner != address(0), "ERC721: balance query for the zero address");

        return _ownedTokensCount[owner].current();
    }

    /**
     * @dev Gets the owner of the specified token ID.
     * @param tokenId uint256 ID of the token to query the owner of
     * @return address currently marked as the owner of the given token ID
     */
    function ownerOf(uint256 tokenId) public view returns (address) {
        address owner = _tokenOwner[tokenId];
        require(owner != address(0), "ERC721: owner query for nonexistent token");

        return owner;
    }

    /**
     * @dev Approves another address to transfer the given token ID
     * The zero address indicates there is no approved address.
     * There can only be one approved address per token at a given time.
     * Can only be called by the token owner or an approved operator.
     * @param to address to be approved for the given token ID
     * @param tokenId uint256 ID of the token to be approved
     */
    function approve(address to, uint256 tokenId) public {
        address owner = ownerOf(tokenId);
        require(to != owner, "ERC721: approval to current owner");

        require(_msgSender() == owner || isApprovedForAll(owner, _msgSender()),
            "ERC721: approve caller is not owner nor approved for all"
        );

        _tokenApprovals[tokenId] = to;
        emit Approval(owner, to, tokenId);
    }

    /**
     * @dev Gets the approved address for a token ID, or zero if no address set
     * Reverts if the token ID does not exist.
     * @param tokenId uint256 ID of the token to query the approval of
     * @return address currently approved for the given token ID
     */
    function getApproved(uint256 tokenId) public view returns (address) {
        require(_exists(tokenId), "ERC721: approved query for nonexistent token");

        return _tokenApprovals[tokenId];
    }

    /**
     * @dev Sets or unsets the approval of a given operator
     * An operator is allowed to transfer all tokens of the sender on their behalf.
     * @param to operator address to set the approval
     * @param approved representing the status of the approval to be set
     */
    function setApprovalForAll(address to, bool approved) public {
        require(to != _msgSender(), "ERC721: approve to caller");

        _operatorApprovals[_msgSender()][to] = approved;
        emit ApprovalForAll(_msgSender(), to, approved);
    }

    /**
     * @dev Tells whether an operator is approved by a given owner.
     * @param owner owner address which you want to query the approval of
     * @param operator operator address which you want to query the approval of
     * @return bool whether the given operator is approved by the given owner
     */
    function isApprovedForAll(address owner, address operator) public view returns (bool) {
        return _operatorApprovals[owner][operator];
    }

    /**
     * @dev Transfers the ownership of a given token ID to another address.
     * Usage of this method is discouraged, use {safeTransferFrom} whenever possible.
     * Requires the msg.sender to be the owner, approved, or operator.
     * @param from current owner of the token
     * @param to address to receive the ownership of the given token ID
     * @param tokenId uint256 ID of the token to be transferred
     */
    function transferFrom(address from, address to, uint256 tokenId) public {
        //solhint-disable-next-line max-line-length
        require(_isApprovedOrOwner(_msgSender(), tokenId), "ERC721: transfer caller is not owner nor approved");

        _transferFrom(from, to, tokenId);
    }

    /**
     * @dev Safely transfers the ownership of a given token ID to another address
     * If the target address is a contract, it must implement {IERC721Receiver-onERC721Received},
     * which is called upon a safe transfer, and return the magic value
     * `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`; otherwise,
     * the transfer is reverted.
     * Requires the msg.sender to be the owner, approved, or operator
     * @param from current owner of the token
     * @param to address to receive the ownership of the given token ID
     * @param tokenId uint256 ID of the token to be transferred
     */
    function safeTransferFrom(address from, address to, uint256 tokenId) public {
        safeTransferFrom(from, to, tokenId, "");
    }

    /**
     * @dev Safely transfers the ownership of a given token ID to another address
     * If the target address is a contract, it must implement {IERC721Receiver-onERC721Received},
     * which is called upon a safe transfer, and return the magic value
     * `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`; otherwise,
     * the transfer is reverted.
     * Requires the _msgSender() to be the owner, approved, or operator
     * @param from current owner of the token
     * @param to address to receive the ownership of the given token ID
     * @param tokenId uint256 ID of the token to be transferred
     * @param _data bytes data to send along with a safe transfer check
     */
    function safeTransferFrom(address from, address to, uint256 tokenId, bytes memory _data) public {
        require(_isApprovedOrOwner(_msgSender(), tokenId), "ERC721: transfer caller is not owner nor approved");
        _safeTransferFrom(from, to, tokenId, _data);
    }

    /**
     * @dev Safely transfers the ownership of a given token ID to another address
     * If the target address is a contract, it must implement `onERC721Received`,
     * which is called upon a safe transfer, and return the magic value
     * `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`; otherwise,
     * the transfer is reverted.
     * Requires the msg.sender to be the owner, approved, or operator
     * @param from current owner of the token
     * @param to address to receive the ownership of the given token ID
     * @param tokenId uint256 ID of the token to be transferred
     * @param _data bytes data to send along with a safe transfer check
     */
    function _safeTransferFrom(address from, address to, uint256 tokenId, bytes memory _data) internal {
        _transferFrom(from, to, tokenId);
        require(_checkOnERC721Received(from, to, tokenId, _data), "ERC721: transfer to non ERC721Receiver implementer");
    }

    /**
     * @dev Returns whether the specified token exists.
     * @param tokenId uint256 ID of the token to query the existence of
     * @return bool whether the token exists
     */
    function _exists(uint256 tokenId) internal view returns (bool) {
        address owner = _tokenOwner[tokenId];
        return owner != address(0);
    }

    /**
     * @dev Returns whether the given spender can transfer a given token ID.
     * @param spender address of the spender to query
     * @param tokenId uint256 ID of the token to be transferred
     * @return bool whether the msg.sender is approved for the given token ID,
     * is an operator of the owner, or is the owner of the token
     */
    function _isApprovedOrOwner(address spender, uint256 tokenId) internal view returns (bool) {
        require(_exists(tokenId), "ERC721: operator query for nonexistent token");
        address owner = ownerOf(tokenId);
        return (spender == owner || getApproved(tokenId) == spender || isApprovedForAll(owner, spender));
    }

    /**
     * @dev Internal function to safely mint a new token.
     * Reverts if the given token ID already exists.
     * If the target address is a contract, it must implement `onERC721Received`,
     * which is called upon a safe transfer, and return the magic value
     * `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`; otherwise,
     * the transfer is reverted.
     * @param to The address that will own the minted token
     * @param tokenId uint256 ID of the token to be minted
     */
    function _safeMint(address to, uint256 tokenId) internal {
        _safeMint(to, tokenId, "");
    }

    /**
     * @dev Internal function to safely mint a new token.
     * Reverts if the given token ID already exists.
     * If the target address is a contract, it must implement `onERC721Received`,
     * which is called upon a safe transfer, and return the magic value
     * `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`; otherwise,
     * the transfer is reverted.
     * @param to The address that will own the minted token
     * @param tokenId uint256 ID of the token to be minted
     * @param _data bytes data to send along with a safe transfer check
     */
    function _safeMint(address to, uint256 tokenId, bytes memory _data) internal {
        _mint(to, tokenId);
        require(_checkOnERC721Received(address(0), to, tokenId, _data), "ERC721: transfer to non ERC721Receiver implementer");
    }

    /**
     * @dev Internal function to mint a new token.
     * Reverts if the given token ID already exists.
     * @param to The address that will own the minted token
     * @param tokenId uint256 ID of the token to be minted
     */
    function _mint(address to, uint256 tokenId) internal {
        require(to != address(0), "ERC721: mint to the zero address");
        require(!_exists(tokenId), "ERC721: token already minted");

        _tokenOwner[tokenId] = to;
        _ownedTokensCount[to].increment();

        emit Transfer(address(0), to, tokenId);
    }

    /**
     * @dev Internal function to burn a specific token.
     * Reverts if the token does not exist.
     * Deprecated, use {_burn} instead.
     * @param owner owner of the token to burn
     * @param tokenId uint256 ID of the token being burned
     */
    function _burn(address owner, uint256 tokenId) internal {
        require(ownerOf(tokenId) == owner, "ERC721: burn of token that is not own");

        _clearApproval(tokenId);

        _ownedTokensCount[owner].decrement();
        _tokenOwner[tokenId] = address(0);

        emit Transfer(owner, address(0), tokenId);
    }

    /**
     * @dev Internal function to burn a specific token.
     * Reverts if the token does not exist.
     * @param tokenId uint256 ID of the token being burned
     */
    function _burn(uint256 tokenId) internal {
        _burn(ownerOf(tokenId), tokenId);
    }

    /**
     * @dev Internal function to transfer ownership of a given token ID to another address.
     * As opposed to {transferFrom}, this imposes no restrictions on msg.sender.
     * @param from current owner of the token
     * @param to address to receive the ownership of the given token ID
     * @param tokenId uint256 ID of the token to be transferred
     */
    function _transferFrom(address from, address to, uint256 tokenId) internal {
        require(ownerOf(tokenId) == from, "ERC721: transfer of token that is not own");
        require(to != address(0), "ERC721: transfer to the zero address");

        _clearApproval(tokenId);

        _ownedTokensCount[from].decrement();
        _ownedTokensCount[to].increment();

        _tokenOwner[tokenId] = to;

        emit Transfer(from, to, tokenId);
    }

    /**
     * @dev Internal function to invoke {IERC721Receiver-onERC721Received} on a target address.
     * The call is not executed if the target address is not a contract.
     *
     * This is an internal detail of the `ERC721` contract and its use is deprecated.
     * @param from address representing the previous owner of the given token ID
     * @param to target address that will receive the tokens
     * @param tokenId uint256 ID of the token to be transferred
     * @param _data bytes optional data to send along with the call
     * @return bool whether the call correctly returned the expected magic value
     */
    function _checkOnERC721Received(address from, address to, uint256 tokenId, bytes memory _data)
        internal returns (bool)
    {
        if (!to.isContract()) {
            return true;
        }
        // solhint-disable-next-line avoid-low-level-calls
        (bool success, bytes memory returndata) = to.call(abi.encodeWithSelector(
            IERC721Receiver(to).onERC721Received.selector,
            _msgSender(),
            from,
            tokenId,
            _data
        ));
        if (!success) {
            if (returndata.length > 0) {
                // solhint-disable-next-line no-inline-assembly
                assembly {
                    let returndata_size := mload(returndata)
                    revert(add(32, returndata), returndata_size)
                }
            } else {
                revert("ERC721: transfer to non ERC721Receiver implementer");
            }
        } else {
            bytes4 retval = abi.decode(returndata, (bytes4));
            return (retval == _ERC721_RECEIVED);
        }
    }

    /**
     * @dev Private function to clear current approval of a given token ID.
     * @param tokenId uint256 ID of the token to be transferred
     */
    function _clearApproval(uint256 tokenId) private {
        if (_tokenApprovals[tokenId] != address(0)) {
            _tokenApprovals[tokenId] = address(0);
        }
    }
}

// File: @openzeppelin-0.5/contracts/token/ERC721/ERC721Enumerable.sol



pragma solidity ^0.5.0;

import "../../GSN/Context.sol";
import "./IERC721Enumerable.sol";
import "./ERC721.sol";
import "../../introspection/ERC165.sol";

/**
 * @title ERC-721 Non-Fungible Token with optional enumeration extension logic
 * @dev See https://eips.ethereum.org/EIPS/eip-721
 */
contract ERC721Enumerable is Context, ERC165, ERC721, IERC721Enumerable {
    // Mapping from owner to list of owned token IDs
    mapping(address => uint256[]) private _ownedTokens;

    // Mapping from token ID to index of the owner tokens list
    mapping(uint256 => uint256) private _ownedTokensIndex;

    // Array with all token ids, used for enumeration
    uint256[] private _allTokens;

    // Mapping from token id to position in the allTokens array
    mapping(uint256 => uint256) private _allTokensIndex;

    /*
     *     bytes4(keccak256('totalSupply()')) == 0x18160ddd
     *     bytes4(keccak256('tokenOfOwnerByIndex(address,uint256)')) == 0x2f745c59
     *     bytes4(keccak256('tokenByIndex(uint256)')) == 0x4f6ccce7
     *
     *     => 0x18160ddd ^ 0x2f745c59 ^ 0x4f6ccce7 == 0x780e9d63
     */
    bytes4 private constant _INTERFACE_ID_ERC721_ENUMERABLE = 0x780e9d63;

    /**
     * @dev Constructor function.
     */
    constructor () public {
        // register the supported interface to conform to ERC721Enumerable via ERC165
        _registerInterface(_INTERFACE_ID_ERC721_ENUMERABLE);
    }

    /**
     * @dev Gets the token ID at a given index of the tokens list of the requested owner.
     * @param owner address owning the tokens list to be accessed
     * @param index uint256 representing the index to be accessed of the requested tokens list
     * @return uint256 token ID at the given index of the tokens list owned by the requested address
     */
    function tokenOfOwnerByIndex(address owner, uint256 index) public view returns (uint256) {
        require(index < balanceOf(owner), "ERC721Enumerable: owner index out of bounds");
        return _ownedTokens[owner][index];
    }

    /**
     * @dev Gets the total amount of tokens stored by the contract.
     * @return uint256 representing the total amount of tokens
     */
    function totalSupply() public view returns (uint256) {
        return _allTokens.length;
    }

    /**
     * @dev Gets the token ID at a given index of all the tokens in this contract
     * Reverts if the index is greater or equal to the total number of tokens.
     * @param index uint256 representing the index to be accessed of the tokens list
     * @return uint256 token ID at the given index of the tokens list
     */
    function tokenByIndex(uint256 index) public view returns (uint256) {
        require(index < totalSupply(), "ERC721Enumerable: global index out of bounds");
        return _allTokens[index];
    }

    /**
     * @dev Internal function to transfer ownership of a given token ID to another address.
     * As opposed to transferFrom, this imposes no restrictions on msg.sender.
     * @param from current owner of the token
     * @param to address to receive the ownership of the given token ID
     * @param tokenId uint256 ID of the token to be transferred
     */
    function _transferFrom(address from, address to, uint256 tokenId) internal {
        super._transferFrom(from, to, tokenId);

        _removeTokenFromOwnerEnumeration(from, tokenId);

        _addTokenToOwnerEnumeration(to, tokenId);
    }

    /**
     * @dev Internal function to mint a new token.
     * Reverts if the given token ID already exists.
     * @param to address the beneficiary that will own the minted token
     * @param tokenId uint256 ID of the token to be minted
     */
    function _mint(address to, uint256 tokenId) internal {
        super._mint(to, tokenId);

        _addTokenToOwnerEnumeration(to, tokenId);

        _addTokenToAllTokensEnumeration(tokenId);
    }

    /**
     * @dev Internal function to burn a specific token.
     * Reverts if the token does not exist.
     * Deprecated, use {ERC721-_burn} instead.
     * @param owner owner of the token to burn
     * @param tokenId uint256 ID of the token being burned
     */
    function _burn(address owner, uint256 tokenId) internal {
        super._burn(owner, tokenId);

        _removeTokenFromOwnerEnumeration(owner, tokenId);
        // Since tokenId will be deleted, we can clear its slot in _ownedTokensIndex to trigger a gas refund
        _ownedTokensIndex[tokenId] = 0;

        _removeTokenFromAllTokensEnumeration(tokenId);
    }

    /**
     * @dev Gets the list of token IDs of the requested owner.
     * @param owner address owning the tokens
     * @return uint256[] List of token IDs owned by the requested address
     */
    function _tokensOfOwner(address owner) internal view returns (uint256[] storage) {
        return _ownedTokens[owner];
    }

    /**
     * @dev Private function to add a token to this extension's ownership-tracking data structures.
     * @param to address representing the new owner of the given token ID
     * @param tokenId uint256 ID of the token to be added to the tokens list of the given address
     */
    function _addTokenToOwnerEnumeration(address to, uint256 tokenId) private {
        _ownedTokensIndex[tokenId] = _ownedTokens[to].length;
        _ownedTokens[to].push(tokenId);
    }

    /**
     * @dev Private function to add a token to this extension's token tracking data structures.
     * @param tokenId uint256 ID of the token to be added to the tokens list
     */
    function _addTokenToAllTokensEnumeration(uint256 tokenId) private {
        _allTokensIndex[tokenId] = _allTokens.length;
        _allTokens.push(tokenId);
    }

    /**
     * @dev Private function to remove a token from this extension's ownership-tracking data structures. Note that
     * while the token is not assigned a new owner, the `_ownedTokensIndex` mapping is _not_ updated: this allows for
     * gas optimizations e.g. when performing a transfer operation (avoiding double writes).
     * This has O(1) time complexity, but alters the order of the _ownedTokens array.
     * @param from address representing the previous owner of the given token ID
     * @param tokenId uint256 ID of the token to be removed from the tokens list of the given address
     */
    function _removeTokenFromOwnerEnumeration(address from, uint256 tokenId) private {
        // To prevent a gap in from's tokens array, we store the last token in the index of the token to delete, and
        // then delete the last slot (swap and pop).

        uint256 lastTokenIndex = _ownedTokens[from].length.sub(1);
        uint256 tokenIndex = _ownedTokensIndex[tokenId];

        // When the token to delete is the last token, the swap operation is unnecessary
        if (tokenIndex != lastTokenIndex) {
            uint256 lastTokenId = _ownedTokens[from][lastTokenIndex];

            _ownedTokens[from][tokenIndex] = lastTokenId; // Move the last token to the slot of the to-delete token
            _ownedTokensIndex[lastTokenId] = tokenIndex; // Update the moved token's index
        }

        // This also deletes the contents at the last position of the array
        _ownedTokens[from].length--;

        // Note that _ownedTokensIndex[tokenId] hasn't been cleared: it still points to the old slot (now occupied by
        // lastTokenId, or just over the end of the array if the token was the last one).
    }

    /**
     * @dev Private function to remove a token from this extension's token tracking data structures.
     * This has O(1) time complexity, but alters the order of the _allTokens array.
     * @param tokenId uint256 ID of the token to be removed from the tokens list
     */
    function _removeTokenFromAllTokensEnumeration(uint256 tokenId) private {
        // To prevent a gap in the tokens array, we store the last token in the index of the token to delete, and
        // then delete the last slot (swap and pop).

        uint256 lastTokenIndex = _allTokens.length.sub(1);
        uint256 tokenIndex = _allTokensIndex[tokenId];

        // When the token to delete is the last token, the swap operation is unnecessary. However, since this occurs so
        // rarely (when the last minted token is burnt) that we still do the swap here to avoid the gas cost of adding
        // an 'if' statement (like in _removeTokenFromOwnerEnumeration)
        uint256 lastTokenId = _allTokens[lastTokenIndex];

        _allTokens[tokenIndex] = lastTokenId; // Move the last token to the slot of the to-delete token
        _allTokensIndex[lastTokenId] = tokenIndex; // Update the moved token's index

        // This also deletes the contents at the last position of the array
        _allTokens.length--;
        _allTokensIndex[tokenId] = 0;
    }
}

// File: @openzeppelin-0.5/contracts/introspection/IERC165.sol



pragma solidity ^0.5.0;

/**
 * @dev Interface of the ERC165 standard, as defined in the
 * https://eips.ethereum.org/EIPS/eip-165[EIP].
 *
 * Implementers can declare support of contract interfaces, which can then be
 * queried by others ({ERC165Checker}).
 *
 * For an implementation, see {ERC165}.
 */
interface IERC165 {
    /**
     * @dev Returns true if this contract implements the interface defined by
     * `interfaceId`. See the corresponding
     * https://eips.ethereum.org/EIPS/eip-165#how-interfaces-are-identified[EIP section]
     * to learn more about how these ids are created.
     *
     * This function call must use less than 30 000 gas.
     */
    function supportsInterface(bytes4 interfaceId) external view returns (bool);
}

// File: @openzeppelin-0.5/contracts/GSN/Context.sol



pragma solidity ^0.5.0;

/*
 * @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 GSN 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.
 */
contract Context {
    // Empty internal constructor, to prevent people from mistakenly deploying
    // an instance of this contract, which should be used via inheritance.
    constructor () internal { }
    // solhint-disable-previous-line no-empty-blocks

    function _msgSender() internal view returns (address payable) {
        return msg.sender;
    }

    function _msgData() internal view returns (bytes memory) {
        this; // silence state mutability warning without generating bytecode - see https://github.com/ethereum/solidity/issues/2691
        return msg.data;
    }
}

// File: @openzeppelin-0.5/contracts/token/ERC721/IERC721.sol



pragma solidity ^0.5.0;

import "../../introspection/IERC165.sol";

/**
 * @dev Required interface of an ERC721 compliant contract.
 */
contract IERC721 is IERC165 {
    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);

    /**
     * @dev Returns the number of NFTs in `owner`'s account.
     */
    function balanceOf(address owner) public view returns (uint256 balance);

    /**
     * @dev Returns the owner of the NFT specified by `tokenId`.
     */
    function ownerOf(uint256 tokenId) public view returns (address owner);

    /**
     * @dev Transfers a specific NFT (`tokenId`) from one account (`from`) to
     * another (`to`).
     *
     *
     *
     * Requirements:
     * - `from`, `to` cannot be zero.
     * - `tokenId` must be owned by `from`.
     * - If the caller is not `from`, it must be have been allowed to move this
     * NFT by either {approve} or {setApprovalForAll}.
     */
    function safeTransferFrom(address from, address to, uint256 tokenId) public;
    /**
     * @dev Transfers a specific NFT (`tokenId`) from one account (`from`) to
     * another (`to`).
     *
     * Requirements:
     * - If the caller is not `from`, it must be approved to move this NFT by
     * either {approve} or {setApprovalForAll}.
     */
    function transferFrom(address from, address to, uint256 tokenId) public;
    function approve(address to, uint256 tokenId) public;
    function getApproved(uint256 tokenId) public view returns (address operator);

    function setApprovalForAll(address operator, bool _approved) public;
    function isApprovedForAll(address owner, address operator) public view returns (bool);


    function safeTransferFrom(address from, address to, uint256 tokenId, bytes memory data) public;
}

// File: @openzeppelin-0.5/contracts/token/ERC721/IERC721Receiver.sol



pragma solidity ^0.5.0;

/**
 * @title ERC721 token receiver interface
 * @dev Interface for any contract that wants to support safeTransfers
 * from ERC721 asset contracts.
 */
contract IERC721Receiver {
    /**
     * @notice Handle the receipt of an NFT
     * @dev The ERC721 smart contract calls this function on the recipient
     * after a {IERC721-safeTransferFrom}. This function MUST return the function selector,
     * otherwise the caller will revert the transaction. The selector to be
     * returned can be obtained as `this.onERC721Received.selector`. This
     * function MAY throw to revert and reject the transfer.
     * Note: the ERC721 contract address is always the message sender.
     * @param operator The address which called `safeTransferFrom` function
     * @param from The address which previously owned the token
     * @param tokenId The NFT identifier which is being transferred
     * @param data Additional data with no specified format
     * @return bytes4 `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`
     */
    function onERC721Received(address operator, address from, uint256 tokenId, bytes memory data)
    public returns (bytes4);
}

// File: @openzeppelin-0.5/contracts/math/SafeMath.sol



pragma solidity ^0.5.0;

/**
 * @dev Wrappers over Solidity's arithmetic operations with added overflow
 * checks.
 *
 * Arithmetic operations in Solidity wrap on overflow. This can easily result
 * in bugs, because programmers usually assume that an overflow raises an
 * error, which is the standard behavior in high level programming languages.
 * `SafeMath` restores this intuition by reverting the transaction when an
 * operation overflows.
 *
 * Using this library instead of the unchecked operations eliminates an entire
 * class of bugs, so it's recommended to use it always.
 */
library SafeMath {
    /**
     * @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) {
        return sub(a, b, "SafeMath: subtraction overflow");
    }

    /**
     * @dev Returns the subtraction of two unsigned integers, reverting with custom message on
     * overflow (when the result is negative).
     *
     * Counterpart to Solidity's `-` operator.
     *
     * Requirements:
     * - Subtraction cannot overflow.
     *
     * _Available since v2.4.0._
     */
    function sub(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b <= a, errorMessage);
        uint256 c = a - b;

        return c;
    }

    /**
     * @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) {
        // 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;
    }

    /**
     * @dev Returns the integer division of two unsigned integers. Reverts 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) {
        return div(a, b, "SafeMath: division by zero");
    }

    /**
     * @dev Returns the integer division of two unsigned integers. Reverts with custom message 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.
     *
     * _Available since v2.4.0._
     */
    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;
    }

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

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * Reverts with custom message 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.
     *
     * _Available since v2.4.0._
     */
    function mod(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b != 0, errorMessage);
        return a % b;
    }
}

// File: @openzeppelin-0.5/contracts/utils/Address.sol



pragma solidity ^0.5.5;

/**
 * @dev Collection of functions related to the address type
 */
library Address {
    /**
     * @dev Returns true if `account` is a contract.
     *
     * [IMPORTANT]
     * ====
     * It is unsafe to assume that an address for which this function returns
     * false is an externally-owned account (EOA) and not a contract.
     *
     * Among others, `isContract` will return false for the following 
     * types of addresses:
     *
     *  - an externally-owned account
     *  - a contract in construction
     *  - an address where a contract will be created
     *  - an address where a contract lived, but was destroyed
     * ====
     */
    function isContract(address account) internal view returns (bool) {
        // According to EIP-1052, 0x0 is the value returned for not-yet created accounts
        // and 0xc5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470 is returned
        // for accounts without code, i.e. `keccak256('')`
        bytes32 codehash;
        bytes32 accountHash = 0xc5d2460186f7233c927e7db2dcc703c0e500b653ca82273b7bfad8045d85a470;
        // solhint-disable-next-line no-inline-assembly
        assembly { codehash := extcodehash(account) }
        return (codehash != accountHash && codehash != 0x0);
    }

    /**
     * @dev Converts an `address` into `address payable`. Note that this is
     * simply a type cast: the actual underlying value is not changed.
     *
     * _Available since v2.4.0._
     */
    function toPayable(address account) internal pure returns (address payable) {
        return address(uint160(account));
    }

    /**
     * @dev Replacement for Solidity's `transfer`: sends `amount` wei to
     * `recipient`, forwarding all available gas and reverting on errors.
     *
     * https://eips.ethereum.org/EIPS/eip-1884[EIP1884] increases the gas cost
     * of certain opcodes, possibly making contracts go over the 2300 gas limit
     * imposed by `transfer`, making them unable to receive funds via
     * `transfer`. {sendValue} removes this limitation.
     *
     * https://diligence.consensys.net/posts/2019/09/stop-using-soliditys-transfer-now/[Learn more].
     *
     * IMPORTANT: because control is transferred to `recipient`, care must be
     * taken to not create reentrancy vulnerabilities. Consider using
     * {ReentrancyGuard} or the
     * https://solidity.readthedocs.io/en/v0.5.11/security-considerations.html#use-the-checks-effects-interactions-pattern[checks-effects-interactions pattern].
     *
     * _Available since v2.4.0._
     */
    function sendValue(address payable recipient, uint256 amount) internal {
        require(address(this).balance >= amount, "Address: insufficient balance");

        // solhint-disable-next-line avoid-call-value
        (bool success, ) = recipient.call.value(amount)("");
        require(success, "Address: unable to send value, recipient may have reverted");
    }
}

// File: @openzeppelin-0.5/contracts/drafts/Counters.sol



pragma solidity ^0.5.0;

import "../math/SafeMath.sol";

/**
 * @title Counters
 * @author Matt Condon (@shrugs)
 * @dev Provides counters that can only be incremented or decremented by one. This can be used e.g. to track the number
 * of elements in a mapping, issuing ERC721 ids, or counting request ids.
 *
 * Include with `using Counters for Counters.Counter;`
 * Since it is not possible to overflow a 256 bit integer with increments of one, `increment` can skip the {SafeMath}
 * overflow check, thereby saving gas. This does assume however correct usage, in that the underlying `_value` is never
 * directly accessed.
 */
library Counters {
    using SafeMath for uint256;

    struct Counter {
        // This variable should never be directly accessed by users of the library: interactions must be restricted to
        // the library's function. As of Solidity v0.5.2, this cannot be enforced, though there is a proposal to add
        // this feature: see https://github.com/ethereum/solidity/issues/4637
        uint256 _value; // default: 0
    }

    function current(Counter storage counter) internal view returns (uint256) {
        return counter._value;
    }

    function increment(Counter storage counter) internal {
        // The {SafeMath} overflow check can be skipped here, see the comment at the top
        counter._value += 1;
    }

    function decrement(Counter storage counter) internal {
        counter._value = counter._value.sub(1);
    }
}

// File: @openzeppelin-0.5/contracts/token/ERC721/IERC721Enumerable.sol



pragma solidity ^0.5.0;

import "./IERC721.sol";

/**
 * @title ERC-721 Non-Fungible Token Standard, optional enumeration extension
 * @dev See https://eips.ethereum.org/EIPS/eip-721
 */
contract IERC721Enumerable is IERC721 {
    function totalSupply() public view returns (uint256);
    function tokenOfOwnerByIndex(address owner, uint256 index) public view returns (uint256 tokenId);

    function tokenByIndex(uint256 index) public view returns (uint256);
}