// File: @chainlink/contracts/src/v0.8/dev/VRFConsumerBase.sol



// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "../interfaces/LinkTokenInterface.sol";

import "./VRFRequestIDBase.sol";

/** ****************************************************************************
 * @notice Interface for contracts using VRF randomness
 * *****************************************************************************
 * @dev PURPOSE
 *
 * @dev Reggie the Random Oracle (not his real job) wants to provide randomness
 * @dev to Vera the verifier in such a way that Vera can be sure he's not
 * @dev making his output up to suit himself. Reggie provides Vera a public key
 * @dev to which he knows the secret key. Each time Vera provides a seed to
 * @dev Reggie, he gives back a value which is computed completely
 * @dev deterministically from the seed and the secret key.
 *
 * @dev Reggie provides a proof by which Vera can verify that the output was
 * @dev correctly computed once Reggie tells it to her, but without that proof,
 * @dev the output is indistinguishable to her from a uniform random sample
 * @dev from the output space.
 *
 * @dev The purpose of this contract is to make it easy for unrelated contracts
 * @dev to talk to Vera the verifier about the work Reggie is doing, to provide
 * @dev simple access to a verifiable source of randomness.
 * *****************************************************************************
 * @dev USAGE
 *
 * @dev Calling contracts must inherit from VRFConsumerBase, and can
 * @dev initialize VRFConsumerBase's attributes in their constructor as
 * @dev shown:
 *
 * @dev   contract VRFConsumer {
 * @dev     constuctor(<other arguments>, address _vrfCoordinator, address _link)
 * @dev       VRFConsumerBase(_vrfCoordinator, _link) public {
 * @dev         <initialization with other arguments goes here>
 * @dev       }
 * @dev   }
 *
 * @dev The oracle will have given you an ID for the VRF keypair they have
 * @dev committed to (let's call it keyHash), and have told you the minimum LINK
 * @dev price for VRF service. Make sure your contract has sufficient LINK, and
 * @dev call requestRandomness(keyHash, fee, seed), where seed is the input you
 * @dev want to generate randomness from.
 *
 * @dev Once the VRFCoordinator has received and validated the oracle's response
 * @dev to your request, it will call your contract's fulfillRandomness method.
 *
 * @dev The randomness argument to fulfillRandomness is the actual random value
 * @dev generated from your seed.
 *
 * @dev The requestId argument is generated from the keyHash and the seed by
 * @dev makeRequestId(keyHash, seed). If your contract could have concurrent
 * @dev requests open, you can use the requestId to track which seed is
 * @dev associated with which randomness. See VRFRequestIDBase.sol for more
 * @dev details. (See "SECURITY CONSIDERATIONS" for principles to keep in mind,
 * @dev if your contract could have multiple requests in flight simultaneously.)
 *
 * @dev Colliding `requestId`s are cryptographically impossible as long as seeds
 * @dev differ. (Which is critical to making unpredictable randomness! See the
 * @dev next section.)
 *
 * *****************************************************************************
 * @dev SECURITY CONSIDERATIONS
 *
 * @dev A method with the ability to call your fulfillRandomness method directly
 * @dev could spoof a VRF response with any random value, so it's critical that
 * @dev it cannot be directly called by anything other than this base contract
 * @dev (specifically, by the VRFConsumerBase.rawFulfillRandomness method).
 *
 * @dev For your users to trust that your contract's random behavior is free
 * @dev from malicious interference, it's best if you can write it so that all
 * @dev behaviors implied by a VRF response are executed *during* your
 * @dev fulfillRandomness method. If your contract must store the response (or
 * @dev anything derived from it) and use it later, you must ensure that any
 * @dev user-significant behavior which depends on that stored value cannot be
 * @dev manipulated by a subsequent VRF request.
 *
 * @dev Similarly, both miners and the VRF oracle itself have some influence
 * @dev over the order in which VRF responses appear on the blockchain, so if
 * @dev your contract could have multiple VRF requests in flight simultaneously,
 * @dev you must ensure that the order in which the VRF responses arrive cannot
 * @dev be used to manipulate your contract's user-significant behavior.
 *
 * @dev Since the ultimate input to the VRF is mixed with the block hash of the
 * @dev block in which the request is made, user-provided seeds have no impact
 * @dev on its economic security properties. They are only included for API
 * @dev compatability with previous versions of this contract.
 *
 * @dev Since the block hash of the block which contains the requestRandomness
 * @dev call is mixed into the input to the VRF *last*, a sufficiently powerful
 * @dev miner could, in principle, fork the blockchain to evict the block
 * @dev containing the request, forcing the request to be included in a
 * @dev different block with a different hash, and therefore a different input
 * @dev to the VRF. However, such an attack would incur a substantial economic
 * @dev cost. This cost scales with the number of blocks the VRF oracle waits
 * @dev until it calls responds to a request.
 */
abstract contract VRFConsumerBase is VRFRequestIDBase {

  /**
   * @notice fulfillRandomness handles the VRF response. Your contract must
   * @notice implement it. See "SECURITY CONSIDERATIONS" above for important
   * @notice principles to keep in mind when implementing your fulfillRandomness
   * @notice method.
   *
   * @dev VRFConsumerBase expects its subcontracts to have a method with this
   * @dev signature, and will call it once it has verified the proof
   * @dev associated with the randomness. (It is triggered via a call to
   * @dev rawFulfillRandomness, below.)
   *
   * @param requestId The Id initially returned by requestRandomness
   * @param randomness the VRF output
   */
  function fulfillRandomness(
    bytes32 requestId,
    uint256 randomness
  )
    internal
    virtual;

  /**
   * @dev In order to keep backwards compatibility we have kept the user
   * seed field around. We remove the use of it because given that the blockhash
   * enters later, it overrides whatever randomness the used seed provides.
   * Given that it adds no security, and can easily lead to misunderstandings,
   * we have removed it from usage and can now provide a simpler API.
   */
  uint256 constant private USER_SEED_PLACEHOLDER = 0;

  /**
   * @notice requestRandomness initiates a request for VRF output given _seed
   *
   * @dev The fulfillRandomness method receives the output, once it's provided
   * @dev by the Oracle, and verified by the vrfCoordinator.
   *
   * @dev The _keyHash must already be registered with the VRFCoordinator, and
   * @dev the _fee must exceed the fee specified during registration of the
   * @dev _keyHash.
   *
   * @dev The _seed parameter is vestigial, and is kept only for API
   * @dev compatibility with older versions. It can't *hurt* to mix in some of
   * @dev your own randomness, here, but it's not necessary because the VRF
   * @dev oracle will mix the hash of the block containing your request into the
   * @dev VRF seed it ultimately uses.
   *
   * @param _keyHash ID of public key against which randomness is generated
   * @param _fee The amount of LINK to send with the request
   *
   * @return requestId unique ID for this request
   *
   * @dev The returned requestId can be used to distinguish responses to
   * @dev concurrent requests. It is passed as the first argument to
   * @dev fulfillRandomness.
   */
  function requestRandomness(
    bytes32 _keyHash,
    uint256 _fee
  )
    internal
    returns (
      bytes32 requestId
    )
  {
    LINK.transferAndCall(vrfCoordinator, _fee, abi.encode(_keyHash, USER_SEED_PLACEHOLDER));
    // This is the seed passed to VRFCoordinator. The oracle will mix this with
    // the hash of the block containing this request to obtain the seed/input
    // which is finally passed to the VRF cryptographic machinery.
    uint256 vRFSeed  = makeVRFInputSeed(_keyHash, USER_SEED_PLACEHOLDER, address(this), nonces[_keyHash]);
    // nonces[_keyHash] must stay in sync with
    // VRFCoordinator.nonces[_keyHash][this], which was incremented by the above
    // successful LINK.transferAndCall (in VRFCoordinator.randomnessRequest).
    // This provides protection against the user repeating their input seed,
    // which would result in a predictable/duplicate output, if multiple such
    // requests appeared in the same block.
    nonces[_keyHash] = nonces[_keyHash] + 1;
    return makeRequestId(_keyHash, vRFSeed);
  }

  LinkTokenInterface immutable internal LINK;
  address immutable private vrfCoordinator;

  // Nonces for each VRF key from which randomness has been requested.
  //
  // Must stay in sync with VRFCoordinator[_keyHash][this]
  mapping(bytes32 /* keyHash */ => uint256 /* nonce */) private nonces;

  /**
   * @param _vrfCoordinator address of VRFCoordinator contract
   * @param _link address of LINK token contract
   *
   * @dev https://docs.chain.link/docs/link-token-contracts
   */
  constructor(
    address _vrfCoordinator,
    address _link
  ) {
    vrfCoordinator = _vrfCoordinator;
    LINK = LinkTokenInterface(_link);
  }

  // rawFulfillRandomness is called by VRFCoordinator when it receives a valid VRF
  // proof. rawFulfillRandomness then calls fulfillRandomness, after validating
  // the origin of the call
  function rawFulfillRandomness(
    bytes32 requestId,
    uint256 randomness
  )
    external
  {
    require(msg.sender == vrfCoordinator, "Only VRFCoordinator can fulfill");
    fulfillRandomness(requestId, randomness);
  }
}

// File: @chainlink/contracts/src/v0.8/dev/VRFRequestIDBase.sol



// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract VRFRequestIDBase {

  /**
   * @notice returns the seed which is actually input to the VRF coordinator
   *
   * @dev To prevent repetition of VRF output due to repetition of the
   * @dev user-supplied seed, that seed is combined in a hash with the
   * @dev user-specific nonce, and the address of the consuming contract. The
   * @dev risk of repetition is mostly mitigated by inclusion of a blockhash in
   * @dev the final seed, but the nonce does protect against repetition in
   * @dev requests which are included in a single block.
   *
   * @param _userSeed VRF seed input provided by user
   * @param _requester Address of the requesting contract
   * @param _nonce User-specific nonce at the time of the request
   */
  function makeVRFInputSeed(
    bytes32 _keyHash,
    uint256 _userSeed,
    address _requester,
    uint256 _nonce
  )
    internal
    pure
    returns (
      uint256
    )
  {
    return uint256(keccak256(abi.encode(_keyHash, _userSeed, _requester, _nonce)));
  }

  /**
   * @notice Returns the id for this request
   * @param _keyHash The serviceAgreement ID to be used for this request
   * @param _vRFInputSeed The seed to be passed directly to the VRF
   * @return The id for this request
   *
   * @dev Note that _vRFInputSeed is not the seed passed by the consuming
   * @dev contract, but the one generated by makeVRFInputSeed
   */
  function makeRequestId(
    bytes32 _keyHash,
    uint256 _vRFInputSeed
  )
    internal
    pure
    returns (
      bytes32
    )
  {
    return keccak256(abi.encodePacked(_keyHash, _vRFInputSeed));
  }
}
// File: @chainlink/contracts/src/v0.8/interfaces/LinkTokenInterface.sol



// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

interface LinkTokenInterface {

  function allowance(
    address owner,
    address spender
  )
    external
    view
    returns (
      uint256 remaining
    );

  function approve(
    address spender,
    uint256 value
  )
    external
    returns (
      bool success
    );

  function balanceOf(
    address owner
  )
    external
    view
    returns (
      uint256 balance
    );

  function decimals()
    external
    view
    returns (
      uint8 decimalPlaces
    );

  function decreaseApproval(
    address spender,
    uint256 addedValue
  )
    external
    returns (
      bool success
    );

  function increaseApproval(
    address spender,
    uint256 subtractedValue
  ) external;

  function name()
    external
    view
    returns (
      string memory tokenName
    );

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

  function totalSupply()
    external
    view
    returns (
      uint256 totalTokensIssued
    );

  function transfer(
    address to,
    uint256 value
  )
    external
    returns (
      bool success
    );

  function transferAndCall(
    address to,
    uint256 value,
    bytes calldata data
  )
    external
    returns (
      bool success
    );

  function transferFrom(
    address from,
    address to,
    uint256 value
  )
    external
    returns (
      bool success
    );

}

// File: @openzeppelin/contracts/access/Ownable.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

import "../utils/Context.sol";
/**
 * @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 () {
        address msgSender = _msgSender();
        _owner = msgSender;
        emit OwnershipTransferred(address(0), 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 {
        emit OwnershipTransferred(_owner, address(0));
        _owner = 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");
        emit OwnershipTransferred(_owner, newOwner);
        _owner = newOwner;
    }
}

// File: @openzeppelin/contracts/security/Pausable.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

import "../utils/Context.sol";

/**
 * @dev Contract module which allows children to implement an emergency stop
 * mechanism that can be triggered by an authorized account.
 *
 * This module is used through inheritance. It will make available the
 * modifiers `whenNotPaused` and `whenPaused`, which can be applied to
 * the functions of your contract. Note that they will not be pausable by
 * simply including this module, only once the modifiers are put in place.
 */
abstract contract Pausable is Context {
    /**
     * @dev Emitted when the pause is triggered by `account`.
     */
    event Paused(address account);

    /**
     * @dev Emitted when the pause is lifted by `account`.
     */
    event Unpaused(address account);

    bool private _paused;

    /**
     * @dev Initializes the contract in unpaused state.
     */
    constructor () {
        _paused = false;
    }

    /**
     * @dev Returns true if the contract is paused, and false otherwise.
     */
    function paused() public view virtual returns (bool) {
        return _paused;
    }

    /**
     * @dev Modifier to make a function callable only when the contract is not paused.
     *
     * Requirements:
     *
     * - The contract must not be paused.
     */
    modifier whenNotPaused() {
        require(!paused(), "Pausable: paused");
        _;
    }

    /**
     * @dev Modifier to make a function callable only when the contract is paused.
     *
     * Requirements:
     *
     * - The contract must be paused.
     */
    modifier whenPaused() {
        require(paused(), "Pausable: not paused");
        _;
    }

    /**
     * @dev Triggers stopped state.
     *
     * Requirements:
     *
     * - The contract must not be paused.
     */
    function _pause() internal virtual whenNotPaused {
        _paused = true;
        emit Paused(_msgSender());
    }

    /**
     * @dev Returns to normal state.
     *
     * Requirements:
     *
     * - The contract must be paused.
     */
    function _unpause() internal virtual whenPaused {
        _paused = false;
        emit Unpaused(_msgSender());
    }
}

// File: @openzeppelin/contracts/security/ReentrancyGuard.sol



// SPDX-License-Identifier: MIT

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 make 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: @openzeppelin/contracts/token/ERC20/IERC20.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

/**
 * @dev Interface of the ERC20 standard as defined in the EIP.
 */
interface IERC20 {
    /**
     * @dev Returns the amount of tokens in existence.
     */
    function totalSupply() external view returns (uint256);

    /**
     * @dev Returns the amount of tokens owned by `account`.
     */
    function balanceOf(address account) external view returns (uint256);

    /**
     * @dev Moves `amount` tokens from the caller's account to `recipient`.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * Emits a {Transfer} event.
     */
    function transfer(address recipient, uint256 amount) external returns (bool);

    /**
     * @dev Returns the remaining number of tokens that `spender` will be
     * allowed to spend on behalf of `owner` through {transferFrom}. This is
     * zero by default.
     *
     * This value changes when {approve} or {transferFrom} are called.
     */
    function allowance(address owner, address spender) external view returns (uint256);

    /**
     * @dev Sets `amount` as the allowance of `spender` over the caller's tokens.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * IMPORTANT: Beware that changing an allowance with this method brings the risk
     * that someone may use both the old and the new allowance by unfortunate
     * transaction ordering. One possible solution to mitigate this race
     * condition is to first reduce the spender's allowance to 0 and set the
     * desired value afterwards:
     * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
     *
     * Emits an {Approval} event.
     */
    function approve(address spender, uint256 amount) external returns (bool);

    /**
     * @dev Moves `amount` tokens from `sender` to `recipient` using the
     * allowance mechanism. `amount` is then deducted from the caller's
     * allowance.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * Emits a {Transfer} event.
     */
    function transferFrom(address sender, address recipient, uint256 amount) external returns (bool);

    /**
     * @dev Emitted when `value` tokens are moved from one account (`from`) to
     * another (`to`).
     *
     * Note that `value` may be zero.
     */
    event Transfer(address indexed from, address indexed to, uint256 value);

    /**
     * @dev Emitted when the allowance of a `spender` for an `owner` is set by
     * a call to {approve}. `value` is the new allowance.
     */
    event Approval(address indexed owner, address indexed spender, uint256 value);
}

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



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

import "./IERC721.sol";
import "./IERC721Receiver.sol";
import "./extensions/IERC721Metadata.sol";
import "../../utils/Address.sol";
import "../../utils/Context.sol";
import "../../utils/Strings.sol";
import "../../utils/introspection/ERC165.sol";

/**
 * @dev Implementation of https://eips.ethereum.org/EIPS/eip-721[ERC721] Non-Fungible Token Standard, including
 * the Metadata extension, but not including the Enumerable extension, which is available separately as
 * {ERC721Enumerable}.
 */
contract ERC721 is Context, ERC165, IERC721, IERC721Metadata {
    using Address for address;
    using Strings for uint256;

    // Token name
    string private _name;

    // Token symbol
    string private _symbol;

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

    // Mapping owner address to token count
    mapping (address => uint256) private _balances;

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

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

    /**
     * @dev Initializes the contract by setting a `name` and a `symbol` to the token collection.
     */
    constructor (string memory name_, string memory symbol_) {
        _name = name_;
        _symbol = symbol_;
    }

    /**
     * @dev See {IERC165-supportsInterface}.
     */
    function supportsInterface(bytes4 interfaceId) public view virtual override(ERC165, IERC165) returns (bool) {
        return interfaceId == type(IERC721).interfaceId
            || interfaceId == type(IERC721Metadata).interfaceId
            || super.supportsInterface(interfaceId);
    }

    /**
     * @dev See {IERC721-balanceOf}.
     */
    function balanceOf(address owner) public view virtual override returns (uint256) {
        require(owner != address(0), "ERC721: balance query for the zero address");
        return _balances[owner];
    }

    /**
     * @dev See {IERC721-ownerOf}.
     */
    function ownerOf(uint256 tokenId) public view virtual override returns (address) {
        address owner = _owners[tokenId];
        require(owner != address(0), "ERC721: owner query for nonexistent token");
        return owner;
    }

    /**
     * @dev See {IERC721Metadata-name}.
     */
    function name() public view virtual override returns (string memory) {
        return _name;
    }

    /**
     * @dev See {IERC721Metadata-symbol}.
     */
    function symbol() public view virtual override returns (string memory) {
        return _symbol;
    }

    /**
     * @dev See {IERC721Metadata-tokenURI}.
     */
    function tokenURI(uint256 tokenId) public view virtual override returns (string memory) {
        require(_exists(tokenId), "ERC721Metadata: URI query for nonexistent token");

        string memory baseURI = _baseURI();
        return bytes(baseURI).length > 0
            ? string(abi.encodePacked(baseURI, tokenId.toString()))
            : '';
    }

    /**
     * @dev Base URI for computing {tokenURI}. Empty by default, can be overriden
     * in child contracts.
     */
    function _baseURI() internal view virtual returns (string memory) {
        return "";
    }

    /**
     * @dev See {IERC721-approve}.
     */
    function approve(address to, uint256 tokenId) public virtual override {
        address owner = ERC721.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"
        );

        _approve(to, tokenId);
    }

    /**
     * @dev See {IERC721-getApproved}.
     */
    function getApproved(uint256 tokenId) public view virtual override returns (address) {
        require(_exists(tokenId), "ERC721: approved query for nonexistent token");

        return _tokenApprovals[tokenId];
    }

    /**
     * @dev See {IERC721-setApprovalForAll}.
     */
    function setApprovalForAll(address operator, bool approved) public virtual override {
        require(operator != _msgSender(), "ERC721: approve to caller");

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

    /**
     * @dev See {IERC721-isApprovedForAll}.
     */
    function isApprovedForAll(address owner, address operator) public view virtual override returns (bool) {
        return _operatorApprovals[owner][operator];
    }

    /**
     * @dev See {IERC721-transferFrom}.
     */
    function transferFrom(address from, address to, uint256 tokenId) public virtual override {
        //solhint-disable-next-line max-line-length
        require(_isApprovedOrOwner(_msgSender(), tokenId), "ERC721: transfer caller is not owner nor approved");

        _transfer(from, to, tokenId);
    }

    /**
     * @dev See {IERC721-safeTransferFrom}.
     */
    function safeTransferFrom(address from, address to, uint256 tokenId) public virtual override {
        safeTransferFrom(from, to, tokenId, "");
    }

    /**
     * @dev See {IERC721-safeTransferFrom}.
     */
    function safeTransferFrom(address from, address to, uint256 tokenId, bytes memory _data) public virtual override {
        require(_isApprovedOrOwner(_msgSender(), tokenId), "ERC721: transfer caller is not owner nor approved");
        _safeTransfer(from, to, tokenId, _data);
    }

    /**
     * @dev Safely transfers `tokenId` token from `from` to `to`, checking first that contract recipients
     * are aware of the ERC721 protocol to prevent tokens from being forever locked.
     *
     * `_data` is additional data, it has no specified format and it is sent in call to `to`.
     *
     * This internal function is equivalent to {safeTransferFrom}, and can be used to e.g.
     * implement alternative mechanisms to perform token transfer, such as signature-based.
     *
     * Requirements:
     *
     * - `from` cannot be the zero address.
     * - `to` cannot be the zero address.
     * - `tokenId` token must exist and be owned by `from`.
     * - If `to` refers to a smart contract, it must implement {IERC721Receiver-onERC721Received}, which is called upon a safe transfer.
     *
     * Emits a {Transfer} event.
     */
    function _safeTransfer(address from, address to, uint256 tokenId, bytes memory _data) internal virtual {
        _transfer(from, to, tokenId);
        require(_checkOnERC721Received(from, to, tokenId, _data), "ERC721: transfer to non ERC721Receiver implementer");
    }

    /**
     * @dev Returns whether `tokenId` exists.
     *
     * Tokens can be managed by their owner or approved accounts via {approve} or {setApprovalForAll}.
     *
     * Tokens start existing when they are minted (`_mint`),
     * and stop existing when they are burned (`_burn`).
     */
    function _exists(uint256 tokenId) internal view virtual returns (bool) {
        return _owners[tokenId] != address(0);
    }

    /**
     * @dev Returns whether `spender` is allowed to manage `tokenId`.
     *
     * Requirements:
     *
     * - `tokenId` must exist.
     */
    function _isApprovedOrOwner(address spender, uint256 tokenId) internal view virtual returns (bool) {
        require(_exists(tokenId), "ERC721: operator query for nonexistent token");
        address owner = ERC721.ownerOf(tokenId);
        return (spender == owner || getApproved(tokenId) == spender || isApprovedForAll(owner, spender));
    }

    /**
     * @dev Safely mints `tokenId` and transfers it to `to`.
     *
     * Requirements:
     *
     * - `tokenId` must not exist.
     * - If `to` refers to a smart contract, it must implement {IERC721Receiver-onERC721Received}, which is called upon a safe transfer.
     *
     * Emits a {Transfer} event.
     */
    function _safeMint(address to, uint256 tokenId) internal virtual {
        _safeMint(to, tokenId, "");
    }

    /**
     * @dev Same as {xref-ERC721-_safeMint-address-uint256-}[`_safeMint`], with an additional `data` parameter which is
     * forwarded in {IERC721Receiver-onERC721Received} to contract recipients.
     */
    function _safeMint(address to, uint256 tokenId, bytes memory _data) internal virtual {
        _mint(to, tokenId);
        require(_checkOnERC721Received(address(0), to, tokenId, _data), "ERC721: transfer to non ERC721Receiver implementer");
    }

    /**
     * @dev Mints `tokenId` and transfers it to `to`.
     *
     * WARNING: Usage of this method is discouraged, use {_safeMint} whenever possible
     *
     * Requirements:
     *
     * - `tokenId` must not exist.
     * - `to` cannot be the zero address.
     *
     * Emits a {Transfer} event.
     */
    function _mint(address to, uint256 tokenId) internal virtual {
        require(to != address(0), "ERC721: mint to the zero address");
        require(!_exists(tokenId), "ERC721: token already minted");

        _beforeTokenTransfer(address(0), to, tokenId);

        _balances[to] += 1;
        _owners[tokenId] = to;

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

    /**
     * @dev Destroys `tokenId`.
     * The approval is cleared when the token is burned.
     *
     * Requirements:
     *
     * - `tokenId` must exist.
     *
     * Emits a {Transfer} event.
     */
    function _burn(uint256 tokenId) internal virtual {
        address owner = ERC721.ownerOf(tokenId);

        _beforeTokenTransfer(owner, address(0), tokenId);

        // Clear approvals
        _approve(address(0), tokenId);

        _balances[owner] -= 1;
        delete _owners[tokenId];

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

    /**
     * @dev Transfers `tokenId` from `from` to `to`.
     *  As opposed to {transferFrom}, this imposes no restrictions on msg.sender.
     *
     * Requirements:
     *
     * - `to` cannot be the zero address.
     * - `tokenId` token must be owned by `from`.
     *
     * Emits a {Transfer} event.
     */
    function _transfer(address from, address to, uint256 tokenId) internal virtual {
        require(ERC721.ownerOf(tokenId) == from, "ERC721: transfer of token that is not own");
        require(to != address(0), "ERC721: transfer to the zero address");

        _beforeTokenTransfer(from, to, tokenId);

        // Clear approvals from the previous owner
        _approve(address(0), tokenId);

        _balances[from] -= 1;
        _balances[to] += 1;
        _owners[tokenId] = to;

        emit Transfer(from, to, tokenId);
    }

    /**
     * @dev Approve `to` to operate on `tokenId`
     *
     * Emits a {Approval} event.
     */
    function _approve(address to, uint256 tokenId) internal virtual {
        _tokenApprovals[tokenId] = to;
        emit Approval(ERC721.ownerOf(tokenId), 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.
     *
     * @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)
        private returns (bool)
    {
        if (to.isContract()) {
            try IERC721Receiver(to).onERC721Received(_msgSender(), from, tokenId, _data) returns (bytes4 retval) {
                return retval == IERC721Receiver(to).onERC721Received.selector;
            } catch (bytes memory reason) {
                if (reason.length == 0) {
                    revert("ERC721: transfer to non ERC721Receiver implementer");
                } else {
                    // solhint-disable-next-line no-inline-assembly
                    assembly {
                        revert(add(32, reason), mload(reason))
                    }
                }
            }
        } else {
            return true;
        }
    }

    /**
     * @dev Hook that is called before any token transfer. This includes minting
     * and burning.
     *
     * Calling conditions:
     *
     * - When `from` and `to` are both non-zero, ``from``'s `tokenId` will be
     * transferred to `to`.
     * - When `from` is zero, `tokenId` will be minted for `to`.
     * - When `to` is zero, ``from``'s `tokenId` will be burned.
     * - `from` cannot be the zero address.
     * - `to` cannot be the zero address.
     *
     * To learn more about hooks, head to xref:ROOT:extending-contracts.adoc#using-hooks[Using Hooks].
     */
    function _beforeTokenTransfer(address from, address to, uint256 tokenId) internal virtual { }
}

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



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

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

/**
 * @dev Required interface of an ERC721 compliant contract.
 */
interface IERC721 is IERC165 {
    /**
     * @dev Emitted when `tokenId` token is transferred from `from` to `to`.
     */
    event Transfer(address indexed from, address indexed to, uint256 indexed tokenId);

    /**
     * @dev Emitted when `owner` enables `approved` to manage the `tokenId` token.
     */
    event Approval(address indexed owner, address indexed approved, uint256 indexed tokenId);

    /**
     * @dev Emitted when `owner` enables or disables (`approved`) `operator` to manage all of its assets.
     */
    event ApprovalForAll(address indexed owner, address indexed operator, bool approved);

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

    /**
     * @dev Returns the owner of the `tokenId` token.
     *
     * Requirements:
     *
     * - `tokenId` must exist.
     */
    function ownerOf(uint256 tokenId) external view returns (address owner);

    /**
     * @dev Safely transfers `tokenId` token from `from` to `to`, checking first that contract recipients
     * are aware of the ERC721 protocol to prevent tokens from being forever locked.
     *
     * Requirements:
     *
     * - `from` cannot be the zero address.
     * - `to` cannot be the zero address.
     * - `tokenId` token must exist and be owned by `from`.
     * - If the caller is not `from`, it must be have been allowed to move this token by either {approve} or {setApprovalForAll}.
     * - If `to` refers to a smart contract, it must implement {IERC721Receiver-onERC721Received}, which is called upon a safe transfer.
     *
     * Emits a {Transfer} event.
     */
    function safeTransferFrom(address from, address to, uint256 tokenId) external;

    /**
     * @dev Transfers `tokenId` token from `from` to `to`.
     *
     * WARNING: Usage of this method is discouraged, use {safeTransferFrom} whenever possible.
     *
     * Requirements:
     *
     * - `from` cannot be the zero address.
     * - `to` cannot be the zero address.
     * - `tokenId` token must be owned by `from`.
     * - If the caller is not `from`, it must be approved to move this token by either {approve} or {setApprovalForAll}.
     *
     * Emits a {Transfer} event.
     */
    function transferFrom(address from, address to, uint256 tokenId) external;

    /**
     * @dev Gives permission to `to` to transfer `tokenId` token to another account.
     * The approval is cleared when the token is transferred.
     *
     * Only a single account can be approved at a time, so approving the zero address clears previous approvals.
     *
     * Requirements:
     *
     * - The caller must own the token or be an approved operator.
     * - `tokenId` must exist.
     *
     * Emits an {Approval} event.
     */
    function approve(address to, uint256 tokenId) external;

    /**
     * @dev Returns the account approved for `tokenId` token.
     *
     * Requirements:
     *
     * - `tokenId` must exist.
     */
    function getApproved(uint256 tokenId) external view returns (address operator);

    /**
     * @dev Approve or remove `operator` as an operator for the caller.
     * Operators can call {transferFrom} or {safeTransferFrom} for any token owned by the caller.
     *
     * Requirements:
     *
     * - The `operator` cannot be the caller.
     *
     * Emits an {ApprovalForAll} event.
     */
    function setApprovalForAll(address operator, bool _approved) external;

    /**
     * @dev Returns if the `operator` is allowed to manage all of the assets of `owner`.
     *
     * See {setApprovalForAll}
     */
    function isApprovedForAll(address owner, address operator) external view returns (bool);

    /**
      * @dev Safely transfers `tokenId` token from `from` to `to`.
      *
      * Requirements:
      *
      * - `from` cannot be the zero address.
      * - `to` cannot be the zero address.
      * - `tokenId` token must exist and be owned by `from`.
      * - If the caller is not `from`, it must be approved to move this token by either {approve} or {setApprovalForAll}.
      * - If `to` refers to a smart contract, it must implement {IERC721Receiver-onERC721Received}, which is called upon a safe transfer.
      *
      * Emits a {Transfer} event.
      */
    function safeTransferFrom(address from, address to, uint256 tokenId, bytes calldata data) external;
}

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



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

/**
 * @title ERC721 token receiver interface
 * @dev Interface for any contract that wants to support safeTransfers
 * from ERC721 asset contracts.
 */
interface IERC721Receiver {
    /**
     * @dev Whenever an {IERC721} `tokenId` token is transferred to this contract via {IERC721-safeTransferFrom}
     * by `operator` from `from`, this function is called.
     *
     * It must return its Solidity selector to confirm the token transfer.
     * If any other value is returned or the interface is not implemented by the recipient, the transfer will be reverted.
     *
     * The selector can be obtained in Solidity with `IERC721.onERC721Received.selector`.
     */
    function onERC721Received(address operator, address from, uint256 tokenId, bytes calldata data) external returns (bytes4);
}

// File: @openzeppelin/contracts/token/ERC721/extensions/ERC721Burnable.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

import "../ERC721.sol";
import "../../../utils/Context.sol";

/**
 * @title ERC721 Burnable Token
 * @dev ERC721 Token that can be irreversibly burned (destroyed).
 */
abstract contract ERC721Burnable is Context, ERC721 {
    /**
     * @dev Burns `tokenId`. See {ERC721-_burn}.
     *
     * Requirements:
     *
     * - The caller must own `tokenId` or be an approved operator.
     */
    function burn(uint256 tokenId) public virtual {
        //solhint-disable-next-line max-line-length
        require(_isApprovedOrOwner(_msgSender(), tokenId), "ERC721Burnable: caller is not owner nor approved");
        _burn(tokenId);
    }
}

// File: @openzeppelin/contracts/token/ERC721/extensions/ERC721Enumerable.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

import "../ERC721.sol";
import "./IERC721Enumerable.sol";

/**
 * @dev This implements an optional extension of {ERC721} defined in the EIP that adds
 * enumerability of all the token ids in the contract as well as all token ids owned by each
 * account.
 */
abstract contract ERC721Enumerable is ERC721, IERC721Enumerable {
    // Mapping from owner to list of owned token IDs
    mapping(address => mapping(uint256 => 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;

    /**
     * @dev See {IERC165-supportsInterface}.
     */
    function supportsInterface(bytes4 interfaceId) public view virtual override(IERC165, ERC721) returns (bool) {
        return interfaceId == type(IERC721Enumerable).interfaceId
            || super.supportsInterface(interfaceId);
    }

    /**
     * @dev See {IERC721Enumerable-tokenOfOwnerByIndex}.
     */
    function tokenOfOwnerByIndex(address owner, uint256 index) public view virtual override returns (uint256) {
        require(index < ERC721.balanceOf(owner), "ERC721Enumerable: owner index out of bounds");
        return _ownedTokens[owner][index];
    }

    /**
     * @dev See {IERC721Enumerable-totalSupply}.
     */
    function totalSupply() public view virtual override returns (uint256) {
        return _allTokens.length;
    }

    /**
     * @dev See {IERC721Enumerable-tokenByIndex}.
     */
    function tokenByIndex(uint256 index) public view virtual override returns (uint256) {
        require(index < ERC721Enumerable.totalSupply(), "ERC721Enumerable: global index out of bounds");
        return _allTokens[index];
    }

    /**
     * @dev Hook that is called before any token transfer. This includes minting
     * and burning.
     *
     * Calling conditions:
     *
     * - When `from` and `to` are both non-zero, ``from``'s `tokenId` will be
     * transferred to `to`.
     * - When `from` is zero, `tokenId` will be minted for `to`.
     * - When `to` is zero, ``from``'s `tokenId` will be burned.
     * - `from` cannot be the zero address.
     * - `to` cannot be the zero address.
     *
     * To learn more about hooks, head to xref:ROOT:extending-contracts.adoc#using-hooks[Using Hooks].
     */
    function _beforeTokenTransfer(address from, address to, uint256 tokenId) internal virtual override {
        super._beforeTokenTransfer(from, to, tokenId);

        if (from == address(0)) {
            _addTokenToAllTokensEnumeration(tokenId);
        } else if (from != to) {
            _removeTokenFromOwnerEnumeration(from, tokenId);
        }
        if (to == address(0)) {
            _removeTokenFromAllTokensEnumeration(tokenId);
        } else if (to != from) {
            _addTokenToOwnerEnumeration(to, tokenId);
        }
    }

    /**
     * @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 {
        uint256 length = ERC721.balanceOf(to);
        _ownedTokens[to][length] = tokenId;
        _ownedTokensIndex[tokenId] = length;
    }

    /**
     * @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 = ERC721.balanceOf(from) - 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
        delete _ownedTokensIndex[tokenId];
        delete _ownedTokens[from][lastTokenIndex];
    }

    /**
     * @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 - 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
        delete _allTokensIndex[tokenId];
        _allTokens.pop();
    }
}

// File: @openzeppelin/contracts/token/ERC721/extensions/ERC721URIStorage.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

import "../ERC721.sol";

/**
 * @dev ERC721 token with storage based token URI management.
 */
abstract contract ERC721URIStorage is ERC721 {
    using Strings for uint256;

    // Optional mapping for token URIs
    mapping (uint256 => string) private _tokenURIs;

    /**
     * @dev See {IERC721Metadata-tokenURI}.
     */
    function tokenURI(uint256 tokenId) public view virtual override returns (string memory) {
        require(_exists(tokenId), "ERC721URIStorage: URI query for nonexistent token");

        string memory _tokenURI = _tokenURIs[tokenId];
        string memory base = _baseURI();

        // If there is no base URI, return the token URI.
        if (bytes(base).length == 0) {
            return _tokenURI;
        }
        // If both are set, concatenate the baseURI and tokenURI (via abi.encodePacked).
        if (bytes(_tokenURI).length > 0) {
            return string(abi.encodePacked(base, _tokenURI));
        }

        return super.tokenURI(tokenId);
    }

    /**
     * @dev Sets `_tokenURI` as the tokenURI of `tokenId`.
     *
     * Requirements:
     *
     * - `tokenId` must exist.
     */
    function _setTokenURI(uint256 tokenId, string memory _tokenURI) internal virtual {
        require(_exists(tokenId), "ERC721URIStorage: URI set of nonexistent token");
        _tokenURIs[tokenId] = _tokenURI;
    }

    /**
     * @dev Destroys `tokenId`.
     * The approval is cleared when the token is burned.
     *
     * Requirements:
     *
     * - `tokenId` must exist.
     *
     * Emits a {Transfer} event.
     */
    function _burn(uint256 tokenId) internal virtual override {
        super._burn(tokenId);

        if (bytes(_tokenURIs[tokenId]).length != 0) {
            delete _tokenURIs[tokenId];
        }
    }
}

// File: @openzeppelin/contracts/token/ERC721/extensions/IERC721Enumerable.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

import "../IERC721.sol";

/**
 * @title ERC-721 Non-Fungible Token Standard, optional enumeration extension
 * @dev See https://eips.ethereum.org/EIPS/eip-721
 */
interface IERC721Enumerable is IERC721 {

    /**
     * @dev Returns the total amount of tokens stored by the contract.
     */
    function totalSupply() external view returns (uint256);

    /**
     * @dev Returns a token ID owned by `owner` at a given `index` of its token list.
     * Use along with {balanceOf} to enumerate all of ``owner``'s tokens.
     */
    function tokenOfOwnerByIndex(address owner, uint256 index) external view returns (uint256 tokenId);

    /**
     * @dev Returns a token ID at a given `index` of all the tokens stored by the contract.
     * Use along with {totalSupply} to enumerate all tokens.
     */
    function tokenByIndex(uint256 index) external view returns (uint256);
}

// File: @openzeppelin/contracts/token/ERC721/extensions/IERC721Metadata.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

import "../IERC721.sol";

/**
 * @title ERC-721 Non-Fungible Token Standard, optional metadata extension
 * @dev See https://eips.ethereum.org/EIPS/eip-721
 */
interface IERC721Metadata is IERC721 {

    /**
     * @dev Returns the token collection name.
     */
    function name() external view returns (string memory);

    /**
     * @dev Returns the token collection symbol.
     */
    function symbol() external view returns (string memory);

    /**
     * @dev Returns the Uniform Resource Identifier (URI) for `tokenId` token.
     */
    function tokenURI(uint256 tokenId) external view returns (string memory);
}

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



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

/**
 * @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) {
        // This method relies on extcodesize, which returns 0 for contracts in
        // construction, since the code is only stored at the end of the
        // constructor execution.

        uint256 size;
        // solhint-disable-next-line no-inline-assembly
        assembly { size := extcodesize(account) }
        return size > 0;
    }

    /**
     * @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].
     */
    function sendValue(address payable recipient, uint256 amount) internal {
        require(address(this).balance >= amount, "Address: insufficient balance");

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

    /**
     * @dev Performs a Solidity function call using a low level `call`. A
     * plain`call` is an unsafe replacement for a function call: use this
     * function instead.
     *
     * If `target` reverts with a revert reason, it is bubbled up by this
     * function (like regular Solidity function calls).
     *
     * Returns the raw returned data. To convert to the expected return value,
     * use https://solidity.readthedocs.io/en/latest/units-and-global-variables.html?highlight=abi.decode#abi-encoding-and-decoding-functions[`abi.decode`].
     *
     * Requirements:
     *
     * - `target` must be a contract.
     * - calling `target` with `data` must not revert.
     *
     * _Available since v3.1._
     */
    function functionCall(address target, bytes memory data) internal returns (bytes memory) {
      return functionCall(target, data, "Address: low-level call failed");
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`], but with
     * `errorMessage` as a fallback revert reason when `target` reverts.
     *
     * _Available since v3.1._
     */
    function functionCall(address target, bytes memory data, string memory errorMessage) internal returns (bytes memory) {
        return functionCallWithValue(target, data, 0, errorMessage);
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
     * but also transferring `value` wei to `target`.
     *
     * Requirements:
     *
     * - the calling contract must have an ETH balance of at least `value`.
     * - the called Solidity function must be `payable`.
     *
     * _Available since v3.1._
     */
    function functionCallWithValue(address target, bytes memory data, uint256 value) internal returns (bytes memory) {
        return functionCallWithValue(target, data, value, "Address: low-level call with value failed");
    }

    /**
     * @dev Same as {xref-Address-functionCallWithValue-address-bytes-uint256-}[`functionCallWithValue`], but
     * with `errorMessage` as a fallback revert reason when `target` reverts.
     *
     * _Available since v3.1._
     */
    function functionCallWithValue(address target, bytes memory data, uint256 value, string memory errorMessage) internal returns (bytes memory) {
        require(address(this).balance >= value, "Address: insufficient balance for call");
        require(isContract(target), "Address: call to non-contract");

        // solhint-disable-next-line avoid-low-level-calls
        (bool success, bytes memory returndata) = target.call{ value: value }(data);
        return _verifyCallResult(success, returndata, errorMessage);
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
     * but performing a static call.
     *
     * _Available since v3.3._
     */
    function functionStaticCall(address target, bytes memory data) internal view returns (bytes memory) {
        return functionStaticCall(target, data, "Address: low-level static call failed");
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-string-}[`functionCall`],
     * but performing a static call.
     *
     * _Available since v3.3._
     */
    function functionStaticCall(address target, bytes memory data, string memory errorMessage) internal view returns (bytes memory) {
        require(isContract(target), "Address: static call to non-contract");

        // solhint-disable-next-line avoid-low-level-calls
        (bool success, bytes memory returndata) = target.staticcall(data);
        return _verifyCallResult(success, returndata, errorMessage);
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
     * but performing a delegate call.
     *
     * _Available since v3.4._
     */
    function functionDelegateCall(address target, bytes memory data) internal returns (bytes memory) {
        return functionDelegateCall(target, data, "Address: low-level delegate call failed");
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-string-}[`functionCall`],
     * but performing a delegate call.
     *
     * _Available since v3.4._
     */
    function functionDelegateCall(address target, bytes memory data, string memory errorMessage) internal returns (bytes memory) {
        require(isContract(target), "Address: delegate call to non-contract");

        // solhint-disable-next-line avoid-low-level-calls
        (bool success, bytes memory returndata) = target.delegatecall(data);
        return _verifyCallResult(success, returndata, errorMessage);
    }

    function _verifyCallResult(bool success, bytes memory returndata, string memory errorMessage) private pure returns(bytes memory) {
        if (success) {
            return returndata;
        } else {
            // Look for revert reason and bubble it up if present
            if (returndata.length > 0) {
                // The easiest way to bubble the revert reason is using memory via assembly

                // solhint-disable-next-line no-inline-assembly
                assembly {
                    let returndata_size := mload(returndata)
                    revert(add(32, returndata), returndata_size)
                }
            } else {
                revert(errorMessage);
            }
        }
    }
}

// File: @openzeppelin/contracts/utils/Context.sol



// SPDX-License-Identifier: MIT

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) {
        this; // silence state mutability warning without generating bytecode - see https://github.com/ethereum/solidity/issues/2691
        return msg.data;
    }
}

// File: @openzeppelin/contracts/utils/Counters.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

/**
 * @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;`
 */
library Counters {
    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 {
        unchecked {
            counter._value += 1;
        }
    }

    function decrement(Counter storage counter) internal {
        uint256 value = counter._value;
        require(value > 0, "Counter: decrement overflow");
        unchecked {
            counter._value = value - 1;
        }
    }
}

// File: @openzeppelin/contracts/utils/Strings.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

/**
 * @dev String operations.
 */
library Strings {
    bytes16 private constant alphabet = "0123456789abcdef";

    /**
     * @dev Converts a `uint256` to its ASCII `string` decimal representation.
     */
    function toString(uint256 value) internal pure returns (string memory) {
        // Inspired by OraclizeAPI's implementation - MIT licence
        // https://github.com/oraclize/ethereum-api/blob/b42146b063c7d6ee1358846c198246239e9360e8/oraclizeAPI_0.4.25.sol

        if (value == 0) {
            return "0";
        }
        uint256 temp = value;
        uint256 digits;
        while (temp != 0) {
            digits++;
            temp /= 10;
        }
        bytes memory buffer = new bytes(digits);
        while (value != 0) {
            digits -= 1;
            buffer[digits] = bytes1(uint8(48 + uint256(value % 10)));
            value /= 10;
        }
        return string(buffer);
    }

    /**
     * @dev Converts a `uint256` to its ASCII `string` hexadecimal representation.
     */
    function toHexString(uint256 value) internal pure returns (string memory) {
        if (value == 0) {
            return "0x00";
        }
        uint256 temp = value;
        uint256 length = 0;
        while (temp != 0) {
            length++;
            temp >>= 8;
        }
        return toHexString(value, length);
    }

    /**
     * @dev Converts a `uint256` to its ASCII `string` hexadecimal representation with fixed length.
     */
    function toHexString(uint256 value, uint256 length) internal pure returns (string memory) {
        bytes memory buffer = new bytes(2 * length + 2);
        buffer[0] = "0";
        buffer[1] = "x";
        for (uint256 i = 2 * length + 1; i > 1; --i) {
            buffer[i] = alphabet[value & 0xf];
            value >>= 4;
        }
        require(value == 0, "Strings: hex length insufficient");
        return string(buffer);
    }

}

// File: @openzeppelin/contracts/utils/introspection/ERC165.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

import "./IERC165.sol";

/**
 * @dev Implementation of the {IERC165} interface.
 *
 * Contracts that want to implement ERC165 should inherit from this contract and override {supportsInterface} to check
 * for the additional interface id that will be supported. For example:
 *
 * ```solidity
 * function supportsInterface(bytes4 interfaceId) public view virtual override returns (bool) {
 *     return interfaceId == type(MyInterface).interfaceId || super.supportsInterface(interfaceId);
 * }
 * ```
 *
 * Alternatively, {ERC165Storage} provides an easier to use but more expensive implementation.
 */
abstract contract ERC165 is IERC165 {
    /**
     * @dev See {IERC165-supportsInterface}.
     */
    function supportsInterface(bytes4 interfaceId) public view virtual override returns (bool) {
        return interfaceId == type(IERC165).interfaceId;
    }
}

// File: @openzeppelin/contracts/utils/introspection/IERC165.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

/**
 * @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: @uniswap/v2-periphery/contracts/interfaces/IUniswapV2Router01.sol



pragma solidity >=0.6.2;

interface IUniswapV2Router01 {
    function factory() external pure returns (address);
    function WETH() external pure returns (address);

    function addLiquidity(
        address tokenA,
        address tokenB,
        uint amountADesired,
        uint amountBDesired,
        uint amountAMin,
        uint amountBMin,
        address to,
        uint deadline
    ) external returns (uint amountA, uint amountB, uint liquidity);
    function addLiquidityETH(
        address token,
        uint amountTokenDesired,
        uint amountTokenMin,
        uint amountETHMin,
        address to,
        uint deadline
    ) external payable returns (uint amountToken, uint amountETH, uint liquidity);
    function removeLiquidity(
        address tokenA,
        address tokenB,
        uint liquidity,
        uint amountAMin,
        uint amountBMin,
        address to,
        uint deadline
    ) external returns (uint amountA, uint amountB);
    function removeLiquidityETH(
        address token,
        uint liquidity,
        uint amountTokenMin,
        uint amountETHMin,
        address to,
        uint deadline
    ) external returns (uint amountToken, uint amountETH);
    function removeLiquidityWithPermit(
        address tokenA,
        address tokenB,
        uint liquidity,
        uint amountAMin,
        uint amountBMin,
        address to,
        uint deadline,
        bool approveMax, uint8 v, bytes32 r, bytes32 s
    ) external returns (uint amountA, uint amountB);
    function removeLiquidityETHWithPermit(
        address token,
        uint liquidity,
        uint amountTokenMin,
        uint amountETHMin,
        address to,
        uint deadline,
        bool approveMax, uint8 v, bytes32 r, bytes32 s
    ) external returns (uint amountToken, uint amountETH);
    function swapExactTokensForTokens(
        uint amountIn,
        uint amountOutMin,
        address[] calldata path,
        address to,
        uint deadline
    ) external returns (uint[] memory amounts);
    function swapTokensForExactTokens(
        uint amountOut,
        uint amountInMax,
        address[] calldata path,
        address to,
        uint deadline
    ) external returns (uint[] memory amounts);
    function swapExactETHForTokens(uint amountOutMin, address[] calldata path, address to, uint deadline)
        external
        payable
        returns (uint[] memory amounts);
    function swapTokensForExactETH(uint amountOut, uint amountInMax, address[] calldata path, address to, uint deadline)
        external
        returns (uint[] memory amounts);
    function swapExactTokensForETH(uint amountIn, uint amountOutMin, address[] calldata path, address to, uint deadline)
        external
        returns (uint[] memory amounts);
    function swapETHForExactTokens(uint amountOut, address[] calldata path, address to, uint deadline)
        external
        payable
        returns (uint[] memory amounts);

    function quote(uint amountA, uint reserveA, uint reserveB) external pure returns (uint amountB);
    function getAmountOut(uint amountIn, uint reserveIn, uint reserveOut) external pure returns (uint amountOut);
    function getAmountIn(uint amountOut, uint reserveIn, uint reserveOut) external pure returns (uint amountIn);
    function getAmountsOut(uint amountIn, address[] calldata path) external view returns (uint[] memory amounts);
    function getAmountsIn(uint amountOut, address[] calldata path) external view returns (uint[] memory amounts);
}

// File: @uniswap/v2-periphery/contracts/interfaces/IUniswapV2Router02.sol



pragma solidity >=0.6.2;

import './IUniswapV2Router01.sol';

interface IUniswapV2Router02 is IUniswapV2Router01 {
    function removeLiquidityETHSupportingFeeOnTransferTokens(
        address token,
        uint liquidity,
        uint amountTokenMin,
        uint amountETHMin,
        address to,
        uint deadline
    ) external returns (uint amountETH);
    function removeLiquidityETHWithPermitSupportingFeeOnTransferTokens(
        address token,
        uint liquidity,
        uint amountTokenMin,
        uint amountETHMin,
        address to,
        uint deadline,
        bool approveMax, uint8 v, bytes32 r, bytes32 s
    ) external returns (uint amountETH);

    function swapExactTokensForTokensSupportingFeeOnTransferTokens(
        uint amountIn,
        uint amountOutMin,
        address[] calldata path,
        address to,
        uint deadline
    ) external;
    function swapExactETHForTokensSupportingFeeOnTransferTokens(
        uint amountOutMin,
        address[] calldata path,
        address to,
        uint deadline
    ) external payable;
    function swapExactTokensForETHSupportingFeeOnTransferTokens(
        uint amountIn,
        uint amountOutMin,
        address[] calldata path,
        address to,
        uint deadline
    ) external;
}

// File: /contracts/MonsterBlockCoreERC721.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

import "@openzeppelin/contracts/token/ERC721/ERC721.sol";
import "@openzeppelin/contracts/token/ERC721/extensions/ERC721Burnable.sol";
import "@openzeppelin/contracts/token/ERC721/extensions/ERC721Enumerable.sol";
import "@openzeppelin/contracts/token/ERC721/extensions/ERC721URIStorage.sol";
import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import "@openzeppelin/contracts/access/Ownable.sol";
import "@openzeppelin/contracts/security/Pausable.sol";
import "@openzeppelin/contracts/security/ReentrancyGuard.sol";
import "@openzeppelin/contracts/utils/Address.sol";

abstract contract MonsterBlockCoreERC721 is ERC721, ERC721Burnable, ERC721Enumerable, ERC721URIStorage, Pausable, Ownable, ReentrancyGuard {
  string public baseURI;
  address internal withdrawContract;
  address internal giveWellAddress;
  address internal charityWalletAddress;

  function strConcat(string memory _a, string memory _b) internal pure returns(string memory) {
    return string(abi.encodePacked(bytes(_a), bytes(_b)));
  }

  function pauseSale() public onlyOwner {
    _pause();
  }

  function unpauseSale() public onlyOwner {
    _unpause();
  }

  function _beforeTokenTransfer(address from, address to, uint256 tokenId) internal override(ERC721, ERC721Enumerable) {
    super._beforeTokenTransfer(from, to, tokenId);
  }

  function supportsInterface(bytes4 interfaceId) public view override(ERC721, ERC721Enumerable) returns (bool) {
    return super.supportsInterface(interfaceId);
  }

  function _burn(uint256 tokenId) internal override(ERC721, ERC721URIStorage) {
    super._burn(tokenId);
  }

  function tokenURI(uint256 tokenId) public view override(ERC721, ERC721URIStorage) returns (string memory) {
    return super.tokenURI(tokenId);
  }

  function setTokenURI(uint256 _tokenId, string memory _tokenURI) public onlyOwner {
    _setTokenURI(_tokenId, _tokenURI);
  }

  function setBaseURI(string memory baseURI_) public onlyOwner {
    baseURI = baseURI_;
  }

  function _baseURI() internal view virtual override returns (string memory) {
      return baseURI;
  }

  function withdraw() public onlyOwner {
    // 10% to GiveWell Maximum Impact Fund
    Address.sendValue(payable(giveWellAddress), address(this).balance / 10);
    // 5% to Charity Wallet
    Address.sendValue(payable(charityWalletAddress), address(this).balance * 5 / 90);
    // Remainder to withdrawal contract
    Address.sendValue(payable(withdrawContract), address(this).balance);
  }

  function withdrawTokens(IERC20 token) public onlyOwner {
		require(address(token) != address(0));
		token.transfer(msg.sender, token.balanceOf(address(this)));
	}

  receive() external payable {}
}

// File: /contracts/MonsterBlocks.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.8.0;

import "./MonsterBlockCoreERC721.sol";
import "@openzeppelin/contracts/utils/Counters.sol";
import "@openzeppelin/contracts/utils/Strings.sol";
import "@chainlink/contracts/src/v0.8/dev/VRFConsumerBase.sol";
import "@uniswap/v2-periphery/contracts/interfaces/IUniswapV2Router02.sol";

contract MonsterBlocks is MonsterBlockCoreERC721, VRFConsumerBase {
  using Counters for Counters.Counter;
  Counters.Counter private _tokenIds;

  uint256 public totalMinted = 0;
  uint256 public totalMintedByOwner = 0;

  IUniswapV2Router02 public uniswapRouter;

  address private VRFCoordinator = 0xf0d54349aDdcf704F77AE15b96510dEA15cb7952;
  address private LinkToken = 0x514910771AF9Ca656af840dff83E8264EcF986CA;
  bytes32 internal keyHash = 0xAA77729D3466CA35AE8D28B3BBAC7CC36A5031EFDC430821C02BC31A238AF445;
  uint256 internal LinkFee = 2 * 10**18; // 2 LINK

  uint256 public constant NFTMintPrice = 55000000000000000; // 0.055 ETH
  uint256 public constant maxSupply = 10000;
  uint256 public constant maxOwnerMintedSupply = 50;
  uint256 public constant maxMintLimit = 20;

  uint256 public constant numberOfTraits = 6;
  uint256 internal constant traitWeighting = 1000;

  uint8[][numberOfTraits] internal traitProbabilities;
  string[numberOfTraits] internal traitCategories;
  string[][numberOfTraits] internal traitNames;

  // DNA is the root random number generated by Chainlink
  mapping(uint256 => uint256) public monsterBlocksDna;
  mapping(bytes32 => uint256[]) internal vrfRequestIds;
  mapping(uint256 => uint256[]) internal stacks;
  mapping(uint256 => bool) internal baseBrickFlourishOverrides;
  mapping(uint256 => bool) internal middleBrickFlourishOverrides;
  mapping(uint256 => bool) internal topBrickFlourishOverrides;
  mapping(address => uint256) internal amountPreMintableByAddress;

  /* Pre-Minting
     Reward your earliest supporters! */

  event PreMintingPaused(address account);
  event PreMintingUnpaused(address account);
  bool private _preMintingPaused;

  function preMintingPaused() public view virtual returns (bool) {
      return _preMintingPaused;
  }

  function pausePreMinting() public onlyOwner {
      _preMintingPaused = true;
      emit PreMintingPaused(_msgSender());
  }

  function unpausePreMinting() public onlyOwner {
      _preMintingPaused = false;
      emit PreMintingUnpaused(_msgSender());
  }

  function preMintMonsterBlock(uint256 _quantity, uint256 _deadline) external payable nonReentrant {
    require(!preMintingPaused(), "Pre-minting paused");
    require(totalMinted + _quantity < maxSupply + 1, "Not enough left");
    require(_quantity < amountPreMintableByAddress[msg.sender] + 1, "Cannot pre-mint this many");
    require(NFTMintPrice * _quantity <= msg.value, "Not enough ETH");

    amountPreMintableByAddress[msg.sender] -= _quantity;

    customerMintMonsterBlock(_quantity, _deadline);
  }

  function pushPreMinters(address[] memory _preMinters, uint256[] memory _amounts) external onlyOwner whenPaused {
    require(_preMinters.length == _amounts.length);

    for(uint8 i = 0; i < _amounts.length; i++) {
      amountPreMintableByAddress[_preMinters[i]] = amountPreMintableByAddress[_preMinters[i]] + _amounts[i];
    }
  }

  function getAmountPreMintable(address _userAddress) public view returns (uint256 amountPreMintable) {
    amountPreMintable = amountPreMintableByAddress[_userAddress];
  }

  /* Owner Minting
     Allow a small number of pre-mints without ETH payment required */

  function ownerMintMonsterBlocks(uint256 _quantity) external onlyOwner {
    require(totalMinted + _quantity < maxSupply + 1, "Not enough left");
    require(totalMintedByOwner + _quantity <= maxOwnerMintedSupply, "Not enough left");
    require(_quantity < maxMintLimit + 1, "Max mint is 20");

    totalMintedByOwner += _quantity;

    mintMonsterBlock(_quantity);
  }

  /* Regular Minting */

  event GenerateMonsterBlock(
    uint256 tokenId
  );

  event UpdateMonsterBlock(
    uint256 indexed id,
    uint256 dna
  );

  function generateMonsterBlock(uint256 _quantity, uint256 _deadline) external payable whenNotPaused nonReentrant {
    require(totalMinted + _quantity < maxSupply + 1, "Not enough left");
    require(_quantity < maxMintLimit + 1, "Max mint is 20");
    require(NFTMintPrice * _quantity <= msg.value, "Not enough ETH");

    customerMintMonsterBlock(_quantity, _deadline);
  }

  function customerMintMonsterBlock(uint256 _quantity, uint256 _deadline) internal {
    /* Every Chainlink call can be used to generate many random numbers
       However, the gas limit for callbacks is 200k
       To balance cost with reliability, use one Chainlink call for every 5 mints */
    uint256 vrfRequestsNeeded = (_quantity + 4) / 5;

    address[] memory path = new address[](2);
    path[0] = uniswapRouter.WETH();
    path[1] = LinkToken;
    uniswapRouter.swapETHForExactTokens { value: msg.value } (LinkFee * vrfRequestsNeeded, path, address(this), _deadline);

    require(LINK.balanceOf(address(this)) >= LinkFee * vrfRequestsNeeded, "Not enough LINK");

    mintMonsterBlock(_quantity);
  }

  function mintMonsterBlock(uint256 _quantity) internal {
    totalMinted += _quantity;

    bytes32 requestId;
    for (uint8 i = 0; i < _quantity; i++) {
      _tokenIds.increment();

      if (i % 5 == 0) { // New Chainlink call for every five mints
        requestId = requestRandomness(keyHash, LinkFee);
      }
      vrfRequestIds[requestId].push(_tokenIds.current());

      _safeMint(msg.sender, _tokenIds.current());

      stacks[_tokenIds.current()] = [_tokenIds.current()];

      emit GenerateMonsterBlock(_tokenIds.current());
    }
  }

  /* Stacking
     Choose a top and bottom block (or tower),
     burn them and create a new tower */

  event StackMonsterBlock(
    uint256 tokenId1,
    uint256 tokenId2,
    uint256 resultTokenId
  );

  function engageStackinator(uint256 _bottomToken, uint256 _topToken) external nonReentrant {
    require(_exists(_bottomToken), "Bottom token does not exist");
    require(_exists(_topToken), "Top token does not exist");
    require(_isApprovedOrOwner(msg.sender, _bottomToken), "Not approved for bottom token");
    require(_isApprovedOrOwner(msg.sender, _topToken), "Not approved for top token");
    require(_bottomToken != _topToken, "Tokens cannot be the same");
    require(stacks[_bottomToken].length > 1 || monsterBlocksDna[_bottomToken] != 0, "No traits for bottom token yet");
    require(stacks[_topToken].length > 1 || monsterBlocksDna[_topToken] != 0, "No traits for top token yet");

    _tokenIds.increment();

    _burn(_bottomToken);
    _burn(_topToken);

    _safeMint(msg.sender, _tokenIds.current());

    for (uint8 i = 0; i < stacks[_bottomToken].length; i++) {
      stacks[_tokenIds.current()].push(stacks[_bottomToken][i]);
    }
    for (uint8 i = 0; i < stacks[_topToken].length; i++) {
      stacks[_tokenIds.current()].push(stacks[_topToken][i]);
    }

    emit StackMonsterBlock(_bottomToken, _topToken, _tokenIds.current());
  }

  /* Constructor */

  constructor(address _withdrawContract, address _giveWellAddress, address _charityWalletAddress)
    ERC721("MonsterBlocks", "MONSTERBLOCK")
    VRFConsumerBase(VRFCoordinator, LinkToken)
  {
    uniswapRouter = IUniswapV2Router02(0x7a250d5630B4cF539739dF2C5dAcb4c659F2488D);

    withdrawContract = _withdrawContract;
    giveWellAddress = _giveWellAddress;
    charityWalletAddress = _charityWalletAddress;

    // Start in paused sale state
    pauseSale();
    pausePreMinting();

    setBaseURI('https://api.monsterblocks.io/metadata/');

    traitProbabilities[0] = [1, 4, 10, 17, 20, 4, 17, 17, 17, 17, 4, 17, 17, 20, 17, 17, 17, 10, 17, 17, 10, 17, 17, 3, 17, 17, 10, 17, 20, 17, 17, 17, 17, 17, 8, 17, 19, 17, 17, 17, 17, 17, 17, 17, 17, 17, 17, 2, 17, 8, 17, 17, 17, 10, 17, 17, 17, 17, 17, 17, 17, 17, 17, 17, 3, 17, 17, 1];
    traitCategories[0] = "Base Brick";

    traitProbabilities[1] = [12, 10, 3, 10, 12, 12, 7, 13, 14, 12, 12, 12, 7, 12, 10, 12, 12, 15, 4, 12, 12, 6, 12, 2, 2, 12, 12, 12, 12, 12, 12, 12, 12, 10, 15, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 15, 12, 12, 12, 12, 12, 15, 12, 12, 12, 12, 12, 15, 12, 12, 12, 12, 15, 6, 3, 12, 12, 12, 15, 12, 6, 12, 15, 12, 12, 12, 12, 14, 6, 12, 12, 12, 8, 12, 12, 15, 14, 14];
    traitCategories[1] = "Middle Brick";

    traitProbabilities[2] = [2, 12, 10, 4, 10, 12, 12, 7, 12, 14, 12, 12, 12, 7, 12, 10, 12, 12, 14, 4, 12, 12, 6, 12, 4, 4, 12, 12, 12, 12, 12, 12, 12, 12, 10, 14, 12, 12, 12, 12, 12, 12, 12, 12, 12, 12, 14, 12, 12, 12, 12, 12, 14, 12, 12, 12, 12, 12, 14, 12, 12, 12, 12, 13, 6, 3, 12, 12, 12, 14, 12, 6, 12, 15, 12, 12, 12, 12, 14, 6, 12, 12, 12, 8, 12, 12, 15, 15, 15];
    traitCategories[2] = "Top Brick";

    traitProbabilities[3] = [19, 19, 19, 19, 19, 22, 19, 19, 3, 19, 15, 19, 20, 22, 19, 19, 19, 19, 22, 15, 19, 19, 19, 3, 19, 19, 22, 10, 3, 3, 19, 19, 19, 9, 19, 22, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 2, 22, 19, 19, 19, 9, 19, 19, 22, 10, 19, 22, 0];
    traitCategories[3] = "Base Flourish";

    traitProbabilities[4] = [19, 19, 19, 19, 19, 22, 19, 19, 3, 19, 15, 19, 20, 22, 19, 19, 19, 19, 22, 15, 19, 19, 19, 3, 19, 19, 22, 10, 3, 3, 19, 19, 19, 9, 19, 22, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 2, 22, 19, 19, 19, 9, 19, 19, 22, 10, 19, 22, 0];
    traitCategories[4] = "Middle Flourish";

    traitProbabilities[5] = [19, 19, 19, 19, 19, 22, 19, 19, 3, 19, 15, 19, 20, 22, 19, 19, 19, 19, 22, 15, 19, 19, 19, 3, 19, 19, 22, 10, 3, 3, 19, 19, 19, 9, 19, 22, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 2, 22, 19, 19, 19, 9, 19, 19, 22, 10, 19, 22, 0];
    traitCategories[5] = "Top Flourish";

    traitNames[0] = [
      'Multivac',
      'Purple Hallucinogenic Frog',
      'Green Hallucinogenic Frog',
      'A Shrewd Mind',
      'Sorrowful Wyvern',
      'Golden Sorrowful Wyvern',
      'Blue Bull',
      'Pink Bull',
      'Crawling Skull',
      'Fly Statue',
      'Frothy Waterkin',
      'Currish Lifter',
      'Craven Maltworm',
      'Mammering Moldwarp',
      'Doghearted Hermit',
      'Saucy Measle',
      'Salacious Lewdster',
      'Golden Vigilant Gator',
      'Vigilant Gator',
      'Primordial Pigdog',
      'The Butcher',
      'Gnarly Vegetation',
      'Unwanted Octopus',
      'Albino Neighborly Chameleon',
      'Neighborly Chameleon',
      'Red Squeamish Behemoth',
      'Stone Squeamish Behemoth',
      'Turquoise Squeamish Behemoth',
      'Orange Lethargic Giraffe',
      'Green Lethargic Giraffe',
      'Green Pensive Serpent',
      'Blue Pensive Serpent',
      'Green Beleaguered Troll',
      'Orange Beleaguered Troll',
      'Gold Contemplative Grizzly',
      'Stone Contemplative Grizzly',
      'Burdened Hobgoblin',
      'Turquoise Militarized Rodent',
      'Amber Militarized Rodent',
      'Concrete Militarized Rodent',
      'Shielded Goblin',
      'Welcoming Demon',
      'Blue Paralyzed Bull',
      'Brown Paralyzed Bull',
      'Yellow Weaponized Hare',
      'Green Weaponized Hare',
      'Obedient Reptile',
      'Red Phillip',
      'Somber Ouroboros',
      'Degenerate Collector',
      'Despondent Skeleton',
      'Merry Beast',
      'Guffawing Skull',
      'Golden Guffawing Skull',
      'Gleeking Footlicker',
      'Gorbellied Varlet',
      'Frothy Wagtail',
      'Red Lumpish Vassal',
      'Grey Lumpish Vassal',
      'Batfowl Statue',
      'Milk Livered Scoundrel',
      'Paunchy Chum',
      'Boxed Cat',
      'Masked Troll',
      'Henrique',
      'Frogwig',
      'Flexible Menace',
      'Shameful Troll'
    ];

    traitNames[1] = [
      'Welcoming Palms',
      'Volcanic Eruption',
      'Vile Spider',
      'Sluggish Tiger',
      'Peculiar Seahorse',
      'Intimidating Amphibian',
      'Hostile Beetle',
      'Golden Hound',
      'Extinct Losers',
      'Chunky Shark',
      'Brown Hound',
      'Cantankerous Snapper',
      'Bodacious Twins',
      'Paranoid Rex',
      'Meek Triceratops',
      'Contemplative Kiwi',
      'Haughty Eagle',
      'Squished Elephant',
      'Jaded Cellmates',
      'Angler Fish',
      'Counterfeit Sphinx',
      'Happy Meal Crown',
      'Primitive Salamander',
      'Trifling Hornrat',
      'Ectoplasm',
      'Red Grisly Harpy',
      'Green Grisly Harpy',
      'Stone Grisly Harpy',
      'Yellow Frantic Coyote',
      'Blue Frantic Coyote',
      'Stone Petrified Owl',
      'Amber Petrified Owl',
      'Burrowing Reptile',
      'Purple Space Walrus',
      'Stone Space Walrus',
      'Jolly Martian',
      'Cement Irate Swine',
      'Red Irate Swine',
      'Plague Rat',
      'Wooden Crate',
      'Red Euphoric Pond Scum',
      'Green Euphoric Pond Scum',
      'Red Pipsqueak',
      'Blue Pipsqueak',
      'Worried Tiki',
      'Sedated Ogre',
      'Medicated Druid',
      'Yellow Ecstatic Shrew',
      'Green Ecstatic Shrew',
      'Green Prancing Lurker',
      'Stone Prancing Lurker',
      'Forlorn Shaman',
      'Marbled Moth',
      'Piggish Vicar',
      'Red Paltry Soothsayer',
      'Green Paltry Soothsayer',
      'Red Moon Hound',
      'Yellow Moon Hound',
      'Unsightly Vermin',
      'Anonymous Eyeball',
      'Weather Satellite',
      'Treasure Chest',
      'Thingamajig',
      'Tesla Coil',
      'Supreme Cyborg',
      'Precious Crystals',
      'Ice Cube',
      'Decommissioned Cyborg',
      'Comet Kohoutek',
      'Boring Columns',
      'Log',
      'Quizzical Pig',
      'Omnipotent Mutant',
      'Psychedelic Bison',
      'Scrunched Brain',
      'Concerned Loaf',
      'Uncultured Swine',
      'Cyclone',
      'Barrel of Fungible Tokens',
      'Blue Odious Bird',
      'Clumsy Eavesdropper',
      'Disembodied Mouth',
      'Just Some Bricks',
      'Metallic Odious Bird',
      'Putrid Underling',
      'Useless Machine',
      'Irritated Aardvark',
      'Hungry Komodo'
    ];
    traitNames[2] = [
      'Gold Bullion',
      'Welcoming Palms',
      'Volcanic Eruption',
      'Vile Spider',
      'Sluggish Tiger',
      'Peculiar Seahorse',
      'Intimidating Amphibian',
      'Hostile Beetle',
      'Golden Hound',
      'Extinct Losers',
      'Chunky Shark',
      'Brown Hound',
      'Cantankerous Snapper',
      'Bodacious Twins',
      'Paranoid Rex',
      'Meek Triceratops',
      'Contemplative Kiwi',
      'Haughty Eagle',
      'Squished Elephant',
      'Jaded Cellmates',
      'Angler Fish',
      'Counterfeit Sphinx',
      'Happy Meal Crown',
      'Primitive Salamander',
      'Trifling Hornrat',
      'Ectoplasm',
      'Red Grisly Harpy',
      'Green Grisly Harpy',
      'Stone Grisly Harpy',
      'Yellow Frantic Coyote',
      'Blue Frantic Coyote',
      'Stone Petrified Owl',
      'Amber Petrified Owl',
      'Burrowing Reptile',
      'Purple Space Walrus',
      'Stone Space Walrus',
      'Jolly Martian',
      'Cement Irate Swine',
      'Red Irate Swine',
      'Plague Rat',
      'Wooden Crate',
      'Red Euphoric Pond Scum',
      'Green Euphoric Pond Scum',
      'Red Pipsqueak',
      'Blue Pipsqueak',
      'Worried Tiki',
      'Sedated Ogre',
      'Medicated Druid',
      'Yellow Ecstatic Shrew',
      'Green Ecstatic Shrew',
      'Green Prancing Lurker',
      'Stone Prancing Lurker',
      'Forlorn Shaman',
      'Marbled Moth',
      'Piggish Vicar',
      'Red Paltry Soothsayer',
      'Green Paltry Soothsayer',
      'Red Moon Hound',
      'Yellow Moon Hound',
      'Unsightly Vermin',
      'Anonymous Eyeball',
      'Weather Satellite',
      'Treasure Chest',
      'Thingamajig',
      'Tesla Coil',
      'Supreme Cyborg',
      'Precious Crystals',
      'Ice Cube',
      'Decommissioned Cyborg',
      'Comet Kohoutek',
      'Boring Columns',
      'Log',
      'Quizzical Pig',
      'Omnipotent Mutant',
      'Psychedelic Bison',
      'Scrunched Brain',
      'Concerned Loaf',
      'Uncultured Swine',
      'Cyclone',
      'Barrel of Fungible Tokens',
      'Blue Odious Bird',
      'Clumsy Eavesdropper',
      'Disembodied Mouth',
      'Just Some Bricks',
      'Metallic Odious Bird',
      'Putrid Underling',
      'Useless Machine',
      'Irritated Aardvark',
      'Hungry Komodo'
    ];

    string[59] memory flourishTraits = [
      'Stone Stern Fists',
      'Blue Stern Fists',
      'Orange Stern Fists',
      'Green Depressive Lizard',
      'Stone Depressive Lizard',
      'Vengeful Dagger',
      'Precious Little Antlers',
      'Prominent Rooster',
      'Gilded Wings',
      'Cretinous Gargoyle',
      'A Real Spark',
      'Royal Flourish',
      'Morning Star',
      'Lush Foliage',
      'Glorious Flappers',
      'Fairy Ears',
      'Expensive Gizmo',
      'Dragonly Wings',
      'Broad Sword',
      'Bear Claw',
      'Cool Hands',
      'Nice Flippers',
      'Feet',
      'Sumptuous Amethyst',
      'Primitive Clubs',
      'Enticing Antennae',
      'Raptor Claws',
      'Skeleton Shield',
      'Sick Flames',
      'Rad Axe',
      'Nice Leaves',
      'Mother Nature',
      'Decent Horns',
      'Detached Eyeball',
      'Impressive Pointers',
      'Butterfly Wings',
      'Fortified Spikes',
      'Skeletal Wings',
      'Tongue',
      'Tentacles',
      'Sumptuous Cactus',
      'Skinless Wings',
      'Satellite Dishes',
      'Roman Columns',
      'Ram Horns',
      'Ominous Cloud',
      'Night and Day',
      'Mounted Cannons',
      'Mortal Chameleon',
      'Helpful Hands',
      'Green Feelers',
      'Fishmonger Hands',
      'Disapproving Eyes',
      'Delightful Wings',
      'Crocodile Mouths',
      'Cheap Ghost',
      'Buzzsaws',
      'Blue Whiskers',
      'None'
    ];
    traitNames[3] = flourishTraits;
    traitNames[4] = flourishTraits;
    traitNames[5] = flourishTraits;

    for (uint256 i = 0; i < 25; i++) {
      baseBrickFlourishOverrides[i] = true;
    }

    for (uint256 i = 0; i < 25; i++) {
      middleBrickFlourishOverrides[i] = true;
    }

    for (uint256 i = 0; i < 26; i++) {
      topBrickFlourishOverrides[i] = true;
    }
  }

  /* Utility & Metadata */

  function latestSerialNumber() external view returns (uint256) {
    return _tokenIds.current();
  }

  function traitMetadata(uint256 _tokenId) external view returns (string memory) {
    require(stacks[_tokenId].length > 1 || monsterBlocksDna[_tokenId] != 0, "No DNA yet");

    string memory resultString = '{';
    for (uint8 i = 0; i < stacks[_tokenId].length; i++) {

      uint256[numberOfTraits] memory mintedTraits;
      for (uint8 j = 0; j < numberOfTraits; j++) {
        mintedTraits[j] = getMintedTrait(stacks[_tokenId][i], j);
      }

      // Override traits with no flourish
      if (baseBrickFlourishOverrides[mintedTraits[0]]) {
        mintedTraits[3] = traitProbabilities[3].length - 1;
      }
      if (middleBrickFlourishOverrides[mintedTraits[1]]) {
        mintedTraits[4] = traitProbabilities[4].length - 1;
      }
      if (topBrickFlourishOverrides[mintedTraits[2]]) {
        mintedTraits[5] = traitProbabilities[5].length - 1;
      }

      for (uint8 j = 0; j < numberOfTraits; j++) {
        if (i > 0 || j > 0) {
          resultString = strConcat(resultString, ', ');
        }
        resultString = strConcat(resultString, '"');
        resultString = strConcat(resultString, traitCategories[j]);
        resultString = strConcat(resultString, '": "');
        resultString = strConcat(resultString, traitNames[j][mintedTraits[j]]);
        resultString = strConcat(resultString, '"');
      }
    }

    resultString = strConcat(resultString, stackMetadata(_tokenId));

    resultString = strConcat(resultString, ', "Tower Height": "');
    resultString = strConcat(resultString, Strings.toString(stacks[_tokenId].length));
    resultString = strConcat(resultString, '"');

    return strConcat(resultString, '}');
  }

  function getMintedTrait(uint256 _tokenId, uint8 _traitIndex) internal view returns (uint256) {
    uint256 currentWeight = traitWeighting;
    uint256 dnaModuloWeight = (uint256(keccak256(abi.encode(monsterBlocksDna[_tokenId], _traitIndex + 1))) % currentWeight) + 1;

    for (uint8 j = 0; j < traitProbabilities[_traitIndex].length; j++) {
      currentWeight -= traitProbabilities[_traitIndex][j];
      if (dnaModuloWeight > currentWeight) {
        return j;
      }
    }

    return traitProbabilities[_traitIndex].length - 1;
  }

  function stackMetadata(uint256 _tokenId) internal view returns (string memory) {
    string memory resultString = '';

    if (stacks[_tokenId].length < 2) {
      return resultString;
    }

    for (uint8 i = 0; i < stacks[_tokenId].length; i++) {
      resultString = strConcat(resultString, ', "Token ');
      resultString = strConcat(resultString, Strings.toString(i + 1));
      resultString = strConcat(resultString, '": "');
      resultString = strConcat(resultString, Strings.toString(stacks[_tokenId][i]));
      resultString = strConcat(resultString, '"');
    }

    return resultString;
  }

  /* Chainlink */

  function setLinkFee(uint256 _LinkFee) external onlyOwner {
    LinkFee = _LinkFee;
  }

  function fulfillRandomness(bytes32 _requestId, uint256 _randomNumber) internal override {
    for (uint8 i = 0; i < vrfRequestIds[_requestId].length; i++) {
      uint256 blockDna;
      if (i == 0) {
        blockDna = _randomNumber;
      } else {
        blockDna = uint256(keccak256(abi.encode(_randomNumber, numberOfTraits * i)));
      }
      monsterBlocksDna[vrfRequestIds[_requestId][i]] = blockDna;

      emit UpdateMonsterBlock(vrfRequestIds[_requestId][i], blockDna);
    }
  }

  // Fail-safe in case Chainlink callback fails - hopefully never required
  function retryChainlinkRequest(uint256 _tokenId) external onlyOwner {
    require(_exists(_tokenId), "Token does not exist");
    require(monsterBlocksDna[_tokenId] == 0, "Already has DNA set - cannot be changed");
    require(LINK.balanceOf(address(this)) >= LinkFee, "Not enough LINK");

    bytes32 requestId = requestRandomness(keyHash, LinkFee);
    vrfRequestIds[requestId].push(_tokenId);
  }
}