/**
 *Submitted for verification at Etherscan.io on 2021-12-21
*/

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.9;

/////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
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/**
 * @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);
}

/**
 * @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;
    }
}

/**
 * @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;
}

/**
 * @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);
}

/**
 * @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);
}

/**
 * @dev Provides information about the current execution context, including the
 * sender of the transaction and its data. While these are generally available
 * via msg.sender and msg.data, they should not be accessed in such a direct
 * manner, since when dealing with meta-transactions the account sending and
 * paying for execution may not be the actual sender (as far as an application
 * is concerned).
 *
 * This contract is only required for intermediate, library-like contracts.
 */
abstract contract Context {
    function _msgSender() internal view virtual returns (address) {
        return msg.sender;
    }

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

/**
 * @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 internal _owner;

    event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);

    /**
     * @dev Initializes the contract setting the deployer as the initial owner.
     */
    constructor() {
        _setOwner(_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 {
        _setOwner(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");
        _setOwner(newOwner);
    }

    function _setOwner(address newOwner) private {
        address oldOwner = _owner;
        _owner = newOwner;
        emit OwnershipTransferred(oldOwner, newOwner);
    }
}

/**
 * @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;
        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");

        (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");

        (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");

        (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");

        (bool success, bytes memory returndata) = target.delegatecall(data);
        return verifyCallResult(success, returndata, errorMessage);
    }

    /**
     * @dev Tool to verifies that a low level call was successful, and revert if it wasn't, either by bubbling the
     * revert reason using the provided one.
     *
     * _Available since v4.3._
     */
    function verifyCallResult(
        bool success,
        bytes memory returndata,
        string memory errorMessage
    ) internal 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

                assembly {
                    let returndata_size := mload(returndata)
                    revert(add(32, returndata), returndata_size)
                }
            } else {
                revert(errorMessage);
            }
        }
    }
}

/**
 * @dev String operations.
 */
library Strings {
    bytes16 private constant _HEX_SYMBOLS = "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] = _HEX_SYMBOLS[value & 0xf];
            value >>= 4;
        }
        require(value == 0, "Strings: hex length insufficient");
        return string(buffer);
    }
}

/**
 * @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}.
 * 
 * This ERC721 Has been adjusted from OpenZepplins to have a totalSupply method. It also is set up to better handle 
 * batch transactions. Namely it does not update _balances on a call to {_mint} and expects the minting method to do so. 
 */
contract ERC721 is Context, ERC165, IERC721, IERC721Metadata {
    using Address for address;
    using Strings for uint256;

    // Token name
    string internal _name;

    // Token symbol
    string internal _symbol;

    // Tracking total minted
    uint256 internal _totalMinted;

    // Tracking total burned
    uint256 internal _totalBurned;

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

    // Mapping owner address to token count
    mapping(address => uint256) internal _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 Returns the total current supply of the contract. 
     *
     * WARNING - Underlying variables do NOT get automatically updated on mints
     * so that we can save gas on transactions that mint multiple tokens.
     *
     */
    function totalSupply() public view virtual returns (uint256) {
        return _totalMinted - _totalBurned;
    }

    /**
     * @dev Returns the total ever minted from this contract. 
     *
     * WARNING - Underlying variable do NOT get automatically updated on mints
     * so that we can save gas on transactions that mint multiple tokens.
     *
     */
    function totalMinted() public view virtual returns (uint256) {
        return _totalMinted;
    }

    /**
     * @dev See {IERC721Metadata-tokenURI}.
     */
    function tokenURI(uint256 tokenId) public view virtual override returns (string memory) {
        require(_exists(tokenId), "ERC721: 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}. If set, the resulting URI for each
     * token will be the concatenation of the `baseURI` and the `tokenId`. 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`. 
     *
     * WARNING - this method does not update totalSupply or _balances, please update that externally. Doing so
     * will allow us to save gas on transactions that mint more than one NFT
     *
     * 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.
     * WARNING: This method does not update totalSupply, please update that externally. Doing so
     * will allow us to save gas on transactions
     */
    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
     * WARNING: This method does not update totalSupply or _balances, please update that externally. Doing so
     * will allow us to save gas on transactions that mint more than one NFT
     *
     * 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);

        _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;
        _totalBurned += 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.onERC721Received.selector;
            } catch (bytes memory reason) {
                if (reason.length == 0) {
                    revert("ERC721: transfer to non ERC721Receiver implementer");
                } else {
                    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` and `to` are never both zero.
     *
     * 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 {}
}

/**
 * @dev Interface for the NFT Royalty Standard
 */
interface IERC2981 is IERC165 {
    /** 
     * ERC165 bytes to add to interface array - set in parent contract
     * implementing this standard
     *
     * bytes4(keccak256("royaltyInfo(uint256,uint256)")) == 0x2a55205a
     * bytes4 private constant _INTERFACE_ID_ERC2981 = 0x2a55205a;
     * _registerInterface(_INTERFACE_ID_ERC2981);
     */

    /**
     * @notice Called with the sale price to determine how much royalty
     *          is owed and to whom.
     * @param _tokenId - the NFT asset queried for royalty information
     * @param _salePrice - the sale price of the NFT asset specified by _tokenId
     * @return receiver - address of who should be sent the royalty payment
     * @return royaltyAmount - the royalty payment amount for _salePrice
     */
    function royaltyInfo(
        uint256 _tokenId,
        uint256 _salePrice
    ) external view returns (
        address receiver,
        uint256 royaltyAmount
    );
}

/**
 * @dev Elliptic Curve Digital Signature Algorithm (ECDSA) operations.
 *
 * These functions can be used to verify that a message was signed by the holder
 * of the private keys of a given address.
 */
library ECDSA {
    enum RecoverError {
        NoError,
        InvalidSignature,
        InvalidSignatureLength,
        InvalidSignatureS,
        InvalidSignatureV
    }

    function _throwError(RecoverError error) private pure {
        if (error == RecoverError.NoError) {
            return; // no error: do nothing
        } else if (error == RecoverError.InvalidSignature) {
            revert("ECDSA: invalid signature");
        } else if (error == RecoverError.InvalidSignatureLength) {
            revert("ECDSA: invalid signature length");
        } else if (error == RecoverError.InvalidSignatureS) {
            revert("ECDSA: invalid signature 's' value");
        } else if (error == RecoverError.InvalidSignatureV) {
            revert("ECDSA: invalid signature 'v' value");
        }
    }

    /**
     * @dev Returns the address that signed a hashed message (`hash`) with
     * `signature` or error string. This address can then be used for verification purposes.
     *
     * The `ecrecover` EVM opcode allows for malleable (non-unique) signatures:
     * this function rejects them by requiring the `s` value to be in the lower
     * half order, and the `v` value to be either 27 or 28.
     *
     * IMPORTANT: `hash` _must_ be the result of a hash operation for the
     * verification to be secure: it is possible to craft signatures that
     * recover to arbitrary addresses for non-hashed data. A safe way to ensure
     * this is by receiving a hash of the original message (which may otherwise
     * be too long), and then calling {toEthSignedMessageHash} on it.
     *
     * Documentation for signature generation:
     * - with https://web3js.readthedocs.io/en/v1.3.4/web3-eth-accounts.html#sign[Web3.js]
     * - with https://docs.ethers.io/v5/api/signer/#Signer-signMessage[ethers]
     *
     * _Available since v4.3._
     */
    function tryRecover(bytes32 hash, bytes memory signature) internal pure returns (address, RecoverError) {
        // Check the signature length
        // - case 65: r,s,v signature (standard)
        // - case 64: r,vs signature (cf https://eips.ethereum.org/EIPS/eip-2098) _Available since v4.1._
        if (signature.length == 65) {
            bytes32 r;
            bytes32 s;
            uint8 v;
            // ecrecover takes the signature parameters, and the only way to get them
            // currently is to use assembly.
            assembly {
                r := mload(add(signature, 0x20))
                s := mload(add(signature, 0x40))
                v := byte(0, mload(add(signature, 0x60)))
            }
            return tryRecover(hash, v, r, s);
        } else if (signature.length == 64) {
            bytes32 r;
            bytes32 vs;
            // ecrecover takes the signature parameters, and the only way to get them
            // currently is to use assembly.
            assembly {
                r := mload(add(signature, 0x20))
                vs := mload(add(signature, 0x40))
            }
            return tryRecover(hash, r, vs);
        } else {
            return (address(0), RecoverError.InvalidSignatureLength);
        }
    }

    /**
     * @dev Returns the address that signed a hashed message (`hash`) with
     * `signature`. This address can then be used for verification purposes.
     *
     * The `ecrecover` EVM opcode allows for malleable (non-unique) signatures:
     * this function rejects them by requiring the `s` value to be in the lower
     * half order, and the `v` value to be either 27 or 28.
     *
     * IMPORTANT: `hash` _must_ be the result of a hash operation for the
     * verification to be secure: it is possible to craft signatures that
     * recover to arbitrary addresses for non-hashed data. A safe way to ensure
     * this is by receiving a hash of the original message (which may otherwise
     * be too long), and then calling {toEthSignedMessageHash} on it.
     */
    function recover(bytes32 hash, bytes memory signature) internal pure returns (address) {
        (address recovered, RecoverError error) = tryRecover(hash, signature);
        _throwError(error);
        return recovered;
    }

    /**
     * @dev Overload of {ECDSA-tryRecover} that receives the `r` and `vs` short-signature fields separately.
     *
     * See https://eips.ethereum.org/EIPS/eip-2098[EIP-2098 short signatures]
     *
     * _Available since v4.3._
     */
    function tryRecover(
        bytes32 hash,
        bytes32 r,
        bytes32 vs
    ) internal pure returns (address, RecoverError) {
        bytes32 s;
        uint8 v;
        assembly {
            s := and(vs, 0x7fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff)
            v := add(shr(255, vs), 27)
        }
        return tryRecover(hash, v, r, s);
    }

    /**
     * @dev Overload of {ECDSA-recover} that receives the `r and `vs` short-signature fields separately.
     *
     * _Available since v4.2._
     */
    function recover(
        bytes32 hash,
        bytes32 r,
        bytes32 vs
    ) internal pure returns (address) {
        (address recovered, RecoverError error) = tryRecover(hash, r, vs);
        _throwError(error);
        return recovered;
    }

    /**
     * @dev Overload of {ECDSA-tryRecover} that receives the `v`,
     * `r` and `s` signature fields separately.
     *
     * _Available since v4.3._
     */
    function tryRecover(
        bytes32 hash,
        uint8 v,
        bytes32 r,
        bytes32 s
    ) internal pure returns (address, RecoverError) {
        // EIP-2 still allows signature malleability for ecrecover(). Remove this possibility and make the signature
        // unique. Appendix F in the Ethereum Yellow paper (https://ethereum.github.io/yellowpaper/paper.pdf), defines
        // the valid range for s in (301): 0 < s < secp256k1n ÷ 2 + 1, and for v in (302): v ∈ {27, 28}. Most
        // signatures from current libraries generate a unique signature with an s-value in the lower half order.
        //
        // If your library generates malleable signatures, such as s-values in the upper range, calculate a new s-value
        // with 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364141 - s1 and flip v from 27 to 28 or
        // vice versa. If your library also generates signatures with 0/1 for v instead 27/28, add 27 to v to accept
        // these malleable signatures as well.
        if (uint256(s) > 0x7FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF5D576E7357A4501DDFE92F46681B20A0) {
            return (address(0), RecoverError.InvalidSignatureS);
        }
        if (v != 27 && v != 28) {
            return (address(0), RecoverError.InvalidSignatureV);
        }

        // If the signature is valid (and not malleable), return the signer address
        address signer = ecrecover(hash, v, r, s);
        if (signer == address(0)) {
            return (address(0), RecoverError.InvalidSignature);
        }

        return (signer, RecoverError.NoError);
    }

    /**
     * @dev Overload of {ECDSA-recover} that receives the `v`,
     * `r` and `s` signature fields separately.
     */
    function recover(
        bytes32 hash,
        uint8 v,
        bytes32 r,
        bytes32 s
    ) internal pure returns (address) {
        (address recovered, RecoverError error) = tryRecover(hash, v, r, s);
        _throwError(error);
        return recovered;
    }

    /**
     * @dev Returns an Ethereum Signed Message, created from a `hash`. This
     * produces hash corresponding to the one signed with the
     * https://eth.wiki/json-rpc/API#eth_sign[`eth_sign`]
     * JSON-RPC method as part of EIP-191.
     *
     * See {recover}.
     */
    function toEthSignedMessageHash(bytes32 hash) internal pure returns (bytes32) {
        // 32 is the length in bytes of hash,
        // enforced by the type signature above
        return keccak256(abi.encodePacked("\x19Ethereum Signed Message:\n32", hash));
    }

    /**
     * @dev Returns an Ethereum Signed Typed Data, created from a
     * `domainSeparator` and a `structHash`. This produces hash corresponding
     * to the one signed with the
     * https://eips.ethereum.org/EIPS/eip-712[`eth_signTypedData`]
     * JSON-RPC method as part of EIP-712.
     *
     * See {recover}.
     */
    function toTypedDataHash(bytes32 domainSeparator, bytes32 structHash) internal pure returns (bytes32) {
        return keccak256(abi.encodePacked("\x19\x01", domainSeparator, structHash));
    }
}

/**
 * @dev External interface of the EaselyPayout contract
 */
interface IEaselyPayout {
    /**
     * @dev Takes in a payable amount and splits it among the given royalties. 
     * Also takes a cut of the payable amount depending on the sender and the primaryPayout address.
     * Ensures that this method never splits over 100% of the payin amount.
     */
    function splitPayable(address primaryPayout, address[] memory royalties, uint256[] memory bps) external payable;
}

/** 
 * @dev Extension of the ERC721Enumerable contract that integrates a marketplace so that simple lazy-sales
 * do not have to be done on another contract. This saves gas fees on secondary sales because
 * buyers will not have to pay a gas fee to setApprovalForAll for another marketplace contract after buying.
 *
 * Easely will help power the lazy-selling as well as lazy minting that take place on 
 * directly on the collection, which is why we take a cut of these transactions. Our cut can 
 * be publically seen in the connected EaselyPayout contract and cannot exceed 5%.
 * 
 * Owners also set an alternate signer which they can change at any time. This alternate signer helps enable
 * sales for large batches of addresses without needing to manually sign hundreds or thousands of hashes.
 */
abstract contract ERC721Marketplace is ERC721, Ownable {
    using ECDSA for bytes32;
    using Strings for uint256;

    /* see {IEaselyPayout} for more */
    address public constant PAYOUT_CONTRACT_ADDRESS = 0x68f5C1e24677Ac4ae845Dde07504EAaD98f82572;
    uint256 public constant TIME_PER_DECREMENT = 300;

    uint256 public constant MAX_ROYALTIES_BPS = 9500;
    /* Optional basis points for the owner for secondary sales of this collection */
    uint256 public constant MAX_SECONDARY_BPS = 1000;
    
    /* Let's the owner enable another address to lazy mint */
    address public alternateSignerAddress;
    /* Optional basis points for the owner for secondary sales of this collection */
    uint256 public ownerRoyaltyBPS;
    /* Optional addresses to distribute revenue of primary sales of this collection */
    address[] public revenueShare;
    /* Optional basis points for revenue share for primary sales of this collection */
    uint256[] public revenueShareBPS;

    /* Mapping to the active version for all signed transactions */
    mapping(address => uint256) internal _addressToActiveVersion;
    /* Cancelled or finalized sales by hash to determine buyabliity */
    mapping(bytes32 => bool) internal _cancelledOrFinalizedSales;

    // Events related to lazy selling
    event SaleCancelled(address indexed seller, bytes32 hash);
    event SaleCompleted(uint256 indexed tokenId, uint256 price, address indexed seller, address indexed buyer, bytes32 hash);

    // Miscellaneous events
    event VersionChanged(address indexed seller, uint256 version);
    event AltSignerChanged(address newSigner);
    event BalanceWithdrawn(uint256 balance);
    event RoyaltyUpdated(uint256 bps);
    event RevenueShareUpdated(address address1, address address2, uint256 bps1, uint256 bps2);

    /**
     * @dev See {IERC165-supportsInterface}.
     */
    function supportsInterface(bytes4 interfaceId) public view virtual override returns (bool) {
        return interfaceId == type(Ownable).interfaceId
            || interfaceId == type(IERC2981).interfaceId
            || super.supportsInterface(interfaceId);
    }

    /**
     * @dev see {IERC2981-supportsInterface}
     */
    function royaltyInfo(uint256 _tokenId, uint256 _salePrice) external view returns (address receiver, uint256 royaltyAmount) {
        uint256 royalty = _salePrice * ownerRoyaltyBPS / 10000;
        return (owner(), royalty);
    }

    /** 
     * @dev See {_currentPrice}
     */
    function getCurrentPrice(uint256[4] memory pricesAndTimestamps) external view returns (uint256) {
        return _currentPrice(pricesAndTimestamps);
    }

    /**
     * @dev Returns the current activeVersion of an address both used to create signatures
     * and to verify signatures of {buyToken} and {buyNewToken}
     */
    function getActiveVersion(address address_) external view returns (uint256) {
        return _addressToActiveVersion[address_];
    }

    /**
     * This function, while callable by anybody will always ONLY withdraw the
     * contract's balance to:
     *
     * the owner's account
     * the addresses the owner has set up for revenue share
     * the easely payout contract cut - capped at 5% but can be lower for some users
     *
     * This is callable by anybody so that Easely can set up automatic payouts
     * after a contract has reached a certain minimum to save creators the gas fees
     * involved in withdrawing balances.
     */
    function withdrawBalance(uint256 withdrawAmount) external {
        require(withdrawAmount <= address(this).balance);
        IEaselyPayout payoutContract = IEaselyPayout(PAYOUT_CONTRACT_ADDRESS);
        payoutContract.splitPayable{ value: withdrawAmount }(owner(), revenueShare, revenueShareBPS);
        emit BalanceWithdrawn(withdrawAmount);
    }

    /**
     * @dev Allows the owner to change who the alternate signer is 
     */
    function setAltSigner(address alt) external onlyOwner {
        alternateSignerAddress = alt;
        emit AltSignerChanged(alt);
    }

    /**
     * @dev see {_setRoyalties}
     */
    function setRevenueShare(address[2] memory newAddresses, uint256[2] memory newBPS) external onlyOwner {
        _setRevenueShare(newAddresses, newBPS);
        emit RevenueShareUpdated(newAddresses[0], newAddresses[1], newBPS[0], newBPS[1]);
    }

    /**
     * @dev see {_setSecondary}
     */
    function setRoyaltiesBPS(uint256 newBPS) external onlyOwner() {
        _setRoyaltiesBPS(newBPS);
        emit RoyaltyUpdated(newBPS);
    }

    /**
     * @dev Usable by any user to update the version that they want their signatures to check. This is helpful if
     * an address wants to mass invalidate their signatures without having to call cancelSale on each one.
     */
    function updateVersion(uint256 version) external {
        _addressToActiveVersion[_msgSender()] = version;
        emit VersionChanged(_msgSender(), version);
    }

    /**
     * @dev Usable by the owner of any token initiate a sale for their token. This does not
     * lock the tokenId and the owner can freely trade their token, but doing so will 
     * invalidate the ability for others to buy.
     */
    function hashToSignToSellToken(
        uint256 version,
        uint256 nonce,
        uint256 tokenId,
        uint256[4] memory pricesAndTimestamps
    ) external view returns (bytes32) {
        require(_msgSender() == ownerOf(tokenId), "Not the owner of the token");
        return _hashForSale(_msgSender(), version, nonce, tokenId, pricesAndTimestamps);
    }

    /**
     * @dev With a hash signed by the method {hashToSignToSellToken} any user sending enough value can buy
     * the token from the seller. Tokens not owned by the contract owner are all considered secondary sales and 
     * will give a cut to the owner of the contract based on the secondaryOwnerBPS.
     */
    function buyToken(
        address seller,
        uint256 version,
        uint256 nonce,
        uint256 tokenId,
        uint256[4] memory pricesAndTimestamps,
        bytes memory signature
    ) external payable {
        uint256 currentPrice = _currentPrice(pricesAndTimestamps);

        require(_addressToActiveVersion[seller] == version, "Incorrect signature version");
        require(msg.value >= currentPrice, "Not enough ETH to buy");

        _markHashSold(seller, version, nonce, tokenId, pricesAndTimestamps, currentPrice, signature);
        _transfer(seller, _msgSender(), tokenId);

        if (seller != owner()) {
            IEaselyPayout(PAYOUT_CONTRACT_ADDRESS).splitPayable{ value: currentPrice }(seller, _ownerRoyalties(), _ownerBPS());
        }
        payable(_msgSender()).transfer(msg.value - currentPrice);
    }

    /**
     * @dev Usable to cancel hashes generated from {hashToSignToSellToken}
     */
    function cancelSale(
        uint256 version,
        uint256 nonce,
        uint256 tokenId,
        uint256[4] memory pricesAndTimestamps
    ) external {
        bytes32 hash = _hashToCheckForSale(_msgSender(), version, nonce, tokenId, pricesAndTimestamps);
        _cancelledOrFinalizedSales[hash] = true;
        emit SaleCancelled(_msgSender(), hash);
    }

    /**
     * @dev Base URI for computing {tokenURI}. If set, the resulting URI for each
     * token will be the concatenation of the `baseURI` and the `tokenId`. Empty
     * by default, can be overriden in child contracts.
     */
    function _baseURI() internal view virtual override returns (string memory) {
        return "ipfs://";
    }

    /**
     * @dev helper method get ownerRoyalties into an array form
     */
    function _ownerRoyalties() internal view returns (address[] memory) {
        address[] memory ownerRoyalties = new address[](1);
        ownerRoyalties[0] = owner();
        return ownerRoyalties;
    }

    /**
     * @dev helper method get secondary BPS into array form
     */
    function _ownerBPS() internal view returns (uint256[] memory) {
        uint256[] memory ownerBPS = new uint256[](1);
        ownerBPS[0] = ownerRoyaltyBPS;
        return ownerBPS;
    }

    /**
     * @dev Current price for a sale which is calculated for the case of a descending sale. So
     * the ending price must be less than the starting price and the timestamp is active.
     * Standard single fare sales will have a matching starting and ending price.
     */
    function _currentPrice(uint256[4] memory pricesAndTimestamps) internal view returns (uint256) {
        uint256 startingPrice = pricesAndTimestamps[0];
        uint256 endingPrice = pricesAndTimestamps[1];
        uint256 startingTimestamp = pricesAndTimestamps[2];
        uint256 endingTimestamp = pricesAndTimestamps[3];

        uint256 currTime = block.timestamp;
        require(currTime >= startingTimestamp, "Has not started yet");
        require(startingTimestamp < endingTimestamp, "Must end after it starts");
        require(startingPrice >= endingPrice, "Ending price cannot be bigger");

        if (startingPrice == endingPrice || currTime > endingTimestamp) {
            return endingPrice;
        }

        uint256 diff = startingPrice - endingPrice;
        uint256 decrements = (currTime - startingTimestamp) / TIME_PER_DECREMENT;
        if (decrements == 0) {
            return startingPrice;
        }

        // decrements will equal 0 before totalDecrements does so we will not divide by 0
        uint256 totalDecrements = (endingTimestamp - startingTimestamp) / TIME_PER_DECREMENT;

        return startingPrice - diff / totalDecrements * decrements;
    }

    /**
     * @dev Sets secondary BPS amount
     */
    function _setRoyaltiesBPS(uint256 newBPS) internal {
        require(ownerRoyaltyBPS <= MAX_SECONDARY_BPS, "Cannot take more than 10% of secondaries");
        ownerRoyaltyBPS = newBPS;
    }

    /**
     * @dev Sets primary revenue share
     */
    function _setRevenueShare(address[2] memory newAddresses, uint256[2] memory newBPS) internal {
        require(newBPS[0] + newBPS[1] <= MAX_ROYALTIES_BPS, "Revenue share too high");
        revenueShare = newAddresses;
        revenueShareBPS = newBPS;
    }

    /**
     * @dev Checks if an address is either the owner, or the approved alternate signer.
     */
    function _checkValidSigner(address signer) internal view {
        require(signer == owner() || signer == alternateSignerAddress, "Not valid signer.");
    }

    /**
     * @dev First checks if a sale is valid by checking that the hash has not been cancelled or already completed
     * and that the correct address has given the signature. If both checks pass we mark the hash as complete and
     * emit an event.
     */
    function _markHashSold(
        address owner,
        uint256 version,
        uint256 nonce,
        uint256 tokenId,
        uint256[4] memory pricesAndTimestamps,
        uint256 salePrice,
        bytes memory signature
    ) internal {
        bytes32 hash = _hashToCheckForSale(owner, version, nonce, tokenId, pricesAndTimestamps);
        require(!_cancelledOrFinalizedSales[hash], "Sale no longer active");
        require(hash.recover(signature) == owner, "Not signed by current token owner");
        _cancelledOrFinalizedSales[hash] = true;

        emit SaleCompleted(tokenId, salePrice, owner, _msgSender(), hash);
    }

    /**
     * @dev Hash an order, returning the hash that a client must sign, including the standard message prefix
     * @return Hash of message prefix and order hash per Ethereum format
     */
    function _hashToCheckForSale(
        address owner,
        uint256 version,
        uint256 nonce,
        uint256 tokenId,
        uint256[4] memory pricesAndTimestamps
    ) internal view returns (bytes32) {
        return ECDSA.toEthSignedMessageHash(_hashForSale(owner, version, nonce, tokenId, pricesAndTimestamps));
    }

    /**
     * @dev Hash an order, returning the hash that a client must sign, including the standard message prefix
     * @return Hash of message prefix and order hash per Ethereum format
     */
    function _hashForSale(
        address owner,
        uint256 version,
        uint256 nonce,
        uint256 tokenId,
        uint256[4] memory pricesAndTimestamps
    ) internal view returns (bytes32) {
        return keccak256(abi.encode(address(this), block.chainid, owner, version, nonce, tokenId, pricesAndTimestamps));
    }
}

/**
 * @dev This implements a lazy-minted, randomized collection of ERC721Marketplace.
 * It requires that the creator knows the total number of NFTs they want and has an IPFS
 * hash that is a directory with all the tokenIds from 0 to the #NFTs - 1.
 *
 * It has two main methods to lazy-mint, one allows the owner or alternate signer to approve single use signatures
 * for specific wallet addresses and the other allows a general mint signature that anyone can use.
 *
 * Minting from this collection is always random, this can be done either with a reveal
 * mechanism that has a random offset, or on-chain randomness if the collection is already revealed. 
 */
contract ERC721RandomizedCollection is ERC721Marketplace {  
    using ECDSA for bytes32;  
    using Strings for uint256;

    bool public burnable;
    bool public isRevealed = false;
    bool private hasInit = false;

    uint256 public constant MAX_SUPPLY_LIMIT = 10 ** 9;
    uint256 public maxSupply;
    // Limits how much any single transaction can be
    uint256 public transactionMax;
    // Limits how much any single wallet can mint on a collection.
    uint256 public maxMint;
    
    mapping(address => uint256) public mintCount;

    // Used to shuffle tokenURI upon reveal
    uint256 public offset;
    // Used when switching from incremental tokenId to 
    uint256 private randOffset;

    // Mapping to enable constant time onchain randomness
    uint256[MAX_SUPPLY_LIMIT] private indices;
    string private ipfsHash;

    // Randomized Collection Events
    event Minted(address indexed buyer, uint256 amount, uint256 unitPrice, bytes32 hash);
    event IpfsRevealed(string ipfsHash, bool locked);

    /**
     * @dev Constructor function
     */
    constructor(
        bool[2] memory bools, address[3] memory addresses, uint256[6] memory uints, string[3] memory strings
    ) ERC721(strings[0], strings[1]) {
        addresses[0] = _msgSender();
        _init(bools, addresses, uints, strings);
    }

    function init(
        bool[2] memory bools, 
        address[3] memory addresses, 
        uint256[6] memory uints, 
        string[3] memory strings
    ) external {
        _init(bools, addresses, uints, strings);
    }

    function _init(
        bool[2] memory bools, 
        address[3] memory addresses, 
        uint256[6] memory uints, 
        string[3] memory strings
    ) internal {
        require(!hasInit, "Already has be initiated");
        hasInit = true;

        burnable = bools[0];
        isRevealed = bools[1];

        _owner = msg.sender;
        address[2] memory revenueShare = [addresses[0], addresses[1]];
        alternateSignerAddress = addresses[2];
                
        _setRoyaltiesBPS(uints[0]);
        _setRevenueShare(revenueShare, [uints[1], uints[2]]);
        maxSupply = uints[3];
        require(maxSupply < MAX_SUPPLY_LIMIT, "Collection is too big");
        
        // Do not allow more than 20 mints a transaction so users cannot exceed gas limit
        if (uints[4] == 0 || uints[4] >= 20){
            transactionMax = 20;
        } else {
            transactionMax = uints[4];
        }
        maxMint = uints[5];

        _name = strings[0];
        _symbol = strings[1];
        ipfsHash = strings[2];
        if (isRevealed) {
            emit IpfsRevealed(ipfsHash, false);
        }
    }

    /**
     * @dev If this collection was created with burnable on, owners of tokens
     * can use this method to burn their tokens. Easely will keep track of 
     * burns in case creators want to reward users for burning tokens.
     */
    function burn(uint256 tokenId) external {
        require(burnable, "Tokens from this collection are not burnable");
        require(ownerOf(tokenId) == _msgSender(), "Cannot burn a token you do not own");

        _burn(tokenId);
    }
    
    /**
     * @dev Method used if the creator wants to keep their collection hidden until
     * a later release date. On reveal, a creator can decide if they want to 
     * lock the unminted tokens or enable them for on-chain randomness minting.
     * 
     * IMPORTANT - this function can only be called ONCE, if a wrong IPFS hash
     * is submitted by the owner, it cannot ever be switched to a different one.
     */
    function lockTokenURI(string calldata revealIPFSHash, bool lockOnReveal) external onlyOwner {
        require(!isRevealed, "The token URI has already been set");
        offset = _random(maxSupply);
        ipfsHash = revealIPFSHash;
        isRevealed = true;

        if (!lockOnReveal) {
            // so we know what index to start generating random numbers from
            randOffset = _totalMinted;
        } else {
            // This will lock the unminted tokens at reveal time
            maxSupply = _totalMinted;
        }

        emit IpfsRevealed(revealIPFSHash, lockOnReveal);
    }

    /**
     * @dev tokenURI of a tokenId, will change to include the tokeId and an offset in
     * the URI once the collection has been revealed.
     */
    function tokenURI(uint256 tokenId) public view virtual override returns (string memory) {
        if (!isRevealed) {
            return string(abi.encodePacked(_baseURI(), ipfsHash));
        }

        require(_exists(tokenId), "URI query for nonexistent token");

        uint256 offsetId = (tokenId + offset) % maxSupply;
        return string(abi.encodePacked(_baseURI(), ipfsHash, "/", offsetId.toString()));
    }

    /**
     * @dev Hash that the owner or approved alternate signer then sign that the approved buyer
     * can use in order to call the {mintAllow} method.
     */
    function hashToSignForAllowList(
        address allowedAddress,
        uint256 version,
        uint256 nonce,
        uint256 price,
        uint256 amount
    ) external view returns (bytes32) {
        _checkValidSigner(_msgSender());
        return _hashForAllowList(allowedAddress, version, nonce, price, amount);
    }

    /**
     * @dev A way to invalidate a signature so the given params cannot be used in the {mintAllow} method.
     */
    function cancelAllowList(
        address allowedAddress,
        uint256 version,
        uint256 nonce,
        uint256 price,
        uint256 amount
    ) external {
        _checkValidSigner(_msgSender());
        bytes32 hash = _hashToCheckForAllowList(allowedAddress, version, nonce, price, amount);
        _cancelledOrFinalizedSales[hash] = true;
        emit SaleCancelled(_msgSender(), hash);
    }
    
    /**
     * @dev Allows a user with an approved signature to mint at a price and quantity specified by the
     * contract. A user is still limited by totalSupply, transactionMax, and mintMax if populated.
     */
    function mintAllow(
        address allowedAddress,
        uint256 version,
        uint256 nonce,
        uint256 price,
        uint256 amount,
        uint256 buyAmount,
        bytes memory signature
    ) external payable {
        require(_totalMinted + buyAmount <= maxSupply, "Over token supply limit");
        require(buyAmount <= amount, "Over signature amount");
        require(buyAmount <= transactionMax, "Over transaction limit");
        require(version == _addressToActiveVersion[owner()], "This presale version is disabled");
        require(allowedAddress == _msgSender(), "Invalid sender");

        uint256 totalPrice = price * buyAmount;
        uint256 msgValue = msg.value;
        require(msgValue >= totalPrice, "Msg value too small");

        bytes32 hash = _hashToCheckForAllowList(allowedAddress, version, nonce, price, amount);
        require(!_cancelledOrFinalizedSales[hash], "Signature no longer active");
        address signer = hash.recover(signature);
        _checkValidSigner(signer);
        _cancelledOrFinalizedSales[hash] = true;

        _mintRandom(_msgSender(), buyAmount);
        emit Minted(_msgSender(), buyAmount, price, hash);
        payable(_msgSender()).transfer(msgValue - totalPrice);
    }

    /**
     * @dev Hash that the owner or approved alternate signer then sign that buyers use 
     * in order to call the {mint} method.
     */
    function hashToSignForMint(uint256 version, uint256 amount, uint256[4] memory pricesAndTimestamps) external view returns (bytes32) {
        _checkValidSigner(_msgSender());
        require(amount <= transactionMax, "Over transaction limit");

        return _hashForMint(version, amount, pricesAndTimestamps);
    }

    /**
     * @dev A way to invalidate a signature so the given params cannot be used in the {mint} method.
     */
    function cancelMint(uint256 version, uint256 amount, uint256[4] memory pricesAndTimestamps) external {
        _checkValidSigner(_msgSender());
        bytes32 hash = _hashToCheckForMint(version, amount, pricesAndTimestamps);
        _cancelledOrFinalizedSales[hash] = true;
        emit SaleCancelled(_msgSender(), hash);
    }

    /**
     * @dev Allows anyone to buy an amount of tokens at a price which matches
     * the signature that the owner or alternate signer has approved
     */
    function mint(
        uint256 version,
        uint256 amount,
        uint256[4] memory pricesAndTimestamps, 
        bytes memory signature
    ) external payable {
        require(_totalMinted + amount <= maxSupply, "Over token supply limit");
        require(amount <= transactionMax, "Over transaction limit");
        require(version == _addressToActiveVersion[owner()], "Invalid version");

        uint256 unitPrice = _currentPrice(pricesAndTimestamps);
        uint256 totalPrice = amount * unitPrice;
        uint256 msgValue = msg.value;
        require(msgValue >= totalPrice, "Msg value too small");

        bytes32 hash = _hashToCheckForMint(version, amount, pricesAndTimestamps);
        require(!_cancelledOrFinalizedSales[hash], "Signature no longer active");
        address signer = hash.recover(signature);
        _checkValidSigner(signer);

        _mintRandom(_msgSender(), amount);
        emit Minted(_msgSender(), amount, unitPrice, hash);

        payable(_msgSender()).transfer(msgValue - totalPrice);
    }

    /**
     * @dev Hash an order that we need to check against the signature to see who the signer is.
     * see {_hashForAllowList} to see the hash that needs to be signed.
     */
    function _hashToCheckForAllowList(
        address allowedAddress,
        uint256 nonce,
        uint256 version,
        uint256 price,
        uint256 amount
    ) internal view returns (bytes32) {
        return ECDSA.toEthSignedMessageHash(_hashForAllowList(allowedAddress, nonce, version, price, amount));
    }

    /**
     * @dev Hash that the owner or alternate wallet must sign to enable a {mintAllow} for a user
     * @return Hash of message prefix and order hash per Ethereum format
     */
    function _hashForAllowList(        
        address allowedAddress,
        uint256 nonce,
        uint256 version,
        uint256 price,
        uint256 amount
    ) internal view returns (bytes32) {
        return keccak256(abi.encode(address(this), block.chainid, owner(), allowedAddress, nonce, version, price, amount));
    }

    /**
     * @dev Hash an order that we need to check against the signature to see who the signer is.
    * see {_hashForMint} to see the hash that needs to be signed.
     */
    function _hashToCheckForMint(
        uint256 version,
        uint256 amount,
        uint256[4] memory pricesAndTimestamps
    ) internal view returns (bytes32) {
        return ECDSA.toEthSignedMessageHash(_hashForMint(version, amount, pricesAndTimestamps));
    }

    /**
     * @dev Hash that the owner or alternate wallet must sign to enable {mint} for all users 
     */
    function _hashForMint(        
        uint256 version,
        uint256 amount,
        uint256[4] memory pricesAndTimestamps
    ) internal view returns (bytes32) {
        return keccak256(abi.encode(address(this), block.chainid, owner(), amount, pricesAndTimestamps, version));
    }

    /// @notice Generates a pseudo random index of our tokens that has not been used so far
    function _mintRandomIndex(address buyer, uint256 amount) internal {
        //  number of tokens left to create
        uint256 supplyLeft = maxSupply - _totalMinted;

        for (uint256 i = 0; i < amount; i++) {
            // generate a random index
            uint256 index = _random(supplyLeft);
            uint256 tokenAtPlace = indices[index];

            uint256 tokenId;
            // if we havent stored a replacement token...
            if (tokenAtPlace == 0) {
                //... we just return the current index
                tokenId = index;
            } else {
                // else we take the replace we stored with logic below
                tokenId = tokenAtPlace;
            }

            // get the highest token id we havent handed out
            uint256 lastTokenAvailable = indices[supplyLeft - 1];
            // we need to store a replacement token for the next time we roll the same index
            // if the last token is still unused...
            if (lastTokenAvailable == 0) {
                // ... we store the last token as index
                indices[index] = supplyLeft - 1;
            } else {
                // ... we store the token that was stored for the last token
                indices[index] = lastTokenAvailable;
            }

            _safeMint(buyer, tokenId + randOffset);
            supplyLeft--;
        }
    }

    /// @notice Generates a pseudo random number based on arguments with decent entropy
    /// @param max The maximum value we want to receive
    /// @return A random number less than the max
    function _random(uint256 max) internal view returns (uint256) {
        if (max == 0) {
            return 0;
        }

        uint256 rand = uint256(
            keccak256(
                abi.encode(
                    msg.sender,
                    block.difficulty,
                    block.timestamp,
                    blockhash(block.number - 1)
                )
            )
        );
        return rand % max;
    }

    /**
     * @dev Wrapper around {_mintRandomIndex} that incrementally if the collection has not 
     * been revealed yet, which also checks the buyer has not exceeded maxMint count 
     */
    function _mintRandom(address buyer, uint256 amount) internal {
        require(maxMint == 0 || mintCount[buyer] + amount <= maxMint, "Buyer over mint maximum");
        mintCount[buyer] += amount;
        uint256 nextTokenId = _totalMinted;

        if (isRevealed) {
            _mintRandomIndex(buyer, amount);
        } else {
            for (uint256 i = 0; i < amount; i++) {
                _safeMint(buyer, nextTokenId + i);
            }
        }

        _balances[buyer] += amount;
        _totalMinted += amount;
    }
}
{"success": true, "error": null, "results": {"detectors": [{"check": "divide-before-multiply", "impact": "Medium", "confidence": "Medium"}, {"check": "reentrancy-no-eth", "impact": "Medium", "confidence": "Medium"}, {"check": "incorrect-equality", "impact": "Medium", "confidence": "High"}, {"check": "weak-prng", "impact": "High", "confidence": "Medium"}, {"check": "unused-return", "impact": "Medium", "confidence": "Medium"}]}}