// File: contracts/BaseErc20.sol



pragma solidity 0.6.8;

/**
 * NOTE: This contract only exists to serve as a testing utility. It is not recommended to be used outside of a testing environment
 */

import {SafeMath} from "@openzeppelin/contracts/math/SafeMath.sol";
import {IERC20} from "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import {Ownable} from "@openzeppelin/contracts/access/Ownable.sol";

/**
 * @title ERC20 Token
 *
 * Basic ERC20 Implementation
 */
contract BaseERC20 is IERC20, Ownable {
    using SafeMath for uint256;

    // ============ Variables ============

    string internal _name;
    string internal _symbol;
    uint256 internal _supply;
    uint8 internal _decimals;

    mapping(address => uint256) private _balances;
    mapping(address => mapping(address => uint256)) private _allowances;

    // ============ Constructor ============

    constructor(
        string memory name,
        string memory symbol,
        uint8 decimals
    ) public {
        _name = name;
        _symbol = symbol;
        _decimals = decimals;
    }

    // ============ Public Functions ============

    function symbol() public view returns (string memory) {
        return _symbol;
    }

    function name() public view returns (string memory) {
        return _name;
    }

    function decimals() public view virtual returns (uint8) {
        return _decimals;
    }

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

    function balanceOf(address who) public view override returns (uint256) {
        return _balances[who];
    }

    function allowance(address owner, address spender)
        public
        view
        override
        returns (uint256)
    {
        return _allowances[owner][spender];
    }

    // ============ Internal Functions ============

    function _mint(address to, uint256 value) internal {
        require(to != address(0), "Cannot mint to zero address");

        _balances[to] = _balances[to].add(value);
        _supply = _supply.add(value);

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

    function _burn(address from, uint256 value) internal {
        require(from != address(0), "Cannot burn to zero");

        _balances[from] = _balances[from].sub(value);
        _supply = _supply.sub(value);

        emit Transfer(from, address(0), value);
    }

    // ============ Token Functions ============

    function transfer(address to, uint256 value)
        public
        virtual
        override
        returns (bool)
    {
        if (_balances[msg.sender] >= value) {
            _balances[msg.sender] = _balances[msg.sender].sub(value);
            _balances[to] = _balances[to].add(value);
            emit Transfer(msg.sender, to, value);
            return true;
        } else {
            return false;
        }
    }

    function transferFrom(
        address from,
        address to,
        uint256 value
    ) public virtual override returns (bool) {
        if (
            _balances[from] >= value && _allowances[from][msg.sender] >= value
        ) {
            _balances[to] = _balances[to].add(value);
            _balances[from] = _balances[from].sub(value);
            _allowances[from][msg.sender] = _allowances[from][msg.sender].sub(
                value
            );
            emit Transfer(from, to, value);
            return true;
        } else {
            return false;
        }
    }

    function approve(address spender, uint256 value)
        public
        override
        returns (bool)
    {
        return _approve(msg.sender, spender, value);
    }

    function _approve(
        address owner,
        address spender,
        uint256 value
    ) internal returns (bool) {
        _allowances[owner][spender] = value;

        emit Approval(owner, spender, value);

        return true;
    }

    function mint(address to, uint256 value) public onlyOwner {
        _mint(to, value);
    }
}

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



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

/**
 * @dev Wrappers over Solidity's arithmetic operations with added overflow
 * checks.
 *
 * Arithmetic operations in Solidity wrap on overflow. This can easily result
 * in bugs, because programmers usually assume that an overflow raises an
 * error, which is the standard behavior in high level programming languages.
 * `SafeMath` restores this intuition by reverting the transaction when an
 * operation overflows.
 *
 * Using this library instead of the unchecked operations eliminates an entire
 * class of bugs, so it's recommended to use it always.
 */
library SafeMath {
    /**
     * @dev Returns the addition of two unsigned integers, reverting on
     * overflow.
     *
     * Counterpart to Solidity's `+` operator.
     *
     * Requirements:
     *
     * - Addition cannot overflow.
     */
    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        require(c >= a, "SafeMath: addition overflow");

        return c;
    }

    /**
     * @dev Returns the subtraction of two unsigned integers, reverting on
     * overflow (when the result is negative).
     *
     * Counterpart to Solidity's `-` operator.
     *
     * Requirements:
     *
     * - Subtraction cannot overflow.
     */
    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        return sub(a, b, "SafeMath: subtraction overflow");
    }

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

        return c;
    }

    /**
     * @dev Returns the multiplication of two unsigned integers, reverting on
     * overflow.
     *
     * Counterpart to Solidity's `*` operator.
     *
     * Requirements:
     *
     * - Multiplication cannot overflow.
     */
    function mul(uint256 a, uint256 b) internal pure returns (uint256) {
        // Gas optimization: this is cheaper than requiring 'a' not being zero, but the
        // benefit is lost if 'b' is also tested.
        // See: https://github.com/OpenZeppelin/openzeppelin-contracts/pull/522
        if (a == 0) {
            return 0;
        }

        uint256 c = a * b;
        require(c / a == b, "SafeMath: multiplication overflow");

        return c;
    }

    /**
     * @dev Returns the integer division of two unsigned integers. Reverts on
     * division by zero. The result is rounded towards zero.
     *
     * Counterpart to Solidity's `/` operator. Note: this function uses a
     * `revert` opcode (which leaves remaining gas untouched) while Solidity
     * uses an invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        return div(a, b, "SafeMath: division by zero");
    }

    /**
     * @dev Returns the integer division of two unsigned integers. Reverts with custom message on
     * division by zero. The result is rounded towards zero.
     *
     * Counterpart to Solidity's `/` operator. Note: this function uses a
     * `revert` opcode (which leaves remaining gas untouched) while Solidity
     * uses an invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function div(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b > 0, errorMessage);
        uint256 c = a / b;
        // assert(a == b * c + a % b); // There is no case in which this doesn't hold

        return c;
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * Reverts when dividing by zero.
     *
     * Counterpart to Solidity's `%` operator. This function uses a `revert`
     * opcode (which leaves remaining gas untouched) while Solidity uses an
     * invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function mod(uint256 a, uint256 b) internal pure returns (uint256) {
        return mod(a, b, "SafeMath: modulo by zero");
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * Reverts with custom message when dividing by zero.
     *
     * Counterpart to Solidity's `%` operator. This function uses a `revert`
     * opcode (which leaves remaining gas untouched) while Solidity uses an
     * invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function mod(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b != 0, errorMessage);
        return a % b;
    }
}

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



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

/**
 * @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/access/Ownable.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

import "../GSN/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.
 */
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 () internal {
        address msgSender = _msgSender();
        _owner = msgSender;
        emit OwnershipTransferred(address(0), msgSender);
    }

    /**
     * @dev Returns the address of the current owner.
     */
    function owner() public view 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/GSN/Context.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

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

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

// File: contracts/ERC721Burnable.sol



// SPDX-License-Identifier: MIT

/**
 * NOTE: This file is a clone of the OpenZeppelin ERC721Burnable.sol contract. It was forked from https://github.com/OpenZeppelin/openzeppelin-contracts
 * at commit 1ada3b633e5bfd9d4ffe0207d64773a11f5a7c40
 *
 * It was cloned in order to ensure it imported from the cloned ERC721.sol file. No other modifications have been made.
 */

pragma solidity 0.6.8;

import "@openzeppelin/contracts/GSN/Context.sol";
import "./ERC721.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: contracts/ERC721.sol



// SPDX-License-Identifier: MIT

/**
 * NOTE: This file is a clone of the OpenZeppelin ERC721.sol contract. It was forked from https://github.com/OpenZeppelin/openzeppelin-contracts
 * at commit 1ada3b633e5bfd9d4ffe0207d64773a11f5a7c40
 *
 *
 * The following functions needed to be modified, prompting this clone:
 *  - `_tokenURIs` visibility was changed from private to internal to support updating URIs after minting
 *  - `_baseURI` visibiility was changed from private to internal to support fetching token URI even after the token was burned
 *  - `_INTERFACE_ID_ERC721_METADATA` is no longer registered as an interface because _tokenURI now returns raw content instead of a JSON file, and supports updatable URIs
 *  - `_approve` visibility was changed from private to internal to support EIP-2612 flavored permits and approval revocation by an approved address
 */

pragma solidity 0.6.8;

import "@openzeppelin/contracts/GSN/Context.sol";
import "@openzeppelin/contracts/token/ERC721/IERC721.sol";
import "@openzeppelin/contracts/token/ERC721/IERC721Metadata.sol";
import "@openzeppelin/contracts/token/ERC721/IERC721Enumerable.sol";
import "@openzeppelin/contracts/token/ERC721/IERC721Receiver.sol";
import "@openzeppelin/contracts/introspection/ERC165.sol";
import "@openzeppelin/contracts/math/SafeMath.sol";
import "@openzeppelin/contracts/utils/Address.sol";
import "@openzeppelin/contracts/utils/EnumerableSet.sol";
import "@openzeppelin/contracts/utils/EnumerableMap.sol";
import "@openzeppelin/contracts/utils/Strings.sol";

/**
 * @title ERC721 Non-Fungible Token Standard basic implementation
 * @dev see https://eips.ethereum.org/EIPS/eip-721
 */
contract ERC721 is
    Context,
    ERC165,
    IERC721,
    IERC721Metadata,
    IERC721Enumerable
{
    using SafeMath for uint256;
    using Address for address;
    using EnumerableSet for EnumerableSet.UintSet;
    using EnumerableMap for EnumerableMap.UintToAddressMap;
    using Strings for uint256;

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

    // Mapping from holder address to their (enumerable) set of owned tokens
    mapping(address => EnumerableSet.UintSet) private _holderTokens;

    // Enumerable mapping from token ids to their owners
    EnumerableMap.UintToAddressMap private _tokenOwners;

    // 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;

    // Token name
    string private _name;

    // Token symbol
    string private _symbol;

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

    // Base URI
    string internal _baseURI;

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

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

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

        // register the supported interfaces to conform to ERC721 via ERC165
        _registerInterface(_INTERFACE_ID_ERC721);
        _registerInterface(_INTERFACE_ID_ERC721_ENUMERABLE);
    }

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

        return _holderTokens[owner].length();
    }

    /**
     * @dev See {IERC721-ownerOf}.
     */
    function ownerOf(uint256 tokenId) public view override returns (address) {
        return
            _tokenOwners.get(
                tokenId,
                "ERC721: owner query for nonexistent token"
            );
    }

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

    /**
     * @dev See {IERC721Metadata-symbol}.
     */
    function symbol() public view 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 _tokenURI = _tokenURIs[tokenId];

        // If there is no base URI, return the token URI.
        if (bytes(_baseURI).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(_baseURI, _tokenURI));
        }
        // If there is a baseURI but no tokenURI, concatenate the tokenID to the baseURI.
        return string(abi.encodePacked(_baseURI, tokenId.toString()));
    }

    /**
     * @dev Returns the base URI set via {_setBaseURI}. This will be
     * automatically added as a prefix in {tokenURI} to each token's URI, or
     * to the token ID if no specific URI is set for that token ID.
     */
    function baseURI() public view returns (string memory) {
        return _baseURI;
    }

    /**
     * @dev See {IERC721Enumerable-tokenOfOwnerByIndex}.
     */
    function tokenOfOwnerByIndex(address owner, uint256 index)
        public
        view
        override
        returns (uint256)
    {
        return _holderTokens[owner].at(index);
    }

    /**
     * @dev See {IERC721Enumerable-totalSupply}.
     */
    function totalSupply() public view override returns (uint256) {
        // _tokenOwners are indexed by tokenIds, so .length() returns the number of tokenIds
        return _tokenOwners.length();
    }

    /**
     * @dev See {IERC721Enumerable-tokenByIndex}.
     */
    function tokenByIndex(uint256 index)
        public
        view
        override
        returns (uint256)
    {
        (uint256 tokenId, ) = _tokenOwners.at(index);
        return tokenId;
    }

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

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

        _approve(to, tokenId);
    }

    /**
     * @dev See {IERC721-getApproved}.
     */
    function getApproved(uint256 tokenId)
        public
        view
        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
        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 returns (bool) {
        return _tokenOwners.contains(tokenId);
    }

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

    /**
     * @dev Safely mints `tokenId` and transfers it to `to`.
     *
     * Requirements:
     d*
     * - `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);

        _holderTokens[to].add(tokenId);

        _tokenOwners.set(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 = ownerOf(tokenId);

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

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

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

        _holderTokens[owner].remove(tokenId);

        _tokenOwners.remove(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(
            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);

        _holderTokens[from].remove(tokenId);
        _holderTokens[to].add(tokenId);

        _tokenOwners.set(tokenId, to);

        emit Transfer(from, to, 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),
            "ERC721Metadata: URI set of nonexistent token"
        );
        _tokenURIs[tokenId] = _tokenURI;
    }

    /**
     * @dev Internal function to set the base URI for all token IDs. It is
     * automatically added as a prefix to the value returned in {tokenURI},
     * or to the token ID if {tokenURI} is empty.
     */
    function _setBaseURI(string memory baseURI_) internal virtual {
        _baseURI = baseURI_;
    }

    /**
     * @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()) {
            return true;
        }
        bytes memory returndata =
            to.functionCall(
                abi.encodeWithSelector(
                    IERC721Receiver(to).onERC721Received.selector,
                    _msgSender(),
                    from,
                    tokenId,
                    _data
                ),
                "ERC721: transfer to non ERC721Receiver implementer"
            );
        bytes4 retval = abi.decode(returndata, (bytes4));
        return (retval == _ERC721_RECEIVED);
    }

    function _approve(address to, uint256 tokenId) internal {
        _tokenApprovals[tokenId] = to;
        emit Approval(ownerOf(tokenId), to, tokenId);
    }

    /**
     * @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 ^0.6.2;

import "../../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/IERC721Metadata.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.2;

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/token/ERC721/IERC721Enumerable.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.2;

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/IERC721Receiver.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

/**
 * @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/introspection/ERC165.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

import "./IERC165.sol";

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

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

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

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

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

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



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.2;

/**
 * @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 in 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");
        return _functionCallWithValue(target, data, value, errorMessage);
    }

    function _functionCallWithValue(address target, bytes memory data, uint256 weiValue, string memory errorMessage) private returns (bytes memory) {
        require(isContract(target), "Address: call to non-contract");

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



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

/**
 * @dev Library for managing
 * https://en.wikipedia.org/wiki/Set_(abstract_data_type)[sets] of primitive
 * types.
 *
 * Sets have the following properties:
 *
 * - Elements are added, removed, and checked for existence in constant time
 * (O(1)).
 * - Elements are enumerated in O(n). No guarantees are made on the ordering.
 *
 * ```
 * contract Example {
 *     // Add the library methods
 *     using EnumerableSet for EnumerableSet.AddressSet;
 *
 *     // Declare a set state variable
 *     EnumerableSet.AddressSet private mySet;
 * }
 * ```
 *
 * As of v3.0.0, only sets of type `address` (`AddressSet`) and `uint256`
 * (`UintSet`) are supported.
 */
library EnumerableSet {
    // To implement this library for multiple types with as little code
    // repetition as possible, we write it in terms of a generic Set type with
    // bytes32 values.
    // The Set implementation uses private functions, and user-facing
    // implementations (such as AddressSet) are just wrappers around the
    // underlying Set.
    // This means that we can only create new EnumerableSets for types that fit
    // in bytes32.

    struct Set {
        // Storage of set values
        bytes32[] _values;

        // Position of the value in the `values` array, plus 1 because index 0
        // means a value is not in the set.
        mapping (bytes32 => uint256) _indexes;
    }

    /**
     * @dev Add a value to a set. O(1).
     *
     * Returns true if the value was added to the set, that is if it was not
     * already present.
     */
    function _add(Set storage set, bytes32 value) private returns (bool) {
        if (!_contains(set, value)) {
            set._values.push(value);
            // The value is stored at length-1, but we add 1 to all indexes
            // and use 0 as a sentinel value
            set._indexes[value] = set._values.length;
            return true;
        } else {
            return false;
        }
    }

    /**
     * @dev Removes a value from a set. O(1).
     *
     * Returns true if the value was removed from the set, that is if it was
     * present.
     */
    function _remove(Set storage set, bytes32 value) private returns (bool) {
        // We read and store the value's index to prevent multiple reads from the same storage slot
        uint256 valueIndex = set._indexes[value];

        if (valueIndex != 0) { // Equivalent to contains(set, value)
            // To delete an element from the _values array in O(1), we swap the element to delete with the last one in
            // the array, and then remove the last element (sometimes called as 'swap and pop').
            // This modifies the order of the array, as noted in {at}.

            uint256 toDeleteIndex = valueIndex - 1;
            uint256 lastIndex = set._values.length - 1;

            // When the value to delete is the last one, the swap operation is unnecessary. However, since this occurs
            // so rarely, we still do the swap anyway to avoid the gas cost of adding an 'if' statement.

            bytes32 lastvalue = set._values[lastIndex];

            // Move the last value to the index where the value to delete is
            set._values[toDeleteIndex] = lastvalue;
            // Update the index for the moved value
            set._indexes[lastvalue] = toDeleteIndex + 1; // All indexes are 1-based

            // Delete the slot where the moved value was stored
            set._values.pop();

            // Delete the index for the deleted slot
            delete set._indexes[value];

            return true;
        } else {
            return false;
        }
    }

    /**
     * @dev Returns true if the value is in the set. O(1).
     */
    function _contains(Set storage set, bytes32 value) private view returns (bool) {
        return set._indexes[value] != 0;
    }

    /**
     * @dev Returns the number of values on the set. O(1).
     */
    function _length(Set storage set) private view returns (uint256) {
        return set._values.length;
    }

   /**
    * @dev Returns the value stored at position `index` in the set. O(1).
    *
    * Note that there are no guarantees on the ordering of values inside the
    * array, and it may change when more values are added or removed.
    *
    * Requirements:
    *
    * - `index` must be strictly less than {length}.
    */
    function _at(Set storage set, uint256 index) private view returns (bytes32) {
        require(set._values.length > index, "EnumerableSet: index out of bounds");
        return set._values[index];
    }

    // AddressSet

    struct AddressSet {
        Set _inner;
    }

    /**
     * @dev Add a value to a set. O(1).
     *
     * Returns true if the value was added to the set, that is if it was not
     * already present.
     */
    function add(AddressSet storage set, address value) internal returns (bool) {
        return _add(set._inner, bytes32(uint256(value)));
    }

    /**
     * @dev Removes a value from a set. O(1).
     *
     * Returns true if the value was removed from the set, that is if it was
     * present.
     */
    function remove(AddressSet storage set, address value) internal returns (bool) {
        return _remove(set._inner, bytes32(uint256(value)));
    }

    /**
     * @dev Returns true if the value is in the set. O(1).
     */
    function contains(AddressSet storage set, address value) internal view returns (bool) {
        return _contains(set._inner, bytes32(uint256(value)));
    }

    /**
     * @dev Returns the number of values in the set. O(1).
     */
    function length(AddressSet storage set) internal view returns (uint256) {
        return _length(set._inner);
    }

   /**
    * @dev Returns the value stored at position `index` in the set. O(1).
    *
    * Note that there are no guarantees on the ordering of values inside the
    * array, and it may change when more values are added or removed.
    *
    * Requirements:
    *
    * - `index` must be strictly less than {length}.
    */
    function at(AddressSet storage set, uint256 index) internal view returns (address) {
        return address(uint256(_at(set._inner, index)));
    }


    // UintSet

    struct UintSet {
        Set _inner;
    }

    /**
     * @dev Add a value to a set. O(1).
     *
     * Returns true if the value was added to the set, that is if it was not
     * already present.
     */
    function add(UintSet storage set, uint256 value) internal returns (bool) {
        return _add(set._inner, bytes32(value));
    }

    /**
     * @dev Removes a value from a set. O(1).
     *
     * Returns true if the value was removed from the set, that is if it was
     * present.
     */
    function remove(UintSet storage set, uint256 value) internal returns (bool) {
        return _remove(set._inner, bytes32(value));
    }

    /**
     * @dev Returns true if the value is in the set. O(1).
     */
    function contains(UintSet storage set, uint256 value) internal view returns (bool) {
        return _contains(set._inner, bytes32(value));
    }

    /**
     * @dev Returns the number of values on the set. O(1).
     */
    function length(UintSet storage set) internal view returns (uint256) {
        return _length(set._inner);
    }

   /**
    * @dev Returns the value stored at position `index` in the set. O(1).
    *
    * Note that there are no guarantees on the ordering of values inside the
    * array, and it may change when more values are added or removed.
    *
    * Requirements:
    *
    * - `index` must be strictly less than {length}.
    */
    function at(UintSet storage set, uint256 index) internal view returns (uint256) {
        return uint256(_at(set._inner, index));
    }
}

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



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

/**
 * @dev Library for managing an enumerable variant of Solidity's
 * https://solidity.readthedocs.io/en/latest/types.html#mapping-types[`mapping`]
 * type.
 *
 * Maps have the following properties:
 *
 * - Entries are added, removed, and checked for existence in constant time
 * (O(1)).
 * - Entries are enumerated in O(n). No guarantees are made on the ordering.
 *
 * ```
 * contract Example {
 *     // Add the library methods
 *     using EnumerableMap for EnumerableMap.UintToAddressMap;
 *
 *     // Declare a set state variable
 *     EnumerableMap.UintToAddressMap private myMap;
 * }
 * ```
 *
 * As of v3.0.0, only maps of type `uint256 -> address` (`UintToAddressMap`) are
 * supported.
 */
library EnumerableMap {
    // To implement this library for multiple types with as little code
    // repetition as possible, we write it in terms of a generic Map type with
    // bytes32 keys and values.
    // The Map implementation uses private functions, and user-facing
    // implementations (such as Uint256ToAddressMap) are just wrappers around
    // the underlying Map.
    // This means that we can only create new EnumerableMaps for types that fit
    // in bytes32.

    struct MapEntry {
        bytes32 _key;
        bytes32 _value;
    }

    struct Map {
        // Storage of map keys and values
        MapEntry[] _entries;

        // Position of the entry defined by a key in the `entries` array, plus 1
        // because index 0 means a key is not in the map.
        mapping (bytes32 => uint256) _indexes;
    }

    /**
     * @dev Adds a key-value pair to a map, or updates the value for an existing
     * key. O(1).
     *
     * Returns true if the key was added to the map, that is if it was not
     * already present.
     */
    function _set(Map storage map, bytes32 key, bytes32 value) private returns (bool) {
        // We read and store the key's index to prevent multiple reads from the same storage slot
        uint256 keyIndex = map._indexes[key];

        if (keyIndex == 0) { // Equivalent to !contains(map, key)
            map._entries.push(MapEntry({ _key: key, _value: value }));
            // The entry is stored at length-1, but we add 1 to all indexes
            // and use 0 as a sentinel value
            map._indexes[key] = map._entries.length;
            return true;
        } else {
            map._entries[keyIndex - 1]._value = value;
            return false;
        }
    }

    /**
     * @dev Removes a key-value pair from a map. O(1).
     *
     * Returns true if the key was removed from the map, that is if it was present.
     */
    function _remove(Map storage map, bytes32 key) private returns (bool) {
        // We read and store the key's index to prevent multiple reads from the same storage slot
        uint256 keyIndex = map._indexes[key];

        if (keyIndex != 0) { // Equivalent to contains(map, key)
            // To delete a key-value pair from the _entries array in O(1), we swap the entry to delete with the last one
            // in the array, and then remove the last entry (sometimes called as 'swap and pop').
            // This modifies the order of the array, as noted in {at}.

            uint256 toDeleteIndex = keyIndex - 1;
            uint256 lastIndex = map._entries.length - 1;

            // When the entry to delete is the last one, the swap operation is unnecessary. However, since this occurs
            // so rarely, we still do the swap anyway to avoid the gas cost of adding an 'if' statement.

            MapEntry storage lastEntry = map._entries[lastIndex];

            // Move the last entry to the index where the entry to delete is
            map._entries[toDeleteIndex] = lastEntry;
            // Update the index for the moved entry
            map._indexes[lastEntry._key] = toDeleteIndex + 1; // All indexes are 1-based

            // Delete the slot where the moved entry was stored
            map._entries.pop();

            // Delete the index for the deleted slot
            delete map._indexes[key];

            return true;
        } else {
            return false;
        }
    }

    /**
     * @dev Returns true if the key is in the map. O(1).
     */
    function _contains(Map storage map, bytes32 key) private view returns (bool) {
        return map._indexes[key] != 0;
    }

    /**
     * @dev Returns the number of key-value pairs in the map. O(1).
     */
    function _length(Map storage map) private view returns (uint256) {
        return map._entries.length;
    }

   /**
    * @dev Returns the key-value pair stored at position `index` in the map. O(1).
    *
    * Note that there are no guarantees on the ordering of entries inside the
    * array, and it may change when more entries are added or removed.
    *
    * Requirements:
    *
    * - `index` must be strictly less than {length}.
    */
    function _at(Map storage map, uint256 index) private view returns (bytes32, bytes32) {
        require(map._entries.length > index, "EnumerableMap: index out of bounds");

        MapEntry storage entry = map._entries[index];
        return (entry._key, entry._value);
    }

    /**
     * @dev Returns the value associated with `key`.  O(1).
     *
     * Requirements:
     *
     * - `key` must be in the map.
     */
    function _get(Map storage map, bytes32 key) private view returns (bytes32) {
        return _get(map, key, "EnumerableMap: nonexistent key");
    }

    /**
     * @dev Same as {_get}, with a custom error message when `key` is not in the map.
     */
    function _get(Map storage map, bytes32 key, string memory errorMessage) private view returns (bytes32) {
        uint256 keyIndex = map._indexes[key];
        require(keyIndex != 0, errorMessage); // Equivalent to contains(map, key)
        return map._entries[keyIndex - 1]._value; // All indexes are 1-based
    }

    // UintToAddressMap

    struct UintToAddressMap {
        Map _inner;
    }

    /**
     * @dev Adds a key-value pair to a map, or updates the value for an existing
     * key. O(1).
     *
     * Returns true if the key was added to the map, that is if it was not
     * already present.
     */
    function set(UintToAddressMap storage map, uint256 key, address value) internal returns (bool) {
        return _set(map._inner, bytes32(key), bytes32(uint256(value)));
    }

    /**
     * @dev Removes a value from a set. O(1).
     *
     * Returns true if the key was removed from the map, that is if it was present.
     */
    function remove(UintToAddressMap storage map, uint256 key) internal returns (bool) {
        return _remove(map._inner, bytes32(key));
    }

    /**
     * @dev Returns true if the key is in the map. O(1).
     */
    function contains(UintToAddressMap storage map, uint256 key) internal view returns (bool) {
        return _contains(map._inner, bytes32(key));
    }

    /**
     * @dev Returns the number of elements in the map. O(1).
     */
    function length(UintToAddressMap storage map) internal view returns (uint256) {
        return _length(map._inner);
    }

   /**
    * @dev Returns the element stored at position `index` in the set. O(1).
    * Note that there are no guarantees on the ordering of values inside the
    * array, and it may change when more values are added or removed.
    *
    * Requirements:
    *
    * - `index` must be strictly less than {length}.
    */
    function at(UintToAddressMap storage map, uint256 index) internal view returns (uint256, address) {
        (bytes32 key, bytes32 value) = _at(map._inner, index);
        return (uint256(key), address(uint256(value)));
    }

    /**
     * @dev Returns the value associated with `key`.  O(1).
     *
     * Requirements:
     *
     * - `key` must be in the map.
     */
    function get(UintToAddressMap storage map, uint256 key) internal view returns (address) {
        return address(uint256(_get(map._inner, bytes32(key))));
    }

    /**
     * @dev Same as {get}, with a custom error message when `key` is not in the map.
     */
    function get(UintToAddressMap storage map, uint256 key, string memory errorMessage) internal view returns (address) {
        return address(uint256(_get(map._inner, bytes32(key), errorMessage)));
    }
}

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



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

/**
 * @dev String operations.
 */
library Strings {
    /**
     * @dev Converts a `uint256` to its ASCII `string` 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);
        uint256 index = digits - 1;
        temp = value;
        while (temp != 0) {
            buffer[index--] = byte(uint8(48 + temp % 10));
            temp /= 10;
        }
        return string(buffer);
    }
}

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



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

/**
 * @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: contracts/Media.sol



// SPDX-License-Identifier: GPL-3.0
pragma solidity 0.6.8;
pragma experimental ABIEncoderV2;

import {ERC721Burnable} from "./ERC721Burnable.sol";
import {ERC721} from "./ERC721.sol";
import {EnumerableSet} from "@openzeppelin/contracts/utils/EnumerableSet.sol";
import {Counters} from "@openzeppelin/contracts/utils/Counters.sol";
import {SafeMath} from "@openzeppelin/contracts/math/SafeMath.sol";
import {Math} from "@openzeppelin/contracts/math/Math.sol";
import {IERC20} from "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import {
    ReentrancyGuard
} from "@openzeppelin/contracts/utils/ReentrancyGuard.sol";
import {Decimal} from "./Decimal.sol";
import {IMarket} from "./interfaces/IMarket.sol";
import "./interfaces/IMedia.sol";

/**
 * @title A media value system, with perpetual equity to creators
 * @notice This contract provides an interface to mint media with a market
 * owned by the creator.
 */
contract Media is IMedia, ERC721Burnable, ReentrancyGuard {
    using Counters for Counters.Counter;
    using SafeMath for uint256;

    /* *******
     * Globals
     * *******
     */

    // Address for the market
    address public marketContract;

    // Mapping from token to previous owner of the token
    mapping(uint256 => address) public previousTokenOwners;

    // Mapping from token id to creator address
    mapping(uint256 => address) public tokenCreators;

    // Mapping from creator address to their (enumerable) set of created tokens
    mapping(address => EnumerableSet.UintSet) private _creatorTokens;

    // Mapping from token id to sha256 hash of content
    mapping(uint256 => bytes32) public tokenContentHashes;

    // Mapping from token id to sha256 hash of metadata
    mapping(uint256 => bytes32) public tokenMetadataHashes;

    // Mapping from token id to metadataURI
    mapping(uint256 => string) private _tokenMetadataURIs;

    // Mapping from contentHash to bool
    mapping(bytes32 => bool) private _contentHashes;

    //keccak256("Permit(address spender,uint256 tokenId,uint256 nonce,uint256 deadline)");
    bytes32 public constant PERMIT_TYPEHASH =
        0x49ecf333e5b8c95c40fdafc95c1ad136e8914a8fb55e9dc8bb01eaa83a2df9ad;

    //keccak256("MintWithSig(bytes32 contentHash,bytes32 metadataHash,uint256 creatorShare,uint256 nonce,uint256 deadline)");
    bytes32 public constant MINT_WITH_SIG_TYPEHASH =
        0x2952e482b8e2b192305f87374d7af45dc2eafafe4f50d26a0c02e90f2fdbe14b;

    // Mapping from address to token id to permit nonce
    mapping(address => mapping(uint256 => uint256)) public permitNonces;

    // Mapping from address to mint with sig nonce
    mapping(address => uint256) public mintWithSigNonces;

    /*
     *     bytes4(keccak256('name()')) == 0x06fdde03
     *     bytes4(keccak256('symbol()')) == 0x95d89b41
     *     bytes4(keccak256('tokenURI(uint256)')) == 0xc87b56dd
     *     bytes4(keccak256('tokenMetadataURI(uint256)')) == 0x157c3df9
     *
     *     => 0x06fdde03 ^ 0x95d89b41 ^ 0xc87b56dd ^ 0x157c3df9 == 0x4e222e66
     */
    bytes4 private constant _INTERFACE_ID_ERC721_METADATA = 0x4e222e66;

    Counters.Counter private _tokenIdTracker;

    /* *********
     * Modifiers
     * *********
     */

    /**
     * @notice Require that the token has not been burned and has been minted
     */
    modifier onlyExistingToken(uint256 tokenId) {
        require(_exists(tokenId), "Media: nonexistent token");
        _;
    }

    /**
     * @notice Require that the token has had a content hash set
     */
    modifier onlyTokenWithContentHash(uint256 tokenId) {
        require(
            tokenContentHashes[tokenId] != 0,
            "Media: token does not have hash of created content"
        );
        _;
    }

    /**
     * @notice Require that the token has had a metadata hash set
     */
    modifier onlyTokenWithMetadataHash(uint256 tokenId) {
        require(
            tokenMetadataHashes[tokenId] != 0,
            "Media: token does not have hash of its metadata"
        );
        _;
    }

    /**
     * @notice Ensure that the provided spender is the approved or the owner of
     * the media for the specified tokenId
     */
    modifier onlyApprovedOrOwner(address spender, uint256 tokenId) {
        require(
            _isApprovedOrOwner(spender, tokenId),
            "Media: Only approved or owner"
        );
        _;
    }

    /**
     * @notice Ensure the token has been created (even if it has been burned)
     */
    modifier onlyTokenCreated(uint256 tokenId) {
        require(
            _tokenIdTracker.current() > tokenId,
            "Media: token with that id does not exist"
        );
        _;
    }

    /**
     * @notice Ensure that the provided URI is not empty
     */
    modifier onlyValidURI(string memory uri) {
        require(
            bytes(uri).length != 0,
            "Media: specified uri must be non-empty"
        );
        _;
    }

    /**
     * @notice On deployment, set the market contract address and register the
     * ERC721 metadata interface
     */
    constructor(address marketContractAddr) public ERC721("Zora", "ZORA") {
        marketContract = marketContractAddr;
        _registerInterface(_INTERFACE_ID_ERC721_METADATA);
    }

    /* **************
     * View Functions
     * **************
     */

    /**
     * @notice return the URI for a particular piece of media with the specified tokenId
     * @dev This function is an override of the base OZ implementation because we
     * will return the tokenURI even if the media has been burned. In addition, this
     * protocol does not support a base URI, so relevant conditionals are removed.
     * @return the URI for a token
     */
    function tokenURI(uint256 tokenId)
        public
        view
        override
        onlyTokenCreated(tokenId)
        returns (string memory)
    {
        string memory _tokenURI = _tokenURIs[tokenId];

        return _tokenURI;
    }

    /**
     * @notice Return the metadata URI for a piece of media given the token URI
     * @return the metadata URI for the token
     */
    function tokenMetadataURI(uint256 tokenId)
        external
        view
        override
        onlyTokenCreated(tokenId)
        returns (string memory)
    {
        return _tokenMetadataURIs[tokenId];
    }

    /* ****************
     * Public Functions
     * ****************
     */

    /**
     * @notice see IMedia
     */
    function mint(MediaData memory data, IMarket.BidShares memory bidShares)
        public
        override
        nonReentrant
    {
        _mintForCreator(msg.sender, data, bidShares);
    }

    /**
     * @notice see IMedia
     */
    function mintWithSig(
        address creator,
        MediaData memory data,
        IMarket.BidShares memory bidShares,
        EIP712Signature memory sig
    ) public override nonReentrant {
        require(
            sig.deadline == 0 || sig.deadline >= block.timestamp,
            "Media: mintWithSig expired"
        );

        bytes32 domainSeparator = _calculateDomainSeparator();

        bytes32 digest =
            keccak256(
                abi.encodePacked(
                    "\x19\x01",
                    domainSeparator,
                    keccak256(
                        abi.encode(
                            MINT_WITH_SIG_TYPEHASH,
                            data.contentHash,
                            data.metadataHash,
                            bidShares.creator.value,
                            mintWithSigNonces[creator]++,
                            sig.deadline
                        )
                    )
                )
            );

        address recoveredAddress = ecrecover(digest, sig.v, sig.r, sig.s);

        require(
            recoveredAddress != address(0) && creator == recoveredAddress,
            "Media: Signature invalid"
        );

        _mintForCreator(recoveredAddress, data, bidShares);
    }

    /**
     * @notice see IMedia
     */
    function auctionTransfer(uint256 tokenId, address recipient)
        external
        override
    {
        require(msg.sender == marketContract, "Media: only market contract");
        previousTokenOwners[tokenId] = ownerOf(tokenId);
        _safeTransfer(ownerOf(tokenId), recipient, tokenId, "");
    }

    /**
     * @notice see IMedia
     */
    function setAsk(uint256 tokenId, IMarket.Ask memory ask)
        public
        override
        nonReentrant
        onlyApprovedOrOwner(msg.sender, tokenId)
    {
        IMarket(marketContract).setAsk(tokenId, ask);
    }

    /**
     * @notice see IMedia
     */
    function removeAsk(uint256 tokenId)
        external
        override
        nonReentrant
        onlyApprovedOrOwner(msg.sender, tokenId)
    {
        IMarket(marketContract).removeAsk(tokenId);
    }

    /**
     * @notice see IMedia
     */
    function setBid(uint256 tokenId, IMarket.Bid memory bid)
        public
        override
        nonReentrant
        onlyExistingToken(tokenId)
    {
        require(msg.sender == bid.bidder, "Market: Bidder must be msg sender");
        IMarket(marketContract).setBid(tokenId, bid, msg.sender);
    }

    /**
     * @notice see IMedia
     */
    function removeBid(uint256 tokenId)
        external
        override
        nonReentrant
        onlyTokenCreated(tokenId)
    {
        IMarket(marketContract).removeBid(tokenId, msg.sender);
    }

    /**
     * @notice see IMedia
     */
    function acceptBid(uint256 tokenId, IMarket.Bid memory bid)
        public
        override
        nonReentrant
        onlyApprovedOrOwner(msg.sender, tokenId)
    {
        IMarket(marketContract).acceptBid(tokenId, bid);
    }

    /**
     * @notice Burn a token.
     * @dev Only callable if the media owner is also the creator.
     */
    function burn(uint256 tokenId)
        public
        override
        nonReentrant
        onlyExistingToken(tokenId)
        onlyApprovedOrOwner(msg.sender, tokenId)
    {
        address owner = ownerOf(tokenId);

        require(
            tokenCreators[tokenId] == owner,
            "Media: owner is not creator of media"
        );

        _burn(tokenId);
    }

    /**
     * @notice Revoke the approvals for a token. The provided `approve` function is not sufficient
     * for this protocol, as it does not allow an approved address to revoke it's own approval.
     * In instances where a 3rd party is interacting on a user's behalf via `permit`, they should
     * revoke their approval once their task is complete as a best practice.
     */
    function revokeApproval(uint256 tokenId) external override nonReentrant {
        require(
            msg.sender == getApproved(tokenId),
            "Media: caller not approved address"
        );
        _approve(address(0), tokenId);
    }

    /**
     * @notice see IMedia
     * @dev only callable by approved or owner
     */
    function updateTokenURI(uint256 tokenId, string calldata tokenURI)
        external
        override
        nonReentrant
        onlyApprovedOrOwner(msg.sender, tokenId)
        onlyTokenWithContentHash(tokenId)
        onlyValidURI(tokenURI)
    {
        _setTokenURI(tokenId, tokenURI);
        emit TokenURIUpdated(tokenId, msg.sender, tokenURI);
    }

    /**
     * @notice see IMedia
     * @dev only callable by approved or owner
     */
    function updateTokenMetadataURI(
        uint256 tokenId,
        string calldata metadataURI
    )
        external
        override
        nonReentrant
        onlyApprovedOrOwner(msg.sender, tokenId)
        onlyTokenWithMetadataHash(tokenId)
        onlyValidURI(metadataURI)
    {
        _setTokenMetadataURI(tokenId, metadataURI);
        emit TokenMetadataURIUpdated(tokenId, msg.sender, metadataURI);
    }

    /**
     * @notice See IMedia
     * @dev This method is loosely based on the permit for ERC-20 tokens in  EIP-2612, but modified
     * for ERC-721.
     */
    function permit(
        address spender,
        uint256 tokenId,
        EIP712Signature memory sig
    ) public override nonReentrant onlyExistingToken(tokenId) {
        require(
            sig.deadline == 0 || sig.deadline >= block.timestamp,
            "Media: Permit expired"
        );
        require(spender != address(0), "Media: spender cannot be 0x0");
        bytes32 domainSeparator = _calculateDomainSeparator();

        bytes32 digest =
            keccak256(
                abi.encodePacked(
                    "\x19\x01",
                    domainSeparator,
                    keccak256(
                        abi.encode(
                            PERMIT_TYPEHASH,
                            spender,
                            tokenId,
                            permitNonces[ownerOf(tokenId)][tokenId]++,
                            sig.deadline
                        )
                    )
                )
            );

        address recoveredAddress = ecrecover(digest, sig.v, sig.r, sig.s);

        require(
            recoveredAddress != address(0) &&
                ownerOf(tokenId) == recoveredAddress,
            "Media: Signature invalid"
        );

        _approve(spender, tokenId);
    }

    /* *****************
     * Private Functions
     * *****************
     */

    /**
     * @notice Creates a new token for `creator`. Its token ID will be automatically
     * assigned (and available on the emitted {IERC721-Transfer} event), and the token
     * URI autogenerated based on the base URI passed at construction.
     *
     * See {ERC721-_safeMint}.
     *
     * On mint, also set the sha256 hashes of the content and its metadata for integrity
     * checks, along with the initial URIs to point to the content and metadata. Attribute
     * the token ID to the creator, mark the content hash as used, and set the bid shares for
     * the media's market.
     *
     * Note that although the content hash must be unique for future mints to prevent duplicate media,
     * metadata has no such requirement.
     */
    function _mintForCreator(
        address creator,
        MediaData memory data,
        IMarket.BidShares memory bidShares
    ) internal onlyValidURI(data.tokenURI) onlyValidURI(data.metadataURI) {
        require(data.contentHash != 0, "Media: content hash must be non-zero");
        require(
            _contentHashes[data.contentHash] == false,
            "Media: a token has already been created with this content hash"
        );
        require(
            data.metadataHash != 0,
            "Media: metadata hash must be non-zero"
        );

        uint256 tokenId = _tokenIdTracker.current();

        _safeMint(creator, tokenId);
        _tokenIdTracker.increment();
        _setTokenContentHash(tokenId, data.contentHash);
        _setTokenMetadataHash(tokenId, data.metadataHash);
        _setTokenMetadataURI(tokenId, data.metadataURI);
        _setTokenURI(tokenId, data.tokenURI);
        _creatorTokens[creator].add(tokenId);
        _contentHashes[data.contentHash] = true;

        tokenCreators[tokenId] = creator;
        previousTokenOwners[tokenId] = creator;
        IMarket(marketContract).setBidShares(tokenId, bidShares);
    }

    function _setTokenContentHash(uint256 tokenId, bytes32 contentHash)
        internal
        virtual
        onlyExistingToken(tokenId)
    {
        tokenContentHashes[tokenId] = contentHash;
    }

    function _setTokenMetadataHash(uint256 tokenId, bytes32 metadataHash)
        internal
        virtual
        onlyExistingToken(tokenId)
    {
        tokenMetadataHashes[tokenId] = metadataHash;
    }

    function _setTokenMetadataURI(uint256 tokenId, string memory metadataURI)
        internal
        virtual
        onlyExistingToken(tokenId)
    {
        _tokenMetadataURIs[tokenId] = metadataURI;
    }

    /**
     * @notice Destroys `tokenId`.
     * @dev We modify the OZ _burn implementation to
     * maintain metadata and to remove the
     * previous token owner from the piece
     */
    function _burn(uint256 tokenId) internal override {
        string memory tokenURI = _tokenURIs[tokenId];

        super._burn(tokenId);

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

        delete previousTokenOwners[tokenId];
    }

    /**
     * @notice transfer a token and remove the ask for it.
     */
    function _transfer(
        address from,
        address to,
        uint256 tokenId
    ) internal override {
        IMarket(marketContract).removeAsk(tokenId);

        super._transfer(from, to, tokenId);
    }

    /**
     * @dev Calculates EIP712 DOMAIN_SEPARATOR based on the current contract and chain ID.
     */
    function _calculateDomainSeparator() internal view returns (bytes32) {
        uint256 chainID;
        /* solium-disable-next-line */
        assembly {
            chainID := chainid()
        }

        return
            keccak256(
                abi.encode(
                    keccak256(
                        "EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)"
                    ),
                    keccak256(bytes("Zora")),
                    keccak256(bytes("1")),
                    chainID,
                    address(this)
                )
            );
    }
}

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



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

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

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

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

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

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

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

// File: @openzeppelin/contracts/math/Math.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

/**
 * @dev Standard math utilities missing in the Solidity language.
 */
library Math {
    /**
     * @dev Returns the largest of two numbers.
     */
    function max(uint256 a, uint256 b) internal pure returns (uint256) {
        return a >= b ? a : b;
    }

    /**
     * @dev Returns the smallest of two numbers.
     */
    function min(uint256 a, uint256 b) internal pure returns (uint256) {
        return a < b ? a : b;
    }

    /**
     * @dev Returns the average of two numbers. The result is rounded towards
     * zero.
     */
    function average(uint256 a, uint256 b) internal pure returns (uint256) {
        // (a + b) / 2 can overflow, so we distribute
        return (a / 2) + (b / 2) + ((a % 2 + b % 2) / 2);
    }
}

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



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

/**
 * @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].
 */
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 () internal {
        _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: contracts/Decimal.sol



/*
    Copyright 2019 dYdX Trading Inc.
    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
    You may obtain a copy of the License at
    http://www.apache.org/licenses/LICENSE-2.0
    Unless required by applicable law or agreed to in writing, software
    distributed under the License is distributed on an "AS IS" BASIS,
    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    See the License for the specific language governing permissions and
    limitations under the License.
*/

pragma solidity 0.6.8;
pragma experimental ABIEncoderV2;

/**
 * NOTE: This file is a clone of the dydx protocol's Decimal.sol contract. It was forked from https://github.com/dydxprotocol/solo
 * at commit 2d8454e02702fe5bc455b848556660629c3cad36
 *
 * It has not been modified other than to use a newer solidity in the pragma to match the rest of the contract suite of this project
 */

import {SafeMath} from "@openzeppelin/contracts/math/SafeMath.sol";
import {Math} from "./Math.sol";

/**
 * @title Decimal
 *
 * Library that defines a fixed-point number with 18 decimal places.
 */
library Decimal {
    using SafeMath for uint256;

    // ============ Constants ============

    uint256 constant BASE_POW = 18;
    uint256 constant BASE = 10**BASE_POW;

    // ============ Structs ============

    struct D256 {
        uint256 value;
    }

    // ============ Functions ============

    function one() internal pure returns (D256 memory) {
        return D256({value: BASE});
    }

    function onePlus(D256 memory d) internal pure returns (D256 memory) {
        return D256({value: d.value.add(BASE)});
    }

    function mul(uint256 target, D256 memory d)
        internal
        pure
        returns (uint256)
    {
        return Math.getPartial(target, d.value, BASE);
    }

    function div(uint256 target, D256 memory d)
        internal
        pure
        returns (uint256)
    {
        return Math.getPartial(target, BASE, d.value);
    }
}

// File: contracts/interfaces/IMarket.sol



// SPDX-License-Identifier: GPL-3.0

pragma solidity 0.6.8;
pragma experimental ABIEncoderV2;

import {Decimal} from "../Decimal.sol";

/**
 * @title Interface for Zora Protocol's Market
 */
interface IMarket {
    struct Bid {
        // Amount of the currency being bid
        uint256 amount;
        // Address to the ERC20 token being used to bid
        address currency;
        // Address of the bidder
        address bidder;
        // Address of the recipient
        address recipient;
        // % of the next sale to award the current owner
        Decimal.D256 sellOnShare;
    }

    struct Ask {
        // Amount of the currency being asked
        uint256 amount;
        // Address to the ERC20 token being asked
        address currency;
    }

    struct BidShares {
        // % of sale value that goes to the _previous_ owner of the nft
        Decimal.D256 prevOwner;
        // % of sale value that goes to the original creator of the nft
        Decimal.D256 creator;
        // % of sale value that goes to the seller (current owner) of the nft
        Decimal.D256 owner;
    }

    event BidCreated(uint256 indexed tokenId, Bid bid);
    event BidRemoved(uint256 indexed tokenId, Bid bid);
    event BidFinalized(uint256 indexed tokenId, Bid bid);
    event AskCreated(uint256 indexed tokenId, Ask ask);
    event AskRemoved(uint256 indexed tokenId, Ask ask);
    event BidShareUpdated(uint256 indexed tokenId, BidShares bidShares);

    function bidForTokenBidder(uint256 tokenId, address bidder)
        external
        view
        returns (Bid memory);

    function currentAskForToken(uint256 tokenId)
        external
        view
        returns (Ask memory);

    function bidSharesForToken(uint256 tokenId)
        external
        view
        returns (BidShares memory);

    function isValidBid(uint256 tokenId, uint256 bidAmount)
        external
        view
        returns (bool);

    function isValidBidShares(BidShares calldata bidShares)
        external
        pure
        returns (bool);

    function splitShare(Decimal.D256 calldata sharePercentage, uint256 amount)
        external
        pure
        returns (uint256);

    function configure(address mediaContractAddress) external;

    function setBidShares(uint256 tokenId, BidShares calldata bidShares)
        external;

    function setAsk(uint256 tokenId, Ask calldata ask) external;

    function removeAsk(uint256 tokenId) external;

    function setBid(
        uint256 tokenId,
        Bid calldata bid,
        address spender
    ) external;

    function removeBid(uint256 tokenId, address bidder) external;

    function acceptBid(uint256 tokenId, Bid calldata expectedBid) external;
}

// File: contracts/interfaces/IMedia.sol



// SPDX-License-Identifier: GPL-3.0

pragma solidity 0.6.8;
pragma experimental ABIEncoderV2;

import {IMarket} from "./IMarket.sol";

/**
 * @title Interface for Zora Protocol's Media
 */
interface IMedia {
    struct EIP712Signature {
        uint256 deadline;
        uint8 v;
        bytes32 r;
        bytes32 s;
    }

    struct MediaData {
        // A valid URI of the content represented by this token
        string tokenURI;
        // A valid URI of the metadata associated with this token
        string metadataURI;
        // A SHA256 hash of the content pointed to by tokenURI
        bytes32 contentHash;
        // A SHA256 hash of the content pointed to by metadataURI
        bytes32 metadataHash;
    }

    event TokenURIUpdated(uint256 indexed _tokenId, address owner, string _uri);
    event TokenMetadataURIUpdated(
        uint256 indexed _tokenId,
        address owner,
        string _uri
    );

    /**
     * @notice Return the metadata URI for a piece of media given the token URI
     */
    function tokenMetadataURI(uint256 tokenId)
        external
        view
        returns (string memory);

    /**
     * @notice Mint new media for msg.sender.
     */
    function mint(MediaData calldata data, IMarket.BidShares calldata bidShares)
        external;

    /**
     * @notice EIP-712 mintWithSig method. Mints new media for a creator given a valid signature.
     */
    function mintWithSig(
        address creator,
        MediaData calldata data,
        IMarket.BidShares calldata bidShares,
        EIP712Signature calldata sig
    ) external;

    /**
     * @notice Transfer the token with the given ID to a given address.
     * Save the previous owner before the transfer, in case there is a sell-on fee.
     * @dev This can only be called by the auction contract specified at deployment
     */
    function auctionTransfer(uint256 tokenId, address recipient) external;

    /**
     * @notice Set the ask on a piece of media
     */
    function setAsk(uint256 tokenId, IMarket.Ask calldata ask) external;

    /**
     * @notice Remove the ask on a piece of media
     */
    function removeAsk(uint256 tokenId) external;

    /**
     * @notice Set the bid on a piece of media
     */
    function setBid(uint256 tokenId, IMarket.Bid calldata bid) external;

    /**
     * @notice Remove the bid on a piece of media
     */
    function removeBid(uint256 tokenId) external;

    function acceptBid(uint256 tokenId, IMarket.Bid calldata bid) external;

    /**
     * @notice Revoke approval for a piece of media
     */
    function revokeApproval(uint256 tokenId) external;

    /**
     * @notice Update the token URI
     */
    function updateTokenURI(uint256 tokenId, string calldata tokenURI) external;

    /**
     * @notice Update the token metadata uri
     */
    function updateTokenMetadataURI(
        uint256 tokenId,
        string calldata metadataURI
    ) external;

    /**
     * @notice EIP-712 permit method. Sets an approved spender given a valid signature.
     */
    function permit(
        address spender,
        uint256 tokenId,
        EIP712Signature calldata sig
    ) external;
}

// File: contracts/Math.sol



pragma solidity 0.6.8;
pragma experimental ABIEncoderV2;

import {SafeMath} from "@openzeppelin/contracts/math/SafeMath.sol";

/**
 * @title Math
 *
 * Library for non-standard Math functions
 * NOTE: This file is a clone of the dydx protocol's Decimal.sol contract.
 * It was forked from https://github.com/dydxprotocol/solo at commit
 * 2d8454e02702fe5bc455b848556660629c3cad36. It has not been modified other than to use a
 * newer solidity in the pragma to match the rest of the contract suite of this project.
 */
library Math {
    using SafeMath for uint256;

    // ============ Library Functions ============

    /*
     * Return target * (numerator / denominator).
     */
    function getPartial(
        uint256 target,
        uint256 numerator,
        uint256 denominator
    ) internal pure returns (uint256) {
        return target.mul(numerator).div(denominator);
    }

    /*
     * Return target * (numerator / denominator), but rounded up.
     */
    function getPartialRoundUp(
        uint256 target,
        uint256 numerator,
        uint256 denominator
    ) internal pure returns (uint256) {
        if (target == 0 || numerator == 0) {
            // SafeMath will check for zero denominator
            return SafeMath.div(0, denominator);
        }
        return target.mul(numerator).sub(1).div(denominator).add(1);
    }

    function to128(uint256 number) internal pure returns (uint128) {
        uint128 result = uint128(number);
        require(result == number, "Math: Unsafe cast to uint128");
        return result;
    }

    function to96(uint256 number) internal pure returns (uint96) {
        uint96 result = uint96(number);
        require(result == number, "Math: Unsafe cast to uint96");
        return result;
    }

    function to32(uint256 number) internal pure returns (uint32) {
        uint32 result = uint32(number);
        require(result == number, "Math: Unsafe cast to uint32");
        return result;
    }

    function min(uint256 a, uint256 b) internal pure returns (uint256) {
        return a < b ? a : b;
    }

    function max(uint256 a, uint256 b) internal pure returns (uint256) {
        return a > b ? a : b;
    }
}

// File: contracts/Market.sol



// SPDX-License-Identifier: GPL-3.0

pragma solidity 0.6.8;
pragma experimental ABIEncoderV2;

import {SafeMath} from "@openzeppelin/contracts/math/SafeMath.sol";
import {IERC721} from "@openzeppelin/contracts/token/ERC721/IERC721.sol";
import {IERC20} from "@openzeppelin/contracts/token/ERC20/IERC20.sol";
import {SafeERC20} from "@openzeppelin/contracts/token/ERC20/SafeERC20.sol";
import {Decimal} from "./Decimal.sol";
import {Media} from "./Media.sol";
import {IMarket} from "./interfaces/IMarket.sol";

/**
 * @title A Market for pieces of media
 * @notice This contract contains all of the market logic for Media
 */
contract Market is IMarket {
    using SafeMath for uint256;
    using SafeERC20 for IERC20;

    /* *******
     * Globals
     * *******
     */
    // Address of the media contract that can call this market
    address public mediaContract;

    // Deployment Address
    address private _owner;

    // Mapping from token to mapping from bidder to bid
    mapping(uint256 => mapping(address => Bid)) private _tokenBidders;

    // Mapping from token to the bid shares for the token
    mapping(uint256 => BidShares) private _bidShares;

    // Mapping from token to the current ask for the token
    mapping(uint256 => Ask) private _tokenAsks;

    /* *********
     * Modifiers
     * *********
     */

    /**
     * @notice require that the msg.sender is the configured media contract
     */
    modifier onlyMediaCaller() {
        require(mediaContract == msg.sender, "Market: Only media contract");
        _;
    }

    /* ****************
     * View Functions
     * ****************
     */
    function bidForTokenBidder(uint256 tokenId, address bidder)
        external
        view
        override
        returns (Bid memory)
    {
        return _tokenBidders[tokenId][bidder];
    }

    function currentAskForToken(uint256 tokenId)
        external
        view
        override
        returns (Ask memory)
    {
        return _tokenAsks[tokenId];
    }

    function bidSharesForToken(uint256 tokenId)
        public
        view
        override
        returns (BidShares memory)
    {
        return _bidShares[tokenId];
    }

    /**
     * @notice Validates that the bid is valid by ensuring that the bid amount can be split perfectly into all the bid shares.
     *  We do this by comparing the sum of the individual share values with the amount and ensuring they are equal. Because
     *  the splitShare function uses integer division, any inconsistencies with the original and split sums would be due to
     *  a bid splitting that does not perfectly divide the bid amount.
     */
    function isValidBid(uint256 tokenId, uint256 bidAmount)
        public
        view
        override
        returns (bool)
    {
        BidShares memory bidShares = bidSharesForToken(tokenId);
        require(
            isValidBidShares(bidShares),
            "Market: Invalid bid shares for token"
        );
        return
            bidAmount != 0 &&
            (bidAmount ==
                splitShare(bidShares.creator, bidAmount)
                    .add(splitShare(bidShares.prevOwner, bidAmount))
                    .add(splitShare(bidShares.owner, bidAmount)));
    }

    /**
     * @notice Validates that the provided bid shares sum to 100
     */
    function isValidBidShares(BidShares memory bidShares)
        public
        pure
        override
        returns (bool)
    {
        return
            bidShares.creator.value.add(bidShares.owner.value).add(
                bidShares.prevOwner.value
            ) == uint256(100).mul(Decimal.BASE);
    }

    /**
     * @notice return a % of the specified amount. This function is used to split a bid into shares
     * for a media's shareholders.
     */
    function splitShare(Decimal.D256 memory sharePercentage, uint256 amount)
        public
        pure
        override
        returns (uint256)
    {
        return Decimal.mul(amount, sharePercentage).div(100);
    }

    /* ****************
     * Public Functions
     * ****************
     */

    constructor() public {
        _owner = msg.sender;
    }

    /**
     * @notice Sets the media contract address. This address is the only permitted address that
     * can call the mutable functions. This method can only be called once.
     */
    function configure(address mediaContractAddress) external override {
        require(msg.sender == _owner, "Market: Only owner");
        require(mediaContract == address(0), "Market: Already configured");
        require(
            mediaContractAddress != address(0),
            "Market: cannot set media contract as zero address"
        );

        mediaContract = mediaContractAddress;
    }

    /**
     * @notice Sets bid shares for a particular tokenId. These bid shares must
     * sum to 100.
     */
    function setBidShares(uint256 tokenId, BidShares memory bidShares)
        public
        override
        onlyMediaCaller
    {
        require(
            isValidBidShares(bidShares),
            "Market: Invalid bid shares, must sum to 100"
        );
        _bidShares[tokenId] = bidShares;
        emit BidShareUpdated(tokenId, bidShares);
    }

    /**
     * @notice Sets the ask on a particular media. If the ask cannot be evenly split into the media's
     * bid shares, this reverts.
     */
    function setAsk(uint256 tokenId, Ask memory ask)
        public
        override
        onlyMediaCaller
    {
        require(
            isValidBid(tokenId, ask.amount),
            "Market: Ask invalid for share splitting"
        );

        _tokenAsks[tokenId] = ask;
        emit AskCreated(tokenId, ask);
    }

    /**
     * @notice removes an ask for a token and emits an AskRemoved event
     */
    function removeAsk(uint256 tokenId) external override onlyMediaCaller {
        emit AskRemoved(tokenId, _tokenAsks[tokenId]);
        delete _tokenAsks[tokenId];
    }

    /**
     * @notice Sets the bid on a particular media for a bidder. The token being used to bid
     * is transferred from the spender to this contract to be held until removed or accepted.
     * If another bid already exists for the bidder, it is refunded.
     */
    function setBid(
        uint256 tokenId,
        Bid memory bid,
        address spender
    ) public override onlyMediaCaller {
        BidShares memory bidShares = _bidShares[tokenId];
        require(
            bidShares.creator.value.add(bid.sellOnShare.value) <=
                uint256(100).mul(Decimal.BASE),
            "Market: Sell on fee invalid for share splitting"
        );
        require(bid.bidder != address(0), "Market: bidder cannot be 0 address");
        require(bid.amount != 0, "Market: cannot bid amount of 0");
        require(
            bid.currency != address(0),
            "Market: bid currency cannot be 0 address"
        );
        require(
            bid.recipient != address(0),
            "Market: bid recipient cannot be 0 address"
        );

        Bid storage existingBid = _tokenBidders[tokenId][bid.bidder];

        // If there is an existing bid, refund it before continuing
        if (existingBid.amount > 0) {
            removeBid(tokenId, bid.bidder);
        }

        IERC20 token = IERC20(bid.currency);

        // We must check the balance that was actually transferred to the market,
        // as some tokens impose a transfer fee and would not actually transfer the
        // full amount to the market, resulting in locked funds for refunds & bid acceptance
        uint256 beforeBalance = token.balanceOf(address(this));
        token.safeTransferFrom(spender, address(this), bid.amount);
        uint256 afterBalance = token.balanceOf(address(this));
        _tokenBidders[tokenId][bid.bidder] = Bid(
            afterBalance.sub(beforeBalance),
            bid.currency,
            bid.bidder,
            bid.recipient,
            bid.sellOnShare
        );
        emit BidCreated(tokenId, bid);

        // If a bid meets the criteria for an ask, automatically accept the bid.
        // If no ask is set or the bid does not meet the requirements, ignore.
        if (
            _tokenAsks[tokenId].currency != address(0) &&
            bid.currency == _tokenAsks[tokenId].currency &&
            bid.amount >= _tokenAsks[tokenId].amount
        ) {
            // Finalize exchange
            _finalizeNFTTransfer(tokenId, bid.bidder);
        }
    }

    /**
     * @notice Removes the bid on a particular media for a bidder. The bid amount
     * is transferred from this contract to the bidder, if they have a bid placed.
     */
    function removeBid(uint256 tokenId, address bidder)
        public
        override
        onlyMediaCaller
    {
        Bid storage bid = _tokenBidders[tokenId][bidder];
        uint256 bidAmount = bid.amount;
        address bidCurrency = bid.currency;

        require(bid.amount > 0, "Market: cannot remove bid amount of 0");

        IERC20 token = IERC20(bidCurrency);

        emit BidRemoved(tokenId, bid);
        delete _tokenBidders[tokenId][bidder];
        token.safeTransfer(bidder, bidAmount);
    }

    /**
     * @notice Accepts a bid from a particular bidder. Can only be called by the media contract.
     * See {_finalizeNFTTransfer}
     * Provided bid must match a bid in storage. This is to prevent a race condition
     * where a bid may change while the acceptBid call is in transit.
     * A bid cannot be accepted if it cannot be split equally into its shareholders.
     * This should only revert in rare instances (example, a low bid with a zero-decimal token),
     * but is necessary to ensure fairness to all shareholders.
     */
    function acceptBid(uint256 tokenId, Bid calldata expectedBid)
        external
        override
        onlyMediaCaller
    {
        Bid memory bid = _tokenBidders[tokenId][expectedBid.bidder];
        require(bid.amount > 0, "Market: cannot accept bid of 0");
        require(
            bid.amount == expectedBid.amount &&
                bid.currency == expectedBid.currency &&
                bid.sellOnShare.value == expectedBid.sellOnShare.value &&
                bid.recipient == expectedBid.recipient,
            "Market: Unexpected bid found."
        );
        require(
            isValidBid(tokenId, bid.amount),
            "Market: Bid invalid for share splitting"
        );

        _finalizeNFTTransfer(tokenId, bid.bidder);
    }

    /**
     * @notice Given a token ID and a bidder, this method transfers the value of
     * the bid to the shareholders. It also transfers the ownership of the media
     * to the bid recipient. Finally, it removes the accepted bid and the current ask.
     */
    function _finalizeNFTTransfer(uint256 tokenId, address bidder) private {
        Bid memory bid = _tokenBidders[tokenId][bidder];
        BidShares storage bidShares = _bidShares[tokenId];

        IERC20 token = IERC20(bid.currency);

        // Transfer bid share to owner of media
        token.safeTransfer(
            IERC721(mediaContract).ownerOf(tokenId),
            splitShare(bidShares.owner, bid.amount)
        );
        // Transfer bid share to creator of media
        token.safeTransfer(
            Media(mediaContract).tokenCreators(tokenId),
            splitShare(bidShares.creator, bid.amount)
        );
        // Transfer bid share to previous owner of media (if applicable)
        token.safeTransfer(
            Media(mediaContract).previousTokenOwners(tokenId),
            splitShare(bidShares.prevOwner, bid.amount)
        );

        // Transfer media to bid recipient
        Media(mediaContract).auctionTransfer(tokenId, bid.recipient);

        // Calculate the bid share for the new owner,
        // equal to 100 - creatorShare - sellOnShare
        bidShares.owner = Decimal.D256(
            uint256(100)
                .mul(Decimal.BASE)
                .sub(_bidShares[tokenId].creator.value)
                .sub(bid.sellOnShare.value)
        );
        // Set the previous owner share to the accepted bid's sell-on fee
        bidShares.prevOwner = bid.sellOnShare;

        // Remove the accepted bid
        delete _tokenBidders[tokenId][bidder];

        emit BidShareUpdated(tokenId, bidShares);
        emit BidFinalized(tokenId, bid);
    }
}

// File: @openzeppelin/contracts/token/ERC20/SafeERC20.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

import "./IERC20.sol";
import "../../math/SafeMath.sol";
import "../../utils/Address.sol";

/**
 * @title SafeERC20
 * @dev Wrappers around ERC20 operations that throw on failure (when the token
 * contract returns false). Tokens that return no value (and instead revert or
 * throw on failure) are also supported, non-reverting calls are assumed to be
 * successful.
 * To use this library you can add a `using SafeERC20 for IERC20;` statement to your contract,
 * which allows you to call the safe operations as `token.safeTransfer(...)`, etc.
 */
library SafeERC20 {
    using SafeMath for uint256;
    using Address for address;

    function safeTransfer(IERC20 token, address to, uint256 value) internal {
        _callOptionalReturn(token, abi.encodeWithSelector(token.transfer.selector, to, value));
    }

    function safeTransferFrom(IERC20 token, address from, address to, uint256 value) internal {
        _callOptionalReturn(token, abi.encodeWithSelector(token.transferFrom.selector, from, to, value));
    }

    /**
     * @dev Deprecated. This function has issues similar to the ones found in
     * {IERC20-approve}, and its usage is discouraged.
     *
     * Whenever possible, use {safeIncreaseAllowance} and
     * {safeDecreaseAllowance} instead.
     */
    function safeApprove(IERC20 token, address spender, uint256 value) internal {
        // safeApprove should only be called when setting an initial allowance,
        // or when resetting it to zero. To increase and decrease it, use
        // 'safeIncreaseAllowance' and 'safeDecreaseAllowance'
        // solhint-disable-next-line max-line-length
        require((value == 0) || (token.allowance(address(this), spender) == 0),
            "SafeERC20: approve from non-zero to non-zero allowance"
        );
        _callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, value));
    }

    function safeIncreaseAllowance(IERC20 token, address spender, uint256 value) internal {
        uint256 newAllowance = token.allowance(address(this), spender).add(value);
        _callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, newAllowance));
    }

    function safeDecreaseAllowance(IERC20 token, address spender, uint256 value) internal {
        uint256 newAllowance = token.allowance(address(this), spender).sub(value, "SafeERC20: decreased allowance below zero");
        _callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, newAllowance));
    }

    /**
     * @dev Imitates a Solidity high-level call (i.e. a regular function call to a contract), relaxing the requirement
     * on the return value: the return value is optional (but if data is returned, it must not be false).
     * @param token The token targeted by the call.
     * @param data The call data (encoded using abi.encode or one of its variants).
     */
    function _callOptionalReturn(IERC20 token, bytes memory data) private {
        // We need to perform a low level call here, to bypass Solidity's return data size checking mechanism, since
        // we're implementing it ourselves. We use {Address.functionCall} to perform this call, which verifies that
        // the target address contains contract code and also asserts for success in the low-level call.

        bytes memory returndata = address(token).functionCall(data, "SafeERC20: low-level call failed");
        if (returndata.length > 0) { // Return data is optional
            // solhint-disable-next-line max-line-length
            require(abi.decode(returndata, (bool)), "SafeERC20: ERC20 operation did not succeed");
        }
    }
}