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// This contract is part of Zellic’s smart contract dataset, which is a collection of publicly available contract code gathered as of March 2023.

// SPDX-License-Identifier: MIT
pragma solidity 0.8.7;

/// @dev Do not manually set balances without updating totalSupply, as the sum of all user balances must not exceed it.

abstract contract ERC20 {

    /*//////////////////////////////////////////////////////////////
                                 EVENTS
    //////////////////////////////////////////////////////////////*/



    event Transfer(address indexed from, address indexed to, uint256 amount);



    event Approval(address indexed owner, address indexed spender, uint256 amount);



    /*//////////////////////////////////////////////////////////////
                            METADATA STORAGE
    //////////////////////////////////////////////////////////////*/



    string public name;



    string public symbol;



    uint8 public immutable decimals;



    /*//////////////////////////////////////////////////////////////
                              ERC20 STORAGE
    //////////////////////////////////////////////////////////////*/



    uint256 public totalSupply;



    mapping(address => uint256) public balanceOf;



    mapping(address => mapping(address => uint256)) public allowance;



    /*//////////////////////////////////////////////////////////////
                            EIP-2612 STORAGE
    //////////////////////////////////////////////////////////////*/



    uint256 internal immutable INITIAL_CHAIN_ID;



    bytes32 internal immutable INITIAL_DOMAIN_SEPARATOR;



    mapping(address => uint256) public nonces;



    /*//////////////////////////////////////////////////////////////
                               CONSTRUCTOR
    //////////////////////////////////////////////////////////////*/



    constructor(

        string memory _name,

        string memory _symbol,

        uint8 _decimals

    ) {

        name = _name;

        symbol = _symbol;

        decimals = _decimals;



        INITIAL_CHAIN_ID = block.chainid;

        INITIAL_DOMAIN_SEPARATOR = computeDomainSeparator();

    }



    /*//////////////////////////////////////////////////////////////
                               ERC20 LOGIC
    //////////////////////////////////////////////////////////////*/



    function approve(address spender, uint256 amount) public virtual returns (bool) {

        allowance[msg.sender][spender] = amount;



        emit Approval(msg.sender, spender, amount);



        return true;

    }



    function transfer(address to, uint256 amount) public virtual returns (bool) {

        balanceOf[msg.sender] -= amount;



        // Cannot overflow because the sum of all user

        // balances can't exceed the max uint256 value.

        unchecked {

            balanceOf[to] += amount;

        }



        emit Transfer(msg.sender, to, amount);



        return true;

    }



    function transferFrom(

        address from,

        address to,

        uint256 amount

    ) public virtual returns (bool) {

        uint256 allowed = allowance[from][msg.sender]; // Saves gas for limited approvals.



        if (allowed != type(uint256).max) allowance[from][msg.sender] = allowed - amount;



        balanceOf[from] -= amount;



        // Cannot overflow because the sum of all user

        // balances can't exceed the max uint256 value.

        unchecked {

            balanceOf[to] += amount;

        }



        emit Transfer(from, to, amount);



        return true;

    }



    /*//////////////////////////////////////////////////////////////
                             EIP-2612 LOGIC
    //////////////////////////////////////////////////////////////*/



    function permit(

        address owner,

        address spender,

        uint256 value,

        uint256 deadline,

        uint8 v,

        bytes32 r,

        bytes32 s

    ) public virtual {

        require(deadline >= block.timestamp, "PERMIT_DEADLINE_EXPIRED");



        // Unchecked because the only math done is incrementing

        // the owner's nonce which cannot realistically overflow.

        unchecked {

            address recoveredAddress = ecrecover(

                keccak256(

                    abi.encodePacked(

                        "\x19\x01",

                        DOMAIN_SEPARATOR(),

                        keccak256(

                            abi.encode(

                                keccak256(

                                    "Permit(address owner,address spender,uint256 value,uint256 nonce,uint256 deadline)"

                                ),

                                owner,

                                spender,

                                value,

                                nonces[owner]++,

                                deadline

                            )

                        )

                    )

                ),

                v,

                r,

                s

            );



            require(recoveredAddress != address(0) && recoveredAddress == owner, "INVALID_SIGNER");



            allowance[recoveredAddress][spender] = value;

        }



        emit Approval(owner, spender, value);

    }



    function DOMAIN_SEPARATOR() public view virtual returns (bytes32) {

        return block.chainid == INITIAL_CHAIN_ID ? INITIAL_DOMAIN_SEPARATOR : computeDomainSeparator();

    }



    function computeDomainSeparator() internal view virtual returns (bytes32) {

        return

            keccak256(

                abi.encode(

                    keccak256("EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)"),

                    keccak256(bytes(name)),

                    keccak256("1"),

                    block.chainid,

                    address(this)

                )

            );

    }



    /*//////////////////////////////////////////////////////////////
                        INTERNAL MINT/BURN LOGIC
    //////////////////////////////////////////////////////////////*/



    function _mint(address to, uint256 amount) internal virtual {

        totalSupply += amount;



        // Cannot overflow because the sum of all user

        // balances can't exceed the max uint256 value.

        unchecked {

            balanceOf[to] += amount;

        }



        emit Transfer(address(0), to, amount);

    }



    function _burn(address from, uint256 amount) internal virtual {

        balanceOf[from] -= amount;



        // Cannot underflow because a user's balance

        // will never be larger than the total supply.

        unchecked {

            totalSupply -= amount;

        }



        emit Transfer(from, address(0), amount);

    }

}



/// @notice Safe ETH and ERC20 transfer library that gracefully handles missing return values.
/// @author Solmate (https://github.com/Rari-Capital/solmate/blob/main/src/utils/SafeTransferLib.sol)
/// @dev Use with caution! Some functions in this library knowingly create dirty bits at the destination of the free memory pointer.

/// @dev Note that none of the functions in this library check that a token has code at all! That responsibility is delegated to the caller.
library SafeTransferLib {
    /*//////////////////////////////////////////////////////////////

                             ETH OPERATIONS

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

    function safeTransferETH(address to, uint256 amount) internal {
        bool success;

        assembly {
            // Transfer the ETH and store if it succeeded or not.
            success := call(gas(), to, amount, 0, 0, 0, 0)
        }

        require(success, "ETH_TRANSFER_FAILED");
    }

    /*//////////////////////////////////////////////////////////////
                            ERC20 OPERATIONS
    //////////////////////////////////////////////////////////////*/



    function safeTransferFrom(

        ERC20 token,

        address from,

        address to,

        uint256 amount

    ) internal {

        bool success;



        assembly {

            // Get a pointer to some free memory.

            let freeMemoryPointer := mload(0x40)



            // Write the abi-encoded calldata into memory, beginning with the function selector.

            mstore(freeMemoryPointer, 0x23b872dd00000000000000000000000000000000000000000000000000000000)

            mstore(add(freeMemoryPointer, 4), from) // Append the "from" argument.

            mstore(add(freeMemoryPointer, 36), to) // Append the "to" argument.

            mstore(add(freeMemoryPointer, 68), amount) // Append the "amount" argument.



            success := and(

                // Set success to whether the call reverted, if not we check it either

                // returned exactly 1 (can't just be non-zero data), or had no return data.

                or(and(eq(mload(0), 1), gt(returndatasize(), 31)), iszero(returndatasize())),

                // We use 100 because the length of our calldata totals up like so: 4 + 32 * 3.

                // We use 0 and 32 to copy up to 32 bytes of return data into the scratch space.

                // Counterintuitively, this call must be positioned second to the or() call in the

                // surrounding and() call or else returndatasize() will be zero during the computation.

                call(gas(), token, 0, freeMemoryPointer, 100, 0, 32)

            )

        }



        require(success, "TRANSFER_FROM_FAILED");

    }



    function safeTransfer(

        ERC20 token,

        address to,

        uint256 amount

    ) internal {

        bool success;



        assembly {

            // Get a pointer to some free memory.

            let freeMemoryPointer := mload(0x40)



            // Write the abi-encoded calldata into memory, beginning with the function selector.

            mstore(freeMemoryPointer, 0xa9059cbb00000000000000000000000000000000000000000000000000000000)

            mstore(add(freeMemoryPointer, 4), to) // Append the "to" argument.

            mstore(add(freeMemoryPointer, 36), amount) // Append the "amount" argument.



            success := and(

                // Set success to whether the call reverted, if not we check it either

                // returned exactly 1 (can't just be non-zero data), or had no return data.

                or(and(eq(mload(0), 1), gt(returndatasize(), 31)), iszero(returndatasize())),

                // We use 68 because the length of our calldata totals up like so: 4 + 32 * 2.

                // We use 0 and 32 to copy up to 32 bytes of return data into the scratch space.

                // Counterintuitively, this call must be positioned second to the or() call in the

                // surrounding and() call or else returndatasize() will be zero during the computation.

                call(gas(), token, 0, freeMemoryPointer, 68, 0, 32)

            )

        }



        require(success, "TRANSFER_FAILED");

    }



    function safeApprove(

        ERC20 token,

        address to,

        uint256 amount

    ) internal {

        bool success;



        assembly {

            // Get a pointer to some free memory.

            let freeMemoryPointer := mload(0x40)



            // Write the abi-encoded calldata into memory, beginning with the function selector.

            mstore(freeMemoryPointer, 0x095ea7b300000000000000000000000000000000000000000000000000000000)

            mstore(add(freeMemoryPointer, 4), to) // Append the "to" argument.

            mstore(add(freeMemoryPointer, 36), amount) // Append the "amount" argument.



            success := and(

                // Set success to whether the call reverted, if not we check it either

                // returned exactly 1 (can't just be non-zero data), or had no return data.

                or(and(eq(mload(0), 1), gt(returndatasize(), 31)), iszero(returndatasize())),

                // We use 68 because the length of our calldata totals up like so: 4 + 32 * 2.

                // We use 0 and 32 to copy up to 32 bytes of return data into the scratch space.

                // Counterintuitively, this call must be positioned second to the or() call in the

                // surrounding and() call or else returndatasize() will be zero during the computation.

                call(gas(), token, 0, freeMemoryPointer, 68, 0, 32)

            )

        }



        require(success, "APPROVE_FAILED");

    }

}

/// @notice Modern, minimalist, and gas efficient ERC-721 implementation.
/// @author Solmate (https://github.com/Rari-Capital/solmate/blob/main/src/tokens/ERC721.sol)
abstract contract ERC721 {
    /*//////////////////////////////////////////////////////////////

                                 EVENTS

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

    event Transfer(address indexed from, address indexed to, uint256 indexed id);

    event Approval(address indexed owner, address indexed spender, uint256 indexed id);

    event ApprovalForAll(address indexed owner, address indexed operator, bool approved);

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

                         METADATA STORAGE/LOGIC
    //////////////////////////////////////////////////////////////*/



    string public name;



    string public symbol;



    function tokenURI(uint256 id) public view virtual returns (string memory);



    /*//////////////////////////////////////////////////////////////
                      ERC721 BALANCE/OWNER STORAGE
    //////////////////////////////////////////////////////////////*/



    mapping(uint256 => address) internal _ownerOf;



    mapping(address => uint256) internal _balanceOf;



    function ownerOf(uint256 id) public view virtual returns (address owner) {

        require((owner = _ownerOf[id]) != address(0), "NOT_MINTED");

    }



    function balanceOf(address owner) public view virtual returns (uint256) {

        require(owner != address(0), "ZERO_ADDRESS");



        return _balanceOf[owner];

    }



    /*//////////////////////////////////////////////////////////////
                         ERC721 APPROVAL STORAGE
    //////////////////////////////////////////////////////////////*/



    mapping(uint256 => address) public getApproved;



    mapping(address => mapping(address => bool)) public isApprovedForAll;



    /*//////////////////////////////////////////////////////////////
                               CONSTRUCTOR
    //////////////////////////////////////////////////////////////*/



    constructor(string memory _name, string memory _symbol) {

        name = _name;

        symbol = _symbol;

    }



    /*//////////////////////////////////////////////////////////////
                              ERC721 LOGIC
    //////////////////////////////////////////////////////////////*/



    function approve(address spender, uint256 id) public virtual {

        address owner = _ownerOf[id];



        require(msg.sender == owner || isApprovedForAll[owner][msg.sender], "NOT_AUTHORIZED");



        getApproved[id] = spender;



        emit Approval(owner, spender, id);

    }



    function setApprovalForAll(address operator, bool approved) public virtual {

        isApprovedForAll[msg.sender][operator] = approved;



        emit ApprovalForAll(msg.sender, operator, approved);

    }



    function transferFrom(

        address from,

        address to,

        uint256 id

    ) public virtual {

        require(from == _ownerOf[id], "WRONG_FROM");



        require(to != address(0), "INVALID_RECIPIENT");



        require(

            msg.sender == from || isApprovedForAll[from][msg.sender] || msg.sender == getApproved[id],

            "NOT_AUTHORIZED"

        );



        // Underflow of the sender's balance is impossible because we check for

        // ownership above and the recipient's balance can't realistically overflow.

        unchecked {

            _balanceOf[from]--;



            _balanceOf[to]++;

        }



        _ownerOf[id] = to;



        delete getApproved[id];



        emit Transfer(from, to, id);

    }



    function safeTransferFrom(

        address from,

        address to,

        uint256 id

    ) public virtual {

        transferFrom(from, to, id);



        require(

            to.code.length == 0 ||

                ERC721TokenReceiver(to).onERC721Received(msg.sender, from, id, "") ==

                ERC721TokenReceiver.onERC721Received.selector,

            "UNSAFE_RECIPIENT"

        );

    }



    function safeTransferFrom(

        address from,

        address to,

        uint256 id,

        bytes calldata data

    ) public virtual {

        transferFrom(from, to, id);



        require(

            to.code.length == 0 ||

                ERC721TokenReceiver(to).onERC721Received(msg.sender, from, id, data) ==

                ERC721TokenReceiver.onERC721Received.selector,

            "UNSAFE_RECIPIENT"

        );

    }



    /*//////////////////////////////////////////////////////////////
                              ERC165 LOGIC
    //////////////////////////////////////////////////////////////*/



    function supportsInterface(bytes4 interfaceId) public view virtual returns (bool) {

        return

            interfaceId == 0x01ffc9a7 || // ERC165 Interface ID for ERC165

            interfaceId == 0x80ac58cd || // ERC165 Interface ID for ERC721

            interfaceId == 0x5b5e139f; // ERC165 Interface ID for ERC721Metadata

    }



    /*//////////////////////////////////////////////////////////////
                        INTERNAL MINT/BURN LOGIC
    //////////////////////////////////////////////////////////////*/



    function _mint(address to, uint256 id) internal virtual {

        require(to != address(0), "INVALID_RECIPIENT");



        require(_ownerOf[id] == address(0), "ALREADY_MINTED");



        // Counter overflow is incredibly unrealistic.

        unchecked {

            _balanceOf[to]++;

        }



        _ownerOf[id] = to;



        emit Transfer(address(0), to, id);

    }



    function _burn(uint256 id) internal virtual {

        address owner = _ownerOf[id];



        require(owner != address(0), "NOT_MINTED");



        // Ownership check above ensures no underflow.

        unchecked {

            _balanceOf[owner]--;

        }



        delete _ownerOf[id];



        delete getApproved[id];



        emit Transfer(owner, address(0), id);

    }



    /*//////////////////////////////////////////////////////////////
                        INTERNAL SAFE MINT LOGIC
    //////////////////////////////////////////////////////////////*/



    function _safeMint(address to, uint256 id) internal virtual {

        _mint(to, id);



        require(

            to.code.length == 0 ||

                ERC721TokenReceiver(to).onERC721Received(msg.sender, address(0), id, "") ==

                ERC721TokenReceiver.onERC721Received.selector,

            "UNSAFE_RECIPIENT"

        );

    }



    function _safeMint(

        address to,

        uint256 id,

        bytes memory data

    ) internal virtual {

        _mint(to, id);



        require(

            to.code.length == 0 ||

                ERC721TokenReceiver(to).onERC721Received(msg.sender, address(0), id, data) ==

                ERC721TokenReceiver.onERC721Received.selector,

            "UNSAFE_RECIPIENT"

        );

    }

}



/// @notice A generic interface for a contract which properly accepts ERC721 tokens.
/// @author Solmate (https://github.com/Rari-Capital/solmate/blob/main/src/tokens/ERC721.sol)
abstract contract ERC721TokenReceiver {
    function onERC721Received(
        address,
        address,
        uint256,
        bytes calldata
    ) external virtual returns (bytes4) {
        return ERC721TokenReceiver.onERC721Received.selector;
    }
}

/// @title Contains 512-bit math functions

/// @notice Facilitates multiplication and division that can have overflow of an intermediate value without any loss of precision  
/// @dev Handles "phantom overflow" i.e., allows multiplication and division where an intermediate value overflows 256 bits        

library FullMath {

    /// @notice Calculates floor(a×b÷denominator) with full precision. Throws if result overflows a uint256 or denominator == 0    
    /// @param a The multiplicand

    /// @param b The multiplier
    /// @param denominator The divisor

    /// @return result The 256-bit result
    /// @dev Credit to Remco Bloemen under MIT license https://xn--2-umb.com/21/muldiv
    function mulDiv(
        uint256 a,
        uint256 b,
        uint256 denominator
    ) internal pure returns (uint256 result) {
        unchecked {
            // 512-bit multiply [prod1 prod0] = a * b
            // Compute the product mod 2**256 and mod 2**256 - 1
            // then use the Chinese Remainder Theorem to reconstruct
            // the 512 bit result. The result is stored in two 256
            // variables such that product = prod1 * 2**256 + prod0
            uint256 prod0; // Least significant 256 bits of the product
            uint256 prod1; // Most significant 256 bits of the product
            assembly {
                let mm := mulmod(a, b, not(0))
                prod0 := mul(a, b)
                prod1 := sub(sub(mm, prod0), lt(mm, prod0))

            }



            // Handle non-overflow cases, 256 by 256 division

            if (prod1 == 0) {
                require(denominator > 0);
                assembly {
                    result := div(prod0, denominator)
                }
                return result;
            }

            // Make sure the result is less than 2**256.
            // Also prevents denominator == 0
            require(denominator > prod1);

            ///////////////////////////////////////////////

            // 512 by 256 division.

            ///////////////////////////////////////////////

            // Make division exact by subtracting the remainder from [prod1 prod0]
            // Compute remainder using mulmod
            uint256 remainder;
            assembly {
                remainder := mulmod(a, b, denominator)
            }
            // Subtract 256 bit number from 512 bit number
            assembly {
                prod1 := sub(prod1, gt(remainder, prod0))

                prod0 := sub(prod0, remainder)

            }



            // Factor powers of two out of denominator

            // Compute largest power of two divisor of denominator.

            // Always >= 1.

            uint256 twos = (0 - denominator) & denominator;
            // Divide denominator by power of two
            assembly {
                denominator := div(denominator, twos)
            }

            // Divide [prod1 prod0] by the factors of two
            assembly {
                prod0 := div(prod0, twos)
            }
            // Shift in bits from prod1 into prod0. For this we need
            // to flip `twos` such that it is 2**256 / twos.
            // If twos is zero, then it becomes one
            assembly {
                twos := add(div(sub(0, twos), twos), 1)

            }

            prod0 |= prod1 * twos;

            // Invert denominator mod 2**256
            // Now that denominator is an odd number, it has an inverse
            // modulo 2**256 such that denominator * inv = 1 mod 2**256.
            // Compute the inverse by starting with a seed that is correct
            // correct for four bits. That is, denominator * inv = 1 mod 2**4
            uint256 inv = (3 * denominator) ^ 2;
            // Now use Newton-Raphson iteration to improve the precision.
            // Thanks to Hensel's lifting lemma, this also works in modular

            // arithmetic, doubling the correct bits in each step.

            inv *= 2 - denominator * inv; // inverse mod 2**8

            inv *= 2 - denominator * inv; // inverse mod 2**16

            inv *= 2 - denominator * inv; // inverse mod 2**32

            inv *= 2 - denominator * inv; // inverse mod 2**64

            inv *= 2 - denominator * inv; // inverse mod 2**128

            inv *= 2 - denominator * inv; // inverse mod 2**256



            // Because the division is now exact we can divide by multiplying

            // with the modular inverse of denominator. This will give us the

            // correct result modulo 2**256. Since the precoditions guarantee

            // that the outcome is less than 2**256, this is the final result.

            // We don't need to compute the high bits of the result and prod1
            // is no longer required.
            result = prod0 * inv;
            return result;
        }
    }

    /// @notice Calculates ceil(a×b÷denominator) with full precision. Throws if result overflows a uint256 or denominator == 0     

    /// @param a The multiplicand
    /// @param b The multiplier

    /// @param denominator The divisor
    /// @return result The 256-bit result

    function mulDivRoundingUp(

        uint256 a,

        uint256 b,

        uint256 denominator

    ) internal pure returns (uint256 result) {

        unchecked {

            result = mulDiv(a, b, denominator);

            if (mulmod(a, b, denominator) > 0) {

                require(result < type(uint256).max);

                result++;

            }

        }

    }

}



/// @notice Allows a buyer to execute an order given they've got

/// an secp256k1 signature from a seller containing verifiable

/// metadata about the trade. The seller can accept native ETH

/// or an ERC-20 if they're whitelisted.
/// @author              ConcaveFi

contract Marketplace {



        // @dev This function ensures this contract can receive ETH

        receive() external payable {}



    function onERC721Received(

        address,

        address,

        uint256,

        bytes memory

    ) public virtual returns (bytes4) {

        return 0x150b7a02;

    }



    //////////////////////////////////////////////////////////////////////

    // CONSTANTS

    //////////////////////////////////////////////////////////////////////



    uint256 internal constant FEE_DIVISOR = 1e4;

    // keccak256("Swap(address seller,address erc721,address erc20,uint256 tokenId,uint256 startPrice,uint256 endPrice,uint256 start,uint256 deadline)")

    bytes32 public constant SWAP_TYPEHASH = 0xce02533ba8247ea665b533936094078425c41815f15e8e856183c2fadc084ea3;



    //////////////////////////////////////////////////////////////////////

    // MUTABLE STORAGE

    //////////////////////////////////////////////////////////////////////



    // @notice Returns the address fees are sent to.

    address payable public feeAddress;



    // @notice Returns the fee charged for selling a token from specific 'collection'

    mapping(address => uint256) public collectionFee;



    // @notice Returns whether a token is allowed to be traded within this contract.

    mapping(address => bool) public allowed;



    // @notice Returns whether a specific signature has been executed before.

    mapping(bytes32 => bool) public executed;



    //////////////////////////////////////////////////////////////////////

    // IMMUTABLE STORAGE

    //////////////////////////////////////////////////////////////////////



    uint256 internal immutable INITIAL_CHAIN_ID;



    bytes32 internal immutable INITIAL_DOMAIN_SEPARATOR;



    constructor() {

        address payable concaveTreasury = payable(0x226e7AF139a0F34c6771DeB252F9988876ac1Ced);

        address lsdCNV = 0x93c3A816242E50Ea8871A29BF62cC3df58787FBD;

        address FRAX = 0x853d955aCEf822Db058eb8505911ED77F175b99e;

        address DAI = 0x6B175474E89094C44Da98b954EedeAC495271d0F;

        address USDC = 0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48;

        address ETH = address(0);



        allowed[lsdCNV] = true;

        collectionFee[lsdCNV] = 150; // 1.5 %

        emit WhitelistUpdated(lsdCNV, true);



        allowed[FRAX] = true;

        emit WhitelistUpdated(FRAX, true);



        allowed[DAI] = true;

        emit WhitelistUpdated(DAI, true);



        allowed[USDC] = true;

        emit WhitelistUpdated(USDC, true);



        allowed[ETH] = true;

        emit WhitelistUpdated(ETH, true);



        feeAddress = concaveTreasury;

        emit FeeAddressUpdated(concaveTreasury);



        INITIAL_CHAIN_ID = block.chainid;

        INITIAL_DOMAIN_SEPARATOR = computeDomainSeparator();

    }



    //////////////////////////////////////////////////////////////////////

    // USER ACTION EVENTS

    //////////////////////////////////////////////////////////////////////



    event OrderExecuted(

        address indexed seller,

        address indexed erc721,

        address indexed erc20,

        uint256 tokenId,

        uint256 price,

        uint256 deadline

    );



    //////////////////////////////////////////////////////////////////////

    // EIP-712 LOGIC

    //////////////////////////////////////////////////////////////////////



    // @notice              Struct containing metadata for a ERC721 <-> ERC20 trade.

    //

    // @param seller        The address of the account that wants to sell their

    //                      'erc721' in exchange for 'price' denominated in 'erc20'.

    //

    // @param erc721        The address of a contract that follows the ERC-721 standard,

    //                      also the address of the collection that holds the token that

    //                      you're purchasing.

    //

    // @param erc20         The address of a contract that follows the ERC-20 standard,

    //                      also the address of the token that the seller wants in exchange

    //                      for their 'erc721'

    //

    // @dev                 If 'erc20' is equal to address(0), we assume the seller wants

    //                      native ETH in exchange for their 'erc721'.

    //

    // @param tokenId       The 'erc721' token identification number, 'tokenId'.

    //

    // @param startPrice    The starting or fixed price the offered 'erc721' is being sold for,

    //                      if ZERO we assume the 'seller' is hosting a dutch auction.

    //

    // @dev                 If a 'endPrice' and 'start' time are both defined, we assume

    //                      the order type is a dutch auction. So 'startPrice' would be

    //                      the price the auction starts at, otherwise 'startPrice' is

    //                      the fixed cost the 'seller' is charging.

    //

    // @param endPrice      The 'endPrice' is the price in which a dutch auction would no

    //                      no longer be valid after.

    //

    // @param start         The time in which the dutch auction starts, if ZERO we assume

    //                      the 'seller' is hosting a dutch auction.

    //

    // @param deadline      The time in which the signature/swap is not valid after.
    struct SwapMetadata {
        address seller;
        address erc721;
        address erc20;
        uint256 tokenId;
        uint256 startPrice;
        uint256 endPrice;
        uint256 start;
        uint256 deadline;
    }

    function computeSigner(
        SwapMetadata calldata data,
        uint8 v,
        bytes32 r,
        bytes32 s
    ) public virtual view returns (address signer) {

        bytes32 hash = keccak256(
            abi.encode(
                SWAP_TYPEHASH,
                data.seller,
                data.erc721,
                data.erc20,
                data.tokenId,
                data.startPrice,
                data.endPrice,
                data.start,
                data.deadline
            )
        );

        signer = ecrecover(keccak256(abi.encodePacked("\x19\x01", DOMAIN_SEPARATOR(), hash)), v, r, s);
    }

    function DOMAIN_SEPARATOR() public virtual view returns (bytes32) {
        return block.chainid == INITIAL_CHAIN_ID ? INITIAL_DOMAIN_SEPARATOR : computeDomainSeparator();
    }

    function computeDomainSeparator() internal view returns (bytes32) {
        return keccak256(
            abi.encode(
                keccak256("EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)"),
                keccak256(bytes("Marketplace")),
                keccak256("1"),
                block.chainid,
                address(this)
            )
        );
    }

    //////////////////////////////////////////////////////////////////////
    // PRICE LOGIC
    //////////////////////////////////////////////////////////////////////

    function computePrice(
        SwapMetadata calldata data
    ) public virtual view returns (uint256 price) {
        data.endPrice == 0 || data.start == 0 ?
            price = data.startPrice :
            price = data.startPrice - FullMath.mulDiv(
                data.startPrice - data.endPrice,
                block.timestamp - data.start,
                data.deadline - data.start
            );
    }

    //////////////////////////////////////////////////////////////////////
    // USER ACTIONS
    //////////////////////////////////////////////////////////////////////

    /// @notice Allows a buyer to execute an order given they've got an secp256k1

    /// signature from a seller containing verifiable swap metadata.
    /// @param data Struct containing metadata for a ERC721 <-> ERC20 trade.

    /// @param v v is part of a valid secp256k1 signature from the seller.
    /// @param r r is part of a valid secp256k1 signature from the seller.

    /// @param s s is part of a valid secp256k1 signature from the seller.
    function swap(
        SwapMetadata calldata data,
        uint8 v,
        bytes32 r,
        bytes32 s
    ) external virtual payable {

        // Make sure both the 'erc721' and the 'erc20' wanted in exchange are both allowed.
        require(allowed[data.erc721] && allowed[data.erc20], "tokenNotWhitelisted()");

        // Make sure the deadline the 'seller' has specified has not elapsed.
        require(data.deadline >= block.timestamp, "orderExpired()");

        bytes32 dataHash = keccak256(abi.encode(data));

        // Make sure the signature has not already been executed.
        require(!executed[dataHash], "signatureExecuted()");

        address signer = computeSigner(data, v, r, s);

        // Make sure the recovered address is not NULL, and is equal to the 'seller'.
        require(signer != address(0) && signer == data.seller, "signatureInvalid()");

        executed[dataHash] = true;

        uint256 price = computePrice(data);

        // Cache the fee that's going to be charged to the 'seller'.

        uint256 fee = FullMath.mulDiv(price, collectionFee[data.erc721], FEE_DIVISOR);



        // If 'erc20' is NULL, we assume the seller wants native ETH.

        if (data.erc20 == address(0)) {



            // Make sure the amount of ETH sent is at least the price specified.

            require(msg.value >= price, "insufficientMsgValue()");



            // Transfer msg.value minus 'fee' from this contract to 'seller'

            SafeTransferLib.safeTransferETH(signer, price - fee);



        // If 'erc20' is not NULL, we assume the seller wants a ERC20.

        } else {



            // Transfer 'erc20' 'price' minus 'fee' from caller to 'seller'.

            SafeTransferLib.safeTransferFrom(ERC20(data.erc20), msg.sender, signer, price - fee);



            // Transfer 'fee' to 'feeAddress'.

            if (fee > 0) SafeTransferLib.safeTransferFrom(ERC20(data.erc20), msg.sender, feeAddress, fee);

        }



        // Transfer 'erc721' from 'seller' to msg.sender/caller.

        ERC721(data.erc721).safeTransferFrom(signer, msg.sender, data.tokenId);



        // Emit event since state was mutated.

        emit OrderExecuted(signer, data.erc721, data.erc20, data.tokenId, price, data.deadline);

    }



    //////////////////////////////////////////////////////////////////////

    // MANAGMENT EVENTS

    //////////////////////////////////////////////////////////////////////



    // @notice emitted when 'feeAddress' is updated.

    event FeeAddressUpdated(

        address newFeeAddress

    );



    // @notice emitted when 'collectionFee' for 'collection' is updated.

    event CollectionFeeUpdated(

        address collection,

        uint256 percent

    );



    // @notice emitted when 'allowed' for a 'token' has been updated.

    event WhitelistUpdated(

        address token,

        bool whitelisted

    );



    // @notice emitted when ETH from fees is collected from the contract.

    event FeeCollection(

        address token,

        uint256 amount

    );



    //////////////////////////////////////////////////////////////////////

    // MANAGMENT MODIFIERS

    //////////////////////////////////////////////////////////////////////



    // @notice only allows 'feeAddress' to call modified function.

    modifier access() {

        require(msg.sender == feeAddress, "ACCESS");

        _;

    }



    //////////////////////////////////////////////////////////////////////

    // MANAGMENT ACTIONS

    //////////////////////////////////////////////////////////////////////



    function updateFeeAddress(address payable account) external virtual access {

        feeAddress = account;

        emit FeeAddressUpdated(account);

    }



    function updateCollectionFee(address collection, uint256 percent) external virtual access {

        collectionFee[collection] = percent;

        emit CollectionFeeUpdated(collection, percent);

    }



    function updateWhitelist(address token) external virtual access {

        bool whitelisted = !allowed[token];

        allowed[token] = whitelisted;

        emit WhitelistUpdated(token, whitelisted);

    }

    function collectEther() external virtual access {

        uint256 balance = address(this).balance;

        SafeTransferLib.safeTransferETH(feeAddress, balance);

        emit FeeCollection(address(0), balance);

    }



    function collectERC20(address token) external virtual access {

        uint256 balance = ERC20(token).balanceOf(address(this));

        SafeTransferLib.safeTransfer(ERC20(token), feeAddress, balance);

        emit FeeCollection(token, balance);

    }



    //////////////////////////////////////////////////////////////////////

    // EXTERNAL SIGNATURE VERIFICATION LOGIC

    //////////////////////////////////////////////////////////////////////



    function isExecuted(

        SwapMetadata calldata data

    ) external view returns (bool) {

        return executed[keccak256(abi.encode(data))];

    }



    function verify(

        SwapMetadata calldata data,

        address buyer,

        uint8 v,

        bytes32 r,

        bytes32 s

    ) external virtual view returns (bool valid) {



        bytes32 dataHash = keccak256(abi.encode(data));



        if (executed[dataHash]) return false;



        // Make sure current time is greater than 'start' if order type is dutch auction.

        if (data.start == 0 || data.endPrice == 0) {

            if (data.start > block.timestamp) return false;

        }



        // Make sure both the 'erc721' and the 'erc20' wanted in exchange are both allowed.

        if (!allowed[data.erc721] || !allowed[data.erc20]) return false;



        // Make sure the deadline the 'seller' has specified has not elapsed.

        if (data.deadline < block.timestamp) return false;



        // Make sure the 'seller' still owns the 'erc721' being offered, and has approved this contract to spend it.

        if (ERC721(data.erc721).ownerOf(data.tokenId) != data.seller || ERC721(data.erc721).getApproved(data.tokenId) != address(this)) return false;



        // Make sure the buyer has 'price' denominated in 'erc20' if 'erc20' is not native ETH.

        if (data.erc20 != address(0)) {

            if (ERC20(data.erc20).balanceOf(buyer) < computePrice(data) && buyer != address(0)) return false;

        }



        address signer = computeSigner(data, v, r, s);



        // Make sure the recovered address is not NULL, and is equal to the 'seller'.

        if (signer == address(0) || signer != data.seller) return false;



        return true;

    }

}