Contract Source Code:

// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts v4.4.1 (security/Pausable.sol)

pragma solidity ^0.8.0;

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

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

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

    bool private _paused;

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

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

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

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

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

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

// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts v4.4.1 (utils/Context.sol)

pragma solidity ^0.8.0;

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

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

// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts v4.4.1 (utils/Strings.sol)

pragma solidity ^0.8.0;

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

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

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

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

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

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

import "./Ownable.sol";
import "./ERC721Receiver.sol";

/// @notice ERC404
///         A gas-efficient, mixed ERC20 / ERC721 implementation
///         with native liquidity and fractionalization.
///
///         This is an experimental standard designed to integrate
///         with pre-existing ERC20 / ERC721 support as smoothly as
///         possible.
///
/// @dev    In order to support full functionality of ERC20 and ERC721
///         supply assumptions are made that slightly constraint usage.
///         Ensure decimals are sufficiently large (standard 18 recommended)
///         as ids are effectively encoded in the lowest range of amounts.
///
///         NFTs are spent on ERC20 functions in a FILO queue, this is by
///         design.
///
abstract contract ERC404 is Ownable {
    // Events
    event ERC20Transfer(
        address indexed from,
        address indexed to,
        uint256 amount
    );
    event Approval(
        address indexed owner,
        address indexed spender,
        uint256 amount
    );
    event Transfer(
        address indexed from,
        address indexed to,
        uint256 indexed id
    );
    event ERC721Approval(
        address indexed owner,
        address indexed spender,
        uint256 indexed id
    );
    event ApprovalForAll(
        address indexed owner,
        address indexed operator,
        bool approved
    );

    // Errors
    error NotFound();
    error AlreadyExists();
    error InvalidRecipient();
    error InvalidSender();
    error UnsafeRecipient();

    // Metadata
    /// @dev Token name
    string public name;

    /// @dev Token symbol
    string public symbol;

    /// @dev Decimals for fractional representation
    uint8 public immutable decimals;

    /// @dev Total supply in fractionalized representation
    uint256 public totalSupply;

    /// @dev Current mint counter, monotonically increasing to ensure accurate ownership
    uint256 public minted;

    // Mappings
    /// @dev Balance of user in fractional representation
    mapping(address => uint256) public balanceOf;

    /// @dev Allowance of user in fractional representation
    mapping(address => mapping(address => uint256)) public allowance;

    /// @dev Approval in native representaion
    mapping(uint256 => address) public getApproved;

    /// @dev Approval for all in native representation
    mapping(address => mapping(address => bool)) public isApprovedForAll;

    /// @dev Owner of id in native representation
    mapping(uint256 => address) internal _ownerOf;

    /// @dev Array of owned ids in native representation
    mapping(address => uint256[]) internal _owned;

    /// @dev Tracks indices for the _owned mapping
    mapping(uint256 => uint256) internal _ownedIndex;

    /// @dev Addresses whitelisted from minting / burning for gas savings (pairs, routers, etc)
    mapping(address => bool) public whitelist;

    // Constructor
    constructor(
        string memory _name,
        string memory _symbol,
        uint8 _decimals,
        uint256 _totalNativeSupply,
        address _owner
    ) Ownable(_owner) {
        name = _name;
        symbol = _symbol;
        decimals = _decimals;
        totalSupply = _totalNativeSupply * (10 ** _decimals);
    }

    /// @notice Initialization function to set pairs / etc
    ///         saving gas by avoiding mint / burn on unnecessary targets
    function setWhitelist(address target, bool state) public onlyOwner {
        whitelist[target] = state;
    }

    /// @notice Function to find owner of a given native token
    function ownerOf(uint256 id) public view virtual returns (address owner) {
        owner = _ownerOf[id];

        if (owner == address(0)) {
            revert NotFound();
        }
    }

    /// @notice tokenURI must be implemented by child contract
    function tokenURI(uint256 id) public view virtual returns (string memory);

    /// @notice Function for token approvals
    /// @dev This function assumes id / native if amount less than or equal to current max id
    function approve(
        address spender,
        uint256 amountOrId
    ) public virtual returns (bool) {
        if (amountOrId <= minted && amountOrId > 0) {
            address owner = _ownerOf[amountOrId];

            if (msg.sender != owner && !isApprovedForAll[owner][msg.sender]) {
                revert Unauthorized();
            }

            getApproved[amountOrId] = spender;

            emit Approval(owner, spender, amountOrId);
        } else {
            allowance[msg.sender][spender] = amountOrId;

            emit Approval(msg.sender, spender, amountOrId);
        }

        return true;
    }

    /// @notice Function native approvals
    function setApprovalForAll(address operator, bool approved) public virtual {
        isApprovedForAll[msg.sender][operator] = approved;

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

    /// @notice Function for mixed transfers
    /// @dev This function assumes id / native if amount less than or equal to current max id
    function transferFrom(
        address from,
        address to,
        uint256 amountOrId
    ) public virtual {
        if (amountOrId <= minted) {
            if (from != _ownerOf[amountOrId]) {
                revert InvalidSender();
            }

            if (to == address(0)) {
                revert InvalidRecipient();
            }

            if (
                msg.sender != from &&
                !isApprovedForAll[from][msg.sender] &&
                msg.sender != getApproved[amountOrId]
            ) {
                revert Unauthorized();
            }

            balanceOf[from] -= _getUnit();

            unchecked {
                balanceOf[to] += _getUnit();
            }

            _ownerOf[amountOrId] = to;
            delete getApproved[amountOrId];

            // update _owned for sender
            uint256 updatedId = _owned[from][_owned[from].length - 1];
            _owned[from][_ownedIndex[amountOrId]] = updatedId;
            // pop
            _owned[from].pop();
            // update index for the moved id
            _ownedIndex[updatedId] = _ownedIndex[amountOrId];
            // push token to to owned
            _owned[to].push(amountOrId);
            // update index for to owned
            _ownedIndex[amountOrId] = _owned[to].length - 1;

            emit Transfer(from, to, amountOrId);
            emit ERC20Transfer(from, to, _getUnit());
        } else {
            uint256 allowed = allowance[from][msg.sender];

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

            _transfer(from, to, amountOrId);
        }
    }

    /// @notice Function for fractional transfers
    function transfer(
        address to,
        uint256 amount
    ) public virtual returns (bool) {
        return _transfer(msg.sender, to, amount);
    }

    /// @notice Function for native transfers with contract support
    function safeTransferFrom(
        address from,
        address to,
        uint256 id
    ) public virtual {
        transferFrom(from, to, id);

        if (
            to.code.length != 0 &&
            ERC721Receiver(to).onERC721Received(msg.sender, from, id, "") !=
            ERC721Receiver.onERC721Received.selector
        ) {
            revert UnsafeRecipient();
        }
    }

    /// @notice Function for native transfers with contract support and callback data
    function safeTransferFrom(
        address from,
        address to,
        uint256 id,
        bytes calldata data
    ) public virtual {
        transferFrom(from, to, id);

        if (
            to.code.length != 0 &&
            ERC721Receiver(to).onERC721Received(msg.sender, from, id, data) !=
            ERC721Receiver.onERC721Received.selector
        ) {
            revert UnsafeRecipient();
        }
    }

    /// @notice Internal function for fractional transfers
    function _transfer(
        address from,
        address to,
        uint256 amount
    ) internal returns (bool) {
        uint256 unit = _getUnit();
        uint256 balanceBeforeSender = balanceOf[from];
        uint256 balanceBeforeReceiver = balanceOf[to];

        balanceOf[from] -= amount;

        unchecked {
            balanceOf[to] += amount;
        }

        // Skip burn for certain addresses to save gas
        if (!whitelist[from]) {
            uint256 tokens_to_burn = (balanceBeforeSender / unit) -
            (balanceOf[from] / unit);
            for (uint256 i = 0; i < tokens_to_burn; i++) {
                _burn(from);
            }
        }

        // Skip minting for certain addresses to save gas
        if (!whitelist[to]) {
            uint256 tokens_to_mint = (balanceOf[to] / unit) -
            (balanceBeforeReceiver / unit);
            for (uint256 i = 0; i < tokens_to_mint; i++) {
                _mint(to);
            }
        }

        emit ERC20Transfer(from, to, amount);
        return true;
    }

    // Internal utility logic
    function _getUnit() internal view returns (uint256) {
        return 10 ** decimals;
    }

    function _mint(address to) internal virtual {
        if (to == address(0)) {
            revert InvalidRecipient();
        }

        unchecked {
            minted++;
        }

        uint256 id = minted;

        if (_ownerOf[id] != address(0)) {
            revert AlreadyExists();
        }

        _ownerOf[id] = to;
        _owned[to].push(id);
        _ownedIndex[id] = _owned[to].length - 1;

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

    function _burn(address from) internal virtual {
        if (from == address(0)) {
            revert InvalidSender();
        }

        uint256 id = _owned[from][_owned[from].length - 1];
        _owned[from].pop();
        delete _ownedIndex[id];
        delete _ownerOf[id];
        delete getApproved[id];

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

    function _setNameSymbol(
        string memory _name,
        string memory _symbol
    ) internal {
        name = _name;
        symbol = _symbol;
    }
}

pragma solidity ^0.8.0;

import "./ERC404.sol";
import "@openzeppelin/contracts/security/Pausable.sol";

abstract contract ERC404Pausable is ERC404, Pausable {

    function _beforeTokenTransfer(
        address from,
        address to,
        uint256 amountOrId
    ) internal virtual {
        require(!paused(), "ERC404Pausable: transfer paused");
    }

    function transferFrom(
        address from,
        address to,
        uint256 amountOrId
    ) public virtual override {
        _beforeTokenTransfer(from, to, amountOrId);
        super.transferFrom(from, to, amountOrId);
    }

    function transfer(
        address to,
        uint256 amount
    ) public virtual override returns (bool) {
        _beforeTokenTransfer(msg.sender, to, amount);
        return super.transfer(to, amount);
    }
}

pragma solidity ^0.8.0;

abstract contract ERC721Receiver {
    function onERC721Received(
        address,
        address,
        uint256,
        bytes calldata
    ) external virtual returns (bytes4) {
        return ERC721Receiver.onERC721Received.selector;
    }
}

pragma solidity ^0.8.0;

abstract contract Ownable {
    event OwnershipTransferred(address indexed user, address indexed newOwner);

    error Unauthorized();
    error InvalidOwner();

    address public owner;

    modifier onlyOwner() virtual {
        if (msg.sender != owner) revert Unauthorized();

        _;
    }

    constructor(address _owner) {
        if (_owner == address(0)) revert InvalidOwner();

        owner = _owner;

        emit OwnershipTransferred(address(0), _owner);
    }

    function transferOwnership(address _owner) public virtual onlyOwner {
        if (_owner == address(0)) revert InvalidOwner();

        owner = _owner;

        emit OwnershipTransferred(msg.sender, _owner);
    }

    function revokeOwnership() public virtual onlyOwner {
        owner = address(0);

        emit OwnershipTransferred(msg.sender, address(0));
    }
}

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

import "@openzeppelin/contracts/utils/Strings.sol";
import "./ERC404/ERC404Pausable.sol";

contract Token404 is ERC404Pausable {
    uint256 public constant MAX_SUPPLY = 10000 * 1E18;
    string private _baseUri;

    mapping(address => bool) public whitelistTransfer;

    constructor() ERC404("404BitcoinCats", "404Cats", 18, 0, msg.sender) {
        whitelistTransfer[address(0)] = true;
        emit ERC20Transfer(address(0), msg.sender, balanceOf[msg.sender] = 0);
    }

    function mint(address to, uint256 amount) public onlyOwner {
        amount = amount * (10 ** decimals);
        require((totalSupply += amount) <= MAX_SUPPLY, 'Error: amount');
        balanceOf[to] += amount;
        emit ERC20Transfer(address(0), to, amount);
    }

    function setBaseUri(string memory newUri) public onlyOwner {
        _baseUri = newUri;
    }

    function setNameSymbol(
        string memory _name,
        string memory _symbol
    ) public onlyOwner {
        _setNameSymbol(_name, _symbol);
    }

    function tokenURI(uint256 tokenId) public view override returns (string memory) {
        if (bytes(_baseUri).length > 0) {
            return string(abi.encodePacked(_baseUri, Strings.toString((tokenId - 1) % 10000 + 1), ".json"));
        } else {
            return 'null';
        }
    }

    function _beforeTokenTransfer(
        address from,
        address to,
        uint256 amountOrId
    ) internal virtual override {
        if (!whitelistTransfer[from]) super._beforeTokenTransfer(from, to, amountOrId);
    }

    function setWhitelistTransfer(address account, bool isAdd) public onlyOwner {
        if (whitelistTransfer[account] != isAdd) {
            whitelistTransfer[account] = isAdd;
        }
    }

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

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