interfaceId

supportsInterface

eventId

renounceEventMinter

name

tokenId

getApproved

approve

tokenEvent

account

removeEventMinter

removeAdmin

totalSupply

from

transferFrom

isAdmin

mintEventToManyUsers

isEventMinter

owner

index

tokenOfOwnerByIndex

unpause

safeTransferFrom

burn

tokenByIndex

baseURI

setBaseURI

paused

ownerOf

tokenDetailsOfOwnerByIndex

addAdmin

balanceOf

initialize

pause

renounceAdmin

__name

__symbol

__baseURI

admins

symbol

addEventMinter

mintToken

approved

setApprovalForAll

_data

sender

tokenURI

operator

isApprovedForAll

eventIds

mintUserToManyEvents

EventToken

Paused

Unpaused

AdminAdded

AdminRemoved

EventMinterAdded

EventMinterRemoved

Transfer

Approval

ApprovalForAll

pragma solidity ^0.5.0;

import "zos-lib/contracts/Initializable.sol";
import "openzeppelin-eth/contracts/token/ERC721/ERC721.sol";
import "openzeppelin-eth/contracts/token/ERC721/ERC721Enumerable.sol";
import "openzeppelin-eth/contracts/token/ERC721/IERC721Metadata.sol";
import "./PoapRoles.sol";
import "./PoapPausable.sol";


// Desired Features
// - Add Event
// - Add Event Organizer
// - Mint token for an event
// - Batch Mint
// - Burn Tokens (only admin?)
// - Pause contract (only admin)
// - ERC721 full interface (base, metadata, enumerable)

contract Poap is Initializable, ERC721, ERC721Enumerable, PoapRoles, PoapPausable {
    event EventToken(uint256 eventId, uint256 tokenId);

    // Token name
    string private _name;

    // Token symbol
    string private _symbol;

    // Base token URI
    string private _baseURI;

    // Last Used id (used to generate new ids)
    uint256 private lastId;

    // EventId for each token
    mapping(uint256 => uint256) private _tokenEvent;


    bytes4 private constant _INTERFACE_ID_ERC721_METADATA = 0x5b5e139f;

    /**
     * @dev Gets the token name
     * @return string representing the token name
     */
    function name() external view returns (string memory) {
        return _name;
    }

    /**
     * @dev Gets the token symbol
     * @return string representing the token symbol
     */
    function symbol() external view returns (string memory) {
        return _symbol;
    }

    function tokenEvent(uint256 tokenId) public view returns (uint256) {
        return _tokenEvent[tokenId];
    }

    /**
     * @dev Gets the token ID at a given index of the tokens list of the requested owner
     * @param owner address owning the tokens list to be accessed
     * @param index uint256 representing the index to be accessed of the requested tokens list
     * @return uint256 token ID at the given index of the tokens list owned by the requested address
     */
    function tokenDetailsOfOwnerByIndex(address owner, uint256 index) public view returns (uint256 tokenId, uint256 eventId) {
        tokenId = tokenOfOwnerByIndex(owner, index);
        eventId = tokenEvent(tokenId);
    }

    /**
     * @dev Gets the token uri
     * @return string representing the token uri
     */
    function tokenURI(uint256 tokenId) external view returns (string memory) {
        uint eventId = _tokenEvent[tokenId];
        return _strConcat(_baseURI, _uint2str(eventId), "/", _uint2str(tokenId), "");
    }

    function setBaseURI(string memory baseURI) public onlyAdmin whenNotPaused {
        _baseURI = baseURI;
    }

    function approve(address to, uint256 tokenId) public whenNotPaused {
        super.approve(to, tokenId);
    }

    function setApprovalForAll(address to, bool approved) public whenNotPaused {
        super.setApprovalForAll(to, approved);
    }

    function transferFrom(address from, address to, uint256 tokenId) public whenNotPaused {
        super.transferFrom(from, to, tokenId);
    }

    /**
     * @dev Function to mint tokens
     * @param eventId EventId for the new token
     * @param to The address that will receive the minted tokens.
     * @return A boolean that indicates if the operation was successful.
     */
    function mintToken(uint256 eventId, address to)
    public whenNotPaused onlyEventMinter(eventId) returns (bool)
    {
        lastId += 1;
        return _mintToken(eventId, lastId, to);
    }

    /**
     * @dev Function to mint tokens with a specific id
     * @param eventId EventId for the new token
     * @param tokenId TokenId for the new token
     * @param to The address that will receive the minted tokens.
     * @return A boolean that indicates if the operation was successful.
     */
    function mintToken(uint256 eventId, uint256 tokenId, address to)
    public whenNotPaused onlyEventMinter(eventId) returns (bool)
    {
        return _mintToken(eventId, tokenId, to);
    }


    /**
     * @dev Function to mint tokens
     * @param eventId EventId for the new token
     * @param to The address that will receive the minted tokens.
     * @return A boolean that indicates if the operation was successful.
     */
    function mintEventToManyUsers(uint256 eventId, address[] memory to)
    public whenNotPaused onlyEventMinter(eventId) returns (bool)
    {
        for (uint256 i = 0; i < to.length; ++i) {
            _mintToken(eventId, lastId + 1 + i, to[i]);
        }
        lastId += to.length;
        return true;
    }

    /**
     * @dev Function to mint tokens
     * @param eventIds EventIds to assing to user
     * @param to The address that will receive the minted tokens.
     * @return A boolean that indicates if the operation was successful.
     */
    function mintUserToManyEvents(uint256[] memory eventIds, address to)
    public whenNotPaused onlyAdmin() returns (bool)
    {
        for (uint256 i = 0; i < eventIds.length; ++i) {
            _mintToken(eventIds[i], lastId + 1 + i, to);
        }
        lastId += eventIds.length;
        return true;
    }

    /**
     * @dev Burns a specific ERC721 token.
     * @param tokenId uint256 id of the ERC721 token to be burned.
     */
    function burn(uint256 tokenId) public {
        require(_isApprovedOrOwner(msg.sender, tokenId) || isAdmin(msg.sender));
        _burn(tokenId);
    }

    function initialize(string memory __name, string memory __symbol, string memory __baseURI, address[] memory admins)
    public initializer
    {
        ERC721.initialize();
        ERC721Enumerable.initialize();
        PoapRoles.initialize(msg.sender);
        PoapPausable.initialize();

        // Add the requested admins
        for (uint256 i = 0; i < admins.length; ++i) {
            _addAdmin(admins[i]);
        }

        _name = __name;
        _symbol = __symbol;
        _baseURI = __baseURI;

        // register the supported interfaces to conform to ERC721 via ERC165
        _registerInterface(_INTERFACE_ID_ERC721_METADATA);
    }

    /**
     * @dev Internal function to burn a specific token
     * Reverts if the token does not exist
     *
     * @param owner owner of the token to burn
     * @param tokenId uint256 ID of the token being burned by the msg.sender
     */
    function _burn(address owner, uint256 tokenId) internal {
        super._burn(owner, tokenId);

        delete _tokenEvent[tokenId];
    }

    /**
     * @dev Function to mint tokens
     * @param eventId EventId for the new token
     * @param tokenId The token id to mint.
     * @param to The address that will receive the minted tokens.
     * @return A boolean that indicates if the operation was successful.
     */
    function _mintToken(uint256 eventId, uint256 tokenId, address to) internal returns (bool) {
        // TODO Verify that the token receiver ('to') do not have already a token for the event ('eventId')
        _mint(to, tokenId);
        _tokenEvent[tokenId] = eventId;
        emit EventToken(eventId, tokenId);
        return true;
    }

    /**
     * @dev Function to convert uint to string
     * Taken from https://github.com/oraclize/ethereum-api/blob/master/oraclizeAPI_0.5.sol
     */
    function _uint2str(uint _i) internal pure returns (string memory _uintAsString) {
        if (_i == 0) {
            return "0";
        }
        uint j = _i;
        uint len;
        while (j != 0) {
            len++;
            j /= 10;
        }
        bytes memory bstr = new bytes(len);
        uint k = len - 1;
        while (_i != 0) {
            bstr[k--] = byte(uint8(48 + _i % 10));
            _i /= 10;
        }
        return string(bstr);
    }

    /**
     * @dev Function to concat strings
     * Taken from https://github.com/oraclize/ethereum-api/blob/master/oraclizeAPI_0.5.sol
     */
    function _strConcat(string memory _a, string memory _b, string memory _c, string memory _d, string memory _e)
    internal pure returns (string memory _concatenatedString)
    {
        bytes memory _ba = bytes(_a);
        bytes memory _bb = bytes(_b);
        bytes memory _bc = bytes(_c);
        bytes memory _bd = bytes(_d);
        bytes memory _be = bytes(_e);
        string memory abcde = new string(_ba.length + _bb.length + _bc.length + _bd.length + _be.length);
        bytes memory babcde = bytes(abcde);
        uint k = 0;
        uint i = 0;
        for (i = 0; i < _ba.length; i++) {
            babcde[k++] = _ba[i];
        }
        for (i = 0; i < _bb.length; i++) {
            babcde[k++] = _bb[i];
        }
        for (i = 0; i < _bc.length; i++) {
            babcde[k++] = _bc[i];
        }
        for (i = 0; i < _bd.length; i++) {
            babcde[k++] = _bd[i];
        }
        for (i = 0; i < _be.length; i++) {
            babcde[k++] = _be[i];
        }
        return string(babcde);
    }

    function removeAdmin(address account) public onlyAdmin {
        _removeAdmin(account);
    }

}


This smart contract is a Poap (Proof of Attendance Protocol) token contract. It allows users to mint and manage unique tokens representing attendance at events. The contract supports token metadata, token ownership, token burning, and token transfer functionalities. It also includes role-based access control and pausing functionality for administrative purposes.

Poap

pragma solidity ^0.5.0;

import "zos-lib/contracts/Initializable.sol";
import "./PoapRoles.sol";

/**
 * @title Pausable
 * @dev Base contract which allows children to implement an emergency stop mechanism.
 */
contract PoapPausable is Initializable, PoapRoles {
    event Paused(address account);
    event Unpaused(address account);

    bool private _paused;

    function initialize() public initializer {
        _paused = false;
    }

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

    /**
     * @dev Modifier to make a function callable only when the contract is not paused.
     */
    modifier whenNotPaused() {
        require(!_paused);
        _;
    }

    /**
     * @dev Modifier to make a function callable only when the contract is paused.
     */
    modifier whenPaused() {
        require(_paused);
        _;
    }

    /**
     * @dev called by the owner to pause, triggers stopped state
     */
    function pause() public onlyAdmin whenNotPaused {
        _paused = true;
        emit Paused(msg.sender);
    }

    /**
     * @dev called by the owner to unpause, returns to normal state
     */
    function unpause() public onlyAdmin whenPaused {
        _paused = false;
        emit Unpaused(msg.sender);
    }

    // For future extensions
    uint256[50] private ______gap;
}


This contract implements a pausable functionality, allowing the owner to pause and unpause certain functions. It emits events when the contract is paused or unpaused.

PoapPausable

Pausable

pragma solidity ^0.5.0;

import "zos-lib/contracts/Initializable.sol";
import "openzeppelin-eth/contracts/access/Roles.sol";

contract PoapRoles is Initializable {
    using Roles for Roles.Role;

    event AdminAdded(address indexed account);
    event AdminRemoved(address indexed account);
    event EventMinterAdded(uint256 indexed eventId, address indexed account);
    event EventMinterRemoved(uint256 indexed eventId, address indexed account);

    Roles.Role private _admins;
    mapping(uint256 => Roles.Role) private _minters;

    function initialize(address sender) public initializer {
        if (!isAdmin(sender)) {
            _addAdmin(sender);
        }
    }

    modifier onlyAdmin() {
        require(isAdmin(msg.sender));
        _;
    }

    modifier onlyEventMinter(uint256 eventId) {
        require(isEventMinter(eventId, msg.sender));
        _;
    }

    function isAdmin(address account) public view returns (bool) {
        return _admins.has(account);
    }

    function isEventMinter(uint256 eventId, address account) public view returns (bool) {
        return isAdmin(account) || _minters[eventId].has(account);
    }

    function addEventMinter(uint256 eventId, address account) public onlyEventMinter(eventId) {
        _addEventMinter(eventId, account);
    }

    function addAdmin(address account) public onlyAdmin {
        _addAdmin(account);
    }

    function renounceEventMinter(uint256 eventId) public {
        _removeEventMinter(eventId, msg.sender);
    }

    function renounceAdmin() public {
        _removeAdmin(msg.sender);
    }

    function removeEventMinter(uint256 eventId, address account) public onlyAdmin {
        _removeEventMinter(eventId, account);
    }

    function _addEventMinter(uint256 eventId, address account) internal {
        _minters[eventId].add(account);
        emit EventMinterAdded(eventId, account);
    }

    function _addAdmin(address account) internal {
        _admins.add(account);
        emit AdminAdded(account);
    }

    function _removeEventMinter(uint256 eventId, address account) internal {
        _minters[eventId].remove(account);
        emit EventMinterRemoved(eventId, account);
    }

    function _removeAdmin(address account) internal {
        _admins.remove(account);
        emit AdminRemoved(account);
    }

    // For future extensions
    uint256[50] private ______gap;
}


The PoapRoles contract manages roles and permissions for admins and event minters. It allows adding and removing admins and event minters, and checks if an account is an admin or event minter for a specific event.

PoapRoles

newLastId

setLastId

lastId

pragma solidity ^0.5.0;

import "zos-lib/contracts/Initializable.sol";
import "openzeppelin-eth/contracts/token/ERC721/ERC721.sol";
import "openzeppelin-eth/contracts/token/ERC721/ERC721Enumerable.sol";
import "openzeppelin-eth/contracts/token/ERC721/IERC721Metadata.sol";
import "./PoapRoles.sol";
import "./PoapPausable.sol";


// Desired Features
// - Add Event
// - Add Event Organizer
// - Mint token for an event
// - Batch Mint
// - Burn Tokens (only admin?)
// - Pause contract (only admin)
// - ERC721 full interface (base, metadata, enumerable)

contract XPoap is Initializable, ERC721, ERC721Enumerable, PoapRoles, PoapPausable {
    event EventToken(uint256 indexed eventId, uint256 tokenId);

    // Token name
    string private _name;

    // Token symbol
    string private _symbol;

    // Base token URI
    string private _baseURI;

    // Last Used id (used to generate new ids)
    uint256 public lastId;

    // EventId for each token
    mapping(uint256 => uint256) private _tokenEvent;


    bytes4 private constant _INTERFACE_ID_ERC721_METADATA = 0x5b5e139f;

    /**
     * @dev Gets the token name
     * @return string representing the token name
     */
    function name() external view returns (string memory) {
        return _name;
    }

    /**
     * @dev Gets the token symbol
     * @return string representing the token symbol
     */
    function symbol() external view returns (string memory) {
        return _symbol;
    }

    function tokenEvent(uint256 tokenId) public view returns (uint256) {
        return _tokenEvent[tokenId];
    }

    /**
     * @dev Gets the token ID at a given index of the tokens list of the requested owner
     * @param owner address owning the tokens list to be accessed
     * @param index uint256 representing the index to be accessed of the requested tokens list
     * @return uint256 token ID at the given index of the tokens list owned by the requested address
     */
    function tokenDetailsOfOwnerByIndex(address owner, uint256 index) public view returns (uint256 tokenId, uint256 eventId) {
        tokenId = tokenOfOwnerByIndex(owner, index);
        eventId = tokenEvent(tokenId);
    }

    /**
     * @dev Gets the token uri
     * @return string representing the token uri
     */
    function tokenURI(uint256 tokenId) external view returns (string memory) {
        uint eventId = _tokenEvent[tokenId];
        return _strConcat(_baseURI, _uint2str(eventId), "/", _uint2str(tokenId), "");
    }

    function setBaseURI(string memory baseURI) public onlyAdmin whenNotPaused {
        _baseURI = baseURI;
    }

    function setLastId(uint256 newLastId) public onlyAdmin whenNotPaused {
        require(lastId < newLastId);
        lastId = newLastId;
    }

    function approve(address to, uint256 tokenId) public whenNotPaused {
        super.approve(to, tokenId);
    }

    function setApprovalForAll(address to, bool approved) public whenNotPaused {
        super.setApprovalForAll(to, approved);
    }

    function transferFrom(address from, address to, uint256 tokenId) public whenNotPaused {
        super.transferFrom(from, to, tokenId);
    }

    /**
     * @dev Function to mint tokens
     * @param eventId EventId for the new token
     * @param to The address that will receive the minted tokens.
     * @return A boolean that indicates if the operation was successful.
     */
    function mintToken(uint256 eventId, address to)
    public whenNotPaused onlyEventMinter(eventId) returns (bool)
    {
        lastId += 1;
        return _mintToken(eventId, lastId, to);
    }

    /**
     * @dev Function to mint tokens
     * @param eventId EventId for the new token
     * @param to The address that will receive the minted tokens.
     * @return A boolean that indicates if the operation was successful.
     */
    function mintEventToManyUsers(uint256 eventId, address[] memory to)
    public whenNotPaused onlyEventMinter(eventId) returns (bool)
    {
        for (uint256 i = 0; i < to.length; ++i) {
            _mintToken(eventId, lastId + 1 + i, to[i]);
        }
        lastId += to.length;
        return true;
    }

    /**
     * @dev Function to mint tokens
     * @param eventIds EventIds to assing to user
     * @param to The address that will receive the minted tokens.
     * @return A boolean that indicates if the operation was successful.
     */
    function mintUserToManyEvents(uint256[] memory eventIds, address to)
    public whenNotPaused onlyAdmin() returns (bool)
    {
        for (uint256 i = 0; i < eventIds.length; ++i) {
            _mintToken(eventIds[i], lastId + 1 + i, to);
        }
        lastId += eventIds.length;
        return true;
    }

    /**
     * @dev Burns a specific ERC721 token.
     * @param tokenId uint256 id of the ERC721 token to be burned.
     */
    function burn(uint256 tokenId) public {
        require(_isApprovedOrOwner(msg.sender, tokenId) || isAdmin(msg.sender));
        _burn(tokenId);
    }

    function initialize(string memory __name, string memory __symbol, string memory __baseURI, address[] memory admins)
    public initializer
    {
        ERC721.initialize();
        ERC721Enumerable.initialize();
        PoapRoles.initialize(msg.sender);
        PoapPausable.initialize();

        // Add the requested admins
        for (uint256 i = 0; i < admins.length; ++i) {
            _addAdmin(admins[i]);
        }

        _name = __name;
        _symbol = __symbol;
        _baseURI = __baseURI;

        // register the supported interfaces to conform to ERC721 via ERC165
        _registerInterface(_INTERFACE_ID_ERC721_METADATA);
    }

    /**
     * @dev Internal function to burn a specific token
     * Reverts if the token does not exist
     *
     * @param owner owner of the token to burn
     * @param tokenId uint256 ID of the token being burned by the msg.sender
     */
    function _burn(address owner, uint256 tokenId) internal {
        super._burn(owner, tokenId);

        delete _tokenEvent[tokenId];
    }

    /**
     * @dev Function to mint tokens
     * @param eventId EventId for the new token
     * @param tokenId The token id to mint.
     * @param to The address that will receive the minted tokens.
     * @return A boolean that indicates if the operation was successful.
     */
    function _mintToken(uint256 eventId, uint256 tokenId, address to) internal returns (bool) {
        // TODO Verify that the token receiver ('to') do not have already a token for the event ('eventId')
        _mint(to, tokenId);
        _tokenEvent[tokenId] = eventId;
        emit EventToken(eventId, tokenId);
        return true;
    }

    /**
     * @dev Function to convert uint to string
     * Taken from https://github.com/oraclize/ethereum-api/blob/master/oraclizeAPI_0.5.sol
     */
    function _uint2str(uint _i) internal pure returns (string memory _uintAsString) {
        if (_i == 0) {
            return "0";
        }
        uint j = _i;
        uint len;
        while (j != 0) {
            len++;
            j /= 10;
        }
        bytes memory bstr = new bytes(len);
        uint k = len - 1;
        while (_i != 0) {
            bstr[k--] = byte(uint8(48 + _i % 10));
            _i /= 10;
        }
        return string(bstr);
    }

    /**
     * @dev Function to concat strings
     * Taken from https://github.com/oraclize/ethereum-api/blob/master/oraclizeAPI_0.5.sol
     */
    function _strConcat(string memory _a, string memory _b, string memory _c, string memory _d, string memory _e)
    internal pure returns (string memory _concatenatedString)
    {
        bytes memory _ba = bytes(_a);
        bytes memory _bb = bytes(_b);
        bytes memory _bc = bytes(_c);
        bytes memory _bd = bytes(_d);
        bytes memory _be = bytes(_e);
        string memory abcde = new string(_ba.length + _bb.length + _bc.length + _bd.length + _be.length);
        bytes memory babcde = bytes(abcde);
        uint k = 0;
        uint i = 0;
        for (i = 0; i < _ba.length; i++) {
            babcde[k++] = _ba[i];
        }
        for (i = 0; i < _bb.length; i++) {
            babcde[k++] = _bb[i];
        }
        for (i = 0; i < _bc.length; i++) {
            babcde[k++] = _bc[i];
        }
        for (i = 0; i < _bd.length; i++) {
            babcde[k++] = _bd[i];
        }
        for (i = 0; i < _be.length; i++) {
            babcde[k++] = _be[i];
        }
        return string(babcde);
    }

}


The XPoap contract is an ERC721 token contract that allows the minting and burning of tokens. It supports token metadata, token ownership, and token enumeration. It also has functionality for setting the base URI, setting the last ID, and transferring tokens. Additionally, it includes functions for minting tokens for events and users, as well as approving and setting approval for token transfers.

XPoap

pragma solidity ^0.5.2;

/**
 * @title Roles
 * @dev Library for managing addresses assigned to a Role.
 */
library Roles {
    struct Role {
        mapping (address => bool) bearer;
    }

    /**
     * @dev give an account access to this role
     */
    function add(Role storage role, address account) internal {
        require(account != address(0));
        require(!has(role, account));

        role.bearer[account] = true;
    }

    /**
     * @dev remove an account's access to this role
     */
    function remove(Role storage role, address account) internal {
        require(account != address(0));
        require(has(role, account));

        role.bearer[account] = false;
    }

    /**
     * @dev check if an account has this role
     * @return bool
     */
    function has(Role storage role, address account) internal view returns (bool) {
        require(account != address(0));
        return role.bearer[account];
    }
}


Roles

pragma solidity ^0.5.2;

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 {
        counter._value += 1;
    }

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


Counters

pragma solidity ^0.5.2;

import "zos-lib/contracts/Initializable.sol";
import "./IERC165.sol";

/**
 * @title ERC165
 * @author Matt Condon (@shrugs)
 * @dev Implements ERC165 using a lookup table.
 */
contract ERC165 is Initializable, IERC165 {
    bytes4 private constant _INTERFACE_ID_ERC165 = 0x01ffc9a7;
    /*
     * 0x01ffc9a7 ===
     *     bytes4(keccak256('supportsInterface(bytes4)'))
     */

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

    /**
     * @dev A contract implementing SupportsInterfaceWithLookup
     * implement ERC165 itself
     */
    function initialize() public initializer {
        _registerInterface(_INTERFACE_ID_ERC165);
    }

    /**
     * @dev implement supportsInterface(bytes4) using a lookup table
     */
    function supportsInterface(bytes4 interfaceId) public view returns (bool) {
        return _supportedInterfaces[interfaceId];
    }

    /**
     * @dev internal method for registering an interface
     */
    function _registerInterface(bytes4 interfaceId) internal {
        require(interfaceId != 0xffffffff);
        _supportedInterfaces[interfaceId] = true;
    }

    uint256[50] private ______gap;
}


ERC165

pragma solidity ^0.5.2;

/**
 * @title IERC165
 * @dev https://eips.ethereum.org/EIPS/eip-165
 */
interface IERC165 {
    /**
     * @notice Query if a contract implements an interface
     * @param interfaceId The interface identifier, as specified in ERC-165
     * @dev Interface identification is specified in ERC-165. This function
     * uses less than 30,000 gas.
     */
    function supportsInterface(bytes4 interfaceId) external view returns (bool);
}


IERC165

pragma solidity ^0.5.2;

/**
 * @title SafeMath
 * @dev Unsigned math operations with safety checks that revert on error
 */
library SafeMath {
    /**
     * @dev Multiplies two unsigned integers, reverts on 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-solidity/pull/522
        if (a == 0) {
            return 0;
        }

        uint256 c = a * b;
        require(c / a == b);

        return c;
    }

    /**
     * @dev Integer division of two unsigned integers truncating the quotient, reverts on division by zero.
     */
    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        // Solidity only automatically asserts when dividing by 0
        require(b > 0);
        uint256 c = a / b;
        // assert(a == b * c + a % b); // There is no case in which this doesn't hold

        return c;
    }

    /**
     * @dev Subtracts two unsigned integers, reverts on overflow (i.e. if subtrahend is greater than minuend).
     */
    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        require(b <= a);
        uint256 c = a - b;

        return c;
    }

    /**
     * @dev Adds two unsigned integers, reverts on overflow.
     */
    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        require(c >= a);

        return c;
    }

    /**
     * @dev Divides two unsigned integers and returns the remainder (unsigned integer modulo),
     * reverts when dividing by zero.
     */
    function mod(uint256 a, uint256 b) internal pure returns (uint256) {
        require(b != 0);
        return a % b;
    }
}


SafeMath

pragma solidity ^0.5.2;

import "zos-lib/contracts/Initializable.sol";
import "./IERC721.sol";
import "./IERC721Receiver.sol";
import "../../math/SafeMath.sol";
import "../../utils/Address.sol";
import "../../drafts/Counters.sol";
import "../../introspection/ERC165.sol";

/**
 * @title ERC721 Non-Fungible Token Standard basic implementation
 * @dev see https://eips.ethereum.org/EIPS/eip-721
 */
contract ERC721 is Initializable, ERC165, IERC721 {
    using SafeMath for uint256;
    using Address for address;
    using Counters for Counters.Counter;

    // 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 token ID to owner
    mapping (uint256 => address) private _tokenOwner;

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

    // Mapping from owner to number of owned token
    mapping (address => Counters.Counter) private _ownedTokensCount;

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

    bytes4 private constant _INTERFACE_ID_ERC721 = 0x80ac58cd;
    /*
     * 0x80ac58cd ===
     *     bytes4(keccak256('balanceOf(address)')) ^
     *     bytes4(keccak256('ownerOf(uint256)')) ^
     *     bytes4(keccak256('approve(address,uint256)')) ^
     *     bytes4(keccak256('getApproved(uint256)')) ^
     *     bytes4(keccak256('setApprovalForAll(address,bool)')) ^
     *     bytes4(keccak256('isApprovedForAll(address,address)')) ^
     *     bytes4(keccak256('transferFrom(address,address,uint256)')) ^
     *     bytes4(keccak256('safeTransferFrom(address,address,uint256)')) ^
     *     bytes4(keccak256('safeTransferFrom(address,address,uint256,bytes)'))
     */

    function initialize() public initializer {
        ERC165.initialize();

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

    function _hasBeenInitialized() internal view returns (bool) {
        return supportsInterface(_INTERFACE_ID_ERC721);
    }

    /**
     * @dev Gets the balance of the specified address
     * @param owner address to query the balance of
     * @return uint256 representing the amount owned by the passed address
     */
    function balanceOf(address owner) public view returns (uint256) {
        require(owner != address(0));
        return _ownedTokensCount[owner].current();
    }

    /**
     * @dev Gets the owner of the specified token ID
     * @param tokenId uint256 ID of the token to query the owner of
     * @return address currently marked as the owner of the given token ID
     */
    function ownerOf(uint256 tokenId) public view returns (address) {
        address owner = _tokenOwner[tokenId];
        require(owner != address(0));
        return owner;
    }

    /**
     * @dev Approves another address to transfer the given token ID
     * The zero address indicates there is no approved address.
     * There can only be one approved address per token at a given time.
     * Can only be called by the token owner or an approved operator.
     * @param to address to be approved for the given token ID
     * @param tokenId uint256 ID of the token to be approved
     */
    function approve(address to, uint256 tokenId) public {
        address owner = ownerOf(tokenId);
        require(to != owner);
        require(msg.sender == owner || isApprovedForAll(owner, msg.sender));

        _tokenApprovals[tokenId] = to;
        emit Approval(owner, to, tokenId);
    }

    /**
     * @dev Gets the approved address for a token ID, or zero if no address set
     * Reverts if the token ID does not exist.
     * @param tokenId uint256 ID of the token to query the approval of
     * @return address currently approved for the given token ID
     */
    function getApproved(uint256 tokenId) public view returns (address) {
        require(_exists(tokenId));
        return _tokenApprovals[tokenId];
    }

    /**
     * @dev Sets or unsets the approval of a given operator
     * An operator is allowed to transfer all tokens of the sender on their behalf
     * @param to operator address to set the approval
     * @param approved representing the status of the approval to be set
     */
    function setApprovalForAll(address to, bool approved) public {
        require(to != msg.sender);
        _operatorApprovals[msg.sender][to] = approved;
        emit ApprovalForAll(msg.sender, to, approved);
    }

    /**
     * @dev Tells whether an operator is approved by a given owner
     * @param owner owner address which you want to query the approval of
     * @param operator operator address which you want to query the approval of
     * @return bool whether the given operator is approved by the given owner
     */
    function isApprovedForAll(address owner, address operator) public view returns (bool) {
        return _operatorApprovals[owner][operator];
    }

    /**
     * @dev Transfers the ownership of a given token ID to another address
     * Usage of this method is discouraged, use `safeTransferFrom` whenever possible
     * Requires the msg.sender to be the owner, approved, or operator
     * @param from current owner of the token
     * @param to address to receive the ownership of the given token ID
     * @param tokenId uint256 ID of the token to be transferred
     */
    function transferFrom(address from, address to, uint256 tokenId) public {
        require(_isApprovedOrOwner(msg.sender, tokenId));

        _transferFrom(from, to, tokenId);
    }

    /**
     * @dev Safely transfers the ownership of a given token ID to another address
     * If the target address is a contract, it must implement `onERC721Received`,
     * which is called upon a safe transfer, and return the magic value
     * `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`; otherwise,
     * the transfer is reverted.
     * Requires the msg.sender to be the owner, approved, or operator
     * @param from current owner of the token
     * @param to address to receive the ownership of the given token ID
     * @param tokenId uint256 ID of the token to be transferred
     */
    function safeTransferFrom(address from, address to, uint256 tokenId) public {
        safeTransferFrom(from, to, tokenId, "");
    }

    /**
     * @dev Safely transfers the ownership of a given token ID to another address
     * If the target address is a contract, it must implement `onERC721Received`,
     * which is called upon a safe transfer, and return the magic value
     * `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`; otherwise,
     * the transfer is reverted.
     * Requires the msg.sender to be the owner, approved, or operator
     * @param from current owner of the token
     * @param to address to receive the ownership of the given token ID
     * @param tokenId uint256 ID of the token to be transferred
     * @param _data bytes data to send along with a safe transfer check
     */
    function safeTransferFrom(address from, address to, uint256 tokenId, bytes memory _data) public {
        transferFrom(from, to, tokenId);
        require(_checkOnERC721Received(from, to, tokenId, _data));
    }

    /**
     * @dev Returns whether the specified token exists
     * @param tokenId uint256 ID of the token to query the existence of
     * @return bool whether the token exists
     */
    function _exists(uint256 tokenId) internal view returns (bool) {
        address owner = _tokenOwner[tokenId];
        return owner != address(0);
    }

    /**
     * @dev Returns whether the given spender can transfer a given token ID
     * @param spender address of the spender to query
     * @param tokenId uint256 ID of the token to be transferred
     * @return bool whether the msg.sender is approved for the given token ID,
     * is an operator of the owner, or is the owner of the token
     */
    function _isApprovedOrOwner(address spender, uint256 tokenId) internal view returns (bool) {
        address owner = ownerOf(tokenId);
        return (spender == owner || getApproved(tokenId) == spender || isApprovedForAll(owner, spender));
    }

    /**
     * @dev Internal function to mint a new token
     * Reverts if the given token ID already exists
     * @param to The address that will own the minted token
     * @param tokenId uint256 ID of the token to be minted
     */
    function _mint(address to, uint256 tokenId) internal {
        require(to != address(0));
        require(!_exists(tokenId));

        _tokenOwner[tokenId] = to;
        _ownedTokensCount[to].increment();

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

    /**
     * @dev Internal function to burn a specific token
     * Reverts if the token does not exist
     * Deprecated, use _burn(uint256) instead.
     * @param owner owner of the token to burn
     * @param tokenId uint256 ID of the token being burned
     */
    function _burn(address owner, uint256 tokenId) internal {
        require(ownerOf(tokenId) == owner);

        _clearApproval(tokenId);

        _ownedTokensCount[owner].decrement();
        _tokenOwner[tokenId] = address(0);

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

    /**
     * @dev Internal function to burn a specific token
     * Reverts if the token does not exist
     * @param tokenId uint256 ID of the token being burned
     */
    function _burn(uint256 tokenId) internal {
        _burn(ownerOf(tokenId), tokenId);
    }

    /**
     * @dev Internal function to transfer ownership of a given token ID to another address.
     * As opposed to transferFrom, this imposes no restrictions on msg.sender.
     * @param from current owner of the token
     * @param to address to receive the ownership of the given token ID
     * @param tokenId uint256 ID of the token to be transferred
     */
    function _transferFrom(address from, address to, uint256 tokenId) internal {
        require(ownerOf(tokenId) == from);
        require(to != address(0));

        _clearApproval(tokenId);

        _ownedTokensCount[from].decrement();
        _ownedTokensCount[to].increment();

        _tokenOwner[tokenId] = to;

        emit Transfer(from, to, tokenId);
    }

    /**
     * @dev Internal function to invoke `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)
        internal returns (bool)
    {
        if (!to.isContract()) {
            return true;
        }

        bytes4 retval = IERC721Receiver(to).onERC721Received(msg.sender, from, tokenId, _data);
        return (retval == _ERC721_RECEIVED);
    }

    /**
     * @dev Private function to clear current approval of a given token ID
     * @param tokenId uint256 ID of the token to be transferred
     */
    function _clearApproval(uint256 tokenId) private {
        if (_tokenApprovals[tokenId] != address(0)) {
            _tokenApprovals[tokenId] = address(0);
        }
    }

    uint256[50] private ______gap;
}


ERC721

ERC721 Non-Fungible Token Standard basic implementation

pragma solidity ^0.5.2;

import "zos-lib/contracts/Initializable.sol";
import "./IERC721Enumerable.sol";
import "./ERC721.sol";
import "../../introspection/ERC165.sol";

/**
 * @title ERC-721 Non-Fungible Token with optional enumeration extension logic
 * @dev See https://eips.ethereum.org/EIPS/eip-721
 */
contract ERC721Enumerable is Initializable, ERC165, ERC721, IERC721Enumerable {
    // Mapping from owner to list of owned token IDs
    mapping(address => uint256[]) private _ownedTokens;

    // Mapping from token ID to index of the owner tokens list
    mapping(uint256 => uint256) private _ownedTokensIndex;

    // Array with all token ids, used for enumeration
    uint256[] private _allTokens;

    // Mapping from token id to position in the allTokens array
    mapping(uint256 => uint256) private _allTokensIndex;

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

    /**
     * @dev Constructor function
     */
    function initialize() public initializer {
        require(ERC721._hasBeenInitialized());
        // register the supported interface to conform to ERC721Enumerable via ERC165
        _registerInterface(_INTERFACE_ID_ERC721_ENUMERABLE);
    }

    function _hasBeenInitialized() internal view returns (bool) {
        return supportsInterface(_INTERFACE_ID_ERC721_ENUMERABLE);
    }

    /**
     * @dev Gets the token ID at a given index of the tokens list of the requested owner
     * @param owner address owning the tokens list to be accessed
     * @param index uint256 representing the index to be accessed of the requested tokens list
     * @return uint256 token ID at the given index of the tokens list owned by the requested address
     */
    function tokenOfOwnerByIndex(address owner, uint256 index) public view returns (uint256) {
        require(index < balanceOf(owner));
        return _ownedTokens[owner][index];
    }

    /**
     * @dev Gets the total amount of tokens stored by the contract
     * @return uint256 representing the total amount of tokens
     */
    function totalSupply() public view returns (uint256) {
        return _allTokens.length;
    }

    /**
     * @dev Gets the token ID at a given index of all the tokens in this contract
     * Reverts if the index is greater or equal to the total number of tokens
     * @param index uint256 representing the index to be accessed of the tokens list
     * @return uint256 token ID at the given index of the tokens list
     */
    function tokenByIndex(uint256 index) public view returns (uint256) {
        require(index < totalSupply());
        return _allTokens[index];
    }

    /**
     * @dev Internal function to transfer ownership of a given token ID to another address.
     * As opposed to transferFrom, this imposes no restrictions on msg.sender.
     * @param from current owner of the token
     * @param to address to receive the ownership of the given token ID
     * @param tokenId uint256 ID of the token to be transferred
     */
    function _transferFrom(address from, address to, uint256 tokenId) internal {
        super._transferFrom(from, to, tokenId);

        _removeTokenFromOwnerEnumeration(from, tokenId);

        _addTokenToOwnerEnumeration(to, tokenId);
    }

    /**
     * @dev Internal function to mint a new token
     * Reverts if the given token ID already exists
     * @param to address the beneficiary that will own the minted token
     * @param tokenId uint256 ID of the token to be minted
     */
    function _mint(address to, uint256 tokenId) internal {
        super._mint(to, tokenId);

        _addTokenToOwnerEnumeration(to, tokenId);

        _addTokenToAllTokensEnumeration(tokenId);
    }

    /**
     * @dev Internal function to burn a specific token
     * Reverts if the token does not exist
     * Deprecated, use _burn(uint256) instead
     * @param owner owner of the token to burn
     * @param tokenId uint256 ID of the token being burned
     */
    function _burn(address owner, uint256 tokenId) internal {
        super._burn(owner, tokenId);

        _removeTokenFromOwnerEnumeration(owner, tokenId);
        // Since tokenId will be deleted, we can clear its slot in _ownedTokensIndex to trigger a gas refund
        _ownedTokensIndex[tokenId] = 0;

        _removeTokenFromAllTokensEnumeration(tokenId);
    }

    /**
     * @dev Gets the list of token IDs of the requested owner
     * @param owner address owning the tokens
     * @return uint256[] List of token IDs owned by the requested address
     */
    function _tokensOfOwner(address owner) internal view returns (uint256[] storage) {
        return _ownedTokens[owner];
    }

    /**
     * @dev Private function to add a token to this extension's ownership-tracking data structures.
     * @param to address representing the new owner of the given token ID
     * @param tokenId uint256 ID of the token to be added to the tokens list of the given address
     */
    function _addTokenToOwnerEnumeration(address to, uint256 tokenId) private {
        _ownedTokensIndex[tokenId] = _ownedTokens[to].length;
        _ownedTokens[to].push(tokenId);
    }

    /**
     * @dev Private function to add a token to this extension's token tracking data structures.
     * @param tokenId uint256 ID of the token to be added to the tokens list
     */
    function _addTokenToAllTokensEnumeration(uint256 tokenId) private {
        _allTokensIndex[tokenId] = _allTokens.length;
        _allTokens.push(tokenId);
    }

    /**
     * @dev Private function to remove a token from this extension's ownership-tracking data structures. Note that
     * while the token is not assigned a new owner, the _ownedTokensIndex mapping is _not_ updated: this allows for
     * gas optimizations e.g. when performing a transfer operation (avoiding double writes).
     * This has O(1) time complexity, but alters the order of the _ownedTokens array.
     * @param from address representing the previous owner of the given token ID
     * @param tokenId uint256 ID of the token to be removed from the tokens list of the given address
     */
    function _removeTokenFromOwnerEnumeration(address from, uint256 tokenId) private {
        // To prevent a gap in from's tokens array, we store the last token in the index of the token to delete, and
        // then delete the last slot (swap and pop).

        uint256 lastTokenIndex = _ownedTokens[from].length.sub(1);
        uint256 tokenIndex = _ownedTokensIndex[tokenId];

        // When the token to delete is the last token, the swap operation is unnecessary
        if (tokenIndex != lastTokenIndex) {
            uint256 lastTokenId = _ownedTokens[from][lastTokenIndex];

            _ownedTokens[from][tokenIndex] = lastTokenId; // Move the last token to the slot of the to-delete token
            _ownedTokensIndex[lastTokenId] = tokenIndex; // Update the moved token's index
        }

        // This also deletes the contents at the last position of the array
        _ownedTokens[from].length--;

        // Note that _ownedTokensIndex[tokenId] hasn't been cleared: it still points to the old slot (now occupied by
        // lastTokenId, or just over the end of the array if the token was the last one).
    }

    /**
     * @dev Private function to remove a token from this extension's token tracking data structures.
     * This has O(1) time complexity, but alters the order of the _allTokens array.
     * @param tokenId uint256 ID of the token to be removed from the tokens list
     */
    function _removeTokenFromAllTokensEnumeration(uint256 tokenId) private {
        // To prevent a gap in the tokens array, we store the last token in the index of the token to delete, and
        // then delete the last slot (swap and pop).

        uint256 lastTokenIndex = _allTokens.length.sub(1);
        uint256 tokenIndex = _allTokensIndex[tokenId];

        // When the token to delete is the last token, the swap operation is unnecessary. However, since this occurs so
        // rarely (when the last minted token is burnt) that we still do the swap here to avoid the gas cost of adding
        // an 'if' statement (like in _removeTokenFromOwnerEnumeration)
        uint256 lastTokenId = _allTokens[lastTokenIndex];

        _allTokens[tokenIndex] = lastTokenId; // Move the last token to the slot of the to-delete token
        _allTokensIndex[lastTokenId] = tokenIndex; // Update the moved token's index

        // This also deletes the contents at the last position of the array
        _allTokens.length--;
        _allTokensIndex[tokenId] = 0;
    }

    uint256[50] private ______gap;
}


ERC721Enumerable

ERC-721 Non-Fungible Token with optional enumeration extension logic

balance

_approved

data

pragma solidity ^0.5.2;

import "zos-lib/contracts/Initializable.sol";
import "../../introspection/IERC165.sol";

/**
 * @title ERC721 Non-Fungible Token Standard basic interface
 * @dev see https://eips.ethereum.org/EIPS/eip-721
 */
contract IERC721 is Initializable, IERC165 {
    event Transfer(address indexed from, address indexed to, uint256 indexed tokenId);
    event Approval(address indexed owner, address indexed approved, uint256 indexed tokenId);
    event ApprovalForAll(address indexed owner, address indexed operator, bool approved);

    function balanceOf(address owner) public view returns (uint256 balance);
    function ownerOf(uint256 tokenId) public view returns (address owner);

    function approve(address to, uint256 tokenId) public;
    function getApproved(uint256 tokenId) public view returns (address operator);

    function setApprovalForAll(address operator, bool _approved) public;
    function isApprovedForAll(address owner, address operator) public view returns (bool);

    function transferFrom(address from, address to, uint256 tokenId) public;
    function safeTransferFrom(address from, address to, uint256 tokenId) public;

    function safeTransferFrom(address from, address to, uint256 tokenId, bytes memory data) public;
}


IERC721

ERC721 Non-Fungible Token Standard basic interface

pragma solidity ^0.5.2;

import "zos-lib/contracts/Initializable.sol";
import "./IERC721.sol";

/**
 * @title ERC-721 Non-Fungible Token Standard, optional enumeration extension
 * @dev See https://eips.ethereum.org/EIPS/eip-721
 */
contract IERC721Enumerable is Initializable, IERC721 {
    function totalSupply() public view returns (uint256);
    function tokenOfOwnerByIndex(address owner, uint256 index) public view returns (uint256 tokenId);

    function tokenByIndex(uint256 index) public view returns (uint256);
}


IERC721Enumerable

ERC-721 Non-Fungible Token Standard, optional enumeration extension

pragma solidity ^0.5.2;

import "zos-lib/contracts/Initializable.sol";
import "./IERC721.sol";

/**
 * @title ERC-721 Non-Fungible Token Standard, optional metadata extension
 * @dev See https://eips.ethereum.org/EIPS/eip-721
 */
contract IERC721Metadata is Initializable, IERC721 {
    function name() external view returns (string memory);
    function symbol() external view returns (string memory);
    function tokenURI(uint256 tokenId) external view returns (string memory);
}


IERC721Metadata

ERC-721 Non-Fungible Token Standard, optional metadata extension

onERC721Received

pragma solidity ^0.5.2;

/**
 * @title ERC721 token receiver interface
 * @dev Interface for any contract that wants to support safeTransfers
 * from ERC721 asset contracts.
 */
contract IERC721Receiver {
    /**
     * @notice Handle the receipt of an NFT
     * @dev The ERC721 smart contract calls this function on the recipient
     * after a `safeTransfer`. This function MUST return the function selector,
     * otherwise the caller will revert the transaction. The selector to be
     * returned can be obtained as `this.onERC721Received.selector`. This
     * function MAY throw to revert and reject the transfer.
     * Note: the ERC721 contract address is always the message sender.
     * @param operator The address which called `safeTransferFrom` function
     * @param from The address which previously owned the token
     * @param tokenId The NFT identifier which is being transferred
     * @param data Additional data with no specified format
     * @return bytes4 `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`
     */
    function onERC721Received(address operator, address from, uint256 tokenId, bytes memory data)
    public returns (bytes4);
}


IERC721Receiver

ERC721 token receiver interface

pragma solidity ^0.5.2;

/**
 * Utility library of inline functions on addresses
 */
library Address {
    /**
     * Returns whether the target address is a contract
     * @dev This function will return false if invoked during the constructor of a contract,
     * as the code is not actually created until after the constructor finishes.
     * @param account address of the account to check
     * @return whether the target address is a contract
     */
    function isContract(address account) internal view returns (bool) {
        uint256 size;
        // XXX Currently there is no better way to check if there is a contract in an address
        // than to check the size of the code at that address.
        // See https://ethereum.stackexchange.com/a/14016/36603
        // for more details about how this works.
        // TODO Check this again before the Serenity release, because all addresses will be
        // contracts then.
        // solhint-disable-next-line no-inline-assembly
        assembly { size := extcodesize(account) }
        return size > 0;
    }
}


Address

pragma solidity >=0.4.24 <0.6.0;


/**
 * @title Initializable
 *
 * @dev Helper contract to support initializer functions. To use it, replace
 * the constructor with a function that has the `initializer` modifier.
 * WARNING: Unlike constructors, initializer functions must be manually
 * invoked. This applies both to deploying an Initializable contract, as well
 * as extending an Initializable contract via inheritance.
 * WARNING: When used with inheritance, manual care must be taken to not invoke
 * a parent initializer twice, or ensure that all initializers are idempotent,
 * because this is not dealt with automatically as with constructors.
 */
contract Initializable {

  /**
   * @dev Indicates that the contract has been initialized.
   */
  bool private initialized;

  /**
   * @dev Indicates that the contract is in the process of being initialized.
   */
  bool private initializing;

  /**
   * @dev Modifier to use in the initializer function of a contract.
   */
  modifier initializer() {
    require(initializing || isConstructor() || !initialized, "Contract instance has already been initialized");

    bool isTopLevelCall = !initializing;
    if (isTopLevelCall) {
      initializing = true;
      initialized = true;
    }

    _;

    if (isTopLevelCall) {
      initializing = false;
    }
  }

  /// @dev Returns true if and only if the function is running in the constructor
  function isConstructor() private view returns (bool) {
    // extcodesize checks the size of the code stored in an address, and
    // address returns the current address. Since the code is still not
    // deployed when running a constructor, any checks on its code size will
    // yield zero, making it an effective way to detect if a contract is
    // under construction or not.
    uint256 cs;
    assembly { cs := extcodesize(address) }
    return cs == 0;
  }

  // Reserved storage space to allow for layout changes in the future.
  uint256[50] private ______gap;
}


Initializable

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