Contract Address Details

// SPDX-License-Identifier: MIT
pragma solidity 0.7.6;

import "../../governance/implementation/GovernedAtGenesis.sol";
import "@openzeppelin/contracts/math/SafeMath.sol";
import "@openzeppelin/contracts/math/Math.sol";
import "@openzeppelin/contracts/utils/ReentrancyGuard.sol";
import "../../utils/implementation/SafePct.sol";

/**
 * @title InitialAirdrop
 * @notice A contract to manage the initial airdrop allocation. 
 * @notice The balance that will be added to this contract must initially be a part of circulating supply 
 **/
contract InitialAirdrop is GovernedAtGenesis, ReentrancyGuard {
    using SafeMath for uint256;
    using SafePct for uint256;

    // constants
    uint256 internal constant CLAIMED_AT_GENESIS_BIPS = 1500;
    uint256 internal constant TOTAL_BIPS = 10000;

    // storage
    address[] public airdropAccounts;
    mapping(address => uint256) public airdropAmountsWei;
    uint256 public totalInitialAirdropWei;          // Total wei to be distributed by this contract (initial airdrop)
    uint256 public initialAirdropStartTs;           // Day 0 when airdrop starts
    uint256 public latestAirdropStartTs;            // Latest day 0 when airdrop starts
    uint256 public totalTransferredAirdropWei;      // All wei already transferred
    uint256 public nextAirdropAccountIndexToTransfer;

    // Errors
    string internal constant ERR_OUT_OF_BALANCE = "balance too low";
    string internal constant ERR_TOO_MANY = "too many";
    string internal constant ERR_NOT_STARTED = "not started";
    string internal constant ERR_ARRAY_MISMATCH = "arrays lengths mismatch";
    string internal constant ERR_ALREADY_SET = "already set";
    string internal constant ERR_WRONG_START_TIMESTAMP = "wrong start timestamp";
    string internal constant ERR_ALREADY_STARTED = "already started";

    // Events
    event AccountsAdded(address[] accounts);
    event AirdropStart(uint256 initialAirdropStartTs);
    event AirdropTransferFailure(address indexed account, uint256 amountWei);

    /**
     * @dev This modifier ensures that this contract's balance matches the expected balance.
     */
    modifier mustBalance {
        _;
        require (_getExpectedBalance() <= address(this).balance, ERR_OUT_OF_BALANCE);
    }

    /**
     * @dev This modifier ensures that the airdrop was already started
     */
    modifier airdropStarted {
        require (initialAirdropStartTs != 0 && initialAirdropStartTs < block.timestamp, ERR_NOT_STARTED);
        _;
    }

    /**
     * @dev This constructor should contain no code as this contract is pre-loaded into the genesis block.
     *   The super constructor is called for testing convenience.
     */
    constructor() GovernedAtGenesis(address(0)) {
        /* empty block */
    }

    /**
     * @notice Method to set addresses and their respective balances in batches to this contract (initial airdrop)
     * @param _accounts         Array of adresses we are adding in batch
     * @param _balances         Array of balances to be airdropped to respective accounts (total amount - 100%)
     * @dev Note that _toAddresses and _balances arrays must be of equal length
     * @dev Note that script must use the same batches to fill data (if restarted), otherwise duplicates may occure
     */
    function setAirdropBalances(address[] calldata _accounts, uint256[] calldata _balances) external onlyGovernance {
        require(_accounts.length <= 1000, ERR_TOO_MANY);
        require(_accounts.length == _balances.length, ERR_ARRAY_MISMATCH);
        require (initialAirdropStartTs == 0, ERR_ALREADY_STARTED);

        if (airdropAmountsWei[_accounts[0]] > 0) return; // batch already added
        for (uint16 i = 0; i < _accounts.length; i++) {
            address airdropAccount = _accounts[i];
            uint256 airdropAmountWei = _balances[i].mulDiv(CLAIMED_AT_GENESIS_BIPS, TOTAL_BIPS);
            airdropAccounts.push(airdropAccount);
            airdropAmountsWei[airdropAccount] = airdropAmountWei;
            totalInitialAirdropWei = totalInitialAirdropWei.add(airdropAmountWei);
        }
        // We added the accounts to airdrop, emit event
        emit AccountsAdded(_accounts);
    }

    /** 
     * @notice Latest start of the initial airdrop at _latestAirdropStartTs timestamp
     * @param _latestAirdropStartTs point in time when latest start is possible
     */
    function setLatestAirdropStart(uint256 _latestAirdropStartTs) external onlyGovernance {
        require(latestAirdropStartTs == 0, ERR_ALREADY_SET);
        latestAirdropStartTs = _latestAirdropStartTs;
    }

    /** 
     * @notice Start the initial airdrop at _initialAirdropStartTs timestamp
     * @param _initialAirdropStartTs point in time when we start
     * @dev should be called immediately after all airdrop accounts and balances are set
     */
    function setAirdropStart(uint256 _initialAirdropStartTs) external onlyGovernance mustBalance {
        require(initialAirdropStartTs == 0 || initialAirdropStartTs > block.timestamp, ERR_ALREADY_STARTED);
        require(initialAirdropStartTs < _initialAirdropStartTs && _initialAirdropStartTs <= latestAirdropStartTs,
            ERR_WRONG_START_TIMESTAMP);
        initialAirdropStartTs = _initialAirdropStartTs;
        emit AirdropStart(_initialAirdropStartTs);
    }

    /**
     * @notice Method for transfering initial airdrop amounts in batches of 50
     */
    function transferAirdrop() external airdropStarted mustBalance nonReentrant {
        uint256 upperBound = Math.min(nextAirdropAccountIndexToTransfer + 50, airdropAccounts.length);
        for (uint256 i = nextAirdropAccountIndexToTransfer; i < upperBound; i++) {
            // Get the account and amount
            address account = airdropAccounts[i];
            uint256 amountWei = airdropAmountsWei[account];
            // update state
            delete airdropAmountsWei[account];
            delete airdropAccounts[i];
            // Update grand total transferred
            totalTransferredAirdropWei = totalTransferredAirdropWei.add(amountWei);
            // Send
            /* solhint-disable avoid-low-level-calls */
            //slither-disable-next-line arbitrary-send
            (bool success, ) = account.call{ value: amountWei, gas: 21000 }("");
            /* solhint-enable avoid-low-level-calls */
            if (!success) {
                emit AirdropTransferFailure(account, amountWei);
            }
        }

        // Update current position
        nextAirdropAccountIndexToTransfer = upperBound;
    }

    /**
     * @notice Return the number of airdrop accounts
     */
    function airdropAccountsLength() external view returns (uint256) {
        return airdropAccounts.length;
    }

    /**
     * @notice Compute the expected balance of this contract.
     * @param _balanceExpectedWei   The computed balance expected.
     */
    function _getExpectedBalance() private view returns(uint256 _balanceExpectedWei) {
        return totalInitialAirdropWei.sub(totalTransferredAirdropWei);
    }

}
        

// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

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

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

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

// SPDX-License-Identifier: MIT
pragma solidity 0.7.6;

import "./GovernedBase.sol";


/**
 * @title Governed At Genesis
 * @dev This contract enforces a fixed governance address when the constructor
 *  is not executed on a contract (for instance when directly loaded to the genesis block).
 *  This is required to fix governance on a contract when the network starts, at such point
 *  where theoretically no accounts yet exist, and leaving it ungoverned could result in a race
 *  to claim governance by an unauthorized address.
 **/
contract GovernedAtGenesis is GovernedBase {
    constructor(address _governance) GovernedBase(_governance) { }

    /**
     * @notice Set governance to a fixed address when constructor is not called.
     **/
    function initialiseFixedAddress() public virtual returns (address) {
        address governanceAddress = address(0xfffEc6C83c8BF5c3F4AE0cCF8c45CE20E4560BD7);
        
        super.initialise(governanceAddress);
        return governanceAddress;
    }

    /**
     * @notice Disallow initialise to be called
     * @param _governance The governance address for initial claiming
     **/
    // solhint-disable-next-line no-unused-vars
    function initialise(address _governance) public override pure {
        assert(false);
    }
}
          

// SPDX-License-Identifier: MIT
pragma solidity 0.7.6;

import "../../userInterfaces/IGovernanceSettings.sol";


/**
 * @title Governed Base
 * @notice This abstract base class defines behaviors for a governed contract.
 * @dev This class is abstract so that specific behaviors can be defined for the constructor.
 *   Contracts should not be left ungoverned, but not all contract will have a constructor
 *   (for example those pre-defined in genesis).
 **/
abstract contract GovernedBase {
    struct TimelockedCall {
        uint256 allowedAfterTimestamp;
        bytes encodedCall;
    }
    
    // solhint-disable-next-line const-name-snakecase
    IGovernanceSettings public constant governanceSettings = 
        IGovernanceSettings(0x1000000000000000000000000000000000000007);

    address private initialGovernance;

    bool private initialised;
    
    bool public productionMode;
    
    bool private executing;
    
    mapping(bytes4 => TimelockedCall) public timelockedCalls;
    
    event GovernanceCallTimelocked(bytes4 selector, uint256 allowedAfterTimestamp, bytes encodedCall);
    event TimelockedGovernanceCallExecuted(bytes4 selector, uint256 timestamp);
    event TimelockedGovernanceCallCanceled(bytes4 selector, uint256 timestamp);
    
    event GovernanceInitialised(address initialGovernance);
    event GovernedProductionModeEntered(address governanceSettings);
    
    modifier onlyGovernance {
        if (executing || !productionMode) {
            _beforeExecute();
            _;
        } else {
            _recordTimelockedCall(msg.data);
        }
    }
    
    modifier onlyImmediateGovernance () {
        _checkOnlyGovernance();
        _;
    }

    constructor(address _initialGovernance) {
        if (_initialGovernance != address(0)) {
            initialise(_initialGovernance);
        }
    }

    /**
     * @notice Execute the timelocked governance calls once the timelock period expires.
     * @dev Only executor can call this method.
     * @param _selector The method selector (only one timelocked call per method is stored).
     */
    function executeGovernanceCall(bytes4 _selector) external {
        require(governanceSettings.isExecutor(msg.sender), "only executor");
        TimelockedCall storage call = timelockedCalls[_selector];
        require(call.allowedAfterTimestamp != 0, "timelock: invalid selector");
        require(block.timestamp >= call.allowedAfterTimestamp, "timelock: not allowed yet");
        bytes memory encodedCall = call.encodedCall;
        delete timelockedCalls[_selector];
        executing = true;
        //solhint-disable-next-line avoid-low-level-calls
        (bool success,) = address(this).call(encodedCall);
        executing = false;
        emit TimelockedGovernanceCallExecuted(_selector, block.timestamp);
        _passReturnOrRevert(success);
    }
    
    /**
     * Cancel a timelocked governance call before it has been executed.
     * @dev Only governance can call this method.
     * @param _selector The method selector.
     */
    function cancelGovernanceCall(bytes4 _selector) external onlyImmediateGovernance {
        require(timelockedCalls[_selector].allowedAfterTimestamp != 0, "timelock: invalid selector");
        emit TimelockedGovernanceCallCanceled(_selector, block.timestamp);
        delete timelockedCalls[_selector];
    }
    
    /**
     * Enter the production mode after all the initial governance settings have been set.
     * This enables timelocks and the governance is afterwards obtained by calling 
     * governanceSettings.getGovernanceAddress(). 
     */
    function switchToProductionMode() external {
        _checkOnlyGovernance();
        require(!productionMode, "already in production mode");
        initialGovernance = address(0);
        productionMode = true;
        emit GovernedProductionModeEntered(address(governanceSettings));
    }

    /**
     * @notice Initialize the governance address if not first initialized.
     */
    function initialise(address _initialGovernance) public virtual {
        require(initialised == false, "initialised != false");
        initialised = true;
        initialGovernance = _initialGovernance;
        emit GovernanceInitialised(_initialGovernance);
    }
    
    /**
     * Returns the current effective governance address.
     */
    function governance() public view returns (address) {
        return productionMode ? governanceSettings.getGovernanceAddress() : initialGovernance;
    }

    function _beforeExecute() private {
        if (executing) {
            // can only be run from executeGovernanceCall(), where we check that only executor can call
            // make sure nothing else gets executed, even in case of reentrancy
            assert(msg.sender == address(this));
            executing = false;
        } else {
            // must be called with: productionMode=false
            // must check governance in this case
            _checkOnlyGovernance();
        }
    }

    function _recordTimelockedCall(bytes calldata _data) private {
        _checkOnlyGovernance();
        bytes4 selector;
        //solhint-disable-next-line no-inline-assembly
        assembly {
            selector := calldataload(_data.offset)
        }
        uint256 timelock = governanceSettings.getTimelock();
        uint256 allowedAt = block.timestamp + timelock;
        timelockedCalls[selector] = TimelockedCall({
            allowedAfterTimestamp: allowedAt,
            encodedCall: _data
        });
        emit GovernanceCallTimelocked(selector, allowedAt, _data);
    }
    
    function _checkOnlyGovernance() private view {
        require(msg.sender == governance(), "only governance");
    }
    
    function _passReturnOrRevert(bool _success) private pure {
        // pass exact return or revert data - needs to be done in assembly
        //solhint-disable-next-line no-inline-assembly
        assembly {
            let size := returndatasize()
            let ptr := mload(0x40)
            mstore(0x40, add(ptr, size))
            returndatacopy(ptr, 0, size)
            if _success {
                return(ptr, size)
            }
            revert(ptr, size)
        }
    }
}
          

// SPDX-License-Identifier: MIT
pragma solidity 0.7.6;


/**
 * A special contract that holds Flare governance address.
 * This contract enables updating governance address and timelock only by hard forking the network,
 * meaning only by updating validator code.
 */
interface IGovernanceSettings {
    /**
     * Get the governance account address.
     * The governance address can only be changed by a hardfork.
     */
    function getGovernanceAddress() external view returns (address);
    
    /**
     * Get the time in seconds that must pass between a governance call and execution.
     * The timelock value can only be changed by a hardfork.
     */
    function getTimelock() external view returns (uint256);
    
    /**
     * Get the addresses of the accounts that are allowed to execute the timelocked governance calls
     * once the timelock period expires.
     * Executors can be changed without a hardfork, via a normal governance call.
     */
    function getExecutors() external view returns (address[] memory);
    
    /**
     * Check whether an address is one of the executors.
     */
    function isExecutor(address _address) external view returns (bool);
}
          

// SPDX-License-Identifier: MIT

pragma solidity 0.7.6;
import {SafeMath} from "@openzeppelin/contracts/math/SafeMath.sol";

/**
 * @dev Compute percentages safely without phantom overflows.
 *
 * Intermediate operations can overflow even when the result will always
 * fit into computed type. Developers usually
 * assume that overflows raise errors. `SafePct` restores this intuition by
 * reverting the transaction when such an operation overflows.
 *
 * Using this library instead of the unchecked operations eliminates an entire
 * class of bugs, so it's recommended to use it always.
 *
 * Can be combined with {SafeMath} and {SignedSafeMath} to extend it to smaller types, by performing
 * all math on `uint256` and `int256` and then downcasting.
 */
library SafePct {
    using SafeMath for uint256;
    /**
     * Requirements:
     *
     * - intermediate operations must revert on overflow
     */
    function mulDiv(uint256 x, uint256 y, uint256 z) internal pure returns (uint256) {
        require(z > 0, "Division by zero");

        if (x == 0) return 0;
        uint256 xy = x * y;
        if (xy / x == y) { // no overflow happened - same as in SafeMath mul
            return xy / z;
        }

        //slither-disable-next-line divide-before-multiply
        uint256 a = x / z;
        uint256 b = x % z; // x = a * z + b

        //slither-disable-next-line divide-before-multiply
        uint256 c = y / z;
        uint256 d = y % z; // y = c * z + d

        return (a.mul(c).mul(z)).add(a.mul(d)).add(b.mul(c)).add(b.mul(d).div(z));
    }
}
          

// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

/**
 * @dev Wrappers over Solidity's arithmetic operations with added overflow
 * checks.
 *
 * Arithmetic operations in Solidity wrap on overflow. This can easily result
 * in bugs, because programmers usually assume that an overflow raises an
 * error, which is the standard behavior in high level programming languages.
 * `SafeMath` restores this intuition by reverting the transaction when an
 * operation overflows.
 *
 * Using this library instead of the unchecked operations eliminates an entire
 * class of bugs, so it's recommended to use it always.
 */
library SafeMath {
    /**
     * @dev Returns the addition of two unsigned integers, with an overflow flag.
     *
     * _Available since v3.4._
     */
    function tryAdd(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        uint256 c = a + b;
        if (c < a) return (false, 0);
        return (true, c);
    }

    /**
     * @dev Returns the substraction of two unsigned integers, with an overflow flag.
     *
     * _Available since v3.4._
     */
    function trySub(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        if (b > a) return (false, 0);
        return (true, a - b);
    }

    /**
     * @dev Returns the multiplication of two unsigned integers, with an overflow flag.
     *
     * _Available since v3.4._
     */
    function tryMul(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        // Gas optimization: this is cheaper than requiring 'a' not being zero, but the
        // benefit is lost if 'b' is also tested.
        // See: https://github.com/OpenZeppelin/openzeppelin-contracts/pull/522
        if (a == 0) return (true, 0);
        uint256 c = a * b;
        if (c / a != b) return (false, 0);
        return (true, c);
    }

    /**
     * @dev Returns the division of two unsigned integers, with a division by zero flag.
     *
     * _Available since v3.4._
     */
    function tryDiv(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        if (b == 0) return (false, 0);
        return (true, a / b);
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers, with a division by zero flag.
     *
     * _Available since v3.4._
     */
    function tryMod(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        if (b == 0) return (false, 0);
        return (true, a % b);
    }

    /**
     * @dev Returns the addition of two unsigned integers, reverting on
     * overflow.
     *
     * Counterpart to Solidity's `+` operator.
     *
     * Requirements:
     *
     * - Addition cannot overflow.
     */
    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        require(c >= a, "SafeMath: addition overflow");
        return c;
    }

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

    /**
     * @dev Returns the multiplication of two unsigned integers, reverting on
     * overflow.
     *
     * Counterpart to Solidity's `*` operator.
     *
     * Requirements:
     *
     * - Multiplication cannot overflow.
     */
    function mul(uint256 a, uint256 b) internal pure returns (uint256) {
        if (a == 0) return 0;
        uint256 c = a * b;
        require(c / a == b, "SafeMath: multiplication overflow");
        return c;
    }

    /**
     * @dev Returns the integer division of two unsigned integers, reverting on
     * division by zero. The result is rounded towards zero.
     *
     * Counterpart to Solidity's `/` operator. Note: this function uses a
     * `revert` opcode (which leaves remaining gas untouched) while Solidity
     * uses an invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        require(b > 0, "SafeMath: division by zero");
        return a / b;
    }

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

    /**
     * @dev Returns the subtraction of two unsigned integers, reverting with custom message on
     * overflow (when the result is negative).
     *
     * CAUTION: This function is deprecated because it requires allocating memory for the error
     * message unnecessarily. For custom revert reasons use {trySub}.
     *
     * Counterpart to Solidity's `-` operator.
     *
     * Requirements:
     *
     * - Subtraction cannot overflow.
     */
    function sub(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b <= a, errorMessage);
        return a - b;
    }

    /**
     * @dev Returns the integer division of two unsigned integers, reverting with custom message on
     * division by zero. The result is rounded towards zero.
     *
     * CAUTION: This function is deprecated because it requires allocating memory for the error
     * message unnecessarily. For custom revert reasons use {tryDiv}.
     *
     * Counterpart to Solidity's `/` operator. Note: this function uses a
     * `revert` opcode (which leaves remaining gas untouched) while Solidity
     * uses an invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function div(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b > 0, errorMessage);
        return a / b;
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * reverting with custom message when dividing by zero.
     *
     * CAUTION: This function is deprecated because it requires allocating memory for the error
     * message unnecessarily. For custom revert reasons use {tryMod}.
     *
     * Counterpart to Solidity's `%` operator. This function uses a `revert`
     * opcode (which leaves remaining gas untouched) while Solidity uses an
     * invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function mod(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b > 0, errorMessage);
        return a % b;
    }
}
          

// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

/**
 * @dev Contract module that helps prevent reentrant calls to a function.
 *
 * Inheriting from `ReentrancyGuard` will make the {nonReentrant} modifier
 * available, which can be applied to functions to make sure there are no nested
 * (reentrant) calls to them.
 *
 * Note that because there is a single `nonReentrant` guard, functions marked as
 * `nonReentrant` may not call one another. This can be worked around by making
 * those functions `private`, and then adding `external` `nonReentrant` entry
 * points to them.
 *
 * TIP: If you would like to learn more about reentrancy and alternative ways
 * to protect against it, check out our blog post
 * https://blog.openzeppelin.com/reentrancy-after-istanbul/[Reentrancy After Istanbul].
 */
abstract contract ReentrancyGuard {
    // Booleans are more expensive than uint256 or any type that takes up a full
    // word because each write operation emits an extra SLOAD to first read the
    // slot's contents, replace the bits taken up by the boolean, and then write
    // back. This is the compiler's defense against contract upgrades and
    // pointer aliasing, and it cannot be disabled.

    // The values being non-zero value makes deployment a bit more expensive,
    // but in exchange the refund on every call to nonReentrant will be lower in
    // amount. Since refunds are capped to a percentage of the total
    // transaction's gas, it is best to keep them low in cases like this one, to
    // increase the likelihood of the full refund coming into effect.
    uint256 private constant _NOT_ENTERED = 1;
    uint256 private constant _ENTERED = 2;

    uint256 private _status;

    constructor () internal {
        _status = _NOT_ENTERED;
    }

    /**
     * @dev Prevents a contract from calling itself, directly or indirectly.
     * Calling a `nonReentrant` function from another `nonReentrant`
     * function is not supported. It is possible to prevent this from happening
     * by making the `nonReentrant` function external, and make it call a
     * `private` function that does the actual work.
     */
    modifier nonReentrant() {
        // On the first call to nonReentrant, _notEntered will be true
        require(_status != _ENTERED, "ReentrancyGuard: reentrant call");

        // Any calls to nonReentrant after this point will fail
        _status = _ENTERED;

        _;

        // By storing the original value once again, a refund is triggered (see
        // https://eips.ethereum.org/EIPS/eip-2200)
        _status = _NOT_ENTERED;
    }
}