// File: /contracts/wrappers/ConvexStakingWrapperFrax.sol



// SPDX-License-Identifier: MIT
pragma solidity 0.6.12;
pragma experimental ABIEncoderV2;

import "./ConvexStakingWrapper.sol";
import "../interfaces/IProxyFactory.sol";
import "../interfaces/IOwner.sol";


interface IFraxFarmDistributor {
    function initialize(address _farm, address _wrapper) external;
}

//Staking wrapper for Frax Finance platform
//use convex LP positions as collateral while still receiving rewards
//
//This version directs all rewards from the vault(fxs gauge) to a distributor contract
//which will feed the rewards back into the vault
contract ConvexStakingWrapperFrax is ConvexStakingWrapper {
    using SafeERC20
    for IERC20;
    using SafeMath
    for uint256;

    address public immutable distroImplementation;
    address public immutable factory;
    address public constant proxyFactory = address(0x66807B5598A848602734B82E432dD88DBE13fC8f);

    address public distroContract;

    constructor(address _distributor, address _factory) public{
        distroImplementation = _distributor;
        factory = _factory;
    }

    modifier onlyOwner() override{
        require(owner() == msg.sender, "Ownable: caller is not the owner");
        _;
    }

    function owner() public view override returns(address) {
        return IOwner(factory).owner();
    }


    function initialize(uint256 _poolId)
    override external {
        require(!isInit,"already init");
        // _owner = msg.sender;
        // emit OwnershipTransferred(address(0), _owner);

        (address _lptoken, address _token, , address _rewards, , ) = IBooster(convexBooster).poolInfo(_poolId);
        curveToken = _lptoken;
        convexToken = _token;
        convexPool = _rewards;
        convexPoolId = _poolId;

        _tokenname = string(abi.encodePacked("Staked ", ERC20(_token).name(), " Frax" ));
        _tokensymbol = string(abi.encodePacked("stk", ERC20(_token).symbol(), "-frax"));
        isShutdown = false;
        isInit = true;

        //add rewards
        addRewards();
        setApprovals();
    }

    function setVault(address _vault) external onlyOwner{
        //set distro contract to take care of rewards
        require(distroContract == address(0), "already set");
        
        //create a distro contract
        distroContract = IProxyFactory(proxyFactory).clone(distroImplementation);
        IFraxFarmDistributor(distroContract).initialize(_vault, address(this));

        //forward rewards from vault to distro
        rewardRedirect[_vault] = distroContract;
    }

}
// File: /contracts/wrappers/ConvexStakingWrapper.sol



// SPDX-License-Identifier: MIT
pragma solidity 0.6.12;
pragma experimental ABIEncoderV2;

import "../interfaces/IRewardStaking.sol";
import "../interfaces/IConvexDeposits.sol";
import "../interfaces/CvxMining.sol";
import "../interfaces/IBooster.sol";
import "../interfaces/IRewardHook.sol";
import "../interfaces/ITokenWrapper.sol";
import '@openzeppelin/contracts/math/SafeMath.sol';
import '@openzeppelin/contracts/token/ERC20/IERC20.sol';
import '@openzeppelin/contracts/token/ERC20/SafeERC20.sol';
import '@openzeppelin/contracts/token/ERC20/ERC20.sol';
import "@openzeppelin/contracts/utils/ReentrancyGuard.sol";


//Example of a tokenize a convex staked position.
//if used as collateral some modifications will be needed to fit the specific platform

//Based on Curve.fi's gauge wrapper implementations at https://github.com/curvefi/curve-dao-contracts/tree/master/contracts/gauges/wrappers
contract ConvexStakingWrapper is ERC20, ReentrancyGuard {
    using SafeERC20
    for IERC20;
    using SafeMath
    for uint256;

    struct EarnedData {
        address token;
        uint256 amount;
    }

    struct RewardType {
        address reward_token;
        address reward_pool;
        uint256 reward_integral;
        uint256 reward_remaining;
        mapping(address => uint256) reward_integral_for;
        mapping(address => uint256) claimable_reward;
    }

    //constants/immutables
    address public constant convexBooster = address(0xF403C135812408BFbE8713b5A23a04b3D48AAE31);
    address public constant crv = address(0xD533a949740bb3306d119CC777fa900bA034cd52);
    address public constant cvx = address(0x4e3FBD56CD56c3e72c1403e103b45Db9da5B9D2B);
    address public curveToken;
    address public convexToken;
    address public convexPool;
    uint256 public convexPoolId;
    address public collateralVault;
    uint256 private constant CRV_INDEX = 0;
    uint256 private constant CVX_INDEX = 1;

    //rewards
    RewardType[] public rewards;
    mapping(address => uint256) public registeredRewards;
    address public rewardHook;
    mapping(address => address) public rewardRedirect;

    //management
    bool public isShutdown;
    bool public isInit;
    address internal _owner;

    string internal _tokenname;
    string internal _tokensymbol;

    event Deposited(address indexed _user, address indexed _account, uint256 _amount, bool _wrapped);
    event Withdrawn(address indexed _user, uint256 _amount, bool _unwrapped);
    event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);
    event RewardInvalidated(address _rewardToken);
    event RewardRedirected(address indexed _account, address _forward);
    event RewardAdded(address _token);
    event Shutdown();
    event HookSet(address _hook);
    event UserCheckpoint(address _userA, address _userB);

    constructor() public
        ERC20(
            "StakedConvexToken",
            "stkCvx"
        ){
    }

    function initialize(uint256 _poolId)
    virtual external {
        require(!isInit,"already init");
        _owner = msg.sender;
        emit OwnershipTransferred(address(0), _owner);

        (address _lptoken, address _token, , address _rewards, , ) = IBooster(convexBooster).poolInfo(_poolId);
        curveToken = _lptoken;
        convexToken = _token;
        convexPool = _rewards;
        convexPoolId = _poolId;

        _tokenname = string(abi.encodePacked("Staked ", ERC20(_token).name() ));
        _tokensymbol = string(abi.encodePacked("stk", ERC20(_token).symbol()));
        isShutdown = false;
        isInit = true;

        // collateralVault = _vault;

        //add rewards
        addRewards();
        setApprovals();
    }

    function owner() public view virtual returns(address) {
        return _owner;
    }

    function name() public view override returns (string memory) {
        return _tokenname;
    }

    function symbol() public view override returns (string memory) {
        return _tokensymbol;
    }

    function decimals() public view override returns (uint8) {
        return 18;
    }

    modifier onlyOwner() virtual{
        require(_owner == msg.sender, "Ownable: caller is not the owner");
        _;
    }

    function transferOwnership(address newOwner) public virtual onlyOwner {
        require(newOwner != address(0), "Ownable: new owner is the zero address");
        emit OwnershipTransferred(_owner, newOwner);
        _owner = newOwner;
    }

    function renounceOwnership() public virtual onlyOwner {
        emit OwnershipTransferred(_owner, address(0));
        _owner = address(0);
    }

    function shutdown() external onlyOwner {
        isShutdown = true;
        emit Shutdown();
    }

    function setApprovals() public {
        IERC20(curveToken).safeApprove(convexBooster, 0);
        IERC20(curveToken).safeApprove(convexBooster, uint256(-1));
        IERC20(convexToken).safeApprove(convexPool, 0);
        IERC20(convexToken).safeApprove(convexPool, uint256(-1));
    }

    function addRewards() public {
        address mainPool = convexPool;

        if (rewards.length == 0) {
            rewards.push(
                RewardType({
                    reward_token: crv,
                    reward_pool: mainPool,
                    reward_integral: 0,
                    reward_remaining: 0
                })
            );
            rewards.push(
                RewardType({
                    reward_token: cvx,
                    reward_pool: address(0),
                    reward_integral: 0,
                    reward_remaining: 0
                })
            );
            registeredRewards[crv] = CRV_INDEX+1; //mark registered at index+1
            registeredRewards[cvx] = CVX_INDEX+1; //mark registered at index+1
            //send to self to warmup state
            IERC20(crv).transfer(address(this),0);
            //send to self to warmup state
            IERC20(cvx).transfer(address(this),0);
            emit RewardAdded(crv);
            emit RewardAdded(cvx);
        }

        uint256 extraCount = IRewardStaking(mainPool).extraRewardsLength();
        for (uint256 i = 0; i < extraCount; i++) {
            address extraPool = IRewardStaking(mainPool).extraRewards(i);
            address extraToken = IRewardStaking(extraPool).rewardToken();
            //from pool 151, extra reward tokens are wrapped
            if(convexPoolId >= 151){
                extraToken = ITokenWrapper(extraToken).token();
            }
            if(extraToken == cvx){
                //update cvx reward pool address
                rewards[CVX_INDEX].reward_pool = extraPool;
            }else if(registeredRewards[extraToken] == 0){
                //add new token to list
                rewards.push(
                    RewardType({
                        reward_token: extraToken,
                        reward_pool: extraPool,
                        reward_integral: 0,
                        reward_remaining: 0
                    })
                );
                registeredRewards[extraToken] = rewards.length; //mark registered at index+1
                emit RewardAdded(extraToken);
            }
        }
    }

    function addTokenReward(address _token) public onlyOwner {

        //check if already registered
        if(registeredRewards[_token] == 0){
            //add new token to list
            rewards.push(
                RewardType({
                    reward_token: _token,
                    reward_pool: address(0),
                    reward_integral: 0,
                    reward_remaining: 0
                })
            );
            //add to registered map
            registeredRewards[_token] = rewards.length; //mark registered at index+1
            //send to self to warmup state
            IERC20(_token).transfer(address(this),0);   
            emit RewardAdded(_token);
        }else{
            //get previous used index of given token
            //this ensures that reviving can only be done on the previous used slot
            uint256 index = registeredRewards[_token];
            if(index > 0){
                //index is registeredRewards minus one
                RewardType storage reward = rewards[index-1];
                //check if it was invalidated
                if(reward.reward_token == address(0)){
                    //revive
                    reward.reward_token = _token;
                    emit RewardAdded(_token);
                }
            }
        }
    }

    //allow invalidating a reward if the token causes trouble in calcRewardIntegral
    function invalidateReward(address _token) public onlyOwner {
        uint256 index = registeredRewards[_token];
        if(index > 0){
            //index is registered rewards minus one
            RewardType storage reward = rewards[index-1];
            require(reward.reward_token == _token, "!mismatch");
            //set reward token address to 0, integral calc will now skip
            reward.reward_token = address(0);
            emit RewardInvalidated(_token);
        }
    }

    function setHook(address _hook) external onlyOwner{
        rewardHook = _hook;
        emit HookSet(_hook);
    }

    function rewardLength() external view returns(uint256) {
        return rewards.length;
    }

    function _getDepositedBalance(address _account) internal virtual view returns(uint256) {
        if (_account == address(0) || _account == collateralVault) {
            return 0;
        }
        //get balance from collateralVault

        return balanceOf(_account);
    }

    function _getTotalSupply() internal virtual view returns(uint256){

        //override and add any supply needed (interest based growth)

        return totalSupply();
    }

    function _calcRewardIntegral(uint256 _index, address[2] memory _accounts, uint256[2] memory _balances, uint256 _supply, bool _isClaim) internal{
         RewardType storage reward = rewards[_index];
         if(reward.reward_token == address(0)){
            return;
         }

        //get difference in balance and remaining rewards
        //getReward is unguarded so we use reward_remaining to keep track of how much was actually claimed
        uint256 bal = IERC20(reward.reward_token).balanceOf(address(this));

        if (_supply > 0 && bal.sub(reward.reward_remaining) > 0) {
            reward.reward_integral = reward.reward_integral + (bal.sub(reward.reward_remaining).mul(1e20).div(_supply));
        }

        //update user integrals
        for (uint256 u = 0; u < _accounts.length; u++) {
            //do not give rewards to address 0
            if (_accounts[u] == address(0)) continue;
            if (_accounts[u] == collateralVault) continue;
            if(_isClaim && u != 0) continue; //only update/claim for first address and use second as forwarding

            uint userI = reward.reward_integral_for[_accounts[u]];
            if(_isClaim || userI < reward.reward_integral){
                if(_isClaim){
                    uint256 receiveable = reward.claimable_reward[_accounts[u]].add(_balances[u].mul( reward.reward_integral.sub(userI)).div(1e20));
                    if(receiveable > 0){
                        reward.claimable_reward[_accounts[u]] = 0;
                        //cheat for gas savings by transfering to the second index in accounts list
                        //if claiming only the 0 index will update so 1 index can hold forwarding info
                        //guaranteed to have an address in u+1 so no need to check
                        IERC20(reward.reward_token).safeTransfer(_accounts[u+1], receiveable);
                        bal = bal.sub(receiveable);
                    }
                }else{
                    reward.claimable_reward[_accounts[u]] = reward.claimable_reward[_accounts[u]].add(_balances[u].mul( reward.reward_integral.sub(userI)).div(1e20));
                }
                reward.reward_integral_for[_accounts[u]] = reward.reward_integral;
            }
        }

        //update remaining reward here since balance could have changed if claiming
        if(bal != reward.reward_remaining){
            reward.reward_remaining = bal;
        }
    }

    function _checkpoint(address[2] memory _accounts) internal nonReentrant{
        uint256 supply = _getTotalSupply();
        uint256[2] memory depositedBalance;
        depositedBalance[0] = _getDepositedBalance(_accounts[0]);
        depositedBalance[1] = _getDepositedBalance(_accounts[1]);
        
        //just in case, dont claim rewards directly if shutdown
        //can still technically claim via unguarded calls but skipping here
        //protects against outside calls reverting
        if(!isShutdown){
            IRewardStaking(convexPool).getReward(address(this), true);
        }
        _claimExtras();

        uint256 rewardCount = rewards.length;
        for (uint256 i = 0; i < rewardCount; i++) {
           _calcRewardIntegral(i,_accounts,depositedBalance,supply,false);
        }
        emit UserCheckpoint(_accounts[0],_accounts[1]);
    }

    function _checkpointAndClaim(address[2] memory _accounts) internal nonReentrant{
        uint256 supply = _getTotalSupply();
        uint256[2] memory depositedBalance;
        depositedBalance[0] = _getDepositedBalance(_accounts[0]); //only do first slot
        
        //just in case, dont claim rewards directly if shutdown
        //can still technically claim via unguarded calls but skipping here
        //protects against outside calls reverting
        if(!isShutdown){
            IRewardStaking(convexPool).getReward(address(this), true);
        }
        _claimExtras();

        uint256 rewardCount = rewards.length;
        for (uint256 i = 0; i < rewardCount; i++) {
           _calcRewardIntegral(i,_accounts,depositedBalance,supply,true);
        }
        emit UserCheckpoint(_accounts[0],_accounts[1]);
    }

    //claim any rewards not part of the convex pool
    function _claimExtras() internal virtual{
        //override and add any external reward claiming
        if(rewardHook != address(0)){
            try IRewardHook(rewardHook).onRewardClaim(){
            }catch{}
        }
    }

    function user_checkpoint(address _account) external returns(bool) {
        _checkpoint([_account, address(0)]);
        return true;
    }

    function totalBalanceOf(address _account) external view returns(uint256){
        return _getDepositedBalance(_account);
    }

    //run earned as a mutable function to claim everything before calculating earned rewards
    function earned(address _account) external returns(EarnedData[] memory claimable) {
        //checkpoint to pull in and tally new rewards
        _checkpoint([_account, address(0)]);
        return _earned(_account);
    }

    function _earned(address _account) internal view returns(EarnedData[] memory claimable) {

        uint256 rewardCount = rewards.length;
        claimable = new EarnedData[](rewardCount);

        for (uint256 i = 0; i < rewardCount; i++) {
            RewardType storage reward = rewards[i];
            if(reward.reward_token == address(0)){
                continue;
            }

            claimable[i].amount = reward.claimable_reward[_account];
            claimable[i].token = reward.reward_token;
        }
        return claimable;
    }

    //set any claimed rewards to automatically go to a different address
    //set address to zero to disable
    function setRewardRedirect(address _to) external nonReentrant{
        rewardRedirect[msg.sender] = _to;
        emit RewardRedirected(msg.sender, _to);
    }

    function getReward(address _account) external {
        //check if there is a redirect address
        if(rewardRedirect[_account] != address(0)){
            _checkpoint([_account, rewardRedirect[_account]]);
        }else{
            //claim directly in checkpoint logic to save a bit of gas
            _checkpoint([_account, _account]);
        }
    }

    function getReward(address _account, address _forwardTo) external {
        require(msg.sender == _account, "!self");
        //claim directly in checkpoint logic to save a bit of gas
        //pack forwardTo into account array to save gas so that a proxy etc doesnt have to double transfer
        _checkpointAndClaim([_account,_forwardTo]);
    }

    //deposit a curve token
    function deposit(uint256 _amount, address _to) external {
        require(!isShutdown, "shutdown");

        //dont need to call checkpoint since _mint() will

        if (_amount > 0) {
            _mint(_to, _amount);
            IERC20(curveToken).safeTransferFrom(msg.sender, address(this), _amount);
            IConvexDeposits(convexBooster).deposit(convexPoolId, _amount, true);
        }

        emit Deposited(msg.sender, _to, _amount, true);
    }

    //stake a convex token
    function stake(uint256 _amount, address _to) external {
        require(!isShutdown, "shutdown");

        //dont need to call checkpoint since _mint() will

        if (_amount > 0) {
            _mint(_to, _amount);
            IERC20(convexToken).safeTransferFrom(msg.sender, address(this), _amount);
            IRewardStaking(convexPool).stake(_amount);
        }

        emit Deposited(msg.sender, _to, _amount, false);
    }

    //withdraw to convex deposit token
    function withdraw(uint256 _amount) external {

        //dont need to call checkpoint since _burn() will

        if (_amount > 0) {
            _burn(msg.sender, _amount);
            IRewardStaking(convexPool).withdraw(_amount, false);
            IERC20(convexToken).safeTransfer(msg.sender, _amount);
        }

        emit Withdrawn(msg.sender, _amount, false);
    }

    //withdraw to underlying curve lp token
    function withdrawAndUnwrap(uint256 _amount) external {
        
        //dont need to call checkpoint since _burn() will

        if (_amount > 0) {
            _burn(msg.sender, _amount);
            IRewardStaking(convexPool).withdrawAndUnwrap(_amount, false);
            IERC20(curveToken).safeTransfer(msg.sender, _amount);
        }

        //events
        emit Withdrawn(msg.sender, _amount, true);
    }

    function _beforeTokenTransfer(address _from, address _to, uint256 _amount) internal override {
        _checkpoint([_from, _to]);
    }

    //helper function
    function earmarkRewards() external returns(bool){
        return IBooster(convexBooster).earmarkRewards(convexPoolId);
    }
}
// File: /contracts/interfaces/ITokenWrapper.sol



// SPDX-License-Identifier: MIT
pragma solidity 0.6.12;

interface ITokenWrapper {
   function token() external view returns (address);
}
// File: /contracts/interfaces/IRewardStaking.sol



// SPDX-License-Identifier: MIT
pragma solidity 0.6.12;

interface IRewardStaking {
    function stakeFor(address, uint256) external;
    function stake( uint256) external;
    function withdraw(uint256 amount, bool claim) external;
    function withdrawAndUnwrap(uint256 amount, bool claim) external;
    function earned(address account) external view returns (uint256);
    function getReward() external;
    function getReward(address _account, bool _claimExtras) external;
    function extraRewardsLength() external view returns (uint256);
    function extraRewards(uint256 _pid) external view returns (address);
    function rewardToken() external view returns (address);
    function balanceOf(address _account) external view returns (uint256);
    function rewardRate() external view returns(uint256);
    function totalSupply() external view returns(uint256);
}
// File: /contracts/interfaces/IRewardHook.sol



// SPDX-License-Identifier: MIT

pragma solidity 0.6.12;

interface IRewardHook {
    function onRewardClaim() external;
}
// File: /contracts/interfaces/IProxyFactory.sol



// SPDX-License-Identifier: MIT
pragma solidity 0.6.12;

interface IProxyFactory {
    function clone(address _target) external returns(address);
}
// File: /contracts/interfaces/IOwner.sol



// SPDX-License-Identifier: MIT
pragma solidity 0.6.12;

interface IOwner {
    function setPendingOwner(address _powner) external;
    function acceptPendingOwner() external;
    function owner() external view returns(address);
    function pendingOwner() external view returns(address);
}
// File: /contracts/interfaces/ICvx.sol



// SPDX-License-Identifier: MIT
pragma solidity 0.6.12;

interface ICvx {
    function reductionPerCliff() external view returns(uint256);
    function totalSupply() external view returns(uint256);
    function totalCliffs() external view returns(uint256);
    function maxSupply() external view returns(uint256);
}
// File: /contracts/interfaces/IConvexDeposits.sol



// SPDX-License-Identifier: MIT
pragma solidity 0.6.12;

interface IConvexDeposits {
    function deposit(uint256 _pid, uint256 _amount, bool _stake) external returns(bool);
    function deposit(uint256 _amount, bool _lock, address _stakeAddress) external;
}
// File: /contracts/interfaces/IBooster.sol



// SPDX-License-Identifier: MIT
pragma solidity 0.6.12;

interface IBooster {
    function owner() external view returns(address);
    function setVoteDelegate(address _voteDelegate) external;
    function vote(uint256 _voteId, address _votingAddress, bool _support) external returns(bool);
    function voteGaugeWeight(address[] calldata _gauge, uint256[] calldata _weight ) external returns(bool);
    function poolInfo(uint256 _pid) external view returns(address _lptoken, address _token, address _gauge, address _crvRewards, address _stash, bool _shutdown);
    function earmarkRewards(uint256 _pid) external returns(bool);
    function earmarkFees() external returns(bool);
}
// File: /contracts/interfaces/CvxMining.sol



// SPDX-License-Identifier: MIT
pragma solidity 0.6.12;

import "../interfaces/ICvx.sol";

library CvxMining{
    ICvx public constant cvx = ICvx(0x4e3FBD56CD56c3e72c1403e103b45Db9da5B9D2B);

    function ConvertCrvToCvx(uint256 _amount) external view returns(uint256){
        uint256 supply = cvx.totalSupply();
        uint256 reductionPerCliff = cvx.reductionPerCliff();
        uint256 totalCliffs = cvx.totalCliffs();
        uint256 maxSupply = cvx.maxSupply();

        uint256 cliff = supply / reductionPerCliff;
        //mint if below total cliffs
        if(cliff < totalCliffs){
            //for reduction% take inverse of current cliff
            uint256 reduction = totalCliffs - cliff;
            //reduce
            _amount = _amount * reduction / totalCliffs;

            //supply cap check
            uint256 amtTillMax = maxSupply - supply;
            if(_amount > amtTillMax){
                _amount = amtTillMax;
            }

            //mint
            return _amount;
        }
        return 0;
    }
}
// File: @openzeppelin/contracts/utils/ReentrancyGuard.sol



// 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;
    }
}

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



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <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 GSN meta-transactions the account sending and
 * paying for execution may not be the actual sender (as far as an application
 * is concerned).
 *
 * This contract is only required for intermediate, library-like contracts.
 */
abstract contract Context {
    function _msgSender() internal view virtual returns (address payable) {
        return msg.sender;
    }

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

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



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.2 <0.8.0;

/**
 * @dev Collection of functions related to the address type
 */
library Address {
    /**
     * @dev Returns true if `account` is a contract.
     *
     * [IMPORTANT]
     * ====
     * It is unsafe to assume that an address for which this function returns
     * false is an externally-owned account (EOA) and not a contract.
     *
     * Among others, `isContract` will return false for the following
     * types of addresses:
     *
     *  - an externally-owned account
     *  - a contract in construction
     *  - an address where a contract will be created
     *  - an address where a contract lived, but was destroyed
     * ====
     */
    function isContract(address account) internal view returns (bool) {
        // This method relies on extcodesize, which returns 0 for contracts in
        // construction, since the code is only stored at the end of the
        // constructor execution.

        uint256 size;
        // solhint-disable-next-line no-inline-assembly
        assembly { size := extcodesize(account) }
        return size > 0;
    }

    /**
     * @dev Replacement for Solidity's `transfer`: sends `amount` wei to
     * `recipient`, forwarding all available gas and reverting on errors.
     *
     * https://eips.ethereum.org/EIPS/eip-1884[EIP1884] increases the gas cost
     * of certain opcodes, possibly making contracts go over the 2300 gas limit
     * imposed by `transfer`, making them unable to receive funds via
     * `transfer`. {sendValue} removes this limitation.
     *
     * https://diligence.consensys.net/posts/2019/09/stop-using-soliditys-transfer-now/[Learn more].
     *
     * IMPORTANT: because control is transferred to `recipient`, care must be
     * taken to not create reentrancy vulnerabilities. Consider using
     * {ReentrancyGuard} or the
     * https://solidity.readthedocs.io/en/v0.5.11/security-considerations.html#use-the-checks-effects-interactions-pattern[checks-effects-interactions pattern].
     */
    function sendValue(address payable recipient, uint256 amount) internal {
        require(address(this).balance >= amount, "Address: insufficient balance");

        // solhint-disable-next-line avoid-low-level-calls, avoid-call-value
        (bool success, ) = recipient.call{ value: amount }("");
        require(success, "Address: unable to send value, recipient may have reverted");
    }

    /**
     * @dev Performs a Solidity function call using a low level `call`. A
     * plain`call` is an unsafe replacement for a function call: use this
     * function instead.
     *
     * If `target` reverts with a revert reason, it is bubbled up by this
     * function (like regular Solidity function calls).
     *
     * Returns the raw returned data. To convert to the expected return value,
     * use https://solidity.readthedocs.io/en/latest/units-and-global-variables.html?highlight=abi.decode#abi-encoding-and-decoding-functions[`abi.decode`].
     *
     * Requirements:
     *
     * - `target` must be a contract.
     * - calling `target` with `data` must not revert.
     *
     * _Available since v3.1._
     */
    function functionCall(address target, bytes memory data) internal returns (bytes memory) {
      return functionCall(target, data, "Address: low-level call failed");
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`], but with
     * `errorMessage` as a fallback revert reason when `target` reverts.
     *
     * _Available since v3.1._
     */
    function functionCall(address target, bytes memory data, string memory errorMessage) internal returns (bytes memory) {
        return functionCallWithValue(target, data, 0, errorMessage);
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
     * but also transferring `value` wei to `target`.
     *
     * Requirements:
     *
     * - the calling contract must have an ETH balance of at least `value`.
     * - the called Solidity function must be `payable`.
     *
     * _Available since v3.1._
     */
    function functionCallWithValue(address target, bytes memory data, uint256 value) internal returns (bytes memory) {
        return functionCallWithValue(target, data, value, "Address: low-level call with value failed");
    }

    /**
     * @dev Same as {xref-Address-functionCallWithValue-address-bytes-uint256-}[`functionCallWithValue`], but
     * with `errorMessage` as a fallback revert reason when `target` reverts.
     *
     * _Available since v3.1._
     */
    function functionCallWithValue(address target, bytes memory data, uint256 value, string memory errorMessage) internal returns (bytes memory) {
        require(address(this).balance >= value, "Address: insufficient balance for call");
        require(isContract(target), "Address: call to non-contract");

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

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
     * but performing a static call.
     *
     * _Available since v3.3._
     */
    function functionStaticCall(address target, bytes memory data) internal view returns (bytes memory) {
        return functionStaticCall(target, data, "Address: low-level static call failed");
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-string-}[`functionCall`],
     * but performing a static call.
     *
     * _Available since v3.3._
     */
    function functionStaticCall(address target, bytes memory data, string memory errorMessage) internal view returns (bytes memory) {
        require(isContract(target), "Address: static call to non-contract");

        // solhint-disable-next-line avoid-low-level-calls
        (bool success, bytes memory returndata) = target.staticcall(data);
        return _verifyCallResult(success, returndata, errorMessage);
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
     * but performing a delegate call.
     *
     * _Available since v3.4._
     */
    function functionDelegateCall(address target, bytes memory data) internal returns (bytes memory) {
        return functionDelegateCall(target, data, "Address: low-level delegate call failed");
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-string-}[`functionCall`],
     * but performing a delegate call.
     *
     * _Available since v3.4._
     */
    function functionDelegateCall(address target, bytes memory data, string memory errorMessage) internal returns (bytes memory) {
        require(isContract(target), "Address: delegate call to non-contract");

        // solhint-disable-next-line avoid-low-level-calls
        (bool success, bytes memory returndata) = target.delegatecall(data);
        return _verifyCallResult(success, returndata, errorMessage);
    }

    function _verifyCallResult(bool success, bytes memory returndata, string memory errorMessage) private pure returns(bytes memory) {
        if (success) {
            return returndata;
        } else {
            // Look for revert reason and bubble it up if present
            if (returndata.length > 0) {
                // The easiest way to bubble the revert reason is using memory via assembly

                // solhint-disable-next-line no-inline-assembly
                assembly {
                    let returndata_size := mload(returndata)
                    revert(add(32, returndata), returndata_size)
                }
            } else {
                revert(errorMessage);
            }
        }
    }
}

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



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

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

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

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

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

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

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

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

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

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

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



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

/**
 * @dev Interface of the ERC20 standard as defined in the EIP.
 */
interface IERC20 {
    /**
     * @dev Returns the amount of tokens in existence.
     */
    function totalSupply() external view returns (uint256);

    /**
     * @dev Returns the amount of tokens owned by `account`.
     */
    function balanceOf(address account) external view returns (uint256);

    /**
     * @dev Moves `amount` tokens from the caller's account to `recipient`.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * Emits a {Transfer} event.
     */
    function transfer(address recipient, uint256 amount) external returns (bool);

    /**
     * @dev Returns the remaining number of tokens that `spender` will be
     * allowed to spend on behalf of `owner` through {transferFrom}. This is
     * zero by default.
     *
     * This value changes when {approve} or {transferFrom} are called.
     */
    function allowance(address owner, address spender) external view returns (uint256);

    /**
     * @dev Sets `amount` as the allowance of `spender` over the caller's tokens.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * IMPORTANT: Beware that changing an allowance with this method brings the risk
     * that someone may use both the old and the new allowance by unfortunate
     * transaction ordering. One possible solution to mitigate this race
     * condition is to first reduce the spender's allowance to 0 and set the
     * desired value afterwards:
     * https://github.com/ethereum/EIPs/issues/20#issuecomment-263524729
     *
     * Emits an {Approval} event.
     */
    function approve(address spender, uint256 amount) external returns (bool);

    /**
     * @dev Moves `amount` tokens from `sender` to `recipient` using the
     * allowance mechanism. `amount` is then deducted from the caller's
     * allowance.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * Emits a {Transfer} event.
     */
    function transferFrom(address sender, address recipient, uint256 amount) external returns (bool);

    /**
     * @dev Emitted when `value` tokens are moved from one account (`from`) to
     * another (`to`).
     *
     * Note that `value` may be zero.
     */
    event Transfer(address indexed from, address indexed to, uint256 value);

    /**
     * @dev Emitted when the allowance of a `spender` for an `owner` is set by
     * a call to {approve}. `value` is the new allowance.
     */
    event Approval(address indexed owner, address indexed spender, uint256 value);
}

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



// SPDX-License-Identifier: MIT

pragma solidity >=0.6.0 <0.8.0;

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

/**
 * @dev Implementation of the {IERC20} interface.
 *
 * This implementation is agnostic to the way tokens are created. This means
 * that a supply mechanism has to be added in a derived contract using {_mint}.
 * For a generic mechanism see {ERC20PresetMinterPauser}.
 *
 * TIP: For a detailed writeup see our guide
 * https://forum.zeppelin.solutions/t/how-to-implement-erc20-supply-mechanisms/226[How
 * to implement supply mechanisms].
 *
 * We have followed general OpenZeppelin guidelines: functions revert instead
 * of returning `false` on failure. This behavior is nonetheless conventional
 * and does not conflict with the expectations of ERC20 applications.
 *
 * Additionally, an {Approval} event is emitted on calls to {transferFrom}.
 * This allows applications to reconstruct the allowance for all accounts just
 * by listening to said events. Other implementations of the EIP may not emit
 * these events, as it isn't required by the specification.
 *
 * Finally, the non-standard {decreaseAllowance} and {increaseAllowance}
 * functions have been added to mitigate the well-known issues around setting
 * allowances. See {IERC20-approve}.
 */
contract ERC20 is Context, IERC20 {
    using SafeMath for uint256;

    mapping (address => uint256) private _balances;

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

    uint256 private _totalSupply;

    string private _name;
    string private _symbol;
    uint8 private _decimals;

    /**
     * @dev Sets the values for {name} and {symbol}, initializes {decimals} with
     * a default value of 18.
     *
     * To select a different value for {decimals}, use {_setupDecimals}.
     *
     * All three of these values are immutable: they can only be set once during
     * construction.
     */
    constructor (string memory name_, string memory symbol_) public {
        _name = name_;
        _symbol = symbol_;
        _decimals = 18;
    }

    /**
     * @dev Returns the name of the token.
     */
    function name() public view virtual returns (string memory) {
        return _name;
    }

    /**
     * @dev Returns the symbol of the token, usually a shorter version of the
     * name.
     */
    function symbol() public view virtual returns (string memory) {
        return _symbol;
    }

    /**
     * @dev Returns the number of decimals used to get its user representation.
     * For example, if `decimals` equals `2`, a balance of `505` tokens should
     * be displayed to a user as `5,05` (`505 / 10 ** 2`).
     *
     * Tokens usually opt for a value of 18, imitating the relationship between
     * Ether and Wei. This is the value {ERC20} uses, unless {_setupDecimals} is
     * called.
     *
     * NOTE: This information is only used for _display_ purposes: it in
     * no way affects any of the arithmetic of the contract, including
     * {IERC20-balanceOf} and {IERC20-transfer}.
     */
    function decimals() public view virtual returns (uint8) {
        return _decimals;
    }

    /**
     * @dev See {IERC20-totalSupply}.
     */
    function totalSupply() public view virtual override returns (uint256) {
        return _totalSupply;
    }

    /**
     * @dev See {IERC20-balanceOf}.
     */
    function balanceOf(address account) public view virtual override returns (uint256) {
        return _balances[account];
    }

    /**
     * @dev See {IERC20-transfer}.
     *
     * Requirements:
     *
     * - `recipient` cannot be the zero address.
     * - the caller must have a balance of at least `amount`.
     */
    function transfer(address recipient, uint256 amount) public virtual override returns (bool) {
        _transfer(_msgSender(), recipient, amount);
        return true;
    }

    /**
     * @dev See {IERC20-allowance}.
     */
    function allowance(address owner, address spender) public view virtual override returns (uint256) {
        return _allowances[owner][spender];
    }

    /**
     * @dev See {IERC20-approve}.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     */
    function approve(address spender, uint256 amount) public virtual override returns (bool) {
        _approve(_msgSender(), spender, amount);
        return true;
    }

    /**
     * @dev See {IERC20-transferFrom}.
     *
     * Emits an {Approval} event indicating the updated allowance. This is not
     * required by the EIP. See the note at the beginning of {ERC20}.
     *
     * Requirements:
     *
     * - `sender` and `recipient` cannot be the zero address.
     * - `sender` must have a balance of at least `amount`.
     * - the caller must have allowance for ``sender``'s tokens of at least
     * `amount`.
     */
    function transferFrom(address sender, address recipient, uint256 amount) public virtual override returns (bool) {
        _transfer(sender, recipient, amount);
        _approve(sender, _msgSender(), _allowances[sender][_msgSender()].sub(amount, "ERC20: transfer amount exceeds allowance"));
        return true;
    }

    /**
     * @dev Atomically increases the allowance granted to `spender` by the caller.
     *
     * This is an alternative to {approve} that can be used as a mitigation for
     * problems described in {IERC20-approve}.
     *
     * Emits an {Approval} event indicating the updated allowance.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     */
    function increaseAllowance(address spender, uint256 addedValue) public virtual returns (bool) {
        _approve(_msgSender(), spender, _allowances[_msgSender()][spender].add(addedValue));
        return true;
    }

    /**
     * @dev Atomically decreases the allowance granted to `spender` by the caller.
     *
     * This is an alternative to {approve} that can be used as a mitigation for
     * problems described in {IERC20-approve}.
     *
     * Emits an {Approval} event indicating the updated allowance.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     * - `spender` must have allowance for the caller of at least
     * `subtractedValue`.
     */
    function decreaseAllowance(address spender, uint256 subtractedValue) public virtual returns (bool) {
        _approve(_msgSender(), spender, _allowances[_msgSender()][spender].sub(subtractedValue, "ERC20: decreased allowance below zero"));
        return true;
    }

    /**
     * @dev Moves tokens `amount` from `sender` to `recipient`.
     *
     * This is internal function is equivalent to {transfer}, and can be used to
     * e.g. implement automatic token fees, slashing mechanisms, etc.
     *
     * Emits a {Transfer} event.
     *
     * Requirements:
     *
     * - `sender` cannot be the zero address.
     * - `recipient` cannot be the zero address.
     * - `sender` must have a balance of at least `amount`.
     */
    function _transfer(address sender, address recipient, uint256 amount) internal virtual {
        require(sender != address(0), "ERC20: transfer from the zero address");
        require(recipient != address(0), "ERC20: transfer to the zero address");

        _beforeTokenTransfer(sender, recipient, amount);

        _balances[sender] = _balances[sender].sub(amount, "ERC20: transfer amount exceeds balance");
        _balances[recipient] = _balances[recipient].add(amount);
        emit Transfer(sender, recipient, amount);
    }

    /** @dev Creates `amount` tokens and assigns them to `account`, increasing
     * the total supply.
     *
     * Emits a {Transfer} event with `from` set to the zero address.
     *
     * Requirements:
     *
     * - `to` cannot be the zero address.
     */
    function _mint(address account, uint256 amount) internal virtual {
        require(account != address(0), "ERC20: mint to the zero address");

        _beforeTokenTransfer(address(0), account, amount);

        _totalSupply = _totalSupply.add(amount);
        _balances[account] = _balances[account].add(amount);
        emit Transfer(address(0), account, amount);
    }

    /**
     * @dev Destroys `amount` tokens from `account`, reducing the
     * total supply.
     *
     * Emits a {Transfer} event with `to` set to the zero address.
     *
     * Requirements:
     *
     * - `account` cannot be the zero address.
     * - `account` must have at least `amount` tokens.
     */
    function _burn(address account, uint256 amount) internal virtual {
        require(account != address(0), "ERC20: burn from the zero address");

        _beforeTokenTransfer(account, address(0), amount);

        _balances[account] = _balances[account].sub(amount, "ERC20: burn amount exceeds balance");
        _totalSupply = _totalSupply.sub(amount);
        emit Transfer(account, address(0), amount);
    }

    /**
     * @dev Sets `amount` as the allowance of `spender` over the `owner` s tokens.
     *
     * This internal function is equivalent to `approve`, and can be used to
     * e.g. set automatic allowances for certain subsystems, etc.
     *
     * Emits an {Approval} event.
     *
     * Requirements:
     *
     * - `owner` cannot be the zero address.
     * - `spender` cannot be the zero address.
     */
    function _approve(address owner, address spender, uint256 amount) internal virtual {
        require(owner != address(0), "ERC20: approve from the zero address");
        require(spender != address(0), "ERC20: approve to the zero address");

        _allowances[owner][spender] = amount;
        emit Approval(owner, spender, amount);
    }

    /**
     * @dev Sets {decimals} to a value other than the default one of 18.
     *
     * WARNING: This function should only be called from the constructor. Most
     * applications that interact with token contracts will not expect
     * {decimals} to ever change, and may work incorrectly if it does.
     */
    function _setupDecimals(uint8 decimals_) internal virtual {
        _decimals = decimals_;
    }

    /**
     * @dev Hook that is called before any transfer of tokens. This includes
     * minting and burning.
     *
     * Calling conditions:
     *
     * - when `from` and `to` are both non-zero, `amount` of ``from``'s tokens
     * will be to transferred to `to`.
     * - when `from` is zero, `amount` tokens will be minted for `to`.
     * - when `to` is zero, `amount` of ``from``'s tokens will be burned.
     * - `from` and `to` are never both zero.
     *
     * To learn more about hooks, head to xref:ROOT:extending-contracts.adoc#using-hooks[Using Hooks].
     */
    function _beforeTokenTransfer(address from, address to, uint256 amount) internal virtual { }
}

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



// 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;
    }
}