// File: convex-platform/contracts/contracts/BaseRewardPool.sol



// SPDX-License-Identifier: MIT
pragma solidity 0.6.12;
/**
 *Submitted for verification at Etherscan.io on 2020-07-17
 */

/*
   ____            __   __        __   _
  / __/__ __ ___  / /_ / /  ___  / /_ (_)__ __
 _\ \ / // // _ \/ __// _ \/ -_)/ __// / \ \ /
/___/ \_, //_//_/\__//_//_/\__/ \__//_/ /_\_\
     /___/

* Synthetix: BaseRewardPool.sol
*
* Docs: https://docs.synthetix.io/
*
*
* MIT License
* ===========
*
* Copyright (c) 2020 Synthetix
*
* Permission is hereby granted, free of charge, to any person obtaining a copy
* of this software and associated documentation files (the "Software"), to deal
* in the Software without restriction, including without limitation the rights
* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
* copies of the Software, and to permit persons to whom the Software is
* furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in all
* copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
*/

import "./Interfaces.sol";
import "./interfaces/MathUtil.sol";
import "./utils/Permission.sol";
import "@openzeppelin/contracts-0.6/math/SafeMath.sol";
import "@openzeppelin/contracts-0.6/token/ERC20/IERC20.sol";
import "@openzeppelin/contracts-0.6/utils/Address.sol";
import "@openzeppelin/contracts-0.6/token/ERC20/SafeERC20.sol";


/**
 * @title   BaseRewardPool
 * @author  Synthetix -> ConvexFinance
 * @notice  Unipool rewards contract that is re-deployed from rFactory for each staking pool.
 * @dev     Changes made here by ConvexFinance are to do with the delayed reward allocation. Curve is queued for
 *          rewards and the distribution only begins once the new rewards are sufficiently large, or the epoch
 *          has ended. Additionally, enables hooks for `extraRewards` that can be enabled at any point to
 *          distribute a child reward token (i.e. a secondary one from Curve, or a separate one).
 */
contract BaseRewardPool is Permission {
    using SafeMath for uint256;
    using SafeERC20 for IERC20;

    IERC20 public immutable rewardToken;
    IERC20 public immutable stakingToken;
    uint256 public constant duration = 7 days;

    address public immutable operator;
    address public immutable rewardManager;

    uint256 public immutable pid;
    uint256 public periodFinish = 0;
    uint256 public rewardRate = 0;
    uint256 public lastUpdateTime;
    uint256 public rewardPerTokenStored;
    uint256 public queuedRewards = 0;
    uint256 public currentRewards = 0;
    uint256 public historicalRewards = 0;
    uint256 public constant newRewardRatio = 830;
    uint256 private _totalSupply;
    mapping(address => uint256) public userRewardPerTokenPaid;
    mapping(address => uint256) public rewards;
    mapping(address => uint256) internal _balances;

    address[] public extraRewards;

    event RewardAdded(uint256 reward);
    event Staked(address indexed user, uint256 amount);
    event Withdrawn(address indexed user, uint256 amount);
    event RewardPaid(address indexed user, uint256 reward);
    event Transfer(address indexed from, address indexed to, uint256 value);

    /**
     * @dev This is called directly from RewardFactory
     * @param pid_           Effectively the pool identifier - used in the Booster
     * @param stakingToken_  Pool LP token
     * @param rewardToken_   Crv
     * @param operator_      Booster
     * @param rewardManager_ RewardFactory
     */
    constructor(
        uint256 pid_,
        address stakingToken_,
        address rewardToken_,
        address operator_,
        address rewardManager_
    ) public {
        pid = pid_;
        stakingToken = IERC20(stakingToken_);
        rewardToken = IERC20(rewardToken_);
        operator = operator_;
        rewardManager = rewardManager_;
    }

    function totalSupply() public view virtual returns (uint256) {
        return _totalSupply;
    }

    function balanceOf(address account) public view virtual returns (uint256) {
        return _balances[account];
    }

    function extraRewardsLength() external view returns (uint256) {
        return extraRewards.length;
    }

    function addExtraReward(address _reward) external returns (bool) {
        require(msg.sender == rewardManager, "!authorized");
        require(_reward != address(0), "!reward setting");

        if (extraRewards.length >= 12) {
            return false;
        }

        extraRewards.push(_reward);
        return true;
    }

    function clearExtraRewards() external {
        require(msg.sender == rewardManager, "!authorized");
        delete extraRewards;
    }

    modifier updateReward(address account) {
        rewardPerTokenStored = rewardPerToken();
        lastUpdateTime = lastTimeRewardApplicable();
        if (account != address(0)) {
            rewards[account] = earned(account);
            userRewardPerTokenPaid[account] = rewardPerTokenStored;
        }
        _;
    }

    function lastTimeRewardApplicable() public view returns (uint256) {
        return MathUtil.min(block.timestamp, periodFinish);
    }

    function rewardPerToken() public view returns (uint256) {
        if (totalSupply() == 0) {
            return rewardPerTokenStored;
        }
        return
            rewardPerTokenStored.add(
                lastTimeRewardApplicable().sub(lastUpdateTime).mul(rewardRate).mul(1e18).div(totalSupply())
            );
    }

    function earned(address account) public view returns (uint256) {
        return
            balanceOf(account).mul(rewardPerToken().sub(userRewardPerTokenPaid[account])).div(1e18).add(
                rewards[account]
            );
    }

    function stake(uint256 _amount) public returns (bool) {
        _processStake(_amount, msg.sender);

        stakingToken.safeTransferFrom(msg.sender, address(this), _amount);
        emit Staked(msg.sender, _amount);

        return true;
    }

    function stakeAll() external returns (bool) {
        uint256 balance = stakingToken.balanceOf(msg.sender);
        stake(balance);
        return true;
    }

    function stakeFor(address _for, uint256 _amount) public returns (bool) {
        _processStake(_amount, _for);

        //take away from sender
        stakingToken.safeTransferFrom(msg.sender, address(this), _amount);
        emit Staked(_for, _amount);

        return true;
    }

    /**
     * @dev Generic internal staking function that basically does 3 things: update rewards based
     *      on previous balance, trigger also on any child contracts, then update balances.
     * @param _amount    Units to add to the users balance
     * @param _receiver  Address of user who will receive the stake
     */
    function _processStake(uint256 _amount, address _receiver) internal updateReward(_receiver) {
        require(_amount > 0, "RewardPool : Cannot stake 0");

        //also stake to linked rewards
        for (uint256 i = 0; i < extraRewards.length; i++) {
            IRewards(extraRewards[i]).stake(_receiver, _amount);
        }

        _totalSupply = _totalSupply.add(_amount);
        _balances[_receiver] = _balances[_receiver].add(_amount);

        emit Transfer(address(0), _receiver, _amount);
    }

    function withdraw(uint256 amount, bool claim) public updateReward(msg.sender) returns (bool) {
        require(amount > 0, "RewardPool : Cannot withdraw 0");

        //also withdraw from linked rewards
        for (uint256 i = 0; i < extraRewards.length; i++) {
            IRewards(extraRewards[i]).withdraw(msg.sender, amount);
        }

        _totalSupply = _totalSupply.sub(amount);
        _balances[msg.sender] = _balances[msg.sender].sub(amount);

        stakingToken.safeTransfer(msg.sender, amount);
        emit Withdrawn(msg.sender, amount);

        if (claim) {
            getReward(msg.sender, true);
        }

        emit Transfer(msg.sender, address(0), amount);

        return true;
    }

    function withdrawAll(bool claim) external {
        withdraw(_balances[msg.sender], claim);
    }

    function withdrawAndUnwrap(uint256 amount, bool claim) public returns (bool) {
        _withdrawAndUnwrapTo(amount, msg.sender, msg.sender);
        //get rewards too
        if (claim) {
            getReward(msg.sender, true);
        }
        return true;
    }

    function _withdrawAndUnwrapTo(
        uint256 amount,
        address from,
        address receiver
    ) internal updateReward(from) returns (bool) {
        //also withdraw from linked rewards
        for (uint256 i = 0; i < extraRewards.length; i++) {
            IRewards(extraRewards[i]).withdraw(from, amount);
        }

        _totalSupply = _totalSupply.sub(amount);
        _balances[from] = _balances[from].sub(amount);

        //tell operator to withdraw from here directly to user
        IDeposit(operator).withdrawTo(pid, amount, receiver);
        emit Withdrawn(from, amount);

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

        return true;
    }

    function withdrawAllAndUnwrap(bool claim) external {
        withdrawAndUnwrap(_balances[msg.sender], claim);
    }

    /**
     * @dev Gives a staker their rewards, with the option of claiming extra rewards
     * @param _account     Account for which to claim
     * @param _claimExtras Get the child rewards too?
     */
    function getReward(address _account, bool _claimExtras) public updateReward(_account) returns (bool) {
        uint256 reward = earned(_account);
        if (reward > 0) {
            rewards[_account] = 0;
            rewardToken.safeTransfer(_account, reward);
            IDeposit(operator).rewardClaimed(pid, _account, reward);
            emit RewardPaid(_account, reward);
        }

        //also get rewards from linked rewards
        if (_claimExtras) {
            for (uint256 i = 0; i < extraRewards.length; i++) {
                IRewards(extraRewards[i]).getReward(_account);
            }
        }
        return true;
    }

    /**
     * @dev Called by a staker to get their allocated rewards
     */
    function getReward() external returns (bool) {
        getReward(msg.sender, true);
        return true;
    }

    /**
     * @dev Sends oLIT to OptionsExerciser for converting it to LIT or liqLit, LIQ and extra rewards are sent to the user
     * @param _account      Account for which to claim
     * @param _claimExtras  Get the child rewards too?
     * @return rewardAmount oLIT amount claimed as reward
     */
    function getRewardFor(address _account, bool _claimExtras)
        public
        updateReward(_account)
        returns (uint256 rewardAmount)
    {
        require(hasPermission(_account, msg.sender), "permission not granted");

        uint256 reward = earned(_account);
        if (reward > 0) {
            rewards[_account] = 0;
            //olit to optionsExerciser
            rewardToken.safeTransfer(msg.sender, reward);
            //liq to account
            IDeposit(operator).rewardClaimed(pid, _account, reward);
            emit RewardPaid(_account, reward);
        }

        //also get rewards from linked rewards
        if (_claimExtras) {
            for (uint256 i = 0; i < extraRewards.length; i++) {
                IRewards(extraRewards[i]).getReward(_account);
            }
        }
        return reward;
    }

    /**
     * @dev Processes queued rewards in isolation, providing the period has finished.
     *      This allows a cheaper way to trigger rewards on low value pools.
     */
    function processIdleRewards() external {
        if (block.timestamp >= periodFinish && queuedRewards > 0) {
            notifyRewardAmount(queuedRewards);
            queuedRewards = 0;
        }
    }

    /**
     * @dev Called by the booster to allocate new Crv rewards to this pool
     *      Curve is queued for rewards and the distribution only begins once the new rewards are sufficiently
     *      large, or the epoch has ended.
     */
    function queueNewRewards(uint256 _rewards) external returns (bool) {
        require(msg.sender == operator, "!authorized");

        _rewards = _rewards.add(queuedRewards);

        if (block.timestamp >= periodFinish) {
            notifyRewardAmount(_rewards);
            queuedRewards = 0;
            return true;
        }

        //et = now - (finish-duration)
        uint256 elapsedTime = block.timestamp.sub(periodFinish.sub(duration));
        //current at now: rewardRate * elapsedTime
        uint256 currentAtNow = rewardRate * elapsedTime;
        uint256 queuedRatio = currentAtNow.mul(1000).div(_rewards);

        //uint256 queuedRatio = currentRewards.mul(1000).div(_rewards);
        if (queuedRatio < newRewardRatio) {
            notifyRewardAmount(_rewards);
            queuedRewards = 0;
        } else {
            queuedRewards = _rewards;
        }
        return true;
    }

    function notifyRewardAmount(uint256 reward) internal updateReward(address(0)) {
        historicalRewards = historicalRewards.add(reward);
        if (block.timestamp >= periodFinish) {
            rewardRate = reward.div(duration);
        } else {
            uint256 remaining = periodFinish.sub(block.timestamp);
            uint256 leftover = remaining.mul(rewardRate);
            reward = reward.add(leftover);
            rewardRate = reward.div(duration);
        }
        currentRewards = reward;
        lastUpdateTime = block.timestamp;
        periodFinish = block.timestamp.add(duration);
        emit RewardAdded(reward);
    }
}

// File: convex-platform/contracts/contracts/Interfaces.sol



// SPDX-License-Identifier: MIT
pragma solidity 0.6.12;



interface ICurveGauge {
    function deposit(uint256) external;
    function balanceOf(address) external view returns (uint256);
    function withdraw(uint256) external;
    function claim_rewards() external;
    function reward_tokens(uint256) external view returns(address);//v2
    function rewarded_token() external view returns(address);//v1
    function lp_token() external view returns(address);
}

interface ICurveVoteEscrow {
    function create_lock(uint256, uint256) external;
    function increase_amount(uint256) external;
    function increase_unlock_time(uint256) external;
    function withdraw() external;
    function smart_wallet_checker() external view returns (address);
    function commit_smart_wallet_checker(address) external;
    function apply_smart_wallet_checker() external;
}

interface IWalletChecker {
    function check(address) external view returns (bool);
    function approveWallet(address) external;
    function dao() external view returns (address);
}

interface IVoting{
    function vote(uint256, bool, bool) external; //voteId, support, executeIfDecided
    function getVote(uint256) external view returns(bool,bool,uint64,uint64,uint64,uint64,uint256,uint256,uint256,bytes memory); 
    function vote_for_gauge_weights(address,uint256) external;
}

interface IMinter{
    function mint(address) external;
}

interface IStaker{
    function deposit(address, address) external returns (bool);
    function withdraw(address) external returns (uint256);
    function withdraw(address, address, uint256) external returns (bool);
    function withdrawAll(address, address) external returns (bool);
    function createLock(uint256, uint256) external returns(bool);
    function increaseAmount(uint256) external returns(bool);
    function increaseTime(uint256) external returns(bool);
    function release() external returns(bool);
    function claimCrv(address) external returns (uint256);
    function claimRewards(address) external returns(bool);
    function claimFees(address,address) external returns (uint256);
    function setStashAccess(address, bool) external returns (bool);
    function vote(uint256,address,bool) external returns(bool);
    function voteGaugeWeight(address,uint256) external returns(bool);
    function balanceOfPool(address) external view returns (uint256);
    function operator() external view returns (address);
    function execute(address _to, uint256 _value, bytes calldata _data) external returns (bool, bytes memory);
    function setVote(bytes32 hash, bool valid) external;
    function migrate(address to) external;
}

interface IRewards{
    function stake(address, uint256) external;
    function stakeFor(address, uint256) external;
    function withdraw(address, uint256) external;
    function exit(address) external;
    function getReward(address) external;
    function queueNewRewards(uint256) external;
    function notifyRewardAmount(uint256) external;
    function addExtraReward(address) external;
    function extraRewardsLength() external view returns (uint256);
    function stakingToken() external view returns (address);
    function rewardToken() external view returns(address);
    function earned(address account) external view returns (uint256);
}

interface IStash{
    function stashRewards() external returns (bool);
    function processStash() external returns (bool);
    function claimRewards() external returns (bool);
    function initialize(uint256 _pid, address _operator, address _staker, address _gauge, address _rewardFactory) external;
    function setExtraReward(address) external;
}

interface IFeeDistributor {
    function claimToken(address user, address token) external returns (uint256);
    function claimTokens(address user, address[] calldata tokens) external returns (uint256[] memory);
    function getTokenTimeCursor(address token) external view returns (uint256);
}

interface ITokenMinter{
    function mint(address,uint256) external;
    function burn(address,uint256) external;
}

interface IDeposit{
    function isShutdown() external view returns(bool);
    function balanceOf(address _account) external view returns(uint256);
    function totalSupply() external view returns(uint256);
    function poolInfo(uint256) external view returns(address,address,address,address,address, bool);
    function rewardClaimed(uint256,address,uint256) external;
    function withdrawTo(uint256,uint256,address) external;
    function claimRewards(uint256,address) external returns(bool);
    function rewardArbitrator() external returns(address);
    function setGaugeRedirect(uint256 _pid) external returns(bool);
    function owner() external returns(address);
    function deposit(uint256 _pid, uint256 _amount, bool _stake) external returns(bool);
}

interface ICrvDeposit{
    function deposit(uint256, bool) external;
    function lockIncentive() external view returns(uint256);
}

interface IRewardFactory{
    function setAccess(address,bool) external;
    function CreateCrvRewards(uint256,address,address) external returns(address);
    function CreateTokenRewards(address,address,address) external returns(address);
    function activeRewardCount(address) external view returns(uint256);
    function addActiveReward(address,uint256) external returns(bool);
    function removeActiveReward(address,uint256) external returns(bool);
}

interface IStashFactory{
    function CreateStash(uint256,address,address,uint256) external returns(address);
    function setImplementation(address, address, address) external;
}

interface ITokenFactory{
    function CreateDepositToken(address) external returns(address);
}

interface IPools{
    function addPool(address _lptoken, address _gauge, uint256 _stashVersion) external returns(bool);
    function forceAddPool(address _lptoken, address _gauge, uint256 _stashVersion) external returns(bool);
    function shutdownPool(uint256 _pid) external returns(bool);
    function poolInfo(uint256) external view returns(address,address,address,address,address,bool);
    function poolLength() external view returns (uint256);
    function gaugeMap(address) external view returns(bool);
    function setPoolManager(address _poolM) external;
    function shutdownSystem() external;
    function setUsedAddress(address[] memory) external;
}

interface IVestedEscrow{
    function fund(address[] calldata _recipient, uint256[] calldata _amount) external returns(bool);
}

interface IRewardDeposit {
    function addReward(address, uint256) external;
}

interface ILocker {
    function lock(address _account, uint256 _amount) external;
    function checkpointEpoch() external;
    function epochCount() external view returns (uint256);
    function balanceAtEpochOf(uint256 _epoch, address _user) external view returns (uint256 amount);
    function totalSupplyAtEpoch(uint256 _epoch) external view returns (uint256 supply);
    function queueNewRewards(address _rewardsToken, uint256 reward) external;
    function getReward(address _account, bool _stake) external;
    function getReward(address _account) external;
}

// File: convex-platform/contracts/contracts/interfaces/MathUtil.sol



// SPDX-License-Identifier: MIT
pragma solidity 0.6.12;

/**
 * @dev Standard math utilities missing in the Solidity language.
 */
library MathUtil {
    /**
     * @dev Returns the smallest of two numbers.
     */
    function min(uint256 a, uint256 b) internal pure returns (uint256) {
        return a < b ? a : b;
    }
}
// File: convex-platform/contracts/contracts/utils/Permission.sol



// SPDX-License-Identifier: MIT
pragma solidity 0.6.12;

/**
 * @title   Permission
 * @author  Liquis Finance
 * @notice  A simple permissions system giving a `caller` the ability to act on behalf of `owner`
 * @dev     Other than ERC20 Allowances, Permissions are boolean giving `caller` the ability
 *          to call a specific contract function without further controls. 
 *          Permission are thought to allow users to give peripheral contracts permission to act
 *          on their behalf in order to improve UX e.g. around claiming rewards.
 */
abstract contract Permission {

    event ModifyPermission(address owner, address caller, bool grant);

    /// @dev Specify whether `caller` can act on behalf of `owner`
    mapping(address => mapping(address => bool)) private _permitted;

    /**
     * @notice Allow (or revoke allowance) `caller` to act on behalf of `msg.sender`
     * @param caller Address of the `caller`
     * @param permitted Allow (true) or revoke (false) permission
     */
    function modifyPermission(address caller, bool permitted) external {
        _permitted[msg.sender][caller] = permitted;
        emit ModifyPermission(msg.sender, caller, permitted);
    }

    /** 
     * @notice Checks permission of `caller` to act on behalf of `owner`
     * @param owner Address of the `owner`
     * @param caller Address of the `caller`
     * @return permission Whether `caller` has the permission
     */
    function hasPermission(address owner, address caller) public view returns (bool) {
        return owner == caller || _permitted[owner][caller];
    }
}

// File: @openzeppelin/contracts-0.6/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;
    }
}

// File: @openzeppelin/contracts-0.6/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-0.6/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-0.6/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");
        }
    }
}