// File: /Users/eric/git/yflusd-protocol/contracts/Treasury.sol



pragma solidity ^0.6.0;

import '@openzeppelin/contracts/math/Math.sol';
import '@openzeppelin/contracts/token/ERC20/IERC20.sol';
import '@openzeppelin/contracts/token/ERC20/SafeERC20.sol';
import '@openzeppelin/contracts/utils/ReentrancyGuard.sol';

import {ICurve} from './curve/Curve.sol';
import {IOracle} from './interfaces/IOracle.sol';
import {ILinkswapPriceOracle} from './interfaces/ILinkswapPriceOracle.sol';
import {IBoardroom} from './interfaces/IBoardroom.sol';
import {IBasisAsset} from './interfaces/IBasisAsset.sol';
import {ISimpleERCFund} from './interfaces/ISimpleERCFund.sol';
import {Babylonian} from './lib/Babylonian.sol';
import {FixedPoint} from './lib/FixedPoint.sol';
import {Safe112} from './lib/Safe112.sol';
import {Operator} from './owner/Operator.sol';
import {Epoch} from './utils/Epoch.sol';
import {ContractGuard} from './utils/ContractGuard.sol';

/**
 * @title YFL USD Treasury contract
 * @notice Monetary policy logic to adjust supplies of YFL USD assets
 * @author Tec05
 */
contract Treasury is ContractGuard, Epoch {
    using FixedPoint for *;
    using SafeERC20 for IERC20;
    using Address for address;
    using SafeMath for uint256;
    using Safe112 for uint112;

    /* ========== STATE VARIABLES ========== */

    // ========== FLAGS
    bool public migrated = false;
    bool public initialized = false;

    // ========== CORE
    address public weth;
    address public fund;
    address public cash;
    address public bond;
    address public share;
    address public curve;
    address public boardroom;

    address public bondOracle;
    address public seigniorageOracle;
    address public linkswapPriceOracle;

    // ========== PARAMS
    uint256 public cashPriceOne;

    uint256 public lastBondOracleEpoch = 0;
    uint256 public bondCap = 0;
    uint256 public accumulatedSeigniorage = 0;
    uint256 public fundAllocationRate = 2; // %

    /* ========== CONSTRUCTOR ========== */

    constructor(
        address _weth,
        address _cash,
        address _bond,
        address _share,
        address _bondOracle,
        address _seigniorageOracle,
        address _linkswapPriceOracle,
        address _boardroom,
        address _fund,
        address _curve,
        uint256 _startTime
    ) public Epoch(1 days, _startTime, 0) {
        weth = _weth;
        cash = _cash;
        bond = _bond;
        share = _share;
        curve = _curve;
        bondOracle = _bondOracle;
        seigniorageOracle = _seigniorageOracle;
        linkswapPriceOracle = _linkswapPriceOracle;

        boardroom = _boardroom;
        fund = _fund;

        cashPriceOne = 10**18;
    }

    /* =================== Modifier =================== */

    modifier checkMigration {
        require(!migrated, 'Treasury: migrated');

        _;
    }

    modifier checkOperator {
        require(
            IBasisAsset(cash).operator() == address(this) &&
                IBasisAsset(bond).operator() == address(this) &&
                IBasisAsset(share).operator() == address(this) &&
                Operator(boardroom).operator() == address(this),
            'Treasury: need more permission'
        );

        _;
    }

    modifier updatePrice {
        _;

        _updateCashPrice();
    }

    /* ========== VIEW FUNCTIONS ========== */

    // budget
    function getReserve() public view returns (uint256) {
        return accumulatedSeigniorage;
    }

    function circulatingSupply() public view returns (uint256) {
        return IERC20(cash).totalSupply().sub(accumulatedSeigniorage);
    }

    function getCeilingPrice() public view returns (uint256) {
        return ICurve(curve).calcCeiling(circulatingSupply());
    }

    // oracle
    function getBondOraclePrice() public view returns (uint256) {
        return _getCashPrice(bondOracle);
    }

    function getSeigniorageOraclePrice() public view returns (uint256) {
        return _getCashPrice(seigniorageOracle);
    }
    
    function getLinkswapPriceOracleUsdPrice(address token, uint256 tokenAmount) public view returns (uint256) {
        return _getLinkswapPriceOracleUsdPrice(token, tokenAmount);
    }

    function _getLinkswapPriceOracleUsdPrice(address token, uint256 tokenAmount) internal view returns (uint256) {
        return ILinkswapPriceOracle(linkswapPriceOracle).calculateUsdAmountFromTokenAmount(token, tokenAmount);
    }

    function _getCashPrice(address oracle) internal view returns (uint256) {
        try IOracle(oracle).consult(cash, 1e18) returns (uint256 price) {
            uint256 wethUsdPrice = _getLinkswapPriceOracleUsdPrice(weth, 1e18);
            return price.mul(wethUsdPrice).div(1e8); // price from price oracle is 8 dp
        } catch {
            revert('Treasury: failed to consult cash price from the oracle');
        }
    }

    /* ========== GOVERNANCE ========== */

    // MIGRATION
    function initialize() public checkOperator {
        require(!initialized, 'Treasury: initialized');

        // set accumulatedSeigniorage to it's balance
        accumulatedSeigniorage = IERC20(cash).balanceOf(address(this));

        initialized = true;
        emit Initialized(msg.sender, block.number);
    }

    function migrate(address target) public onlyOperator checkOperator {
        require(!migrated, 'Treasury: migrated');

        // cash
        Operator(cash).transferOperator(target);
        Operator(cash).transferOwnership(target);
        IERC20(cash).transfer(target, IERC20(cash).balanceOf(address(this)));

        // bond
        Operator(bond).transferOperator(target);
        Operator(bond).transferOwnership(target);
        IERC20(bond).transfer(target, IERC20(bond).balanceOf(address(this)));

        // share
        Operator(share).transferOperator(target);
        Operator(share).transferOwnership(target);
        IERC20(share).transfer(target, IERC20(share).balanceOf(address(this)));

        migrated = true;
        emit Migration(target);
    }

    // FUND
    function setFund(address newFund) public onlyOperator {
        address oldFund = fund;
        fund = newFund;
        emit ContributionPoolChanged(msg.sender, oldFund, newFund);
    }

    function setFundAllocationRate(uint256 newRate) public onlyOperator {
        uint256 oldRate = fundAllocationRate;
        fundAllocationRate = newRate;
        emit ContributionPoolRateChanged(msg.sender, oldRate, newRate);
    }

    // ORACLE
    function setBondOracle(address newOracle) public onlyOperator {
        address oldOracle = bondOracle;
        bondOracle = newOracle;
        emit BondOracleChanged(msg.sender, oldOracle, newOracle);
    }

    function setSeigniorageOracle(address newOracle) public onlyOperator {
        address oldOracle = seigniorageOracle;
        seigniorageOracle = newOracle;
        emit SeigniorageOracleChanged(msg.sender, oldOracle, newOracle);
    }
    
    function setLinkswapPriceOracle(address newOracle) public onlyOperator {
        address oldOracle = linkswapPriceOracle;
        linkswapPriceOracle = newOracle;
        emit LinkswapPriceOracleChanged(msg.sender, oldOracle, newOracle);
    }

    // TWEAK
    function setCeilingCurve(address newCurve) public onlyOperator {
        address oldCurve = newCurve;
        curve = newCurve;
        emit CeilingCurveChanged(msg.sender, oldCurve, newCurve);
    }

    /* ========== MUTABLE FUNCTIONS ========== */

    function _updateConversionLimit(uint256 cashPrice) internal {
        uint256 currentEpoch = Epoch(bondOracle).getLastEpoch(); // lastest update time
        if (lastBondOracleEpoch != currentEpoch) {
            uint256 percentage = cashPriceOne.sub(cashPrice);
            uint256 bondSupply = IERC20(bond).totalSupply();

            bondCap = circulatingSupply().mul(percentage).div(1e18);
            bondCap = bondCap.sub(Math.min(bondCap, bondSupply));

            lastBondOracleEpoch = currentEpoch;
        }
    }

    function _updateCashPrice() internal {
        if (Epoch(bondOracle).callable()) {
            try IOracle(bondOracle).update() {} catch {}
        }
        if (Epoch(seigniorageOracle).callable()) {
            try IOracle(seigniorageOracle).update() {} catch {}
        }
    }

    function buyBonds(uint256 amount, uint256 targetPrice)
        external
        onlyOneBlock
        checkMigration
        checkStartTime
        checkOperator
        updatePrice
    {
        require(amount > 0, 'Treasury: cannot purchase bonds with zero amount');

        uint256 cashPrice = _getCashPrice(bondOracle);
        require(cashPrice <= targetPrice, 'Treasury: cash price moved');
        require(
            cashPrice < cashPriceOne, // price < $1
            'Treasury: cashPrice not eligible for bond purchase'
        );
        _updateConversionLimit(cashPrice);

        amount = Math.min(amount, bondCap.mul(cashPrice).div(1e18));
        require(amount > 0, 'Treasury: amount exceeds bond cap');

        IBasisAsset(cash).burnFrom(msg.sender, amount);
        IBasisAsset(bond).mint(msg.sender, amount.mul(1e18).div(cashPrice));

        emit BoughtBonds(msg.sender, amount);
    }

    function redeemBonds(uint256 amount)
        external
        onlyOneBlock
        checkMigration
        checkStartTime
        checkOperator
        updatePrice
    {
        require(amount > 0, 'Treasury: cannot redeem bonds with zero amount');

        uint256 cashPrice = _getCashPrice(bondOracle);
        require(
            cashPrice > getCeilingPrice(), // price > $1.05
            'Treasury: cashPrice not eligible for bond purchase'
        );
        require(
            IERC20(cash).balanceOf(address(this)) >= amount,
            'Treasury: treasury has no more budget'
        );

        accumulatedSeigniorage = accumulatedSeigniorage.sub(
            Math.min(accumulatedSeigniorage, amount)
        );

        IBasisAsset(bond).burnFrom(msg.sender, amount);
        IERC20(cash).safeTransfer(msg.sender, amount);

        emit RedeemedBonds(msg.sender, amount);
    }

    function allocateSeigniorage()
        external
        onlyOneBlock
        checkMigration
        checkStartTime
        checkEpoch
        checkOperator
    {
        _updateCashPrice();
        uint256 cashPrice = _getCashPrice(seigniorageOracle);
        if (cashPrice <= getCeilingPrice()) {
            return; // just advance epoch instead revert
        }

        // circulating supply
        uint256 percentage = cashPrice.sub(cashPriceOne);
        uint256 seigniorage = circulatingSupply().mul(percentage).div(1e18);
        IBasisAsset(cash).mint(address(this), seigniorage);

        // ======================== BIP-3
        uint256 fundReserve = seigniorage.mul(fundAllocationRate).div(100);
        if (fundReserve > 0) {
            IERC20(cash).safeApprove(fund, fundReserve);
            ISimpleERCFund(fund).deposit(
                cash,
                fundReserve,
                'Treasury: Seigniorage Allocation'
            );
            emit ContributionPoolFunded(now, fundReserve);
        }

        seigniorage = seigniorage.sub(fundReserve);

        // ======================== BIP-4
        uint256 treasuryReserve =
            Math.min(
                seigniorage,
                IERC20(bond).totalSupply().sub(accumulatedSeigniorage)
            );
        if (treasuryReserve > 0) {
            if (treasuryReserve == seigniorage) {
                treasuryReserve = treasuryReserve.mul(80).div(100);
            }
            accumulatedSeigniorage = accumulatedSeigniorage.add(
                treasuryReserve
            );
            emit TreasuryFunded(now, treasuryReserve);
        }

        // boardroom
        uint256 boardroomReserve = seigniorage.sub(treasuryReserve);
        if (boardroomReserve > 0) {
            IERC20(cash).safeApprove(boardroom, boardroomReserve);
            IBoardroom(boardroom).allocateSeigniorage(boardroomReserve);
            emit BoardroomFunded(now, boardroomReserve);
        }
    }

    /* ========== EVENTS ========== */

    // GOV
    event Initialized(address indexed executor, uint256 at);
    event Migration(address indexed target);
    event ContributionPoolChanged(
        address indexed operator,
        address oldFund,
        address newFund
    );
    event ContributionPoolRateChanged(
        address indexed operator,
        uint256 oldRate,
        uint256 newRate
    );
    event BondOracleChanged(
        address indexed operator,
        address oldOracle,
        address newOracle
    );
    event SeigniorageOracleChanged(
        address indexed operator,
        address oldOracle,
        address newOracle
    );
    event LinkswapPriceOracleChanged(
        address indexed operator,
        address oldOracle,
        address newOracle
    );
    event CeilingCurveChanged(
        address indexed operator,
        address oldCurve,
        address newCurve
    );

    // CORE
    event RedeemedBonds(address indexed from, uint256 amount);
    event BoughtBonds(address indexed from, uint256 amount);
    event TreasuryFunded(uint256 timestamp, uint256 seigniorage);
    event BoardroomFunded(uint256 timestamp, uint256 seigniorage);
    event ContributionPoolFunded(uint256 timestamp, uint256 seigniorage);
}
// File: /Users/eric/git/yflusd-protocol/contracts/curve/Curve.sol



pragma solidity ^0.6.0;

interface ICurve {
    function minSupply() external view returns (uint256);

    function maxSupply() external view returns (uint256);

    function minCeiling() external view returns (uint256);

    function maxCeiling() external view returns (uint256);

    function calcCeiling(uint256 _supply) external view returns (uint256);
}

abstract contract Curve is ICurve {
    /* ========== EVENTS ========== */

    event MinSupplyChanged(
        address indexed operator,
        uint256 _old,
        uint256 _new
    );

    event MaxSupplyChanged(
        address indexed operator,
        uint256 _old,
        uint256 _new
    );

    event MinCeilingChanged(
        address indexed operator,
        uint256 _old,
        uint256 _new
    );

    event MaxCeilingChanged(
        address indexed operator,
        uint256 _old,
        uint256 _new
    );

    /* ========== STATE VARIABLES ========== */

    uint256 public override minSupply;
    uint256 public override maxSupply;

    uint256 public override minCeiling;
    uint256 public override maxCeiling;

    /* ========== GOVERNANCE ========== */

    function setMinSupply(uint256 _newMinSupply) public virtual {
        uint256 oldMinSupply = minSupply;
        minSupply = _newMinSupply;
        emit MinSupplyChanged(msg.sender, oldMinSupply, _newMinSupply);
    }

    function setMaxSupply(uint256 _newMaxSupply) public virtual {
        uint256 oldMaxSupply = maxSupply;
        maxSupply = _newMaxSupply;
        emit MaxSupplyChanged(msg.sender, oldMaxSupply, _newMaxSupply);
    }

    function setMinCeiling(uint256 _newMinCeiling) public virtual {
        uint256 oldMinCeiling = _newMinCeiling;
        minCeiling = _newMinCeiling;
        emit MinCeilingChanged(msg.sender, oldMinCeiling, _newMinCeiling);
    }

    function setMaxCeiling(uint256 _newMaxCeiling) public virtual {
        uint256 oldMaxCeiling = _newMaxCeiling;
        maxCeiling = _newMaxCeiling;
        emit MaxCeilingChanged(msg.sender, oldMaxCeiling, _newMaxCeiling);
    }

    function calcCeiling(uint256 _supply)
        external
        view
        virtual
        override
        returns (uint256);
}

// File: /Users/eric/git/yflusd-protocol/contracts/interfaces/IBasisAsset.sol



pragma solidity ^0.6.0;

interface IBasisAsset {
    function mint(address recipient, uint256 amount) external returns (bool);

    function burn(uint256 amount) external;

    function burnFrom(address from, uint256 amount) external;

    function isOperator() external returns (bool);

    function operator() external view returns (address);
}

// File: /Users/eric/git/yflusd-protocol/contracts/interfaces/IBoardroom.sol



pragma solidity ^0.6.0;

interface IBoardroom {
    function allocateSeigniorage(uint256 amount) external;
}

// File: /Users/eric/git/yflusd-protocol/contracts/interfaces/ILinkswapPriceOracle.sol



pragma solidity ^0.6.0;

interface ILinkswapPriceOracle {
    function update() external;

    // tokenAmount is to 18 dp, usdAmount is to 8 dp
    // token must be LINK / WETH / YFL
    function calculateTokenAmountFromUsdAmount(address token, uint256 usdAmount)
        external
        view
        returns (uint256 tokenAmount);

    // token must be LINK / WETH
    function calculateUsdAmountFromTokenAmount(address token, uint256 tokenAmount)
        external
        view
        returns (uint256 usdAmount);
}

// File: /Users/eric/git/yflusd-protocol/contracts/interfaces/IOracle.sol



pragma solidity ^0.6.0;

interface IOracle {
    function update() external;

    function consult(address token, uint256 amountIn)
        external
        view
        returns (uint256 amountOut);
    // function getReserves() external view returns (uint112 reserve0, uint112 reserve1, uint32 blockTimestamp);
}

// File: /Users/eric/git/yflusd-protocol/contracts/interfaces/ISimpleERCFund.sol



pragma solidity ^0.6.0;

interface ISimpleERCFund {
    function deposit(
        address token,
        uint256 amount,
        string memory reason
    ) external;

    function withdraw(
        address token,
        uint256 amount,
        address to,
        string memory reason
    ) external;
}

// File: /Users/eric/git/yflusd-protocol/contracts/lib/Babylonian.sol



pragma solidity ^0.6.0;

library Babylonian {
    function sqrt(uint256 y) internal pure returns (uint256 z) {
        if (y > 3) {
            z = y;
            uint256 x = y / 2 + 1;
            while (x < z) {
                z = x;
                x = (y / x + x) / 2;
            }
        } else if (y != 0) {
            z = 1;
        }
        // else z = 0
    }
}

// File: /Users/eric/git/yflusd-protocol/contracts/lib/FixedPoint.sol



pragma solidity ^0.6.0;

import './Babylonian.sol';

// a library for handling binary fixed point numbers (https://en.wikipedia.org/wiki/Q_(number_format))
library FixedPoint {
    // range: [0, 2**112 - 1]
    // resolution: 1 / 2**112
    struct uq112x112 {
        uint224 _x;
    }

    // range: [0, 2**144 - 1]
    // resolution: 1 / 2**112
    struct uq144x112 {
        uint256 _x;
    }

    uint8 private constant RESOLUTION = 112;
    uint256 private constant Q112 = uint256(1) << RESOLUTION;
    uint256 private constant Q224 = Q112 << RESOLUTION;

    // encode a uint112 as a UQ112x112
    function encode(uint112 x) internal pure returns (uq112x112 memory) {
        return uq112x112(uint224(x) << RESOLUTION);
    }

    // encodes a uint144 as a UQ144x112
    function encode144(uint144 x) internal pure returns (uq144x112 memory) {
        return uq144x112(uint256(x) << RESOLUTION);
    }

    // divide a UQ112x112 by a uint112, returning a UQ112x112
    function div(uq112x112 memory self, uint112 x)
        internal
        pure
        returns (uq112x112 memory)
    {
        require(x != 0, 'FixedPoint: DIV_BY_ZERO');
        return uq112x112(self._x / uint224(x));
    }

    // multiply a UQ112x112 by a uint, returning a UQ144x112
    // reverts on overflow
    function mul(uq112x112 memory self, uint256 y)
        internal
        pure
        returns (uq144x112 memory)
    {
        uint256 z;
        require(
            y == 0 || (z = uint256(self._x) * y) / y == uint256(self._x),
            'FixedPoint: MULTIPLICATION_OVERFLOW'
        );
        return uq144x112(z);
    }

    // returns a UQ112x112 which represents the ratio of the numerator to the denominator
    // equivalent to encode(numerator).div(denominator)
    function fraction(uint112 numerator, uint112 denominator)
        internal
        pure
        returns (uq112x112 memory)
    {
        require(denominator > 0, 'FixedPoint: DIV_BY_ZERO');
        return uq112x112((uint224(numerator) << RESOLUTION) / denominator);
    }

    // decode a UQ112x112 into a uint112 by truncating after the radix point
    function decode(uq112x112 memory self) internal pure returns (uint112) {
        return uint112(self._x >> RESOLUTION);
    }

    // decode a UQ144x112 into a uint144 by truncating after the radix point
    function decode144(uq144x112 memory self) internal pure returns (uint144) {
        return uint144(self._x >> RESOLUTION);
    }

    // take the reciprocal of a UQ112x112
    function reciprocal(uq112x112 memory self)
        internal
        pure
        returns (uq112x112 memory)
    {
        require(self._x != 0, 'FixedPoint: ZERO_RECIPROCAL');
        return uq112x112(uint224(Q224 / self._x));
    }

    // square root of a UQ112x112
    function sqrt(uq112x112 memory self)
        internal
        pure
        returns (uq112x112 memory)
    {
        return uq112x112(uint224(Babylonian.sqrt(uint256(self._x)) << 56));
    }
}

// File: /Users/eric/git/yflusd-protocol/contracts/lib/Safe112.sol



pragma solidity ^0.6.0;

library Safe112 {
    function add(uint112 a, uint112 b) internal pure returns (uint256) {
        uint256 c = a + b;
        require(c >= a, 'Safe112: addition overflow');

        return c;
    }

    function sub(uint112 a, uint112 b) internal pure returns (uint256) {
        return sub(a, b, 'Safe112: subtraction overflow');
    }

    function sub(
        uint112 a,
        uint112 b,
        string memory errorMessage
    ) internal pure returns (uint112) {
        require(b <= a, errorMessage);
        uint112 c = a - b;

        return c;
    }

    function mul(uint112 a, uint112 b) internal pure returns (uint256) {
        if (a == 0) {
            return 0;
        }

        uint256 c = a * b;
        require(c / a == b, 'Safe112: multiplication overflow');

        return c;
    }

    function div(uint112 a, uint112 b) internal pure returns (uint256) {
        return div(a, b, 'Safe112: division by zero');
    }

    function div(
        uint112 a,
        uint112 b,
        string memory errorMessage
    ) internal pure returns (uint112) {
        // Solidity only automatically asserts when dividing by 0
        require(b > 0, errorMessage);
        uint112 c = a / b;

        return c;
    }

    function mod(uint112 a, uint112 b) internal pure returns (uint256) {
        return mod(a, b, 'Safe112: modulo by zero');
    }

    function mod(
        uint112 a,
        uint112 b,
        string memory errorMessage
    ) internal pure returns (uint112) {
        require(b != 0, errorMessage);
        return a % b;
    }
}

// File: /Users/eric/git/yflusd-protocol/contracts/owner/Operator.sol



pragma solidity ^0.6.0;

import '@openzeppelin/contracts/GSN/Context.sol';
import '@openzeppelin/contracts/access/Ownable.sol';

contract Operator is Context, Ownable {
    address private _operator;

    event OperatorTransferred(
        address indexed previousOperator,
        address indexed newOperator
    );

    constructor() internal {
        _operator = _msgSender();
        emit OperatorTransferred(address(0), _operator);
    }

    function operator() public view returns (address) {
        return _operator;
    }

    modifier onlyOperator() {
        require(
            _operator == msg.sender,
            'operator: caller is not the operator'
        );
        _;
    }

    function isOperator() public view returns (bool) {
        return _msgSender() == _operator;
    }

    function transferOperator(address newOperator_) public onlyOwner {
        _transferOperator(newOperator_);
    }

    function _transferOperator(address newOperator_) internal {
        require(
            newOperator_ != address(0),
            'operator: zero address given for new operator'
        );
        emit OperatorTransferred(address(0), newOperator_);
        _operator = newOperator_;
    }
}

// File: /Users/eric/git/yflusd-protocol/contracts/utils/ContractGuard.sol



pragma solidity ^0.6.0;

contract ContractGuard {
    mapping(uint256 => mapping(address => bool)) private _status;

    function checkSameOriginReentranted() internal view returns (bool) {
        return _status[block.number][tx.origin];
    }

    function checkSameSenderReentranted() internal view returns (bool) {
        return _status[block.number][msg.sender];
    }

    modifier onlyOneBlock() {
        require(
            !checkSameOriginReentranted(),
            'ContractGuard: one block, one function'
        );
        require(
            !checkSameSenderReentranted(),
            'ContractGuard: one block, one function'
        );

        _;

        _status[block.number][tx.origin] = true;
        _status[block.number][msg.sender] = true;
    }
}

// File: /Users/eric/git/yflusd-protocol/contracts/utils/Epoch.sol



pragma solidity ^0.6.0;

import '@openzeppelin/contracts/math/Math.sol';
import '@openzeppelin/contracts/math/SafeMath.sol';

import '../owner/Operator.sol';

contract Epoch is Operator {
    using SafeMath for uint256;

    uint256 private period;
    uint256 private startTime;
    uint256 private lastExecutedAt;

    /* ========== CONSTRUCTOR ========== */

    constructor(
        uint256 _period,
        uint256 _startTime,
        uint256 _startEpoch
    ) public {
        require(_startTime > block.timestamp, 'Epoch: invalid start time');
        period = _period;
        startTime = _startTime;
        lastExecutedAt = startTime.add(_startEpoch.mul(period));
    }

    /* ========== Modifier ========== */

    modifier checkStartTime {
        require(now >= startTime, 'Epoch: not started yet');

        _;
    }

    modifier checkEpoch {
        require(now > startTime, 'Epoch: not started yet');
        require(callable(), 'Epoch: not allowed');

        _;

        lastExecutedAt = block.timestamp;
    }

    /* ========== VIEW FUNCTIONS ========== */

    function callable() public view returns (bool) {
        return getCurrentEpoch() >= getNextEpoch();
    }

    // epoch
    function getLastEpoch() public view returns (uint256) {
        return lastExecutedAt.sub(startTime).div(period);
    }

    function getCurrentEpoch() public view returns (uint256) {
        return Math.max(startTime, block.timestamp).sub(startTime).div(period);
    }

    function getNextEpoch() public view returns (uint256) {
        if (startTime == lastExecutedAt) {
            return getLastEpoch();
        }
        return getLastEpoch().add(1);
    }

    function nextEpochPoint() public view returns (uint256) {
        return startTime.add(getNextEpoch().mul(period));
    }

    // params
    function getPeriod() public view returns (uint256) {
        return period;
    }

    function getStartTime() public view returns (uint256) {
        return startTime;
    }

    /* ========== GOVERNANCE ========== */

    function setPeriod(uint256 _period) external onlyOperator {
        period = _period;
    }
}

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



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

/*
 * @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/access/Ownable.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

import "../GSN/Context.sol";
/**
 * @dev Contract module which provides a basic access control mechanism, where
 * there is an account (an owner) that can be granted exclusive access to
 * specific functions.
 *
 * By default, the owner account will be the one that deploys the contract. This
 * can later be changed with {transferOwnership}.
 *
 * This module is used through inheritance. It will make available the modifier
 * `onlyOwner`, which can be applied to your functions to restrict their use to
 * the owner.
 */
contract Ownable is Context {
    address private _owner;

    event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);

    /**
     * @dev Initializes the contract setting the deployer as the initial owner.
     */
    constructor () internal {
        address msgSender = _msgSender();
        _owner = msgSender;
        emit OwnershipTransferred(address(0), msgSender);
    }

    /**
     * @dev Returns the address of the current owner.
     */
    function owner() public view returns (address) {
        return _owner;
    }

    /**
     * @dev Throws if called by any account other than the owner.
     */
    modifier onlyOwner() {
        require(_owner == _msgSender(), "Ownable: caller is not the owner");
        _;
    }

    /**
     * @dev Leaves the contract without owner. It will not be possible to call
     * `onlyOwner` functions anymore. Can only be called by the current owner.
     *
     * NOTE: Renouncing ownership will leave the contract without an owner,
     * thereby removing any functionality that is only available to the owner.
     */
    function renounceOwnership() public virtual onlyOwner {
        emit OwnershipTransferred(_owner, address(0));
        _owner = address(0);
    }

    /**
     * @dev Transfers ownership of the contract to a new account (`newOwner`).
     * Can only be called by the current 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;
    }
}

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



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

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

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

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

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



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

/**
 * @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, 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) {
        return sub(a, b, "SafeMath: subtraction overflow");
    }

    /**
     * @dev Returns the subtraction of two unsigned integers, reverting with custom message on
     * overflow (when the result is negative).
     *
     * 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);
        uint256 c = a - b;

        return c;
    }

    /**
     * @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) {
        // 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 0;
        }

        uint256 c = a * b;
        require(c / a == b, "SafeMath: multiplication overflow");

        return c;
    }

    /**
     * @dev Returns the integer division of two unsigned integers. Reverts 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) {
        return div(a, b, "SafeMath: division by zero");
    }

    /**
     * @dev Returns the integer division of two unsigned integers. Reverts with custom message 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, string memory errorMessage) internal pure returns (uint256) {
        require(b > 0, errorMessage);
        uint256 c = a / b;
        // assert(a == b * c + a % b); // There is no case in which this doesn't hold

        return c;
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * Reverts 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) {
        return mod(a, b, "SafeMath: modulo by zero");
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * Reverts with custom message 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, string memory errorMessage) internal pure returns (uint256) {
        require(b != 0, errorMessage);
        return a % b;
    }
}

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



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

/**
 * @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/SafeERC20.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

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/utils/Address.sol



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.2;

/**
 * @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 in 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");
        return _functionCallWithValue(target, data, value, errorMessage);
    }

    function _functionCallWithValue(address target, bytes memory data, uint256 weiValue, string memory errorMessage) private returns (bytes memory) {
        require(isContract(target), "Address: call to non-contract");

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



// SPDX-License-Identifier: MIT

pragma solidity ^0.6.0;

/**
 * @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].
 */
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;
    }
}