// SPDX-License-Identifier: MIT
pragma solidity 0.6.12;

/**
 * @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;
    }
}

/*
 * @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;
    }
}

/**
 * @dev Interface of the ERC20 standard as defined in the EIP.
 */
interface IERC20Lib {
    /**
     * @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: contracts/utils/Address.sol
/**
 * @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"
        );
        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: contracts/token/ERC20/ERC20.sol
/**
 * @dev Implementation of the {IERC20} interface.
 *
 * This implementation is agnostic to the way tokens are created. This means
 * that a supply mechanism has to be added in a derived contract using {_mint}.
 * For a generic mechanism see {ERC20PresetMinterPauser}.
 *
 * TIP: For a detailed writeup see our guide
 * https://forum.zeppelin.solutions/t/how-to-implement-erc20-supply-mechanisms/226[How
 * to implement supply mechanisms].
 *
 * We have followed general OpenZeppelin guidelines: functions revert instead
 * of returning `false` on failure. This behavior is nonetheless conventional
 * and does not conflict with the expectations of ERC20 applications.
 *
 * Additionally, an {Approval} event is emitted on calls to {transferFrom}.
 * This allows applications to reconstruct the allowance for all accounts just
 * by listening to said events. Other implementations of the EIP may not emit
 * these events, as it isn't required by the specification.
 *
 * Finally, the non-standard {decreaseAllowance} and {increaseAllowance}
 * functions have been added to mitigate the well-known issues around setting
 * allowances. See {IERC20-approve}.
 */
contract ERC20Lib is Context, IERC20Lib {
    using SafeMath for uint256;
    using Address for address;

    mapping(address => uint256) private _balances;

    mapping(address => mapping(address => uint256)) private _allowances;

    uint256 private _totalSupply;

    string private _name;
    string private _symbol;
    uint8 private _decimals;

    /**
     * @dev Sets the values for {name} and {symbol}, initializes {decimals} with
     * a default value of 18.
     *
     * To select a different value for {decimals}, use {_setupDecimals}.
     *
     * All three of these values are immutable: they can only be set once during
     * construction.
     */
    constructor(string memory name, string memory symbol) public {
        _name = name;
        _symbol = symbol;
        _decimals = 18;
    }

    /**
     * @dev Returns the name of the token.
     */
    function name() public view returns (string memory) {
        return _name;
    }

    /**
     * @dev Returns the symbol of the token, usually a shorter version of the
     * name.
     */
    function symbol() public view returns (string memory) {
        return _symbol;
    }

    /**
     * @dev Returns the number of decimals used to get its user representation.
     * For example, if `decimals` equals `2`, a balance of `505` tokens should
     * be displayed to a user as `5,05` (`505 / 10 ** 2`).
     *
     * Tokens usually opt for a value of 18, imitating the relationship between
     * Ether and Wei. This is the value {ERC20} uses, unless {_setupDecimals} is
     * called.
     *
     * NOTE: This information is only used for _display_ purposes: it in
     * no way affects any of the arithmetic of the contract, including
     * {IERC20-balanceOf} and {IERC20-transfer}.
     */
    function decimals() public view returns (uint8) {
        return _decimals;
    }

    /**
     * @dev See {IERC20-totalSupply}.
     */
    function totalSupply() public override view returns (uint256) {
        return _totalSupply;
    }

    /**
     * @dev See {IERC20-balanceOf}.
     */
    function balanceOf(address account) public override view returns (uint256) {
        return _balances[account];
    }

    /**
     * @dev See {IERC20-transfer}.
     *
     * Requirements:
     *
     * - `recipient` cannot be the zero address.
     * - the caller must have a balance of at least `amount`.
     */
    function transfer(address recipient, uint256 amount)
        public
        virtual
        override
        returns (bool)
    {
        _transfer(_msgSender(), recipient, amount);
        return true;
    }

    /**
     * @dev See {IERC20-allowance}.
     */
    function allowance(address owner, address spender)
        public
        virtual
        override
        view
        returns (uint256)
    {
        return _allowances[owner][spender];
    }

    /**
     * @dev See {IERC20-approve}.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     */
    function approve(address spender, uint256 amount)
        public
        virtual
        override
        returns (bool)
    {
        _approve(_msgSender(), spender, amount);
        return true;
    }

    /**
     * @dev See {IERC20-transferFrom}.
     *
     * Emits an {Approval} event indicating the updated allowance. This is not
     * required by the EIP. See the note at the beginning of {ERC20};
     *
     * Requirements:
     * - `sender` and `recipient` cannot be the zero address.
     * - `sender` must have a balance of at least `amount`.
     * - the caller must have allowance for ``sender``'s tokens of at least
     * `amount`.
     */
    function transferFrom(
        address sender,
        address recipient,
        uint256 amount
    ) public virtual override returns (bool) {
        _transfer(sender, recipient, amount);
        _approve(
            sender,
            _msgSender(),
            _allowances[sender][_msgSender()].sub(
                amount,
                "ERC20: transfer amount exceeds allowance"
            )
        );
        return true;
    }

    /**
     * @dev Atomically increases the allowance granted to `spender` by the caller.
     *
     * This is an alternative to {approve} that can be used as a mitigation for
     * problems described in {IERC20-approve}.
     *
     * Emits an {Approval} event indicating the updated allowance.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     */
    function increaseAllowance(address spender, uint256 addedValue)
        public
        virtual
        returns (bool)
    {
        _approve(
            _msgSender(),
            spender,
            _allowances[_msgSender()][spender].add(addedValue)
        );
        return true;
    }

    /**
     * @dev Atomically decreases the allowance granted to `spender` by the caller.
     *
     * This is an alternative to {approve} that can be used as a mitigation for
     * problems described in {IERC20-approve}.
     *
     * Emits an {Approval} event indicating the updated allowance.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     * - `spender` must have allowance for the caller of at least
     * `subtractedValue`.
     */
    function decreaseAllowance(address spender, uint256 subtractedValue)
        public
        virtual
        returns (bool)
    {
        _approve(
            _msgSender(),
            spender,
            _allowances[_msgSender()][spender].sub(
                subtractedValue,
                "ERC20: decreased allowance below zero"
            )
        );
        return true;
    }

    /**
     * @dev Moves tokens `amount` from `sender` to `recipient`.
     *
     * This is internal function is equivalent to {transfer}, and can be used to
     * e.g. implement automatic token fees, slashing mechanisms, etc.
     *
     * Emits a {Transfer} event.
     *
     * Requirements:
     *
     * - `sender` cannot be the zero address.
     * - `recipient` cannot be the zero address.
     * - `sender` must have a balance of at least `amount`.
     */
    function _transfer(
        address sender,
        address recipient,
        uint256 amount
    ) internal virtual {
        require(sender != address(0), "ERC20: transfer from the zero address");
        require(recipient != address(0), "ERC20: transfer to the zero address");

        _beforeTokenTransfer(sender, recipient, amount);

        _balances[sender] = _balances[sender].sub(
            amount,
            "ERC20: transfer amount exceeds balance"
        );
        _balances[recipient] = _balances[recipient].add(amount);
        emit Transfer(sender, recipient, amount);
    }

    /** @dev Creates `amount` tokens and assigns them to `account`, increasing
     * the total supply.
     *
     * Emits a {Transfer} event with `from` set to the zero address.
     *
     * Requirements
     *
     * - `to` cannot be the zero address.
     */
    function _mint(address account, uint256 amount) internal virtual {
        require(account != address(0), "ERC20: mint to the zero address");

        _beforeTokenTransfer(address(0), account, amount);

        _totalSupply = _totalSupply.add(amount);
        _balances[account] = _balances[account].add(amount);
        emit Transfer(address(0), account, amount);
    }

    /**
     * @dev Destroys `amount` tokens from `account`, reducing the
     * total supply.
     *
     * Emits a {Transfer} event with `to` set to the zero address.
     *
     * Requirements
     *
     * - `account` cannot be the zero address.
     * - `account` must have at least `amount` tokens.
     */
    function _burn(address account, uint256 amount) internal virtual {
        require(account != address(0), "ERC20: burn from the zero address");

        _beforeTokenTransfer(account, address(0), amount);

        _balances[account] = _balances[account].sub(
            amount,
            "ERC20: burn amount exceeds balance"
        );
        _totalSupply = _totalSupply.sub(amount);
        emit Transfer(account, address(0), amount);
    }

    /**
     * @dev Sets `amount` as the allowance of `spender` over the `owner` s tokens.
     *
     * This internal function is equivalent to `approve`, and can be used to
     * e.g. set automatic allowances for certain subsystems, etc.
     *
     * Emits an {Approval} event.
     *
     * Requirements:
     *
     * - `owner` cannot be the zero address.
     * - `spender` cannot be the zero address.
     */
    function _approve(
        address owner,
        address spender,
        uint256 amount
    ) internal virtual {
        require(owner != address(0), "ERC20: approve from the zero address");
        require(spender != address(0), "ERC20: approve to the zero address");

        _allowances[owner][spender] = amount;
        emit Approval(owner, spender, amount);
    }

    /**
     * @dev Sets {decimals} to a value other than the default one of 18.
     *
     * WARNING: This function should only be called from the constructor. Most
     * applications that interact with token contracts will not expect
     * {decimals} to ever change, and may work incorrectly if it does.
     */
    function _setupDecimals(uint8 decimals_) internal {
        _decimals = decimals_;
    }

    /**
     * @dev Hook that is called before any transfer of tokens. This includes
     * minting and burning.
     *
     * Calling conditions:
     *
     * - when `from` and `to` are both non-zero, `amount` of ``from``'s tokens
     * will be to transferred to `to`.
     * - when `from` is zero, `amount` tokens will be minted for `to`.
     * - when `to` is zero, `amount` of ``from``'s tokens will be burned.
     * - `from` and `to` are never both zero.
     *
     * To learn more about hooks, head to xref:ROOT:extending-contracts.adoc#using-hooks[Using Hooks].
     */
    function _beforeTokenTransfer(
        address from,
        address to,
        uint256 amount
    ) internal virtual {}
}

/**
 * @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 SafeERC20Lib {
    using SafeMath for uint256;
    using Address for address;

    function safeTransfer(
        IERC20Lib token,
        address to,
        uint256 value
    ) internal {
        _callOptionalReturn(
            token,
            abi.encodeWithSelector(token.transfer.selector, to, value)
        );
    }

    function safeTransferFrom(
        IERC20Lib 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(
        IERC20Lib 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(
        IERC20Lib 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(
        IERC20Lib 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(IERC20Lib 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"
            );
        }
    }
}

interface IVault is IERC20Lib {
    function underlying() external view returns (address);

    function decimals() external view returns (uint8);

    function earn() external;

    function deposit(uint256) external;

    function depositAll() external;

    function withdraw(uint256) external;

    function withdrawAll() external;

    function distribute() external;

    function salvage(address, uint256) external;

    function getRatio() external view returns (uint256);
}

interface ICurveFi_2 {
    function get_virtual_price() external view returns (uint256);

    function add_liquidity(uint256[2] calldata amounts, uint256 min_mint_amount)
        external;

    function remove_liquidity_imbalance(
        uint256[2] calldata amounts,
        uint256 max_burn_amount
    ) external;

    function remove_liquidity(uint256 _amount, uint256[2] calldata amounts)
        external;

    function exchange(
        int128 from,
        int128 to,
        uint256 _from_amount,
        uint256 _min_to_amount
    ) external;

    function balances(int128) external view returns (uint256);
}

interface ICurve3Pool {
    function get_virtual_price() external view returns (uint256);

    function add_liquidity(uint256[3] calldata amounts, uint256 min_mint_amount)
        external;

    function remove_liquidity_imbalance(
        uint256[3] calldata amounts,
        uint256 max_burn_amount
    ) external;

    function remove_liquidity(uint256 _amount, uint256[3] calldata amounts)
        external;

    function exchange(
        int128 from,
        int128 to,
        uint256 _from_amount,
        uint256 _min_to_amount
    ) external;

    function balances(uint256) external view returns (uint256);
}

interface ICurveFi_4 {
    function get_virtual_price() external view returns (uint256);

    function add_liquidity(uint256[4] calldata amounts, uint256 min_mint_amount)
        external;

    function remove_liquidity_imbalance(
        uint256[4] calldata amounts,
        uint256 max_burn_amount
    ) external;

    function remove_liquidity(uint256 _amount, uint256[4] calldata amounts)
        external;

    function exchange(
        int128 from,
        int128 to,
        uint256 _from_amount,
        uint256 _min_to_amount
    ) external;

    function balances(int128) external view returns (uint256);
}

interface ICurveZap_4 {
    function add_liquidity(
        uint256[4] calldata uamounts,
        uint256 min_mint_amount
    ) external;

    function remove_liquidity(uint256 _amount, uint256[4] calldata min_uamounts)
        external;

    function remove_liquidity_imbalance(
        uint256[4] calldata uamounts,
        uint256 max_burn_amount
    ) external;

    function calc_withdraw_one_coin(uint256 _token_amount, int128 i)
        external
        returns (uint256);

    function remove_liquidity_one_coin(
        uint256 _token_amount,
        int128 i,
        uint256 min_uamount
    ) external;

    function remove_liquidity_one_coin(
        uint256 _token_amount,
        int128 i,
        uint256 min_uamount,
        bool donate_dust
    ) external;

    function withdraw_donated_dust() external;

    function coins(int128 arg0) external returns (address);

    function underlying_coins(int128 arg0) external returns (address);

    function curve() external returns (address);

    function token() external returns (address);
}

interface ICurveZap {
    function remove_liquidity_one_coin(
        uint256 _token_amount,
        int128 i,
        uint256 min_uamount
    ) external;
}

interface ICurveGauge {
    function deposit(uint256 _value) external;

    function deposit(uint256 _value, address addr) external;

    function balanceOf(address arg0) external view returns (uint256);

    function withdraw(uint256 _value) external;

    function withdraw(uint256 _value, bool claim_rewards) external;

    function claim_rewards() external;

    function claim_rewards(address addr) external;

    function claimable_tokens(address addr) external returns (uint256);

    function claimable_reward(address addr) external view returns (uint256);

    function integrate_fraction(address arg0) external view returns (uint256);
}

interface ICurveMintr {
    function mint(address) external;

    function minted(address arg0, address arg1) external view returns (uint256);
}

interface ICurveVotingEscrow {
    function locked(address arg0)
        external
        view
        returns (int128 amount, uint256 end);

    function locked__end(address _addr) external view returns (uint256);

    function create_lock(uint256, uint256) external;

    function increase_amount(uint256) external;

    function increase_unlock_time(uint256 _unlock_time) external;

    function withdraw() external;

    function smart_wallet_checker() external returns (address);
}

interface ICurveSmartContractChecker {
    function wallets(address) external returns (bool);

    function approveWallet(address _wallet) external;
}

interface UniswapRouterV2 {
    function swapExactTokensForTokens(
        uint256 amountIn,
        uint256 amountOutMin,
        address[] calldata path,
        address to,
        uint256 deadline
    ) external returns (uint256[] memory amounts);

    function addLiquidity(
        address tokenA,
        address tokenB,
        uint256 amountADesired,
        uint256 amountBDesired,
        uint256 amountAMin,
        uint256 amountBMin,
        address to,
        uint256 deadline
    )
        external
        returns (
            uint256 amountA,
            uint256 amountB,
            uint256 liquidity
        );

    function addLiquidityETH(
        address token,
        uint256 amountTokenDesired,
        uint256 amountTokenMin,
        uint256 amountETHMin,
        address to,
        uint256 deadline
    )
        external
        payable
        returns (
            uint256 amountToken,
            uint256 amountETH,
            uint256 liquidity
        );

    function removeLiquidity(
        address tokenA,
        address tokenB,
        uint256 liquidity,
        uint256 amountAMin,
        uint256 amountBMin,
        address to,
        uint256 deadline
    ) external returns (uint256 amountA, uint256 amountB);

    function getAmountsOut(uint256 amountIn, address[] calldata path)
        external
        view
        returns (uint256[] memory amounts);

    function getAmountsIn(uint256 amountOut, address[] calldata path)
        external
        view
        returns (uint256[] memory amounts);

    function swapETHForExactTokens(
        uint256 amountOut,
        address[] calldata path,
        address to,
        uint256 deadline
    ) external payable returns (uint256[] memory amounts);

    function swapExactETHForTokens(
        uint256 amountOutMin,
        address[] calldata path,
        address to,
        uint256 deadline
    ) external payable returns (uint256[] memory amounts);
}

interface IUniswapV2Pair {
    event Approval(
        address indexed owner,
        address indexed spender,
        uint256 value
    );
    event Transfer(address indexed from, address indexed to, uint256 value);

    function name() external pure returns (string memory);

    function symbol() external pure returns (string memory);

    function decimals() external pure returns (uint8);

    function totalSupply() external view returns (uint256);

    function balanceOf(address owner) external view returns (uint256);

    function allowance(address owner, address spender)
        external
        view
        returns (uint256);

    function approve(address spender, uint256 value) external returns (bool);

    function transfer(address to, uint256 value) external returns (bool);

    function transferFrom(
        address from,
        address to,
        uint256 value
    ) external returns (bool);

    function DOMAIN_SEPARATOR() external view returns (bytes32);

    function PERMIT_TYPEHASH() external pure returns (bytes32);

    function nonces(address owner) external view returns (uint256);

    function permit(
        address owner,
        address spender,
        uint256 value,
        uint256 deadline,
        uint8 v,
        bytes32 r,
        bytes32 s
    ) external;

    event Mint(address indexed sender, uint256 amount0, uint256 amount1);
    event Burn(
        address indexed sender,
        uint256 amount0,
        uint256 amount1,
        address indexed to
    );
    event Swap(
        address indexed sender,
        uint256 amount0In,
        uint256 amount1In,
        uint256 amount0Out,
        uint256 amount1Out,
        address indexed to
    );
    event Sync(uint112 reserve0, uint112 reserve1);

    function MINIMUM_LIQUIDITY() external pure returns (uint256);

    function factory() external view returns (address);

    function token0() external view returns (address);

    function token1() external view returns (address);

    function getReserves()
        external
        view
        returns (
            uint112 reserve0,
            uint112 reserve1,
            uint32 blockTimestampLast
        );

    function price0CumulativeLast() external view returns (uint256);

    function price1CumulativeLast() external view returns (uint256);

    function kLast() external view returns (uint256);

    function mint(address to) external returns (uint256 liquidity);

    function burn(address to)
        external
        returns (uint256 amount0, uint256 amount1);

    function swap(
        uint256 amount0Out,
        uint256 amount1Out,
        address to,
        bytes calldata data
    ) external;

    function skim(address to) external;

    function sync() external;
}

interface IUniswapV2Factory {
    event PairCreated(
        address indexed token0,
        address indexed token1,
        address pair,
        uint256
    );

    function getPair(address tokenA, address tokenB)
        external
        view
        returns (address pair);

    function allPairs(uint256) external view returns (address pair);

    function allPairsLength() external view returns (uint256);

    function feeTo() external view returns (address);

    function feeToSetter() external view returns (address);

    function createPair(address tokenA, address tokenB)
        external
        returns (address pair);
}

interface ICurveEURS {
    function add_liquidity(
        uint256[2] calldata amounts,
        uint256 min_mint_amount
    ) external returns (uint256);
}

interface ITimelock {
    function MINIMUM_DELAY() external view returns (uint);
    function MAXIMUM_DELAY() external view returns (uint);
    function delay() external view returns (uint);
}

// Base of all strategy contracts
abstract contract VoxStrategyBase {
    using SafeERC20Lib for IERC20Lib;
    using Address for address;
    using SafeMath for uint256;

    // Performance fees
    uint256 public treasuryFee = 1000;
    uint256 public constant treasuryFeeMax = 2000;
    uint256 public constant treasuryFeeBase = 10000;

    uint256 public strategyFee = 500;
    uint256 public constant strategyFeeMax = 1000;
    uint256 public constant strategyFeeBase = 10000;

    uint256 public developerFee = 250;
    uint256 public constant developerFeeMax = 500;
    uint256 public constant developerFeeBase = 10000;

    // Tokens
    address public underlying;
    address public constant weth = 0xC02aaA39b223FE8D0A0e5C4F27eAD9083C756Cc2;
    address public constant wbtc = 0x2260FAC5E5542a773Aa44fBCfeDf7C193bc2C599;

    // User accounts
    address public governance;
    address public strategist;
    address public treasury;
    address public devfund;
    address public timelock;
    address public vault;

    // Timelock
    uint256 public constant minTimelockInterval = 12 hours;

    // Dex
    address public univ2Router2 = 0x7a250d5630B4cF539739dF2C5dAcb4c659F2488D;

    constructor(
        address _underlying,
        address _governance,
        address _strategist,
        address _treasury,
        address _devfund,
        address _timelock,
        address _vault
    ) public {
        require(_underlying != address(0));
        require(_governance != address(0));
        require(_strategist != address(0));
        require(_treasury != address(0));
        require(_devfund != address(0));
        require(_timelock != address(0));
        require(_vault != address(0));

        underlying = _underlying;
        governance = _governance;
        strategist = _strategist;
        treasury = _treasury;
        devfund = _devfund;
        timelock = _timelock;
        vault = _vault;
    }

    // **** Modifiers **** //

    modifier restricted {
        require(
            (msg.sender == tx.origin && !address(msg.sender).isContract()) ||
                msg.sender == governance ||
                msg.sender == strategist
        );
        _;
    }

    modifier isTimelock {
        require(msg.sender == timelock, "!timelock");
        _;
    }

    modifier isGovernance {
        require(msg.sender == governance, "!governance");
        _;
    }

    // **** Views **** //

    function balanceOfUnderlying() public view returns (uint256) {
        return IERC20Lib(underlying).balanceOf(address(this));
    }

    function balanceOfPool() public virtual view returns (uint256);

    function balanceOf() public view returns (uint256) {
        return balanceOfUnderlying().add(balanceOfPool());
    }

    function getName() external virtual pure returns (string memory);

    // **** Setters **** //

    function setTreasuryFee(uint256 _treasuryFee) external isTimelock {
        require(_treasuryFee <= treasuryFeeMax, "!treasury fee max");
        treasuryFee = _treasuryFee;
    }

    function setStrategyFee(uint256 _strategyFee) external isTimelock {
        require(_strategyFee <= strategyFeeMax, "!strategy fee max");
        strategyFee = _strategyFee;
    }

    function setDeveloperFee(uint256 _developerFee) external isTimelock {
        require(_developerFee <= developerFeeMax, "!developer fee max");
        developerFee = _developerFee;
    }

    function setStrategist(address _strategist) external isGovernance {
        strategist = _strategist;
    }

    function setGovernance(address _governance) external isGovernance {
        governance = _governance;
    }

    function setTreasury(address _treasury) external isGovernance {
        treasury = _treasury;
    }

    function setDevFund(address _devfund) external {
        require(msg.sender == devfund, "!devfund");
        devfund = _devfund;
    }

    function setTimelock(address _timelock) external isTimelock {
        timelock = _timelock;
    }

    function setVault(address _vault) external isTimelock {
        require(
            IVault(_vault).underlying() == address(underlying),
            "vault does not support this underlying"
        );
        vault = _vault;
    }

    // **** State mutations **** //
    function deposit() public virtual;

    // Governance only function to salvage non-underlying assets
    function salvage(IERC20Lib _asset)
        external
        isGovernance
        returns (uint256 balance)
    {
        require(underlying != address(_asset), "underlying");
        balance = _asset.balanceOf(address(this));
        _asset.safeTransfer(treasury, balance);
    }

    // Withdraw partial funds, normally used with a vault withdrawal
    function withdraw(uint256 _amount) external {
        require(vault != address(0), "!vault"); // additional protection so we don't burn the funds
        require(msg.sender == vault, "!vault");
        uint256 _balance = IERC20Lib(underlying).balanceOf(address(this));
        if (_balance < _amount) {
            _amount = _withdrawSome(_amount.sub(_balance));
            _amount = _amount.add(_balance);
        }

        IERC20Lib(underlying).safeTransfer(vault, _amount);
    }

    // Withdraw all funds, normally used when migrating strategies
    function withdrawAll() external restricted returns (uint256 balance) {
        require(vault != address(0), "!vault"); // additional protection so we don't burn the funds
        _withdrawAll();
        balance = IERC20Lib(underlying).balanceOf(address(this));
        IERC20Lib(underlying).safeTransfer(vault, balance);
    }

    function _withdrawAll() internal {
        _withdrawSome(balanceOfPool());
    }

    function _withdrawSome(uint256 _amount) internal virtual returns (uint256);

    function harvest() public virtual;

    function _distributeAndDeposit() internal {
        uint256 _underlying = IERC20Lib(underlying).balanceOf(address(this));

        if (_underlying > 0) {
            // Treasury fees
            IERC20Lib(underlying).safeTransfer(
                treasury,
                _underlying.mul(treasuryFee).div(treasuryFeeBase)
            );

            // Strategy fee
            IERC20Lib(underlying).safeTransfer(
                strategist,
                _underlying.mul(strategyFee).div(strategyFeeBase)
            );

            // Developer fee
            IERC20Lib(underlying).safeTransfer(
                devfund,
                _underlying.mul(developerFee).div(developerFeeBase)
            );

            deposit();
        }
    }

    // **** Emergency functions ****

    function execute(address _target, bytes memory _data)
        public
        payable
        isTimelock
        returns (bytes memory response)
    {
        require(_target != address(0), "!target");

        // call contract in current context
        assembly {
            let succeeded := delegatecall(
                sub(gas(), 5000),
                _target,
                add(_data, 0x20),
                mload(_data),
                0,
                0
            )
            let size := returndatasize()

            response := mload(0x40)
            mstore(
                0x40,
                add(response, and(add(add(size, 0x20), 0x1f), not(0x1f)))
            )
            mstore(response, size)
            returndatacopy(add(response, 0x20), 0, size)

            switch iszero(succeeded)
                case 1 {
                    // throw if delegatecall failed
                    revert(add(response, 0x20), size)
                }
        }
    }

    // **** Internal functions ****
    function _swapUniswap(
        address _from,
        address _to,
        uint256 _amount
    ) internal {
        require(_to != address(0));

        // Swap with uniswap
        IERC20Lib(_from).safeApprove(univ2Router2, 0);
        IERC20Lib(_from).safeApprove(univ2Router2, _amount);

        address[] memory path;

        if (_from == weth || _to == weth) {
            path = new address[](2);
            path[0] = _from;
            path[1] = _to;
        } else {
            path = new address[](3);
            path[0] = _from;
            path[1] = weth;
            path[2] = _to;
        }

        UniswapRouterV2(univ2Router2).swapExactTokensForTokens(
            _amount,
            0,
            path,
            address(this),
            now.add(60)
        );
    }
}

// Base contract for Curve based vault strategies
abstract contract VoxStrategyCurveBase is VoxStrategyBase {
    // curve dao
    address public curve;
    address public gauge;
    address public mintr = 0xd061D61a4d941c39E5453435B6345Dc261C2fcE0;

    // stablecoins
    address public dai = 0x6B175474E89094C44Da98b954EedeAC495271d0F;
    address public usdc = 0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48;
    address public usdt = 0xdAC17F958D2ee523a2206206994597C13D831ec7;
    address public susd = 0x57Ab1ec28D129707052df4dF418D58a2D46d5f51;

    // bitcoins
    address public renbtc = 0xEB4C2781e4ebA804CE9a9803C67d0893436bB27D;

    // rewards
    address public crv = 0xD533a949740bb3306d119CC777fa900bA034cd52;

    // keep
    address public keep = 0x85Eee30c52B0b379b046Fb0F85F4f3Dc3009aFEC;
    address public keep_rewards = 0x6828bcF74279eE32f2723eC536c22c51Eed383C6;

    // How much CRV tokens to keep
    uint256 public keepCRV = 0;
    uint256 public keepCRVMax = 10000;

    constructor(
        address _curve,
        address _gauge,
        address _underlying,
        address _governance,
        address _strategist,
        address _treasury,
        address _devfund,
        address _timelock,
        address _vault
    )
        public
        VoxStrategyBase(
            _underlying,
            _governance,
            _strategist,
            _treasury,
            _devfund,
            _timelock,
            _vault
        )
    {
        curve = _curve;
        gauge = _gauge;
    }

    // **** Getters ****

    function balanceOfPool() public override view returns (uint256) {
        return ICurveGauge(gauge).balanceOf(address(this));
    }

    function getHarvestable() external returns (uint256) {
        return ICurveGauge(gauge).claimable_tokens(address(this));
    }

    function getMostPremium() public virtual view returns (address, uint256);

    // **** Setters ****

    function setKeepCRV(uint256 _keepCRV) external {
        require(msg.sender == governance, "!governance");
        keepCRV = _keepCRV;
    }

    // **** State Mutation functions ****

    function deposit() public override {
        uint256 _underlying = IERC20Lib(underlying).balanceOf(address(this));
        if (_underlying > 0) {
            IERC20Lib(underlying).safeApprove(gauge, 0);
            IERC20Lib(underlying).safeApprove(gauge, _underlying);
            ICurveGauge(gauge).deposit(_underlying);
        }
    }

    function _withdrawSome(uint256 _amount)
        internal
        override
        returns (uint256)
    {
        ICurveGauge(gauge).withdraw(_amount);
        return _amount;
    }
}

contract VoxStrategyCurveEURS is VoxStrategyCurveBase {
    // Curve stuff
    address public eurs_pool = 0x0Ce6a5fF5217e38315f87032CF90686C96627CAA;
    address public eurs_gauge = 0x90Bb609649E0451E5aD952683D64BD2d1f245840;
    address public eurs_crv = 0x194eBd173F6cDacE046C53eACcE9B953F28411d1;

    address public eurs = 0xdB25f211AB05b1c97D595516F45794528a807ad8;

    // Tokens we're collecting as well
    address public snx = 0xC011a73ee8576Fb46F5E1c5751cA3B9Fe0af2a6F;

    constructor(
        address _governance,
        address _strategist,
        address _treasury,
        address _devfund,
        address _timelock,
        address _vault
    )
        public
        VoxStrategyCurveBase(
            eurs_pool,
            eurs_gauge,
            eurs_crv,
            _governance,
            _strategist,
            _treasury,
            _devfund,
            _timelock,
            _vault
        )
    {}

    // **** Views ****

    function getMostPremium() public override view returns (address, uint256) {
        return (eurs, 0);
    }

    function getName() external override pure returns (string memory) {
        return "VoxStrategyCurveEURS";
    }

    // **** State Mutations ****

    function harvest() public override restricted {
        // stablecoin we want to convert to
        (address to, uint256 toIndex) = getMostPremium();

        // Collects crv tokens
        // Don't bother voting in v1
        ICurveMintr(mintr).mint(gauge);
        uint256 _crv = IERC20Lib(crv).balanceOf(address(this));
        if (_crv > 0) {
            // x% is sent back to the rewards holder
            // to be used to lock up in as veCRV in a future date
            uint256 _keepCRV = _crv.mul(keepCRV).div(keepCRVMax);
            if (_keepCRV > 0) {
                IERC20Lib(crv).safeTransfer(treasury, _keepCRV);
            }
            _crv = _crv.sub(_keepCRV);
            _swapUniswap(crv, to, _crv);
        }

        // Collects SNX tokens
        ICurveGauge(gauge).claim_rewards(address(this));
        uint256 _snx = IERC20Lib(snx).balanceOf(address(this));
        if (_snx > 0) {
            _swapUniswap(snx, to, _snx);
        }

        // Adds liquidity to curve.fi's pool
        // to get back want (eurs)
        uint256 _to = IERC20Lib(to).balanceOf(address(this));
        if (_to > 0) {
            IERC20Lib(to).safeApprove(curve, 0);
            IERC20Lib(to).safeApprove(curve, _to);
            uint256[2] memory liquidity;
            liquidity[toIndex] = _to;
            ICurveEURS(curve).add_liquidity(liquidity, 0);
        }

        _distributeAndDeposit();
    }
}