// 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/math/SafeMath.sol



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/utils/Address.sol



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/GSN/Context.sol



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/token/ERC20/ERC20.sol



pragma solidity ^0.6.0;





/**
 * @dev Implementation of the {IERC20} interface.
 *
 * This implementation is agnostic to the way tokens are created. This means
 * that a supply mechanism has to be added in a derived contract using {_mint}.
 * For a generic mechanism see {ERC20PresetMinterPauser}.
 *
 * TIP: For a detailed writeup see our guide
 * https://forum.zeppelin.solutions/t/how-to-implement-erc20-supply-mechanisms/226[How
 * to implement supply mechanisms].
 *
 * We have followed general OpenZeppelin guidelines: functions revert instead
 * of returning `false` on failure. This behavior is nonetheless conventional
 * and does not conflict with the expectations of ERC20 applications.
 *
 * Additionally, an {Approval} event is emitted on calls to {transferFrom}.
 * This allows applications to reconstruct the allowance for all accounts just
 * by listening to said events. Other implementations of the EIP may not emit
 * these events, as it isn't required by the specification.
 *
 * Finally, the non-standard {decreaseAllowance} and {increaseAllowance}
 * functions have been added to mitigate the well-known issues around setting
 * allowances. See {IERC20-approve}.
 */
contract ERC20 is Context, IERC20 {
    using SafeMath for uint256;
    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 view override returns (uint256) {
        return _totalSupply;
    }

    /**
     * @dev See {IERC20-balanceOf}.
     */
    function balanceOf(address account) public view override returns (uint256) {
        return _balances[account];
    }

    /**
     * @dev See {IERC20-transfer}.
     *
     * Requirements:
     *
     * - `recipient` cannot be the zero address.
     * - the caller must have a balance of at least `amount`.
     */
    function transfer(address recipient, uint256 amount) public virtual override returns (bool) {
        _transfer(_msgSender(), recipient, amount);
        return true;
    }

    /**
     * @dev See {IERC20-allowance}.
     */
    function allowance(address owner, address spender) public view virtual override returns (uint256) {
        return _allowances[owner][spender];
    }

    /**
     * @dev See {IERC20-approve}.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     */
    function approve(address spender, uint256 amount) public virtual override returns (bool) {
        _approve(_msgSender(), spender, amount);
        return true;
    }

    /**
     * @dev See {IERC20-transferFrom}.
     *
     * Emits an {Approval} event indicating the updated allowance. This is not
     * required by the EIP. See the note at the beginning of {ERC20};
     *
     * Requirements:
     * - `sender` and `recipient` cannot be the zero address.
     * - `sender` must have a balance of at least `amount`.
     * - the caller must have allowance for ``sender``'s tokens of at least
     * `amount`.
     */
    function transferFrom(address sender, address recipient, uint256 amount) public virtual override returns (bool) {
        _transfer(sender, recipient, amount);
        _approve(sender, _msgSender(), _allowances[sender][_msgSender()].sub(amount, "ERC20: transfer amount exceeds allowance"));
        return true;
    }

    /**
     * @dev Atomically increases the allowance granted to `spender` by the caller.
     *
     * This is an alternative to {approve} that can be used as a mitigation for
     * problems described in {IERC20-approve}.
     *
     * Emits an {Approval} event indicating the updated allowance.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     */
    function increaseAllowance(address spender, uint256 addedValue) public virtual returns (bool) {
        _approve(_msgSender(), spender, _allowances[_msgSender()][spender].add(addedValue));
        return true;
    }

    /**
     * @dev Atomically decreases the allowance granted to `spender` by the caller.
     *
     * This is an alternative to {approve} that can be used as a mitigation for
     * problems described in {IERC20-approve}.
     *
     * Emits an {Approval} event indicating the updated allowance.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     * - `spender` must have allowance for the caller of at least
     * `subtractedValue`.
     */
    function decreaseAllowance(address spender, uint256 subtractedValue) public virtual returns (bool) {
        _approve(_msgSender(), spender, _allowances[_msgSender()][spender].sub(subtractedValue, "ERC20: decreased allowance below zero"));
        return true;
    }

    /**
     * @dev Moves tokens `amount` from `sender` to `recipient`.
     *
     * This is internal function is equivalent to {transfer}, and can be used to
     * e.g. implement automatic token fees, slashing mechanisms, etc.
     *
     * Emits a {Transfer} event.
     *
     * Requirements:
     *
     * - `sender` cannot be the zero address.
     * - `recipient` cannot be the zero address.
     * - `sender` must have a balance of at least `amount`.
     */
    function _transfer(address sender, address recipient, uint256 amount) internal virtual {
        require(sender != address(0), "ERC20: transfer from the zero address");
        require(recipient != address(0), "ERC20: transfer to the zero address");

        _beforeTokenTransfer(sender, recipient, amount);

        _balances[sender] = _balances[sender].sub(amount, "ERC20: transfer amount exceeds balance");
        _balances[recipient] = _balances[recipient].add(amount);
        emit Transfer(sender, recipient, amount);
    }

    /** @dev Creates `amount` tokens and assigns them to `account`, increasing
     * the total supply.
     *
     * Emits a {Transfer} event with `from` set to the zero address.
     *
     * Requirements
     *
     * - `to` cannot be the zero address.
     */
    function _mint(address account, uint256 amount) internal virtual {
        require(account != address(0), "ERC20: mint to the zero address");

        _beforeTokenTransfer(address(0), account, amount);

        _totalSupply = _totalSupply.add(amount);
        _balances[account] = _balances[account].add(amount);
        emit Transfer(address(0), account, amount);
    }

    /**
     * @dev Destroys `amount` tokens from `account`, reducing the
     * total supply.
     *
     * Emits a {Transfer} event with `to` set to the zero address.
     *
     * Requirements
     *
     * - `account` cannot be the zero address.
     * - `account` must have at least `amount` tokens.
     */
    function _burn(address account, uint256 amount) internal virtual {
        require(account != address(0), "ERC20: burn from the zero address");

        _beforeTokenTransfer(account, address(0), amount);

        _balances[account] = _balances[account].sub(amount, "ERC20: burn amount exceeds balance");
        _totalSupply = _totalSupply.sub(amount);
        emit Transfer(account, address(0), amount);
    }

    /**
     * @dev Sets `amount` as the allowance of `spender` over the `owner` s tokens.
     *
     * This internal function is equivalent to `approve`, and can be used to
     * e.g. set automatic allowances for certain subsystems, etc.
     *
     * Emits an {Approval} event.
     *
     * Requirements:
     *
     * - `owner` cannot be the zero address.
     * - `spender` cannot be the zero address.
     */
    function _approve(address owner, address spender, uint256 amount) internal virtual {
        require(owner != address(0), "ERC20: approve from the zero address");
        require(spender != address(0), "ERC20: approve to the zero address");

        _allowances[owner][spender] = amount;
        emit Approval(owner, spender, amount);
    }

    /**
     * @dev Sets {decimals} to a value other than the default one of 18.
     *
     * WARNING: This function should only be called from the constructor. Most
     * applications that interact with token contracts will not expect
     * {decimals} to ever change, and may work incorrectly if it does.
     */
    function _setupDecimals(uint8 decimals_) internal {
        _decimals = decimals_;
    }

    /**
     * @dev Hook that is called before any transfer of tokens. This includes
     * minting and burning.
     *
     * Calling conditions:
     *
     * - when `from` and `to` are both non-zero, `amount` of ``from``'s tokens
     * will be to transferred to `to`.
     * - when `from` is zero, `amount` tokens will be minted for `to`.
     * - when `to` is zero, `amount` of ``from``'s tokens will be burned.
     * - `from` and `to` are never both zero.
     *
     * To learn more about hooks, head to xref:ROOT:extending-contracts.adoc#using-hooks[Using Hooks].
     */
    function _beforeTokenTransfer(address from, address to, uint256 amount) internal virtual { }
}

// File: @openzeppelin/contracts/token/ERC20/SafeERC20.sol



pragma solidity ^0.6.0;




/**
 * @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: contracts/interface/IVault.sol

pragma solidity ^0.6.2;


interface IVault is IERC20 {
    function balance() external view returns (uint256);

    function balanceOfToken() external view returns (uint256);

    function token() external view returns (address);

    function claimInsurance() external; // NOTE: Only yDelegatedVault implements this

    function getRatio() external view returns (uint256);

    function deposit(uint256) external;

    function withdraw(uint256) external;

    function earn() external;
}

// File: contracts/interface/IStakingRewards.sol

pragma solidity ^0.6.2;

interface IStakingRewards {
    function balanceOf(address account) external view returns (uint256);

    function earned(address account) external view returns (uint256);

    function exit() external;

    function getReward() external;

    function getRewardForDuration() external view returns (uint256);

    function lastTimeRewardApplicable() external view returns (uint256);

    function lastUpdateTime() external view returns (uint256);

    function notifyRewardAmount(uint256 reward) external;

    function periodFinish() external view returns (uint256);

    function rewardPerToken() external view returns (uint256);

    function rewardPerTokenStored() external view returns (uint256);

    function rewardRate() external view returns (uint256);

    function rewards(address) external view returns (uint256);

    function rewardsDistribution() external view returns (address);

    function rewardsDuration() external view returns (uint256);

    function rewardsToken() external view returns (address);

    function stake(uint256 amount) external;

    function stakeWithPermit(
        uint256 amount,
        uint256 deadline,
        uint8 v,
        bytes32 r,
        bytes32 s
    ) external;

    function stakingToken() external view returns (address);

    function totalSupply() external view returns (uint256);

    function userRewardPerTokenPaid(address) external view returns (uint256);

    function withdraw(uint256 amount) external;
}

// File: contracts/interface/UniswapRouterV2.sol

pragma solidity ^0.6.2;

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);
}

// File: contracts/interface/IController.sol

pragma solidity ^0.6.0;

interface IController {
    function vaults(address) external view returns (address);

    function comAddr() external view returns (address);

    function devAddr() external view returns (address);

    function burnAddr() external view returns (address);

    function want(address) external view returns (address); // NOTE: Only StrategyControllerV2 implements this

    function balanceOf(address) external view returns (uint256);

    function withdraw(address, uint256) external;

    function freeWithdraw(address, uint256) external;

    function earn(address, uint256) external;
}

// File: contracts/strategies/StrategyBase.sol

pragma solidity ^0.6.7;










// Strategy Contract Basics
abstract contract StrategyBase {
    using SafeERC20 for IERC20;
    using Address for address;
    using SafeMath for uint256;

    // Fees 30% in total
    // - 0%   devFundFee for development fund
    // - 0%     comFundFee for community fund
    // - 30%   used to burn/repurchase btfs which will be sent to profit pool
    uint256 public devFundFee = 0;
    uint256 public constant devFundMax = 10000;

    uint256 public comFundFee = 0;
    uint256 public constant comFundMax = 10000;

    uint256 public burnFee = 3000;
    uint256 public constant burnMax = 10000;

    // Withdrawal fee up to 0.5%
    uint256 public withdrawalFee = 0;
    uint256 public constant withdrawalMax = 10000;

    // Tokens
    address public token;
    address public constant weth = 0xC02aaA39b223FE8D0A0e5C4F27eAD9083C756Cc2;

    // stablecoins
    address public constant dai = 0x6B175474E89094C44Da98b954EedeAC495271d0F;
    address public constant usdc = 0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48;
    address public constant usdt = 0xdAC17F958D2ee523a2206206994597C13D831ec7;
    address public constant susd = 0x57Ab1ec28D129707052df4dF418D58a2D46d5f51;
    address public constant tusd = 0x0000000000085d4780B73119b644AE5ecd22b376;

    address public btf;

    // User accounts
    address public governance;
    address public controller;
    address public strategist;
    address public timelock;

    // Dex
    address public univ2Router2 = 0x7a250d5630B4cF539739dF2C5dAcb4c659F2488D;

    // usdc -> btf pair exists?
    bool public swapUsdcForBtf = false;

    constructor(
        address _btf,
        address _token,
        address _governance,
        address _strategist,
        address _controller,
        address _timelock
    ) public {
        require(_btf != address(0));
        require(_token != address(0));
        require(_governance != address(0));
        require(_strategist != address(0));
        require(_controller != address(0));
        require(_timelock != address(0));

        btf = _btf;
        token = _token;
        governance = _governance;
        strategist = _strategist;
        controller = _controller;
        timelock = _timelock;
    }

    // **** Modifiers **** //

    modifier onlyBenevolent {
        require(
            msg.sender == tx.origin ||
            msg.sender == governance ||
            msg.sender == strategist
        );
        _;
    }

    // **** Views **** //

    function balanceOfWant() public view returns (uint256) {
        return IERC20(token).balanceOf(address(this));
    }

    function balanceOfPool() public virtual view returns (uint256);

    function balanceOf() public view returns (uint256) {
        return balanceOfWant().add(balanceOfPool());
    }

    // weth->btf path
    function getSwapPathOfBtf() public view returns (address[] memory){
        address[] memory path;

        if (swapUsdcForBtf) {
            path = new address[](3);
            path[0] = weth;
            path[1] = usdc;
            path[2] = btf;
        } else {
            path = new address[](2);
            path[0] = weth;
            path[1] = btf;
        }
        return path;
    }

    function getName() external virtual pure returns (string memory);

    // **** Setters **** //

    function setBtf(address _btf) public {
        require(msg.sender == governance, "!governance");
        btf = _btf;
    }

    function setDevFundFee(uint256 _devFundFee) external {
        require(msg.sender == timelock, "!timelock");
        devFundFee = _devFundFee;
    }

    function setComFundFee(uint256 _comFundFee) external {
        require(msg.sender == timelock, "!timelock");
        comFundFee = _comFundFee;
    }

    function setBurnFee(uint256 _burnFee) external {
        require(msg.sender == timelock, "!timelock");
        burnFee = _burnFee;
    }

    function setWithdrawalFee(uint256 _withdrawalFee) external {
        require(msg.sender == timelock, "!timelock");
        withdrawalFee = _withdrawalFee;
    }

    function setStrategist(address _strategist) external {
        require(msg.sender == governance, "!governance");
        strategist = _strategist;
    }

    function setGovernance(address _governance) external {
        require(msg.sender == governance, "!governance");
        governance = _governance;
    }

    function setTimelock(address _timelock) external {
        require(msg.sender == timelock, "!timelock");
        timelock = _timelock;
    }

    function setController(address _controller) external {
        require(msg.sender == timelock, "!timelock");
        controller = _controller;
    }

    function setSwapUsdcForBtf(bool _swapUsdcForBtf) external {
        require(msg.sender == governance, "!governance");
        swapUsdcForBtf = _swapUsdcForBtf;
    }

    // **** State mutations **** //
    function deposit() public virtual;

    // Controller only function for creating additional rewards from dust
    function withdraw(IERC20 _asset) external returns (uint256 balance) {
        require(msg.sender == controller, "!controller");
        require(token != address(_asset), "token");
        balance = _asset.balanceOf(address(this));
        _asset.safeTransfer(controller, balance);
    }

    // Contoller only function for withdrawing for free
    // This is used to swap between vaults
    function freeWithdraw(uint256 _amount) external {
        require(msg.sender == controller, "!controller");
        uint256 _balance = IERC20(token).balanceOf(address(this));
        if (_balance < _amount) {
            _amount = _withdrawSome(_amount.sub(_balance));
            _amount = _amount.add(_balance);
        }
        IERC20(token).safeTransfer(msg.sender, _amount);
    }

    // Withdraw partial funds, normally used with a vault withdrawal
    function withdraw(uint256 _amount) external {
        require(msg.sender == controller, "!controller");
        uint256 _balance = IERC20(token).balanceOf(address(this));
        if (_balance < _amount) {
            _amount = _withdrawSome(_amount.sub(_balance));
            _amount = _amount.add(_balance);
        }

        if (withdrawalFee > 0) {
            uint256 _fee = _amount.mul(withdrawalFee).div(withdrawalMax);
            IERC20(token).safeTransfer(IController(controller).comAddr(), _fee);
            _amount = _amount.sub(_fee);
        }

        address _vault = IController(controller).vaults(address(token));
        require(_vault != address(0), "!vault");
        // additional protection so we don't burn the funds

        IERC20(token).safeTransfer(_vault, _amount);
    }

    // Withdraw all funds, normally used when migrating strategies
    function withdrawAll() external returns (uint256 balance) {
        require(msg.sender == controller, "!controller");
        _withdrawAll();

        balance = IERC20(token).balanceOf(address(this));

        address _vault = IController(controller).vaults(address(token));
        require(_vault != address(0), "!vault");
        // additional protection so we don't burn the funds
        IERC20(token).safeTransfer(_vault, balance);
    }

    function _withdrawAll() internal {
        _withdrawSome(balanceOfPool());
    }

    function _withdrawSome(uint256 _amount) internal virtual returns (uint256);

    function harvest() public virtual;

    // **** Emergency functions ****

    function execute(address _target, bytes memory _data)
    public
    payable
    returns (bytes memory response)
    {
        require(msg.sender == timelock, "!timelock");
        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[] memory path,
        uint256 _amount
    ) internal {
        // Swap with uniswap
        IERC20(path[0]).safeApprove(univ2Router2, 0);
        IERC20(path[0]).safeApprove(univ2Router2, _amount);

        UniswapRouterV2(univ2Router2).swapExactTokensForTokens(
            _amount,
            0,
            path,
            address(this),
            now.add(60)
        );
    }

    function _swapUniswap(
        address _from,
        address _to,
        uint256 _amount
    ) internal {
        require(_to != address(0));

        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;
        }

        _swapUniswap(path, _amount);
    }
}

// File: contracts/interface/Curve.sol


pragma solidity ^0.6.7;

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 ICurveFi_3 {
    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;
}

// File: contracts/interface/yVault.sol

pragma solidity ^0.6.2;

interface yERC20 {
    function deposit(uint256 _amount) external;

    function withdraw(uint256 _amount) external;

    function getPricePerFullShare() external view returns (uint256);
}

// File: contracts/strategies/curve/StrategyCurveBase.sol


pragma solidity ^0.6.2;



abstract contract StrategyCurveBase is StrategyBase {
    // Curve stuff
    address public want;
    address public gauge;
    // curve pool
    address public curve;
    address public mintr;

    // bitcoins
    address public constant wbtc = 0x2260FAC5E5542a773Aa44fBCfeDf7C193bc2C599;
    address public constant renbtc = 0xEB4C2781e4ebA804CE9a9803C67d0893436bB27D;

    // rewards
    address public constant crv = 0xD533a949740bb3306d119CC777fa900bA034cd52;

    constructor(
        address _want,
        address _gauge,
        address _curve,
        address _mintr,
        address _btf,
        address _governance,
        address _strategist,
        address _controller,
        address _timelock
    )
    public
    StrategyBase(_btf, _want, _governance, _strategist, _controller, _timelock)
    {
        want = _want;
        gauge = _gauge;
        curve = _curve;
        mintr = _mintr;
    }

    // **** 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));
    }

    // **** State Mutation functions ****

    function deposit() public override {
        uint256 _want = IERC20(want).balanceOf(address(this));
        if (_want > 0) {
            IERC20(want).safeApprove(gauge, 0);
            IERC20(want).safeApprove(gauge, _want);
            ICurveGauge(gauge).deposit(_want);
        }
    }

    function _withdrawSome(uint256 _amount)
    internal
    override
    returns (uint256)
    {
        ICurveGauge(gauge).withdraw(_amount);
        return _amount;
    }
}

// File: contracts/strategies/curve/StrategyCurveYcrvV1.sol


pragma solidity ^0.6.2;





contract StrategyCurveYcrvV1 is StrategyCurveBase {
    // Curve stuff
    address public _ycrv = 0xdF5e0e81Dff6FAF3A7e52BA697820c5e32D806A8;
    address public _gauge = 0xFA712EE4788C042e2B7BB55E6cb8ec569C4530c1;
    // y pool
    address public _curve = 0x45F783CCE6B7FF23B2ab2D70e416cdb7D6055f51;
    address public _mintr = 0xd061D61a4d941c39E5453435B6345Dc261C2fcE0;

    // y
    address public yDai = 0x16de59092dAE5CcF4A1E6439D611fd0653f0Bd01;

    constructor(
        address _btf,
        address _governance,
        address _strategist,
        address _controller,
        address _timelock
    )
    public
    StrategyCurveBase(_ycrv, _gauge, _curve, _mintr, _btf, _governance, _strategist, _controller, _timelock)
    {
    }

    // **** Views ****

    function getMostPremium()
    public
    pure
    returns (address, uint256)
    {
        return (dai, 0);
    }

    function getName() external override pure returns (string memory) {
        return "StrategyCurveYcrvV1";
    }

    // **** State Mutation functions ****

    function harvest() public override onlyBenevolent {
        // Anyone can harvest it at any given time.
        // I understand the possibility of being frontrun
        // But ETH is a dark forest, and I wanna see how this plays out
        // i.e. will be be heavily frontrunned?
        //      if so, a new strategy will be deployed.

        // Collects crv tokens
        // Don't bother voting in v1
        ICurveMintr(mintr).mint(gauge);
        uint256 _crv = IERC20(crv).balanceOf(address(this));
        if (_crv > 0) {
            _swapUniswap(crv, weth, _crv);
        }

        // Adds liquidity to curve.fi's pool
        uint256 _weth = IERC20(weth).balanceOf(address(this));
        if (_weth > 0) {
            if (devFundFee > 0) {
                uint256 _devFundFee = _weth.mul(devFundFee).div(devFundMax);
                IERC20(weth).transfer(
                    IController(controller).devAddr(),
                    _devFundFee
                );
            }

            // Burn some btfs first
            if (burnFee > 0) {
                uint256 _burnFee = _weth.mul(burnFee).div(burnMax);
                _swapUniswap(getSwapPathOfBtf(), _burnFee);
                IERC20(btf).transfer(
                    IController(controller).burnAddr(),
                    IERC20(btf).balanceOf(address(this))
                );
            }

            if (comFundFee > 0) {
                uint256 _comFundFee = _weth.mul(comFundFee).div(comFundMax);
                IERC20(weth).transfer(
                    IController(controller).comAddr(),
                    _comFundFee
                );
            }

            _weth = IERC20(weth).balanceOf(address(this));
            _swapUniswap(weth, dai, _weth);

            yCurveFromDai();
        }

        // get back want (3crv)
        uint256 _want = IERC20(want).balanceOf(address(this));
        if (_want > 0) {
            deposit();
        }
    }

    /**
    * Converts all DAI to yCRV using the CRV protocol.
    */
    function yCurveFromDai() internal {
        uint256 daiBalance = IERC20(dai).balanceOf(address(this));
        if (daiBalance > 0) {
            IERC20(dai).safeApprove(yDai, 0);
            IERC20(dai).safeApprove(yDai, daiBalance);
            yERC20(yDai).deposit(daiBalance);
        }
        uint256 yDaiBalance = IERC20(yDai).balanceOf(address(this));
        if (yDaiBalance > 0) {
            IERC20(yDai).safeApprove(curve, 0);
            IERC20(yDai).safeApprove(curve, yDaiBalance);
            // we can accept 0 as minimum, this will be called only by trusted roles
            uint256 minimum = 0;
            ICurveFi_4(curve).add_liquidity([yDaiBalance, 0, 0, 0], minimum);
        }
        // now we have yCRV
    }
}