// YieldTopia-Pegg $USDY Token
// 1:1 Ratio With USD - Always.
// Created by the YieldTopia ecosystem
// Website: https//YieldTopia.finance
// Whitepaper: https://docs.yieldtopia.finance
// Inspired by Origin Protocol who proved this method. 

// SPDX-License-Identifier: MIT
// OpenZeppelin Contracts (last updated v4.6.0) (token/ERC20/IERC20.sol)

pragma solidity ^0.8.0;

/**
 * @dev Interface of the ERC20 standard as defined in the EIP.
 */
interface IERC20 {
    /**
     * @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);

    /**
     * @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 `to`.
     *
     * Returns a boolean value indicating whether the operation succeeded.
     *
     * Emits a {Transfer} event.
     */
    function transfer(address to, 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 `from` to `to` 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 from,
        address to,
        uint256 amount
    ) external returns (bool);
}

pragma solidity ^0.8.1;

/**
 * @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
     * ====
     *
     * [IMPORTANT]
     * ====
     * You shouldn't rely on `isContract` to protect against flash loan attacks!
     *
     * Preventing calls from contracts is highly discouraged. It breaks composability, breaks support for smart wallets
     * like Gnosis Safe, and does not provide security since it can be circumvented by calling from a contract
     * constructor.
     * ====
     */
    function isContract(address account) internal view returns (bool) {
        // This method relies on extcodesize/address.code.length, which returns 0
        // for contracts in construction, since the code is only stored at the end
        // of the constructor execution.

        return account.code.length > 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");

        (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 functionCallWithValue(target, data, 0, "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");
        (bool success, bytes memory returndata) = target.call{value: value}(data);
        return verifyCallResultFromTarget(target, success, returndata, errorMessage);
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
     * but performing a static call.
     *
     * _Available since v3.3._
     */
    function functionStaticCall(address target, bytes memory data) internal view returns (bytes memory) {
        return functionStaticCall(target, data, "Address: low-level static call failed");
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-string-}[`functionCall`],
     * but performing a static call.
     *
     * _Available since v3.3._
     */
    function functionStaticCall(
        address target,
        bytes memory data,
        string memory errorMessage
    ) internal view returns (bytes memory) {
        (bool success, bytes memory returndata) = target.staticcall(data);
        return verifyCallResultFromTarget(target, success, returndata, errorMessage);
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-}[`functionCall`],
     * but performing a delegate call.
     *
     * _Available since v3.4._
     */
    function functionDelegateCall(address target, bytes memory data) internal returns (bytes memory) {
        return functionDelegateCall(target, data, "Address: low-level delegate call failed");
    }

    /**
     * @dev Same as {xref-Address-functionCall-address-bytes-string-}[`functionCall`],
     * but performing a delegate call.
     *
     * _Available since v3.4._
     */
    function functionDelegateCall(
        address target,
        bytes memory data,
        string memory errorMessage
    ) internal returns (bytes memory) {
        (bool success, bytes memory returndata) = target.delegatecall(data);
        return verifyCallResultFromTarget(target, success, returndata, errorMessage);
    }

    /**
     * @dev Tool to verify that a low level call to smart-contract was successful, and revert (either by bubbling
     * the revert reason or using the provided one) in case of unsuccessful call or if target was not a contract.
     *
     * _Available since v4.8._
     */
    function verifyCallResultFromTarget(
        address target,
        bool success,
        bytes memory returndata,
        string memory errorMessage
    ) internal view returns (bytes memory) {
        if (success) {
            if (returndata.length == 0) {
                // only check isContract if the call was successful and the return data is empty
                // otherwise we already know that it was a contract
                require(isContract(target), "Address: call to non-contract");
            }
            return returndata;
        } else {
            _revert(returndata, errorMessage);
        }
    }

    /**
     * @dev Tool to verify that a low level call was successful, and revert if it wasn't, either by bubbling the
     * revert reason or using the provided one.
     *
     * _Available since v4.3._
     */
    function verifyCallResult(
        bool success,
        bytes memory returndata,
        string memory errorMessage
    ) internal pure returns (bytes memory) {
        if (success) {
            return returndata;
        } else {
            _revert(returndata, errorMessage);
        }
    }

    function _revert(bytes memory returndata, string memory errorMessage) private pure {
        // 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
            /// @solidity memory-safe-assembly
            assembly {
                let returndata_size := mload(returndata)
                revert(add(32, returndata), returndata_size)
            }
        } else {
            revert(errorMessage);
        }
    }
}

// CAUTION
// This version of SafeMath should only be used with Solidity 0.8 or later,
// because it relies on the compiler's built in overflow checks.

/**
 * @dev Wrappers over Solidity's arithmetic operations.
 *
 * NOTE: `SafeMath` is generally not needed starting with Solidity 0.8, since the compiler
 * now has built in overflow checking.
 */
library SafeMath {
    /**
     * @dev Returns the addition of two unsigned integers, with an overflow flag.
     *
     * _Available since v3.4._
     */
    function tryAdd(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        unchecked {
            uint256 c = a + b;
            if (c < a) return (false, 0);
            return (true, c);
        }
    }

    /**
     * @dev Returns the subtraction of two unsigned integers, with an overflow flag.
     *
     * _Available since v3.4._
     */
    function trySub(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        unchecked {
            if (b > a) return (false, 0);
            return (true, a - b);
        }
    }

    /**
     * @dev Returns the multiplication of two unsigned integers, with an overflow flag.
     *
     * _Available since v3.4._
     */
    function tryMul(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        unchecked {
            // Gas optimization: this is cheaper than requiring 'a' not being zero, but the
            // benefit is lost if 'b' is also tested.
            // See: https://github.com/OpenZeppelin/openzeppelin-contracts/pull/522
            if (a == 0) return (true, 0);
            uint256 c = a * b;
            if (c / a != b) return (false, 0);
            return (true, c);
        }
    }

    /**
     * @dev Returns the division of two unsigned integers, with a division by zero flag.
     *
     * _Available since v3.4._
     */
    function tryDiv(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        unchecked {
            if (b == 0) return (false, 0);
            return (true, a / b);
        }
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers, with a division by zero flag.
     *
     * _Available since v3.4._
     */
    function tryMod(uint256 a, uint256 b) internal pure returns (bool, uint256) {
        unchecked {
            if (b == 0) return (false, 0);
            return (true, a % b);
        }
    }

    /**
     * @dev Returns the addition of two unsigned integers, reverting on
     * overflow.
     *
     * Counterpart to Solidity's `+` operator.
     *
     * Requirements:
     *
     * - Addition cannot overflow.
     */
    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        return a + b;
    }

    /**
     * @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 a - b;
    }

    /**
     * @dev Returns the multiplication of two unsigned integers, reverting on
     * overflow.
     *
     * Counterpart to Solidity's `*` operator.
     *
     * Requirements:
     *
     * - Multiplication cannot overflow.
     */
    function mul(uint256 a, uint256 b) internal pure returns (uint256) {
        return a * b;
    }

    /**
     * @dev Returns the integer division of two unsigned integers, reverting on
     * division by zero. The result is rounded towards zero.
     *
     * Counterpart to Solidity's `/` operator.
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        return a / b;
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * reverting when dividing by zero.
     *
     * Counterpart to Solidity's `%` operator. This function uses a `revert`
     * opcode (which leaves remaining gas untouched) while Solidity uses an
     * invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function mod(uint256 a, uint256 b) internal pure returns (uint256) {
        return a % b;
    }

    /**
     * @dev Returns the subtraction of two unsigned integers, reverting with custom message on
     * overflow (when the result is negative).
     *
     * CAUTION: This function is deprecated because it requires allocating memory for the error
     * message unnecessarily. For custom revert reasons use {trySub}.
     *
     * Counterpart to Solidity's `-` operator.
     *
     * Requirements:
     *
     * - Subtraction cannot overflow.
     */
    function sub(
        uint256 a,
        uint256 b,
        string memory errorMessage
    ) internal pure returns (uint256) {
        unchecked {
            require(b <= a, errorMessage);
            return a - b;
        }
    }

    /**
     * @dev Returns the integer division of two unsigned integers, reverting with custom message on
     * division by zero. The result is rounded towards zero.
     *
     * 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) {
        unchecked {
            require(b > 0, errorMessage);
            return a / b;
        }
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * reverting with custom message when dividing by zero.
     *
     * CAUTION: This function is deprecated because it requires allocating memory for the error
     * message unnecessarily. For custom revert reasons use {tryMod}.
     *
     * Counterpart to Solidity's `%` operator. This function uses a `revert`
     * opcode (which leaves remaining gas untouched) while Solidity uses an
     * invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     *
     * - The divisor cannot be zero.
     */
    function mod(
        uint256 a,
        uint256 b,
        string memory errorMessage
    ) internal pure returns (uint256) {
        unchecked {
            require(b > 0, errorMessage);
            return a % b;
        }
    }
}

// Based on StableMath from Stability Labs Pty. Ltd.
// https://github.com/mstable/mStable-contracts/blob/master/contracts/shared/StableMath.sol

library StableMath {
    using SafeMath for uint256;

    /**
     * @dev Scaling unit for use in specific calculations,
     * where 1 * 10**18, or 1e18 represents a unit '1'
     */
    uint256 private constant FULL_SCALE = 1e18;

    /***************************************
                    Helpers
    ****************************************/

    /**
     * @dev Adjust the scale of an integer
     * @param to Decimals to scale to
     * @param from Decimals to scale from
     */
    function scaleBy(
        uint256 x,
        uint256 to,
        uint256 from
    ) internal pure returns (uint256) {
        if (to > from) {
            x = x.mul(10**(to - from));
        } else if (to < from) {
            x = x.div(10**(from - to));
        }
        return x;
    }

    /***************************************
               Precise Arithmetic
    ****************************************/

    /**
     * @dev Multiplies two precise units, and then truncates by the full scale
     * @param x Left hand input to multiplication
     * @param y Right hand input to multiplication
     * @return Result after multiplying the two inputs and then dividing by the shared
     *         scale unit
     */
    function mulTruncate(uint256 x, uint256 y) internal pure returns (uint256) {
        return mulTruncateScale(x, y, FULL_SCALE);
    }

    /**
     * @dev Multiplies two precise units, and then truncates by the given scale. For example,
     * when calculating 90% of 10e18, (10e18 * 9e17) / 1e18 = (9e36) / 1e18 = 9e18
     * @param x Left hand input to multiplication
     * @param y Right hand input to multiplication
     * @param scale Scale unit
     * @return Result after multiplying the two inputs and then dividing by the shared
     *         scale unit
     */
    function mulTruncateScale(
        uint256 x,
        uint256 y,
        uint256 scale
    ) internal pure returns (uint256) {
        // e.g. assume scale = fullScale
        // z = 10e18 * 9e17 = 9e36
        uint256 z = x.mul(y);
        // return 9e36 / 1e18 = 9e18
        return z.div(scale);
    }

    /**
     * @dev Multiplies two precise units, and then truncates by the full scale, rounding up the result
     * @param x Left hand input to multiplication
     * @param y Right hand input to multiplication
     * @return Result after multiplying the two inputs and then dividing by the shared
     *          scale unit, rounded up to the closest base unit.
     */
    function mulTruncateCeil(uint256 x, uint256 y)
        internal
        pure
        returns (uint256)
    {
        // e.g. 8e17 * 17268172638 = 138145381104e17
        uint256 scaled = x.mul(y);
        // e.g. 138145381104e17 + 9.99...e17 = 138145381113.99...e17
        uint256 ceil = scaled.add(FULL_SCALE.sub(1));
        // e.g. 13814538111.399...e18 / 1e18 = 13814538111
        return ceil.div(FULL_SCALE);
    }

    /**
     * @dev Precisely divides two units, by first scaling the left hand operand. Useful
     *      for finding percentage weightings, i.e. 8e18/10e18 = 80% (or 8e17)
     * @param x Left hand input to division
     * @param y Right hand input to division
     * @return Result after multiplying the left operand by the scale, and
     *         executing the division on the right hand input.
     */
    function divPrecisely(uint256 x, uint256 y)
        internal
        pure
        returns (uint256)
    {
        // e.g. 8e18 * 1e18 = 8e36
        uint256 z = x.mul(FULL_SCALE);
        // e.g. 8e36 / 10e18 = 8e17
        return z.div(y);
    }
}

pragma solidity ^0.8.0;

/**
 * @dev Optional functions from the ERC20 standard.
 * Converted from openzeppelin/contracts/token/ERC20/ERC20Detailed.sol
 */
abstract contract InitializableERC20Detailed is IERC20 {
    // Storage gap to skip storage from prior to USDY reset
    uint256[100] private _____gap;

    string private _name;
    string private _symbol;
    uint8 private _decimals;

    /**
     * @dev Sets the values for `name`, `symbol`, and `decimals`. All three of
     * these values are immutable: they can only be set once during
     * construction.
     * @notice To avoid variable shadowing appended `Arg` after arguments name.
     */
    function _initialize(
        string memory nameArg,
        string memory symbolArg,
        uint8 decimalsArg
    ) internal {
        _name = nameArg;
        _symbol = symbolArg;
        _decimals = decimalsArg;
    }

    /**
     * @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.
     *
     * 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;
    }
}


// OpenZeppelin Contracts v4.4.1 (token/ERC20/extensions/draft-IERC20Permit.sol)

pragma solidity ^0.8.0;

/**
 * @dev Interface of the ERC20 Permit extension allowing approvals to be made via signatures, as defined in
 * https://eips.ethereum.org/EIPS/eip-2612[EIP-2612].
 *
 * Adds the {permit} method, which can be used to change an account's ERC20 allowance (see {IERC20-allowance}) by
 * presenting a message signed by the account. By not relying on {IERC20-approve}, the token holder account doesn't
 * need to send a transaction, and thus is not required to hold Ether at all.
 */
interface IERC20Permit {
    /**
     * @dev Sets `value` as the allowance of `spender` over ``owner``'s tokens,
     * given ``owner``'s signed approval.
     *
     * IMPORTANT: The same issues {IERC20-approve} has related to transaction
     * ordering also apply here.
     *
     * Emits an {Approval} event.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     * - `deadline` must be a timestamp in the future.
     * - `v`, `r` and `s` must be a valid `secp256k1` signature from `owner`
     * over the EIP712-formatted function arguments.
     * - the signature must use ``owner``'s current nonce (see {nonces}).
     *
     * For more information on the signature format, see the
     * https://eips.ethereum.org/EIPS/eip-2612#specification[relevant EIP
     * section].
     */
    function permit(
        address owner,
        address spender,
        uint256 value,
        uint256 deadline,
        uint8 v,
        bytes32 r,
        bytes32 s
    ) external;

    /**
     * @dev Returns the current nonce for `owner`. This value must be
     * included whenever a signature is generated for {permit}.
     *
     * Every successful call to {permit} increases ``owner``'s nonce by one. This
     * prevents a signature from being used multiple times.
     */
    function nonces(address owner) external view returns (uint256);

    /**
     * @dev Returns the domain separator used in the encoding of the signature for {permit}, as defined by {EIP712}.
     */
    // solhint-disable-next-line func-name-mixedcase
    function DOMAIN_SEPARATOR() external view returns (bytes32);
}

/**
 * @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 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'
        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) + value;
        _callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, newAllowance));
    }

    function safeDecreaseAllowance(
        IERC20 token,
        address spender,
        uint256 value
    ) internal {
        unchecked {
            uint256 oldAllowance = token.allowance(address(this), spender);
            require(oldAllowance >= value, "SafeERC20: decreased allowance below zero");
            uint256 newAllowance = oldAllowance - value;
            _callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, newAllowance));
        }
    }

    function safePermit(
        IERC20Permit token,
        address owner,
        address spender,
        uint256 value,
        uint256 deadline,
        uint8 v,
        bytes32 r,
        bytes32 s
    ) internal {
        uint256 nonceBefore = token.nonces(owner);
        token.permit(owner, spender, value, deadline, v, r, s);
        uint256 nonceAfter = token.nonces(owner);
        require(nonceAfter == nonceBefore + 1, "SafeERC20: permit did not succeed");
    }

    /**
     * @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
            require(abi.decode(returndata, (bool)), "SafeERC20: ERC20 operation did not succeed");
        }
    }
}

/**
 * @dev This is a base contract to aid in writing upgradeable contracts, or any kind of contract that will be deployed
 * behind a proxy. Since proxied contracts do not make use of a constructor, it's common to move constructor logic to an
 * external initializer function, usually called `initialize`. It then becomes necessary to protect this initializer
 * function so it can only be called once. The {initializer} modifier provided by this contract will have this effect.
 *
 * The initialization functions use a version number. Once a version number is used, it is consumed and cannot be
 * reused. This mechanism prevents re-execution of each "step" but allows the creation of new initialization steps in
 * case an upgrade adds a module that needs to be initialized.
 *
 * For example:
 *
 * [.hljs-theme-light.nopadding]
 * ```
 * contract MyToken is ERC20Upgradeable {
 *     function initialize() initializer public {
 *         __ERC20_init("MyToken", "MTK");
 *     }
 * }
 * contract MyTokenV2 is MyToken, ERC20PermitUpgradeable {
 *     function initializeV2() reinitializer(2) public {
 *         __ERC20Permit_init("MyToken");
 *     }
 * }
 * ```
 *
 * TIP: To avoid leaving the proxy in an uninitialized state, the initializer function should be called as early as
 * possible by providing the encoded function call as the `_data` argument to {ERC1967Proxy-constructor}.
 *
 * CAUTION: When used with inheritance, manual care must be taken to not invoke a parent initializer twice, or to ensure
 * that all initializers are idempotent. This is not verified automatically as constructors are by Solidity.
 *
 * [CAUTION]
 * ====
 * Avoid leaving a contract uninitialized.
 *
 * An uninitialized contract can be taken over by an attacker. This applies to both a proxy and its implementation
 * contract, which may impact the proxy. To prevent the implementation contract from being used, you should invoke
 * the {_disableInitializers} function in the constructor to automatically lock it when it is deployed:
 *
 * [.hljs-theme-light.nopadding]
 * ```
 * /// @custom:oz-upgrades-unsafe-allow constructor
 * constructor() {
 *     _disableInitializers();
 * }
 * ```
 * ====
 */
abstract contract Initializable {
    /**
     * @dev Indicates that the contract has been initialized.
     * @custom:oz-retyped-from bool
     */
    uint8 private _initialized;

    /**
     * @dev Indicates that the contract is in the process of being initialized.
     */
    bool private _initializing;

    /**
     * @dev Triggered when the contract has been initialized or reinitialized.
     */
    event Initialized(uint8 version);

    /**
     * @dev A modifier that defines a protected initializer function that can be invoked at most once. In its scope,
     * `onlyInitializing` functions can be used to initialize parent contracts. Equivalent to `reinitializer(1)`.
     */
    modifier initializer() {
        bool isTopLevelCall = !_initializing;
        require(
            (isTopLevelCall && _initialized < 1) || (!Address.isContract(address(this)) && _initialized == 1),
            "Initializable: contract is already initialized"
        );
        _initialized = 1;
        if (isTopLevelCall) {
            _initializing = true;
        }
        _;
        if (isTopLevelCall) {
            _initializing = false;
            emit Initialized(1);
        }
    }

    /**
     * @dev A modifier that defines a protected reinitializer function that can be invoked at most once, and only if the
     * contract hasn't been initialized to a greater version before. In its scope, `onlyInitializing` functions can be
     * used to initialize parent contracts.
     *
     * `initializer` is equivalent to `reinitializer(1)`, so a reinitializer may be used after the original
     * initialization step. This is essential to configure modules that are added through upgrades and that require
     * initialization.
     *
     * Note that versions can jump in increments greater than 1; this implies that if multiple reinitializers coexist in
     * a contract, executing them in the right order is up to the developer or operator.
     */
    modifier reinitializer(uint8 version) {
        require(!_initializing && _initialized < version, "Initializable: contract is already initialized");
        _initialized = version;
        _initializing = true;
        _;
        _initializing = false;
        emit Initialized(version);
    }

    /**
     * @dev Modifier to protect an initialization function so that it can only be invoked by functions with the
     * {initializer} and {reinitializer} modifiers, directly or indirectly.
     */
    modifier onlyInitializing() {
        require(_initializing, "Initializable: contract is not initializing");
        _;
    }

    /**
     * @dev Locks the contract, preventing any future reinitialization. This cannot be part of an initializer call.
     * Calling this in the constructor of a contract will prevent that contract from being initialized or reinitialized
     * to any version. It is recommended to use this to lock implementation contracts that are designed to be called
     * through proxies.
     */
    function _disableInitializers() internal virtual {
        require(!_initializing, "Initializable: contract is initializing");
        if (_initialized < type(uint8).max) {
            _initialized = type(uint8).max;
            emit Initialized(type(uint8).max);
        }
    }
}


pragma solidity ^0.8.0;

/**
 * @title USDY Governable Contract
 * @dev Copy of the openzeppelin Ownable.sol contract with nomenclature change
 *      from owner to governor and renounce methods removed. Does not use
 *      Context.sol like Ownable.sol does for simplification.
 * @author Origin Protocol Inc
 */
contract Governable {
    // Storage position of the owner and pendingOwner of the contract
    // keccak256("OUSD.governor");
    bytes32 private constant governorPosition =
        0x7bea13895fa79d2831e0a9e28edede30099005a50d652d8957cf8a607ee6ca4a;

    // keccak256("OUSD.pending.governor");
    bytes32 private constant pendingGovernorPosition =
        0x44c4d30b2eaad5130ad70c3ba6972730566f3e6359ab83e800d905c61b1c51db;

    // keccak256("OUSD.reentry.status");
    bytes32 private constant reentryStatusPosition =
        0x53bf423e48ed90e97d02ab0ebab13b2a235a6bfbe9c321847d5c175333ac4535;

    // See OpenZeppelin ReentrancyGuard implementation
    uint256 constant _NOT_ENTERED = 1;
    uint256 constant _ENTERED = 2;

    event PendingGovernorshipTransfer(
        address indexed previousGovernor,
        address indexed newGovernor
    );

    event GovernorshipTransferred(
        address indexed previousGovernor,
        address indexed newGovernor
    );

    /**
     * @dev Initializes the contract setting the deployer as the initial Governor.
     */
    constructor() {
        _setGovernor(0x18b6fa7CA8289F5D0Ad4d02bBfFBe10D5bE6D3dF);
        emit GovernorshipTransferred(address(0), _governor());
    }

    /**
     * @dev Returns the address of the current Governor.
     */
    function governor() public view returns (address) {
        return _governor();
    }

    /**
     * @dev Returns the address of the current Governor.
     */
    function _governor() internal view returns (address governorOut) {
        bytes32 position = governorPosition;
        assembly {
            governorOut := sload(position)
        }
    }

    /**
     * @dev Returns the address of the pending Governor.
     */
    function _pendingGovernor()
        internal
        view
        returns (address pendingGovernor)
    {
        bytes32 position = pendingGovernorPosition;
        assembly {
            pendingGovernor := sload(position)
        }
    }

    /**
     * @dev Throws if called by any account other than the Governor.
     */
    modifier onlyGovernor() {
        require(isGovernor(), "Caller is not the Governor");
        _;
    }

    /**
     * @dev Returns true if the caller is the current Governor.
     */
    function isGovernor() public view returns (bool) {
        return msg.sender == _governor();
    }

    function _setGovernor(address newGovernor) internal {
        bytes32 position = governorPosition;
        assembly {
            sstore(position, newGovernor)
        }
    }

    /**
     * @dev Prevents a contract from calling itself, directly or indirectly.
     * Calling a `nonReentrant` function from another `nonReentrant`
     * function is not supported. It is possible to prevent this from happening
     * by making the `nonReentrant` function external, and make it call a
     * `private` function that does the actual work.
     */
    modifier nonReentrant() {
        bytes32 position = reentryStatusPosition;
        uint256 _reentry_status;
        assembly {
            _reentry_status := sload(position)
        }

        // On the first call to nonReentrant, _notEntered will be true
        require(_reentry_status != _ENTERED, "Reentrant call");

        // Any calls to nonReentrant after this point will fail
        assembly {
            sstore(position, _ENTERED)
        }

        _;

        // By storing the original value once again, a refund is triggered (see
        // https://eips.ethereum.org/EIPS/eip-2200)
        assembly {
            sstore(position, _NOT_ENTERED)
        }
    }

    function _setPendingGovernor(address newGovernor) internal {
        bytes32 position = pendingGovernorPosition;
        assembly {
            sstore(position, newGovernor)
        }
    }

    /**
     * @dev Transfers Governance of the contract to a new account (`newGovernor`).
     * Can only be called by the current Governor. Must be claimed for this to complete
     * @param _newGovernor Address of the new Governor
     */
    function transferGovernance(address _newGovernor) external onlyGovernor {
        _setPendingGovernor(_newGovernor);
        emit PendingGovernorshipTransfer(_governor(), _newGovernor);
    }

    /**
     * @dev Claim Governance of the contract to a new account (`newGovernor`).
     * Can only be called by the new Governor.
     */
    function claimGovernance() external {
        require(
            msg.sender == _pendingGovernor(),
            "Only the pending Governor can complete the claim"
        );
        _changeGovernor(msg.sender);
    }

    /**
     * @dev Change Governance of the contract to a new account (`newGovernor`).
     * @param _newGovernor Address of the new Governor
     */
    function _changeGovernor(address _newGovernor) internal {
        require(_newGovernor != address(0), "New Governor is address(0)");
        emit GovernorshipTransferred(_governor(), _newGovernor);
        _setGovernor(_newGovernor);
    }
}


/**
 * @title USDY Token Contract
 * @dev ERC20 compatible contract for USDY
 * @dev Implements an elastic supply
 * @author Origin Protocol Inc
 */
 

/**
 * NOTE that this is an ERC20 token but the invariant that the sum of
 * balanceOf(x) for all x is not >= totalSupply(). This is a consequence of the
 * rebasing design. Any integrations with USDY should be aware.
 */

contract USDY is Initializable, InitializableERC20Detailed, Governable {
    using SafeMath for uint256;
    using StableMath for uint256;

    event TotalSupplyUpdatedHighres(
        uint256 totalSupply,
        uint256 rebasingCredits,
        uint256 rebasingCreditsPerToken
    );

    enum RebaseOptions {
        NotSet,
        OptOut,
        OptIn
    }

    uint256 private constant MAX_SUPPLY = ~uint128(0); // (2^128) - 1
    uint256 public _totalSupply;
    mapping(address => mapping(address => uint256)) private _allowances;
    address public vaultAddress = address(0);
    mapping(address => uint256) private _creditBalances;
    uint256 private _rebasingCredits;
    uint256 private _rebasingCreditsPerToken;
    // Frozen address/credits are non rebasing (value is held in contracts which
    // do not receive yield unless they explicitly opt in)
    uint256 public nonRebasingSupply;
    mapping(address => uint256) public nonRebasingCreditsPerToken;
    mapping(address => RebaseOptions) public rebaseState;
    mapping(address => uint256) public isUpgraded;

    uint256 private constant RESOLUTION_INCREASE = 1e9;

    function initialize(
        string calldata _nameArg,
        string calldata _symbolArg,
        address _vaultAddress
    ) external onlyGovernor initializer {
        InitializableERC20Detailed._initialize(_nameArg, _symbolArg, 18);
        _rebasingCreditsPerToken = 1e18;
        vaultAddress = _vaultAddress;
    }

    // The governor role will be transfered to the DAO / Governance contract once the beta period passes
    function setVaultByGovernor(address _newvault) external onlyGovernor {
        vaultAddress = _newvault ;
    }

    /**
     * @dev Verifies that the caller is the Vault contract
     */
    modifier onlyVault() {
        require(vaultAddress == msg.sender, "Caller is not the Vault");
        _;
    }

    /**
     * @return The total supply of USDY.
     */
    function totalSupply() public view override returns (uint256) {
        return _totalSupply;
    }

    /**
     * @return Low resolution rebasingCreditsPerToken
     */
    function rebasingCreditsPerToken() public view returns (uint256) {
        return _rebasingCreditsPerToken / RESOLUTION_INCREASE;
    }

    /**
     * @return Low resolution total number of rebasing credits
     */
    function rebasingCredits() public view returns (uint256) {
        return _rebasingCredits / RESOLUTION_INCREASE;
    }

    /**
     * @return High resolution rebasingCreditsPerToken
     */
    function rebasingCreditsPerTokenHighres() public view returns (uint256) {
        return _rebasingCreditsPerToken;
    }

    /**
     * @return High resolution total number of rebasing credits
     */
    function rebasingCreditsHighres() public view returns (uint256) {
        return _rebasingCredits;
    }

    /**
     * @dev Gets the balance of the specified address.
     * @param _account Address to query the balance of.
     * @return A uint256 representing the amount of base units owned by the
     *         specified address.
     */
    function balanceOf(address _account)
        public
        view
        override
        returns (uint256)
    {
        if (_creditBalances[_account] == 0) return 0;
        return
            _creditBalances[_account].divPrecisely(_creditsPerToken(_account));
    }

    /**
     * @dev Gets the credits balance of the specified address.
     * @dev Backwards compatible with old low res credits per token.
     * @param _account The address to query the balance of.
     * @return (uint256, uint256) Credit balance and credits per token of the
     *         address
     */
    function creditsBalanceOf(address _account)
        public
        view
        returns (uint256, uint256)
    {
        uint256 cpt = _creditsPerToken(_account);
        if (cpt == 1e27) {
            // For a period before the resolution upgrade, we created all new
            // contract accounts at high resolution. Since they are not changing
            // as a result of this upgrade, we will return their true values
            return (_creditBalances[_account], cpt);
        } else {
            return (
                _creditBalances[_account] / RESOLUTION_INCREASE,
                cpt / RESOLUTION_INCREASE
            );
        }
    }

    /**
     * @dev Gets the credits balance of the specified address.
     * @param _account The address to query the balance of.
     * @return (uint256, uint256, bool) Credit balance, credits per token of the
     *         address, and isUpgraded
     */
    function creditsBalanceOfHighres(address _account)
        public
        view
        returns (
            uint256,
            uint256,
            bool
        )
    {
        return (
            _creditBalances[_account],
            _creditsPerToken(_account),
            isUpgraded[_account] == 1
        );
    }

    /**
     * @dev Transfer tokens to a specified address.
     * @param _to the address to transfer to.
     * @param _value the amount to be transferred.
     * @return true on success.
     */
    function transfer(address _to, uint256 _value)
        public
        override
        returns (bool)
    {
        require(_to != address(0), "Transfer to zero address");
        require(
            _value <= balanceOf(msg.sender),
            "Transfer greater than balance"
        );

        _executeTransfer(msg.sender, _to, _value);

        emit Transfer(msg.sender, _to, _value);

        return true;
    }

    /**
     * @dev Transfer tokens from one address to another.
     * @param _from The address you want to send tokens from.
     * @param _to The address you want to transfer to.
     * @param _value The amount of tokens to be transferred.
     */
    function transferFrom(
        address _from,
        address _to,
        uint256 _value
    ) public override returns (bool) {
        require(_to != address(0), "Transfer to zero address");
        require(_value <= balanceOf(_from), "Transfer greater than balance");

        _allowances[_from][msg.sender] = _allowances[_from][msg.sender].sub(
            _value
        );

        _executeTransfer(_from, _to, _value);

        emit Transfer(_from, _to, _value);

        return true;
    }

    /**
     * @dev Update the count of non rebasing credits in response to a transfer
     * @param _from The address you want to send tokens from.
     * @param _to The address you want to transfer to.
     * @param _value Amount of USDY to transfer
     */
    function _executeTransfer(
        address _from,
        address _to,
        uint256 _value
    ) internal {
        bool isNonRebasingTo = _isNonRebasingAccount(_to);
        bool isNonRebasingFrom = _isNonRebasingAccount(_from);

        // Credits deducted and credited might be different due to the
        // differing creditsPerToken used by each account
        uint256 creditsCredited = _value.mulTruncate(_creditsPerToken(_to));
        uint256 creditsDeducted = _value.mulTruncate(_creditsPerToken(_from));

        _creditBalances[_from] = _creditBalances[_from].sub(
            creditsDeducted,
            "Transfer amount exceeds balance"
        );
        _creditBalances[_to] = _creditBalances[_to].add(creditsCredited);

        if (isNonRebasingTo && !isNonRebasingFrom) {
            // Transfer to non-rebasing account from rebasing account, credits
            // are removed from the non rebasing tally
            nonRebasingSupply = nonRebasingSupply.add(_value);
            // Update rebasingCredits by subtracting the deducted amount
            _rebasingCredits = _rebasingCredits.sub(creditsDeducted);
        } else if (!isNonRebasingTo && isNonRebasingFrom) {
            // Transfer to rebasing account from non-rebasing account
            // Decreasing non-rebasing credits by the amount that was sent
            nonRebasingSupply = nonRebasingSupply.sub(_value);
            // Update rebasingCredits by adding the credited amount
            _rebasingCredits = _rebasingCredits.add(creditsCredited);
        }
    }

    /**
     * @dev Function to check the amount of tokens that _owner has allowed to
     *      `_spender`.
     * @param _owner The address which owns the funds.
     * @param _spender The address which will spend the funds.
     * @return The number of tokens still available for the _spender.
     */
    function allowance(address _owner, address _spender)
        public
        view
        override
        returns (uint256)
    {
        return _allowances[_owner][_spender];
    }

    /**
     * @dev Approve the passed address to spend the specified amount of tokens
     *      on behalf of msg.sender. This method is included for ERC20
     *      compatibility. `increaseAllowance` and `decreaseAllowance` should be
     *      used instead.
     *
     *      Changing an allowance with this method brings the risk that someone
     *      may transfer both the old and the new allowance - if they are both
     *      greater than zero - if a transfer transaction is mined before the
     *      later approve() call is mined.
     * @param _spender The address which will spend the funds.
     * @param _value The amount of tokens to be spent.
     */
    function approve(address _spender, uint256 _value)
        public
        override
        returns (bool)
    {
        _allowances[msg.sender][_spender] = _value;
        emit Approval(msg.sender, _spender, _value);
        return true;
    }

    /**
     * @dev Increase the amount of tokens that an owner has allowed to
     *      `_spender`.
     *      This method should be used instead of approve() to avoid the double
     *      approval vulnerability described above.
     * @param _spender The address which will spend the funds.
     * @param _addedValue The amount of tokens to increase the allowance by.
     */
    function increaseAllowance(address _spender, uint256 _addedValue)
        public
        returns (bool)
    {
        _allowances[msg.sender][_spender] = _allowances[msg.sender][_spender]
            .add(_addedValue);
        emit Approval(msg.sender, _spender, _allowances[msg.sender][_spender]);
        return true;
    }

    /**
     * @dev Decrease the amount of tokens that an owner has allowed to
            `_spender`.
     * @param _spender The address which will spend the funds.
     * @param _subtractedValue The amount of tokens to decrease the allowance
     *        by.
     */
    function decreaseAllowance(address _spender, uint256 _subtractedValue)
        public
        returns (bool)
    {
        uint256 oldValue = _allowances[msg.sender][_spender];
        if (_subtractedValue >= oldValue) {
            _allowances[msg.sender][_spender] = 0;
        } else {
            _allowances[msg.sender][_spender] = oldValue.sub(_subtractedValue);
        }
        emit Approval(msg.sender, _spender, _allowances[msg.sender][_spender]);
        return true;
    }

    /**
     * @dev Mints new tokens, increasing totalSupply.
     */
    function mint(address _account, uint256 _amount) external onlyVault {
        _mint(_account, _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 nonReentrant {
        require(_account != address(0), "Mint to the zero address");

        bool isNonRebasingAccount = _isNonRebasingAccount(_account);

        uint256 creditAmount = _amount.mulTruncate(_creditsPerToken(_account));
        _creditBalances[_account] = _creditBalances[_account].add(creditAmount);

        // If the account is non rebasing and doesn't have a set creditsPerToken
        // then set it i.e. this is a mint from a fresh contract
        if (isNonRebasingAccount) {
            nonRebasingSupply = nonRebasingSupply.add(_amount);
        } else {
            _rebasingCredits = _rebasingCredits.add(creditAmount);
        }

        _totalSupply = _totalSupply.add(_amount);

        require(_totalSupply < MAX_SUPPLY, "Max supply");

        emit Transfer(address(0), _account, _amount);
    }

    /**
     * @dev Burns tokens, decreasing totalSupply.
     */
    function burn(address account, uint256 amount) external onlyVault {
        _burn(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 nonReentrant {
        require(_account != address(0), "Burn from the zero address");
        if (_amount == 0) {
            return;
        }

        bool isNonRebasingAccount = _isNonRebasingAccount(_account);
        uint256 creditAmount = _amount.mulTruncate(_creditsPerToken(_account));
        uint256 currentCredits = _creditBalances[_account];

        // Remove the credits, burning rounding errors
        if (
            currentCredits == creditAmount || currentCredits - 1 == creditAmount
        ) {
            // Handle dust from rounding
            _creditBalances[_account] = 0;
        } else if (currentCredits > creditAmount) {
            _creditBalances[_account] = _creditBalances[_account].sub(
                creditAmount
            );
        } else {
            revert("Remove exceeds balance");
        }

        // Remove from the credit tallies and non-rebasing supply
        if (isNonRebasingAccount) {
            nonRebasingSupply = nonRebasingSupply.sub(_amount);
        } else {
            _rebasingCredits = _rebasingCredits.sub(creditAmount);
        }

        _totalSupply = _totalSupply.sub(_amount);

        emit Transfer(_account, address(0), _amount);
    }

    /**
     * @dev Get the credits per token for an account. Returns a fixed amount
     *      if the account is non-rebasing.
     * @param _account Address of the account.
     */
    function _creditsPerToken(address _account)
        internal
        view
        returns (uint256)
    {
        if (nonRebasingCreditsPerToken[_account] != 0) {
            return nonRebasingCreditsPerToken[_account];
        } else {
            return _rebasingCreditsPerToken;
        }
    }

    /**
     * @dev Is an account using rebasing accounting or non-rebasing accounting?
     *      Also, ensure contracts are non-rebasing if they have not opted in.
     * @param _account Address of the account.
     */
    function _isNonRebasingAccount(address _account) internal returns (bool) {
        bool isContract = Address.isContract(_account);
        if (isContract && rebaseState[_account] == RebaseOptions.NotSet) {
            _ensureRebasingMigration(_account);
        }
        return nonRebasingCreditsPerToken[_account] > 0;
    }

    /**
     * @dev Ensures internal account for rebasing and non-rebasing credits and
     *      supply is updated following deployment of frozen yield change.
     */
    function _ensureRebasingMigration(address _account) internal {
        if (nonRebasingCreditsPerToken[_account] == 0) {
            if (_creditBalances[_account] == 0) {
                // Since there is no existing balance, we can directly set to
                // high resolution, and do not have to do any other bookkeeping
                nonRebasingCreditsPerToken[_account] = 1e27;
            } else {
                // Migrate an existing account:

                // Set fixed credits per token for this account
                nonRebasingCreditsPerToken[_account] = _rebasingCreditsPerToken;
                // Update non rebasing supply
                nonRebasingSupply = nonRebasingSupply.add(balanceOf(_account));
                // Update credit tallies
                _rebasingCredits = _rebasingCredits.sub(
                    _creditBalances[_account]
                );
            }
        }
    }

    /**
     * @dev Add a contract address to the non-rebasing exception list. The
     * address's balance will be part of rebases and the account will be exposed
     * to upside and downside.
     */
    function rebaseOptIn() public nonReentrant {
        require(_isNonRebasingAccount(msg.sender), "Account has not opted out");

        // Convert balance into the same amount at the current exchange rate
        uint256 newCreditBalance = _creditBalances[msg.sender]
            .mul(_rebasingCreditsPerToken)
            .div(_creditsPerToken(msg.sender));

        // Decreasing non rebasing supply
        nonRebasingSupply = nonRebasingSupply.sub(balanceOf(msg.sender));

        _creditBalances[msg.sender] = newCreditBalance;

        // Increase rebasing credits, totalSupply remains unchanged so no
        // adjustment necessary
        _rebasingCredits = _rebasingCredits.add(_creditBalances[msg.sender]);

        rebaseState[msg.sender] = RebaseOptions.OptIn;

        // Delete any fixed credits per token
        delete nonRebasingCreditsPerToken[msg.sender];
    }

    /**
     * @dev Explicitly mark that an address is non-rebasing.
     */
    function rebaseOptOut() public nonReentrant {
        require(!_isNonRebasingAccount(msg.sender), "Account has not opted in");

        // Increase non rebasing supply
        nonRebasingSupply = nonRebasingSupply.add(balanceOf(msg.sender));
        // Set fixed credits per token
        nonRebasingCreditsPerToken[msg.sender] = _rebasingCreditsPerToken;

        // Decrease rebasing credits, total supply remains unchanged so no
        // adjustment necessary
        _rebasingCredits = _rebasingCredits.sub(_creditBalances[msg.sender]);

        // Mark explicitly opted out of rebasing
        rebaseState[msg.sender] = RebaseOptions.OptOut;
    }

    /**
     * @dev Modify the supply without minting new tokens. This uses a change in
     *      the exchange rate between "credits" and USDY tokens to change balances.
     * @param _newTotalSupply New total supply of USDY.
     */
    function changeSupply(uint256 _newTotalSupply)
        external
        onlyVault
        nonReentrant
    {
        require(_totalSupply > 0, "Cannot increase 0 supply");

        if (_totalSupply == _newTotalSupply) {
            emit TotalSupplyUpdatedHighres(
                _totalSupply,
                _rebasingCredits,
                _rebasingCreditsPerToken
            );
            return;
        }

        _totalSupply = _newTotalSupply > MAX_SUPPLY
            ? MAX_SUPPLY
            : _newTotalSupply;

        _rebasingCreditsPerToken = _rebasingCredits.divPrecisely(
            _totalSupply.sub(nonRebasingSupply)
        );

        require(_rebasingCreditsPerToken > 0, "Invalid change in supply");

        _totalSupply = _rebasingCredits
            .divPrecisely(_rebasingCreditsPerToken)
            .add(nonRebasingSupply);

        emit TotalSupplyUpdatedHighres(
            _totalSupply,
            _rebasingCredits,
            _rebasingCreditsPerToken
        );
    }
}