Cryo Explorer Ethereum Mainnet

pragma solidity 0.5.16;


interface ISavingsManager {

    /** @dev Admin privs */
    function withdrawUnallocatedInterest(address _mAsset, address _recipient) external;

    /** @dev Public privs */
    function collectAndDistributeInterest(address _mAsset) external;

}

interface ISavingsContract {

    /** @dev Manager privs */
    function depositInterest(uint256 _amount) external;

    /** @dev Saver privs */
    function depositSavings(uint256 _amount) external returns (uint256 creditsIssued);
    function redeem(uint256 _amount) external returns (uint256 massetReturned);

}

/**
  * @title  ModuleKeys
  * @author Stability Labs Pty. Ltd.
  * @notice Provides system wide access to the byte32 represntations of system modules
  *         This allows each system module to be able to reference and update one another in a
  *         friendly way
  * @dev    keccak256() values are hardcoded to avoid re-evaluation of the constants at runtime.
  */
contract ModuleKeys {


    // keccak256("Governance");
    bytes32 internal constant KEY_GOVERNANCE = 0x9409903de1e6fd852dfc61c9dacb48196c48535b60e25abf92acc92dd689078d;
    //keccak256("Staking");
    bytes32 internal constant KEY_STAKING = 0x1df41cd916959d1163dc8f0671a666ea8a3e434c13e40faef527133b5d167034;
    //keccak256("ProxyAdmin");
    bytes32 internal constant KEY_PROXY_ADMIN = 0x96ed0203eb7e975a4cbcaa23951943fa35c5d8288117d50c12b3d48b0fab48d1;

    // keccak256("OracleHub");
    bytes32 internal constant KEY_ORACLE_HUB = 0x8ae3a082c61a7379e2280f3356a5131507d9829d222d853bfa7c9fe1200dd040;
    // keccak256("Manager");
    bytes32 internal constant KEY_MANAGER = 0x6d439300980e333f0256d64be2c9f67e86f4493ce25f82498d6db7f4be3d9e6f;
    //keccak256("Recollateraliser");
    bytes32 internal constant KEY_RECOLLATERALISER = 0x39e3ed1fc335ce346a8cbe3e64dd525cf22b37f1e2104a755e761c3c1eb4734f;
    //keccak256("MetaToken");
    bytes32 internal constant KEY_META_TOKEN = 0xea7469b14936af748ee93c53b2fe510b9928edbdccac3963321efca7eb1a57a2;
    // keccak256("SavingsManager");
    bytes32 internal constant KEY_SAVINGS_MANAGER = 0x12fe936c77a1e196473c4314f3bed8eeac1d757b319abb85bdda70df35511bf1;
}

/**
  * @title INexus
  * @dev Basic interface for interacting with the Nexus i.e. SystemKernel
  */
interface INexus {
    function governor() external view returns (address);
    function getModule(bytes32 key) external view returns (address);

    function proposeModule(bytes32 _key, address _addr) external;
    function cancelProposedModule(bytes32 _key) external;
    function acceptProposedModule(bytes32 _key) external;
    function acceptProposedModules(bytes32[] calldata _keys) external;

    function requestLockModule(bytes32 _key) external;
    function cancelLockModule(bytes32 _key) external;
    function lockModule(bytes32 _key) external;
}

/**
 * @title   Module
 * @author  Stability Labs Pty. Ltd.
 * @dev     Subscribes to module updates from a given publisher by reading from its registry
 */
contract Module is ModuleKeys {

    INexus public nexus;

    /**
     * @dev Initialises the Module by setting publisher addresses,
     *      and reading all available system module information
     */
    constructor(address _nexus) internal {
        require(_nexus != address(0), "Nexus is zero address");
        nexus = INexus(_nexus);
    }

    /**
     * @dev Modifier to allow function calls only from the Governor.
     */
    modifier onlyGovernor() {
        require(msg.sender == _governor(), "Only governor can execute");
        _;
    }

    /**
     * @dev Modifier to allow function calls only from the Governance.
     *      Governance is either Governor address or Governance address.
     */
    modifier onlyGovernance() {
        require(
            msg.sender == _governor() || msg.sender == _governance(),
            "Only governance can execute"
        );
        _;
    }

    /**
     * @dev Modifier to allow function calls only from the ProxyAdmin.
     */
    modifier onlyProxyAdmin() {
        require(
            msg.sender == _proxyAdmin(), "Only ProxyAdmin can execute"
        );
        _;
    }

    /**
     * @dev Modifier to allow function calls only from the Manager.
     */
    modifier onlyManager() {
        require(msg.sender == _manager(), "Only manager can execute");
        _;
    }

    /**
     * @dev Returns Governor address from the Nexus
     * @return Address of Governor Contract
     */
    function _governor() internal view returns (address) {
        return nexus.governor();
    }

    /**
     * @dev Returns Governance Module address from the Nexus
     * @return Address of the Governance (Phase 2)
     */
    function _governance() internal view returns (address) {
        return nexus.getModule(KEY_GOVERNANCE);
    }

    /**
     * @dev Return Staking Module address from the Nexus
     * @return Address of the Staking Module contract
     */
    function _staking() internal view returns (address) {
        return nexus.getModule(KEY_STAKING);
    }

    /**
     * @dev Return ProxyAdmin Module address from the Nexus
     * @return Address of the ProxyAdmin Module contract
     */
    function _proxyAdmin() internal view returns (address) {
        return nexus.getModule(KEY_PROXY_ADMIN);
    }

    /**
     * @dev Return MetaToken Module address from the Nexus
     * @return Address of the MetaToken Module contract
     */
    function _metaToken() internal view returns (address) {
        return nexus.getModule(KEY_META_TOKEN);
    }

    /**
     * @dev Return OracleHub Module address from the Nexus
     * @return Address of the OracleHub Module contract
     */
    function _oracleHub() internal view returns (address) {
        return nexus.getModule(KEY_ORACLE_HUB);
    }

    /**
     * @dev Return Manager Module address from the Nexus
     * @return Address of the Manager Module contract
     */
    function _manager() internal view returns (address) {
        return nexus.getModule(KEY_MANAGER);
    }

    /**
     * @dev Return SavingsManager Module address from the Nexus
     * @return Address of the SavingsManager Module contract
     */
    function _savingsManager() internal view returns (address) {
        return nexus.getModule(KEY_SAVINGS_MANAGER);
    }

    /**
     * @dev Return Recollateraliser Module address from the Nexus
     * @return  Address of the Recollateraliser Module contract (Phase 2)
     */
    function _recollateraliser() internal view returns (address) {
        return nexus.getModule(KEY_RECOLLATERALISER);
    }
}

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);
}

/**
 * @dev Wrappers over Solidity's arithmetic operations with added overflow
 * checks.
 */
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.
     *
     * _Available since v2.4.0._
     */
    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.
     *
     * _Available since v2.4.0._
     */
    function div(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        // Solidity only automatically asserts when dividing by 0
        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.
     *
     * _Available since v2.4.0._
     */
    function mod(uint256 a, uint256 b, string memory errorMessage) internal pure returns (uint256) {
        require(b != 0, errorMessage);
        return a % b;
    }
}

/**
 * @title   StableMath
 * @author  Stability Labs Pty. Ltd.
 * @notice  A library providing safe mathematical operations to multiply and
 *          divide with standardised precision.
 * @dev     Derives from OpenZeppelin's SafeMath lib and uses generic system
 *          wide variables for managing precision.
 */
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;

    /**
     * @notice Token Ratios are used when converting between units of bAsset, mAsset and MTA
     * Reasoning: Takes into account token decimals, and difference in base unit (i.e. grams to Troy oz for gold)
     * @dev bAsset ratio unit for use in exact calculations,
     * where (1 bAsset unit * bAsset.ratio) / ratioScale == x mAsset unit
     */
    uint256 private constant RATIO_SCALE = 1e8;

    /**
     * @dev Provides an interface to the scaling unit
     * @return Scaling unit (1e18 or 1 * 10**18)
     */
    function getFullScale() internal pure returns (uint256) {
        return FULL_SCALE;
    }

    /**
     * @dev Provides an interface to the ratio unit
     * @return Ratio scale unit (1e8 or 1 * 10**8)
     */
    function getRatioScale() internal pure returns (uint256) {
        return RATIO_SCALE;
    }

    /**
     * @dev Scales a given integer to the power of the full scale.
     * @param x   Simple uint256 to scale
     * @return    Scaled value a to an exact number
     */
    function scaleInteger(uint256 x)
        internal
        pure
        returns (uint256)
    {
        return x.mul(FULL_SCALE);
    }

    /***************************************
              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)
    {
        uint256 z = x.mul(y);
        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)
    {
        uint256 scaled = x.mul(y);
        uint256 ceil = scaled.add(FULL_SCALE.sub(1));
        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)
    {
        uint256 z = x.mul(FULL_SCALE);
        return z.div(y);
    }


    /***************************************
                  RATIO FUNCS
    ****************************************/

    /**
     * @dev Multiplies and truncates a token ratio, essentially flooring the result
     *      i.e. How much mAsset is this bAsset worth?
     * @param x     Left hand operand to multiplication (i.e Exact quantity)
     * @param ratio bAsset ratio
     * @return      Result after multiplying the two inputs and then dividing by the ratio scale
     */
    function mulRatioTruncate(uint256 x, uint256 ratio)
        internal
        pure
        returns (uint256 c)
    {
        return mulTruncateScale(x, ratio, RATIO_SCALE);
    }

    /**
     * @dev Multiplies and truncates a token ratio, rounding up the result
     *      i.e. How much mAsset is this bAsset worth?
     * @param x     Left hand input to multiplication (i.e Exact quantity)
     * @param ratio bAsset ratio
     * @return      Result after multiplying the two inputs and then dividing by the shared
     *              ratio scale, rounded up to the closest base unit.
     */
    function mulRatioTruncateCeil(uint256 x, uint256 ratio)
        internal
        pure
        returns (uint256)
    {
        uint256 scaled = x.mul(ratio);
        uint256 ceil = scaled.add(RATIO_SCALE.sub(1));
        return ceil.div(RATIO_SCALE);
    }


    /**
     * @dev Precisely divides two ratioed units, by first scaling the left hand operand
     *      i.e. How much bAsset is this mAsset worth?
     * @param x     Left hand operand in division
     * @param ratio bAsset ratio
     * @return      Result after multiplying the left operand by the scale, and
     *              executing the division on the right hand input.
     */
    function divRatioPrecisely(uint256 x, uint256 ratio)
        internal
        pure
        returns (uint256 c)
    {
        uint256 y = x.mul(RATIO_SCALE);
        return y.div(ratio);
    }

    /***************************************
                    HELPERS
    ****************************************/

    /**
     * @dev Calculates minimum of two numbers
     * @param x     Left hand input
     * @param y     Right hand input
     * @return      Minimum of the two inputs
     */
    function min(uint256 x, uint256 y)
        internal
        pure
        returns (uint256)
    {
        return x > y ? y : x;
    }

    /**
     * @dev Calculated maximum of two numbers
     * @param x     Left hand input
     * @param y     Right hand input
     * @return      Maximum of the two inputs
     */
    function max(uint256 x, uint256 y)
        internal
        pure
        returns (uint256)
    {
        return x > y ? x : y;
    }

    /**
     * @dev Clamps a value to an upper bound
     * @param x           Left hand input
     * @param upperBound  Maximum possible value to return
     * @return            Input x clamped to a maximum value, upperBound
     */
    function clamp(uint256 x, uint256 upperBound)
        internal
        pure
        returns (uint256)
    {
        return x > upperBound ? upperBound : x;
    }
}

/**
 * @title   SavingsContract
 * @author  Stability Labs Pty. Ltd.
 * @notice  Savings contract uses the ever increasing "exchangeRate" to increase
 *          the value of the Savers "credits" relative to the amount of additional
 *          underlying collateral that has been deposited into this contract ("interest")
 * @dev     VERSION: 1.0
 *          DATE:    2020-03-28
 */
contract SavingsContract is ISavingsContract, Module {

    using SafeMath for uint256;
    using StableMath for uint256;

    // Core events for depositing and withdrawing
    event ExchangeRateUpdated(uint256 newExchangeRate, uint256 interestCollected);
    event SavingsDeposited(address indexed saver, uint256 savingsDeposited, uint256 creditsIssued);
    event CreditsRedeemed(address indexed redeemer, uint256 creditsRedeemed, uint256 savingsCredited);
    event AutomaticInterestCollectionSwitched(bool automationEnabled);

    // Underlying asset is mUSD
    IERC20 private mUSD;

    // Amount of underlying savings in the contract
    uint256 public totalSavings;
    // Total number of savings credits issued
    uint256 public totalCredits;

    // Rate between 'savings credits' and mUSD
    // e.g. 1 credit (1e18) mulTruncate(exchangeRate) = mUSD, starts at 1:1
    // exchangeRate increases over time and is essentially a percentage based value
    uint256 public exchangeRate = 1e18;
    // Amount of credits for each saver
    mapping(address => uint256) public creditBalances;
    bool private automateInterestCollection = true;

    constructor(address _nexus, IERC20 _mUSD)
        public
        Module(_nexus)
    {
        require(address(_mUSD) != address(0), "mAsset address is zero");
        mUSD = _mUSD;
    }

    /** @dev Only the savings managaer (pulled from Nexus) can execute this */
    modifier onlySavingsManager() {
        require(msg.sender == _savingsManager(), "Only savings manager can execute");
        _;
    }

    /** @dev Enable or disable the automation of fee collection during deposit process */
    function automateInterestCollectionFlag(bool _enabled)
        external
        onlyGovernor
    {
        automateInterestCollection = _enabled;
        emit AutomaticInterestCollectionSwitched(_enabled);
    }

    /***************************************
                    INTEREST
    ****************************************/

    /**
     * @dev Deposit interest (add to savings) and update exchange rate of contract.
     *      Exchange rate is calculated as the ratio between new savings q and credits:
     *                    exchange rate = savings / credits
     *
     * @param _amount   Units of underlying to add to the savings vault
     */
    function depositInterest(uint256 _amount)
        external
        onlySavingsManager
    {
        require(_amount > 0, "Must deposit something");

        // Transfer the interest from sender to here
        require(mUSD.transferFrom(msg.sender, address(this), _amount), "Must receive tokens");
        totalSavings = totalSavings.add(_amount);

        // Calc new exchange rate, protect against initialisation case
        if(totalCredits > 0) {
            // new exchange rate is relationship between totalCredits & totalSavings
            // exchangeRate = totalSavings/totalCredits
            exchangeRate = totalSavings.divPrecisely(totalCredits);

            emit ExchangeRateUpdated(exchangeRate, _amount);
        }
    }


    /***************************************
                    SAVING
    ****************************************/

    /**
     * @dev Deposit the senders savings to the vault, and credit them internally with "credits".
     *      Credit amount is calculated as a ratio of deposit amount and exchange rate:
     *                    credits = underlying / exchangeRate
     *      If automation is enabled, we will first update the internal exchange rate by
     *      collecting any interest generated on the underlying.
     * @param _amount          Units of underlying to deposit into savings vault
     * @return creditsIssued   Units of credits issued internally
     */
    function depositSavings(uint256 _amount)
        external
        returns (uint256 creditsIssued)
    {
        require(_amount > 0, "Must deposit something");

        if(automateInterestCollection) {
            // Collect recent interest generated by basket and update exchange rate
            ISavingsManager(_savingsManager()).collectAndDistributeInterest(address(mUSD));
        }

        // Transfer tokens from sender to here
        require(mUSD.transferFrom(msg.sender, address(this), _amount), "Must receive tokens");
        totalSavings = totalSavings.add(_amount);

        // Calc how many credits they receive based on currentRatio
        creditsIssued = _massetToCredit(_amount);
        totalCredits = totalCredits.add(creditsIssued);

        // add credits to balances
        creditBalances[msg.sender] = creditBalances[msg.sender].add(creditsIssued);

        emit SavingsDeposited(msg.sender, _amount, creditsIssued);
    }

    /**
     * @dev Redeem specific number of the senders "credits" in exchange for underlying.
     *      Payout amount is calculated as a ratio of credits and exchange rate:
     *                    payout = credits * exchangeRate
     * @param _credits         Amount of credits to redeem
     * @return massetReturned  Units of underlying mAsset paid out
     */
    function redeem(uint256 _credits)
        external
        returns (uint256 massetReturned)
    {
        require(_credits > 0, "Must withdraw something");

        uint256 saverCredits = creditBalances[msg.sender];
        require(saverCredits >= _credits, "Saver has no credits");

        creditBalances[msg.sender] = saverCredits.sub(_credits);
        totalCredits = totalCredits.sub(_credits);

        // Calc payout based on currentRatio
        massetReturned = _creditToMasset(_credits);
        totalSavings = totalSavings.sub(massetReturned);

        // Transfer tokens from here to sender
        require(mUSD.transfer(msg.sender, massetReturned), "Must send tokens");

        emit CreditsRedeemed(msg.sender, _credits, massetReturned);
    }

    /**
     * @dev Converts masset amount into credits based on exchange rate
     *               c = masset / exchangeRate
     */
    function _massetToCredit(uint256 _amount)
        internal
        view
        returns (uint256 credits)
    {
        credits = _amount.divPrecisely(exchangeRate);
    }

    /**
     * @dev Converts masset amount into credits based on exchange rate
     *               m = credits * exchangeRate
     */
    function _creditToMasset(uint256 _credits)
        internal
        view
        returns (uint256 massetAmount)
    {
        massetAmount = _credits.mulTruncate(exchangeRate);
    }
}