/*
   ____            __   __        __   _
  / __/__ __ ___  / /_ / /  ___  / /_ (_)__ __
 _\ \ / // // _ \/ __// _ \/ -_)/ __// / \ \ /
/___/ \_, //_//_/\__//_//_/\__/ \__//_/ /_\_\
     /___/

* Synthetix: CollateralErc20.sol
*
* Latest source (may be newer): https://github.com/Synthetixio/synthetix/blob/master/contracts/CollateralErc20.sol
* Docs: https://docs.synthetix.io/contracts/CollateralErc20
*
* Contract Dependencies: 
*	- Collateral
*	- IAddressResolver
*	- ICollateralErc20
*	- ICollateralLoan
*	- MixinResolver
*	- MixinSystemSettings
*	- Owned
*	- State
* Libraries: 
*	- SafeDecimalMath
*	- SafeMath
*
* MIT License
* ===========
*
* Copyright (c) 2020 Synthetix
*
* Permission is hereby granted, free of charge, to any person obtaining a copy
* of this software and associated documentation files (the "Software"), to deal
* in the Software without restriction, including without limitation the rights
* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
* copies of the Software, and to permit persons to whom the Software is
* furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in all
* copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
*/



pragma solidity ^0.5.16;


// https://docs.synthetix.io/contracts/source/contracts/owned
contract Owned {
    address public owner;
    address public nominatedOwner;

    constructor(address _owner) public {
        require(_owner != address(0), "Owner address cannot be 0");
        owner = _owner;
        emit OwnerChanged(address(0), _owner);
    }

    function nominateNewOwner(address _owner) external onlyOwner {
        nominatedOwner = _owner;
        emit OwnerNominated(_owner);
    }

    function acceptOwnership() external {
        require(msg.sender == nominatedOwner, "You must be nominated before you can accept ownership");
        emit OwnerChanged(owner, nominatedOwner);
        owner = nominatedOwner;
        nominatedOwner = address(0);
    }

    modifier onlyOwner {
        _onlyOwner();
        _;
    }

    function _onlyOwner() private view {
        require(msg.sender == owner, "Only the contract owner may perform this action");
    }

    event OwnerNominated(address newOwner);
    event OwnerChanged(address oldOwner, address newOwner);
}


// https://docs.synthetix.io/contracts/source/interfaces/iaddressresolver
interface IAddressResolver {
    function getAddress(bytes32 name) external view returns (address);

    function getSynth(bytes32 key) external view returns (address);

    function requireAndGetAddress(bytes32 name, string calldata reason) external view returns (address);
}


// https://docs.synthetix.io/contracts/source/interfaces/isynth
interface ISynth {
    // Views
    function currencyKey() external view returns (bytes32);

    function transferableSynths(address account) external view returns (uint);

    // Mutative functions
    function transferAndSettle(address to, uint value) external returns (bool);

    function transferFromAndSettle(
        address from,
        address to,
        uint value
    ) external returns (bool);

    // Restricted: used internally to Synthetix
    function burn(address account, uint amount) external;

    function issue(address account, uint amount) external;
}


// https://docs.synthetix.io/contracts/source/interfaces/iissuer
interface IIssuer {
    // Views
    function anySynthOrSNXRateIsInvalid() external view returns (bool anyRateInvalid);

    function availableCurrencyKeys() external view returns (bytes32[] memory);

    function availableSynthCount() external view returns (uint);

    function availableSynths(uint index) external view returns (ISynth);

    function canBurnSynths(address account) external view returns (bool);

    function collateral(address account) external view returns (uint);

    function collateralisationRatio(address issuer) external view returns (uint);

    function collateralisationRatioAndAnyRatesInvalid(address _issuer)
        external
        view
        returns (uint cratio, bool anyRateIsInvalid);

    function debtBalanceOf(address issuer, bytes32 currencyKey) external view returns (uint debtBalance);

    function issuanceRatio() external view returns (uint);

    function lastIssueEvent(address account) external view returns (uint);

    function maxIssuableSynths(address issuer) external view returns (uint maxIssuable);

    function minimumStakeTime() external view returns (uint);

    function remainingIssuableSynths(address issuer)
        external
        view
        returns (
            uint maxIssuable,
            uint alreadyIssued,
            uint totalSystemDebt
        );

    function synths(bytes32 currencyKey) external view returns (ISynth);

    function getSynths(bytes32[] calldata currencyKeys) external view returns (ISynth[] memory);

    function synthsByAddress(address synthAddress) external view returns (bytes32);

    function totalIssuedSynths(bytes32 currencyKey, bool excludeEtherCollateral) external view returns (uint);

    function transferableSynthetixAndAnyRateIsInvalid(address account, uint balance)
        external
        view
        returns (uint transferable, bool anyRateIsInvalid);

    // Restricted: used internally to Synthetix
    function issueSynths(address from, uint amount) external;

    function issueSynthsOnBehalf(
        address issueFor,
        address from,
        uint amount
    ) external;

    function issueMaxSynths(address from) external;

    function issueMaxSynthsOnBehalf(address issueFor, address from) external;

    function burnSynths(address from, uint amount) external;

    function burnSynthsOnBehalf(
        address burnForAddress,
        address from,
        uint amount
    ) external;

    function burnSynthsToTarget(address from) external;

    function burnSynthsToTargetOnBehalf(address burnForAddress, address from) external;

    function liquidateDelinquentAccount(
        address account,
        uint susdAmount,
        address liquidator
    ) external returns (uint totalRedeemed, uint amountToLiquidate);
}


// Inheritance


// Internal references


// https://docs.synthetix.io/contracts/source/contracts/addressresolver
contract AddressResolver is Owned, IAddressResolver {
    mapping(bytes32 => address) public repository;

    constructor(address _owner) public Owned(_owner) {}

    /* ========== RESTRICTED FUNCTIONS ========== */

    function importAddresses(bytes32[] calldata names, address[] calldata destinations) external onlyOwner {
        require(names.length == destinations.length, "Input lengths must match");

        for (uint i = 0; i < names.length; i++) {
            bytes32 name = names[i];
            address destination = destinations[i];
            repository[name] = destination;
            emit AddressImported(name, destination);
        }
    }

    /* ========= PUBLIC FUNCTIONS ========== */

    function rebuildCaches(MixinResolver[] calldata destinations) external {
        for (uint i = 0; i < destinations.length; i++) {
            destinations[i].rebuildCache();
        }
    }

    /* ========== VIEWS ========== */

    function areAddressesImported(bytes32[] calldata names, address[] calldata destinations) external view returns (bool) {
        for (uint i = 0; i < names.length; i++) {
            if (repository[names[i]] != destinations[i]) {
                return false;
            }
        }
        return true;
    }

    function getAddress(bytes32 name) external view returns (address) {
        return repository[name];
    }

    function requireAndGetAddress(bytes32 name, string calldata reason) external view returns (address) {
        address _foundAddress = repository[name];
        require(_foundAddress != address(0), reason);
        return _foundAddress;
    }

    function getSynth(bytes32 key) external view returns (address) {
        IIssuer issuer = IIssuer(repository["Issuer"]);
        require(address(issuer) != address(0), "Cannot find Issuer address");
        return address(issuer.synths(key));
    }

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

    event AddressImported(bytes32 name, address destination);
}


// solhint-disable payable-fallback

// https://docs.synthetix.io/contracts/source/contracts/readproxy
contract ReadProxy is Owned {
    address public target;

    constructor(address _owner) public Owned(_owner) {}

    function setTarget(address _target) external onlyOwner {
        target = _target;
        emit TargetUpdated(target);
    }

    function() external {
        // The basics of a proxy read call
        // Note that msg.sender in the underlying will always be the address of this contract.
        assembly {
            calldatacopy(0, 0, calldatasize)

            // Use of staticcall - this will revert if the underlying function mutates state
            let result := staticcall(gas, sload(target_slot), 0, calldatasize, 0, 0)
            returndatacopy(0, 0, returndatasize)

            if iszero(result) {
                revert(0, returndatasize)
            }
            return(0, returndatasize)
        }
    }

    event TargetUpdated(address newTarget);
}


// Inheritance


// Internal references


// https://docs.synthetix.io/contracts/source/contracts/mixinresolver
contract MixinResolver {
    AddressResolver public resolver;

    mapping(bytes32 => address) private addressCache;

    constructor(address _resolver) internal {
        resolver = AddressResolver(_resolver);
    }

    /* ========== INTERNAL FUNCTIONS ========== */

    function combineArrays(bytes32[] memory first, bytes32[] memory second)
        internal
        pure
        returns (bytes32[] memory combination)
    {
        combination = new bytes32[](first.length + second.length);

        for (uint i = 0; i < first.length; i++) {
            combination[i] = first[i];
        }

        for (uint j = 0; j < second.length; j++) {
            combination[first.length + j] = second[j];
        }
    }

    /* ========== PUBLIC FUNCTIONS ========== */

    // Note: this function is public not external in order for it to be overridden and invoked via super in subclasses
    function resolverAddressesRequired() public view returns (bytes32[] memory addresses) {}

    function rebuildCache() public {
        bytes32[] memory requiredAddresses = resolverAddressesRequired();
        // The resolver must call this function whenver it updates its state
        for (uint i = 0; i < requiredAddresses.length; i++) {
            bytes32 name = requiredAddresses[i];
            // Note: can only be invoked once the resolver has all the targets needed added
            address destination = resolver.requireAndGetAddress(
                name,
                string(abi.encodePacked("Resolver missing target: ", name))
            );
            addressCache[name] = destination;
            emit CacheUpdated(name, destination);
        }
    }

    /* ========== VIEWS ========== */

    function isResolverCached() external view returns (bool) {
        bytes32[] memory requiredAddresses = resolverAddressesRequired();
        for (uint i = 0; i < requiredAddresses.length; i++) {
            bytes32 name = requiredAddresses[i];
            // false if our cache is invalid or if the resolver doesn't have the required address
            if (resolver.getAddress(name) != addressCache[name] || addressCache[name] == address(0)) {
                return false;
            }
        }

        return true;
    }

    /* ========== INTERNAL FUNCTIONS ========== */

    function requireAndGetAddress(bytes32 name) internal view returns (address) {
        address _foundAddress = addressCache[name];
        require(_foundAddress != address(0), string(abi.encodePacked("Missing address: ", name)));
        return _foundAddress;
    }

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

    event CacheUpdated(bytes32 name, address destination);
}


// https://docs.synthetix.io/contracts/source/interfaces/iflexiblestorage
interface IFlexibleStorage {
    // Views
    function getUIntValue(bytes32 contractName, bytes32 record) external view returns (uint);

    function getUIntValues(bytes32 contractName, bytes32[] calldata records) external view returns (uint[] memory);

    function getIntValue(bytes32 contractName, bytes32 record) external view returns (int);

    function getIntValues(bytes32 contractName, bytes32[] calldata records) external view returns (int[] memory);

    function getAddressValue(bytes32 contractName, bytes32 record) external view returns (address);

    function getAddressValues(bytes32 contractName, bytes32[] calldata records) external view returns (address[] memory);

    function getBoolValue(bytes32 contractName, bytes32 record) external view returns (bool);

    function getBoolValues(bytes32 contractName, bytes32[] calldata records) external view returns (bool[] memory);

    function getBytes32Value(bytes32 contractName, bytes32 record) external view returns (bytes32);

    function getBytes32Values(bytes32 contractName, bytes32[] calldata records) external view returns (bytes32[] memory);

    // Mutative functions
    function deleteUIntValue(bytes32 contractName, bytes32 record) external;

    function deleteIntValue(bytes32 contractName, bytes32 record) external;

    function deleteAddressValue(bytes32 contractName, bytes32 record) external;

    function deleteBoolValue(bytes32 contractName, bytes32 record) external;

    function deleteBytes32Value(bytes32 contractName, bytes32 record) external;

    function setUIntValue(
        bytes32 contractName,
        bytes32 record,
        uint value
    ) external;

    function setUIntValues(
        bytes32 contractName,
        bytes32[] calldata records,
        uint[] calldata values
    ) external;

    function setIntValue(
        bytes32 contractName,
        bytes32 record,
        int value
    ) external;

    function setIntValues(
        bytes32 contractName,
        bytes32[] calldata records,
        int[] calldata values
    ) external;

    function setAddressValue(
        bytes32 contractName,
        bytes32 record,
        address value
    ) external;

    function setAddressValues(
        bytes32 contractName,
        bytes32[] calldata records,
        address[] calldata values
    ) external;

    function setBoolValue(
        bytes32 contractName,
        bytes32 record,
        bool value
    ) external;

    function setBoolValues(
        bytes32 contractName,
        bytes32[] calldata records,
        bool[] calldata values
    ) external;

    function setBytes32Value(
        bytes32 contractName,
        bytes32 record,
        bytes32 value
    ) external;

    function setBytes32Values(
        bytes32 contractName,
        bytes32[] calldata records,
        bytes32[] calldata values
    ) external;
}


// Internal references


// https://docs.synthetix.io/contracts/source/contracts/mixinsystemsettings
contract MixinSystemSettings is MixinResolver {
    bytes32 internal constant SETTING_CONTRACT_NAME = "SystemSettings";

    bytes32 internal constant SETTING_WAITING_PERIOD_SECS = "waitingPeriodSecs";
    bytes32 internal constant SETTING_PRICE_DEVIATION_THRESHOLD_FACTOR = "priceDeviationThresholdFactor";
    bytes32 internal constant SETTING_ISSUANCE_RATIO = "issuanceRatio";
    bytes32 internal constant SETTING_FEE_PERIOD_DURATION = "feePeriodDuration";
    bytes32 internal constant SETTING_TARGET_THRESHOLD = "targetThreshold";
    bytes32 internal constant SETTING_LIQUIDATION_DELAY = "liquidationDelay";
    bytes32 internal constant SETTING_LIQUIDATION_RATIO = "liquidationRatio";
    bytes32 internal constant SETTING_LIQUIDATION_PENALTY = "liquidationPenalty";
    bytes32 internal constant SETTING_RATE_STALE_PERIOD = "rateStalePeriod";
    bytes32 internal constant SETTING_EXCHANGE_FEE_RATE = "exchangeFeeRate";
    bytes32 internal constant SETTING_MINIMUM_STAKE_TIME = "minimumStakeTime";
    bytes32 internal constant SETTING_AGGREGATOR_WARNING_FLAGS = "aggregatorWarningFlags";
    bytes32 internal constant SETTING_TRADING_REWARDS_ENABLED = "tradingRewardsEnabled";
    bytes32 internal constant SETTING_DEBT_SNAPSHOT_STALE_TIME = "debtSnapshotStaleTime";
    bytes32 internal constant SETTING_CROSS_DOMAIN_MESSAGE_GAS_LIMIT = "crossDomainMessageGasLimit";

    bytes32 internal constant CONTRACT_FLEXIBLESTORAGE = "FlexibleStorage";

    constructor(address _resolver) internal MixinResolver(_resolver) {}

    function resolverAddressesRequired() public view returns (bytes32[] memory addresses) {
        addresses = new bytes32[](1);
        addresses[0] = CONTRACT_FLEXIBLESTORAGE;
    }

    function flexibleStorage() internal view returns (IFlexibleStorage) {
        return IFlexibleStorage(requireAndGetAddress(CONTRACT_FLEXIBLESTORAGE));
    }

    function getCrossDomainMessageGasLimit() internal view returns (uint) {
        return flexibleStorage().getUIntValue(SETTING_CONTRACT_NAME, SETTING_CROSS_DOMAIN_MESSAGE_GAS_LIMIT);
    }

    function getTradingRewardsEnabled() internal view returns (bool) {
        return flexibleStorage().getBoolValue(SETTING_CONTRACT_NAME, SETTING_TRADING_REWARDS_ENABLED);
    }

    function getWaitingPeriodSecs() internal view returns (uint) {
        return flexibleStorage().getUIntValue(SETTING_CONTRACT_NAME, SETTING_WAITING_PERIOD_SECS);
    }

    function getPriceDeviationThresholdFactor() internal view returns (uint) {
        return flexibleStorage().getUIntValue(SETTING_CONTRACT_NAME, SETTING_PRICE_DEVIATION_THRESHOLD_FACTOR);
    }

    function getIssuanceRatio() internal view returns (uint) {
        // lookup on flexible storage directly for gas savings (rather than via SystemSettings)
        return flexibleStorage().getUIntValue(SETTING_CONTRACT_NAME, SETTING_ISSUANCE_RATIO);
    }

    function getFeePeriodDuration() internal view returns (uint) {
        // lookup on flexible storage directly for gas savings (rather than via SystemSettings)
        return flexibleStorage().getUIntValue(SETTING_CONTRACT_NAME, SETTING_FEE_PERIOD_DURATION);
    }

    function getTargetThreshold() internal view returns (uint) {
        // lookup on flexible storage directly for gas savings (rather than via SystemSettings)
        return flexibleStorage().getUIntValue(SETTING_CONTRACT_NAME, SETTING_TARGET_THRESHOLD);
    }

    function getLiquidationDelay() internal view returns (uint) {
        return flexibleStorage().getUIntValue(SETTING_CONTRACT_NAME, SETTING_LIQUIDATION_DELAY);
    }

    function getLiquidationRatio() internal view returns (uint) {
        return flexibleStorage().getUIntValue(SETTING_CONTRACT_NAME, SETTING_LIQUIDATION_RATIO);
    }

    function getLiquidationPenalty() internal view returns (uint) {
        return flexibleStorage().getUIntValue(SETTING_CONTRACT_NAME, SETTING_LIQUIDATION_PENALTY);
    }

    function getRateStalePeriod() internal view returns (uint) {
        return flexibleStorage().getUIntValue(SETTING_CONTRACT_NAME, SETTING_RATE_STALE_PERIOD);
    }

    function getExchangeFeeRate(bytes32 currencyKey) internal view returns (uint) {
        return
            flexibleStorage().getUIntValue(
                SETTING_CONTRACT_NAME,
                keccak256(abi.encodePacked(SETTING_EXCHANGE_FEE_RATE, currencyKey))
            );
    }

    function getMinimumStakeTime() internal view returns (uint) {
        return flexibleStorage().getUIntValue(SETTING_CONTRACT_NAME, SETTING_MINIMUM_STAKE_TIME);
    }

    function getAggregatorWarningFlags() internal view returns (address) {
        return flexibleStorage().getAddressValue(SETTING_CONTRACT_NAME, SETTING_AGGREGATOR_WARNING_FLAGS);
    }

    function getDebtSnapshotStaleTime() internal view returns (uint) {
        return flexibleStorage().getUIntValue(SETTING_CONTRACT_NAME, SETTING_DEBT_SNAPSHOT_STALE_TIME);
    }
}


pragma experimental ABIEncoderV2;

interface ICollateralLoan {
    struct Loan {
        // ID for the loan
        uint id;
        //  Acccount that created the loan
        address payable account;
        //  Amount of collateral deposited
        uint collateral;
        // The synth that was borowed
        bytes32 currency;
        //  Amount of synths borrowed
        uint amount;
        // Indicates if the position was short sold
        bool short;
        // interest amounts accrued
        uint accruedInterest;
        // last interest index
        uint interestIndex;
        // time of last interaction.
        uint lastInteraction;
    }
}


/**
 * @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) {
        require(b <= a, "SafeMath: subtraction overflow");
        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-solidity/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) {
        // Solidity only automatically asserts when dividing by 0
        require(b > 0, "SafeMath: division by zero");
        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) {
        require(b != 0, "SafeMath: modulo by zero");
        return a % b;
    }
}


// Libraries


// https://docs.synthetix.io/contracts/source/libraries/safedecimalmath
library SafeDecimalMath {
    using SafeMath for uint;

    /* Number of decimal places in the representations. */
    uint8 public constant decimals = 18;
    uint8 public constant highPrecisionDecimals = 27;

    /* The number representing 1.0. */
    uint public constant UNIT = 10**uint(decimals);

    /* The number representing 1.0 for higher fidelity numbers. */
    uint public constant PRECISE_UNIT = 10**uint(highPrecisionDecimals);
    uint private constant UNIT_TO_HIGH_PRECISION_CONVERSION_FACTOR = 10**uint(highPrecisionDecimals - decimals);

    /**
     * @return Provides an interface to UNIT.
     */
    function unit() external pure returns (uint) {
        return UNIT;
    }

    /**
     * @return Provides an interface to PRECISE_UNIT.
     */
    function preciseUnit() external pure returns (uint) {
        return PRECISE_UNIT;
    }

    /**
     * @return The result of multiplying x and y, interpreting the operands as fixed-point
     * decimals.
     *
     * @dev A unit factor is divided out after the product of x and y is evaluated,
     * so that product must be less than 2**256. As this is an integer division,
     * the internal division always rounds down. This helps save on gas. Rounding
     * is more expensive on gas.
     */
    function multiplyDecimal(uint x, uint y) internal pure returns (uint) {
        /* Divide by UNIT to remove the extra factor introduced by the product. */
        return x.mul(y) / UNIT;
    }

    /**
     * @return The result of safely multiplying x and y, interpreting the operands
     * as fixed-point decimals of the specified precision unit.
     *
     * @dev The operands should be in the form of a the specified unit factor which will be
     * divided out after the product of x and y is evaluated, so that product must be
     * less than 2**256.
     *
     * Unlike multiplyDecimal, this function rounds the result to the nearest increment.
     * Rounding is useful when you need to retain fidelity for small decimal numbers
     * (eg. small fractions or percentages).
     */
    function _multiplyDecimalRound(
        uint x,
        uint y,
        uint precisionUnit
    ) private pure returns (uint) {
        /* Divide by UNIT to remove the extra factor introduced by the product. */
        uint quotientTimesTen = x.mul(y) / (precisionUnit / 10);

        if (quotientTimesTen % 10 >= 5) {
            quotientTimesTen += 10;
        }

        return quotientTimesTen / 10;
    }

    /**
     * @return The result of safely multiplying x and y, interpreting the operands
     * as fixed-point decimals of a precise unit.
     *
     * @dev The operands should be in the precise unit factor which will be
     * divided out after the product of x and y is evaluated, so that product must be
     * less than 2**256.
     *
     * Unlike multiplyDecimal, this function rounds the result to the nearest increment.
     * Rounding is useful when you need to retain fidelity for small decimal numbers
     * (eg. small fractions or percentages).
     */
    function multiplyDecimalRoundPrecise(uint x, uint y) internal pure returns (uint) {
        return _multiplyDecimalRound(x, y, PRECISE_UNIT);
    }

    /**
     * @return The result of safely multiplying x and y, interpreting the operands
     * as fixed-point decimals of a standard unit.
     *
     * @dev The operands should be in the standard unit factor which will be
     * divided out after the product of x and y is evaluated, so that product must be
     * less than 2**256.
     *
     * Unlike multiplyDecimal, this function rounds the result to the nearest increment.
     * Rounding is useful when you need to retain fidelity for small decimal numbers
     * (eg. small fractions or percentages).
     */
    function multiplyDecimalRound(uint x, uint y) internal pure returns (uint) {
        return _multiplyDecimalRound(x, y, UNIT);
    }

    /**
     * @return The result of safely dividing x and y. The return value is a high
     * precision decimal.
     *
     * @dev y is divided after the product of x and the standard precision unit
     * is evaluated, so the product of x and UNIT must be less than 2**256. As
     * this is an integer division, the result is always rounded down.
     * This helps save on gas. Rounding is more expensive on gas.
     */
    function divideDecimal(uint x, uint y) internal pure returns (uint) {
        /* Reintroduce the UNIT factor that will be divided out by y. */
        return x.mul(UNIT).div(y);
    }

    /**
     * @return The result of safely dividing x and y. The return value is as a rounded
     * decimal in the precision unit specified in the parameter.
     *
     * @dev y is divided after the product of x and the specified precision unit
     * is evaluated, so the product of x and the specified precision unit must
     * be less than 2**256. The result is rounded to the nearest increment.
     */
    function _divideDecimalRound(
        uint x,
        uint y,
        uint precisionUnit
    ) private pure returns (uint) {
        uint resultTimesTen = x.mul(precisionUnit * 10).div(y);

        if (resultTimesTen % 10 >= 5) {
            resultTimesTen += 10;
        }

        return resultTimesTen / 10;
    }

    /**
     * @return The result of safely dividing x and y. The return value is as a rounded
     * standard precision decimal.
     *
     * @dev y is divided after the product of x and the standard precision unit
     * is evaluated, so the product of x and the standard precision unit must
     * be less than 2**256. The result is rounded to the nearest increment.
     */
    function divideDecimalRound(uint x, uint y) internal pure returns (uint) {
        return _divideDecimalRound(x, y, UNIT);
    }

    /**
     * @return The result of safely dividing x and y. The return value is as a rounded
     * high precision decimal.
     *
     * @dev y is divided after the product of x and the high precision unit
     * is evaluated, so the product of x and the high precision unit must
     * be less than 2**256. The result is rounded to the nearest increment.
     */
    function divideDecimalRoundPrecise(uint x, uint y) internal pure returns (uint) {
        return _divideDecimalRound(x, y, PRECISE_UNIT);
    }

    /**
     * @dev Convert a standard decimal representation to a high precision one.
     */
    function decimalToPreciseDecimal(uint i) internal pure returns (uint) {
        return i.mul(UNIT_TO_HIGH_PRECISION_CONVERSION_FACTOR);
    }

    /**
     * @dev Convert a high precision decimal to a standard decimal representation.
     */
    function preciseDecimalToDecimal(uint i) internal pure returns (uint) {
        uint quotientTimesTen = i / (UNIT_TO_HIGH_PRECISION_CONVERSION_FACTOR / 10);

        if (quotientTimesTen % 10 >= 5) {
            quotientTimesTen += 10;
        }

        return quotientTimesTen / 10;
    }
}


// Inheritance


// https://docs.synthetix.io/contracts/source/contracts/state
contract State is Owned {
    // the address of the contract that can modify variables
    // this can only be changed by the owner of this contract
    address public associatedContract;

    constructor(address _associatedContract) internal {
        // This contract is abstract, and thus cannot be instantiated directly
        require(owner != address(0), "Owner must be set");

        associatedContract = _associatedContract;
        emit AssociatedContractUpdated(_associatedContract);
    }

    /* ========== SETTERS ========== */

    // Change the associated contract to a new address
    function setAssociatedContract(address _associatedContract) external onlyOwner {
        associatedContract = _associatedContract;
        emit AssociatedContractUpdated(_associatedContract);
    }

    /* ========== MODIFIERS ========== */

    modifier onlyAssociatedContract {
        require(msg.sender == associatedContract, "Only the associated contract can perform this action");
        _;
    }

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

    event AssociatedContractUpdated(address associatedContract);
}


// Inheritance


// Libraries


contract CollateralState is Owned, State, ICollateralLoan {
    using SafeMath for uint;
    using SafeDecimalMath for uint;

    mapping(address => Loan[]) public loans;

    constructor(address _owner, address _associatedContract) public Owned(_owner) State(_associatedContract) {}

    /* ========== VIEWS ========== */
    // If we do not find the loan, this returns a struct with 0'd values.
    function getLoan(address account, uint256 loanID) external view returns (Loan memory) {
        Loan[] memory accountLoans = loans[account];
        for (uint i = 0; i < accountLoans.length; i++) {
            if (accountLoans[i].id == loanID) {
                return (accountLoans[i]);
            }
        }
    }

    function getNumLoans(address account) external view returns (uint numLoans) {
        return loans[account].length;
    }

    /* ========== MUTATIVE FUNCTIONS ========== */

    function createLoan(Loan memory loan) public onlyAssociatedContract {
        loans[loan.account].push(loan);
    }

    function updateLoan(Loan memory loan) public onlyAssociatedContract {
        Loan[] storage accountLoans = loans[loan.account];
        for (uint i = 0; i < accountLoans.length; i++) {
            if (accountLoans[i].id == loan.id) {
                loans[loan.account][i] = loan;
            }
        }
    }
}


interface ICollateralManager {
    // Manager information
    function hasCollateral(address collateral) external view returns (bool);

    function isSynthManaged(bytes32 currencyKey) external view returns (bool);

    // State information
    function long(bytes32 synth) external view returns (uint amount);

    function short(bytes32 synth) external view returns (uint amount);

    function totalLong() external view returns (uint susdValue, bool anyRateIsInvalid);

    function totalShort() external view returns (uint susdValue, bool anyRateIsInvalid);

    function getBorrowRate() external view returns (uint borrowRate, bool anyRateIsInvalid);

    function getShortRate(bytes32 synth) external view returns (uint shortRate, bool rateIsInvalid);

    function getRatesAndTime(uint index)
        external
        view
        returns (
            uint entryRate,
            uint lastRate,
            uint lastUpdated,
            uint newIndex
        );

    function getShortRatesAndTime(bytes32 currency, uint index)
        external
        view
        returns (
            uint entryRate,
            uint lastRate,
            uint lastUpdated,
            uint newIndex
        );

    function exceedsDebtLimit(uint amount, bytes32 currency) external view returns (bool canIssue, bool anyRateIsInvalid);

    function areSynthsAndCurrenciesSet(bytes32[] calldata requiredSynthNamesInResolver, bytes32[] calldata synthKeys)
        external
        view
        returns (bool);

    function areShortableSynthsSet(bytes32[] calldata requiredSynthNamesInResolver, bytes32[] calldata synthKeys)
        external
        view
        returns (bool);

    // Loans
    function getNewLoanId() external returns (uint id);

    // Manager mutative
    function addCollaterals(address[] calldata collaterals) external;

    function removeCollaterals(address[] calldata collaterals) external;

    function addSynths(bytes32[] calldata synthNamesInResolver, bytes32[] calldata synthKeys) external;

    function removeSynths(bytes32[] calldata synths, bytes32[] calldata synthKeys) external;

    function addShortableSynths(bytes32[2][] calldata requiredSynthAndInverseNamesInResolver, bytes32[] calldata synthKeys)
        external;

    function removeShortableSynths(bytes32[] calldata synths) external;

    // State mutative
    function updateBorrowRates(uint rate) external;

    function updateShortRates(bytes32 currency, uint rate) external;

    function incrementLongs(bytes32 synth, uint amount) external;

    function decrementLongs(bytes32 synth, uint amount) external;

    function incrementShorts(bytes32 synth, uint amount) external;

    function decrementShorts(bytes32 synth, uint amount) external;
}


// https://docs.synthetix.io/contracts/source/interfaces/isystemstatus
interface ISystemStatus {
    struct Status {
        bool canSuspend;
        bool canResume;
    }

    struct Suspension {
        bool suspended;
        // reason is an integer code,
        // 0 => no reason, 1 => upgrading, 2+ => defined by system usage
        uint248 reason;
    }

    // Views
    function accessControl(bytes32 section, address account) external view returns (bool canSuspend, bool canResume);

    function requireSystemActive() external view;

    function requireIssuanceActive() external view;

    function requireExchangeActive() external view;

    function requireSynthActive(bytes32 currencyKey) external view;

    function requireSynthsActive(bytes32 sourceCurrencyKey, bytes32 destinationCurrencyKey) external view;

    function synthSuspension(bytes32 currencyKey) external view returns (bool suspended, uint248 reason);

    // Restricted functions
    function suspendSynth(bytes32 currencyKey, uint256 reason) external;

    function updateAccessControl(
        bytes32 section,
        address account,
        bool canSuspend,
        bool canResume
    ) external;
}


// https://docs.synthetix.io/contracts/source/interfaces/ifeepool
interface IFeePool {
    // Views

    // solhint-disable-next-line func-name-mixedcase
    function FEE_ADDRESS() external view returns (address);

    function feesAvailable(address account) external view returns (uint, uint);

    function feePeriodDuration() external view returns (uint);

    function isFeesClaimable(address account) external view returns (bool);

    function targetThreshold() external view returns (uint);

    function totalFeesAvailable() external view returns (uint);

    function totalRewardsAvailable() external view returns (uint);

    // Mutative Functions
    function claimFees() external returns (bool);

    function claimOnBehalf(address claimingForAddress) external returns (bool);

    function closeCurrentFeePeriod() external;

    // Restricted: used internally to Synthetix
    function appendAccountIssuanceRecord(
        address account,
        uint lockedAmount,
        uint debtEntryIndex
    ) external;

    function recordFeePaid(uint sUSDAmount) external;

    function setRewardsToDistribute(uint amount) external;
}


// https://docs.synthetix.io/contracts/source/interfaces/ierc20
interface IERC20 {
    // ERC20 Optional Views
    function name() external view returns (string memory);

    function symbol() external view returns (string memory);

    function decimals() external view returns (uint8);

    // Views
    function totalSupply() external view returns (uint);

    function balanceOf(address owner) external view returns (uint);

    function allowance(address owner, address spender) external view returns (uint);

    // Mutative functions
    function transfer(address to, uint value) external returns (bool);

    function approve(address spender, uint value) external returns (bool);

    function transferFrom(
        address from,
        address to,
        uint value
    ) external returns (bool);

    // Events
    event Transfer(address indexed from, address indexed to, uint value);

    event Approval(address indexed owner, address indexed spender, uint value);
}


// https://docs.synthetix.io/contracts/source/interfaces/iexchangerates
interface IExchangeRates {
    // Structs
    struct RateAndUpdatedTime {
        uint216 rate;
        uint40 time;
    }

    struct InversePricing {
        uint entryPoint;
        uint upperLimit;
        uint lowerLimit;
        bool frozenAtUpperLimit;
        bool frozenAtLowerLimit;
    }

    // Views
    function aggregators(bytes32 currencyKey) external view returns (address);

    function aggregatorWarningFlags() external view returns (address);

    function anyRateIsInvalid(bytes32[] calldata currencyKeys) external view returns (bool);

    function canFreezeRate(bytes32 currencyKey) external view returns (bool);

    function currentRoundForRate(bytes32 currencyKey) external view returns (uint);

    function currenciesUsingAggregator(address aggregator) external view returns (bytes32[] memory);

    function effectiveValue(
        bytes32 sourceCurrencyKey,
        uint sourceAmount,
        bytes32 destinationCurrencyKey
    ) external view returns (uint value);

    function effectiveValueAndRates(
        bytes32 sourceCurrencyKey,
        uint sourceAmount,
        bytes32 destinationCurrencyKey
    )
        external
        view
        returns (
            uint value,
            uint sourceRate,
            uint destinationRate
        );

    function effectiveValueAtRound(
        bytes32 sourceCurrencyKey,
        uint sourceAmount,
        bytes32 destinationCurrencyKey,
        uint roundIdForSrc,
        uint roundIdForDest
    ) external view returns (uint value);

    function getCurrentRoundId(bytes32 currencyKey) external view returns (uint);

    function getLastRoundIdBeforeElapsedSecs(
        bytes32 currencyKey,
        uint startingRoundId,
        uint startingTimestamp,
        uint timediff
    ) external view returns (uint);

    function inversePricing(bytes32 currencyKey)
        external
        view
        returns (
            uint entryPoint,
            uint upperLimit,
            uint lowerLimit,
            bool frozenAtUpperLimit,
            bool frozenAtLowerLimit
        );

    function lastRateUpdateTimes(bytes32 currencyKey) external view returns (uint256);

    function oracle() external view returns (address);

    function rateAndTimestampAtRound(bytes32 currencyKey, uint roundId) external view returns (uint rate, uint time);

    function rateAndUpdatedTime(bytes32 currencyKey) external view returns (uint rate, uint time);

    function rateAndInvalid(bytes32 currencyKey) external view returns (uint rate, bool isInvalid);

    function rateForCurrency(bytes32 currencyKey) external view returns (uint);

    function rateIsFlagged(bytes32 currencyKey) external view returns (bool);

    function rateIsFrozen(bytes32 currencyKey) external view returns (bool);

    function rateIsInvalid(bytes32 currencyKey) external view returns (bool);

    function rateIsStale(bytes32 currencyKey) external view returns (bool);

    function rateStalePeriod() external view returns (uint);

    function ratesAndUpdatedTimeForCurrencyLastNRounds(bytes32 currencyKey, uint numRounds)
        external
        view
        returns (uint[] memory rates, uint[] memory times);

    function ratesAndInvalidForCurrencies(bytes32[] calldata currencyKeys)
        external
        view
        returns (uint[] memory rates, bool anyRateInvalid);

    function ratesForCurrencies(bytes32[] calldata currencyKeys) external view returns (uint[] memory);

    // Mutative functions
    function freezeRate(bytes32 currencyKey) external;
}


interface IVirtualSynth {
    // Views
    function balanceOfUnderlying(address account) external view returns (uint);

    function rate() external view returns (uint);

    function readyToSettle() external view returns (bool);

    function secsLeftInWaitingPeriod() external view returns (uint);

    function settled() external view returns (bool);

    function synth() external view returns (ISynth);

    // Mutative functions
    function settle(address account) external;
}


// https://docs.synthetix.io/contracts/source/interfaces/iexchanger
interface IExchanger {
    // Views
    function calculateAmountAfterSettlement(
        address from,
        bytes32 currencyKey,
        uint amount,
        uint refunded
    ) external view returns (uint amountAfterSettlement);

    function isSynthRateInvalid(bytes32 currencyKey) external view returns (bool);

    function maxSecsLeftInWaitingPeriod(address account, bytes32 currencyKey) external view returns (uint);

    function settlementOwing(address account, bytes32 currencyKey)
        external
        view
        returns (
            uint reclaimAmount,
            uint rebateAmount,
            uint numEntries
        );

    function hasWaitingPeriodOrSettlementOwing(address account, bytes32 currencyKey) external view returns (bool);

    function feeRateForExchange(bytes32 sourceCurrencyKey, bytes32 destinationCurrencyKey)
        external
        view
        returns (uint exchangeFeeRate);

    function getAmountsForExchange(
        uint sourceAmount,
        bytes32 sourceCurrencyKey,
        bytes32 destinationCurrencyKey
    )
        external
        view
        returns (
            uint amountReceived,
            uint fee,
            uint exchangeFeeRate
        );

    function priceDeviationThresholdFactor() external view returns (uint);

    function waitingPeriodSecs() external view returns (uint);

    // Mutative functions
    function exchange(
        address from,
        bytes32 sourceCurrencyKey,
        uint sourceAmount,
        bytes32 destinationCurrencyKey,
        address destinationAddress
    ) external returns (uint amountReceived);

    function exchangeOnBehalf(
        address exchangeForAddress,
        address from,
        bytes32 sourceCurrencyKey,
        uint sourceAmount,
        bytes32 destinationCurrencyKey
    ) external returns (uint amountReceived);

    function exchangeWithTracking(
        address from,
        bytes32 sourceCurrencyKey,
        uint sourceAmount,
        bytes32 destinationCurrencyKey,
        address destinationAddress,
        address originator,
        bytes32 trackingCode
    ) external returns (uint amountReceived);

    function exchangeOnBehalfWithTracking(
        address exchangeForAddress,
        address from,
        bytes32 sourceCurrencyKey,
        uint sourceAmount,
        bytes32 destinationCurrencyKey,
        address originator,
        bytes32 trackingCode
    ) external returns (uint amountReceived);

    function exchangeWithVirtual(
        address from,
        bytes32 sourceCurrencyKey,
        uint sourceAmount,
        bytes32 destinationCurrencyKey,
        address destinationAddress,
        bytes32 trackingCode
    ) external returns (uint amountReceived, IVirtualSynth vSynth);

    function settle(address from, bytes32 currencyKey)
        external
        returns (
            uint reclaimed,
            uint refunded,
            uint numEntries
        );

    function setLastExchangeRateForSynth(bytes32 currencyKey, uint rate) external;

    function suspendSynthWithInvalidRate(bytes32 currencyKey) external;
}


// https://docs.synthetix.io/contracts/source/interfaces/istakingrewards
interface IShortingRewards {
    // Views
    function lastTimeRewardApplicable() external view returns (uint256);

    function rewardPerToken() external view returns (uint256);

    function earned(address account) external view returns (uint256);

    function getRewardForDuration() external view returns (uint256);

    function totalSupply() external view returns (uint256);

    function balanceOf(address account) external view returns (uint256);

    // Mutative

    function enrol(address account, uint256 amount) external;

    function withdraw(address account, uint256 amount) external;

    function getReward(address account) external;

    function exit(address account) external;
}


// Inheritance


// Libraries


// Internal references


contract Collateral is ICollateralLoan, Owned, MixinSystemSettings {
    /* ========== LIBRARIES ========== */
    using SafeMath for uint;
    using SafeDecimalMath for uint;

    /* ========== CONSTANTS ========== */

    bytes32 private constant sUSD = "sUSD";

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

    // The synth corresponding to the collateral.
    bytes32 public collateralKey;

    // Stores loans
    CollateralState public state;

    address public manager;

    // The synths that this contract can issue.
    bytes32[] public synths;

    // Map from currency key to synth contract name.
    mapping(bytes32 => bytes32) public synthsByKey;

    // Map from currency key to the shorting rewards contract
    mapping(bytes32 => address) public shortingRewards;

    // ========== SETTER STATE VARIABLES ==========

    // The minimum collateral ratio required to avoid liquidation.
    uint public minCratio;

    // The minimum amount of collateral to create a loan.
    uint public minCollateral;

    // The fee charged for issuing a loan.
    uint public issueFeeRate;

    // The maximum number of loans that an account can create with this collateral.
    uint public maxLoansPerAccount = 50;

    // Time in seconds that a user must wait between interacting with a loan.
    // Provides front running and flash loan protection.
    uint public interactionDelay = 300;

    bool public canOpenLoans = true;

    /* ========== ADDRESS RESOLVER CONFIGURATION ========== */

    bytes32 private constant CONTRACT_SYSTEMSTATUS = "SystemStatus";
    bytes32 private constant CONTRACT_EXRATES = "ExchangeRates";
    bytes32 private constant CONTRACT_EXCHANGER = "Exchanger";
    bytes32 private constant CONTRACT_FEEPOOL = "FeePool";
    bytes32 private constant CONTRACT_SYNTHSUSD = "SynthsUSD";

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

    constructor(
        CollateralState _state,
        address _owner,
        address _manager,
        address _resolver,
        bytes32 _collateralKey,
        uint _minCratio,
        uint _minCollateral
    ) public Owned(_owner) MixinSystemSettings(_resolver) {
        manager = _manager;
        state = _state;
        collateralKey = _collateralKey;
        minCratio = _minCratio;
        minCollateral = _minCollateral;
    }

    /* ========== VIEWS ========== */

    function resolverAddressesRequired() public view returns (bytes32[] memory addresses) {
        bytes32[] memory existingAddresses = MixinSystemSettings.resolverAddressesRequired();
        bytes32[] memory newAddresses = new bytes32[](5);
        newAddresses[0] = CONTRACT_FEEPOOL;
        newAddresses[1] = CONTRACT_EXRATES;
        newAddresses[2] = CONTRACT_EXCHANGER;
        newAddresses[3] = CONTRACT_SYSTEMSTATUS;
        newAddresses[4] = CONTRACT_SYNTHSUSD;

        bytes32[] memory combined = combineArrays(existingAddresses, newAddresses);

        addresses = combineArrays(combined, synths);
    }

    /* ---------- Related Contracts ---------- */

    function _systemStatus() internal view returns (ISystemStatus) {
        return ISystemStatus(requireAndGetAddress(CONTRACT_SYSTEMSTATUS));
    }

    function _synth(bytes32 synthName) internal view returns (ISynth) {
        return ISynth(requireAndGetAddress(synthName));
    }

    function _synthsUSD() internal view returns (ISynth) {
        return ISynth(requireAndGetAddress(CONTRACT_SYNTHSUSD));
    }

    function _exchangeRates() internal view returns (IExchangeRates) {
        return IExchangeRates(requireAndGetAddress(CONTRACT_EXRATES));
    }

    function _exchanger() internal view returns (IExchanger) {
        return IExchanger(requireAndGetAddress(CONTRACT_EXCHANGER));
    }

    function _feePool() internal view returns (IFeePool) {
        return IFeePool(requireAndGetAddress(CONTRACT_FEEPOOL));
    }

    function _manager() internal view returns (ICollateralManager) {
        return ICollateralManager(manager);
    }

    /* ---------- Public Views ---------- */

    function collateralRatio(Loan memory loan) public view returns (uint cratio) {
        uint cvalue = _exchangeRates().effectiveValue(collateralKey, loan.collateral, sUSD);
        uint dvalue = _exchangeRates().effectiveValue(loan.currency, loan.amount.add(loan.accruedInterest), sUSD);
        cratio = cvalue.divideDecimal(dvalue);
    }

    // The maximum number of synths issuable for this amount of collateral
    function maxLoan(uint amount, bytes32 currency) public view returns (uint max) {
        max = issuanceRatio().multiplyDecimal(_exchangeRates().effectiveValue(collateralKey, amount, currency));
    }

    /**
     * r = target issuance ratio
     * D = debt value in sUSD
     * V = Collateral VALUE in sUSD
     * P = liquidation penalty
     * Calculates amount of synths = (D - V * r) / (1 - (1 + P) * r)
     * Note: if you pass a loan in here that is not eligible for liquidation it will revert.
     * We check the ratio first in liquidateInternal and only pass eligible loans in.
     */
    function liquidationAmount(Loan memory loan) public view returns (uint amount) {
        uint liquidationPenalty = getLiquidationPenalty();
        uint debtValue = loan.amount.add(loan.accruedInterest).multiplyDecimal(
            _exchangeRates().rateForCurrency(loan.currency)
        );
        uint collateralValue = loan.collateral.multiplyDecimal(_exchangeRates().rateForCurrency(collateralKey));
        uint unit = SafeDecimalMath.unit();

        uint dividend = debtValue.sub(collateralValue.divideDecimal(minCratio));
        uint divisor = unit.sub(unit.add(liquidationPenalty).divideDecimal(minCratio));

        return dividend.divideDecimal(divisor);
    }

    // amount is the amount of synths we are liquidating
    function collateralRedeemed(bytes32 currency, uint amount) public view returns (uint collateral) {
        uint liquidationPenalty = getLiquidationPenalty();
        collateral = _exchangeRates().effectiveValue(currency, amount, collateralKey);

        collateral = collateral.multiplyDecimal(SafeDecimalMath.unit().add(liquidationPenalty));
    }

    function areSynthsAndCurrenciesSet(bytes32[] calldata _synthNamesInResolver, bytes32[] calldata _synthKeys)
        external
        view
        returns (bool)
    {
        if (synths.length != _synthNamesInResolver.length) {
            return false;
        }

        for (uint i = 0; i < _synthNamesInResolver.length; i++) {
            bytes32 synthName = _synthNamesInResolver[i];
            if (synths[i] != synthName) {
                return false;
            }
            if (synthsByKey[_synthKeys[i]] != synths[i]) {
                return false;
            }
        }

        return true;
    }

    /* ---------- UTILITIES ---------- */

    // Check the account has enough of the synth to make the payment
    function _checkSynthBalance(
        address payer,
        bytes32 key,
        uint amount
    ) internal view {
        require(IERC20(address(_synth(synthsByKey[key]))).balanceOf(payer) >= amount, "Not enough synth balance");
    }

    // We set the interest index to 0 to indicate the loan has been closed.
    function _checkLoanAvailable(Loan memory _loan) internal view {
        require(_loan.interestIndex > 0, "Loan does not exist");
        require(_loan.lastInteraction.add(interactionDelay) <= block.timestamp, "Loan recently interacted with");
    }

    function issuanceRatio() internal view returns (uint ratio) {
        ratio = SafeDecimalMath.unit().divideDecimalRound(minCratio);
    }

    /* ========== MUTATIVE FUNCTIONS ========== */

    /* ---------- Synths ---------- */

    function addSynths(bytes32[] calldata _synthNamesInResolver, bytes32[] calldata _synthKeys) external onlyOwner {
        require(_synthNamesInResolver.length == _synthKeys.length, "Input array length mismatch");

        for (uint i = 0; i < _synthNamesInResolver.length; i++) {
            bytes32 synthName = _synthNamesInResolver[i];
            synths.push(synthName);
            synthsByKey[_synthKeys[i]] = synthName;
        }

        // ensure cache has the latest
        rebuildCache();
    }

    /* ---------- Rewards Contracts ---------- */

    function addRewardsContracts(address rewardsContract, bytes32 synth) external onlyOwner {
        shortingRewards[synth] = rewardsContract;
    }

    /* ---------- SETTERS ---------- */

    function setMinCratio(uint _minCratio) external onlyOwner {
        require(_minCratio > SafeDecimalMath.unit(), "Must be greater than 1");
        minCratio = _minCratio;
        emit MinCratioRatioUpdated(minCratio);
    }

    function setIssueFeeRate(uint _issueFeeRate) external onlyOwner {
        issueFeeRate = _issueFeeRate;
        emit IssueFeeRateUpdated(issueFeeRate);
    }

    function setInteractionDelay(uint _interactionDelay) external onlyOwner {
        require(_interactionDelay <= SafeDecimalMath.unit() * 3600, "Max 1 hour");
        interactionDelay = _interactionDelay;
        emit InteractionDelayUpdated(interactionDelay);
    }

    function setManager(address _newManager) external onlyOwner {
        manager = _newManager;
        emit ManagerUpdated(manager);
    }

    function setCanOpenLoans(bool _canOpenLoans) external onlyOwner {
        canOpenLoans = _canOpenLoans;
        emit CanOpenLoansUpdated(canOpenLoans);
    }

    /* ---------- LOAN INTERACTIONS ---------- */

    function openInternal(
        uint collateral,
        uint amount,
        bytes32 currency,
        bool short
    ) internal returns (uint id) {
        // 0. Check the system is active.
        _systemStatus().requireIssuanceActive();

        require(canOpenLoans, "Opening is disabled");

        // 1. Make sure the collateral rate is valid.
        require(!_exchangeRates().rateIsInvalid(collateralKey), "Collateral rate is invalid");

        // 2. We can only issue certain synths.
        require(synthsByKey[currency] > 0, "Not allowed to issue this synth");

        // 3. Make sure the synth rate is not invalid.
        require(!_exchangeRates().rateIsInvalid(currency), "Currency rate is invalid");

        // 4. Collateral >= minimum collateral size.
        require(collateral >= minCollateral, "Not enough collateral to open");

        // 5. Cap the number of loans so that the array doesn't get too big.
        require(state.getNumLoans(msg.sender) < maxLoansPerAccount, "Max loans exceeded");

        // 6. Check we haven't hit the debt cap for non snx collateral.
        (bool canIssue, bool anyRateIsInvalid) = _manager().exceedsDebtLimit(amount, currency);

        require(canIssue && !anyRateIsInvalid, "Debt limit or invalid rate");

        // 7. Require requested loan < max loan
        require(amount <= maxLoan(collateral, currency), "Exceeds max borrowing power");

        // 8. This fee is denominated in the currency of the loan
        uint issueFee = amount.multiplyDecimalRound(issueFeeRate);

        // 9. Calculate the minting fee and subtract it from the loan amount
        uint loanAmountMinusFee = amount.sub(issueFee);

        // 10. Get a Loan ID
        id = _manager().getNewLoanId();

        // 11. Create the loan struct.
        Loan memory loan = Loan({
            id: id,
            account: msg.sender,
            collateral: collateral,
            currency: currency,
            amount: amount,
            short: short,
            accruedInterest: 0,
            interestIndex: 0,
            lastInteraction: block.timestamp
        });

        // 12. Accrue interest on the loan.
        loan = accrueInterest(loan);

        // 13. Save the loan to storage
        state.createLoan(loan);

        // 14. Pay the minting fees to the fee pool
        _payFees(issueFee, currency);

        // 15. If its short, convert back to sUSD, otherwise issue the loan.
        if (short) {
            _synthsUSD().issue(msg.sender, _exchangeRates().effectiveValue(currency, loanAmountMinusFee, sUSD));
            _manager().incrementShorts(currency, amount);

            if (shortingRewards[currency] != address(0)) {
                IShortingRewards(shortingRewards[currency]).enrol(msg.sender, amount);
            }
        } else {
            _synth(synthsByKey[currency]).issue(msg.sender, loanAmountMinusFee);
            _manager().incrementLongs(currency, amount);
        }

        // 16. Emit event
        emit LoanCreated(msg.sender, id, amount, collateral, currency, issueFee);
    }

    function closeInternal(address borrower, uint id) internal returns (uint collateral) {
        // 0. Check the system is active.
        _systemStatus().requireIssuanceActive();

        // 1. Make sure the collateral rate is valid
        require(!_exchangeRates().rateIsInvalid(collateralKey), "Collateral rate is invalid");

        // 2. Get the loan.
        Loan memory loan = state.getLoan(borrower, id);

        // 3. Check loan is open and the last interaction time.
        _checkLoanAvailable(loan);

        // 4. Accrue interest on the loan.
        loan = accrueInterest(loan);

        // 5. Work out the total amount owing on the loan.
        uint total = loan.amount.add(loan.accruedInterest);

        // 6. Check they have enough balance to close the loan.
        _checkSynthBalance(loan.account, loan.currency, total);

        // 7. Burn the synths
        require(
            !_exchanger().hasWaitingPeriodOrSettlementOwing(borrower, loan.currency),
            "Waiting secs or settlement owing"
        );
        _synth(synthsByKey[loan.currency]).burn(borrower, total);

        // 8. Tell the manager.
        if (loan.short) {
            _manager().decrementShorts(loan.currency, loan.amount);

            if (shortingRewards[loan.currency] != address(0)) {
                IShortingRewards(shortingRewards[loan.currency]).withdraw(borrower, loan.amount);
            }
        } else {
            _manager().decrementLongs(loan.currency, loan.amount);
        }

        // 9. Assign the collateral to be returned.
        collateral = loan.collateral;

        // 10. Pay fees
        _payFees(loan.accruedInterest, loan.currency);

        // 11. Record loan as closed
        loan.amount = 0;
        loan.collateral = 0;
        loan.accruedInterest = 0;
        loan.interestIndex = 0;
        loan.lastInteraction = block.timestamp;
        state.updateLoan(loan);

        // 12. Emit the event
        emit LoanClosed(borrower, id);
    }

    function closeByLiquidationInternal(
        address borrower,
        address liquidator,
        Loan memory loan
    ) internal returns (uint collateral) {
        // 1. Work out the total amount owing on the loan.
        uint total = loan.amount.add(loan.accruedInterest);

        // 2. Store this for the event.
        uint amount = loan.amount;

        // 3. Return collateral to the child class so it knows how much to transfer.
        collateral = loan.collateral;

        // 4. Burn the synths
        require(!_exchanger().hasWaitingPeriodOrSettlementOwing(liquidator, loan.currency), "Waiting or settlement owing");
        _synth(synthsByKey[loan.currency]).burn(liquidator, total);

        // 5. Tell the manager.
        if (loan.short) {
            _manager().decrementShorts(loan.currency, loan.amount);

            if (shortingRewards[loan.currency] != address(0)) {
                IShortingRewards(shortingRewards[loan.currency]).withdraw(borrower, loan.amount);
            }
        } else {
            _manager().decrementLongs(loan.currency, loan.amount);
        }

        // 6. Pay fees
        _payFees(loan.accruedInterest, loan.currency);

        // 7. Record loan as closed
        loan.amount = 0;
        loan.collateral = 0;
        loan.accruedInterest = 0;
        loan.interestIndex = 0;
        loan.lastInteraction = block.timestamp;
        state.updateLoan(loan);

        // 8. Emit the event.
        emit LoanClosedByLiquidation(borrower, loan.id, liquidator, amount, collateral);
    }

    function depositInternal(
        address account,
        uint id,
        uint amount
    ) internal {
        // 0. Check the system is active.
        _systemStatus().requireIssuanceActive();

        // 1. Make sure the collateral rate is valid.
        require(!_exchangeRates().rateIsInvalid(collateralKey), "Collateral rate is invalid");

        // 2. They sent some value > 0
        require(amount > 0, "Deposit must be greater than 0");

        // 3. Get the loan
        Loan memory loan = state.getLoan(account, id);

        // 4. Check loan is open and last interaction time.
        _checkLoanAvailable(loan);

        // 5. Accrue interest
        loan = accrueInterest(loan);

        // 6. Add the collateral
        loan.collateral = loan.collateral.add(amount);

        // 7. Update the last interaction time.
        loan.lastInteraction = block.timestamp;

        // 8. Store the loan
        state.updateLoan(loan);

        // 9. Emit the event
        emit CollateralDeposited(account, id, amount, loan.collateral);
    }

    function withdrawInternal(uint id, uint amount) internal returns (uint withdraw) {
        // 0. Check the system is active.
        _systemStatus().requireIssuanceActive();

        // 1. Make sure the collateral rate is valid.
        require(!_exchangeRates().rateIsInvalid(collateralKey), "Collateral rate is invalid");

        // 2. Get the loan.
        Loan memory loan = state.getLoan(msg.sender, id);

        // 3. Check loan is open and last interaction time.
        _checkLoanAvailable(loan);

        // 4. Accrue interest.
        loan = accrueInterest(loan);

        // 5. Subtract the collateral.
        loan.collateral = loan.collateral.sub(amount);

        // 6. Update the last interaction time.
        loan.lastInteraction = block.timestamp;

        // 7. Check that the new amount does not put them under the minimum c ratio.
        require(collateralRatio(loan) > minCratio, "Cratio too low");

        // 8. Store the loan.
        state.updateLoan(loan);

        // 9. Assign the return variable.
        withdraw = amount;

        // 10. Emit the event.
        emit CollateralWithdrawn(msg.sender, id, amount, loan.collateral);
    }

    function liquidateInternal(
        address borrower,
        uint id,
        uint payment
    ) internal returns (uint collateralLiquidated) {
        // 0. Check the system is active.
        _systemStatus().requireIssuanceActive();

        // 1. Make sure the collateral rate is valid.
        require(!_exchangeRates().rateIsInvalid(collateralKey), "Collateral rate is invalid");

        // 2. Check the payment amount.
        require(payment > 0, "Payment must be greater than 0");

        // 3. Get the loan.
        Loan memory loan = state.getLoan(borrower, id);

        // 4. Check loan is open and last interaction time.
        _checkLoanAvailable(loan);

        // 5. Accrue interest.
        loan = accrueInterest(loan);

        // 6. Check they have enough balance to make the payment.
        _checkSynthBalance(msg.sender, loan.currency, payment);

        // 7. Check they are eligible for liquidation.
        require(collateralRatio(loan) < minCratio, "Cratio above liquidation ratio");

        // 8. Determine how much needs to be liquidated to fix their c ratio.
        uint liqAmount = liquidationAmount(loan);

        // 9. Only allow them to liquidate enough to fix the c ratio.
        uint amountToLiquidate = liqAmount < payment ? liqAmount : payment;

        // 10. Work out the total amount owing on the loan.
        uint amountOwing = loan.amount.add(loan.accruedInterest);

        // 11. If its greater than the amount owing, we need to close the loan.
        if (amountToLiquidate >= amountOwing) {
            return closeByLiquidationInternal(borrower, msg.sender, loan);
        }

        // 12. Process the payment to workout interest/principal split.
        loan = _processPayment(loan, amountToLiquidate);

        // 13. Work out how much collateral to redeem.
        collateralLiquidated = collateralRedeemed(loan.currency, amountToLiquidate);
        loan.collateral = loan.collateral.sub(collateralLiquidated);

        // 14. Update the last interaction time.
        loan.lastInteraction = block.timestamp;

        // 15. Burn the synths from the liquidator.
        require(!_exchanger().hasWaitingPeriodOrSettlementOwing(msg.sender, loan.currency), "Waiting or settlement owing");
        _synth(synthsByKey[loan.currency]).burn(msg.sender, amountToLiquidate);

        // 16. Store the loan.
        state.updateLoan(loan);

        // 17. Emit the event
        emit LoanPartiallyLiquidated(borrower, id, msg.sender, amountToLiquidate, collateralLiquidated);
    }

    function repayInternal(
        address borrower,
        address repayer,
        uint id,
        uint payment
    ) internal {
        // 0. Check the system is active.
        _systemStatus().requireIssuanceActive();

        // 1. Make sure the collateral rate is valid.
        require(!_exchangeRates().rateIsInvalid(collateralKey), "Collateral rate is invalid");

        // 2. Check the payment amount.
        require(payment > 0, "Payment must be greater than 0");

        // 3. Get loan
        Loan memory loan = state.getLoan(borrower, id);

        // 4. Check loan is open and last interaction time.
        _checkLoanAvailable(loan);

        // 5. Accrue interest.
        loan = accrueInterest(loan);

        // 6. Check the spender has enough synths to make the repayment
        _checkSynthBalance(repayer, loan.currency, payment);

        // 7. Process the payment.
        loan = _processPayment(loan, payment);

        // 8. Update the last interaction time.
        loan.lastInteraction = block.timestamp;

        // 9. Burn synths from the payer
        require(!_exchanger().hasWaitingPeriodOrSettlementOwing(repayer, loan.currency), "Waiting or settlement owing");
        _synth(synthsByKey[loan.currency]).burn(repayer, payment);

        // 10. Store the loan
        state.updateLoan(loan);

        // 11. Emit the event.
        emit LoanRepaymentMade(borrower, repayer, id, payment, loan.amount);
    }

    function drawInternal(uint id, uint amount) internal {
        // 0. Check the system is active.
        _systemStatus().requireIssuanceActive();

        // 1. Make sure the collateral rate is valid.
        require(!_exchangeRates().rateIsInvalid(collateralKey), "Collateral rate is invalid");

        // 2. Get loan.
        Loan memory loan = state.getLoan(msg.sender, id);

        // 3. Check loan is open and last interaction time.
        _checkLoanAvailable(loan);

        // 4. Accrue interest.
        loan = accrueInterest(loan);

        // 5. Add the requested amount.
        loan.amount = loan.amount.add(amount);

        // 6. If it is below the minimum, don't allow this draw.
        require(collateralRatio(loan) > minCratio, "Cannot draw this much");

        // 7. This fee is denominated in the currency of the loan
        uint issueFee = amount.multiplyDecimalRound(issueFeeRate);

        // 8. Calculate the minting fee and subtract it from the draw amount
        uint amountMinusFee = amount.sub(issueFee);

        // 9. If its short, let the child handle it, otherwise issue the synths.
        if (loan.short) {
            _manager().incrementShorts(loan.currency, amount);
            _synthsUSD().issue(msg.sender, _exchangeRates().effectiveValue(loan.currency, amountMinusFee, sUSD));

            if (shortingRewards[loan.currency] != address(0)) {
                IShortingRewards(shortingRewards[loan.currency]).enrol(msg.sender, amount);
            }
        } else {
            _manager().incrementLongs(loan.currency, amount);
            _synth(synthsByKey[loan.currency]).issue(msg.sender, amountMinusFee);
        }

        // 10. Pay the minting fees to the fee pool
        _payFees(issueFee, loan.currency);

        // 11. Update the last interaction time.
        loan.lastInteraction = block.timestamp;

        // 12. Store the loan
        state.updateLoan(loan);

        // 13. Emit the event.
        emit LoanDrawnDown(msg.sender, id, amount);
    }

    // Update the cumulative interest rate for the currency that was interacted with.
    function accrueInterest(Loan memory loan) internal returns (Loan memory loanAfter) {
        loanAfter = loan;

        // 1. Get the rates we need.
        (uint entryRate, uint lastRate, uint lastUpdated, uint newIndex) = loan.short
            ? _manager().getShortRatesAndTime(loan.currency, loan.interestIndex)
            : _manager().getRatesAndTime(loan.interestIndex);

        // 2. Get the instantaneous rate.
        (uint rate, bool invalid) = loan.short
            ? _manager().getShortRate(synthsByKey[loan.currency])
            : _manager().getBorrowRate();

        require(!invalid, "Rates are invalid");

        // 3. Get the time since we last updated the rate.
        uint timeDelta = block.timestamp.sub(lastUpdated).mul(SafeDecimalMath.unit());

        // 4. Get the latest cumulative rate. F_n+1 = F_n + F_last
        uint latestCumulative = lastRate.add(rate.multiplyDecimal(timeDelta));

        // 5. If the loan was just opened, don't record any interest. Otherwise multiple by the amount outstanding.
        uint interest = loan.interestIndex == 0 ? 0 : loan.amount.multiplyDecimal(latestCumulative.sub(entryRate));

        // 7. Update rates with the lastest cumulative rate. This also updates the time.
        loan.short
            ? _manager().updateShortRates(loan.currency, latestCumulative)
            : _manager().updateBorrowRates(latestCumulative);

        // 8. Update loan
        loanAfter.accruedInterest = loan.accruedInterest.add(interest);
        loanAfter.interestIndex = newIndex;
        state.updateLoan(loanAfter);
    }

    // Works out the amount of interest and principal after a repayment is made.
    function _processPayment(Loan memory loanBefore, uint payment) internal returns (Loan memory loanAfter) {
        loanAfter = loanBefore;

        if (payment > 0 && loanBefore.accruedInterest > 0) {
            uint interestPaid = payment > loanBefore.accruedInterest ? loanBefore.accruedInterest : payment;
            loanAfter.accruedInterest = loanBefore.accruedInterest.sub(interestPaid);
            payment = payment.sub(interestPaid);

            _payFees(interestPaid, loanBefore.currency);
        }

        // If there is more payment left after the interest, pay down the principal.
        if (payment > 0) {
            loanAfter.amount = loanBefore.amount.sub(payment);

            // And get the manager to reduce the total long/short balance.
            if (loanAfter.short) {
                _manager().decrementShorts(loanAfter.currency, payment);

                if (shortingRewards[loanAfter.currency] != address(0)) {
                    IShortingRewards(shortingRewards[loanAfter.currency]).withdraw(loanAfter.account, payment);
                }
            } else {
                _manager().decrementLongs(loanAfter.currency, payment);
            }
        }
    }

    // Take an amount of fees in a certain synth and convert it to sUSD before paying the fee pool.
    function _payFees(uint amount, bytes32 synth) internal {
        if (amount > 0) {
            if (synth != sUSD) {
                amount = _exchangeRates().effectiveValue(synth, amount, sUSD);
            }
            _synthsUSD().issue(_feePool().FEE_ADDRESS(), amount);
            _feePool().recordFeePaid(amount);
        }
    }

    // ========== EVENTS ==========
    // Setters
    event MinCratioRatioUpdated(uint minCratio);
    event MinCollateralUpdated(uint minCollateral);
    event IssueFeeRateUpdated(uint issueFeeRate);
    event MaxLoansPerAccountUpdated(uint maxLoansPerAccount);
    event InteractionDelayUpdated(uint interactionDelay);
    event ManagerUpdated(address manager);
    event CanOpenLoansUpdated(bool canOpenLoans);

    // Loans
    event LoanCreated(address indexed account, uint id, uint amount, uint collateral, bytes32 currency, uint issuanceFee);
    event LoanClosed(address indexed account, uint id);
    event CollateralDeposited(address indexed account, uint id, uint amountDeposited, uint collateralAfter);
    event CollateralWithdrawn(address indexed account, uint id, uint amountWithdrawn, uint collateralAfter);
    event LoanRepaymentMade(address indexed account, address indexed repayer, uint id, uint amountRepaid, uint amountAfter);
    event LoanDrawnDown(address indexed account, uint id, uint amount);
    event LoanPartiallyLiquidated(
        address indexed account,
        uint id,
        address liquidator,
        uint amountLiquidated,
        uint collateralLiquidated
    );
    event LoanClosedByLiquidation(
        address indexed account,
        uint id,
        address indexed liquidator,
        uint amountLiquidated,
        uint collateralLiquidated
    );
}


interface ICollateralErc20 {
    function open(uint collateral, uint amount, bytes32 currency) external;

    function close(uint id) external;

    function deposit(address borrower, uint id, uint collateral) external;

    function withdraw(uint id, uint amount) external;

    function repay(address borrower, uint id, uint amount) external;

    function draw(uint id, uint amount) external;

    function liquidate(address borrower, uint id, uint amount) external;
}


// Inheritance


// Internal references


// This contract handles the specific ERC20 implementation details of managing a loan.
contract CollateralErc20 is ICollateralErc20, Collateral {
    // The underlying asset for this ERC20 collateral
    address public underlyingContract;

    uint public underlyingContractDecimals;

    constructor(
        CollateralState _state,
        address _owner,
        address _manager,
        address _resolver,
        bytes32 _collateralKey,
        uint _minCratio,
        uint _minCollateral,
        address _underlyingContract,
        uint _underlyingDecimals
    ) public Collateral(_state, _owner, _manager, _resolver, _collateralKey, _minCratio, _minCollateral) {
        underlyingContract = _underlyingContract;

        underlyingContractDecimals = _underlyingDecimals;
    }

    function open(
        uint collateral,
        uint amount,
        bytes32 currency
    ) external {
        require(collateral <= IERC20(underlyingContract).allowance(msg.sender, address(this)), "Allowance not high enough");

        // only transfer the actual collateral
        IERC20(underlyingContract).transferFrom(msg.sender, address(this), collateral);

        // scale up before entering the system.
        uint scaledCollateral = scaleUpCollateral(collateral);

        openInternal(scaledCollateral, amount, currency, false);
    }

    function close(uint id) external {
        uint collateral = closeInternal(msg.sender, id);

        // scale down before transferring back.
        uint scaledCollateral = scaleDownCollateral(collateral);

        IERC20(underlyingContract).transfer(msg.sender, scaledCollateral);
    }

    function deposit(
        address borrower,
        uint id,
        uint amount
    ) external {
        require(amount <= IERC20(underlyingContract).allowance(msg.sender, address(this)), "Allowance not high enough");

        IERC20(underlyingContract).transferFrom(msg.sender, address(this), amount);

        // scale up before entering the system.
        uint scaledAmount = scaleUpCollateral(amount);

        depositInternal(borrower, id, scaledAmount);
    }

    function withdraw(uint id, uint amount) external {
        // scale up before entering the system.
        uint scaledAmount = scaleUpCollateral(amount);

        uint withdrawnAmount = withdrawInternal(id, scaledAmount);

        // scale down before transferring back.
        uint scaledWithdraw = scaleDownCollateral(withdrawnAmount);

        IERC20(underlyingContract).transfer(msg.sender, scaledWithdraw);
    }

    function repay(
        address borrower,
        uint id,
        uint amount
    ) external {
        repayInternal(borrower, msg.sender, id, amount);
    }

    function draw(uint id, uint amount) external {
        drawInternal(id, amount);
    }

    function liquidate(
        address borrower,
        uint id,
        uint amount
    ) external {
        uint collateralLiquidated = liquidateInternal(borrower, id, amount);

        // scale down before transferring back.
        uint scaledCollateral = scaleDownCollateral(collateralLiquidated);

        IERC20(underlyingContract).transfer(msg.sender, scaledCollateral);
    }

    function scaleUpCollateral(uint collateral) public view returns (uint scaledUp) {
        uint conversionFactor = 10**uint(SafeMath.sub(18, underlyingContractDecimals));

        scaledUp = uint(uint(collateral).mul(conversionFactor));
    }

    function scaleDownCollateral(uint collateral) public view returns (uint scaledDown) {
        uint conversionFactor = 10**uint(SafeMath.sub(18, underlyingContractDecimals));

        scaledDown = collateral.div(conversionFactor);
    }
}