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

* Synthetix: ExchangerWithVirtualSynth.sol
*
* Latest source (may be newer): https://github.com/Synthetixio/synthetix/blob/master/contracts/ExchangerWithVirtualSynth.sol
* Docs: https://docs.synthetix.io/contracts/ExchangerWithVirtualSynth
*
* Contract Dependencies: 
*	- ERC20
*	- Exchanger
*	- IAddressResolver
*	- IERC20
*	- IExchanger
*	- IVirtualSynth
*	- MixinResolver
*	- MixinSystemSettings
*	- Owned
* 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


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

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

    /* ========== MUTATIVE 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++) {
            repository[names[i]] = destinations[i];
        }
    }

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

    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));
    }
}


// Inheritance


// Internal references


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

    mapping(bytes32 => address) private addressCache;

    bytes32[] public resolverAddressesRequired;

    uint public constant MAX_ADDRESSES_FROM_RESOLVER = 24;

    constructor(address _resolver, bytes32[MAX_ADDRESSES_FROM_RESOLVER] memory _addressesToCache) internal {
        // This contract is abstract, and thus cannot be instantiated directly
        require(owner != address(0), "Owner must be set");

        for (uint i = 0; i < _addressesToCache.length; i++) {
            if (_addressesToCache[i] != bytes32(0)) {
                resolverAddressesRequired.push(_addressesToCache[i]);
            } else {
                // End early once an empty item is found - assumes there are no empty slots in
                // _addressesToCache
                break;
            }
        }
        resolver = AddressResolver(_resolver);
        // Do not sync the cache as addresses may not be in the resolver yet
    }

    /* ========== SETTERS ========== */
    function setResolverAndSyncCache(AddressResolver _resolver) external onlyOwner {
        resolver = _resolver;

        for (uint i = 0; i < resolverAddressesRequired.length; i++) {
            bytes32 name = resolverAddressesRequired[i];
            // Note: can only be invoked once the resolver has all the targets needed added
            addressCache[name] = resolver.requireAndGetAddress(name, "Resolver missing target");
        }
    }

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

    function requireAndGetAddress(bytes32 name, string memory reason) internal view returns (address) {
        address _foundAddress = addressCache[name];
        require(_foundAddress != address(0), reason);
        return _foundAddress;
    }

    // Note: this could be made external in a utility contract if addressCache was made public
    // (used for deployment)
    function isResolverCached(AddressResolver _resolver) external view returns (bool) {
        if (resolver != _resolver) {
            return false;
        }

        // otherwise, check everything
        for (uint i = 0; i < resolverAddressesRequired.length; i++) {
            bytes32 name = resolverAddressesRequired[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;
    }

    // Note: can be made external into a utility contract (used for deployment)
    function getResolverAddressesRequired()
        external
        view
        returns (bytes32[MAX_ADDRESSES_FROM_RESOLVER] memory addressesRequired)
    {
        for (uint i = 0; i < resolverAddressesRequired.length; i++) {
            addressesRequired[i] = resolverAddressesRequired[i];
        }
    }

    /* ========== INTERNAL FUNCTIONS ========== */
    function appendToAddressCache(bytes32 name) internal {
        resolverAddressesRequired.push(name);
        require(resolverAddressesRequired.length < MAX_ADDRESSES_FROM_RESOLVER, "Max resolver cache size met");
        // Because this is designed to be called internally in constructors, we don't
        // check the address exists already in the resolver
        addressCache[name] = resolver.getAddress(name);
    }
}


// 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 private constant CONTRACT_FLEXIBLESTORAGE = "FlexibleStorage";

    constructor() internal {
        appendToAddressCache(CONTRACT_FLEXIBLESTORAGE);
    }

    function flexibleStorage() internal view returns (IFlexibleStorage) {
        return IFlexibleStorage(requireAndGetAddress(CONTRACT_FLEXIBLESTORAGE, "Missing FlexibleStorage address"));
    }

    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);
    }
}


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
    ) 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;
}


/**
 * @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;
    }
}


// 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/iexchangestate
interface IExchangeState {
    // Views
    struct ExchangeEntry {
        bytes32 src;
        uint amount;
        bytes32 dest;
        uint amountReceived;
        uint exchangeFeeRate;
        uint timestamp;
        uint roundIdForSrc;
        uint roundIdForDest;
    }

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

    function getEntryAt(
        address account,
        bytes32 currencyKey,
        uint index
    )
        external
        view
        returns (
            bytes32 src,
            uint amount,
            bytes32 dest,
            uint amountReceived,
            uint exchangeFeeRate,
            uint timestamp,
            uint roundIdForSrc,
            uint roundIdForDest
        );

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

    // Mutative functions
    function appendExchangeEntry(
        address account,
        bytes32 src,
        uint amount,
        bytes32 dest,
        uint amountReceived,
        uint exchangeFeeRate,
        uint timestamp,
        uint roundIdForSrc,
        uint roundIdForDest
    ) external;

    function removeEntries(address account, bytes32 currencyKey) external;
}


// 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;
}


// https://docs.synthetix.io/contracts/source/interfaces/isynthetix
interface ISynthetix {
    // 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 collateral(address account) external view returns (uint);

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

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

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

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

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

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

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

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

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

    function transferableSynthetix(address account) external view returns (uint transferable);

    // Mutative Functions
    function burnSynths(uint amount) external;

    function burnSynthsOnBehalf(address burnForAddress, uint amount) external;

    function burnSynthsToTarget() external;

    function burnSynthsToTargetOnBehalf(address burnForAddress) external;

    function exchange(
        bytes32 sourceCurrencyKey,
        uint sourceAmount,
        bytes32 destinationCurrencyKey
    ) external returns (uint amountReceived);

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

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

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

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

    function issueMaxSynths() external;

    function issueMaxSynthsOnBehalf(address issueForAddress) external;

    function issueSynths(uint amount) external;

    function issueSynthsOnBehalf(address issueForAddress, uint amount) external;

    function mint() external returns (bool);

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

    function liquidateDelinquentAccount(address account, uint susdAmount) external returns (bool);

    // Restricted Functions

    function mintSecondary(address account, uint amount) external;

    function mintSecondaryRewards(uint amount) external;

    function burnSecondary(address account, uint amount) 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/idelegateapprovals
interface IDelegateApprovals {
    // Views
    function canBurnFor(address authoriser, address delegate) external view returns (bool);

    function canIssueFor(address authoriser, address delegate) external view returns (bool);

    function canClaimFor(address authoriser, address delegate) external view returns (bool);

    function canExchangeFor(address authoriser, address delegate) external view returns (bool);

    // Mutative
    function approveAllDelegatePowers(address delegate) external;

    function removeAllDelegatePowers(address delegate) external;

    function approveBurnOnBehalf(address delegate) external;

    function removeBurnOnBehalf(address delegate) external;

    function approveIssueOnBehalf(address delegate) external;

    function removeIssueOnBehalf(address delegate) external;

    function approveClaimOnBehalf(address delegate) external;

    function removeClaimOnBehalf(address delegate) external;

    function approveExchangeOnBehalf(address delegate) external;

    function removeExchangeOnBehalf(address delegate) external;
}


// https://docs.synthetix.io/contracts/source/interfaces/itradingrewards
interface ITradingRewards {
    /* ========== VIEWS ========== */

    function getAvailableRewards() external view returns (uint);

    function getUnassignedRewards() external view returns (uint);

    function getRewardsToken() external view returns (address);

    function getPeriodController() external view returns (address);

    function getCurrentPeriod() external view returns (uint);

    function getPeriodIsClaimable(uint periodID) external view returns (bool);

    function getPeriodIsFinalized(uint periodID) external view returns (bool);

    function getPeriodRecordedFees(uint periodID) external view returns (uint);

    function getPeriodTotalRewards(uint periodID) external view returns (uint);

    function getPeriodAvailableRewards(uint periodID) external view returns (uint);

    function getUnaccountedFeesForAccountForPeriod(address account, uint periodID) external view returns (uint);

    function getAvailableRewardsForAccountForPeriod(address account, uint periodID) external view returns (uint);

    function getAvailableRewardsForAccountForPeriods(address account, uint[] calldata periodIDs)
        external
        view
        returns (uint totalRewards);

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

    function claimRewardsForPeriod(uint periodID) external;

    function claimRewardsForPeriods(uint[] calldata periodIDs) external;

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

    function recordExchangeFeeForAccount(uint usdFeeAmount, address account) external;

    function closeCurrentPeriodWithRewards(uint rewards) external;

    function recoverEther(address payable recoverAddress) external;

    function recoverTokens(address tokenAddress, address recoverAddress) external;

    function recoverUnassignedRewardTokens(address recoverAddress) external;

    function recoverAssignedRewardTokensAndDestroyPeriod(address recoverAddress, uint periodID) external;

    function setPeriodController(address newPeriodController) external;
}


// https://docs.synthetix.io/contracts/source/interfaces/idebtcache
interface IDebtCache {
    // Views

    function cachedDebt() external view returns (uint);

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

    function cacheTimestamp() external view returns (uint);

    function cacheInvalid() external view returns (bool);

    function cacheStale() external view returns (bool);

    function currentSynthDebts(bytes32[] calldata currencyKeys)
        external
        view
        returns (uint[] memory debtValues, bool anyRateIsInvalid);

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

    function currentDebt() external view returns (uint debt, bool anyRateIsInvalid);

    function cacheInfo()
        external
        view
        returns (
            uint debt,
            uint timestamp,
            bool isInvalid,
            bool isStale
        );

    // Mutative functions

    function takeDebtSnapshot() external;

    function updateCachedSynthDebts(bytes32[] calldata currencyKeys) external;
}


/**
 * @dev Interface of the ERC20 standard as defined in the EIP. Does not include
 * the optional functions; to access them see `ERC20Detailed`.
 */
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.
     *
     * > 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);
}


// Inheritance


// Libraries


// Internal references


// Note: use OZ's IERC20 here as using ours will complain about conflicting names
// during the build (VirtualSynth has IERC20 from the OZ ERC20 implementation)


// Used to have strongly-typed access to internal mutative functions in Synthetix
interface ISynthetixInternal {
    function emitExchangeTracking(
        bytes32 trackingCode,
        bytes32 toCurrencyKey,
        uint256 toAmount
    ) external;

    function emitSynthExchange(
        address account,
        bytes32 fromCurrencyKey,
        uint fromAmount,
        bytes32 toCurrencyKey,
        uint toAmount,
        address toAddress
    ) external;

    function emitExchangeReclaim(
        address account,
        bytes32 currencyKey,
        uint amount
    ) external;

    function emitExchangeRebate(
        address account,
        bytes32 currencyKey,
        uint amount
    ) external;
}


interface IExchangerInternalDebtCache {
    function updateCachedSynthDebtsWithRates(bytes32[] calldata currencyKeys, uint[] calldata currencyRates) external;

    function updateCachedSynthDebts(bytes32[] calldata currencyKeys) external;
}


// https://docs.synthetix.io/contracts/source/contracts/exchanger
contract Exchanger is Owned, MixinResolver, MixinSystemSettings, IExchanger {
    using SafeMath for uint;
    using SafeDecimalMath for uint;

    struct ExchangeEntrySettlement {
        bytes32 src;
        uint amount;
        bytes32 dest;
        uint reclaim;
        uint rebate;
        uint srcRoundIdAtPeriodEnd;
        uint destRoundIdAtPeriodEnd;
        uint timestamp;
    }

    bytes32 private constant sUSD = "sUSD";

    // SIP-65: Decentralized circuit breaker
    uint public constant CIRCUIT_BREAKER_SUSPENSION_REASON = 65;

    mapping(bytes32 => uint) public lastExchangeRate;

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

    bytes32 private constant CONTRACT_SYSTEMSTATUS = "SystemStatus";
    bytes32 private constant CONTRACT_EXCHANGESTATE = "ExchangeState";
    bytes32 private constant CONTRACT_EXRATES = "ExchangeRates";
    bytes32 private constant CONTRACT_SYNTHETIX = "Synthetix";
    bytes32 private constant CONTRACT_FEEPOOL = "FeePool";
    bytes32 private constant CONTRACT_TRADING_REWARDS = "TradingRewards";
    bytes32 private constant CONTRACT_DELEGATEAPPROVALS = "DelegateApprovals";
    bytes32 private constant CONTRACT_ISSUER = "Issuer";
    bytes32 private constant CONTRACT_DEBTCACHE = "DebtCache";

    bytes32[24] private addressesToCache = [
        CONTRACT_SYSTEMSTATUS,
        CONTRACT_EXCHANGESTATE,
        CONTRACT_EXRATES,
        CONTRACT_SYNTHETIX,
        CONTRACT_FEEPOOL,
        CONTRACT_TRADING_REWARDS,
        CONTRACT_DELEGATEAPPROVALS,
        CONTRACT_ISSUER,
        CONTRACT_DEBTCACHE
    ];

    constructor(address _owner, address _resolver)
        public
        Owned(_owner)
        MixinResolver(_resolver, addressesToCache)
        MixinSystemSettings()
    {}

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

    function systemStatus() internal view returns (ISystemStatus) {
        return ISystemStatus(requireAndGetAddress(CONTRACT_SYSTEMSTATUS, "Missing SystemStatus address"));
    }

    function exchangeState() internal view returns (IExchangeState) {
        return IExchangeState(requireAndGetAddress(CONTRACT_EXCHANGESTATE, "Missing ExchangeState address"));
    }

    function exchangeRates() internal view returns (IExchangeRates) {
        return IExchangeRates(requireAndGetAddress(CONTRACT_EXRATES, "Missing ExchangeRates address"));
    }

    function synthetix() internal view returns (ISynthetix) {
        return ISynthetix(requireAndGetAddress(CONTRACT_SYNTHETIX, "Missing Synthetix address"));
    }

    function feePool() internal view returns (IFeePool) {
        return IFeePool(requireAndGetAddress(CONTRACT_FEEPOOL, "Missing FeePool address"));
    }

    function tradingRewards() internal view returns (ITradingRewards) {
        return ITradingRewards(requireAndGetAddress(CONTRACT_TRADING_REWARDS, "Missing TradingRewards address"));
    }

    function delegateApprovals() internal view returns (IDelegateApprovals) {
        return IDelegateApprovals(requireAndGetAddress(CONTRACT_DELEGATEAPPROVALS, "Missing DelegateApprovals address"));
    }

    function issuer() internal view returns (IIssuer) {
        return IIssuer(requireAndGetAddress(CONTRACT_ISSUER, "Missing Issuer address"));
    }

    function debtCache() internal view returns (IExchangerInternalDebtCache) {
        return IExchangerInternalDebtCache(requireAndGetAddress(CONTRACT_DEBTCACHE, "Missing DebtCache address"));
    }

    function maxSecsLeftInWaitingPeriod(address account, bytes32 currencyKey) public view returns (uint) {
        return secsLeftInWaitingPeriodForExchange(exchangeState().getMaxTimestamp(account, currencyKey));
    }

    function waitingPeriodSecs() external view returns (uint) {
        return getWaitingPeriodSecs();
    }

    function tradingRewardsEnabled() external view returns (bool) {
        return getTradingRewardsEnabled();
    }

    function priceDeviationThresholdFactor() external view returns (uint) {
        return getPriceDeviationThresholdFactor();
    }

    function settlementOwing(address account, bytes32 currencyKey)
        public
        view
        returns (
            uint reclaimAmount,
            uint rebateAmount,
            uint numEntries
        )
    {
        (reclaimAmount, rebateAmount, numEntries, ) = _settlementOwing(account, currencyKey);
    }

    // Internal function to emit events for each individual rebate and reclaim entry
    function _settlementOwing(address account, bytes32 currencyKey)
        internal
        view
        returns (
            uint reclaimAmount,
            uint rebateAmount,
            uint numEntries,
            ExchangeEntrySettlement[] memory
        )
    {
        // Need to sum up all reclaim and rebate amounts for the user and the currency key
        numEntries = exchangeState().getLengthOfEntries(account, currencyKey);

        // For each unsettled exchange
        ExchangeEntrySettlement[] memory settlements = new ExchangeEntrySettlement[](numEntries);
        for (uint i = 0; i < numEntries; i++) {
            uint reclaim;
            uint rebate;
            // fetch the entry from storage
            IExchangeState.ExchangeEntry memory exchangeEntry = _getExchangeEntry(account, currencyKey, i);

            // determine the last round ids for src and dest pairs when period ended or latest if not over
            (uint srcRoundIdAtPeriodEnd, uint destRoundIdAtPeriodEnd) = getRoundIdsAtPeriodEnd(exchangeEntry);

            // given these round ids, determine what effective value they should have received
            uint destinationAmount = exchangeRates().effectiveValueAtRound(
                exchangeEntry.src,
                exchangeEntry.amount,
                exchangeEntry.dest,
                srcRoundIdAtPeriodEnd,
                destRoundIdAtPeriodEnd
            );

            // and deduct the fee from this amount using the exchangeFeeRate from storage
            uint amountShouldHaveReceived = _getAmountReceivedForExchange(destinationAmount, exchangeEntry.exchangeFeeRate);

            // SIP-65 settlements where the amount at end of waiting period is beyond the threshold, then
            // settle with no reclaim or rebate
            if (!_isDeviationAboveThreshold(exchangeEntry.amountReceived, amountShouldHaveReceived)) {
                if (exchangeEntry.amountReceived > amountShouldHaveReceived) {
                    // if they received more than they should have, add to the reclaim tally
                    reclaim = exchangeEntry.amountReceived.sub(amountShouldHaveReceived);
                    reclaimAmount = reclaimAmount.add(reclaim);
                } else if (amountShouldHaveReceived > exchangeEntry.amountReceived) {
                    // if less, add to the rebate tally
                    rebate = amountShouldHaveReceived.sub(exchangeEntry.amountReceived);
                    rebateAmount = rebateAmount.add(rebate);
                }
            }

            settlements[i] = ExchangeEntrySettlement({
                src: exchangeEntry.src,
                amount: exchangeEntry.amount,
                dest: exchangeEntry.dest,
                reclaim: reclaim,
                rebate: rebate,
                srcRoundIdAtPeriodEnd: srcRoundIdAtPeriodEnd,
                destRoundIdAtPeriodEnd: destRoundIdAtPeriodEnd,
                timestamp: exchangeEntry.timestamp
            });
        }

        return (reclaimAmount, rebateAmount, numEntries, settlements);
    }

    function _getExchangeEntry(
        address account,
        bytes32 currencyKey,
        uint index
    ) internal view returns (IExchangeState.ExchangeEntry memory) {
        (
            bytes32 src,
            uint amount,
            bytes32 dest,
            uint amountReceived,
            uint exchangeFeeRate,
            uint timestamp,
            uint roundIdForSrc,
            uint roundIdForDest
        ) = exchangeState().getEntryAt(account, currencyKey, index);

        return
            IExchangeState.ExchangeEntry({
                src: src,
                amount: amount,
                dest: dest,
                amountReceived: amountReceived,
                exchangeFeeRate: exchangeFeeRate,
                timestamp: timestamp,
                roundIdForSrc: roundIdForSrc,
                roundIdForDest: roundIdForDest
            });
    }

    function hasWaitingPeriodOrSettlementOwing(address account, bytes32 currencyKey) external view returns (bool) {
        if (maxSecsLeftInWaitingPeriod(account, currencyKey) != 0) {
            return true;
        }

        (uint reclaimAmount, , , ) = _settlementOwing(account, currencyKey);

        return reclaimAmount > 0;
    }

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

    function calculateAmountAfterSettlement(
        address from,
        bytes32 currencyKey,
        uint amount,
        uint refunded
    ) public view returns (uint amountAfterSettlement) {
        amountAfterSettlement = amount;

        // balance of a synth will show an amount after settlement
        uint balanceOfSourceAfterSettlement = IERC20(address(issuer().synths(currencyKey))).balanceOf(from);

        // when there isn't enough supply (either due to reclamation settlement or because the number is too high)
        if (amountAfterSettlement > balanceOfSourceAfterSettlement) {
            // then the amount to exchange is reduced to their remaining supply
            amountAfterSettlement = balanceOfSourceAfterSettlement;
        }

        if (refunded > 0) {
            amountAfterSettlement = amountAfterSettlement.add(refunded);
        }
    }

    function isSynthRateInvalid(bytes32 currencyKey) external view returns (bool) {
        return _isSynthRateInvalid(currencyKey, exchangeRates().rateForCurrency(currencyKey));
    }

    /* ========== MUTATIVE FUNCTIONS ========== */
    function exchange(
        address from,
        bytes32 sourceCurrencyKey,
        uint sourceAmount,
        bytes32 destinationCurrencyKey,
        address destinationAddress
    ) external onlySynthetixorSynth returns (uint amountReceived) {
        uint fee;
        (amountReceived, fee, ) = _exchange(
            from,
            sourceCurrencyKey,
            sourceAmount,
            destinationCurrencyKey,
            destinationAddress,
            false
        );

        _processTradingRewards(fee, destinationAddress);
    }

    function exchangeOnBehalf(
        address exchangeForAddress,
        address from,
        bytes32 sourceCurrencyKey,
        uint sourceAmount,
        bytes32 destinationCurrencyKey
    ) external onlySynthetixorSynth returns (uint amountReceived) {
        require(delegateApprovals().canExchangeFor(exchangeForAddress, from), "Not approved to act on behalf");

        uint fee;
        (amountReceived, fee, ) = _exchange(
            exchangeForAddress,
            sourceCurrencyKey,
            sourceAmount,
            destinationCurrencyKey,
            exchangeForAddress,
            false
        );

        _processTradingRewards(fee, exchangeForAddress);
    }

    function exchangeWithTracking(
        address from,
        bytes32 sourceCurrencyKey,
        uint sourceAmount,
        bytes32 destinationCurrencyKey,
        address destinationAddress,
        address originator,
        bytes32 trackingCode
    ) external onlySynthetixorSynth returns (uint amountReceived) {
        uint fee;
        (amountReceived, fee, ) = _exchange(
            from,
            sourceCurrencyKey,
            sourceAmount,
            destinationCurrencyKey,
            destinationAddress,
            false
        );

        _processTradingRewards(fee, originator);

        _emitTrackingEvent(trackingCode, destinationCurrencyKey, amountReceived);
    }

    function exchangeOnBehalfWithTracking(
        address exchangeForAddress,
        address from,
        bytes32 sourceCurrencyKey,
        uint sourceAmount,
        bytes32 destinationCurrencyKey,
        address originator,
        bytes32 trackingCode
    ) external onlySynthetixorSynth returns (uint amountReceived) {
        require(delegateApprovals().canExchangeFor(exchangeForAddress, from), "Not approved to act on behalf");

        uint fee;
        (amountReceived, fee, ) = _exchange(
            exchangeForAddress,
            sourceCurrencyKey,
            sourceAmount,
            destinationCurrencyKey,
            exchangeForAddress,
            false
        );

        _processTradingRewards(fee, originator);

        _emitTrackingEvent(trackingCode, destinationCurrencyKey, amountReceived);
    }

    function exchangeWithVirtual(
        address from,
        bytes32 sourceCurrencyKey,
        uint sourceAmount,
        bytes32 destinationCurrencyKey,
        address destinationAddress
    ) external onlySynthetixorSynth returns (uint amountReceived, IVirtualSynth vSynth) {
        uint fee;
        (amountReceived, fee, vSynth) = _exchange(
            from,
            sourceCurrencyKey,
            sourceAmount,
            destinationCurrencyKey,
            destinationAddress,
            true
        );

        _processTradingRewards(fee, destinationAddress);
    }

    function _emitTrackingEvent(
        bytes32 trackingCode,
        bytes32 toCurrencyKey,
        uint256 toAmount
    ) internal {
        ISynthetixInternal(address(synthetix())).emitExchangeTracking(trackingCode, toCurrencyKey, toAmount);
    }

    function _processTradingRewards(uint fee, address originator) internal {
        if (fee > 0 && originator != address(0) && getTradingRewardsEnabled()) {
            tradingRewards().recordExchangeFeeForAccount(fee, originator);
        }
    }

    function _suspendIfRateInvalid(bytes32 currencyKey, uint rate) internal returns (bool circuitBroken) {
        if (_isSynthRateInvalid(currencyKey, rate)) {
            systemStatus().suspendSynth(currencyKey, CIRCUIT_BREAKER_SUSPENSION_REASON);
            circuitBroken = true;
        } else {
            lastExchangeRate[currencyKey] = rate;
        }
    }

    function _updateSNXIssuedDebtOnExchange(bytes32[2] memory currencyKeys, uint[2] memory currencyRates) internal {
        bool includesSUSD = currencyKeys[0] == sUSD || currencyKeys[1] == sUSD;
        uint numKeys = includesSUSD ? 2 : 3;

        bytes32[] memory keys = new bytes32[](numKeys);
        keys[0] = currencyKeys[0];
        keys[1] = currencyKeys[1];

        uint[] memory rates = new uint[](numKeys);
        rates[0] = currencyRates[0];
        rates[1] = currencyRates[1];

        if (!includesSUSD) {
            keys[2] = sUSD; // And we'll also update sUSD to account for any fees if it wasn't one of the exchanged currencies
            rates[2] = SafeDecimalMath.unit();
        }

        // Note that exchanges can't invalidate the debt cache, since if a rate is invalid,
        // the exchange will have failed already.
        debtCache().updateCachedSynthDebtsWithRates(keys, rates);
    }

    function _settleAndCalcSourceAmountRemaining(
        uint sourceAmount,
        address from,
        bytes32 sourceCurrencyKey
    ) internal returns (uint sourceAmountAfterSettlement) {
        (, uint refunded, uint numEntriesSettled) = _internalSettle(from, sourceCurrencyKey, false);

        sourceAmountAfterSettlement = sourceAmount;

        // when settlement was required
        if (numEntriesSettled > 0) {
            // ensure the sourceAmount takes this into account
            sourceAmountAfterSettlement = calculateAmountAfterSettlement(from, sourceCurrencyKey, sourceAmount, refunded);
        }
    }

    function _exchange(
        address from,
        bytes32 sourceCurrencyKey,
        uint sourceAmount,
        bytes32 destinationCurrencyKey,
        address destinationAddress,
        bool virtualSynth
    )
        internal
        returns (
            uint amountReceived,
            uint fee,
            IVirtualSynth vSynth
        )
    {
        _ensureCanExchange(sourceCurrencyKey, sourceAmount, destinationCurrencyKey);

        uint sourceAmountAfterSettlement = _settleAndCalcSourceAmountRemaining(sourceAmount, from, sourceCurrencyKey);

        // If, after settlement the user has no balance left (highly unlikely), then return to prevent
        // emitting events of 0 and don't revert so as to ensure the settlement queue is emptied
        if (sourceAmountAfterSettlement == 0) {
            return (0, 0, IVirtualSynth(0));
        }

        uint exchangeFeeRate;
        uint sourceRate;
        uint destinationRate;

        // Note: `fee` is denominated in the destinationCurrencyKey.
        (amountReceived, fee, exchangeFeeRate, sourceRate, destinationRate) = _getAmountsForExchangeMinusFees(
            sourceAmountAfterSettlement,
            sourceCurrencyKey,
            destinationCurrencyKey
        );

        // SIP-65: Decentralized Circuit Breaker
        if (
            _suspendIfRateInvalid(sourceCurrencyKey, sourceRate) ||
            _suspendIfRateInvalid(destinationCurrencyKey, destinationRate)
        ) {
            return (0, 0, IVirtualSynth(0));
        }

        // Note: We don't need to check their balance as the burn() below will do a safe subtraction which requires
        // the subtraction to not overflow, which would happen if their balance is not sufficient.

        vSynth = _convert(
            sourceCurrencyKey,
            from,
            sourceAmountAfterSettlement,
            destinationCurrencyKey,
            amountReceived,
            destinationAddress,
            virtualSynth
        );

        // Remit the fee if required
        if (fee > 0) {
            // Normalize fee to sUSD
            // Note: `fee` is being reused to avoid stack too deep errors.
            fee = exchangeRates().effectiveValue(destinationCurrencyKey, fee, sUSD);

            // Remit the fee in sUSDs
            issuer().synths(sUSD).issue(feePool().FEE_ADDRESS(), fee);

            // Tell the fee pool about this
            feePool().recordFeePaid(fee);
        }

        // Note: As of this point, `fee` is denominated in sUSD.

        // Nothing changes as far as issuance data goes because the total value in the system hasn't changed.
        // But we will update the debt snapshot in case exchange rates have fluctuated since the last exchange
        // in these currencies
        _updateSNXIssuedDebtOnExchange([sourceCurrencyKey, destinationCurrencyKey], [sourceRate, destinationRate]);

        // Let the DApps know there was a Synth exchange
        ISynthetixInternal(address(synthetix())).emitSynthExchange(
            from,
            sourceCurrencyKey,
            sourceAmountAfterSettlement,
            destinationCurrencyKey,
            amountReceived,
            destinationAddress
        );

        // persist the exchange information for the dest key
        appendExchange(
            destinationAddress,
            sourceCurrencyKey,
            sourceAmountAfterSettlement,
            destinationCurrencyKey,
            amountReceived,
            exchangeFeeRate
        );
    }

    function _convert(
        bytes32 sourceCurrencyKey,
        address from,
        uint sourceAmountAfterSettlement,
        bytes32 destinationCurrencyKey,
        uint amountReceived,
        address recipient,
        bool virtualSynth
    ) internal returns (IVirtualSynth vSynth) {
        // Burn the source amount
        issuer().synths(sourceCurrencyKey).burn(from, sourceAmountAfterSettlement);

        // Issue their new synths
        ISynth dest = issuer().synths(destinationCurrencyKey);

        if (virtualSynth) {
            vSynth = _createVirtualSynth(dest, recipient, amountReceived);
            dest.issue(address(vSynth), amountReceived);
        } else {
            dest.issue(recipient, amountReceived);
        }
    }

    function _createVirtualSynth(
        ISynth,
        address,
        uint
    ) internal returns (IVirtualSynth) {
        revert("Not supported in this layer");
    }

    // Note: this function can intentionally be called by anyone on behalf of anyone else (the caller just pays the gas)
    function settle(address from, bytes32 currencyKey)
        external
        returns (
            uint reclaimed,
            uint refunded,
            uint numEntriesSettled
        )
    {
        systemStatus().requireSynthActive(currencyKey);
        return _internalSettle(from, currencyKey, true);
    }

    function suspendSynthWithInvalidRate(bytes32 currencyKey) external {
        systemStatus().requireSystemActive();
        require(issuer().synths(currencyKey) != ISynth(0), "No such synth");
        require(_isSynthRateInvalid(currencyKey, exchangeRates().rateForCurrency(currencyKey)), "Synth price is valid");
        systemStatus().suspendSynth(currencyKey, CIRCUIT_BREAKER_SUSPENSION_REASON);
    }

    // SIP-78
    function setLastExchangeRateForSynth(bytes32 currencyKey, uint rate) external onlyExchangeRates {
        require(rate > 0, "Rate must be above 0");
        lastExchangeRate[currencyKey] = rate;
    }

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

    function _ensureCanExchange(
        bytes32 sourceCurrencyKey,
        uint sourceAmount,
        bytes32 destinationCurrencyKey
    ) internal view {
        require(sourceCurrencyKey != destinationCurrencyKey, "Can't be same synth");
        require(sourceAmount > 0, "Zero amount");

        bytes32[] memory synthKeys = new bytes32[](2);
        synthKeys[0] = sourceCurrencyKey;
        synthKeys[1] = destinationCurrencyKey;
        require(!exchangeRates().anyRateIsInvalid(synthKeys), "Src/dest rate invalid or not found");
    }

    function _isSynthRateInvalid(bytes32 currencyKey, uint currentRate) internal view returns (bool) {
        if (currentRate == 0) {
            return true;
        }

        uint lastRateFromExchange = lastExchangeRate[currencyKey];

        if (lastRateFromExchange > 0) {
            return _isDeviationAboveThreshold(lastRateFromExchange, currentRate);
        }

        // if no last exchange for this synth, then we need to look up last 3 rates (+1 for current rate)
        (uint[] memory rates, ) = exchangeRates().ratesAndUpdatedTimeForCurrencyLastNRounds(currencyKey, 4);

        // start at index 1 to ignore current rate
        for (uint i = 1; i < rates.length; i++) {
            // ignore any empty rates in the past (otherwise we will never be able to get validity)
            if (rates[i] > 0 && _isDeviationAboveThreshold(rates[i], currentRate)) {
                return true;
            }
        }

        return false;
    }

    function _isDeviationAboveThreshold(uint base, uint comparison) internal view returns (bool) {
        if (base == 0 || comparison == 0) {
            return true;
        }

        uint factor;
        if (comparison > base) {
            factor = comparison.divideDecimal(base);
        } else {
            factor = base.divideDecimal(comparison);
        }

        return factor >= getPriceDeviationThresholdFactor();
    }

    function _internalSettle(
        address from,
        bytes32 currencyKey,
        bool updateCache
    )
        internal
        returns (
            uint reclaimed,
            uint refunded,
            uint numEntriesSettled
        )
    {
        require(maxSecsLeftInWaitingPeriod(from, currencyKey) == 0, "Cannot settle during waiting period");

        (
            uint reclaimAmount,
            uint rebateAmount,
            uint entries,
            ExchangeEntrySettlement[] memory settlements
        ) = _settlementOwing(from, currencyKey);

        if (reclaimAmount > rebateAmount) {
            reclaimed = reclaimAmount.sub(rebateAmount);
            reclaim(from, currencyKey, reclaimed);
        } else if (rebateAmount > reclaimAmount) {
            refunded = rebateAmount.sub(reclaimAmount);
            refund(from, currencyKey, refunded);
        }

        if (updateCache) {
            bytes32[] memory key = new bytes32[](1);
            key[0] = currencyKey;
            debtCache().updateCachedSynthDebts(key);
        }

        // emit settlement event for each settled exchange entry
        for (uint i = 0; i < settlements.length; i++) {
            emit ExchangeEntrySettled(
                from,
                settlements[i].src,
                settlements[i].amount,
                settlements[i].dest,
                settlements[i].reclaim,
                settlements[i].rebate,
                settlements[i].srcRoundIdAtPeriodEnd,
                settlements[i].destRoundIdAtPeriodEnd,
                settlements[i].timestamp
            );
        }

        numEntriesSettled = entries;

        // Now remove all entries, even if no reclaim and no rebate
        exchangeState().removeEntries(from, currencyKey);
    }

    function reclaim(
        address from,
        bytes32 currencyKey,
        uint amount
    ) internal {
        // burn amount from user
        issuer().synths(currencyKey).burn(from, amount);
        ISynthetixInternal(address(synthetix())).emitExchangeReclaim(from, currencyKey, amount);
    }

    function refund(
        address from,
        bytes32 currencyKey,
        uint amount
    ) internal {
        // issue amount to user
        issuer().synths(currencyKey).issue(from, amount);
        ISynthetixInternal(address(synthetix())).emitExchangeRebate(from, currencyKey, amount);
    }

    function secsLeftInWaitingPeriodForExchange(uint timestamp) internal view returns (uint) {
        uint _waitingPeriodSecs = getWaitingPeriodSecs();
        if (timestamp == 0 || now >= timestamp.add(_waitingPeriodSecs)) {
            return 0;
        }

        return timestamp.add(_waitingPeriodSecs).sub(now);
    }

    function feeRateForExchange(bytes32 sourceCurrencyKey, bytes32 destinationCurrencyKey)
        external
        view
        returns (uint exchangeFeeRate)
    {
        exchangeFeeRate = _feeRateForExchange(sourceCurrencyKey, destinationCurrencyKey);
    }

    function _feeRateForExchange(
        bytes32, // API for source in case pricing model evolves to include source rate /* sourceCurrencyKey */
        bytes32 destinationCurrencyKey
    ) internal view returns (uint exchangeFeeRate) {
        return getExchangeFeeRate(destinationCurrencyKey);
    }

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

    function _getAmountsForExchangeMinusFees(
        uint sourceAmount,
        bytes32 sourceCurrencyKey,
        bytes32 destinationCurrencyKey
    )
        internal
        view
        returns (
            uint amountReceived,
            uint fee,
            uint exchangeFeeRate,
            uint sourceRate,
            uint destinationRate
        )
    {
        uint destinationAmount;
        (destinationAmount, sourceRate, destinationRate) = exchangeRates().effectiveValueAndRates(
            sourceCurrencyKey,
            sourceAmount,
            destinationCurrencyKey
        );
        exchangeFeeRate = _feeRateForExchange(sourceCurrencyKey, destinationCurrencyKey);
        amountReceived = _getAmountReceivedForExchange(destinationAmount, exchangeFeeRate);
        fee = destinationAmount.sub(amountReceived);
    }

    function _getAmountReceivedForExchange(uint destinationAmount, uint exchangeFeeRate)
        internal
        pure
        returns (uint amountReceived)
    {
        amountReceived = destinationAmount.multiplyDecimal(SafeDecimalMath.unit().sub(exchangeFeeRate));
    }

    function appendExchange(
        address account,
        bytes32 src,
        uint amount,
        bytes32 dest,
        uint amountReceived,
        uint exchangeFeeRate
    ) internal {
        IExchangeRates exRates = exchangeRates();
        uint roundIdForSrc = exRates.getCurrentRoundId(src);
        uint roundIdForDest = exRates.getCurrentRoundId(dest);
        exchangeState().appendExchangeEntry(
            account,
            src,
            amount,
            dest,
            amountReceived,
            exchangeFeeRate,
            now,
            roundIdForSrc,
            roundIdForDest
        );

        emit ExchangeEntryAppended(
            account,
            src,
            amount,
            dest,
            amountReceived,
            exchangeFeeRate,
            roundIdForSrc,
            roundIdForDest
        );
    }

    function getRoundIdsAtPeriodEnd(IExchangeState.ExchangeEntry memory exchangeEntry)
        internal
        view
        returns (uint srcRoundIdAtPeriodEnd, uint destRoundIdAtPeriodEnd)
    {
        IExchangeRates exRates = exchangeRates();
        uint _waitingPeriodSecs = getWaitingPeriodSecs();

        srcRoundIdAtPeriodEnd = exRates.getLastRoundIdBeforeElapsedSecs(
            exchangeEntry.src,
            exchangeEntry.roundIdForSrc,
            exchangeEntry.timestamp,
            _waitingPeriodSecs
        );
        destRoundIdAtPeriodEnd = exRates.getLastRoundIdBeforeElapsedSecs(
            exchangeEntry.dest,
            exchangeEntry.roundIdForDest,
            exchangeEntry.timestamp,
            _waitingPeriodSecs
        );
    }

    // ========== MODIFIERS ==========

    modifier onlySynthetixorSynth() {
        ISynthetix _synthetix = synthetix();
        require(
            msg.sender == address(_synthetix) || _synthetix.synthsByAddress(msg.sender) != bytes32(0),
            "Exchanger: Only synthetix or a synth contract can perform this action"
        );
        _;
    }

    modifier onlyExchangeRates() {
        IExchangeRates _exchangeRates = exchangeRates();
        require(msg.sender == address(_exchangeRates), "Restricted to ExchangeRates");
        _;
    }

    // ========== EVENTS ==========
    event ExchangeEntryAppended(
        address indexed account,
        bytes32 src,
        uint256 amount,
        bytes32 dest,
        uint256 amountReceived,
        uint256 exchangeFeeRate,
        uint256 roundIdForSrc,
        uint256 roundIdForDest
    );

    event ExchangeEntrySettled(
        address indexed from,
        bytes32 src,
        uint256 amount,
        bytes32 dest,
        uint256 reclaim,
        uint256 rebate,
        uint256 srcRoundIdAtPeriodEnd,
        uint256 destRoundIdAtPeriodEnd,
        uint256 exchangeTimestamp
    );
}


/**
 * @dev Implementation of the `IERC20` interface.
 *
 * This implementation is agnostic to the way tokens are created. This means
 * that a supply mechanism has to be added in a derived contract using `_mint`.
 * For a generic mechanism see `ERC20Mintable`.
 *
 * *For a detailed writeup see our guide [How to implement supply
 * mechanisms](https://forum.zeppelin.solutions/t/how-to-implement-erc20-supply-mechanisms/226).*
 *
 * We have followed general OpenZeppelin guidelines: functions revert instead
 * of returning `false` on failure. This behavior is nonetheless conventional
 * and does not conflict with the expectations of ERC20 applications.
 *
 * Additionally, an `Approval` event is emitted on calls to `transferFrom`.
 * This allows applications to reconstruct the allowance for all accounts just
 * by listening to said events. Other implementations of the EIP may not emit
 * these events, as it isn't required by the specification.
 *
 * Finally, the non-standard `decreaseAllowance` and `increaseAllowance`
 * functions have been added to mitigate the well-known issues around setting
 * allowances. See `IERC20.approve`.
 */
contract ERC20 is IERC20 {
    using SafeMath for uint256;

    mapping (address => uint256) private _balances;

    mapping (address => mapping (address => uint256)) private _allowances;

    uint256 private _totalSupply;

    /**
     * @dev See `IERC20.totalSupply`.
     */
    function totalSupply() public view returns (uint256) {
        return _totalSupply;
    }

    /**
     * @dev See `IERC20.balanceOf`.
     */
    function balanceOf(address account) public view returns (uint256) {
        return _balances[account];
    }

    /**
     * @dev See `IERC20.transfer`.
     *
     * Requirements:
     *
     * - `recipient` cannot be the zero address.
     * - the caller must have a balance of at least `amount`.
     */
    function transfer(address recipient, uint256 amount) public returns (bool) {
        _transfer(msg.sender, recipient, amount);
        return true;
    }

    /**
     * @dev See `IERC20.allowance`.
     */
    function allowance(address owner, address spender) public view returns (uint256) {
        return _allowances[owner][spender];
    }

    /**
     * @dev See `IERC20.approve`.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     */
    function approve(address spender, uint256 value) public returns (bool) {
        _approve(msg.sender, spender, value);
        return true;
    }

    /**
     * @dev See `IERC20.transferFrom`.
     *
     * Emits an `Approval` event indicating the updated allowance. This is not
     * required by the EIP. See the note at the beginning of `ERC20`;
     *
     * Requirements:
     * - `sender` and `recipient` cannot be the zero address.
     * - `sender` must have a balance of at least `value`.
     * - the caller must have allowance for `sender`'s tokens of at least
     * `amount`.
     */
    function transferFrom(address sender, address recipient, uint256 amount) public returns (bool) {
        _transfer(sender, recipient, amount);
        _approve(sender, msg.sender, _allowances[sender][msg.sender].sub(amount));
        return true;
    }

    /**
     * @dev Atomically increases the allowance granted to `spender` by the caller.
     *
     * This is an alternative to `approve` that can be used as a mitigation for
     * problems described in `IERC20.approve`.
     *
     * Emits an `Approval` event indicating the updated allowance.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     */
    function increaseAllowance(address spender, uint256 addedValue) public returns (bool) {
        _approve(msg.sender, spender, _allowances[msg.sender][spender].add(addedValue));
        return true;
    }

    /**
     * @dev Atomically decreases the allowance granted to `spender` by the caller.
     *
     * This is an alternative to `approve` that can be used as a mitigation for
     * problems described in `IERC20.approve`.
     *
     * Emits an `Approval` event indicating the updated allowance.
     *
     * Requirements:
     *
     * - `spender` cannot be the zero address.
     * - `spender` must have allowance for the caller of at least
     * `subtractedValue`.
     */
    function decreaseAllowance(address spender, uint256 subtractedValue) public returns (bool) {
        _approve(msg.sender, spender, _allowances[msg.sender][spender].sub(subtractedValue));
        return true;
    }

    /**
     * @dev Moves tokens `amount` from `sender` to `recipient`.
     *
     * This is internal function is equivalent to `transfer`, and can be used to
     * e.g. implement automatic token fees, slashing mechanisms, etc.
     *
     * Emits a `Transfer` event.
     *
     * Requirements:
     *
     * - `sender` cannot be the zero address.
     * - `recipient` cannot be the zero address.
     * - `sender` must have a balance of at least `amount`.
     */
    function _transfer(address sender, address recipient, uint256 amount) internal {
        require(sender != address(0), "ERC20: transfer from the zero address");
        require(recipient != address(0), "ERC20: transfer to the zero address");

        _balances[sender] = _balances[sender].sub(amount);
        _balances[recipient] = _balances[recipient].add(amount);
        emit Transfer(sender, recipient, amount);
    }

    /** @dev Creates `amount` tokens and assigns them to `account`, increasing
     * the total supply.
     *
     * Emits a `Transfer` event with `from` set to the zero address.
     *
     * Requirements
     *
     * - `to` cannot be the zero address.
     */
    function _mint(address account, uint256 amount) internal {
        require(account != address(0), "ERC20: mint to the zero address");

        _totalSupply = _totalSupply.add(amount);
        _balances[account] = _balances[account].add(amount);
        emit Transfer(address(0), account, amount);
    }

     /**
     * @dev Destoys `amount` tokens from `account`, reducing the
     * total supply.
     *
     * Emits a `Transfer` event with `to` set to the zero address.
     *
     * Requirements
     *
     * - `account` cannot be the zero address.
     * - `account` must have at least `amount` tokens.
     */
    function _burn(address account, uint256 value) internal {
        require(account != address(0), "ERC20: burn from the zero address");

        _totalSupply = _totalSupply.sub(value);
        _balances[account] = _balances[account].sub(value);
        emit Transfer(account, address(0), value);
    }

    /**
     * @dev Sets `amount` as the allowance of `spender` over the `owner`s tokens.
     *
     * This is internal function is equivalent to `approve`, and can be used to
     * e.g. set automatic allowances for certain subsystems, etc.
     *
     * Emits an `Approval` event.
     *
     * Requirements:
     *
     * - `owner` cannot be the zero address.
     * - `spender` cannot be the zero address.
     */
    function _approve(address owner, address spender, uint256 value) internal {
        require(owner != address(0), "ERC20: approve from the zero address");
        require(spender != address(0), "ERC20: approve to the zero address");

        _allowances[owner][spender] = value;
        emit Approval(owner, spender, value);
    }

    /**
     * @dev Destoys `amount` tokens from `account`.`amount` is then deducted
     * from the caller's allowance.
     *
     * See `_burn` and `_approve`.
     */
    function _burnFrom(address account, uint256 amount) internal {
        _burn(account, amount);
        _approve(account, msg.sender, _allowances[account][msg.sender].sub(amount));
    }
}


// Inheritance


// Libraries


// Internal references


// Note: use OZ's IERC20 here as using ours will complain about conflicting names
// during the build


// https://docs.synthetix.io/contracts/source/contracts/virtualsynth
contract VirtualSynth is ERC20, IVirtualSynth {
    using SafeMath for uint;
    using SafeDecimalMath for uint;

    ISynth public synth;
    IAddressResolver public resolver;

    bool public settled = false;

    uint8 public constant decimals = 18;

    // track initial supply so we can calculate the rate even after all supply is burned
    uint public initialSupply;

    // track final settled amount of the synth so we can calculate the rate after settlement
    uint public settledAmount;

    constructor(
        ISynth _synth,
        IAddressResolver _resolver,
        address _recipient,
        uint _amount
    ) public ERC20() {
        synth = _synth;
        resolver = _resolver;

        // Assumption: the synth will be issued to us within the same transaction,
        // and this supply matches that
        _mint(_recipient, _amount);

        initialSupply = _amount;
    }

    // INTERNALS

    function exchanger() internal view returns (IExchanger) {
        return IExchanger(resolver.requireAndGetAddress("Exchanger", "Exchanger contract not found"));
    }

    function secsLeft() internal view returns (uint) {
        return exchanger().maxSecsLeftInWaitingPeriod(address(this), synth.currencyKey());
    }

    function calcRate() internal view returns (uint) {
        if (initialSupply == 0) {
            return 0;
        }

        uint synthBalance;

        if (!settled) {
            synthBalance = IERC20(address(synth)).balanceOf(address(this));
            (uint reclaim, uint rebate, ) = exchanger().settlementOwing(address(this), synth.currencyKey());

            if (reclaim > 0) {
                synthBalance = synthBalance.sub(reclaim);
            } else if (rebate > 0) {
                synthBalance = synthBalance.add(rebate);
            }
        } else {
            synthBalance = settledAmount;
        }

        return synthBalance.divideDecimalRound(initialSupply);
    }

    function balanceUnderlying(address account) internal view returns (uint) {
        uint vBalanceOfAccount = balanceOf(account);

        return vBalanceOfAccount.multiplyDecimalRound(calcRate());
    }

    function settleSynth() internal {
        if (settled) {
            return;
        }
        settled = true;

        exchanger().settle(address(this), synth.currencyKey());

        settledAmount = IERC20(address(synth)).balanceOf(address(this));

        emit Settled(totalSupply(), settledAmount);
    }

    // VIEWS

    function name() external view returns (string memory) {
        return string(abi.encodePacked("Virtual Synth ", synth.currencyKey()));
    }

    function symbol() external view returns (string memory) {
        return string(abi.encodePacked("v", synth.currencyKey()));
    }

    // get the rate of the vSynth to the synth.
    function rate() external view returns (uint) {
        return calcRate();
    }

    // show the balance of the underlying synth that the given address has, given
    // their proportion of totalSupply
    function balanceOfUnderlying(address account) external view returns (uint) {
        return balanceUnderlying(account);
    }

    function secsLeftInWaitingPeriod() external view returns (uint) {
        return secsLeft();
    }

    function readyToSettle() external view returns (bool) {
        return secsLeft() == 0;
    }

    // PUBLIC FUNCTIONS

    // Perform settlement of the underlying exchange if required,
    // then burn the accounts vSynths and transfer them their owed balanceOfUnderlying
    function settle(address account) external {
        settleSynth();

        IERC20(address(synth)).transfer(account, balanceUnderlying(account));

        _burn(account, balanceOf(account));
    }

    event Settled(uint totalSupply, uint amountAfterSettled);
}


// Inheritance


// Internal references


// https://docs.synthetix.io/contracts/source/contracts/exchangerwithvirtualsynth
contract ExchangerWithVirtualSynth is Exchanger {
    constructor(address _owner, address _resolver) public Exchanger(_owner, _resolver) {}

    function _createVirtualSynth(
        ISynth synth,
        address recipient,
        uint amount
    ) internal returns (IVirtualSynth vSynth) {
        vSynth = new VirtualSynth(synth, resolver, recipient, amount);
        emit VirtualSynthCreated(address(vSynth), address(synth), synth.currencyKey(), amount);
    }

    event VirtualSynthCreated(address vSynth, address synth, bytes32 currencyKey, uint amount);
}