contract PurgeableSynth is Synth {

    using SafeDecimalMath for uint;

    // The maximum allowed amount of tokenSupply in equivalent sUSD value for this synth to permit purging
    uint public maxSupplyToPurgeInUSD = 100000 * SafeDecimalMath.unit(); // 100,000

    // Track exchange rates so we can determine if supply in USD is below threshold at purge time
    ExchangeRates public exchangeRates;

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

    constructor(address _proxy, TokenState _tokenState, address _synthetixProxy, IFeePool _feePool,
        string _tokenName, string _tokenSymbol, address _owner, bytes32 _currencyKey, ExchangeRates _exchangeRates, uint _totalSupply
    )
        Synth(_proxy, _tokenState, _synthetixProxy, _feePool, _tokenName, _tokenSymbol, _owner, _currencyKey, _totalSupply)
        public
    {
        exchangeRates = _exchangeRates;
    }

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

    /**
     * @notice Function that allows owner to exchange any number of holders back to sUSD (for frozen or deprecated synths)
     * @param addresses The list of holders to purge
     */
    function purge(address[] addresses)
        external
        optionalProxy_onlyOwner
    {
        uint maxSupplyToPurge = exchangeRates.effectiveValue("sUSD", maxSupplyToPurgeInUSD, currencyKey);

        // Only allow purge when total supply is lte the max or the rate is frozen in ExchangeRates
        require(
            totalSupply <= maxSupplyToPurge || exchangeRates.rateIsFrozen(currencyKey),
            "Cannot purge as total supply is above threshold and rate is not frozen."
        );

        for (uint i = 0; i < addresses.length; i++) {
            address holder = addresses[i];

            uint amountHeld = balanceOf(holder);

            if (amountHeld > 0) {
                ISynthetix(synthetixProxy).synthInitiatedExchange(holder, currencyKey, amountHeld, "sUSD", holder);
                emitPurged(holder, amountHeld);
            }

        }

    }

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

    function setExchangeRates(ExchangeRates _exchangeRates)
        external
        optionalProxy_onlyOwner
    {
        exchangeRates = _exchangeRates;
    }

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

    event Purged(address indexed account, uint value);
    bytes32 constant PURGED_SIG = keccak256("Purged(address,uint256)");
    function emitPurged(address account, uint value) internal {
        proxy._emit(abi.encode(value), 2, PURGED_SIG, bytes32(account), 0, 0);
    }
}
/*
 -----------------------------------------------------------------
 FILE INFORMATION
 -----------------------------------------------------------------
 
 file:       PurgeableSynth.sol
 version:    1.0
 author:     Justin J. Moses
 date:       2019-05-22
 
 -----------------------------------------------------------------
 MODULE DESCRIPTION
 -----------------------------------------------------------------
 
 Purgeable synths are a subclass of Synth that allows the owner
 to exchange all holders of the Synth back into sUSD.
 
 In order to reduce gas load on the system, and to repurpose older synths
 no longer used, purge allows the owner to purge all holders balances into sUSD
 
 -----------------------------------------------------------------
 */
function setExchangeRates(ExchangeRates _exchangeRates)
    external
    optionalProxy_onlyOwner
{
    exchangeRates = _exchangeRates;
}

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