1. Summary

Fixed discount collateral auctions are similar to their English counterpart in that they are used to preserve the overall system health by liquidating under-collateralized SAFEs and selling off collateral in exchange for system coins. This auction type automatically calculates an amount of collateral to send back to a bidder, taking into account the amount of system coins the bidder submits as well as the current system coin redemptionPrice (and optionally its market price) and collateral market price.

2. Contract Variables & Functions

Variables

• contractEnabled - settlement flag (1 or 0).

• AUCTION_HOUSE_TYPE - flag set to bytes32("COLLATERAL")

• AUCTION_TYPE - flag set to bytes32("FIXED_DISCOUNT").

• authorizedAccounts[usr: address] - addresses allowed to call modifyParameters() and disableContract().

• safeEngine - storage of the SAFEEngine's address.

• bids[id: uint] - storage of all bids.

• collateralType - id of the collateral type for which the CollateralAuctionHouse is responsible.

• minimumBid - minimum amount of system coins that must be submitted by each bidder.

• totalAuctionLength - auction length (default: uint48(-1)).

• auctionsStarted - total auction count, used to track auction ids.

• lastReadRedemptionPrice - the last read redemption price. Can (and most probably is) be different than the latest OracleRelayer._redemptionPrice

• discount - discount compared to the collateral market price; used when calculating the amount of collateral to send to a bidder.

• lowerCollateralMedianDeviation - max collateralMedian collateral price deviation (compared to the FSM price) used when the median price is lower than the collateralFSM price and the contract needs to pick which one to use

• upperCollateralMedianDeviation - max collateralMedian collateral price deviation (compared to the FSM price) used when the median price is higher than the collateralFSM price and the contract needs to pick which one to use

• lowerSystemCoinMedianDeviation - max systemCoinOracle price deviation (compared to the redemptionPrice) used when the system coin's redemptionPrice price is higher than its market price and the contract needs to pick which one to use

• upperSystemCoinMedianDeviation - max systemCoinOracle price deviation (compared to the redemptionPrice) used when the system coin's redemptionPrice price is lower than its market price and the contract needs to pick which one to use

• minSystemCoinMedianDeviation - minimum deviation between the system coin's market and redemption prices that must be passed in order for the contract to use the deviated price instead of the redemption one

• oracleRelayer - address of the OracleRelayer

• collateralFSM - the collateral type's FSM address

• collateralMedian - collateral type medianizer address

• systemCoinOracle - market price oracle for the system coin

• liquidationEngine - the address of the LiquidationEngine

• RAD - number with 45 decimals (e.g a safeEngine.coinBalance)

• WAD - number with 18 decimals (e.g a bid submitted when someone wants to buy collateral from an auction)

• RAY - number with 27 decimals

Data Structures

• Bid - state of a specific auction

  • raisedAmount - amount of system coins raised up until this point

  • soldAmount - amount of collateral sold up until this point

  • amountToSell - quantity up for auction / collateral for sale

  • amountToRaise - total system coins wanted from the auction

  • auctionDeadline - max auction duration

  • forgoneCollateralReceiver - address of the SAFE being auctioned

  • auctionIncomeRecipient - recipient of auction income / receives system coin income (this is the AccountingEngine contract)

Modifiers

• isAuthorized - checks whether an address is part of authorizedAddresses (and thus can call authed functions).

Functions

• modifyParameters(bytes32 parameter, uint256 data) - update an uint256 parameter.

• modifyParameters(bytes32 parameter, address data) - update an address parameter.

• addAuthorization(usr: address) - add an address to authorizedAddresses.

• removeAuthorization(usr: address) - remove an address from authorizedAddresses.

• getCollateralMedianPrice() public view returns (priceFeed: uint256) - get the collateral's median price from collateralMedian

• getSystemCoinMarketPrice() public view returns (priceFeed: uint256) - get the system coin's market price from systemCoinOracle

• getFinalTokenPrices(systemCoinRedemptionPrice: uint256) public view returns (uint256, uint256) - get the collateral and system coin prices that can be currently used to determine the amount of collateral bought by bidders

• getFinalBaseCollateralPrice(collateralFsmPriceFeedValue: uint256, collateralMedianPriceFeedValue: uint256) public view returns (uint256) - get the final collateral price (without the discount applied) that will be used to determine the amount of collateral bought by a bid

• getDiscountedCollateralPrice(collateralFsmPriceFeedValue: bytes32, collateralMedianPriceFeedValue: bytes32, systemCoinPriceFeedValue: uint256, customDiscount: uint256) public view returns (uint256) - get the (discounted) collateral price using either the FSM or median price for the collateral and the redemption/market price for the system coin

• startAuction(forgoneCollateralReceiver: address, auctionIncomeRecipient: address, amountToRaise: uint256, amountToSell: uint256, initialBid: uint256 ) - function used by LiquidationEngine to start an auction / put collateral up for auction

• getApproximateCollateralBought(id: uint256, wad: uint256) - get the amount of collateral that can be bought from a specific auction by bidding wad and assuming that the latest system coin redemptionPrice is equal to lastReadRedemptionPrice

• getCollateralBought(id: uint256, wad: uint256) returns (uint256, uint256) - get the amount of collateral that can be bought from a specific auction by bidding wad amount of system coins (where wad will be scaled by RAY to transfer the correct amount of RAD system coins from safeEngine.coinBalance)

• buyCollateral(id: uint256, wad: uint256) - buy collateral from an auction and offer wad amount of system coins in return (where wad is scaled by RAY in order to transfer RAD amount of coins from the bidder's safeEngine.coinBalance)

• settleAuction(id: uint256) - settle an auction that has passed its auctionDeadline and return any unsold collateral to the forgoneCollateralReceiver

• terminateAuctionPrematurely(id: uint256) - normally used during GlobalSettlement to terminate an auction early and send unsold collateral to the msg.sender as well as call LiquidationEngine in order to subtract bids[auctionId].amountToRaise from LiquidationEngine.currentOnAuctionSystemCoins

• bidAmount(id: uint256) public view returns (uint256) - always returns zero

• remainingAmountToSell(id: uint256) public view returns (uint256) - return the remaining collateral amount to sell from a specific auction

• forgoneCollateralReceiver(uint id) public view returns (address) - returns theforgoneCollateralReceiver for a specific auction

• raisedAmount(id: uint256) - returns the currently raised amount of system coins for a specific auction.

• amountToRaise(uint id) public view returns (uint256) - returns the amount of system coins to raise for a specific auction

Events

• AddAuthorization - emitted when a new address becomes authorized. Contains:

  • account - the new authorized account

• RemoveAuthorization - emitted when an address is de-authorized. Contains:

  • account - the address that was de-authorized

• StartAuction- emitted when startAuction(address, address, uint256, uint256, uint256) is successfully executed. Contains:

  • id - auction id

  • auctionsStarted - amount of auctions started up until now

  • amountToSell - amount of collateral sold in the auction

  • initialBid - starting bid for the auction (usually zero).

  • amountToRaise - amount of system coins that should be raised by the auction.

  • forgoneCollateralReceiver - receiver of leftover collateral (usually the SAFE whose collateral was confiscated by the LiquidationEngine).

  • auctionIncomeRecipient - receiver of system coins (usually the AccountingEngine).

  • auctionDeadline - the auction's deadline

• ModifyParameters - emitted when a parameter is modified

• BuyCollateral - emitted when someone buys collateral from an auction. Contains:

  • id - the ID of the auction from which collateral was bought

  • wad - the bid size

  • boughtCollateral - the amount of collateral that was bought

• SettleAuction - emitted when someone settles an auction. Contains:

  • id - the ID of the auction settled

  • leftoverCollateral - the amount of collateral that hasn't been sold by the now settled auction

• TerminateAuctionPrematurely - emitted when an auction is terminated before its deadline. Contains:

  • id - the ID of the auction that was terminated

  • sender - the address that terminated the auction

  • collateralAmount - the amount of collateral still unauctioned

3. Walkthrough

The fixed discount auction is a straightforward way (compared to English auctions) to put SAFE collateral up for sale in exchange for system coins used to settle bad debt. Bidders are only required to allow the auction house to transfer their safeEngine.coinBalance and can then callbuyCollateral(id: uint256, wad: uint256) in order to exchange their system coins for collateral which is sold at a discount compared to its latest recorded market price. Bidders can also review the amount of collateral they can get from a specific auction by calling getCollateralBought(id: uint256, wad: uint256) or getApproximateCollateralBought(id: uint256, wad: uint256). Note that getCollateralBought is not marked as view because it reads (and also updates) the redemptionPrice from the OracleRelayer whereas getApproximateCollateralBought uses thelastReadRedemptionPrice.

There are several parameters that come together when the contract calculates the amount of collateral to send to a bidder:

• The amount of system coins wad used when calling buyCollateral. wad must be bigger than zero and bigger than or equal to minimumBid. In case wad is bigger than minimumBid, the contract will only request what it needs in order to fill remainingToRaise. Note that in order to avoid dusty auctions resulting from computing an adjustedBid (because of precision loss from division) the contract will automatically request 10**-18 extra system coins

• The difference between the collateral's FSM price (the delayed price) and the latest market price stored in the medianizer (which the FSM is connected to). The auction house uses the FSM price by default so that, in case of an oracle attack, governance can react and temporarily disable the FSM (in case they have power over that system component).

  • During the FSM's delay, the market price (and thus the medianizer value) might change significantly and thus bidders might have to wait until a new median price feed is pushed into the FSM so that they can profit from auctions. In order to avoid scenarios where bidders have to wait for an FSM update and at the same time protect collateral auctions from an oracle attack, we added two variables: lowerCollateralMedianDeviationand upperCollateralMedianDeviationwhich allow the auction house to pick the medianizer price (when calculating the amount of collateral bought by a bidder) in case it deviated within certain bounds compared to the FSM price.

• The difference between the system coin's redemptionPrice and its market price (provided only if systemCoinOracle is not null and it returns a valid price). The auction house uses the redemptionPrice by default, although, during market shocks, there may be a significant difference between the system coin's redemption and market prices which will discourage bidding (if the market price is above redemption). This is why governance can set lowerSystemCoinMedianDeviation and upperSystemCoinMedianDeviation in order to allow the contract to use the system coin market price if it deviated within certain bounds from the redemptionPrice. Governance can also set minSystemCoinMedianDeviation so that the contract only chooses the market price in case it deviated above a certain threshold.

Similar to English auctions, when the auction is settled (or terminated prematurely), the contract will call the LiquidationEngine in order to removeCoinsFromAuction (subtract bids[auctionId].amountToRaise from LiquidationEngine.currentOnAuctionSystemCoins).

4. Gotchas

• In case someone bids an amount that is higher than the remaining amount of system coins which still need to be raised by an auction, the contract will only charge for the remaining amount plus 10**-18 extra system coins (meant to prevent dusty auctions)

• You must always submit a bid that is higher than or equal to minimum(minimumBid, subtract(bids[id].amountToRaise, bids[id].raisedAmount)). The contract will take care of charging only for the amount needed to cover the total remainingamountToRaise

• The auctions use the system coin redemptionPrice by default (not its market price)

5. Examples

// Scenario 1
​
minimumBid                     = 5 * WAD  // 5 coins
discount                       = 0.95E18  // 5%
lowerCollateralMedianDeviation = 0.90E18  // 10%
upperCollateralMedianDeviation = 0.95E18  // 5%
lowerSystemCoinMedianDeviation = WAD      // 0%
upperSystemCoinMedianDeviation = WAD      // 0%
minSystemCoinMedianDeviation   = 0.999E18 // 0.1%
​
collateralFsmPriceFeedValue    = 100 * WAD
collateralMedianPriceFeedValue = 89 * WAD
​
systemCoinRedemptionPrice      = 5 * RAY
systemCoinMarketPrice          = 5.01 * RAY
​
amountToSell                   = WAD
amountToRaise                  = 10 * RAD
​
submittedBid                   = 5 * WAD
​
/* 
    Because the collateral price fetched from the median is smaller than
    the collateral's FSM price and it is also deviated more than what
    lowerCollateralMedianDeviation permits (11%), the final collateral price 
    used by the contract will be 
    0.9 (lower collateral deviation) * 100 (collateral FSM price) = 90 USD
*/
finalCollateralPrice            = 90 * WAD
​
/*
    Even if the system coin market price is deviated from the redemption price,
    both the lower and the upper systemCoinMedianDeviation are 0% so the contract
    will use the redemption price
*/
finalSystemCoinPrice            = 5 * RAY
​
/*
    Determining the amount of collateral bought given the 5 system coin bid
*/
discountedSystemCoinCollateralPrice 
    = (finalCollateralPrice * RAY / finalSystemCoinPrice) * discount / WAD 
    = 17.1 * WAD
​
boughtCollateralAmount 
    = submittedBid * WAD / discountedSystemCoinCollateralPrice
    = 0.292397661 * WAD

// Scenario 2
​
minimumBid                     = 5 * WAD  // 5 coins
discount                       = 0.95E18  // 5%
lowerCollateralMedianDeviation = 0.90E18  // 10%
upperCollateralMedianDeviation = 0.95E18  // 5%
lowerSystemCoinMedianDeviation = 0.95E18  // 5%
upperSystemCoinMedianDeviation = 0.98E18  // 2%
minSystemCoinMedianDeviation   = 0.999E18 // 0.1%
​
collateralFsmPriceFeedValue    = 100 * WAD
collateralMedianPriceFeedValue = 89 * WAD
​
systemCoinRedemptionPrice      = 5 * RAY
systemCoinMarketPrice          = 5.1 * RAY
​
amountToSell                   = WAD
amountToRaise                  = 10 * RAD
​
submittedBid                   = 15 * WAD
​
/*
    Given that the submittedBid is higher than the total remaining amount to raise,
    the contract will adjust the bid (by rounding up)
*/
adjustedBid = amountToRaise / RAY + 1 = 10 * WAD + 1
​
/*
    Similar to Scenario 1, the final collateral price used by the contract 
    will be 90 USD
*/
finalCollateralPrice           = 90 * WAD
​
/*
    The system coin market price is 2% deviated compared to the redemption price
    and upperSystemCoinMedianDeviation is also 2% so the contract will pick
    5 (redemption price) * 1.02 (allowed deviation) = 5.1 USD as the system
    coin price
*/
finalSystemCoinPrice           = 5.1 * RAY
​
/*
    Determining the amount of collateral bought given the 10 WAD + 1 
    adjusted system coin bid
*/
discountedSystemCoinCollateralPrice 
    = (finalCollateralPrice * RAY / finalSystemCoinPrice) * discount / WAD 
    = 16.764705882 * WAD
​
boughtCollateralAmount 
    = adjustedBid * WAD / discountedSystemCoinCollateralPrice
    = 0.596491228082733148 * WAD