🏆Prize Pools

Pool deposits and award accrued interest periodically as a prize

Introduction

Prize Pools allow funds to be pooled together into a no-loss yield source, such as Compound, and have the yield safely exposed to a separate Prize Strategy. They are the primary way through which users interact with PoolTogether prize games.

Prize Pools provide controls to the owner so that participation can be made fair. See Fairness for more information.

There is a different type of prize pool for each yield source. For example, if you wish to use Compound you will use the Compound Prize Pool.

All Prize Pools share the functionality below.

Owner

When a Prize Pool is created, the creator is set as the pool's "owner". The owner is able to:

• Add additional pool tokens

• Change the Prize Strategy

• Set the credit rate and credit limit

• Shutdown the prize pool

• Transfer ownership

• Renounce ownership

The prize pool is not upgradeable and therefore the owner can never seize the funds deposited into the prize pool

Limits

When a Prize Pool is created it is initialized with some hard-coded limits to protect users. See Fairness for more details.

Maximum Timelock Duration

The maximum timelock duration ensures that a user has to wait at most X amount of time to withdraw their funds loss-lessly. If the owner of a pool sets the credit rate to be way too low, this limit ensures users will still be able to withdraw.

If using the Single Random Winner Prize Strategy, it would make sense to set the maximum timelock duration to 2x the prize period. That way the owner has some flexibility when adjusting the credit limit and credit rate.

Maximum Credit Limit

The maximum credit limit ensures that the credit limit cannot be set higher than this number. This prevents the owner of the Prize Pool from capturing *all* of a user's deposit at withdrawal time.

Token Model

A Prize Pool accepts a single type of ERC20 token for deposits. This token depends on the implementation: for a Compound Prize Pool bound to cDai it will be Dai, for a yEarn yUSDC vault it will be USDC. This is the underlying asset of the Prize Pool.

Prize Pools use Controlled Tokens for their internal accounting. These tokens are minted when depositing or awarding prizes. Controlled Tokens are burned when users withdraw. They are exchanged at a ratio of 1:1 to the asset.

Controlled Tokens

A Controlled Token is a standard ERC20 that is bound to a Token Controller.

The Token Controller has the privileged ability to mint and burn tokens on user's behalf, and has a callback that listens for token transfers. Controlled Tokens are expected to trigger this callback on any mint, burns or transfers.

The Prize Pool must be the Token Controller for the controlled tokens that it is initialized with at construction.

The default Compound Prize Pool Builder creates a Ticket controlled token and a Sponsorship controlled token. These tokens can be looked up on the corresponding Prize Strategy.

Minting

When a user deposits into a Prize Pool they must request what type of controlled token they receive in exchange. This token will be minted to them at an exchange rate of 1:1 for the asset.

Burning

When a user wishes to withdraw from a Prize Pool they must burn controlled tokens.

Depositing

Users can deposit into the Prize Pool using the depositTo function. A user is instantly minted tokens upon deposit.

function depositTo(
    address to,
    uint256 amount,
    address controlledToken,
    address referrer
) external;

Parameter	Description
to	The address to whom the controlled tokens should be minted
amount	The amount of the underlying asset the user wishes to deposit. The Prize Pool contract should have been pre-approved by the caller to transfer the underlying ERC20 tokens.
controlledToken	The address of the token that they wish to mint. For our default Prize Strategy this will either be the Ticket address or the Sponsorship address. Those addresses can be looked up on the Prize Strategy.
referrer	The address that should receive referral awards, if any.

Depositing fires the event:

event Deposited(
    address indexed operator,
    address indexed to,
    address indexed token,
    uint256 amount
);

Event Data	Description
operator	The caller that made the deposit
to	The address that received the minted tokens
token	The address of the controlled token that was minted
amount	The amount of both the underlying asset that was transferred and the tokens that were minted.

Withdrawing

When a user withdraws they may need to contribute to the prize according to the fairness rules. They may either cover the contribution by time-locking their funds, or cover the contribution explicitly using funds.

Withdraw with Timelock

Funds can be withdrawn losslessly by time-locking the funds. The withdrawal amount will be unlocked at a later date at which point the funds can be swept back to the user. The timelock duration is calculated based on the users accrued credit, the credit rate, and the fairness fee.

If the user has sufficient credit, the unlockTimestamp may be "now" and the funds are instantly swept to the from address.

Tip: You can call this function in a constant way to see when the users funds will be unlocked.

To start a lossless withdrawal a user may call:

function withdrawWithTimelockFrom(
    address from,
    uint256 amount,
    address controlledToken
) external returns (uint256 unlockTimestamp);

Parameter Name	Parameter Description
from	The user from whom to withdraw. This means you may withdraw on another user's behalf if they have given you an ERC20 allowance.
amount	The amount of collateral to withdraw.
controlledToken	The type of controlled token to withdraw.

Checking Timelock Balances

To see how many funds have been timelocked for a user you can call:

function timelockBalanceOf(address user) external view returns (uint256)

After funds have been time-locked, you can see at what timestamp they'll be available:

function timelockBalanceAvailableAt(address user) external view returns (uint256)

Sweeping Timelocked Funds

When a user's withdrawal timelocks have ended, the funds may be swept to their wallets:

function sweepTimelockBalances(
    address[] memory users
) external returns (uint256 totalWithdrawal);

The function accepts an array of addresses and will attempt to sweep the time-locked funds for each one. The funds will be transferred back to the users wallets.

Depositing Using Timelocked Funds

If a user wishes to re-deposit their timelocked funds, they can do so using this function:

function timelockDepositTo(
    address to,
    uint256 amount,
    address controlledToken
)
    external;

Parameter	Description
to	The address to whom the controlled tokens should be minted
amount	The amount of the underlying asset the user wishes to deposit. The Prize Pool contract should have been pre-approved by the caller to transfer the underlying ERC20 tokens.
controlledToken	The address of the token that they wish to mint. For our default Prize Strategy this will either be the Ticket address or the Sponsorship address. Those addresses can be looked up on the Prize Strategy.

Withdraw Instantly

If a user would like their tickets right away, they may pay an early exit fee to the prize. The early exit fee is determined by the Prize Strategy.

The instant withdrawal function returns the amount of the withdrawal that was retained as payment. This means you can call this function in a constant way to check to see what the exit fee will be. When it comes time to run the tx, that exit fee can be passed as the maximumExitFee to ensure it doesn't exceed the expected limit.

function withdrawInstantlyFrom(
    address from,
    uint256 amount,
    address controlledToken,
    uint256 maximumExitFee
  )
    external
    returns (uint256 exitFee);

Parameter Name	Parameter Description
from	The address to withdraw from. This means you can withdraw on another user's behalf if you have an allowance for the controlled token.
amount	The amount to withdraw
controlledToken	The controlled token to withdraw from
maximumExitFee	The maximum early exit fee the caller is willing to pay. This prevents the Prize Strategy from changing the fee on-the-fly.

Awarding

Only the Prize Strategy can call the award functions. These functions allow prizes to be disbursed to users.

Awarding Yield

Yield that accrues in the Prize Pool can be awarded by the Prize Strategy. The yield must first be captured and then it can be awarded.

To capture the yield the prize strategy can call the captureAwardBalance function:

function captureAwardBalance() external onlyPrizeStrategy returns (uint256);

This function will:

• add the current yield balance to the available award balance

• capture a portion for the reserve

• return the total available award balance.

To award the captured yield to an address, the Prize strategy uses the award function. The yield must be awarded as one of the controlled tokens configured in the Prize Pool.

function award(
    address to,
    uint256 amount,
    address controlledToken
) external onlyPrizeStrategy;

Parameter Name	Parameter Description
to	The address to receive the newly minted tokens
amount	The amount of tokens to mint
controlledToken	The type of token to mint

Awarding ERC20s

The Prize Strategy can award ERC20 tokens that are held by the Prize Pool.

function awardExternalERC20(
    address to,
    address externalToken,
    uint256 amount
) external onlyPrizeStrategy;

However, some tokens are be blacklisted if they need to be held to generate yield (i.e. Compound cTokens).

Parameter Name	Parameter Description
to	The address to receive the transfer
externalToken	The ERC20 to transfer
amount	The amount of tokens to transfer

Awarding ERC721s (NFTs)

The Prize Strategy can award ERC721 tokens that are held by the Prize Pool.

function awardExternalERC721(
    address to,
    address externalToken,
    uint256[] calldata tokenIds
  )
    external
    onlyPrizeStrategy;

Parameter Name	Parameter Description
to	The address to receive the NFTs
externalToken	The ERC721 contract address
tokenIds	The NFT token ids to transfer.

Credit

Credit accrues differently for each of the Prize Pool's controlled tokens, so each token will have its own credit rate and credit limit.

Credit Balance

To get a users credit balance for a controlled token:

function balanceOfCredit(
    address user,
    address controlledToken
) external returns (uint256);

Parameter Name	Parameter Description
user	The user whose credit balance should be returned
controlledToken	The token for which the credit balance should be pulled

Credit Rate

The credit rate for a controlled token can be checked like so:

function creditRateOf(
    address controlledToken
) external view returns (
    uint128 creditLimitMantissa,
    uint128 creditRateMantissa
);

Parameter Name	Parameter Description
controlledToken	The controlled token whose credit limit and rate should be returned.

Note that the returned values are "mantissas": i.e. fixed point numbers with 18 decimal places.