The contract: how and why it works

A smart contract is a program executed by the nodes of the Solana network, not on the project's server. Its code is the same for everyone, and every action is a public transaction. The FAIRDRUM contract is written in Rust with the Anchor library; its source code is open.

The program address will appear here once the site is connected to the contract.

An important consequence: if the fairdrum.to site disappears, players' funds stay in the contract, and they can be withdrawn with any program able to send the withdraw command signed by your wallet.

• The vault. A single wrapped-SOL account that only the program itself controls. It holds everything: players' balances, the prize bank, the jackpots, the reserve and the administrator's share that has not been collected yet.
• The shared game account. A record of how the amount in the vault is divided: prize bank, jackpot, mini jackpot, super jackpot, seed reserve, administrator's share, jackpot start amounts, per-game round counters.
• The player account. Created by the register command, which the site sends together with the first top-up, and tied to the wallet address: game balance, time of the last top-up, unfinished round, unpaid part of a jackpot.
• The session account (if “signature-free play” is on): a temporary key, an expiry time and a bet limit.

The bookkeeping is built so that the sum of all balances and pots equals the amount in the vault. A test checks this equality after each of tens of thousands of random operations.

1. The bet. The spin_commit command deducts the round from your balance and, in the same transaction, creates a random-number request with the ORAO oracle. The request address is derived from your wallet and the round number, so someone else's request cannot be substituted.
2. The number. Several independent ORAO nodes sign the random number and write it to the network. Until it is written, nobody knows it: not you, not the project, not any single node.
3. Settlement. The spin_settle command reads the number from the oracle's record, checks that the request is the one for your round, and computes the payout. The site sends it itself; if the page is closed, the project's keeper program will do it, or you will on your next visit.
4. Display. Cards, mines, the rocket, gems are ways of showing a payout that has already been computed. Your taps in the game do not affect it.

Why the result cannot be known in advance or forged. The final number of a round is the oracle's number mixed with the hash of the block in which the bet is recorded. At the moment of the bet this hash does not yet exist, and the oracle's number arrives later and from a third party: nobody can compute the outcome in advance — not the player, not the project, not the oracle itself. The game, the jackpot start amounts and the prize cap are recorded at the moment of the bet, and the settlement formula is in open code. A late settlement is never more favourable than a timely one. The rules can be changed only by replacing the program's code, and that is visible to everyone (see the section on the administrator).

A round is 1 to 100 bets of 0.01 SOL each (up to 1000 on the wheel). The oracle's number is expanded into an independent random value for each bet, and for each one the contract answers these questions in turn:

1. Super jackpot, jackpot, mini jackpot? The chance of each pot equals the bet's contribution to it divided by the pot's start amount. A pot is drawn only when it holds at least 25% of its start amount.
2. A game prize? Each game has its own prize chance and its own table of six tiers with weights. For example, on the wheel a prize lands in 10% of bets. The tables of all games are shown on their pages.
3. “Spin again”? With a 11.6% chance the bet is repeated for free at the expense of the prize bank.
4. Otherwise the bet wins no prize. It is split the same way as any other: prize bank, jackpots and the administrator's share.

The games' tables differ, but the expected payout from them is the same (to within hundredths of a percent; with a prize bank of 40 SOL or more, when prizes do not hit the cap): about 77.5% of the bet in regular prizes, and together with “Spin again” repeats about 88% of paid bets. The compiler checks this equality: if any game's expectation turns out higher than the wheel's, or lower by more than half a percent, the program will not build.

• A single prize is at most a quarter of the prize bank. The contract cannot promise more than it has, and one big win does not empty the bank for everyone else. Whatever is not paid out stays in the bank.
• Withdrawal 12 hours after the last top-up. The rule is the same for everyone and is written as a constant: it can only be changed by a new version of the program.
• Jackpot start amounts change with a 24-hour delay. A round's odds are fixed at the moment of the bet, so they cannot be changed “to fit a bet already placed”.
• 20 minutes to settle. If the oracle has not responded, the bet is refunded to the player in full: until the number exists, nobody knows the outcome, so a loss cannot be avoided this way. If it responded but the settlement was late, the regular prizes are paid, and the jackpots are not drawn in that round. Vault protection. Before every payout and after every settlement the contract checks that the vault holds no less than it owes to players and pots; otherwise the operation is rejected. Guardian. If the oracle has stopped responding, the acceptance of new bets is paused automatically; settlements and withdrawals keep working.
• A jackpot is paid in installments: the winner is owed 95% of the pot, of which 20% at once and 8% per week. The unpaid part is recorded on the winner's account.
• One unfinished round per player. Until the round is settled, a new bet is not accepted.

• Contract tests: a 10% administrator share in every game, a closely matching return across all games with a 50 SOL bank, the quarter-of-the-bank rule and the prize cap as of the moment of the bet, the withdrawal lock, the delayed change of start amounts, a late settlement being no more favourable than a timely one.
• Conservation-of-funds check: tens of thousands of random operations, and after each one the amount in the vault is compared with the sum of the balances and pots.
• Long runs: two million bets for each game — how much was paid to players and how much the administrator received.
• Real transactions on a local Solana node: a top-up, a withdrawal refused before the lock expires, a contribution to the reserve, a change of start amounts.
• Site tests: the site computes the odds with the same formulas as the contract and checks itself against samples recorded by the contract.

What has not been tested or done is listed plainly on the “Why FAIRDRUM” page. How to verify any round yourself is on the “Verify a round” page.