夺宝开奖
怎么做到 公平？

import java.security.SecureRandom;

public final class BackendDrawEngine {
  private static final SecureRandom RNG = new SecureRandom();

  public DrawResult draw(long roundId, int ticketAll) {
    if (ticketAll <= 0) {
      throw new IllegalArgumentException("ticketAll must be positive");
    }

    long randomValue = RNG.nextLong();              // 64-bit signed long, may be negative
    int winningIndex = Math.floorMod(randomValue, ticketAll); // 0..ticketAll-1
    int ticketNo = winningIndex + 1;             // business ticket: 1..ticketAll

    return new DrawResult(roundId, randomValue, ticketAll, winningIndex, ticketNo);
  }
}

record DrawResult(
  long roundId,
  long randomValue,
  int ticketAll,
  int winningIndex,
  int ticketNo
) {}

// ── 售完触发 / on sold-out (后端) ──
chainService.requestVrf(roundId)
  └→ DrawCoordinator.requestRandom(roundId)         // 链上调 Chainlink VRF
       └→ VRFCoordinatorV2Plus.requestRandomWords(...)

// ── 1-3 分钟后 VRF 链上回调 (合约) ──
DrawCoordinator.fulfillRandomWords(reqId, [word])
  └→ emit RandomWordsFulfilled(roundId, word)        // 链上事件

// ── chain-service 监听到事件 → Kafka → game-service ──
gameService.onVrfFulfilled(roundId, word):
  winNo  = (BigInteger(word) % soldCount) + 1
  winner = ticket[winNo].uid
  walletService.credit(winner, prize)                // 后端钱包派奖
  publish(game.draw.completed{roundId, winner})

// ── 用户链上买票 / every purchase is on-chain ──
TreasureContract.buy(ticketCount, optionalNumber)
  └→ transfer USDT into contract
  └→ assign ticketNo by Fisher-Yates
  └→ store numberOwner[ticketNo] = buyer

// ── 售完即开 / sold out triggers VRF ──
if remainingTicket == 0:
  requestId = VRFCoordinatorV2Plus.requestRandomWords(...)

// ── VRF 回调后合约开奖和分账 / fulfill on-chain ──
rawFulfillRandomWords(requestId, randomWords):
  randomWord = randomWords[0]
  winningIndex = randomWord % ticketAll
  ticketNo = winningIndex + 1
  winner = numberOwner[ticketNo]
  settleDraw(winner, rewardConfig)        // 中奖人、推荐人、平台、社区、滚动奖池
  launchNextRoundIfNeeded()

// ── 售完触发 / on sold-out (后端) ──
tickets = loadTickets(roundId)
ticketRoot = merkleRoot(tickets)              // leaf = hash(roundId, ticketNo, userId, wallet)
DrawCoordinator.commitTickets(roundId, ticketRoot, soldCount)

// ── root 上链后才能请求 VRF ──
DrawCoordinator.requestRandom(roundId)
  require(ticketRoot[roundId] != 0)
  require(vrfRequestId[roundId] == 0)

// ── VRF 回调锁定中奖票号 ──
fulfillRandomWords(reqId, [word]):
  winNo = (word % soldCount) + 1
  store(roundId, winNo)

// ── 中奖者或代领服务提交 proof，合约验票后付款 ──
claim(roundId, ticketNo, userId, wallet, proof):
  leaf = hash(roundId, ticketNo, userId, wallet)
  require(ticketNo == winNo)
  require(MerkleProof.verify(proof, ticketRoot, leaf))
  require(!claimed[roundId])
  claimed[roundId] = true
  Vault.payout(wallet, prize)