Every winner is chosen
100% fairly

1// src/lib/services/WinnerSelection.ts
2import { randomInt } from 'crypto';
3import { GiveawayEntry } from '@/components/dashboard/giveaways/types';
4
5/**
6 * Select winners using weighted random selection
7 * @param entries Array of giveaway entries with user IDs and entry counts
8 * @param winnerCount Number of winners to select
9 * @param hasFraudDetection Whether the server has fraud detection enabled (premium+)
10 * @param cooldownExcludedUserIds Optional array of user IDs to exclude due to winner cooldown
11 * @param geoExcludedUserIds Optional array of user IDs to exclude due to geo restriction
12 *        (warn-mode entrants and block-mode strays that slipped past the submit gate)
13 * @param referralExcludedUserIds Optional array of user IDs to exclude because they are
14 *        below the giveaway's Invite-to-Win referral requirement (premium feature)
15 * @returns Array of selected winner user IDs
16 */
17export function selectWinners(
18  entries: GiveawayEntry[],
19  winnerCount: number,
20  hasFraudDetection: boolean = true, // Default to true for backward compatibility
21  cooldownExcludedUserIds: string[] = [], // Users excluded due to recent wins (cooldown)
22  geoExcludedUserIds: string[] = [], // Users excluded due to geo restriction
23  referralExcludedUserIds: string[] = [] // Users below the Invite-to-Win referral requirement
24): string[] {
25  if (!entries || entries.length === 0) {
26    return [];
27  }
28
29  // Create a Set for O(1) lookup of cooldown excluded users
30  const cooldownExcludedSet = new Set(cooldownExcludedUserIds);
31
32  // Filter out:
33  // 1. Red-flagged entries ONLY if server has fraud detection enabled (premium+)
34  // 2. Users on winner cooldown (won from this server recently)
35  // 3. Users whose entry country falls under the giveaway's geo restriction
36  // 4. Users below the giveaway's Invite-to-Win referral requirement
37  let eligibleEntries = hasFraudDetection
38    ? entries.filter(entry => entry.fraud_flag !== 'red')
39    : entries; // Free servers: include all entries (ignore fraud flags)
40
41  // Apply cooldown exclusions (silent exclusion - users can still enter but won't win)
42  if (cooldownExcludedSet.size > 0) {
43    const beforeCount = eligibleEntries.length;
44    eligibleEntries = eligibleEntries.filter(entry => !cooldownExcludedSet.has(entry.user_id));
45    const excluded = beforeCount - eligibleEntries.length;
46    if (excluded > 0) {
47      console.log(`[Winner Selection] Excluded ${excluded} entries due to winner cooldown`);
48    }
49  }
50
51  // Apply geo exclusions (warn mode: entered with disclosure, can't win)
52  const geoExcludedSet = new Set(geoExcludedUserIds);
53  if (geoExcludedSet.size > 0) {
54    const beforeCount = eligibleEntries.length;
55    eligibleEntries = eligibleEntries.filter(entry => !geoExcludedSet.has(entry.user_id));
56    const excluded = beforeCount - eligibleEntries.length;
57    if (excluded > 0) {
58      console.log(`[Winner Selection] Excluded ${excluded} entries due to geo restriction`);
59    }
60  }
61
62  // Apply Invite-to-Win exclusions (entrants below the referral requirement;
63  // participants were told up-front they must invite X friends to be eligible)
64  const referralExcludedSet = new Set(referralExcludedUserIds);
65  if (referralExcludedSet.size > 0) {
66    const beforeCount = eligibleEntries.length;
67    eligibleEntries = eligibleEntries.filter(entry => !referralExcludedSet.has(entry.user_id));
68    const excluded = beforeCount - eligibleEntries.length;
69    if (excluded > 0) {
70      console.log(`[Winner Selection] Excluded ${excluded} entries below the Invite-to-Win referral requirement`);
71    }
72  }
73
74  // If no eligible entries remain (after fraud + cooldown + geo + referral filtering), return empty array
75  if (eligibleEntries.length === 0) {
76    console.warn('[Winner Selection] No eligible entries after filtering (fraud flags + cooldown + geo + referral-requirement exclusions)');
77    return [];
78  }
79
80  // Create weighted pool of entries based on entry count
81  const entryPool: string[] = [];
82
83  for (const entry of eligibleEntries) {
84    // Add user_id to the pool based on their entry count
85    for (let i = 0; i < entry.entries_count; i++) {
86      entryPool.push(entry.user_id);
87    }
88  }
89
90  // Randomly select winners without duplicates
91  const winners: string[] = [];
92  const winnerSet = new Set<string>();
93
94  // Limit winner count to available unique participants (only eligible entries)
95  const uniqueParticipants = new Set(eligibleEntries.map(entry => entry.user_id));
96  const availableWinnerCount = Math.min(winnerCount, uniqueParticipants.size);
97  
98  // If there are no entries or the pool is empty, return empty array
99  if (availableWinnerCount === 0 || entryPool.length === 0) {
100    return [];
101  }
102  
103  // Select winners
104  while (winners.length < availableWinnerCount && entryPool.length > 0) {
105    // Select a random entry from the pool
106    const randomIndex = randomInt(entryPool.length);
107    const selectedUserId = entryPool[randomIndex];
108
109    // If this user hasn't already been selected as a winner, add them
110    if (!winnerSet.has(selectedUserId)) {
111      winners.push(selectedUserId);
112      winnerSet.add(selectedUserId);
113    }
114
115    // Remove via swap-and-pop (O(1)) instead of splice (O(n)).
116    // Selection is uniform random so order of remaining elements doesn't matter.
117    entryPool[randomIndex] = entryPool[entryPool.length - 1];
118    entryPool.pop();
119  }
120  
121  return winners;
122}