Every winner is chosen
100% fairly

1
2// src/lib/services/winnerSelection.ts
3import { GiveawayEntry } from '@/types/giveaway';
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 * @returns Array of selected winner user IDs
10 */
11export function selectWinners(entries: GiveawayEntry[], winnerCount: number): string[] {
12  // Input validation
13  if (!entries || entries.length === 0) {
14    return [];
15  }
16  
17  // Create weighted pool of entries based on entry count
18  const entryPool: string[] = [];
19  
20  for (const entry of entries) {
21    // Add user_id to the pool based on their entry count
22    for (let i = 0; i < entry.entries_count; i++) {
23      entryPool.push(entry.user_id);
24    }
25  }
26  
27  // Randomly select winners without duplicates
28  const winners: string[] = [];
29  const winnerSet = new Set<string>();
30  
31  // Limit winner count to available unique participants
32  const uniqueParticipants = new Set(entries.map(entry => entry.user_id));
33  const availableWinnerCount = Math.min(winnerCount, uniqueParticipants.size);
34  
35  // If there are no entries or the pool is empty, return empty array
36  if (availableWinnerCount === 0 || entryPool.length === 0) {
37    return [];
38  }
39  
40  // Select winners
41  while (winners.length < availableWinnerCount && entryPool.length > 0) {
42    // Select a random entry from the pool
43    const randomIndex = Math.floor(Math.random() * entryPool.length);
44    const selectedUserId = entryPool[randomIndex];
45    
46    // If this user hasn't already been selected as a winner, add them
47    if (!winnerSet.has(selectedUserId)) {
48      winners.push(selectedUserId);
49      winnerSet.add(selectedUserId);
50    }
51    
52    // Remove the selected entry from the pool
53    entryPool.splice(randomIndex, 1);
54  }
55  
56  return winners;
57}