ANTOOZA-BACKEND/Application/Features/Admin/Channel/List/GenerateDonationCode/Handler.cs

using System.Security.Cryptography;
using Application.Abstractions.Data;
using Application.Abstractions.Messaging;
using Microsoft.EntityFrameworkCore;

namespace Application.Features.Admin.Channel.List.GenerateDonationCode;

internal sealed class Handler(IAppDbContext db) : IQueryHandler<Query, string>
{
    // 혼동되는 문자(0/O, 1/I/L) 제외 31자. 6자리 조합 31^6 ≈ 8.8억.
    private const string Alphabet = "23456789ABCDEFGHJKMNPQRSTUVWXYZ";

    private const int CodeLength = 6;

    private const int MaxAttempts = 10;

    public async Task<string> Handle(Query request, CancellationToken ct)
    {
        for (var attempt = 0; attempt < MaxAttempts; attempt++)
        {
            var candidate = Generate();
            var exists = await db.Channel.AnyAsync(c => c.DonationCode == candidate, ct);

            if (!exists)
            {
                return candidate;
            }
        }

        // 31^6 ≈ 8.8억 중 10회 모두 충돌은 극히 낮은 확률. 호출자 측 유효성 검증으로 한 번 더 안전망.
        return Generate();
    }

    private static string Generate()
    {
        var chars = new char[CodeLength];
        var bytes = new byte[CodeLength];
        RandomNumberGenerator.Fill(bytes);

        for (var i = 0; i < CodeLength; i++)
        {
            chars[i] = Alphabet[bytes[i] % Alphabet.Length];
        }

        return new string(chars);
    }
}