LehrerApp/LehrerApp.Api/SnapshotStore.cs

using LiteDB;
using LehrerApp.Sync.Models;

namespace LehrerApp.Api;

public class SnapshotStore(string dataPath) : IDisposable
{
    private readonly LiteDatabase _db =
        new(Path.Combine(dataPath, "snapshots.db"));
    private readonly Timer _cleanup;

    public SnapshotStore(string dataPath, bool unused = false) : this(dataPath)
    {
        Directory.CreateDirectory(dataPath);
        _cleanup = new(_ => Clean(), null, TimeSpan.FromHours(1), TimeSpan.FromHours(1));
    }

    private ILiteCollection<SnapshotEntry> Col => _db.GetCollection<SnapshotEntry>("snap");

    public SnapshotUploadResponse Store(string userId, SnapshotUploadRequest req)
    {
        // Zweiter Upload-Schritt (Schlüssel nachreichen, mit dem im ersten Schritt vergebenen Code
        // verschlüsselt): denselben Eintrag aktualisieren statt einen neuen mit neuem Code
        // anzulegen — sonst würde der Schlüssel dauerhaft mit dem falschen Code verknüpft bleiben
        // und die Entschlüsselung auf dem Empfängergerät schlägt fehl (Bug, siehe TODO.md 10.3.1).
        if (!string.IsNullOrEmpty(req.Code))
        {
            var existing = Col.FindOne(e => e.UserId == userId && e.Code == req.Code.ToUpperInvariant());
            if (existing is not null)
            {
                existing.EncryptedPayload = req.EncryptedPayload;
                existing.EncryptedSyncKey = req.EncryptedSyncKey;
                Col.Update(existing);
                return new() { Code = existing.Code, ExpiresAt = existing.ExpiresAt };
            }
        }

        Col.DeleteMany(e => e.UserId == userId);
        var code  = NewCode();
        var entry = new SnapshotEntry { Code = code, UserId = userId,
            EncryptedPayload = req.EncryptedPayload,
            EncryptedSyncKey = req.EncryptedSyncKey,
            SourceDeviceType = req.DeviceType,
            CreatedAt = DateTime.UtcNow, ExpiresAt = DateTime.UtcNow.AddHours(24) };
        Col.Insert(entry);
        return new() { Code = code, ExpiresAt = entry.ExpiresAt };
    }

    public SnapshotDownloadResponse? Retrieve(string userId, string code)
    {
        var e = Col.FindOne(x => x.UserId == userId && x.Code == code.ToUpperInvariant());
        if (e is null || e.ExpiresAt < DateTime.UtcNow) { if (e is not null) Col.Delete(e.Id); return null; }
        Col.Delete(e.Id);
        return new() { EncryptedPayload = e.EncryptedPayload, EncryptedSyncKey = e.EncryptedSyncKey,
                       CreatedAt = e.CreatedAt, SourceDeviceType = e.SourceDeviceType };
    }

    private void Clean() => Col.DeleteMany(e => e.ExpiresAt < DateTime.UtcNow);

    private static string NewCode()
    {
        string[] animals = ["TIGER","ADLER","DACHS","LUCHS","FALKE","IGEL","ELCH","FUCHS","RABE","WOLF","BISON","LAMM","EULE","BIBER","STORCH"];
        string[] colors  = ["BLAU","GRUEN","ROT","GOLD","GRAU","CYAN","ROSA","LILA","SAND","MINT"];
        return $"{animals[Random.Shared.Next(animals.Length)]}-{Random.Shared.Next(10,99)}-{colors[Random.Shared.Next(colors.Length)]}";
    }

    public void Dispose() { _cleanup?.Dispose(); _db.Dispose(); }
}

internal class SnapshotEntry
{
    public ObjectId Id { get; set; } = ObjectId.NewObjectId();
    public string Code { get; set; } = "";
    public string UserId { get; set; } = "";
    public string EncryptedPayload { get; set; } = "";
    public string EncryptedSyncKey { get; set; } = "";
    public DeviceType SourceDeviceType { get; set; }
    public DateTime CreatedAt { get; set; }
    public DateTime ExpiresAt { get; set; }
}