using System.Net;
using System.Net.Http.Json;
using LehrerApp.Core.Models;
using LehrerApp.Data;
using LehrerApp.Sync.Crypto;
using LehrerApp.Sync.Models;
using Xunit;

namespace LehrerApp.Sync.Tests;

public sealed class SyncEngineTests
{
    private static LiteDbContext NewInMemoryContext() => new(new MemoryStream());
    private static readonly byte[] Key = SyncCrypto.GenerateKey();

    [Fact]
    public async Task SyncNowAsync_InkompatibleServerVersion_BrichtVorWeiterenRequestsAb()
    {
        using var temp = new TempEventQueue();
        var pending = temp.Queue.Enqueue("this-device", DeviceType.Desktop,
            "Lesson", Guid.NewGuid().ToString(), "Save", "lokale-daten");
        var handler = new FakeHttpMessageHandler(req =>
        {
            var response = new HttpResponseMessage(HttpStatusCode.UpgradeRequired);
            response.Headers.Add(SyncProtocol.VersionHeaderName, "2");
            return response;
        });
        var engine = MakeEngine(temp, handler);

        var result = await engine.SyncNowAsync();

        Assert.False(result.Success);
        Assert.Contains("Sync-Version 1", result.Reason);
        Assert.Contains("Server Version 2", result.Reason);
        Assert.Equal(SyncState.IncompatibleVersion, engine.Status.State);
        Assert.Equal(pending.EventId, Assert.Single(temp.Queue.GetPending()).EventId);
        var request = Assert.Single(handler.Requests);
        Assert.Equal("/api/sync/push", request.RequestUri!.AbsolutePath);
        Assert.Equal(SyncProtocol.CurrentVersion,
            request.Headers.GetValues(SyncProtocol.VersionHeaderName).Single());
    }

    /// Regression: EventApplier.ApplyAsync fing früher nur LiteException ab. Jede andere Ausnahme
    /// (z.B. eine ungültige/korrupte Payload eines einzelnen Ereignisses) fiel unbehandelt aus
    /// SyncEngine.PullAsync heraus, BEVOR _queue.SetLastServerSeq() erreicht wurde — der nächste
    /// Sync-Versuch lud denselben Batch erneut und scheiterte am selben Ereignis erneut: ein
    /// dauerhaft blockierter Sync, bei dem selbst bereits im selben Batch erfolgreich angewendete
    /// Ereignisse (wie hier "goodStudent") nie durchkamen.
    [Fact]
    public async Task SyncNowAsync_KorruptesEreignisImPullBatch_BlockiertNachfolgendeEreignisseNicht()
    {
        using var temp = new TempEventQueue();
        using var db = NewInMemoryContext();
        var applier = new EventApplier(db, Key);
        var resolver = new ConflictResolver(temp.Queue);
        var attachments = new AttachmentSyncer(db, new HttpClient(), Key);

        var goodStudent = new Student { FirstName = "Anna", LastName = "Beispiel" };
        var badEvent = new SyncEvent
        {
            DeviceId = "other-device", DeviceType = DeviceType.Desktop,
            EntityType = nameof(Student), EntityId = Guid.NewGuid().ToString(),
            Operation = "Save", Payload = "offensichtlich-keine-gueltige-verschluesselte-payload",
            SequenceNr = 1,
        };
        var goodEvent = new SyncEvent
        {
            DeviceId = "other-device", DeviceType = DeviceType.Desktop,
            EntityType = nameof(Student), EntityId = goodStudent.Id.ToString(),
            Operation = "Save", Payload = SyncCrypto.EncryptObject(goodStudent, Key),
            SequenceNr = 2,
        };

        var handler = new FakeHttpMessageHandler(req =>
        {
            if (req.RequestUri!.AbsolutePath == "/api/sync/pull")
                return new HttpResponseMessage(HttpStatusCode.OK)
                {
                    Content = JsonContent.Create(new PullResponse
                    { Events = [badEvent, goodEvent], ServerSequenceNr = 2 }),
                };
            return new HttpResponseMessage(HttpStatusCode.OK);
        });
        var http = new HttpClient(handler) { BaseAddress = new Uri("https://example.invalid") };
        var engine = new SyncEngine(temp.Queue, resolver, applier, attachments, http,
            new SyncConfig { DeviceId = "this-device" });

        var result = await engine.SyncNowAsync();

        Assert.True(result.Success);
        Assert.NotNull(db.Students.FindById(goodStudent.Id));
        // Nicht bei 0 stecken geblieben - der Batch gilt als vollständig verarbeitet, auch wenn ein
        // einzelnes Ereignis darin übersprungen werden musste.
        Assert.Equal(2, temp.Queue.GetLastServerSeq());
    }

    // ── DataChanged: Hook für den Desktop-Client, die sichtbare Seite neu zu laden (TODO 10.1.11) ─

    [Fact]
    public async Task DataChanged_PullMitEreignissen_Feuert()
    {
        using var temp = new TempEventQueue();
        var goodStudent = new Student { FirstName = "Anna", LastName = "Beispiel" };
        var evt = new SyncEvent
        {
            DeviceId = "other-device", DeviceType = DeviceType.Desktop,
            EntityType = nameof(Student), EntityId = goodStudent.Id.ToString(),
            Operation = "Save", Payload = SyncCrypto.EncryptObject(goodStudent, Key), SequenceNr = 1,
        };
        var handler = new FakeHttpMessageHandler(req =>
        {
            if (req.RequestUri!.AbsolutePath == "/api/sync/pull")
                return new HttpResponseMessage(HttpStatusCode.OK)
                { Content = JsonContent.Create(new PullResponse { Events = [evt], ServerSequenceNr = 1 }) };
            return new HttpResponseMessage(HttpStatusCode.OK)
            { Content = JsonContent.Create(new PushResponse()) };
        });
        var engine = MakeEngine(temp, handler);
        var fired = false;
        engine.DataChanged += () => fired = true;

        await engine.SyncNowAsync();

        Assert.True(fired);
    }

    [Fact]
    public async Task DataChanged_PullOhneEreignisse_FeuertNicht()
    {
        using var temp = new TempEventQueue();
        var handler = new FakeHttpMessageHandler(req => new HttpResponseMessage(HttpStatusCode.OK)
        {
            Content = JsonContent.Create(req.RequestUri!.AbsolutePath == "/api/sync/pull"
                ? new PullResponse()
                : (object)new PushResponse()),
        });
        var engine = MakeEngine(temp, handler);
        var fired = false;
        engine.DataChanged += () => fired = true;

        await engine.SyncNowAsync();

        Assert.False(fired);
    }

    /// Regression: PushAsync setzte den lokalen Pull-Cursor bisher direkt aus PushResponse.
    /// ServerSequenceNr - dem GLOBALEN Zähler über alle Geräte NACH diesem Push, nicht dem Stand,
    /// den DIESES Gerät tatsächlich per Pull erhalten hat. Hatte der Server zum Push-Zeitpunkt
    /// bereits ein noch nicht abgeholtes Ereignis eines ANDEREN Geräts mit niedrigerer ServerSeq,
    /// sprang der Cursor beim eigenen Push darüber hinweg - der direkt anschließende PullAsync
    /// fragte dann schon mit einem "since" danach und bekam 0 Ereignisse, obwohl das fremde
    /// Ereignis nie angewendet wurde. Genau das vom Nutzer beobachtete Symptom ("Client 1 konnte
    /// übermitteln, Client 2 bekommt weiterhin 'keine Änderungen'"), diesmal über einen anderen
    /// Pfad als die bereits behobene EventStore.Pull-Wasserzeichen-Berechnung.
    [Fact]
    public async Task SyncNowAsync_EigenerPushWaehrendFremdesEreignisNochAussteht_LiefertFremdesEreignisTrotzdem()
    {
        using var temp = new TempEventQueue();
        using var db = NewInMemoryContext();
        var applier = new EventApplier(db, Key, versions: temp.Queue);
        var goodStudent = new Student { FirstName = "Anna", LastName = "Beispiel" };
        var foreignEvent = new SyncEvent
        {
            DeviceId = "other-device", DeviceType = DeviceType.Desktop,
            EntityType = nameof(Student), EntityId = goodStudent.Id.ToString(),
            Operation = "Save", Payload = SyncCrypto.EncryptObject(goodStudent, Key),
            SequenceNr = 5,
        };
        temp.Queue.Enqueue("this-device", DeviceType.Desktop, "Lesson", Guid.NewGuid().ToString(), "Save", "x");
        var handler = new FakeHttpMessageHandler(req =>
        {
            if (req.RequestUri!.AbsolutePath == "/api/sync/push")
                // Globaler Höchststand (10) schließt das fremde, von DIESEM Gerät noch nicht
                // abgeholte Ereignis (Seq 5) bereits mit ein - genau das durfte SyncEngine NICHT
                // als eigenen Pull-Cursor übernehmen.
                return new HttpResponseMessage(HttpStatusCode.OK)
                { Content = JsonContent.Create(new PushResponse { ServerSequenceNr = 10 }) };
            if (req.RequestUri!.AbsolutePath == "/api/sync/pull")
            {
                var query = req.RequestUri.Query.TrimStart('?').Split('&')
                    .Select(p => p.Split('=')).ToDictionary(p => p[0], p => p[1]);
                var since = long.Parse(query["since"]);
                return new HttpResponseMessage(HttpStatusCode.OK)
                {
                    Content = JsonContent.Create(since < 5
                        ? new PullResponse { Events = [foreignEvent], ServerSequenceNr = 5 }
                        : new PullResponse { ServerSequenceNr = since }),
                };
            }
            return new HttpResponseMessage(HttpStatusCode.OK);
        });
        var engine = MakeEngine(temp, handler, applier);

        var result = await engine.SyncNowAsync();

        Assert.True(result.Success);
        Assert.NotNull(db.Students.FindById(goodStudent.Id));
    }

    // ── PushAsync: Dedup, BasedOnServerSeq, AssignedServerSeqs (TODO 10.3.4) ────────────────

    [Fact]
    public async Task PushAsync_MehrereAusstehendeEreignisseDerselbenEntitaet_SendetNurDasJuengste()
    {
        using var temp = new TempEventQueue();
        var entityId = Guid.NewGuid().ToString();
        temp.Queue.Enqueue("this-device", DeviceType.Desktop, "Lesson", entityId, "Save", "alt");
        var newest = temp.Queue.Enqueue("this-device", DeviceType.Desktop, "Lesson", entityId, "Save", "neu");
        List<SyncEvent>? pushed = null;
        var handler = new FakeHttpMessageHandler(req =>
        {
            if (req.RequestUri!.AbsolutePath == "/api/sync/push")
            {
                pushed = req.Content!.ReadFromJsonAsync<List<SyncEvent>>().GetAwaiter().GetResult();
                return new HttpResponseMessage(HttpStatusCode.OK)
                { Content = JsonContent.Create(new PushResponse { ServerSequenceNr = 1 }) };
            }
            return new HttpResponseMessage(HttpStatusCode.OK)
            { Content = JsonContent.Create(new PullResponse()) };
        });
        var engine = MakeEngine(temp, handler);

        var result = await engine.SyncNowAsync();

        Assert.True(result.Success);
        var evt = Assert.Single(pushed!);
        Assert.Equal(newest.EventId, evt.EventId);
        Assert.Equal("neu", evt.Payload);
        Assert.Equal(0, temp.Queue.PendingCount());
    }

    [Fact]
    public async Task SyncNowAsync_MehrAlsEinPushBatch_LeertQueueInEinemSync()
    {
        using var temp = new TempEventQueue();
        for (var i = 0; i < SyncProtocol.PushBatchSize + 5; i++)
            temp.Queue.Enqueue("this-device", DeviceType.Desktop,
                "Lesson", Guid.NewGuid().ToString(), "Save", $"payload-{i}");

        var pushedBatchSizes = new List<int>();
        long serverSeq = 0;
        var handler = new FakeHttpMessageHandler(req =>
        {
            if (req.RequestUri!.AbsolutePath == "/api/sync/push")
            {
                var events = req.Content!.ReadFromJsonAsync<List<SyncEvent>>().GetAwaiter().GetResult()!;
                pushedBatchSizes.Add(events.Count);
                var assigned = events.ToDictionary(e => e.EventId, _ => ++serverSeq);
                return new HttpResponseMessage(HttpStatusCode.OK)
                {
                    Content = JsonContent.Create(new PushResponse
                    { ServerSequenceNr = serverSeq, AssignedServerSeqs = assigned }),
                };
            }
            return new HttpResponseMessage(HttpStatusCode.OK)
            { Content = JsonContent.Create(new PullResponse()) };
        });
        var engine = MakeEngine(temp, handler);

        var result = await engine.SyncNowAsync();

        Assert.True(result.Success);
        Assert.Equal(SyncProtocol.PushBatchSize + 5, result.EventsPushed);
        Assert.Equal([SyncProtocol.PushBatchSize, 5], pushedBatchSizes);
        Assert.Equal(0, temp.Queue.PendingCount());
    }

    [Fact]
    public async Task SyncNowAsync_VollerPullBatch_LaedtFolgebatchImSelbenSync()
    {
        using var temp = new TempEventQueue();
        using var db = NewInMemoryContext();
        var allEvents = Enumerable.Range(1, SyncProtocol.PullBatchSize + 3)
            .Select(sequence =>
            {
                var student = new Student { FirstName = $"Vorname-{sequence}", LastName = "Beispiel" };
                return new SyncEvent
                {
                    DeviceId = "other-device", DeviceType = DeviceType.Desktop,
                    EntityType = nameof(Student), EntityId = student.Id.ToString(),
                    Operation = "Save", Payload = SyncCrypto.EncryptObject(student, Key),
                    SequenceNr = sequence,
                };
            }).ToList();
        var pullRequests = 0;
        var handler = new FakeHttpMessageHandler(req =>
        {
            if (req.RequestUri!.AbsolutePath == "/api/sync/pull")
            {
                pullRequests++;
                var since = long.Parse(req.RequestUri.Query.TrimStart('?').Split('&')
                    .Select(part => part.Split('=')).Single(part => part[0] == "since")[1]);
                var events = allEvents.Where(e => e.SequenceNr > since)
                    .Take(SyncProtocol.PullBatchSize).ToList();
                return new HttpResponseMessage(HttpStatusCode.OK)
                {
                    Content = JsonContent.Create(new PullResponse
                    {
                        Events = events,
                        ServerSequenceNr = events.Count == 0 ? since : events[^1].SequenceNr,
                    }),
                };
            }
            return new HttpResponseMessage(HttpStatusCode.OK)
            { Content = JsonContent.Create(new PushResponse()) };
        });
        var engine = MakeEngine(temp, handler, new EventApplier(db, Key, versions: temp.Queue));
        var dataChangedCount = 0;
        engine.DataChanged += () => dataChangedCount++;

        var result = await engine.SyncNowAsync();

        Assert.True(result.Success);
        Assert.Equal(SyncProtocol.PullBatchSize + 3, result.EventsPulled);
        Assert.Equal(2, pullRequests);
        Assert.Equal(SyncProtocol.PullBatchSize + 3, db.Students.Count());
        Assert.Equal(SyncProtocol.PullBatchSize + 3, temp.Queue.GetLastServerSeq());
        Assert.Equal(1, dataChangedCount);
    }

    [Fact]
    public async Task PushAsync_SetztBasedOnServerSeqAusLokalerVersionsverfolgung()
    {
        using var temp = new TempEventQueue();
        var entityId = Guid.NewGuid().ToString();
        temp.Queue.SetKnownServerSeq("Lesson", entityId, 5);
        temp.Queue.Enqueue("this-device", DeviceType.Desktop, "Lesson", entityId, "Save", "x");
        List<SyncEvent>? pushed = null;
        var handler = new FakeHttpMessageHandler(req =>
        {
            if (req.RequestUri!.AbsolutePath == "/api/sync/push")
            {
                pushed = req.Content!.ReadFromJsonAsync<List<SyncEvent>>().GetAwaiter().GetResult();
                return new HttpResponseMessage(HttpStatusCode.OK)
                { Content = JsonContent.Create(new PushResponse { ServerSequenceNr = 6 }) };
            }
            return new HttpResponseMessage(HttpStatusCode.OK)
            { Content = JsonContent.Create(new PullResponse()) };
        });
        var engine = MakeEngine(temp, handler);

        await engine.SyncNowAsync();

        Assert.Equal(5, Assert.Single(pushed!).BasedOnServerSeq);
    }

    [Fact]
    public async Task PushAsync_ErfolgreicherPush_AktualisiertLokaleVersionsverfolgung()
    {
        using var temp = new TempEventQueue();
        var entityId = Guid.NewGuid().ToString();
        var evt = temp.Queue.Enqueue("this-device", DeviceType.Desktop, "Lesson", entityId, "Save", "x");
        var handler = new FakeHttpMessageHandler(req =>
        {
            if (req.RequestUri!.AbsolutePath == "/api/sync/push")
                return new HttpResponseMessage(HttpStatusCode.OK)
                {
                    Content = JsonContent.Create(new PushResponse
                    { ServerSequenceNr = 7, AssignedServerSeqs = new() { [evt.EventId] = 7 } }),
                };
            return new HttpResponseMessage(HttpStatusCode.OK)
            { Content = JsonContent.Create(new PullResponse()) };
        });
        var engine = MakeEngine(temp, handler);

        await engine.SyncNowAsync();

        Assert.Equal(7, temp.Queue.GetKnownServerSeq("Lesson", entityId));
    }

    /// Regression: ein wegen neuerem Server-Stand abgelehnter Push blieb bisher unsichtbar - das
    /// Ereignis verschwand einfach nicht aus der Queue, ohne dass der Nutzer je erfuhr, dass es
    /// bereits neuere Daten gab (siehe TODO 10.3.4). RemoteWon-Fall: der Server-Stand gewinnt, wird
    /// sofort angewendet, und die lokale Änderung wird verworfen.
    [Fact]
    public async Task PushAsync_AbgelehnterPushRemoteWon_WendetServerStandAnUndVerwirftLokal()
    {
        using var temp = new TempEventQueue();
        using var db = NewInMemoryContext();
        var entityId = Guid.NewGuid();
        // Companion verliert immer gegen Desktop (siehe ConflictResolver.DetermineWinner).
        var local = temp.Queue.Enqueue("companion-device", DeviceType.Companion,
            nameof(Student), entityId.ToString(), "Save", "veraltete-lokale-payload");
        var remoteStudent = new Student { Id = entityId, FirstName = "Anna", LastName = "Beispiel" };
        var remoteEvent = new SyncEvent
        {
            DeviceId = "other-device", DeviceType = DeviceType.Desktop,
            EntityType = nameof(Student), EntityId = entityId.ToString(),
            Operation = "Save", Payload = SyncCrypto.EncryptObject(remoteStudent, Key),
            SequenceNr = 99,
        };
        var handler = new FakeHttpMessageHandler(req =>
        {
            if (req.RequestUri!.AbsolutePath == "/api/sync/push")
                return new HttpResponseMessage(HttpStatusCode.OK)
                {
                    Content = JsonContent.Create(new PushResponse
                    { ServerSequenceNr = 99, ConflictingEventIds = [local.EventId] }),
                };
            if (req.RequestUri!.AbsolutePath == $"/api/sync/entity/{nameof(Student)}/{entityId}")
                return new HttpResponseMessage(HttpStatusCode.OK) { Content = JsonContent.Create(remoteEvent) };
            return new HttpResponseMessage(HttpStatusCode.OK)
            { Content = JsonContent.Create(new PullResponse()) };
        });
        var applier = new EventApplier(db, Key, versions: temp.Queue);
        var engine = MakeEngine(temp, handler, applier);
        var dataChanged = false;
        engine.DataChanged += () => dataChanged = true;

        await engine.SyncNowAsync();

        var conflict = Assert.Single(temp.Queue.GetUnreviewed());
        Assert.Equal("RemoteWon", conflict.Resolution);
        Assert.NotNull(db.Students.FindById(entityId));
        Assert.Equal(0, temp.Queue.PendingCount());
        // DataChanged muss auch bei einem RemoteWon-Konflikt feuern - dabei wird lokal genauso
        // Daten angewendet wie bei einem regulären Pull (TODO 10.1.11).
        Assert.True(dataChanged);
        Assert.Equal(99, temp.Queue.GetKnownServerSeq(nameof(Student), entityId.ToString()));
    }

    /// Gegenstück: LocalWon-Fall - die lokale Änderung bleibt (unbestätigt) in der Queue, damit der
    /// nächste Sync-Versuch sie mit dem soeben aktualisierten BasedOnServerSeq erneut versucht.
    [Fact]
    public async Task PushAsync_AbgelehnterPushLocalWon_BleibtUnbestaetigtInDerQueue()
    {
        using var temp = new TempEventQueue();
        using var db = NewInMemoryContext();
        var entityId = Guid.NewGuid();
        // Desktop gewinnt immer gegen Companion (siehe ConflictResolver.DetermineWinner).
        var local = temp.Queue.Enqueue("this-device", DeviceType.Desktop,
            nameof(Student), entityId.ToString(), "Save", SyncCrypto.EncryptObject(
                new Student { Id = entityId, FirstName = "Lokal", LastName = "Beispiel" }, Key));
        var remoteEvent = new SyncEvent
        {
            DeviceId = "other-device", DeviceType = DeviceType.Companion,
            EntityType = nameof(Student), EntityId = entityId.ToString(),
            Operation = "Save", Payload = "irrelevant-verliert-ohnehin",
            SequenceNr = 42,
        };
        var handler = new FakeHttpMessageHandler(req =>
        {
            if (req.RequestUri!.AbsolutePath == "/api/sync/push")
                return new HttpResponseMessage(HttpStatusCode.OK)
                {
                    Content = JsonContent.Create(new PushResponse
                    { ServerSequenceNr = 42, ConflictingEventIds = [local.EventId] }),
                };
            if (req.RequestUri!.AbsolutePath == $"/api/sync/entity/{nameof(Student)}/{entityId}")
                return new HttpResponseMessage(HttpStatusCode.OK) { Content = JsonContent.Create(remoteEvent) };
            return new HttpResponseMessage(HttpStatusCode.OK)
            { Content = JsonContent.Create(new PullResponse()) };
        });
        var applier = new EventApplier(db, Key, versions: temp.Queue);
        var engine = MakeEngine(temp, handler, applier);

        await engine.SyncNowAsync();

        var conflict = Assert.Single(temp.Queue.GetUnreviewed());
        Assert.Equal("LocalWon", conflict.Resolution);
        Assert.Equal(1, temp.Queue.PendingCount());
        Assert.Equal(42, temp.Queue.GetKnownServerSeq(nameof(Student), entityId.ToString()));
    }

    /// Regression (TODO 10.3.5): nach einem Kontowechsel (z.B. Login-Korrektur der Groß-/
    /// Kleinschreibung, TODO 10.2.5) referenziert die lokale Versionsverfolgung ServerSeq-Werte
    /// eines FREMDEN Kontos. Der Server lehnt den Push ab (BasedOnServerSeq-Mismatch), kennt die
    /// Entität unter der aktuellen userId aber selbst gar nicht (404 bei GetLatestForEntity) - vor
    /// diesem Fix blieb das Ereignis dadurch dauerhaft und ohne jede Selbstheilung stecken (derselbe
    /// 404 bei jedem weiteren Sync-Versuch). Jetzt wird der stale Cache-Eintrag gelöscht, sodass
    /// der NÄCHSTE Push die Entität korrekt als neu behandelt und vom (für sie leeren) Server-Konto
    /// angenommen wird.
    [Fact]
    public async Task PushAsync_AbgelehnterPushServerKenntEntitaetNicht_LoeschtStaleCacheUndErholtSichSelbst()
    {
        using var temp = new TempEventQueue();
        using var db = NewInMemoryContext();
        var entityId = Guid.NewGuid().ToString();
        // Stale Cache-Eintrag aus einem früheren (fremden) Konto - der Server unter der jetzigen
        // userId hat davon nie etwas gehört.
        temp.Queue.SetKnownServerSeq("Unit", entityId, 422);
        var local = temp.Queue.Enqueue("this-device", DeviceType.Desktop, "Unit", entityId, "Save", "x");
        var pushAttempts = 0;
        var handler = new FakeHttpMessageHandler(req =>
        {
            if (req.RequestUri!.AbsolutePath == "/api/sync/push")
            {
                pushAttempts++;
                // Erster Versuch: BasedOnServerSeq=422 (stale) passt nicht zum leeren Server-Konto
                // -> abgelehnt. Zweiter Versuch (nach Cache-Löschung): BasedOnServerSeq=null passt
                // zur ebenfalls unbekannten Entität -> angenommen.
                return new HttpResponseMessage(HttpStatusCode.OK)
                {
                    Content = JsonContent.Create(pushAttempts == 1
                        ? new PushResponse { ServerSequenceNr = 0, ConflictingEventIds = [local.EventId] }
                        : new PushResponse
                        {
                            ServerSequenceNr = 1,
                            AssignedServerSeqs = new() { [local.EventId] = 1 },
                        }),
                };
            }
            if (req.RequestUri!.AbsolutePath == $"/api/sync/entity/Unit/{entityId}")
                return new HttpResponseMessage(HttpStatusCode.NotFound);
            return new HttpResponseMessage(HttpStatusCode.OK)
            { Content = JsonContent.Create(new PullResponse()) };
        });
        var engine = MakeEngine(temp, handler);

        // SyncResult.Conflicts spiegelt nur PULL-Konflikte wider (siehe SyncEngine.SyncNowAsync) -
        // Push-Konflikte werden ausschließlich geloggt, daher hier über EventsPushed==0 geprüft.
        var first = await engine.SyncNowAsync();
        Assert.Equal(0, first.EventsPushed);
        Assert.Empty(temp.Queue.GetUnreviewed());
        Assert.Null(temp.Queue.GetKnownServerSeq("Unit", entityId));
        Assert.Equal(1, temp.Queue.PendingCount());

        var second = await engine.SyncNowAsync();

        Assert.Equal(1, second.EventsPushed);
        Assert.Equal(0, temp.Queue.PendingCount());
    }

    private static SyncEngine MakeEngine(TempEventQueue temp, FakeHttpMessageHandler handler,
        EventApplier? applier = null)
    {
        var db = NewInMemoryContext();
        var http = new HttpClient(handler) { BaseAddress = new Uri("https://example.invalid") };
        return new SyncEngine(temp.Queue, new ConflictResolver(temp.Queue),
            applier ?? new EventApplier(db, Key), new AttachmentSyncer(db, http, Key), http,
            new SyncConfig { DeviceId = "this-device" });
    }

    private sealed class TempEventQueue : IDisposable
    {
        private readonly string _directory = Path.Combine(
            Path.GetTempPath(), $"lehrerapp-sync-tests-engine-{Guid.NewGuid():N}");
        public EventQueue Queue { get; }

        public TempEventQueue()
        {
            Directory.CreateDirectory(_directory);
            Queue = new EventQueue(Path.Combine(_directory, "queue.db"));
        }

        public void Dispose()
        {
            Queue.Dispose();
            if (Directory.Exists(_directory)) Directory.Delete(_directory, recursive: true);
        }
    }
}