using System.Net.Http.Json;
using System.Net.WebSockets;
using HSchool.Protocol;

namespace HSchool.AppHost.Tests;

/// <summary>
/// Talks to the realtime channel the way the browser does: binary frames over a WebSocket.
/// </summary>
[Collection(AppHostCollection.Name)]
public class GameSocketTests(AppHostFixture fixture)
{
    private static readonly TimeSpan DefaultTimeout = TimeSpan.FromSeconds(30);
    private static readonly DateTime StartDate = new(2012, 4, 3, 6, 0, 0, DateTimeKind.Utc);

    [Fact]
    public async Task Handshake_AnswersWithAWelcomeFrame()
    {
        using var socket = await ConnectAsync();

        var welcome = ProtocolCodec.ReadWelcome(await ReceiveUntilAsync(socket, MessageType.ServerWelcome));

        Assert.Equal(ProtocolConstants.Version, welcome.ProtocolVersion);
        Assert.Equal(20, welcome.TickRate);
        Assert.Equal(2, welcome.MaxSchools);
    }

    [Fact]
    public async Task ChangingSpeed_DoesNotResumeAPausedSchool()
    {
        using var client = fixture.App.CreateHttpClient("server");
        await SchoolApiTests.ResetAsync(client);
        var school = await SchoolApiTests.CreateAsync(client, "Пауза и скорость", StartDate);

        using var socket = await OpenSchoolAsync(school.Id);
        await ReceiveClockAsync(socket);

        await SendAsync(socket, buffer =>
            ProtocolCodec.WriteSetRunning(buffer, new ClientSetRunningMessage(Running: false)));
        var paused = await ReceiveClockWhereAsync(socket, clock => !clock.Running);

        await SendAsync(socket, buffer =>
            ProtocolCodec.WriteSetSpeed(buffer, new ClientSetSpeedMessage(SpeedIndex: 3)));
        var afterSpeedChange = await ReceiveClockWhereAsync(socket, clock => clock.SpeedIndex == 3);
        var later = await ReceiveClockAfterAsync(socket, TimeSpan.FromSeconds(1));

        Assert.False(afterSpeedChange.Running);
        Assert.False(later.Running);
        Assert.Equal(paused.GameTimeUnixMs, later.GameTimeUnixMs);
    }

    [Fact]
    public async Task NoSchoolOpen_MeansNoClockFramesButTheCalendarKeepsRunning()
    {
        using var client = fixture.App.CreateHttpClient("server");
        await SchoolApiTests.ResetAsync(client);
        var school = await SchoolApiTests.CreateAsync(client, "Живёт сама", StartDate);

        using var socket = await ConnectAsync();
        await ReceiveUntilAsync(socket, MessageType.ServerWelcome);

        // A connection that opened nothing gets no clock frames…
        await Assert.ThrowsAsync<TimeoutException>(() =>
            ReceiveUntilAsync(socket, MessageType.ServerClock, TimeSpan.FromSeconds(1)));

        // …but the school moved on anyway, which is what the menu cards show.
        var reloaded = await FindAsync(client, school.Id);
        Assert.True(reloaded.Running);
        Assert.True(reloaded.GameTime > StartDate, $"The calendar stood still at {reloaded.GameTime:O}.");
    }

    [Fact]
    public async Task OpeningASchool_StreamsItsClock()
    {
        using var client = fixture.App.CreateHttpClient("server");
        await SchoolApiTests.ResetAsync(client);
        var school = await SchoolApiTests.CreateAsync(client, "Ход времени", StartDate);

        using var socket = await OpenSchoolAsync(school.Id);
        var first = await ReceiveClockAsync(socket);

        Assert.Equal(school.Id, first.SchoolId);
        Assert.True(first.Running);
        Assert.Equal(1, first.SpeedIndex);

        // 1 game minute per real second at ×1, so one second of ticks has to move the calendar.
        var later = await ReceiveClockAfterAsync(socket, TimeSpan.FromSeconds(1));
        var elapsed = ToDate(later) - ToDate(first);

        Assert.InRange(elapsed.TotalMinutes, 0.5, 2);
    }

    [Fact]
    public async Task OpeningASchool_SendsAMapSnapshot()
    {
        using var client = fixture.App.CreateHttpClient("server");
        await SchoolApiTests.ResetAsync(client);
        var school = await SchoolApiTests.CreateAsync(client, "Снимок карты", StartDate);

        using var socket = await OpenSchoolAsync(school.Id);
        var snapshot = ProtocolCodec.ReadMapSnapshot(await ReceiveUntilAsync(socket, MessageType.ServerMapSnapshot));

        Assert.Equal(school.Id, snapshot.SchoolId);
        Assert.Contains(snapshot.Nodes, node => node.Id == "yard" && node.Kind == 0);
        var office = Assert.Single(snapshot.Nodes, node => node.Id == "principals-office");
        Assert.Equal("Кабинет директора", office.Name);
        Assert.Equal("floor-1", office.ParentId);
        Assert.Contains("Директор", office.Positions);
        Assert.NotEmpty(office.Items);
        var classroom = Assert.Single(snapshot.Nodes, node => node.Id == "classroom-101");
        Assert.Equal(16, classroom.PupilSlots);
        Assert.Contains(classroom.Items, item => item.Name == "Парта" && item.Count == 16);
        Assert.DoesNotContain(classroom.Items, item => item.Name == "Стул");
    }

    [Fact]
    public async Task OpeningASchool_LabelsTheSnapshotInTheHelloLocale()
    {
        using var client = fixture.App.CreateHttpClient("server");
        await SchoolApiTests.ResetAsync(client);
        var school = await SchoolApiTests.CreateAsync(client, "English snapshot", StartDate);

        using var socket = await OpenSchoolAsync(school.Id, ProtocolConstants.LocaleEnglish);
        var snapshot = ProtocolCodec.ReadMapSnapshot(await ReceiveUntilAsync(socket, MessageType.ServerMapSnapshot));

        Assert.Equal("Principal's office", Assert.Single(snapshot.Nodes, node => node.Id == "principals-office").Name);
    }

    [Fact]
    public async Task OpeningASchool_WithACustomMap_ReturnsThatLayout()
    {
        using var client = fixture.App.CreateHttpClient("server");
        await SchoolApiTests.ResetAsync(client);
        var school = await SchoolApiTests.CreateWithMapAsync(client, "Упрощённая", StartDate, SchoolApiTests.SimpleCustomMap);

        using var socket = await OpenSchoolAsync(school.Id);
        var snapshot = ProtocolCodec.ReadMapSnapshot(await ReceiveUntilAsync(socket, MessageType.ServerMapSnapshot));

        Assert.DoesNotContain(snapshot.Nodes, node => node.Id == "corridor-1");
        var office = Assert.Single(snapshot.Nodes, node => node.Id == "office");
        Assert.Equal("floor-1", office.ParentId);
    }

    [Fact]
    public async Task ReloadFromDisk_RestoresACustomMap()
    {
        using var client = fixture.App.CreateHttpClient("server");
        await SchoolApiTests.ResetAsync(client);
        var school = await SchoolApiTests.CreateWithMapAsync(client, "Карта с диска", StartDate, SchoolApiTests.SimpleCustomMap);

        using var reload = await client.PostAsync("/api/dev/reload-schools", content: null, TestContext.Current.CancellationToken);
        reload.EnsureSuccessStatusCode();

        using var socket = await OpenSchoolAsync(school.Id);
        var snapshot = ProtocolCodec.ReadMapSnapshot(await ReceiveUntilAsync(socket, MessageType.ServerMapSnapshot));

        Assert.Contains(snapshot.Nodes, node => node.Id == "office");
        Assert.DoesNotContain(snapshot.Nodes, node => node.Id == "corridor-1");
    }

    [Fact]
    public async Task OpeningASchoolDuringAMathLesson_PutsOccupancyOnPresenceNotTheSnapshot()
    {
        using var client = fixture.App.CreateHttpClient("server");
        await SchoolApiTests.ResetAsync(client);
        var start = new DateTime(2012, 4, 3, 10, 20, 0, DateTimeKind.Utc);
        var school = await SchoolApiTests.CreateAsync(client, "Кто где сейчас", start);

        using var socket = await OpenSchoolAsync(school.Id);
        var snapshot = ProtocolCodec.ReadMapSnapshot(await ReceiveUntilAsync(socket, MessageType.ServerMapSnapshot));
        Assert.Contains(snapshot.Nodes, node => node.Id == "classroom-101");

        await SendAsync(socket, buffer =>
            ProtocolCodec.WriteSetRunning(buffer, new ClientSetRunningMessage(Running: false)));
        await ReceiveClockWhereAsync(socket, clock => !clock.Running);

        var staffing = await client.GetFromJsonAsync<StaffingSnapshot>(
            $"/api/schools/{school.Id}/staffing",
            TestContext.Current.CancellationToken);
        Assert.NotNull(staffing);
        var applicant = staffing.Applicants[0];
        using var hire = await client.PostAsJsonAsync(
            $"/api/schools/{school.Id}/staff/hire",
            new { personId = applicant.Id, position = "Teacher" },
            TestContext.Current.CancellationToken);
        hire.EnsureSuccessStatusCode();
        using var assign = await client.PostAsJsonAsync(
            $"/api/schools/{school.Id}/staff/{Uri.EscapeDataString(applicant.Id)}/subjects",
            new { subject = "Mathematics" },
            TestContext.Current.CancellationToken);
        assign.EnsureSuccessStatusCode();

        var presence = await ReceivePresenceWhereAsync(
            socket,
            frame => frame.Nodes.Any(node => node.ActivitySubject.Length > 0));
        var occupied = Assert.Single(presence.Nodes, node => node.ActivitySubject.Length > 0);

        Assert.Equal("Математика", occupied.ActivitySubject);
        Assert.NotEmpty(occupied.ActivityClass);

        var directory = await client.GetFromJsonAsync<DirectorySnapshot>(
            $"/api/schools/{school.Id}/directory",
            TestContext.Current.CancellationToken);
        Assert.NotNull(directory);
        Assert.Contains(directory.People, person => person.Id == applicant.Id && person.FullName == applicant.FullName);

        // Resume and check that people ride the frame at all — that is what "occupancy is on
        // presence, not the snapshot" means. Do not wait for the freshly hired teacher in
        // particular: whether somebody hired mid-day comes in today depends on their day plan,
        // and the frozen moment drifts with however long this test's own HTTP calls took (the
        // school runs at one game minute per real second until it is paused). Waiting for that
        // one person made this test fail under load.
        await SendAsync(socket, buffer =>
            ProtocolCodec.WriteSetRunning(buffer, new ClientSetRunningMessage(Running: true)));
        // Presence still goes out while paused (empty people, lesson labels from the table).
        // Waiting for people first drains those frames and can spend the 40-frame budget
        // before the resume even lands. Clock.Running is the signal that time is moving.
        await ReceiveClockWhereAsync(socket, clock => clock.Running);
        presence = await ReceivePresenceWhereAsync(socket, frame => frame.People.Count > 0);
        Assert.All(presence.People, person => Assert.False(string.IsNullOrWhiteSpace(person.NodeId)));
        Assert.Contains(presence.People, person => directory.People.Any(row => row.Id == person.Id));
    }

    [Fact]
    public async Task SkipEmpty_DuringWorkHours_IsIgnored()
    {
        using var client = fixture.App.CreateHttpClient("server");
        await SchoolApiTests.ResetAsync(client);
        var start = new DateTime(2012, 4, 3, 10, 20, 0, DateTimeKind.Utc);
        var school = await SchoolApiTests.CreateAsync(client, "Пропуск в учебное время", start);

        using var socket = await OpenSchoolAsync(school.Id);
        await SendAsync(socket, buffer =>
            ProtocolCodec.WriteSetRunning(buffer, new ClientSetRunningMessage(Running: false)));
        var paused = await ReceiveClockWhereAsync(socket, clock => !clock.Running);

        Assert.False(paused.SkipAllowed);
        Assert.Equal(0, paused.SkipTargetUnixMs);

        await SendAsync(socket, buffer => ProtocolCodec.WriteSkipEmpty(buffer));
        var later = await ReceiveClockAfterAsync(socket, TimeSpan.FromSeconds(1));

        Assert.Equal(paused.GameTimeUnixMs, later.GameTimeUnixMs);
        Assert.False(later.SkipAllowed);
    }

    [Fact]
    public async Task Pausing_FreezesTheClock()
    {
        using var client = fixture.App.CreateHttpClient("server");
        await SchoolApiTests.ResetAsync(client);
        var school = await SchoolApiTests.CreateAsync(client, "Пауза", StartDate);

        using var socket = await OpenSchoolAsync(school.Id);
        await ReceiveClockAsync(socket);

        await SendAsync(socket, buffer =>
            ProtocolCodec.WriteSetRunning(buffer, new ClientSetRunningMessage(Running: false)));

        var paused = await ReceiveClockWhereAsync(socket, clock => !clock.Running);
        var later = await ReceiveClockAfterAsync(socket, TimeSpan.FromSeconds(1));

        Assert.False(later.Running);
        Assert.Equal(paused.GameTimeUnixMs, later.GameTimeUnixMs);
    }

    [Fact]
    public async Task SpeedIndex_ChangesHowFastTheCalendarMoves()
    {
        using var client = fixture.App.CreateHttpClient("server");
        await SchoolApiTests.ResetAsync(client);
        var school = await SchoolApiTests.CreateAsync(client, "Быстрая", StartDate);

        using var socket = await OpenSchoolAsync(school.Id);
        await ReceiveClockAsync(socket);

        await SendAsync(socket, buffer =>
            ProtocolCodec.WriteSetSpeed(buffer, new ClientSetSpeedMessage(SpeedIndex: 4)));

        var fast = await ReceiveClockWhereAsync(socket, clock => clock.SpeedIndex == 4);
        var later = await ReceiveClockAfterAsync(socket, TimeSpan.FromSeconds(1));
        var elapsed = ToDate(later) - ToDate(fast);

        // ×10 means 10 game minutes per real second.
        Assert.InRange(elapsed.TotalMinutes, 6, 14);
    }

    [Fact]
    public async Task LeavingASchool_KeepsItsClockRunning()
    {
        using var client = fixture.App.CreateHttpClient("server");
        await SchoolApiTests.ResetAsync(client);
        var school = await SchoolApiTests.CreateAsync(client, "Выход", StartDate);

        using (var socket = await OpenSchoolAsync(school.Id))
        {
            await ReceiveClockAsync(socket);
            await SendAsync(socket, buffer => ProtocolCodec.WriteCloseSchool(buffer));
        }

        var afterLeaving = await FindAsync(client, school.Id);
        await Task.Delay(TimeSpan.FromSeconds(1), TestContext.Current.CancellationToken);
        var later = await FindAsync(client, school.Id);

        Assert.True(later.Running);
        Assert.True(later.GameTime > afterLeaving.GameTime, "The calendar stopped when the client left.");
    }

    [Fact]
    public async Task APausedSchool_StaysPausedAfterLeaving()
    {
        using var client = fixture.App.CreateHttpClient("server");
        await SchoolApiTests.ResetAsync(client);
        var school = await SchoolApiTests.CreateAsync(client, "Оставлена на паузе", StartDate);

        using (var socket = await OpenSchoolAsync(school.Id))
        {
            await ReceiveClockAsync(socket);
            await SendAsync(socket, buffer =>
                ProtocolCodec.WriteSetRunning(buffer, new ClientSetRunningMessage(Running: false)));
            await ReceiveClockWhereAsync(socket, clock => !clock.Running);
            await SendAsync(socket, buffer => ProtocolCodec.WriteCloseSchool(buffer));
        }

        var afterLeaving = await FindAsync(client, school.Id);
        await Task.Delay(TimeSpan.FromSeconds(1), TestContext.Current.CancellationToken);
        var later = await FindAsync(client, school.Id);

        Assert.False(later.Running);
        Assert.Equal(afterLeaving.GameTime, later.GameTime);
    }

    [Fact]
    public async Task DeletingTheOpenSchool_TellsTheClient()
    {
        using var client = fixture.App.CreateHttpClient("server");
        await SchoolApiTests.ResetAsync(client);
        var school = await SchoolApiTests.CreateAsync(client, "Исчезнет", StartDate);

        using var socket = await OpenSchoolAsync(school.Id);
        await ReceiveClockAsync(socket);

        using var deleted = await client.DeleteAsync($"/api/schools/{school.Id}", TestContext.Current.CancellationToken);
        deleted.EnsureSuccessStatusCode();

        var gone = ProtocolCodec.ReadSchoolGone(await ReceiveUntilAsync(socket, MessageType.ServerSchoolGone));

        Assert.Equal(school.Id, gone.SchoolId);
    }

    [Fact]
    public async Task OpeningASchoolThatDoesNotExist_IsAnsweredWithSchoolGone()
    {
        using var socket = await ConnectAsync();
        await ReceiveUntilAsync(socket, MessageType.ServerWelcome);

        await SendAsync(socket, buffer =>
            ProtocolCodec.WriteOpenSchool(buffer, new ClientOpenSchoolMessage(999999)));

        var gone = ProtocolCodec.ReadSchoolGone(await ReceiveUntilAsync(socket, MessageType.ServerSchoolGone));

        Assert.Equal(999999, gone.SchoolId);
    }

    [Fact]
    public async Task PausingOneSchool_DoesNotStopAnother()
    {
        using var client = fixture.App.CreateHttpClient("server");
        await SchoolApiTests.ResetAsync(client);
        var paused = await SchoolApiTests.CreateAsync(client, "На паузе", StartDate);
        var running = await SchoolApiTests.CreateAsync(client, "Идёт дальше", StartDate);

        using var socket = await OpenSchoolAsync(paused.Id);
        await ReceiveClockAsync(socket);
        await SendAsync(socket, buffer =>
            ProtocolCodec.WriteSetRunning(buffer, new ClientSetRunningMessage(Running: false)));
        await ReceiveClockWhereAsync(socket, clock => !clock.Running);

        var pausedAt = await FindAsync(client, paused.Id);
        await Task.Delay(TimeSpan.FromSeconds(1), TestContext.Current.CancellationToken);
        var pausedLater = await FindAsync(client, paused.Id);
        var runningLater = await FindAsync(client, running.Id);

        Assert.False(pausedLater.Running);
        Assert.Equal(pausedAt.GameTime, pausedLater.GameTime);
        Assert.True(runningLater.Running);
        Assert.True(runningLater.GameTime > running.GameTime, "Pausing one school stopped the other.");
    }

    [Fact]
    public async Task Guest_SetRunning_DoesNotChangeOwnerRunning()
    {
        using var ownerClient = await SchoolApiTests.CreateIsolatedClientAsync(fixture.App, "GuestClockOwner");
        await SchoolApiTests.WipeAllSavesAsync(ownerClient);
        var school = await SchoolApiTests.CreateAsync(ownerClient, "Гостевые часы", StartDate);

        var guestCookie = await SchoolApiTests.LoginAndGetCookieAsync(
            fixture.App.CreateHttpClient("server"),
            "GuestClockViewer");

        using var socket = new ClientWebSocket();
        socket.Options.SetRequestHeader("Cookie", guestCookie);
        var http = fixture.App.GetEndpoint("server", "http");
        var uri = new UriBuilder(http) { Scheme = "ws", Path = "/ws/game" }.Uri;
        await socket.ConnectAsync(uri, TestContext.Current.CancellationToken).WaitAsync(DefaultTimeout);
        await SendAsync(socket, buffer =>
            ProtocolCodec.WriteHello(buffer, new ClientHelloMessage(ProtocolConstants.Version, ProtocolConstants.LocaleRussian)));
        await ReceiveUntilAsync(socket, MessageType.ServerWelcome);
        await SendAsync(socket, buffer =>
            ProtocolCodec.WriteOpenSchool(buffer, new ClientOpenSchoolMessage(school.Id)));
        _ = await ReceiveClockAsync(socket);
        await SendAsync(socket, buffer =>
            ProtocolCodec.WriteSetRunning(buffer, new ClientSetRunningMessage(Running: false)));
        await Task.Delay(TimeSpan.FromMilliseconds(300), TestContext.Current.CancellationToken);

        var ownerView = await FindAsync(ownerClient, school.Id);
        Assert.True(ownerView.Running);

        using var delete = await ownerClient.DeleteAsync($"/api/schools/{school.Id}", TestContext.Current.CancellationToken);
        delete.EnsureSuccessStatusCode();
    }

    [Fact]
    public async Task ReloadFromDisk_RestoresAPausedClock()
    {
        using var client = fixture.App.CreateHttpClient("server");
        await SchoolApiTests.ResetAsync(client);
        var school = await SchoolApiTests.CreateAsync(client, "Снимок паузы", StartDate);

        using (var socket = await OpenSchoolAsync(school.Id))
        {
            await ReceiveClockAsync(socket);
            await SendAsync(socket, buffer =>
                ProtocolCodec.WriteSetRunning(buffer, new ClientSetRunningMessage(Running: false)));
            await ReceiveClockWhereAsync(socket, clock => !clock.Running);
        }

        var paused = await FindAsync(client, school.Id);

        using var reload = await client.PostAsync("/api/dev/reload-schools", content: null, TestContext.Current.CancellationToken);
        reload.EnsureSuccessStatusCode();

        var restored = await FindAsync(client, school.Id);
        Assert.Equal(paused.Name, restored.Name);
        Assert.False(restored.Running);
        Assert.Equal(paused.GameTime, restored.GameTime);
        Assert.Equal(paused.SpeedIndex, restored.SpeedIndex);
    }

    [Fact]
    public async Task Ping_IsAnsweredWithTheSameTimestamp()
    {
        using var socket = await ConnectAsync();
        await ReceiveUntilAsync(socket, MessageType.ServerWelcome);

        const long ClientTime = 1_700_000_000_123;
        await SendAsync(socket, buffer => ProtocolCodec.WritePing(buffer, new ClientPingMessage(ClientTime)));

        var pong = ProtocolCodec.ReadPong(await ReceiveUntilAsync(socket, MessageType.ServerPong));

        Assert.Equal(ClientTime, pong.ClientTimeMs);
    }

    [Fact]
    public async Task OldProtocolVersion_IsRejected()
    {
        using var httpClient = fixture.App.CreateHttpClient("server");
        var cookie = await SchoolApiTests.LoginAndGetCookieAsync(httpClient);
        using var socket = new ClientWebSocket();
        socket.Options.SetRequestHeader("Cookie", cookie);
        var http = fixture.App.GetEndpoint("server", "http");
        var uri = new UriBuilder(http) { Scheme = "ws", Path = "/ws/game" }.Uri;
        await socket.ConnectAsync(uri, TestContext.Current.CancellationToken).WaitAsync(DefaultTimeout);

        await SendAsync(socket, buffer => ProtocolCodec.WriteHello(
            buffer,
            new ClientHelloMessage((byte)(ProtocolConstants.Version - 1), ProtocolConstants.LocaleRussian)));

        var buffer = new byte[ProtocolConstants.MaxMessageSize];
        var result = await socket.ReceiveAsync(buffer, TestContext.Current.CancellationToken);

        Assert.Equal(WebSocketMessageType.Close, result.MessageType);
        Assert.Equal(WebSocketCloseStatus.ProtocolError, socket.CloseStatus);
    }

    [Fact]
    public async Task VersionMismatch_IsRejected()
    {
        using var httpClient = fixture.App.CreateHttpClient("server");
        var cookie = await SchoolApiTests.LoginAndGetCookieAsync(httpClient);
        using var socket = new ClientWebSocket();
        socket.Options.SetRequestHeader("Cookie", cookie);
        var http = fixture.App.GetEndpoint("server", "http");
        var uri = new UriBuilder(http) { Scheme = "ws", Path = "/ws/game" }.Uri;
        await socket.ConnectAsync(uri, TestContext.Current.CancellationToken).WaitAsync(DefaultTimeout);

        await SendAsync(socket, buffer => ProtocolCodec.WriteHello(
            buffer,
            new ClientHelloMessage((byte)(ProtocolConstants.Version + 1), ProtocolConstants.LocaleRussian)));

        var buffer = new byte[ProtocolConstants.MaxMessageSize];
        var result = await socket.ReceiveAsync(buffer, TestContext.Current.CancellationToken);

        Assert.Equal(WebSocketMessageType.Close, result.MessageType);
        Assert.Equal(WebSocketCloseStatus.ProtocolError, socket.CloseStatus);
    }

    private static DateTime ToDate(ServerClockMessage clock) =>
        DateTimeOffset.FromUnixTimeMilliseconds(clock.GameTimeUnixMs).UtcDateTime;

    private async Task<SchoolApiTests.SchoolResponse> FindAsync(HttpClient client, int schoolId)
    {
        var state = await SchoolApiTests.GetSchoolsAsync(client);
        var school = state.Schools.SingleOrDefault(candidate => candidate.Id == schoolId);

        Assert.NotNull(school);
        return school;
    }

    private async Task<ClientWebSocket> OpenSchoolAsync(int schoolId, byte locale = ProtocolConstants.LocaleRussian)
    {
        using var httpClient = fixture.App.CreateHttpClient("server");
        var socket = await ConnectAsync(httpClient, locale);
        await ReceiveUntilAsync(socket, MessageType.ServerWelcome);
        await SendAsync(socket, buffer =>
            ProtocolCodec.WriteOpenSchool(buffer, new ClientOpenSchoolMessage(schoolId)));
        return socket;
    }

    private async Task<ClientWebSocket> ConnectAsync(byte locale = ProtocolConstants.LocaleRussian)
    {
        using var httpClient = fixture.App.CreateHttpClient("server");
        var userName = $"Ws-{Guid.NewGuid():N}"[..12];
        var cookie = await SchoolApiTests.LoginAndGetCookieAsync(httpClient, userName);

        var socket = new ClientWebSocket();
        socket.Options.SetRequestHeader("Cookie", cookie);
        var http = fixture.App.GetEndpoint("server", "http");
        var uri = new UriBuilder(http) { Scheme = "ws", Path = "/ws/game" }.Uri;
        await socket.ConnectAsync(uri, TestContext.Current.CancellationToken).WaitAsync(DefaultTimeout);
        await SendAsync(socket, buffer =>
            ProtocolCodec.WriteHello(buffer, new ClientHelloMessage(ProtocolConstants.Version, locale)));
        return socket;
    }

    private async Task<ClientWebSocket> ConnectAsync(HttpClient client, byte locale = ProtocolConstants.LocaleRussian)
    {
        var userName = $"Ws-{Guid.NewGuid():N}"[..12];
        var cookie = await SchoolApiTests.LoginAndGetCookieAsync(client, userName);

        var socket = new ClientWebSocket();
        socket.Options.SetRequestHeader("Cookie", cookie);
        var http = fixture.App.GetEndpoint("server", "http");
        var uri = new UriBuilder(http) { Scheme = "ws", Path = "/ws/game" }.Uri;
        await socket.ConnectAsync(uri, TestContext.Current.CancellationToken).WaitAsync(DefaultTimeout);
        await SendAsync(socket, buffer =>
            ProtocolCodec.WriteHello(buffer, new ClientHelloMessage(ProtocolConstants.Version, locale)));
        return socket;
    }

    private async Task<ClientWebSocket> ConnectRawAsync()
    {
        var http = fixture.App.GetEndpoint("server", "http");
        var uri = new UriBuilder(http) { Scheme = "ws", Path = "/ws/game" }.Uri;

        var socket = new ClientWebSocket();
        await socket.ConnectAsync(uri, TestContext.Current.CancellationToken).WaitAsync(DefaultTimeout);
        return socket;
    }

    private static async Task SendAsync(WebSocket socket, Func<byte[], int> write)
    {
        var buffer = new byte[ProtocolCodec.MaxFrameSize];
        var length = write(buffer);

        await socket.SendAsync(
            buffer.AsMemory(0, length),
            WebSocketMessageType.Binary,
            endOfMessage: true,
            TestContext.Current.CancellationToken);
    }

    private static async Task<ServerClockMessage> ReceiveClockAsync(WebSocket socket) =>
        ProtocolCodec.ReadClock(await ReceiveUntilAsync(socket, MessageType.ServerClock));

    /// <summary>Drains clock frames until one satisfies <paramref name="predicate"/>.</summary>
    private static async Task<ServerClockMessage> ReceiveClockWhereAsync(
        WebSocket socket,
        Func<ServerClockMessage, bool> predicate)
    {
        for (var attempt = 0; attempt < 40; attempt++)
        {
            var clock = await ReceiveClockAsync(socket);
            if (predicate(clock))
            {
                return clock;
            }
        }

        throw new InvalidOperationException("No clock frame matched within 40 frames.");
    }

    private static async Task<ServerPresenceMessage> ReceivePresenceWhereAsync(
        WebSocket socket,
        Func<ServerPresenceMessage, bool> predicate)
    {
        for (var attempt = 0; attempt < 40; attempt++)
        {
            var presence = ProtocolCodec.ReadPresence(await ReceiveUntilAsync(socket, MessageType.ServerPresence));
            if (predicate(presence))
            {
                return presence;
            }
        }

        throw new InvalidOperationException("No presence frame matched within 40 frames.");
    }

    /// <summary>Keeps reading clock frames for <paramref name="duration"/> and returns the last one.</summary>
    private static async Task<ServerClockMessage> ReceiveClockAfterAsync(WebSocket socket, TimeSpan duration)
    {
        var deadline = DateTime.UtcNow + duration;
        var clock = await ReceiveClockAsync(socket);

        while (DateTime.UtcNow < deadline)
        {
            clock = await ReceiveClockAsync(socket);
        }

        return clock;
    }

    /// <summary>Reads frames until one of <paramref name="expected"/> shows up.</summary>
    private static async Task<byte[]> ReceiveUntilAsync(WebSocket socket, MessageType expected, TimeSpan? timeout = null)
    {
        using var cts = CancellationTokenSource.CreateLinkedTokenSource(TestContext.Current.CancellationToken);
        cts.CancelAfter(timeout ?? DefaultTimeout);

        var buffer = new byte[64 * 1024];
        var chunks = new List<byte>();

        while (true)
        {
            WebSocketReceiveResult result;
            try
            {
                result = await socket.ReceiveAsync(buffer, cts.Token);
            }
            catch (OperationCanceledException) when (!TestContext.Current.CancellationToken.IsCancellationRequested)
            {
                throw new TimeoutException($"No {expected} frame arrived within {timeout ?? DefaultTimeout}.");
            }

            if (result.MessageType == WebSocketMessageType.Close)
            {
                throw new InvalidOperationException($"Socket closed while waiting for {expected}: {socket.CloseStatus}.");
            }

            chunks.AddRange(buffer.AsSpan(0, result.Count).ToArray());
            if (!result.EndOfMessage)
            {
                continue;
            }

            var frame = chunks.ToArray();
            chunks.Clear();
            if (ProtocolCodec.PeekMessageType(frame) == expected)
            {
                return frame;
            }
        }
    }

    private sealed record StaffingSnapshot(IReadOnlyList<ApplicantSnapshot> Applicants);

    private sealed record ApplicantSnapshot(string Id, string FullName);

    private sealed record DirectorySnapshot(IReadOnlyList<DirectoryPersonSnapshot> People);

    private sealed record DirectoryPersonSnapshot(string Id, string FullName);
}