h-school/tests/HSchool.Protocol.Tests/ProtocolCodecTests.cs

namespace HSchool.Protocol.Tests;

/// <summary>
/// The wire format is a contract with the browser client. Round-trips prove the C# side is
/// self-consistent; the explicit byte-layout tests are what keeps
/// <c>src/HSchool.Client/src/net/protocol.ts</c> honest.
/// </summary>
public class ProtocolCodecTests
{
    [Fact]
    public void Hello_RoundTripsAndIsThreeBytes()
    {
        var message = new ClientHelloMessage(ProtocolConstants.Version, ProtocolConstants.LocaleEnglish);
        Span<byte> buffer = stackalloc byte[ProtocolCodec.MaxFrameSize];

        var length = ProtocolCodec.WriteHello(buffer, message);

        Assert.Equal(3, length);
        Assert.Equal((byte)MessageType.ClientHello, buffer[0]);
        Assert.Equal(ProtocolConstants.Version, buffer[1]);
        Assert.Equal(ProtocolConstants.LocaleEnglish, buffer[2]);
        Assert.Equal(message, ProtocolCodec.ReadHello(buffer[..length]));
    }

    [Fact]
    public void Ping_RoundTripsAndIsNineBytes()
    {
        var message = new ClientPingMessage(1_700_000_000_123);
        Span<byte> buffer = stackalloc byte[ProtocolCodec.MaxFrameSize];

        var length = ProtocolCodec.WritePing(buffer, message);

        Assert.Equal(9, length);
        Assert.Equal(message, ProtocolCodec.ReadPing(buffer[..length]));
    }

    [Fact]
    public void OpenSchool_RoundTripsAndIsFiveBytes()
    {
        var message = new ClientOpenSchoolMessage(0x01020304);
        Span<byte> buffer = stackalloc byte[ProtocolCodec.MaxFrameSize];

        var length = ProtocolCodec.WriteOpenSchool(buffer, message);

        Assert.Equal(5, length);
        Assert.Equal(message, ProtocolCodec.ReadOpenSchool(buffer[..length]));
    }

    [Fact]
    public void CloseSchool_IsASingleByte()
    {
        Span<byte> buffer = stackalloc byte[ProtocolCodec.MaxFrameSize];

        var length = ProtocolCodec.WriteCloseSchool(buffer);

        Assert.Equal(1, length);
        Assert.Equal(MessageType.ClientCloseSchool, ProtocolCodec.PeekMessageType(buffer[..length]));
    }

    [Theory]
    [InlineData(true)]
    [InlineData(false)]
    public void SetRunning_RoundTripsAndIsTwoBytes(bool running)
    {
        var message = new ClientSetRunningMessage(running);
        Span<byte> buffer = stackalloc byte[ProtocolCodec.MaxFrameSize];

        var length = ProtocolCodec.WriteSetRunning(buffer, message);

        Assert.Equal(2, length);
        Assert.Equal(message, ProtocolCodec.ReadSetRunning(buffer[..length]));
    }

    [Theory]
    [InlineData(0)]
    [InlineData(4)]
    public void SetSpeed_RoundTripsAndIsTwoBytes(byte speedIndex)
    {
        var message = new ClientSetSpeedMessage(speedIndex);
        Span<byte> buffer = stackalloc byte[ProtocolCodec.MaxFrameSize];

        var length = ProtocolCodec.WriteSetSpeed(buffer, message);

        Assert.Equal(2, length);
        Assert.Equal(message, ProtocolCodec.ReadSetSpeed(buffer[..length]));
    }

    [Fact]
    public void Welcome_RoundTripsAndIsFourBytes()
    {
        var message = new ServerWelcomeMessage(ProtocolConstants.Version, 20, 6);
        Span<byte> buffer = stackalloc byte[ProtocolCodec.MaxFrameSize];

        var length = ProtocolCodec.WriteWelcome(buffer, message);

        Assert.Equal(4, length);
        Assert.Equal(message, ProtocolCodec.ReadWelcome(buffer[..length]));
    }

    [Fact]
    public void Pong_RoundTripsAndIsThirteenBytes()
    {
        var message = new ServerPongMessage(5, 99);
        Span<byte> buffer = stackalloc byte[ProtocolCodec.MaxFrameSize];

        var length = ProtocolCodec.WritePong(buffer, message);

        Assert.Equal(13, length);
        Assert.Equal(message, ProtocolCodec.ReadPong(buffer[..length]));
    }

    [Fact]
    public void Clock_RoundTripsAndIsFifteenBytes()
    {
        var message = new ServerClockMessage(7, 1_333_432_800_000, Running: true, SpeedIndex: 2);
        Span<byte> buffer = stackalloc byte[ProtocolCodec.MaxFrameSize];

        var length = ProtocolCodec.WriteClock(buffer, message);

        Assert.Equal(15, length);
        Assert.Equal(message, ProtocolCodec.ReadClock(buffer[..length]));
    }

    [Fact]
    public void SchoolGone_RoundTripsAndIsFiveBytes()
    {
        var message = new ServerSchoolGoneMessage(3);
        Span<byte> buffer = stackalloc byte[ProtocolCodec.MaxFrameSize];

        var length = ProtocolCodec.WriteSchoolGone(buffer, message);

        Assert.Equal(5, length);
        Assert.Equal(message, ProtocolCodec.ReadSchoolGone(buffer[..length]));
    }

    [Fact]
    public void MapSnapshot_RoundTripsAndWritesHeaderOffsets()
    {
        var message = new ServerMapSnapshotMessage(7, [
            new MapSnapshotNode(0, "yard", "", "Двор", 0, [], []),
            new MapSnapshotNode(
                3,
                "office",
                "floor-1",
                "Кабинет директора",
                0,
                [new MapSnapshotItem("Стул", 2)],
                ["Директор"]),
        ]);
        var buffer = new byte[ProtocolConstants.MaxMessageSize];

        var length = ProtocolCodec.WriteMapSnapshot(buffer, message);

        Assert.Equal((byte)MessageType.ServerMapSnapshot, buffer[0]);
        Assert.Equal(7, BitConverter.ToInt32(buffer.AsSpan(1, 4)));
        Assert.Equal((ushort)2, BitConverter.ToUInt16(buffer.AsSpan(5, 2)));

        var read = ProtocolCodec.ReadMapSnapshot(buffer.AsSpan(0, length));
        Assert.Equal(message.SchoolId, read.SchoolId);
        Assert.Equal(2, read.Nodes.Count);
        Assert.Equal("yard", read.Nodes[0].Id);
        Assert.Equal("", read.Nodes[0].ParentId);
        Assert.Equal("Двор", read.Nodes[0].Name);
        Assert.Equal(0, read.Nodes[0].PupilSlots);
        Assert.Equal([new MapSnapshotItem("Стул", 2)], read.Nodes[1].Items);
        Assert.Equal(["Директор"], read.Nodes[1].Positions);
    }

    [Fact]
    public void MapSnapshot_WritesPupilSlotsAndStackedItemCount()
    {
        var message = new ServerMapSnapshotMessage(1, [
            new MapSnapshotNode(
                3,
                "classroom-1a",
                "floor-1",
                "Класс 1A",
                16,
                [new MapSnapshotItem("Парта", 16)],
                []),
        ]);
        var buffer = new byte[ProtocolCodec.MapSnapshotSize(message)];

        var length = ProtocolCodec.WriteMapSnapshot(buffer, message);
        var read = ProtocolCodec.ReadMapSnapshot(buffer.AsSpan(0, length));

        Assert.Equal(16, read.Nodes[0].PupilSlots);
        Assert.Equal([new MapSnapshotItem("Парта", 16)], read.Nodes[0].Items);
    }

    [Fact]
    public void MapSnapshotSize_IsExactlyWhatTheWriterProduces()
    {
        var message = new ServerMapSnapshotMessage(7, [
            new MapSnapshotNode(0, "yard", "", "Двор", 0, [], []),
            new MapSnapshotNode(2, "floor-1", "main", "Этаж 1", 0, [], []),
            new MapSnapshotNode(
                3,
                "office",
                "floor-1",
                "Кабинет директора",
                0,
                [new MapSnapshotItem("Стол", 1), new MapSnapshotItem("Стул", 2)],
                ["Директор"]),
        ]);

        var size = ProtocolCodec.MapSnapshotSize(message);
        var buffer = new byte[size];

        Assert.Equal(size, ProtocolCodec.WriteMapSnapshot(buffer, message));
    }

    [Fact]
    public void MapSnapshot_OfALargeSchoolSurvivesTheEightKilobyteLimit()
    {
        // A player who keeps clicking "Add room" in the create editor gets past 8 KiB somewhere
        // around sixty furnished rooms. Sizing the buffer from the message is what keeps that
        // school openable instead of killing its worker thread on the first snapshot.
        var nodes = new List<MapSnapshotNode> { new(0, "yard", "", "Двор", 0, [], []) };
        for (var i = 1; i <= 200; i++)
        {
            nodes.Add(new MapSnapshotNode(
                3,
                $"principals-office-{i}",
                "floor-1",
                "Кабинет директора",
                0,
                [new MapSnapshotItem("Кресло директора", 1), new MapSnapshotItem("Стол", 1), new MapSnapshotItem("Стул", 2)],
                ["Директор"]));
        }

        var message = new ServerMapSnapshotMessage(1, nodes);
        var buffer = new byte[ProtocolCodec.MapSnapshotSize(message)];

        Assert.True(buffer.Length > ProtocolConstants.MaxMessageSize);

        var length = ProtocolCodec.WriteMapSnapshot(buffer, message);
        var read = ProtocolCodec.ReadMapSnapshot(buffer.AsSpan(0, length));

        Assert.Equal(nodes.Count, read.Nodes.Count);
        Assert.Equal("principals-office-200", read.Nodes[^1].Id);
        Assert.Equal(
            [new MapSnapshotItem("Кресло директора", 1), new MapSnapshotItem("Стол", 1), new MapSnapshotItem("Стул", 2)],
            read.Nodes[^1].Items);
    }

    [Fact]
    public void Numbers_AreLittleEndian()
    {
        Span<byte> buffer = stackalloc byte[ProtocolCodec.MaxFrameSize];
        ProtocolCodec.WriteOpenSchool(buffer, new ClientOpenSchoolMessage(0x01020304));

        Assert.Equal((byte)MessageType.ClientOpenSchool, buffer[0]);
        Assert.Equal(new byte[] { 0x04, 0x03, 0x02, 0x01 }, buffer[1..5].ToArray());
    }

    [Fact]
    public void MaxFrameSize_FitsEveryFixedSizeMessage()
    {
        // Handlers size clock/welcome/pong buffers from this constant; map snapshots use MaxMessageSize.
        Span<byte> buffer = stackalloc byte[ProtocolCodec.MaxFrameSize];
        var clock = ProtocolCodec.WriteClock(buffer, new ServerClockMessage(1, long.MaxValue, true, 4));

        Assert.True(clock <= ProtocolCodec.MaxFrameSize);
    }

    [Fact]
    public void PeekMessageType_ReadsTheFirstByte()
    {
        Span<byte> buffer = stackalloc byte[ProtocolCodec.MaxFrameSize];
        ProtocolCodec.WritePing(buffer, new ClientPingMessage(1));

        Assert.Equal(MessageType.ClientPing, ProtocolCodec.PeekMessageType(buffer));
        Assert.Equal(MessageType.None, ProtocolCodec.PeekMessageType([]));
    }

    [Fact]
    public void TruncatedFrame_Throws()
    {
        byte[] frame = [(byte)MessageType.ServerClock, 1, 2];

        Assert.Throws<ProtocolException>(() => ProtocolCodec.ReadClock(frame));
    }

    [Fact]
    public void WrongMessageId_Throws()
    {
        Span<byte> buffer = stackalloc byte[ProtocolCodec.MaxFrameSize];
        var length = ProtocolCodec.WritePing(buffer, new ClientPingMessage(1));
        var frame = buffer[..length].ToArray();

        Assert.Throws<ProtocolException>(() => ProtocolCodec.ReadOpenSchool(frame));
    }

    [Fact]
    public void UndersizedBuffer_Throws()
    {
        var buffer = new byte[2];

        Assert.Throws<ProtocolException>(() =>
            ProtocolCodec.WriteClock(buffer, new ServerClockMessage(1, 0, false, 1)));
    }
}