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_RoundTripsAndIsTwoBytes()
    {
        var message = new ClientHelloMessage(ProtocolConstants.Version);
        Span<byte> buffer = stackalloc byte[ProtocolCodec.MaxFrameSize];

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

        Assert.Equal(2, length);
        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 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_FitsEveryMessage()
    {
        // The handlers size their buffers from this constant; the clock frame is the largest one.
        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)));
    }
}