// Create a pipe with default options (uses ArrayPool internally)
var pipe = new Pipe();

PipeWriter writer = pipe.Writer;  // Producer side
PipeReader reader = pipe.Reader;  // Consumer side

async Task FillPipeAsync(Stream source, PipeWriter writer,
    CancellationToken ct)
{
    const int minimumBufferSize = 512;

    while (true)
    {
        // Request a buffer from the pipe's memory pool
        Memory<byte> memory = writer.GetMemory(minimumBufferSize);

        int bytesRead = await source.ReadAsync(memory, ct);
        if (bytesRead == 0)
            break;  // End of stream

        // Tell the pipe how many bytes were written
        writer.Advance(bytesRead);

        // Flush makes data available to the reader.
        // FlushAsync may pause here if the reader is slow (backpressure).
        FlushResult result = await writer.FlushAsync(ct);
        if (result.IsCompleted)
            break;  // Reader stopped consuming
    }

    // Signal completion -- reader will see IsCompleted = true
    await writer.CompleteAsync();
}

async Task ReadPipeAsync(PipeReader reader, CancellationToken ct)
{
    while (true)
    {
        ReadResult result = await reader.ReadAsync(ct);
        ReadOnlySequence<byte> buffer = result.Buffer;

        // Try to parse messages from the buffer
        while (TryParseMessage(ref buffer, out var message))
        {
            await ProcessMessageAsync(message, ct);
        }

        // Tell the pipe how much was consumed and how much was examined.
        // consumed: data that has been fully processed (will be freed)
        // examined: data that has been looked at (won't trigger re-read
        //           until new data arrives)
        reader.AdvanceTo(buffer.Start, buffer.End);

        if (result.IsCompleted)
            break;  // Writer finished and all data consumed
    }

    await reader.CompleteAsync();
}

var pipe = new Pipe(new PipeOptions(
    pauseWriterThreshold: 64 * 1024,   // Pause writer at 64 KB buffered
    resumeWriterThreshold: 32 * 1024,  // Resume writer when buffer drops to 32 KB
    minimumSegmentSize: 4096,
    useSynchronizationContext: false));

static bool TryParseMessage(
    ref ReadOnlySequence<byte> buffer,
    out ReadOnlySequence<byte> payload)
{
    payload = default;

    // Need at least 4 bytes for the length prefix
    if (buffer.Length < 4)
        return false;

    // Read length from first 4 bytes
    int length;
    if (buffer.FirstSpan.Length >= 4)
    {
        length = BinaryPrimitives.ReadInt32BigEndian(buffer.FirstSpan);
    }
    else
    {
        // Slow path: length header spans multiple segments
        Span<byte> lengthBytes = stackalloc byte[4];
        buffer.Slice(0, 4).CopyTo(lengthBytes);
        length = BinaryPrimitives.ReadInt32BigEndian(lengthBytes);
    }

    // Validate length to prevent allocation attacks
    if (length < 0 || length > 1_048_576)  // 1 MB max
        throw new ProtocolViolationException(
            $"Invalid message length: {length}");

    // Check if the full message is available
    long totalLength = 4 + length;
    if (buffer.Length < totalLength)
        return false;

    // Extract the payload (zero-copy slice)
    payload = buffer.Slice(4, length);

    // Advance the buffer past this message
    buffer = buffer.Slice(totalLength);
    return true;
}

static bool TryReadLine(
    ref ReadOnlySequence<byte> buffer,
    out ReadOnlySequence<byte> line)
{
    // Look for the newline delimiter
    SequencePosition? position = buffer.PositionOf((byte)'\n');
    if (position is null)
    {
        line = default;
        return false;
    }

    // Slice up to (not including) the delimiter
    line = buffer.Slice(0, position.Value);

    // Advance past the delimiter
    buffer = buffer.Slice(buffer.GetPosition(1, position.Value));
    return true;
}

static string DecodeUtf8(ReadOnlySequence<byte> sequence)
{
    // Fast path: single contiguous segment
    if (sequence.IsSingleSegment)
    {
        return Encoding.UTF8.GetString(sequence.FirstSpan);
    }

    // Slow path: multi-segment -- rent a contiguous buffer
    int length = (int)sequence.Length;
    byte[] rented = ArrayPool<byte>.Shared.Rent(length);
    try
    {
        sequence.CopyTo(rented);
        return Encoding.UTF8.GetString(rented, 0, length);
    }
    finally
    {
        ArrayPool<byte>.Shared.Return(rented);
    }
}

// Wrap a NetworkStream for pipeline-based reading
await using var networkStream = tcpClient.GetStream();
var reader = PipeReader.Create(networkStream, new StreamPipeReaderOptions(
    bufferSize: 4096,
    minimumReadSize: 1024,
    leaveOpen: true)); // Caller manages networkStream lifetime

try
{
    await ProcessProtocolAsync(reader, cancellationToken);
}
finally
{
    await reader.CompleteAsync();
}

// Wrap a stream for pipeline-based writing
var writer = PipeWriter.Create(networkStream, new StreamPipeWriterOptions(
    minimumBufferSize: 4096,
    leaveOpen: true)); // Caller manages networkStream lifetime

try
{
    await WriteResponseAsync(writer, response, cancellationToken);
}
finally
{
    await writer.CompleteAsync();
}

// Custom connection middleware for protocol-level processing
builder.WebHost.ConfigureKestrel(options =>
{
    options.ListenLocalhost(9000, listenOptions =>
    {
        listenOptions.UseConnectionHandler<MyProtocolHandler>();
    });
});

public sealed class MyProtocolHandler : ConnectionHandler
{
    public override async Task OnConnectedAsync(
        ConnectionContext connection)
    {
        var reader = connection.Transport.Input;
        var writer = connection.Transport.Output;
        var ct = connection.ConnectionClosed;

        try
        {
            while (true)
            {
                ReadResult result = await reader.ReadAsync(ct);
                ReadOnlySequence<byte> buffer = result.Buffer;

                while (TryParseMessage(ref buffer, out var payload))
                {
                    var response = ProcessRequest(payload);
                    await WriteResponseAsync(writer, response);
                }

                reader.AdvanceTo(buffer.Start, buffer.End);

                if (result.IsCompleted)
                    break;
            }
        }
        finally
        {
            await reader.CompleteAsync();
            await writer.CompleteAsync();
        }
    }

    private static async Task WriteResponseAsync(
        PipeWriter writer, ReadOnlyMemory<byte> response)
    {
        // Write length prefix + payload
        var memory = writer.GetMemory(4 + response.Length);
        BinaryPrimitives.WriteInt32BigEndian(
            memory.Span, response.Length);
        response.CopyTo(memory[4..]);
        writer.Advance(4 + response.Length);
        await writer.FlushAsync();
    }
}

public interface IDuplexPipe
{
    PipeReader Input { get; }
    PipeWriter Output { get; }
}

static bool TryParseHeader(
    ref ReadOnlySequence<byte> buffer,
    out int messageType,
    out int length)
{
    var reader = new SequenceReader<byte>(buffer);

    if (!reader.TryRead(out byte typeByte) ||
        !reader.TryReadBigEndian(out int len))
    {
        messageType = 0;
        length = 0;
        return false;
    }

    messageType = typeByte;
    length = len;
    buffer = buffer.Slice(reader.Position);
    return true;
}