// Port of Go client_test.go: TestNoClientLeakOnSlowConsumer, TestClientSlowConsumerWithoutConnect
// Reference: golang/nats-server/server/client_test.go lines 2181, 2236

using System.Net;
using System.Net.Sockets;
using System.Text;
using Microsoft.Extensions.Logging.Abstractions;
using NATS.Server;

namespace NATS.Server.Tests;

/// <summary>
/// Tests for slow consumer detection and client cleanup when pending bytes exceed MaxPending.
/// Reference: Go TestNoClientLeakOnSlowConsumer (line 2181) and TestClientSlowConsumerWithoutConnect (line 2236)
/// </summary>
public class ClientSlowConsumerTests
{
    private static int GetFreePort()
    {
        using var sock = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        sock.Bind(new IPEndPoint(IPAddress.Loopback, 0));
        return ((IPEndPoint)sock.LocalEndPoint!).Port;
    }

    private static async Task<string> ReadUntilAsync(Socket sock, string expected, int timeoutMs = 5000)
    {
        using var cts = new CancellationTokenSource(timeoutMs);
        var sb = new StringBuilder();
        var buf = new byte[4096];
        while (!sb.ToString().Contains(expected))
        {
            var n = await sock.ReceiveAsync(buf, SocketFlags.None, cts.Token);
            if (n == 0) break;
            sb.Append(Encoding.ASCII.GetString(buf, 0, n));
        }
        return sb.ToString();
    }

    /// <summary>
    /// Slow_consumer_detected_when_pending_exceeds_limit: Creates a server with a small
    /// MaxPending so that flooding a non-reading subscriber triggers slow consumer detection.
    /// Verifies that SlowConsumers and SlowConsumerClients stats are incremented, and the
    /// slow consumer connection is closed cleanly (no leak).
    ///
    /// Reference: Go TestNoClientLeakOnSlowConsumer (line 2181) and
    ///            TestClientSlowConsumerWithoutConnect (line 2236)
    ///
    /// The Go tests use write deadline manipulation to force a timeout. Here we use a
    /// small MaxPending (1KB) so the outbound buffer overflows quickly when flooded
    /// with 1KB messages.
    /// </summary>
    [Fact]
    public async Task Slow_consumer_detected_when_pending_exceeds_limit()
    {
        // MaxPending set to 1KB — any subscriber that falls more than 1KB behind
        // will be classified as a slow consumer and disconnected.
        const long maxPendingBytes = 1024;
        const int payloadSize = 512; // each message payload
        const int floodCount = 50;   // enough to exceed the 1KB limit

        var port = GetFreePort();
        using var cts = new CancellationTokenSource();
        var server = new NatsServer(
            new NatsOptions
            {
                Port = port,
                MaxPending = maxPendingBytes,
            },
            NullLoggerFactory.Instance);
        _ = server.StartAsync(cts.Token);
        await server.WaitForReadyAsync();

        try
        {
            // Connect the slow subscriber — it will not read any MSG frames
            using var slowSub = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
            await slowSub.ConnectAsync(IPAddress.Loopback, port);

            var buf = new byte[4096];
            await slowSub.ReceiveAsync(buf, SocketFlags.None); // INFO

            // Subscribe to "flood" subject and confirm with PING/PONG
            await slowSub.SendAsync(Encoding.ASCII.GetBytes("CONNECT {\"verbose\":false}\r\nSUB flood 1\r\nPING\r\n"));
            var pong = await ReadUntilAsync(slowSub, "PONG");
            pong.ShouldContain("PONG");

            // Connect the publisher
            using var pub = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
            await pub.ConnectAsync(IPAddress.Loopback, port);
            await pub.ReceiveAsync(buf, SocketFlags.None); // INFO
            await pub.SendAsync(Encoding.ASCII.GetBytes("CONNECT {\"verbose\":false}\r\n"));

            // Flood the slow subscriber with messages — it will not drain
            var payload = new string('X', payloadSize);
            var pubSb = new StringBuilder();
            for (int i = 0; i < floodCount; i++)
            {
                pubSb.Append($"PUB flood {payloadSize}\r\n{payload}\r\n");
            }
            pubSb.Append("PING\r\n");
            await pub.SendAsync(Encoding.ASCII.GetBytes(pubSb.ToString()));

            // Wait for publisher's PONG confirming all publishes were processed
            await ReadUntilAsync(pub, "PONG", timeoutMs: 5000);

            // Give the server time to detect and close the slow consumer
            await Task.Delay(500);

            // Verify slow consumer stats were incremented
            var stats = server.Stats;
            Interlocked.Read(ref stats.SlowConsumers).ShouldBeGreaterThan(0);
            Interlocked.Read(ref stats.SlowConsumerClients).ShouldBeGreaterThan(0);

            // Verify the slow subscriber was disconnected (connection closed by server).
            // Drain the slow subscriber socket until 0 bytes (TCP FIN from server).
            // The server may send a -ERR 'Slow Consumer' before closing, so we read
            // until the connection is terminated.
            slowSub.ReceiveTimeout = 3000;
            int n;
            bool connectionClosed = false;
            try
            {
                while (true)
                {
                    n = slowSub.Receive(buf);
                    if (n == 0)
                    {
                        connectionClosed = true;
                        break;
                    }
                }
            }
            catch (SocketException)
            {
                // Socket was forcibly closed — counts as connection closed
                connectionClosed = true;
            }
            connectionClosed.ShouldBeTrue();

            // Verify the slow subscriber is no longer in the server's client list
            // The server removes the client after detecting the slow consumer condition
            await Task.Delay(300);
            server.ClientCount.ShouldBe(1); // only the publisher remains
        }
        finally
        {
            await cts.CancelAsync();
            server.Dispose();
        }
    }
}