// Reference: golang/nats-server/server/route.go:3085 — removeAllRoutesExcept
// Reference: golang/nats-server/server/route.go:3113 — removeRoute
// Tests for cluster split detection and route partition handling.

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

namespace NATS.Server.Clustering.Tests.Routes;

/// <summary>
/// Tests for <see cref="RouteManager.RemoveRoute"/>,
/// <see cref="RouteManager.RemoveAllRoutesExcept"/>, and
/// <see cref="RouteManager.DetectClusterSplit"/>.
/// Go reference: server/route.go removeRoute (3113), removeAllRoutesExcept (3085).
/// </summary>
public class ClusterSplitTests
{
    // -- Helpers --

    private static RouteManager CreateManager(string serverId = "local-server")
        => new(
            new ClusterOptions { Host = "127.0.0.1", Port = 0 },
            new ServerStats(),
            serverId,
            _ => { },
            _ => { },
            NullLogger<RouteManager>.Instance);

    /// <summary>
    /// Creates a connected socket pair (listener + client) so that
    /// <see cref="RouteConnection"/> can build its internal <see cref="NetworkStream"/>.
    /// Returns both the RouteConnection (which owns the server-side socket) and the
    /// client-side socket (kept alive for the duration of the test).
    /// </summary>
    private static (RouteConnection Route, Socket ClientSocket) CreateConnectedRoute()
    {
        var listener = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        listener.Bind(new IPEndPoint(IPAddress.Loopback, 0));
        listener.Listen(1);
        var port = ((IPEndPoint)listener.LocalEndPoint!).Port;

        var client = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        client.Connect(IPAddress.Loopback, port);
        var server = listener.Accept();
        listener.Dispose();

        return (new RouteConnection(server), client);
    }

    // -- RemoveRoute tests --

    [Fact]
    public void RemoveRoute_ExistingRoute_ReturnsTrue()
    {
        // Go ref: route.go removeRoute — returns true when a route was found.
        var manager = CreateManager();
        var (conn, clientSocket) = CreateConnectedRoute();

        try
        {
            manager.RegisterRoute("server-A", conn);
            manager.RouteCount.ShouldBe(1);

            var removed = manager.RemoveRoute("server-A");

            removed.ShouldBeTrue();
            manager.RouteCount.ShouldBe(0);
        }
        finally
        {
            clientSocket.Dispose();
        }
    }

    [Fact]
    public void RemoveRoute_NonExistent_ReturnsFalse()
    {
        // Go ref: route.go removeRoute — returns false when server ID is unknown.
        var manager = CreateManager();

        var removed = manager.RemoveRoute("no-such-server");

        removed.ShouldBeFalse();
        manager.RouteCount.ShouldBe(0);
    }

    [Fact]
    public void RemoveRoute_RemovesFromConnectedIds()
    {
        // Go ref: route.go removeRoute — must update the connectedServerIds set
        // so topology snapshots no longer list the removed peer.
        var manager = CreateManager();
        var (conn, clientSocket) = CreateConnectedRoute();

        try
        {
            manager.RegisterRoute("server-B", conn);

            var snapshotBefore = manager.BuildTopologySnapshot();
            snapshotBefore.ConnectedServerIds.ShouldContain("server-B");

            manager.RemoveRoute("server-B");

            var snapshotAfter = manager.BuildTopologySnapshot();
            snapshotAfter.ConnectedServerIds.ShouldNotContain("server-B");
        }
        finally
        {
            clientSocket.Dispose();
        }
    }

    // -- RemoveAllRoutesExcept tests --

    [Fact]
    public void RemoveAllRoutesExcept_KeepsSpecified()
    {
        // Go ref: route.go removeAllRoutesExcept — only removes routes whose
        // server ID is not in the keep set.
        var manager = CreateManager();
        var (connA, clientA) = CreateConnectedRoute();
        var (connB, clientB) = CreateConnectedRoute();
        var (connC, clientC) = CreateConnectedRoute();

        try
        {
            manager.RegisterRoute("server-A", connA);
            manager.RegisterRoute("server-B", connB);
            manager.RegisterRoute("server-C", connC);
            manager.RouteCount.ShouldBe(3);

            var removed = manager.RemoveAllRoutesExcept(new HashSet<string> { "server-A", "server-B" });

            removed.ShouldBe(1);
            manager.RouteCount.ShouldBe(2);

            var snapshot = manager.BuildTopologySnapshot();
            snapshot.ConnectedServerIds.ShouldContain("server-A");
            snapshot.ConnectedServerIds.ShouldContain("server-B");
            snapshot.ConnectedServerIds.ShouldNotContain("server-C");
        }
        finally
        {
            clientA.Dispose();
            clientB.Dispose();
            clientC.Dispose();
        }
    }

    [Fact]
    public void RemoveAllRoutesExcept_RemovesAll_WhenEmptyKeepSet()
    {
        // Go ref: route.go removeAllRoutesExcept — empty keep set removes every route.
        var manager = CreateManager();
        var (connA, clientA) = CreateConnectedRoute();
        var (connB, clientB) = CreateConnectedRoute();

        try
        {
            manager.RegisterRoute("server-A", connA);
            manager.RegisterRoute("server-B", connB);
            manager.RouteCount.ShouldBe(2);

            var removed = manager.RemoveAllRoutesExcept(new HashSet<string>());

            removed.ShouldBe(2);
            manager.RouteCount.ShouldBe(0);
        }
        finally
        {
            clientA.Dispose();
            clientB.Dispose();
        }
    }

    [Fact]
    public void RemoveAllRoutesExcept_NoRoutes_ReturnsZero()
    {
        // Go ref: route.go removeAllRoutesExcept — no-op when no routes exist.
        var manager = CreateManager();

        var removed = manager.RemoveAllRoutesExcept(new HashSet<string> { "server-A" });

        removed.ShouldBe(0);
        manager.RouteCount.ShouldBe(0);
    }

    // -- DetectClusterSplit tests --

    [Fact]
    public void DetectClusterSplit_AllPresent_NotSplit()
    {
        // Go ref: cluster split detection — no missing peers means no partition.
        var manager = CreateManager();
        var (connA, clientA) = CreateConnectedRoute();
        var (connB, clientB) = CreateConnectedRoute();

        try
        {
            manager.RegisterRoute("peer-1", connA);
            manager.RegisterRoute("peer-2", connB);

            var result = manager.DetectClusterSplit(new HashSet<string> { "peer-1", "peer-2" });

            result.IsSplit.ShouldBeFalse();
            result.MissingPeers.ShouldBeEmpty();
            result.UnexpectedPeers.ShouldBeEmpty();
        }
        finally
        {
            clientA.Dispose();
            clientB.Dispose();
        }
    }

    [Fact]
    public void DetectClusterSplit_MissingPeers_IsSplit()
    {
        // Go ref: cluster split detection — missing expected peers indicates partition.
        var manager = CreateManager();
        var (connA, clientA) = CreateConnectedRoute();

        try
        {
            manager.RegisterRoute("peer-1", connA);
            // peer-2 and peer-3 are expected but not connected.

            var result = manager.DetectClusterSplit(new HashSet<string> { "peer-1", "peer-2", "peer-3" });

            result.IsSplit.ShouldBeTrue();
            result.MissingPeers.Count.ShouldBe(2);
            result.MissingPeers.ShouldContain("peer-2");
            result.MissingPeers.ShouldContain("peer-3");
            result.UnexpectedPeers.ShouldBeEmpty();
        }
        finally
        {
            clientA.Dispose();
        }
    }

    [Fact]
    public void DetectClusterSplit_UnexpectedPeers_Listed()
    {
        // Go ref: unexpected connected peers are enumerated (extras don't cause IsSplit).
        var manager = CreateManager();
        var (connA, clientA) = CreateConnectedRoute();
        var (connX, clientX) = CreateConnectedRoute();

        try
        {
            manager.RegisterRoute("peer-1", connA);
            manager.RegisterRoute("unexpected-peer", connX);

            var result = manager.DetectClusterSplit(new HashSet<string> { "peer-1" });

            result.IsSplit.ShouldBeFalse();
            result.MissingPeers.ShouldBeEmpty();
            result.UnexpectedPeers.Count.ShouldBe(1);
            result.UnexpectedPeers.ShouldContain("unexpected-peer");
        }
        finally
        {
            clientA.Dispose();
            clientX.Dispose();
        }
    }

    [Fact]
    public void DetectClusterSplit_NoPeers_NoExpected_NotSplit()
    {
        // Go ref: cluster split detection — empty state with no expectations is healthy.
        var manager = CreateManager();

        var result = manager.DetectClusterSplit(new HashSet<string>());

        result.IsSplit.ShouldBeFalse();
        result.MissingPeers.ShouldBeEmpty();
        result.UnexpectedPeers.ShouldBeEmpty();
    }
}