using System.Buffers.Binary;
using System.Net;
using AcDream.Core.Net.Messages;
using AcDream.Core.Net.Packets;

namespace AcDream.Core.Net.Tests.Transport;

/// <summary>
/// An <see cref="IWorldSessionTransport"/> that binds a REAL
/// <see cref="WorldSession"/> to an <see cref="AceSessionModel"/> through a
/// <see cref="LossyLink"/> — no sockets anywhere. Outbound sends run through
/// the link into the model; the model's emitted datagrams run through the
/// link into the queue that <c>Receive</c>/<c>ReceiveAsync</c> serve.
///
/// <para>
/// The handshake is scripted against the model's events so a test can run a
/// genuine <c>Connect()</c> / <c>EnterWorld()</c> / <c>Tick()</c> /
/// <c>Dispose()</c> lifecycle:
/// <list type="number">
///   <item>LoginRequest → the model answers with a ConnectRequest carrying
///         the 32-byte optional (server time, cookie, client id, both ISAAC
///         seeds) — the same layout the existing negotiation fixture
///         (<c>WorldSessionNegotiationShutdownTests.BuildConnectRequest</c>)
///         pins.</item>
///   <item>ConnectResponse (cookie match) → the model enqueues a
///         CharacterList (0xF658) with one selectable character.</item>
///   <item>CharacterEnterWorldRequest (0xF7C8) → ServerReady (0xF7DF).</item>
///   <item>CharacterLogOff (0xF653 request) → the opcode-only 0xF653
///         confirmation, so <c>Dispose()</c> completes its retail graceful
///         logout instead of burning the 35 s confirmation timeout.</item>
/// </list>
/// Because the model seeds its C2S verifier and S2C keystream from the same
/// seeds it hands out in the ConnectRequest, post-handshake encrypted
/// traffic verifies in both directions.
/// </para>
///
/// <para>
/// Thread-safety: the model is single-threaded, so every model interaction
/// happens under one lock. <c>ReceiveAsync</c> (the session's background
/// receive loop) waits on a semaphore counting queued deliverables.
/// </para>
/// </summary>
internal sealed class FakeAceTransport : IWorldSessionTransport
{
    public const uint DefaultClientSeed = 0x2B6D6F87u;
    public const uint DefaultServerSeed = 0x9A3C51E4u;
    public const uint DefaultClientId = 0x1234u;
    public const ulong DefaultCookie = 0xFEEDFACECAFEBABEUL;
    public const uint DefaultCharacterId = 0x50000001u;
    public const string DefaultCharacterName = "+Acdream";
    public const string DefaultAccountName = "testaccount";

    private readonly object _gate = new();
    private readonly SemaphoreSlim _deliverable = new(0);
    private readonly Queue<byte[]> _toClient = new();
    private readonly IPEndPoint _serverEndpoint = new(IPAddress.Loopback, 9000);

    public VirtualClock Clock { get; }
    public LossyLink Link { get; }
    public AceSessionModel Model { get; }

    /// <summary>
    /// N5: virtual-clock advance applied at the top of every BLOCKING
    /// <see cref="Receive"/> call — the session-thread Connect()/EnterWorld()
    /// pump path only; the async in-world receive owner never touches the
    /// clock. The lossy-decorator lifecycle test needs time to move during
    /// the blocking handshake pumps: with the clock frozen there, a dropped
    /// handshake-window datagram could never be NAK-healed (the 0.6 s gate
    /// never opens and the model never emits a later sequenced packet to
    /// expose the gap) — a fixture artifact, not a transport property.
    /// Zero (the default) preserves the pre-N5 fixture behavior exactly.
    /// Single-threaded by construction: blocking receives happen on the same
    /// thread that owns the clock in every test that sets this.
    /// </summary>
    public TimeSpan AutoAdvanceOnBlockingReceive { get; set; }

    public FakeAceTransport(VirtualClock? clock = null, LossyLink? link = null)
    {
        Clock = clock ?? new VirtualClock();
        Link = link ?? new LossyLink();
        Model = new AceSessionModel(
            Clock,
            DefaultClientSeed,
            DefaultServerSeed,
            DefaultClientId,
            DefaultCookie);

        Model.LoginRequestReceived += () => Model.SendConnectRequest();
        Model.ConnectResponseAccepted += () =>
            Model.EnqueueGameMessage(BuildCharacterListBody(), GameMessageGroup.UIQueue);
        Model.MessageDispatched += OnClientMessage;
    }

    private void OnClientMessage(byte[] body)
    {
        if (body.Length < 4)
            return;
        uint opcode = BinaryPrimitives.ReadUInt32LittleEndian(body);
        switch (opcode)
        {
            case CharacterEnterWorld.EnterWorldRequestOpcode: // 0xF7C8
                // Server replies CharacterEnterWorldServerReady (0xF7DF) —
                // WorldSession.EnterWorld blocks on this opcode.
                Model.EnqueueGameMessage(BuildOpcodeOnlyBody(0xF7DFu), GameMessageGroup.UIQueue);
                break;
            case CharacterLogOff.Opcode: // 0xF653 request (opcode + character id)
                // ACE echoes the opcode-only confirmation; WorldSession.Dispose
                // waits for it before sending the transport Disconnect.
                Model.EnqueueGameMessage(BuildOpcodeOnlyBody(CharacterLogOff.Opcode), GameMessageGroup.UIQueue);
                break;
        }
    }

    // ---- IWorldSessionTransport ----

    public void Send(ReadOnlySpan<byte> datagram) => SendCore(datagram);

    // WorldSession sends the ConnectResponse to port+1; the double serves
    // both listeners from one model, like ACE's single-process server.
    public void Send(IPEndPoint remote, ReadOnlySpan<byte> datagram) => SendCore(datagram);

    private void SendCore(ReadOnlySpan<byte> datagram)
    {
        lock (_gate)
        {
            foreach (byte[] delivered in Link.Transmit(LinkDirection.ClientToServer, datagram))
                Model.Receive(delivered);
            PumpServerLocked();
        }
    }

    /// <summary>
    /// Run one server frame (model Update + S2C link delivery) without any
    /// client traffic — the hook tests use after advancing the clock or
    /// enqueuing server-side messages.
    /// </summary>
    public void PumpServer()
    {
        lock (_gate)
        {
            PumpServerLocked();
        }
    }

    /// <summary>
    /// N2 test hook: deliver raw bytes straight into the client's receive
    /// queue, bypassing both the model and the link. Used for late
    /// byte-identical redelivery of a dropped S2C datagram (ACE's cached
    /// packet shape), duplicate injections, and crafted sequence-0 control
    /// packets.
    /// </summary>
    public void InjectServerDatagram(byte[] datagram)
    {
        lock (_gate)
        {
            _toClient.Enqueue((byte[])datagram.Clone());
            _deliverable.Release();
        }
    }

    private void PumpServerLocked()
    {
        Model.Update();
        foreach (byte[] outbound in Model.TakePendingDatagrams())
        {
            foreach (byte[] delivered in Link.Transmit(LinkDirection.ServerToClient, outbound))
            {
                _toClient.Enqueue(delivered);
                _deliverable.Release();
            }
        }
    }

    public int Receive(Span<byte> destination, TimeSpan timeout, out IPEndPoint? from)
    {
        if (AutoAdvanceOnBlockingReceive > TimeSpan.Zero)
            Clock.Advance(AutoAdvanceOnBlockingReceive);
        lock (_gate)
        {
            PumpServerLocked();
        }

        if (timeout < TimeSpan.Zero)
            timeout = TimeSpan.Zero;
        if (!_deliverable.Wait(timeout))
        {
            from = null;
            return -1; // NetClient.Receive's timeout contract
        }

        from = _serverEndpoint;
        lock (_gate)
        {
            byte[] datagram = _toClient.Dequeue();
            datagram.CopyTo(destination);
            return datagram.Length;
        }
    }

    public async ValueTask<NetReceiveResult> ReceiveAsync(
        Memory<byte> destination,
        CancellationToken cancellationToken)
    {
        await _deliverable.WaitAsync(cancellationToken).ConfigureAwait(false);
        lock (_gate)
        {
            byte[] datagram = _toClient.Dequeue();
            datagram.CopyTo(destination);
            return new NetReceiveResult(datagram.Length, _serverEndpoint);
        }
    }

    public void Dispose()
    {
        // WorldSession disposes the transport only after cancelling and
        // joining its receive task, so no waiter can be parked on the
        // semaphore here. SemaphoreSlim without AvailableWaitHandle holds no
        // unmanaged state — deliberately left to the GC to keep a hypothetical
        // late waiter from hitting ObjectDisposedException.
    }

    // ---- scripted server content ----

    /// <summary>
    /// Minimal CharacterList (0xF658) matching <c>CharacterList.Parse</c>:
    /// status, active characters, deleted characters, slot count, account,
    /// turbine chat, ToD flag.
    /// </summary>
    private static byte[] BuildCharacterListBody()
    {
        var writer = new PacketWriter(96);
        writer.WriteUInt32(CharacterList.Opcode);
        writer.WriteUInt32(0);                    // status
        writer.WriteUInt32(1);                    // active count
        writer.WriteUInt32(DefaultCharacterId);
        writer.WriteString16L(DefaultCharacterName);
        writer.WriteUInt32(0);                    // secondsGreyedOut
        writer.WriteUInt32(0);                    // deleted count
        writer.WriteUInt32(11);                   // slot count
        writer.WriteString16L(DefaultAccountName);
        writer.WriteUInt32(1);                    // useTurbineChat
        writer.WriteUInt32(1);                    // hasThroneOfDestiny
        return writer.ToArray();
    }

    private static byte[] BuildOpcodeOnlyBody(uint opcode)
    {
        byte[] body = new byte[4];
        BinaryPrimitives.WriteUInt32LittleEndian(body, opcode);
        return body;
    }
}