#nullable enable
using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Net.Sockets;
using System.Text;
using System.Threading.Tasks;
using GameCult.Logging;
using MessagePack;
using NUnit.Framework;

namespace GameCult.Networking.Tests
{
    /// <summary>
    /// A schema server's poll serves every peer whatever one peer sends. A message that cannot be
    /// decoded or handled is that peer's failure: its session ends, the drop is counted and logged,
    /// and the drain goes on to the other peers.
    /// </summary>
    public class CultNetRudpSchemaServerPeerInputTests
    {
        private const uint ConnectionId = 0x10203096;
        private const string Canary = "canary-5f1c9e0b";

        private sealed class Client
        {
            public Socket Socket = null!;
            public CultNetRudpSession Session = null!;
            public EndPoint EndPoint => Socket.LocalEndPoint!;
        }

        private sealed class CapturingLogger : ILogger
        {
            public List<string> Errors { get; } = new();
            public void LogInfo(string message) { }
            public void LogWarning(string message) { }
            public void LogError(string message) => Errors.Add(message);
            public void LogDebug(string message) { }
        }

        private static Socket Bind()
        {
            var socket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
            socket.Bind(new IPEndPoint(IPAddress.Loopback, 0));
            return socket;
        }

        private static List<CultNetRudpPacket> Drain(Socket socket)
        {
            var packets = new List<CultNetRudpPacket>();
            var buffer = new byte[65535];
            EndPoint from = new IPEndPoint(IPAddress.Any, 0);
            while (socket.Poll(20_000, SelectMode.SelectRead))
            {
                var received = socket.ReceiveFrom(buffer, ref from);
                packets.Add(CultNetRudpPacketCodec.Decode(buffer.AsSpan(0, received).ToArray()));
            }
            return packets;
        }

        private static async Task<Client> Connect(RudpCultNetSchemaServer server)
        {
            var client = new Client
            {
                Socket = Bind(),
                Session = new CultNetRudpSession(new CultNetRudpSessionOptions { ConnectionId = ConnectionId })
            };
            client.Socket.SendTo(CultNetRudpPacketCodec.Encode(client.Session.CreateConnect(0)), server.LocalEndPoint);
            await server.PollAvailableAsync(8);
            client.Session.Receive(Drain(client.Socket).Single(p => p.PacketType == CultNetRudpPacketType.Accept), 0);
            return client;
        }

        private static void SendSchema(RudpCultNetSchemaServer server, Client client, byte[] payload) =>
            client.Socket.SendTo(
                CultNetRudpPacketCodec.Encode(client.Session.Send("schema", payload, new CultNetRudpSendOptions { Reliable = true, Ordered = true })),
                server.LocalEndPoint);

        private static byte[] Malformed() => new byte[] { 0xc1 };

        // Well-formed MessagePack naming a schema version no runtime knows. The decoder's own error
        // quotes that version.
        private static byte[] UnknownSchemaVersion() =>
            MessagePackSerializer.Serialize(
                new Dictionary<string, object> { ["schemaVersion"] = "cultnet." + Canary + ".v0", ["messageId"] = Canary },
                CultNetSchemaMessageSerialization.Options);

        private sealed class Scene : IDisposable
        {
            public RudpCultNetSchemaServer Server = null!;
            public Client Bad = null!;
            public Client Good = null!;
            public CapturingLogger Logger = new();
            public List<(string MessageId, EndPoint From)> Served = new();
            public List<EndPoint> Ended = new();

            public void Dispose()
            {
                Server.Dispose();
                Bad.Socket.Dispose();
                Good.Socket.Dispose();
            }
        }

        // The bad peer's frame is read first, so the drain reaches the good peer only by getting past it.
        private static async Task<Scene> BadThenGood(byte[] badPayload)
        {
            var scene = new Scene
            {
                Server = new RudpCultNetSchemaServer(new RudpCultNetSchemaServerOptions
                {
                    RuntimeId = "csharp-peer-input",
                    ConnectionId = ConnectionId
                })
            };
            scene.Server.Logger = scene.Logger;
            scene.Server.OnCultNet<CultNetShardCatalogRequestMessage>((message, peer) =>
                scene.Served.Add((message.MessageId, peer.RemoteEndPoint)));
            scene.Server.PeerDisconnected += peer => scene.Ended.Add(((RudpCultNetSchemaServerPeer)peer).RemoteEndPoint);
            scene.Bad = await Connect(scene.Server);
            scene.Good = await Connect(scene.Server);

            SendSchema(scene.Server, scene.Bad, badPayload);
            SendSchema(scene.Server, scene.Good, CultNetSchemaMessageSerialization.Serialize(
                new CultNetShardCatalogRequestMessage { MessageId = "good-request" }));
            return scene;
        }

        [Test]
        public async Task AMalformedPayloadDoesNotStopTheDrainServingAnotherPeer()
        {
            using var scene = await BadThenGood(Malformed());

            Assert.DoesNotThrowAsync(async () => await scene.Server.PollAvailableAsync(16));

            Assert.That(scene.Served, Is.EqualTo(new[] { ("good-request", scene.Good.EndPoint) }));
        }

        [Test]
        public async Task AnUnknownSchemaVersionDoesNotStopTheDrainServingAnotherPeer()
        {
            using var scene = await BadThenGood(UnknownSchemaVersion());

            Assert.DoesNotThrowAsync(async () => await scene.Server.PollAvailableAsync(16));

            Assert.That(scene.Served, Is.EqualTo(new[] { ("good-request", scene.Good.EndPoint) }));
        }

        [Test]
        public async Task PollOnceDoesNotThrowForAPeersUndecodableMessage()
        {
            using var scene = await BadThenGood(Malformed());

            Assert.DoesNotThrowAsync(async () =>
            {
                for (var attempt = 0; attempt < 16; attempt++)
                    await scene.Server.PollOnceAsync();
            });

            Assert.That(scene.Served, Is.EqualTo(new[] { ("good-request", scene.Good.EndPoint) }));
        }

        [Test]
        public async Task APeerWhoseMessageCannotBeDecodedHasItsSessionEnded()
        {
            using var scene = await BadThenGood(Malformed());
            var dropped = scene.Server.Stats.PacketsDropped;

            try { await scene.Server.PollAvailableAsync(16); }
            catch (MessagePackSerializationException) { Assert.Fail("the poll threw for a peer's input"); }

            // The frame was already acknowledged, so the session cannot be kept: the peer is told,
            // the listener forgets it, and the drop is counted.
            Assert.That(scene.Ended, Is.EqualTo(new[] { scene.Bad.EndPoint }));
            Assert.That(scene.Server.Peers.Select(peer => peer.RemoteEndPoint), Is.EqualTo(new[] { scene.Good.EndPoint }));
            Assert.That(scene.Server.Stats.PacketsDropped, Is.EqualTo(dropped + 1));
            var goodbye = Drain(scene.Bad.Socket).Single(packet => packet.PacketType == CultNetRudpPacketType.Disconnect);
            Assert.That(scene.Bad.Session.Receive(goodbye, 1).Disconnected, Is.True);
            Assert.That(Drain(scene.Good.Socket).Select(packet => packet.PacketType), Has.None.EqualTo(CultNetRudpPacketType.Disconnect));
            Assert.That(scene.Logger.Errors, Has.Count.EqualTo(1));
        }

        // The peer's malformed message and a well-formed one after it arrive in one datagram's
        // delivery: the second is held for order, and both are released together. The session ends
        // on the first, so the second belongs to no session and is never handled.
        [Test]
        public async Task ARefusedPeersFramesStillQueuedAreNotHandled()
        {
            var logger = new CapturingLogger();
            using var server = new RudpCultNetSchemaServer(new RudpCultNetSchemaServerOptions
            {
                RuntimeId = "csharp-peer-input",
                ConnectionId = ConnectionId
            });
            server.Logger = logger;
            var served = new List<string>();
            server.OnCultNet<CultNetShardCatalogRequestMessage>((message, _) => served.Add(message.MessageId));
            var bad = await Connect(server);
            var options = new CultNetRudpSendOptions { Reliable = true, Ordered = true };
            var malformed = bad.Session.Send("schema", Malformed(), options);
            var after = bad.Session.Send("schema", CultNetSchemaMessageSerialization.Serialize(
                new CultNetShardCatalogRequestMessage { MessageId = "after-the-refused-one" }), options);
            bad.Socket.SendTo(CultNetRudpPacketCodec.Encode(after), server.LocalEndPoint);
            bad.Socket.SendTo(CultNetRudpPacketCodec.Encode(malformed), server.LocalEndPoint);

            await server.PollAvailableAsync(16);

            Assert.That(served, Is.Empty);
            Assert.That(server.Peers, Is.Empty);
            Assert.That(logger.Errors, Has.Count.EqualTo(1));
            bad.Socket.Dispose();
        }

        private static async Task<(RudpCultNetSchemaServer Server, Client Client, CapturingLogger Logger)> ServerWhoseHandlerAnswers(
            byte[] response, int? maxFragmentBytes, int? unsendableAfter = null)
        {
            var logger = new CapturingLogger();
            var server = new RudpCultNetSchemaServer(new RudpCultNetSchemaServerOptions
            {
                RuntimeId = "csharp-peer-input",
                ConnectionId = ConnectionId,
                MaxFragmentBytes = maxFragmentBytes
            });
            server.Logger = logger;
            server.OnCultNet<CultNetShardCatalogRequestMessage>((_, peer) =>
            {
                if (unsendableAfter is { } count)
                    server.Transport.UnsendableAfter[peer.RemoteEndPoint] = count;
                server.Transport.SendSchema(peer.TransportPeer, response);
            });
            var client = await Connect(server);
            SendSchema(server, client, CultNetSchemaMessageSerialization.Serialize(
                new CultNetShardCatalogRequestMessage { MessageId = "ask" }));
            return (server, client, logger);
        }

        // A handler whose own response can never be sent (larger than a datagram, unfragmented) fails
        // on the server's account, not the peer's. The session still ends, but its goodbye does not say
        // the peer's packet was refused.
        [Test]
        public async Task AHandlerFaultEndsTheSessionWithAReasonThatDoesNotBlameThePeer()
        {
            var (server, client, logger) = await ServerWhoseHandlerAnswers(new byte[70_000], maxFragmentBytes: null);
            using (server)
            {
                await server.PollAvailableAsync(16);

                var goodbye = Drain(client.Socket).Single(packet => packet.PacketType == CultNetRudpPacketType.Disconnect);
                Assert.That(Encoding.UTF8.GetString(goodbye.Payload!), Is.EqualTo("server could not handle a message"));
                Assert.That(server.Peers, Is.Empty);
                Assert.That(logger.Errors, Has.Count.EqualTo(1));
                Assert.That(logger.Errors[0], Does.Contain("handler failed (SocketException)"));
                Assert.That(logger.Errors[0], Does.Contain("that peer's session ended."));
            }
            client.Socket.Dispose();
        }

        // A response that fails after its first fragment has already ended the session over the send,
        // with a goodbye that names the send failure. The backstop ends nothing more and says so.
        [Test]
        public async Task AHandlerWhoseResponseEndedTheSessionIsLoggedAsAlreadyEnded()
        {
            var (server, client, logger) = await ServerWhoseHandlerAnswers(new byte[3000], maxFragmentBytes: 1000, unsendableAfter: 1);
            using (server)
            {
                await server.PollAvailableAsync(16);
                await server.PollAvailableAsync(16);

                var goodbyes = Drain(client.Socket).Where(packet => packet.PacketType == CultNetRudpPacketType.Disconnect).ToList();
                Assert.That(goodbyes, Has.Count.EqualTo(1));
                Assert.That(Encoding.UTF8.GetString(goodbyes[0].Payload!), Does.StartWith("packet could not be sent: "));
                Assert.That(server.Peers, Is.Empty);
                Assert.That(logger.Errors, Has.Count.EqualTo(1));
                Assert.That(logger.Errors[0], Does.Contain("that peer's session had already ended."));
            }
            client.Socket.Dispose();
        }

        [Test]
        public async Task TheDropIsLoggedByErrorTypeWithoutEchoingWhatThePeerSent()
        {
            using var scene = await BadThenGood(UnknownSchemaVersion());

            try { await scene.Server.PollAvailableAsync(16); }
            catch (MessagePackSerializationException) { Assert.Fail("the poll threw for a peer's input"); }

            // The decoder's own error message quotes the unknown version, which is the peer's input.
            Assert.That(scene.Logger.Errors, Has.Count.EqualTo(1));
            Assert.That(scene.Logger.Errors[0], Does.Contain(nameof(MessagePackSerializationException)));
            Assert.That(scene.Logger.Errors[0], Does.Not.Contain(Canary));
        }
    }
}