using System.Collections.Concurrent;
using Friflo.Engine.ECS;
using MrGameEng.Net;
using Xunit;

namespace MrGameEng.Net.Tests;

public class ReplicationTests
{
    private struct TestPosition : IComponent
    {
        public float X;
        public float Y;
    }

    private struct TestHealth : IComponent
    {
        public int Value;
    }

    private sealed class FakeConnection : INetConnection
    {
        public int Id { get; init; }
        public bool IsOpen { get; set; } = true;
        public readonly ConcurrentQueue<byte[]> Sent = new();

        public void Send(ReadOnlySpan<byte> message) => Sent.Enqueue(message.ToArray());

        public bool TryReceive(out byte[] message) => Sent.TryDequeue(out message!);

        public void Close() => IsOpen = false;
    }

    private static ReplicationSchema MakeSchema() =>
        new ReplicationSchema().Register<TestPosition>().Register<TestHealth>();

    [Fact]
    public void FirstSnapshot_SpawnsEntities_OnTheClient()
    {
        var serverStore = new EntityStore();
        var clientStore = new EntityStore();
        var server = new ReplicationServer(MakeSchema(), serverStore);
        var client = new ReplicationClient(MakeSchema(), clientStore);
        var connection = new FakeConnection();

        serverStore.CreateEntity(
            new NetId { Value = server.NextNetId() },
            new TestPosition { X = 10f, Y = 20f },
            new TestHealth { Value = 7 }
        );
        serverStore.CreateEntity(
            new NetId { Value = server.NextNetId() },
            new TestPosition { X = -3f, Y = 4f }
        );

        server.Send([connection]);
        client.Pump(connection);

        Assert.Equal(2, client.EntityCount);
        var first = FindByNetId(clientStore, 1);
        Assert.Equal(10f, first.GetComponent<TestPosition>().X);
        Assert.Equal(7, first.GetComponent<TestHealth>().Value);
        var second = FindByNetId(clientStore, 2);
        Assert.False(second.HasComponent<TestHealth>());
    }

    [Fact]
    public void UnchangedWorld_SendsNothing()
    {
        var serverStore = new EntityStore();
        var server = new ReplicationServer(MakeSchema(), serverStore);
        var connection = new FakeConnection();
        serverStore.CreateEntity(
            new NetId { Value = server.NextNetId() },
            new TestPosition { X = 1f, Y = 1f }
        );

        server.Send([connection]);
        Assert.Single(connection.Sent);

        server.Send([connection]);
        Assert.Single(connection.Sent); // дельта пустая — второго сообщения нет
    }

    [Fact]
    public void ChangedComponent_IsTheOnlyThingResent()
    {
        var serverStore = new EntityStore();
        var clientStore = new EntityStore();
        var server = new ReplicationServer(MakeSchema(), serverStore);
        var client = new ReplicationClient(MakeSchema(), clientStore);
        var connection = new FakeConnection();

        var entity = serverStore.CreateEntity(
            new NetId { Value = server.NextNetId() },
            new TestPosition { X = 1f, Y = 1f },
            new TestHealth { Value = 100 }
        );
        server.Send([connection]);
        client.Pump(connection);

        entity.AddComponent(new TestPosition { X = 5f, Y = 6f }); // здоровье не трогаем
        server.Send([connection]);

        Assert.True(connection.Sent.TryDequeue(out var delta));
        // Запись: type(1) + version(1) + count(4) + netId(4) + op(1) + mask(4) + TestPosition(8) — без TestHealth.
        Assert.Equal(23, delta!.Length);

        client.Apply(delta);
        var replicated = FindByNetId(clientStore, 1);
        Assert.Equal(5f, replicated.GetComponent<TestPosition>().X);
        Assert.Equal(100, replicated.GetComponent<TestHealth>().Value);
    }

    [Fact]
    public void DeletedEntity_DespawnsOnTheClient()
    {
        var serverStore = new EntityStore();
        var clientStore = new EntityStore();
        var server = new ReplicationServer(MakeSchema(), serverStore);
        var client = new ReplicationClient(MakeSchema(), clientStore);
        var connection = new FakeConnection();

        var entity = serverStore.CreateEntity(
            new NetId { Value = server.NextNetId() },
            new TestPosition { X = 1f, Y = 1f }
        );
        server.Send([connection]);
        client.Pump(connection);
        Assert.Equal(1, client.EntityCount);

        entity.DeleteEntity();
        server.Send([connection]);
        client.Pump(connection);

        Assert.Equal(0, client.EntityCount);
    }

    [Fact]
    public void LateJoiner_GetsFullState()
    {
        var serverStore = new EntityStore();
        var server = new ReplicationServer(MakeSchema(), serverStore);
        var early = new FakeConnection { Id = 1 };
        var late = new FakeConnection { Id = 2 };

        serverStore.CreateEntity(
            new NetId { Value = server.NextNetId() },
            new TestPosition { X = 9f, Y = 9f }
        );
        server.Send([early]);
        server.Send([early, late]); // мир не менялся: early — тишина, late — полный стейт

        Assert.Single(early.Sent);
        Assert.Single(late.Sent);

        var clientStore = new EntityStore();
        var client = new ReplicationClient(MakeSchema(), clientStore);
        client.Pump(late);
        Assert.Equal(1, client.EntityCount);
        Assert.Equal(9f, FindByNetId(clientStore, 1).GetComponent<TestPosition>().X);
    }

    [Fact]
    public void EntitySpawned_FiresOncePerEntity()
    {
        var serverStore = new EntityStore();
        var clientStore = new EntityStore();
        var server = new ReplicationServer(MakeSchema(), serverStore);
        var client = new ReplicationClient(MakeSchema(), clientStore);
        var connection = new FakeConnection();
        var spawns = 0;
        client.EntitySpawned += _ => spawns++;

        var entity = serverStore.CreateEntity(
            new NetId { Value = server.NextNetId() },
            new TestPosition { X = 1f, Y = 1f }
        );
        server.Send([connection]);
        client.Pump(connection);
        entity.AddComponent(new TestPosition { X = 2f, Y = 2f });
        server.Send([connection]);
        client.Pump(connection);

        Assert.Equal(1, spawns);
    }

    [Fact]
    public void Clear_DeletesEveryReplicatedEntity()
    {
        var serverStore = new EntityStore();
        var clientStore = new EntityStore();
        var server = new ReplicationServer(MakeSchema(), serverStore);
        var client = new ReplicationClient(MakeSchema(), clientStore);
        var connection = new FakeConnection();
        serverStore.CreateEntity(
            new NetId { Value = server.NextNetId() },
            new TestPosition { X = 1f, Y = 2f }
        );
        serverStore.CreateEntity(
            new NetId { Value = server.NextNetId() },
            new TestPosition { X = 3f, Y = 4f }
        );
        server.Send([connection]);
        client.Pump(connection);
        Assert.Equal(2, client.EntityCount);

        client.Clear();

        Assert.Equal(0, client.EntityCount);
        foreach (var entity in clientStore.Entities)
        {
            Assert.False(entity.HasComponent<NetId>());
        }
    }

    [Fact]
    public void MismatchedProtocolVersion_IsDropped()
    {
        var serverStore = new EntityStore();
        var clientStore = new EntityStore();
        var server = new ReplicationServer(MakeSchema(), serverStore);
        var client = new ReplicationClient(MakeSchema(), clientStore);
        var connection = new FakeConnection();
        serverStore.CreateEntity(
            new NetId { Value = server.NextNetId() },
            new TestPosition { X = 1f, Y = 2f }
        );
        server.Send([connection]);
        Assert.True(connection.Sent.TryDequeue(out var snapshot));

        // Портим байт версии (индекс 1: type=0, version=1) — клиент обязан отбросить снапшот целиком.
        snapshot![1] = 0xFF;
        client.Apply(snapshot);

        Assert.Equal(0, client.EntityCount);
    }

    [Fact]
    public void TruncatedSnapshot_IsIgnoredWithoutThrowing()
    {
        var serverStore = new EntityStore();
        var clientStore = new EntityStore();
        var server = new ReplicationServer(MakeSchema(), serverStore);
        var client = new ReplicationClient(MakeSchema(), clientStore);
        var connection = new FakeConnection();
        serverStore.CreateEntity(
            new NetId { Value = server.NextNetId() },
            new TestPosition { X = 5f, Y = 6f },
            new TestHealth { Value = 9 }
        );
        server.Send([connection]);
        Assert.True(connection.Sent.TryDequeue(out var snapshot));

        // Режем хвост: заголовок и счётчик целы, но данные компонентов оборваны.
        var truncated = snapshot!.AsSpan(0, snapshot.Length - 6).ToArray();
        client.Apply(truncated); // не должно бросить

        // Записи могли частично примениться, но клиент остался живым и консистентным.
        Assert.True(client.EntityCount <= 1);
    }

    private static Entity FindByNetId(EntityStore store, int netId)
    {
        foreach (var entity in store.Entities)
        {
            if (entity.HasComponent<NetId>() && entity.GetComponent<NetId>().Value == netId)
            {
                return entity;
            }
        }

        throw new InvalidOperationException($"Entity with NetId {netId} not found.");
    }
}