using FluentAssertions;
using R3;
using Xunit;

namespace OECS.Tests;

public class ReactivityTests
{
    private struct Position { public float X; public float Y; }
    private struct Health { public int Value; }
    private struct Frozen { }

    /// <summary>
    /// Helper that collects changes into a list and exposes an R3 Observer.
    /// </summary>
    private sealed class ChangeCollector : IObserver<EntityChange>
    {
        public List<EntityChange> Changes = new();

        public void OnNext(EntityChange value) => Changes.Add(value);
        public void OnError(Exception error) { }
        public void OnCompleted() { }
    }

    [Fact]
    public void EntityCreation_PostsEntityAdded()
    {
        var world = new World();
        var collector = new ChangeCollector();
        using var sub = world.ObserveEntityChanges().Subscribe(collector.ToObserver());

        var entity = world.CreateEntity();
        world.PostChanges();

        collector.Changes.Should().ContainSingle()
            .Which.Should().Match<EntityChange>(c =>
                c.Entity == entity && c.Kind == ChangeKind.EntityAdded);
    }

    [Fact]
    public void EntityDestruction_PostsEntityRemoved()
    {
        var world = new World();
        var entity = world.CreateEntity();
        world.PostChanges(); // Clear pending

        var collector = new ChangeCollector();
        using var sub = world.ObserveEntityChanges().Subscribe(collector.ToObserver());

        world.DestroyEntity(entity);
        world.PostChanges();

        collector.Changes.Should().ContainSingle()
            .Which.Should().Match<EntityChange>(c =>
                c.Entity == entity && c.Kind == ChangeKind.EntityRemoved);
    }

    [Fact]
    public void ComponentAdd_PostsComponentAdded()
    {
        var world = new World();
        var entity = world.CreateEntity();
        world.PostChanges(); // Clear pending

        var collector = new ChangeCollector();
        using var sub = world.ObserveComponentChanges<Position>().Subscribe(collector.ToObserver());

        world.AddComponent(entity, new Position { X = 1, Y = 2 });
        world.PostChanges();

        collector.Changes.Should().ContainSingle()
            .Which.Should().Match<EntityChange>(c =>
                c.Entity == entity &&
                c.Kind == ChangeKind.ComponentAdded &&
                c.ComponentType == typeof(Position));
    }

    [Fact]
    public void ComponentRemove_PostsComponentRemoved()
    {
        var world = new World();
        var entity = world.CreateEntity();
        world.AddComponent(entity, new Position { X = 1, Y = 2 });
        world.PostChanges(); // Clear pending

        var collector = new ChangeCollector();
        using var sub = world.ObserveComponentChanges<Position>().Subscribe(collector.ToObserver());

        world.RemoveComponent<Position>(entity);
        world.PostChanges();

        collector.Changes.Should().ContainSingle()
            .Which.Should().Match<EntityChange>(c =>
                c.Entity == entity &&
                c.Kind == ChangeKind.ComponentRemoved &&
                c.ComponentType == typeof(Position));
    }

    [Fact]
    public void MarkModified_PostsComponentModified()
    {
        var world = new World();
        var entity = world.CreateEntity();
        world.AddComponent(entity, new Position { X = 0, Y = 0 });
        world.PostChanges(); // Clear pending

        var collector = new ChangeCollector();
        using var sub = world.ObserveComponentChanges<Position>().Subscribe(collector.ToObserver());

        world.MarkModified<Position>(entity);
        world.PostChanges();

        collector.Changes.Should().ContainSingle()
            .Which.Should().Match<EntityChange>(c =>
                c.Entity == entity &&
                c.Kind == ChangeKind.ComponentModified &&
                c.ComponentType == typeof(Position));
    }

    [Fact]
    public void Changes_AreBatchedPerPost()
    {
        var world = new World();
        var collector = new ChangeCollector();
        using var sub = world.ObserveEntityChanges().Subscribe(collector.ToObserver());

        var a = world.CreateEntity();
        var b = world.CreateEntity();
        world.AddComponent(a, new Position { X = 1, Y = 2 });
        world.AddComponent(b, new Health { Value = 100 });

        // No Post yet — subscribers should not have received anything.
        collector.Changes.Should().BeEmpty();

        world.PostChanges();

        // All changes arrive in one batch.
        collector.Changes.Should().HaveCount(4);
    }

    [Fact]
    public void Subscribers_ReceiveChangesInOrder()
    {
        var world = new World();
        var collector = new ChangeCollector();
        using var sub = world.ObserveEntityChanges().Subscribe(collector.ToObserver());

        var entity = world.CreateEntity();
        world.AddComponent(entity, new Position { X = 0, Y = 0 });
        world.MarkModified<Position>(entity);
        world.RemoveComponent<Position>(entity);
        world.DestroyEntity(entity);

        world.PostChanges();

        collector.Changes.Select(c => c.Kind).Should().Equal(
            ChangeKind.EntityAdded,
            ChangeKind.ComponentAdded,
            ChangeKind.ComponentModified,
            ChangeKind.ComponentRemoved,
            ChangeKind.EntityRemoved
        );
    }

    [Fact]
    public void DisposingSubscription_StopsNotifications()
    {
        var world = new World();
        var collector = new ChangeCollector();
        var sub = world.ObserveEntityChanges().Subscribe(collector.ToObserver());

        var entity = world.CreateEntity();
        world.PostChanges();
        collector.Changes.Should().HaveCount(1);

        sub.Dispose();

        world.DestroyEntity(entity);
        world.PostChanges();

        // No new changes after dispose.
        collector.Changes.Should().HaveCount(1);
    }

    [Fact]
    public void ObserveComponentChanges_OnlyReceivesMatchingType()
    {
        var world = new World();
        var posCollector = new ChangeCollector();
        var hpCollector = new ChangeCollector();

        using var sub1 = world.ObserveComponentChanges<Position>().Subscribe(posCollector.ToObserver());
        using var sub2 = world.ObserveComponentChanges<Health>().Subscribe(hpCollector.ToObserver());

        var entity = world.CreateEntity();
        world.AddComponent(entity, new Position { X = 1, Y = 2 });
        world.AddComponent(entity, new Health { Value = 100 });
        world.PostChanges();

        posCollector.Changes.Should().HaveCount(1);
        posCollector.Changes[0].ComponentType.Should().Be(typeof(Position));

        hpCollector.Changes.Should().HaveCount(1);
        hpCollector.Changes[0].ComponentType.Should().Be(typeof(Health));
    }

    [Fact]
    public void ObserveQuery_OnlyReceivesMatchingChanges()
    {
        var world = new World();
        var query = world.Query().With<Position>().Without<Frozen>().Build();
        var collector = new ChangeCollector();

        using var sub = world.ObserveQuery(query).Subscribe(collector.ToObserver());

        var entity = world.CreateEntity();
        world.AddComponent(entity, new Position { X = 1, Y = 2 }); // Should match
        world.AddComponent(entity, new Frozen());                    // Should NOT match (not in With)
        world.AddComponent(entity, new Health { Value = 100 });      // Should NOT match (not in With)
        world.PostChanges();

        collector.Changes.Should().HaveCount(1);
        collector.Changes[0].ComponentType.Should().Be(typeof(Position));
    }

    [Fact]
    public void Deduplication_PreventsDuplicateChanges()
    {
        var world = new World();
        var collector = new ChangeCollector();
        using var sub = world.ObserveComponentChanges<Position>().Subscribe(collector.ToObserver());

        var entity = world.CreateEntity();
        world.AddComponent(entity, new Position { X = 0, Y = 0 });
        world.MarkModified<Position>(entity);
        world.MarkModified<Position>(entity); // Duplicate
        world.MarkModified<Position>(entity); // Duplicate
        world.PostChanges();

        // Should have exactly 2 changes: ComponentAdded + ComponentModified (deduplicated).
        collector.Changes.Should().HaveCount(2);
        collector.Changes[0].Kind.Should().Be(ChangeKind.ComponentAdded);
        collector.Changes[1].Kind.Should().Be(ChangeKind.ComponentModified);
    }

    [Fact]
    public void Changes_ArePostedAfterEachSystem_InSystemGroup()
    {
        var world = new World();
        var group = new SystemGroup(world);

        var collector = new ChangeCollector();
        using var sub = world.ObserveEntityChanges().Subscribe(collector.ToObserver());

        // System that creates an entity.
        group.Add(new EntityCreatorSystem(world));

        // Before running, no changes should be posted.
        collector.Changes.Should().BeEmpty();

        group.RunLogical();

        // After the system group runs, changes should be posted.
        collector.Changes.Should().Contain(c => c.Kind == ChangeKind.EntityAdded);
    }

    private class EntityCreatorSystem : ISystem
    {
        private readonly World _world;

        public EntityCreatorSystem(World world)
        {
            _world = world;
        }

        public void Run(World world)
        {
            _world.CreateEntity();
        }
    }
}