using FluentAssertions;
using R3;
using Xunit;

namespace OECS.Tests;

/// <summary>
/// Tests for edge cases and previously uncovered code paths.
/// </summary>
public class EdgeCaseTests
{
    // ── Test components ──────────────────────────────────────────────

    private struct Position { public float X; public float Y; }
    private struct Velocity { public float X; public float Y; }
    private struct Health { public int Value; }
    private struct Frozen { }
    private struct Burning { }
    private struct Poisoned { }
    private struct Cursed { }

    // ── SparseSet resize correctness ─────────────────────────────────

    [Fact]
    public void SparseSet_Resize_InitializesNewSlotsToMinusOne()
    {
        // This test verifies that when the sparse array grows, new slots
        // are filled with -1 (not left as default 0, which would falsely
        // indicate the entity has a component).
        var world = new World();

        // Create many entities to force sparse array resizes.
        var entities = new List<Entity>();
        for (int i = 0; i < 2000; i++)
        {
            entities.Add(world.CreateEntity());
        }

        // Add Position to every other entity.
        for (int i = 0; i < entities.Count; i += 2)
        {
            world.AddComponent(entities[i], new Position { X = i, Y = i });
        }

        // Entities without Position should correctly report false.
        for (int i = 1; i < entities.Count; i += 2)
        {
            world.HasComponent<Position>(entities[i]).Should().BeFalse(
                $"entity {entities[i]} should not have Position");
        }
    }

    [Fact]
    public void SparseSet_Resize_PreservesExistingData()
    {
        var world = new World();

        // Create entities before any component adds to trigger resizes
        // after data is already present.
        var first = world.CreateEntity();
        world.AddComponent(first, new Position { X = 1, Y = 2 });

        // Create many more to force resize.
        for (int i = 0; i < 2000; i++)
        {
            var e = world.CreateEntity();
            world.AddComponent(e, new Position { X = i, Y = i });
        }

        // Original entity should still have its data.
        ref var pos = ref world.GetComponent<Position>(first);
        pos.X.Should().Be(1);
        pos.Y.Should().Be(2);
    }

    // ── Entity version wrapping ──────────────────────────────────────

    [Fact]
    public void Entity_VersionWraps_From255To1()
    {
        var world = new World();
        var original = world.CreateEntity();
        uint id = original.Id;

        // Destroy and recreate the entity 255 times to wrap the version.
        for (int i = 0; i < 255; i++)
        {
            world.DestroyEntity(original);
            original = world.CreateEntity();
            original.Id.Should().Be(id);
        }

        // After 255 recycles, version should have wrapped from 255 to 1.
        original.Version.Should().Be(1);
        world.IsAlive(original).Should().BeTrue();
    }

    // ── DestroyEntity with multiple incoming relationships ───────────

    [Fact]
    public void DestroyEntity_WithMultipleIncomingRelationships_DoesNotThrow()
    {
        var world = new World();
        var target = world.CreateEntity();

        // Create multiple sources pointing to the same target.
        var sources = new List<Entity>();
        for (int i = 0; i < 10; i++)
        {
            var source = world.CreateEntity();
            sources.Add(source);
            world.AddComponent(source, new Relationship<ChildOf, ParentOf>
            {
                Source = source,
                Target = target
            });
        }

        // Destroying the target should cascade-remove all incoming
        // relationships without throwing.
        var act = () => world.DestroyEntity(target);
        act.Should().NotThrow();

        // All source entities should have had their relationship removed.
        foreach (var source in sources)
        {
            world.HasComponent<Relationship<ChildOf, ParentOf>>(source).Should().BeFalse();
        }
    }

    // ── Component removal change tracking on destroy ─────────────────

    [Fact]
    public void DestroyEntity_PostsComponentRemoved_ForEachComponent()
    {
        var world = new World();
        var entity = world.CreateEntity();
        world.AddComponent(entity, new Position { X = 1, Y = 2 });
        world.AddComponent(entity, new Health { Value = 100 });
        world.PostChanges(); // Clear pending

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

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

        // Should see: ComponentRemoved(Position), ComponentRemoved(Health), EntityRemoved
        collector.Changes.Should().Contain(c =>
            c.Kind == ChangeKind.ComponentRemoved && c.ComponentType == typeof(Position));
        collector.Changes.Should().Contain(c =>
            c.Kind == ChangeKind.ComponentRemoved && c.ComponentType == typeof(Health));
        collector.Changes.Should().Contain(c =>
            c.Kind == ChangeKind.EntityRemoved && c.Entity == entity);
    }

    // ── QueryDescriptor equality ─────────────────────────────────────

    [Fact]
    public void QueryDescriptor_EqualQueries_AreEqual()
    {
        var q1 = new QueryBuilder().With<Position>().With<Velocity>().Without<Frozen>().Build();
        var q2 = new QueryBuilder().With<Position>().With<Velocity>().Without<Frozen>().Build();

        q1.Should().Be(q2);
        q1.GetHashCode().Should().Be(q2.GetHashCode());
    }

    [Fact]
    public void QueryDescriptor_DifferentWith_AreNotEqual()
    {
        var q1 = new QueryBuilder().With<Position>().Build();
        var q2 = new QueryBuilder().With<Velocity>().Build();

        q1.Should().NotBe(q2);
    }

    [Fact]
    public void QueryDescriptor_DifferentWithout_AreNotEqual()
    {
        var q1 = new QueryBuilder().With<Position>().Without<Frozen>().Build();
        var q2 = new QueryBuilder().With<Position>().Without<Burning>().Build();

        q1.Should().NotBe(q2);
    }

    [Fact]
    public void QueryDescriptor_DifferentCount_AreNotEqual()
    {
        var q1 = new QueryBuilder().With<Position>().Build();
        var q2 = new QueryBuilder().With<Position>().With<Velocity>().Build();

        q1.Should().NotBe(q2);
    }

    // ── Query with 4, 5, 6 components ────────────────────────────────

    [Fact]
    public void Query_FourComponents_Works()
    {
        var world = new World();
        var entity = world.CreateEntity();
        world.AddComponent(entity, new Position { X = 1, Y = 2 });
        world.AddComponent(entity, new Velocity { X = 3, Y = 4 });
        world.AddComponent(entity, new Health { Value = 100 });
        world.AddComponent(entity, new Frozen());

        var query = world.Query()
            .With<Position>().With<Velocity>().With<Health>().With<Frozen>()
            .Build();

        var count = 0;
        world.ForEach(query, (Entity e, ref Position pos, ref Velocity vel, ref Health hp, ref Frozen f) =>
        {
            count++;
            pos.X.Should().Be(1);
            vel.Y.Should().Be(4);
            hp.Value.Should().Be(100);
        });
        count.Should().Be(1);
    }

    [Fact]
    public void Query_FiveComponents_Works()
    {
        var world = new World();
        var entity = world.CreateEntity();
        world.AddComponent(entity, new Position { X = 1, Y = 2 });
        world.AddComponent(entity, new Velocity { X = 3, Y = 4 });
        world.AddComponent(entity, new Health { Value = 100 });
        world.AddComponent(entity, new Frozen());
        world.AddComponent(entity, new Burning());

        var query = world.Query()
            .With<Position>().With<Velocity>().With<Health>().With<Frozen>().With<Burning>()
            .Build();

        var count = 0;
        world.ForEach(query, (Entity e, ref Position pos, ref Velocity vel, ref Health hp, ref Frozen f, ref Burning b) =>
        {
            count++;
        });
        count.Should().Be(1);
    }

    [Fact]
    public void Query_SixComponents_Works()
    {
        var world = new World();
        var entity = world.CreateEntity();
        world.AddComponent(entity, new Position { X = 1, Y = 2 });
        world.AddComponent(entity, new Velocity { X = 3, Y = 4 });
        world.AddComponent(entity, new Health { Value = 100 });
        world.AddComponent(entity, new Frozen());
        world.AddComponent(entity, new Burning());
        world.AddComponent(entity, new Poisoned());

        var query = world.Query()
            .With<Position>().With<Velocity>().With<Health>()
            .With<Frozen>().With<Burning>().With<Poisoned>()
            .Build();

        var count = 0;
        world.ForEach(query, (Entity e, ref Position pos, ref Velocity vel, ref Health hp,
            ref Frozen f, ref Burning b, ref Poisoned p) =>
        {
            count++;
        });
        count.Should().Be(1);
    }

    // ── Query with only Without ──────────────────────────────────────

    [Fact]
    public void Query_OnlyWithout_FiltersCorrectly()
    {
        var world = new World();
        var a = world.CreateEntity();
        var b = world.CreateEntity();

        world.AddComponent(a, new Position { X = 1, Y = 2 });
        world.AddComponent(b, new Position { X = 3, Y = 4 });
        world.AddComponent(b, new Frozen());

        // Without<Frozen> filters out entity b which has Frozen.
        var query = world.Query().Without<Frozen>().Build();
        var results = new List<Entity>();

        world.ForEach(query, (Entity e, ref Position pos) =>
        {
            results.Add(e);
        });

        results.Should().BeEquivalentTo([a]);
    }

    // ── World.Dispose ────────────────────────────────────────────────

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

        var act = () => world.Dispose();
        act.Should().NotThrow();
    }

    [Fact]
    public void World_Dispose_CanBeCalledMultipleTimes()
    {
        var world = new World();

        world.Dispose();
        var act = () => world.Dispose();
        act.Should().NotThrow();
    }

    // ── CommandQueue error accumulation ──────────────────────────────

    [Fact]
    public void CommandQueue_Errors_AccumulateAcrossDrainCycles()
    {
        var world = new World();

        world.Commands.Enqueue(new FailingCommand { Message = "first" });
        world.ExecuteCommands();
        world.Commands.Errors.Should().HaveCount(1);

        world.Commands.Enqueue(new FailingCommand { Message = "second" });
        world.ExecuteCommands();
        world.Commands.Errors.Should().HaveCount(2);

        world.Commands.Errors[0].Message.Should().Be("first");
        world.Commands.Errors[1].Message.Should().Be("second");
    }

    [Fact]
    public void CommandQueue_ClearErrors_ResetsErrorList()
    {
        var world = new World();

        world.Commands.Enqueue(new FailingCommand { Message = "boom" });
        world.ExecuteCommands();
        world.Commands.Errors.Should().HaveCount(1);

        world.Commands.ClearErrors();
        world.Commands.Errors.Should().BeEmpty();
    }

    // ── Multiple World instances ──────────────────────────────────────

    [Fact]
    public void MultipleWorlds_AreIndependent()
    {
        var world1 = new World();
        var world2 = new World();

        var e1 = world1.CreateEntity();
        var e2 = world2.CreateEntity();

        world1.AddComponent(e1, new Position { X = 1, Y = 2 });
        world2.AddComponent(e2, new Position { X = 3, Y = 4 });

        // Each world sees its own entity's components.
        ref var pos1 = ref world1.GetComponent<Position>(e1);
        pos1.X.Should().Be(1);

        ref var pos2 = ref world2.GetComponent<Position>(e2);
        pos2.X.Should().Be(3);

        // Worlds are independent: destroying in one doesn't affect the other.
        world1.DestroyEntity(e1);
        world1.IsAlive(e1).Should().BeFalse();
        world2.IsAlive(e2).Should().BeTrue();
    }

    // ── EntityAllocator large scale ──────────────────────────────────

    [Fact]
    public void EntityAllocator_HandlesManyEntities()
    {
        var world = new World();
        var entities = new List<Entity>();

        for (int i = 0; i < 5000; i++)
        {
            var e = world.CreateEntity();
            entities.Add(e);
            world.IsAlive(e).Should().BeTrue();
        }

        // Destroy half.
        for (int i = 0; i < 2500; i++)
        {
            world.DestroyEntity(entities[i]);
            world.IsAlive(entities[i]).Should().BeFalse();
        }

        // Create more — should reuse freed IDs.
        for (int i = 0; i < 2500; i++)
        {
            var e = world.CreateEntity();
            world.IsAlive(e).Should().BeTrue();
        }
    }

    // ── Structural mutation during ForEach ────────────────────────────

    [Fact]
    public void ForEach_AddingComponent_DoesNotAffectCurrentIteration()
    {
        var world = new World();
        var a = world.CreateEntity();
        var b = world.CreateEntity();
        world.AddComponent(a, new Position { X = 1, Y = 2 });
        world.AddComponent(b, new Position { X = 3, Y = 4 });

        var query = world.Query().With<Position>().Build();
        var seen = new List<Entity>();

        world.ForEach(query, (Entity e, ref Position pos) =>
        {
            seen.Add(e);
            // Add a component to the other entity during iteration.
            // This should not affect the current iteration.
            world.AddComponent(e, new Velocity { X = 1, Y = 1 });
        });

        seen.Should().BeEquivalentTo([a, b]);
    }

    [Fact]
    public void ForEach_DestroyingEntity_DoesNotThrow()
    {
        var world = new World();
        var a = world.CreateEntity();
        var b = world.CreateEntity();
        world.AddComponent(a, new Position { X = 1, Y = 2 });
        world.AddComponent(b, new Position { X = 3, Y = 4 });

        var query = world.Query().With<Position>().Build();

        // Destroying an entity during iteration: the sparse set uses
        // swap-remove, so the destroyed entity's slot is replaced by
        // the last element. This is safe as long as we don't re-iterate
        // the destroyed entity (which we won't since it's swap-removed).
        var act = () =>
        {
            world.ForEach(query, (Entity e, ref Position pos) =>
            {
                world.DestroyEntity(e);
            });
        };
        act.Should().NotThrow();
    }

    // ── Helpers ──────────────────────────────────────────────────────

    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() { }
    }

    // Phantom types for relationship tests.
    public struct ChildOf { }
    public struct ParentOf { }
}