using FluentAssertions;
using Xunit;

namespace OECS.Tests;

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

        a.Should().NotBe(b);
        a.Id.Should().NotBe(b.Id);
    }

    [Fact]
    public void Null_IsSentinel()
    {
        Entity.Null.IsNull.Should().BeTrue();
        Entity.Null.Id.Should().Be(0);
        Entity.Null.Version.Should().Be(0);
    }

    [Fact]
    public void DestroyEntity_RecyclesId_WithIncrementedVersion()
    {
        var world = new World();
        var original = world.CreateEntity();
        uint originalId = original.Id;
        uint originalVersion = original.Version;

        world.DestroyEntity(original);
        world.IsAlive(original).Should().BeFalse();

        var recycled = world.CreateEntity();
        recycled.Id.Should().Be(originalId);
        recycled.Version.Should().BeGreaterThan(originalVersion);
    }

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

        world.IsAlive(entity).Should().BeTrue();
        world.DestroyEntity(entity);
        world.IsAlive(entity).Should().BeFalse();
    }

    [Fact]
    public void IsAlive_ReturnsFalse_ForNull()
    {
        var world = new World();
        world.IsAlive(Entity.Null).Should().BeFalse();
    }

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

        var str = entity.ToString();
        str.Should().Contain(entity.Id.ToString());
        str.Should().Contain(entity.Version.ToString());
    }

    [Fact]
    public void Equality_ChecksBothIdAndVersion()
    {
        var world = new World();
        var a = world.CreateEntity();
        world.DestroyEntity(a);
        var b = world.CreateEntity(); // Same ID, different version

        a.Should().NotBe(b);
        (a == b).Should().BeFalse();
    }
}

public class ComponentTests
{
    private struct Position { public float X; public float Y; }
    private struct Velocity { public float X; public float Y; }
    private struct Health { public int Value; }

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

        world.AddComponent(entity, new Position { X = 1, Y = 2 });
        ref var pos = ref world.GetComponent<Position>(entity);

        pos.X.Should().Be(1);
        pos.Y.Should().Be(2);
    }

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

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

        ref var pos = ref world.GetComponent<Position>(entity);
        pos.X.Should().Be(3);
        pos.Y.Should().Be(4);
    }

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

        world.HasComponent<Position>(entity).Should().BeFalse();
        world.AddComponent(entity, new Position());
        world.HasComponent<Position>(entity).Should().BeTrue();
    }

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

        world.AddComponent(entity, new Position { X = 1, Y = 2 });
        world.RemoveComponent<Position>(entity);
        world.HasComponent<Position>(entity).Should().BeFalse();
    }

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

        // Should not throw.
        world.RemoveComponent<Position>(entity);
    }

    [Fact]
    public void MultipleComponents_OnSameEntity()
    {
        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.HasComponent<Position>(entity).Should().BeTrue();
        world.HasComponent<Velocity>(entity).Should().BeTrue();
        world.HasComponent<Health>(entity).Should().BeTrue();

        ref var pos = ref world.GetComponent<Position>(entity);
        ref var vel = ref world.GetComponent<Velocity>(entity);
        ref var hp = ref world.GetComponent<Health>(entity);

        pos.X.Should().Be(1);
        vel.Y.Should().Be(4);
        hp.Value.Should().Be(100);
    }

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

        world.AddComponent(entity, new Position());
        world.AddComponent(entity, new Velocity());

        world.DestroyEntity(entity);

        // After destroy, the entity is not alive, so component queries
        // on the stale handle are irrelevant. But a new entity with the
        // same ID should not have the old components.
        var newEntity = world.CreateEntity();
        world.HasComponent<Position>(newEntity).Should().BeFalse();
        world.HasComponent<Velocity>(newEntity).Should().BeFalse();
    }

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

        var act = () => world.GetComponent<Position>(entity);
        act.Should().Throw<InvalidOperationException>()
            .WithMessage("*does not have*Position*");
    }

    [Fact]
    public void GetComponent_Throws_WhenEntityIdOutOfRange()
    {
        var world = new World();
        // Create an entity with a very large ID by exhausting the allocator.
        // This is hard to do without internals, so we test via a destroyed
        // entity that has a valid ID but no component.
        var entity = world.CreateEntity();

        var act = () => world.GetComponent<Position>(entity);
        act.Should().Throw<InvalidOperationException>()
            .WithMessage("*does not have*Position*");
    }

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

        bool found = world.TryGetComponent<Position>(entity, out var pos);

        found.Should().BeTrue();
        pos.X.Should().Be(1);
        pos.Y.Should().Be(2);
    }

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

        bool found = world.TryGetComponent<Position>(entity, out var pos);

        found.Should().BeFalse();
        pos.Should().Be(default(Position));
    }

    [Fact]
    public void AddComponent_Throws_WhenEntityNotAlive()
    {
        var world = new World();
        var entity = world.CreateEntity();
        world.DestroyEntity(entity);

        var act = () => world.AddComponent(entity, new Position());
        act.Should().Throw<InvalidOperationException>();
    }

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

        world.AddComponent(entity, new Position { X = 0, Y = 0 });
        ref var pos = ref world.GetComponent<Position>(entity);
        pos.X = 42;

        ref var pos2 = ref world.GetComponent<Position>(entity);
        pos2.X.Should().Be(42);
    }
}