using FluentAssertions;
using Xunit;

namespace OECS.Tests;

// ── Phantom types for relationship differentiation ───────────────────

public struct ChildOf { }
public struct ParentOf { }
public struct OwnedBy { }
public struct MemberOf { }
public struct Thing { }

// ── Tests ────────────────────────────────────────────────────────────

public class RelationshipTests
{
    [Fact]
    public void AddRelationship_UpdatesReverseIndex()
    {
        var world = new World();
        var child = world.CreateEntity();
        var parent = world.CreateEntity();

        world.AddComponent(child, new Relationship<ChildOf, ParentOf>
        {
            Target = parent
        });

        var sources = world.GetSources<Relationship<ChildOf, ParentOf>>(parent);
        sources.Should().BeEquivalentTo([child]);
    }

    [Fact]
    public void RemoveRelationship_UpdatesReverseIndex()
    {
        var world = new World();
        var child = world.CreateEntity();
        var parent = world.CreateEntity();

        world.AddComponent(child, new Relationship<ChildOf, ParentOf>
        {
            Target = parent
        });

        world.RemoveComponent<Relationship<ChildOf, ParentOf>>(child);

        var sources = world.GetSources<Relationship<ChildOf, ParentOf>>(parent);
        sources.Should().BeEmpty();
    }

    [Fact]
    public void ReplaceRelationship_UpdatesReverseIndex()
    {
        var world = new World();
        var child = world.CreateEntity();
        var parent1 = world.CreateEntity();
        var parent2 = world.CreateEntity();

        world.AddComponent(child, new Relationship<ChildOf, ParentOf>
        {
            Target = parent1
        });

        // Replace with a new target.
        world.AddComponent(child, new Relationship<ChildOf, ParentOf>
        {
            Target = parent2
        });

        world.GetSources<Relationship<ChildOf, ParentOf>>(parent1).Should().BeEmpty();
        world.GetSources<Relationship<ChildOf, ParentOf>>(parent2).Should().BeEquivalentTo([child]);
    }

    [Fact]
    public void DestroySourceEntity_CleansUpRelationships()
    {
        var world = new World();
        var child = world.CreateEntity();
        var parent = world.CreateEntity();

        world.AddComponent(child, new Relationship<ChildOf, ParentOf>
        {
            Target = parent
        });

        world.DestroyEntity(child);

        // The relationship should be gone from the reverse index.
        world.GetSources<Relationship<ChildOf, ParentOf>>(parent).Should().BeEmpty();

        // The destroyed entity should not be alive.
        world.IsAlive(child).Should().BeFalse();
    }

    [Fact]
    public void DestroyTargetEntity_CleansUpIncomingRelationships()
    {
        var world = new World();
        var child = world.CreateEntity();
        var parent = world.CreateEntity();

        world.AddComponent(child, new Relationship<ChildOf, ParentOf>
        {
            Target = parent
        });

        world.DestroyEntity(parent);

        // The relationship component should be removed from the source entity.
        world.HasComponent<Relationship<ChildOf, ParentOf>>(child).Should().BeFalse();

        // The reverse index should be empty for the destroyed target.
        world.GetSources<Relationship<ChildOf, ParentOf>>(parent).Should().BeEmpty();
    }

    [Fact]
    public void MultipleSources_ForSameTarget()
    {
        var world = new World();
        var child1 = world.CreateEntity();
        var child2 = world.CreateEntity();
        var child3 = world.CreateEntity();
        var parent = world.CreateEntity();

        world.AddComponent(child1, new Relationship<ChildOf, ParentOf>
        {
            Target = parent
        });
        world.AddComponent(child2, new Relationship<ChildOf, ParentOf>
        {
            Target = parent
        });
        world.AddComponent(child3, new Relationship<ChildOf, ParentOf>
        {
            Target = parent
        });

        var sources = world.GetSources<Relationship<ChildOf, ParentOf>>(parent);
        sources.Should().BeEquivalentTo([child1, child2, child3]);
    }

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

        world.AddComponent(entity, new Relationship<OwnedBy, Thing>
        {
            Target = owner
        });
        world.AddComponent(entity, new Relationship<MemberOf, Thing>
        {
            Target = group
        });

        world.GetSources<Relationship<OwnedBy, Thing>>(owner).Should().BeEquivalentTo([entity]);
        world.GetSources<Relationship<MemberOf, Thing>>(group).Should().BeEquivalentTo([entity]);

        // Cross-check: OwnedBy sources should not appear in MemberOf lookups.
        world.GetSources<Relationship<MemberOf, Thing>>(owner).Should().BeEmpty();
    }

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

        var sources = world.GetSources<Relationship<ChildOf, ParentOf>>(entity);
        sources.Should().BeEmpty();
    }

    [Fact]
    public void Relationship_CanBeQueried_LikeNormalComponent()
    {
        var world = new World();
        var child = world.CreateEntity();
        var parent = world.CreateEntity();

        world.AddComponent(child, new Relationship<ChildOf, ParentOf>
        {
            Target = parent
        });

        var results = new List<Entity>();

        foreach (var it in world.Select<Relationship<ChildOf, ParentOf>>())
        {
            results.Add(it.Entity);
            it.Val1.Target.Should().Be(parent);
        }

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

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

        // Add sources in a specific order.
        var sources = new Entity[5];
        for (int i = 0; i < 5; i++)
        {
            sources[i] = world.CreateEntity();
            world.AddComponent(sources[i], new Relationship<ChildOf, ParentOf>
            {
                Target = parent
            });
        }

        var result = world.GetSources<Relationship<ChildOf, ParentOf>>(parent);
        result.Should().Equal(sources);
    }

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

        var a = world.CreateEntity();
        var b = world.CreateEntity();
        var c = world.CreateEntity();

        world.AddComponent(a, new Relationship<ChildOf, ParentOf> { Target = parent });
        world.AddComponent(b, new Relationship<ChildOf, ParentOf> { Target = parent });
        world.AddComponent(c, new Relationship<ChildOf, ParentOf> { Target = parent });

        // Remove the middle source.
        world.RemoveComponent<Relationship<ChildOf, ParentOf>>(b);

        // Remaining sources should preserve insertion order.
        var result = world.GetSources<Relationship<ChildOf, ParentOf>>(parent);
        result.Should().Equal([a, c]);
    }

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

        var a = world.CreateEntity();
        var b = world.CreateEntity();
        var c = world.CreateEntity();

        world.AddComponent(a, new Relationship<ChildOf, ParentOf> { Target = parent });
        world.AddComponent(b, new Relationship<ChildOf, ParentOf> { Target = parent });
        world.AddComponent(c, new Relationship<ChildOf, ParentOf> { Target = parent });

        // Reverse the order.
        world.ReorderSources<Relationship<ChildOf, ParentOf>>(parent, [c, b, a]);

        var result = world.GetSources<Relationship<ChildOf, ParentOf>>(parent);
        result.Should().Equal([c, b, a]);
    }

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

        // Should not throw.
        world.ReorderSources<Relationship<ChildOf, ParentOf>>(entity, []);
    }

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

        // a → b (a is child of b)
        world.AddComponent(a, new Relationship<ChildOf, ParentOf>
        {
            Target = b
        });

        // c → a (c is child of a)
        world.AddComponent(c, new Relationship<ChildOf, ParentOf>
        {
            Target = a
        });

        // Destroy a. This should:
        // 1. Remove the a→b relationship (a is source).
        // 2. Remove the c→a relationship (a is target, so c's component is removed).
        world.DestroyEntity(a);

        world.IsAlive(a).Should().BeFalse();
        world.HasComponent<Relationship<ChildOf, ParentOf>>(c).Should().BeFalse();
        world.GetSources<Relationship<ChildOf, ParentOf>>(b).Should().BeEmpty();
    }
}