using NUnit.Framework;
using FluentAssertions;

using Encompass;

using System;
using System.Linq;
using System.Collections.Generic;
using Encompass.Exceptions;

namespace Tests
{
    struct MockComponent : IComponent
    {
        public int myInt;
        public string myString;
    }

    public class EngineTest
    {
        static List<MockComponent> resultComponents;
        static MockComponent resultComponent;

        static List<MockMessage> resultMessages = new List<MockMessage>();

        [Reads(typeof(MockComponent))]
        public class ReadComponentsTestEngine : Engine
        {
            public override void Update(double dt)
            {
                resultComponents = ReadComponents<MockComponent>().ToList();
            }
        }

        static List<(MockComponent, Entity)> resultComponentsIncludingEntity;
        static (MockComponent, Entity) resultComponentIncludingEntity;

        [Reads(typeof(MockComponent))]
        public class ReadComponentsIncludingEntityEngine : Engine
        {
            public override void Update(double dt)
            {
                resultComponentsIncludingEntity = ReadComponentsIncludingEntity<MockComponent>().ToList();
            }
        }

        [Reads(typeof(MockComponent))]
        public class ReadComponentTestEngine : Engine
        {
            public override void Update(double dt)
            {
                resultComponent = ReadComponent<MockComponent>();
            }
        }

        [Reads(typeof(MockComponent))]
        public class ReadComponentIncludingEntityEngine : Engine
        {
            public override void Update(double dt)
            {
                resultComponentIncludingEntity = ReadComponentIncludingEntity<MockComponent>();
            }
        }

        [Test]
        public void ReadComponents()
        {
            var worldBuilder = new WorldBuilder();
            worldBuilder.AddEngine(new ReadComponentsTestEngine());

            var entity = worldBuilder.CreateEntity();
            var entityB = worldBuilder.CreateEntity();

            MockComponent mockComponent;
            mockComponent.myInt = 0;
            mockComponent.myString = "hello";

            MockComponent mockComponentB;
            mockComponentB.myInt = 1;
            mockComponentB.myString = "howdy";

            worldBuilder.SetComponent(entity, mockComponent);
            worldBuilder.SetComponent(entityB, mockComponentB);

            var world = worldBuilder.Build();

            world.Update(0.01f);

            resultComponents.Should().Contain(mockComponent);
            resultComponents.Should().Contain(mockComponentB);
        }

        [Test]
        public void ReadComponentsIncludingEntity()
        {
            var worldBuilder = new WorldBuilder();
            worldBuilder.AddEngine(new ReadComponentsIncludingEntityEngine());

            var entity = worldBuilder.CreateEntity();
            var entityB = worldBuilder.CreateEntity();

            MockComponent mockComponent;
            mockComponent.myInt = 0;
            mockComponent.myString = "hello";

            MockComponent mockComponentB;
            mockComponentB.myInt = 1;
            mockComponentB.myString = "howdy";

            worldBuilder.SetComponent(entity, mockComponent);
            worldBuilder.SetComponent(entityB, mockComponentB);

            var world = worldBuilder.Build();

            world.Update(0.01f);

            var resultComponentValues = resultComponentsIncludingEntity.Select((kv) => kv.Item1);
            resultComponentValues.Should().Contain(mockComponent);
            resultComponentValues.Should().Contain(mockComponentB);

            var resultEntities = resultComponentsIncludingEntity.Select((kv) => kv.Item2);
            resultEntities.Should().Contain(entity);
            resultEntities.Should().Contain(entityB);

            resultComponentsIncludingEntity.Should().Contain((mockComponent, entity));
            resultComponentsIncludingEntity.Should().Contain((mockComponentB, entityB));
        }

        [Test]
        public void ReadComponent()
        {
            var worldBuilder = new WorldBuilder();
            worldBuilder.AddEngine(new ReadComponentTestEngine());

            var entity = worldBuilder.CreateEntity();

            MockComponent mockComponent;
            mockComponent.myInt = 0;
            mockComponent.myString = "hello";

            worldBuilder.SetComponent(entity, mockComponent);

            var world = worldBuilder.Build();

            world.Update(0.01f);

            Assert.AreEqual(mockComponent, resultComponent);
        }

        [Test]
        public void ReadComponentWhenMultipleComponents()
        {
            var worldBuilder = new WorldBuilder();
            worldBuilder.AddEngine(new ReadComponentTestEngine());

            var entity = worldBuilder.CreateEntity();
            var entityB = worldBuilder.CreateEntity();

            MockComponent mockComponent;
            mockComponent.myInt = 0;
            mockComponent.myString = "hello";

            MockComponent mockComponentB;
            mockComponentB.myInt = 1;
            mockComponentB.myString = "howdy";

            worldBuilder.SetComponent(entity, mockComponent);
            worldBuilder.SetComponent(entityB, mockComponentB);

            var world = worldBuilder.Build();

            world.Update(0.01);

            Assert.That(resultComponent, Is.EqualTo(mockComponent).Or.EqualTo(mockComponentB));
        }

        [Test]
        public void ReadComponentWithEntity()
        {
            var worldBuilder = new WorldBuilder();
            worldBuilder.AddEngine(new ReadComponentIncludingEntityEngine());

            var entity = worldBuilder.CreateEntity();

            MockComponent mockComponent;
            mockComponent.myInt = 0;
            mockComponent.myString = "hello";

            worldBuilder.SetComponent(entity, mockComponent);

            var world = worldBuilder.Build();

            world.Update(0.01f);

            (mockComponent, entity).Should().BeEquivalentTo(resultComponentIncludingEntity);
        }

        [Reads(typeof(MockComponent))]
        [Writes(typeof(MockComponent))]
        public class UpdateComponentTestEngine : Engine
        {
            public override void Update(double dt)
            {
                var (component, entity) = ReadComponentIncludingEntity<MockComponent>();

                component.myInt = 420;
                component.myString = "blaze it";
                SetComponent(entity, component);
            }
        }

        // this test needs to be improved...

        [Test]
        public void UpdateComponent()
        {
            var worldBuilder = new WorldBuilder();

            worldBuilder.AddEngine(new UpdateComponentTestEngine());
            worldBuilder.AddEngine(new ReadComponentTestEngine());

            var entity = worldBuilder.CreateEntity();

            MockComponent mockComponent;
            mockComponent.myInt = 0;
            mockComponent.myString = "hello";

            worldBuilder.SetComponent(entity, mockComponent);

            var world = worldBuilder.Build();

            world.Update(0.01);
            world.Update(0.01);

            Assert.AreEqual(420, resultComponent.myInt);
            Assert.AreEqual("blaze it", resultComponent.myString);
        }

        [Reads(typeof(MockComponent))]
        public class UndeclaredUpdateComponentTestEngine : Engine
        {
            public override void Update(double dt)
            {
                var (component, entity) = ReadComponentIncludingEntity<MockComponent>();

                component.myInt = 420;
                component.myString = "blaze it";
                SetComponent(entity, component);

                component = ReadComponent<MockComponent>();
            }
        }

        [Test]
        public void UpdateUndeclaredComponent()
        {
            var worldBuilder = new WorldBuilder();
            worldBuilder.AddEngine(new UndeclaredUpdateComponentTestEngine());

            var entity = worldBuilder.CreateEntity();

            MockComponent mockComponent;
            mockComponent.myInt = 0;
            mockComponent.myString = "hello";

            worldBuilder.SetComponent(entity, mockComponent);

            var world = worldBuilder.Build();

            var ex = Assert.Throws<IllegalWriteException>(() => world.Update(0.01f));
            Assert.That(ex.Message, Is.EqualTo("Engine UndeclaredUpdateComponentTestEngine tried to update undeclared Component MockComponent"));
        }

        struct MockMessage : IMessage
        {
            public string myString;
        }

        [Sends(typeof(MockMessage))]
        public class MessageEmitEngine : Engine
        {
            public override void Update(double dt)
            {
                MockMessage message;
                message.myString = "howdy";

                this.SendMessage(message);
            }
        }

        [Receives(typeof(MockMessage))]
        public class MessageReadEngine : Engine
        {
            public override void Update(double dt)
            {
                resultMessages = this.ReadMessages<MockMessage>().ToList();
            }
        }

        [Test]
        public void EmitAndReadMessage()
        {
            var worldBuilder = new WorldBuilder();
            worldBuilder.AddEngine(new MessageEmitEngine());
            worldBuilder.AddEngine(new MessageReadEngine());

            var world = worldBuilder.Build();

            world.Update(0.01f);

            Assert.AreEqual(resultMessages.First().myString, "howdy");
        }

        public class UndeclaredMessageEmitEngine : Engine
        {
            public override void Update(double dt)
            {
                MockMessage message;
                message.myString = "howdy";

                this.SendMessage(message);
            }
        }

        static IEnumerable<MockMessage> emptyReadMessagesResult;

        [Receives(typeof(MockMessage))]
        class ReadMessagesWhenNoneExistEngine : Engine
        {
            public override void Update(double dt)
            {
                emptyReadMessagesResult = ReadMessages<MockMessage>();
            }
        }

        [Test]
        public void ReadMessagesWhenNoneHaveBeenEmitted()
        {
            var worldBuilder = new WorldBuilder();
            worldBuilder.AddEngine(new ReadMessagesWhenNoneExistEngine());

            var world = worldBuilder.Build();

            world.Update(0.01f);

            emptyReadMessagesResult.Should().BeEmpty();
        }

        [Test]
        public void EmitUndeclaredMessage()
        {
            var worldBuilder = new WorldBuilder();
            worldBuilder.AddEngine(new UndeclaredMessageEmitEngine());

            var world = worldBuilder.Build();

            var ex = Assert.Throws<IllegalSendException>(() => world.Update(0.01f));
            Assert.That(ex.Message, Is.EqualTo("Engine UndeclaredMessageEmitEngine tried to send undeclared Message MockMessage"));
        }

        static bool someTest;

        [Sends(typeof(MockMessage))]
        class EmitMockMessageEngine : Engine
        {
            public override void Update(double dt)
            {
                MockMessage message;
                message.myString = "howdy";

                this.SendMessage(message);
            }
        }

        [Receives(typeof(MockMessage))]
        class SomeMessageTestEngine : Engine
        {
            public override void Update(double dt)
            {
                someTest = this.SomeMessage<MockMessage>();
            }
        }

        [Test]
        public void SomeMessage()
        {
            var worldBuilder = new WorldBuilder();
            worldBuilder.AddEngine(new EmitMockMessageEngine());
            worldBuilder.AddEngine(new SomeMessageTestEngine());

            var world = worldBuilder.Build();

            world.Update(0.01f);

            Assert.That(someTest, Is.True);
        }

        class UndeclaredSomeMessageEngine : Engine
        {
            public override void Update(double dt)
            {
                someTest = this.SomeMessage<MockMessage>();
            }
        }

        [Test]
        public void UndeclaredSomeMessage()
        {
            var worldBuilder = new WorldBuilder();
            worldBuilder.AddEngine(new EmitMockMessageEngine());
            worldBuilder.AddEngine(new UndeclaredSomeMessageEngine());

            var world = worldBuilder.Build();

            Assert.Throws<IllegalReadException>(() => world.Update(0.01f));
        }

        class SomeComponentTestEngine : Engine
        {
            public override void Update(double dt)
            {
                Assert.IsTrue(SomeComponent<MockComponent>());
            }
        }

        [Test]
        public void SomeComponent()
        {
            var worldBuilder = new WorldBuilder();

            var entity = worldBuilder.CreateEntity();
            worldBuilder.SetComponent(entity, new MockComponent());

            var world = worldBuilder.Build();

            world.Update(0.01);
        }

        static (MockComponent, Entity) pairA;
        static (MockComponent, Entity) pairB;

        [Reads(typeof(MockComponent))]
        class SameValueComponentReadEngine : Engine
        {
            public override void Update(double dt)
            {
                var components = ReadComponentsIncludingEntity<MockComponent>();

                pairA = components.First();
                pairB = components.Last();
            }
        }

        // Tests that components with identical values should be distinguishable by their entities
        [Test]
        public void SameValueComponents()
        {
            var worldBuilder = new WorldBuilder();
            worldBuilder.AddEngine(new SameValueComponentReadEngine());

            MockComponent componentA;
            componentA.myInt = 20;
            componentA.myString = "hello";

            MockComponent componentB;
            componentB.myInt = 20;
            componentB.myString = "hello";

            var entity = worldBuilder.CreateEntity();
            worldBuilder.SetComponent(entity, componentA);

            var entityB = worldBuilder.CreateEntity();
            worldBuilder.SetComponent(entityB, componentB);

            var world = worldBuilder.Build();
            world.Update(0.01f);

            Assert.That(EngineTest.pairA, Is.Not.EqualTo(EngineTest.pairB));
            Assert.That(EngineTest.pairA.Item1, Is.EqualTo(EngineTest.pairB.Item1));
        }

        static IEnumerable<(MockComponent, Entity)> emptyComponentReadResult;

        [Reads(typeof(MockComponent))]
        class ReadEmptyMockComponentsEngine : Engine
        {
            public override void Update(double dt)
            {
                emptyComponentReadResult = ReadComponentsIncludingEntity<MockComponent>();
            }
        }

        [Test]
        public void ReadComponentsOfTypeWhereNoneExist()
        {
            var worldBuilder = new WorldBuilder();
            worldBuilder.AddEngine(new ReadEmptyMockComponentsEngine());

            var world = worldBuilder.Build();
            world.Update(0.01f);

            Assert.That(emptyComponentReadResult, Is.Empty);
        }

        struct DestroyerComponent : IComponent { }

        [Reads(typeof(DestroyerComponent))]
        class DestroyerEngine : Engine
        {
            public override void Update(double dt)
            {
                foreach (var (component, entity) in ReadComponentsIncludingEntity<DestroyerComponent>())
                {
                    Destroy(entity);
                }
            }
        }

        static List<(MockComponent, Entity)> results;

        [Reads(typeof(MockComponent))]
        class ReaderEngine : Engine
        {
            public override void Update(double dt)
            {
                results = ReadComponentsIncludingEntity<MockComponent>().ToList();
            }
        }

        [Test]
        public void DestroyEntity()
        {
            var worldBuilder = new WorldBuilder();
            worldBuilder.AddEngine(new DestroyerEngine());
            worldBuilder.AddEngine(new ReaderEngine());

            var entity = worldBuilder.CreateEntity();
            var entityB = worldBuilder.CreateEntity();
            var entityC = worldBuilder.CreateEntity();

            DestroyerComponent destroyerComponent;
            MockComponent mockComponent;
            mockComponent.myInt = 2;
            mockComponent.myString = "blah";

            worldBuilder.SetComponent(entity, destroyerComponent);
            worldBuilder.SetComponent(entity, mockComponent);

            worldBuilder.SetComponent(entityB, destroyerComponent);
            worldBuilder.SetComponent(entityB, mockComponent);

            worldBuilder.SetComponent(entityC, mockComponent);

            var world = worldBuilder.Build();

            world.Update(0.01);
            world.Update(0.01);

            Assert.That(results, Does.Not.Contain((mockComponent, entity)));
            Assert.That(results, Does.Not.Contain((mockComponent, entity)));
            Assert.That(results, Does.Contain((mockComponent, entityC)));
        }

        [Receives(typeof(DestroyComponentMessage))]
        class DestroyEntityEngine : Engine
        {
            public override void Update(double dt)
            {
                foreach (var message in ReadMessages<DestroyComponentMessage>())
                {
                    Destroy(message.entity);
                }
            }
        }

        [Test]
        public void DestroyEntityWithoutID()
        {
            var worldBuilder = new WorldBuilder();
            worldBuilder.AddEngine(new AddComponentEngine());
            worldBuilder.AddEngine(new DestroyEntityEngine());
            worldBuilder.AddEngine(new ReaderEngine());

            var mockComponent = new MockComponent { };
            var entity = worldBuilder.CreateEntity();
            worldBuilder.SetComponent(entity, mockComponent);
            worldBuilder.SendMessage(new DestroyComponentMessage { entity = entity });

            var world = worldBuilder.Build();
            world.Update(0.01);

            Assert.DoesNotThrow(() => world.Update(0.01));
            Assert.That(results, Does.Not.Contain((mockComponent, entity)));
        }

        [Reads(typeof(DestroyerComponent), typeof(MockComponent))]
        class DestroyAndAddComponentEngine : Engine
        {
            public override void Update(double dt)
            {
                foreach (var (componentPair, entity) in ReadComponentsIncludingEntity<DestroyerComponent>())
                {
                    RemoveComponent<MockComponent>(entity);
                    Destroy(entity);
                }
            }
        }

        [Test]
        public void DestroyEntityWhileRemovingComponent()
        {
            var worldBuilder = new WorldBuilder();
            worldBuilder.AddEngine(new DestroyAndAddComponentEngine());
            worldBuilder.AddEngine(new ReaderEngine());

            var entity = worldBuilder.CreateEntity();

            worldBuilder.SetComponent(entity, new DestroyerComponent());
            worldBuilder.SetComponent(entity, new MockComponent());

            var world = worldBuilder.Build();

            Assert.DoesNotThrow(() => world.Update(0.01));
        }

        [Reads(typeof(MockComponent))]
        [WritesPending(typeof(MockComponent))]
        [Writes(typeof(MockComponent))]
        class AddAndRemoveMockComponentEngine : Engine
        {
            public override void Update(double dt)
            {
                foreach (var (mockComponent, entity) in ReadComponentsIncludingEntity<MockComponent>())
                {
                    RemoveComponent<MockComponent>(entity);
                    SetComponent(entity, new MockComponent());
                }
            }
        }

        static Entity entityResult;

        [ReadsPending(typeof(MockComponent))]
        class GetEntityFromPendingReadComponents : Engine
        {
            public override void Update(double dt)
            {
                var (_, entity) = ReadComponentIncludingEntity<MockComponent>();
            }
        }

        [Test]
        public void GetEntityFromPendingComponentID()
        {
            var worldBuilder = new WorldBuilder();
            worldBuilder.AddEngine(new AddAndRemoveMockComponentEngine());
            worldBuilder.AddEngine(new GetEntityFromPendingReadComponents());

            var entity = worldBuilder.CreateEntity();
            worldBuilder.SetComponent(entity, new MockComponent());

            var world = worldBuilder.Build();

            Assert.DoesNotThrow(() => world.Update(0.01));
        }

        [Reads(typeof(MockComponent))]
        class DelayedMessageEngine : Engine
        {
            public override void Update(double dt)
            {
                foreach (var (component, entity) in ReadComponentsIncludingEntity<MockComponent>())
                {
                    RemoveComponent<MockComponent>(entity);
                    SendMessage(new MockMessage { }, 1);
                }
            }
        }

        [Test]
        public void EngineSendMessageDelayed()
        {
            resultMessages.Clear();

            var worldBuilder = new WorldBuilder();
            worldBuilder.AddEngine(new ActivateTimeDilationEngine());
            worldBuilder.AddEngine(new DelayedMessageEngine());
            worldBuilder.AddEngine(new MessageReadEngine());

            var entity = worldBuilder.CreateEntity();
            worldBuilder.SetComponent(entity, new MockComponent { });

            var world = worldBuilder.Build();

            world.Update(0.01);

            resultMessages.Should().BeEmpty();

            world.Update(0.5);

            resultMessages.Should().BeEmpty();

            world.Update(0.5);

            resultMessages.Should().BeEmpty();

            world.Update(2);

            resultMessages.Should().NotBeEmpty();
            resultMessages.First().Should().BeOfType<MockMessage>();
        }

        [Reads(typeof(MockComponent))]
        class DelayedMessageIgnoringTimeDilationEngine : Engine
        {
            public override void Update(double dt)
            {
                foreach (var (component, entity) in ReadComponentsIncludingEntity<MockComponent>())
                {
                    RemoveComponent<MockComponent>(entity);
                    SendMessageIgnoringTimeDilation(new MockMessage { }, 1);
                }
            }
        }

        [Test]
        public void EngineSendMessageDelayedIgnoringTimeDilation()
        {
            resultMessages.Clear();

            var worldBuilder = new WorldBuilder();
            worldBuilder.AddEngine(new ActivateTimeDilationEngine());
            worldBuilder.AddEngine(new DelayedMessageIgnoringTimeDilationEngine());
            worldBuilder.AddEngine(new MessageReadEngine());

            var entity = worldBuilder.CreateEntity();
            worldBuilder.SetComponent(entity, new MockComponent { });

            var world = worldBuilder.Build();

            world.Update(0.01);

            resultMessages.Should().BeEmpty();

            world.Update(0.5);

            resultMessages.Should().BeEmpty();

            world.Update(0.5);

            resultMessages.Should().NotBeEmpty();
            resultMessages.First().Should().BeOfType<MockMessage>();
        }

        [Receives(typeof(MockMessage))]
        [WritesPending(typeof(MockComponent))]
        [Writes(typeof(MockComponent))]
        class ActivateComponentEngine : Engine
        {
            public override void Update(double dt)
            {
                foreach (var message in ReadMessages<MockMessage>())
                {
                    var entity = CreateEntity();
                    SetComponent(entity, new MockComponent { });
                }
            }
        }

        [ReadsPending(typeof(MockComponent))]
        class RemoveComponentEngine : Engine
        {
            public override void Update(double dt)
            {
                foreach (var entity in ReadEntities<MockComponent>())
                {
                    RemoveComponent<MockComponent>(entity);
                }
            }
        }

        [Test]
        public void EngineAddAndRemoveComponentSameFrame()
        {
            var worldBuilder = new WorldBuilder();
            worldBuilder.AddEngine(new ActivateComponentEngine());
            worldBuilder.AddEngine(new RemoveComponentEngine());

            worldBuilder.SendMessage(new MockMessage { });

            var world = worldBuilder.Build();

            Assert.DoesNotThrow(() => world.Update(0.01));
        }

        struct DestroyComponentMessage : IMessage { public Entity entity; }

        [Reads(typeof(MockComponent))]
        [Writes(typeof(MockComponent))]
        class AddComponentEngine : Engine
        {
            public override void Update(double dt)
            {
                foreach (var entity in ReadEntities<MockComponent>())
                {
                    SetComponent(entity, new MockComponent { });
                }
            }
        }

        static Entity readEntity;

        [Reads(typeof(MockComponent))]
        class ReadEntityByComponentTypeEngine : Engine
        {
            public override void Update(double dt)
            {
                readEntity = ReadEntity<MockComponent>();
            }
        }

        [Test]
        public void GetEntityByComponentType()
        {
            var worldBuilder = new WorldBuilder();
            worldBuilder.AddEngine(new ReadEntityByComponentTypeEngine());

            var entity = worldBuilder.CreateEntity();
            worldBuilder.SetComponent(entity, new MockComponent { });

            var world = worldBuilder.Build();
            world.Update(0.01);

            entity.Should().BeEquivalentTo(readEntity);
        }



        static Entity[] readEntities;

        [Reads(typeof(MockComponent))]
        class ReadEntitiesWithComponentTypeEngine : Engine
        {
            public override void Update(double dt)
            {
                readEntities = ReadEntities<MockComponent>().ToArray();
            }
        }

        [Test]
        public void ReadEntities()
        {
            var worldBuilder = new WorldBuilder();
            worldBuilder.AddEngine(new ReadEntitiesWithComponentTypeEngine());
            worldBuilder.AddEngine(new DestroyAllWithEngine());

            var entity = worldBuilder.CreateEntity();
            worldBuilder.SetComponent(entity, new MockComponent { });

            var entityB = worldBuilder.CreateEntity();
            worldBuilder.SetComponent(entityB, new MockComponent { });

            var world = worldBuilder.Build();
            world.Update(0.01);

            readEntities.Should().Contain(entity);
            readEntities.Should().Contain(entityB);
        }

        [Reads(typeof(MockComponent))]
        class DestroyWithEngine : Engine
        {
            public override void Update(double dt)
            {
                if (SomeComponent<MockComponent>())
                {
                    DestroyWith<MockComponent>();
                }
            }
        }

        [Test]
        public void DestroyWith()
        {
            var worldBuilder = new WorldBuilder();
            worldBuilder.AddEngine(new ReadEntitiesWithComponentTypeEngine());
            worldBuilder.AddEngine(new DestroyWithEngine());

            var entity = worldBuilder.CreateEntity();
            worldBuilder.SetComponent(entity, new MockComponent { });

            var world = worldBuilder.Build();
            world.Update(0.01);
            world.Update(0.01); // update twice so the read happens after destroy

            readEntities.Should().BeEmpty();
        }

        [Reads(typeof(MockComponent))]
        class DestroyAllWithEngine : Engine
        {
            public override void Update(double dt)
            {
                DestroyAllWith<MockComponent>();
            }
        }

        [Test]
        public void DestroyAllWith()
        {
            var worldBuilder = new WorldBuilder();
            worldBuilder.AddEngine(new ReadEntitiesWithComponentTypeEngine());
            worldBuilder.AddEngine(new DestroyAllWithEngine());

            var entity = worldBuilder.CreateEntity();
            worldBuilder.SetComponent(entity, new MockComponent { });

            var entityB = worldBuilder.CreateEntity();
            worldBuilder.SetComponent(entityB, new MockComponent { });

            var world = worldBuilder.Build();
            world.Update(0.01);
            world.Update(0.01); // update twice so the read happens after destroy

            readEntities.Should().BeEmpty();
        }

        [Reads(typeof(MockComponent))]
        class RemoveComponentByTypeEngine : Engine
        {
            public override void Update(double dt)
            {
                foreach (var (_, entity) in ReadComponentsIncludingEntity<MockComponent>())
                {
                    RemoveComponent<MockComponent>(entity);
                }
            }
        }

        [Test]
        public void RemoveComponentByType()
        {
            var worldBuilder = new WorldBuilder();
            worldBuilder.AddEngine(new ReadComponentsTestEngine());
            worldBuilder.AddEngine(new RemoveComponentByTypeEngine());

            var entity = worldBuilder.CreateEntity();
            worldBuilder.SetComponent(entity, new MockComponent { });

            var entityB = worldBuilder.CreateEntity();
            worldBuilder.SetComponent(entity, new MockComponent { });

            var world = worldBuilder.Build();

            world.Update(0.01);
            world.Update(0.01);

            resultComponents.Should().BeEmpty();
        }

        static double dilatedDeltaTime;

        class ActivateTimeDilationEngine : Engine
        {
            public override void Update(double dt)
            {
                if (!TimeDilationActive)
                {
                    ActivateTimeDilation(0.2, 1, 1, 1);
                }
                else
                {
                    dilatedDeltaTime = dt;
                }
            }
        }

        [Test]
        public void ActivateTimeDilation()
        {
            var worldBuilder = new WorldBuilder();
            worldBuilder.AddEngine(new ActivateTimeDilationEngine());

            var world = worldBuilder.Build();

            world.Update(0.01); // activate time dilation

            world.Update(0.5);

            dilatedDeltaTime.Should().BeApproximately(0.3, 0.01);

            world.Update(0.5);

            dilatedDeltaTime.Should().BeApproximately(0.1, 0.01);

            world.Update(1);

            world.Update(0.5);

            dilatedDeltaTime.Should().BeApproximately(0.3, 0.01);
        }

        class ActivateTimeDilationLowerFactorEngine : Engine
        {
            private bool activated = false;

            public override void Update(double dt)
            {
                if (!activated)
                {
                    ActivateTimeDilation(0.2, 1, 1, 1);
                    activated = true;
                }
            }
        }

        class ActivateTimeDilationHigherFactorEngine : Engine
        {
            private bool activated = false;

            public override void Update(double dt)
            {
                if (!activated)
                {
                    ActivateTimeDilation(0.5, 1, 1, 1);
                    activated = true;
                }
            }
        }

        static bool timeDilationActive;
        class ReadDilatedDeltaTimeEngine : Engine
        {
            public override void Update(double dt)
            {
                dilatedDeltaTime = dt;
                timeDilationActive = TimeDilationActive;
            }
        }

        [Test]
        public void MultipleActivateTimeDilation()
        {
            var worldBuilder = new WorldBuilder();
            worldBuilder.AddEngine(new ReadDilatedDeltaTimeEngine());
            worldBuilder.AddEngine(new ActivateTimeDilationLowerFactorEngine());
            worldBuilder.AddEngine(new ActivateTimeDilationHigherFactorEngine());

            var world = worldBuilder.Build();

            world.Update(0.01); // activate time dilation

            world.Update(0.5); // 0.3 and 0.375

            dilatedDeltaTime.Should().BeApproximately(0.3375, 0.01);
            timeDilationActive.Should().BeTrue();

            world.Update(5);

            dilatedDeltaTime.Should().BeApproximately(5, 0.01);
            timeDilationActive.Should().BeFalse();
        }

        static double undilatedDeltaTime;

        [IgnoresTimeDilation]
        class IgnoresTimeDilationEngine : Engine
        {
            public override void Update(double dt)
            {
                undilatedDeltaTime = dt;
            }
        }

        [Test]
        public void IgnoresTimeDilation()
        {
            var worldBuilder = new WorldBuilder();
            worldBuilder.AddEngine(new ActivateTimeDilationEngine());
            worldBuilder.AddEngine(new IgnoresTimeDilationEngine());

            var world = worldBuilder.Build();

            world.Update(0.01); // activate time dilation

            world.Update(0.5);

            undilatedDeltaTime.Should().Be(0.5);
        }
    }
}