moonshine-kind 0.5.0

use std::borrow::Borrow;
use std::{
    cmp::Ordering,
    fmt,
    hash::{Hash, Hasher},
    marker::PhantomData,
    ops::{Deref, DerefMut},
};

use bevy_derive::{Deref, DerefMut};
use bevy_ecs::change_detection::MaybeLocation;
use bevy_ecs::component::Mutable;
use bevy_ecs::query::EcsAccessType;
use bevy_ecs::relationship::RelationshipSourceCollection;
use bevy_ecs::{
    archetype::Archetype,
    change_detection::Tick,
    component::{ComponentId, Components},
    entity::{EntityMapper, MapEntities},
    prelude::*,
    query::{FilteredAccess, IterQueryData, QueryData, ReadOnlyQueryData, WorldQuery},
    storage::{Table, TableRow},
    system::EntityCommands,
    world::unsafe_world_cell::UnsafeWorldCell,
};
use bevy_reflect::Reflect;

use crate::{Any, CastInto, Kind};

/// Represents an [`Entity`] of [`Kind`] `T`.
///
/// `Instance<Any>` is functionally equivalent to an entity.
///
/// # Usage
/// An `Instance<T>` can be used to access entities in a "kind-safe" manner to improve safety and readability.
///
/// This type is designed to behave exactly like an [`Entity`].
///
/// This means you may use it as a [`Query`] parameter, pass it to [`Commands`] to access [`InstanceCommands<T>`],
/// or store it as a type-safe reference to an [`Entity`].
///
/// Note that an `Instance<T>` has `'static` lifetime and does not contain any [`Component`] data.
/// It *only* contains type information.
///
/// # Example
/// ```
/// # use bevy::prelude::*;
/// # use moonshine_kind::prelude::*;
///
/// #[derive(Component)]
/// struct Apple;
///
/// #[derive(Component)]
/// struct Orange;
///
/// struct Fruit;
///
/// impl Kind for Fruit {
///     type Filter = Or<(With<Apple>, With<Orange>)>;
/// }
///
/// #[derive(Resource, Deref, DerefMut)]
/// struct FruitBasket(Vec<Instance<Fruit>>);
///
/// fn collect_fruits(mut basket: ResMut<FruitBasket>, fruits: Query<Instance<Fruit>>) {
///     for fruit in fruits.iter() {
///         println!("{fruit:?}");
///         basket.push(fruit);
///     }
/// }
///
/// # bevy_ecs::system::assert_is_system(collect_fruits);
/// ```
#[derive(Reflect)]
pub struct Instance<T: Kind>(Entity, #[reflect(ignore)] PhantomData<T>);

impl<T: Kind> Instance<T> {
    /// Same as [`Entity::PLACEHOLDER`], but for an [`Instance<T>`].
    pub const PLACEHOLDER: Self = Self(Entity::PLACEHOLDER, PhantomData);

    /// Creates a new instance of kind `T` from some [`Entity`].
    ///
    /// # Usage
    /// This function is useful when you **know** an `Entity` is of a specific kind and you
    /// need an `Instance<T>` with no way to validate it.
    ///
    /// See [`Instance::from_entity`] for a safer alternative.
    ///
    /// # Safety
    /// Assumes `entity` is a valid instance of kind `T`.
    ///
    /// # Example
    /// ```
    /// # use bevy::prelude::*;
    /// # use moonshine_kind::prelude::*;
    ///
    /// #[derive(Component)]
    /// struct Apple;
    ///
    /// fn init_apple(entity: Entity, commands: &mut Commands) -> Instance<Apple> {
    ///     commands.entity(entity).insert(Apple);
    ///     // SAFE: `entity` will be a valid instance of `Apple`.
    ///     unsafe { Instance::from_entity_unchecked(entity) }
    /// }
    /// ```
    pub unsafe fn from_entity_unchecked(entity: Entity) -> Self {
        Self(entity, PhantomData)
    }

    /// Returns the [`Entity`] of this instance.
    pub fn entity(&self) -> Entity {
        self.0
    }

    /// Converts this instance into an instance of another kind [`Kind`] `U`.
    ///
    /// # Usage
    /// A kind `T` is safety convertible to another kind `U` if `T` implements [`CastInto<U>`].
    pub fn cast_into<U: Kind>(self) -> Instance<U>
    where
        T: CastInto<U>,
    {
        unsafe { T::cast(self) }
    }

    /// Converts this instance into an instance of [`Kind`] [`Any`].
    ///
    /// # Usage
    ///
    /// Any [`Instance<T>`] can be safely cast into an [`Instance<Any>`] using this function.
    pub fn cast_into_any(self) -> Instance<Any> {
        // SAFE: All instances are of kind `Any`.
        unsafe { self.cast_into_unchecked() }
    }

    /// Converts this instance into an instance of another kind [`Kind`] `U` without any validation.
    ///
    /// # Usage
    /// This function is useful when you **know** an `Instance<T>` is convertible to a specific type and you
    /// need an `Instance<U>` with no way to validate it.
    ///
    /// Always prefer to explicitly declare safe casts with the [`CastInto`] trait and use [`Instance::cast_into`].
    ///
    /// # Safety
    /// Assumes this instance is also a valid `Instance<U>`.
    pub unsafe fn cast_into_unchecked<U: Kind>(self) -> Instance<U> {
        Instance::from_entity_unchecked(self.entity())
    }

    /// Returns a mutable reference to the internal [`Entity`] of this [`Instance`].
    ///
    /// # Safety
    /// You are responsible to ensure the entity is still a valid instance of [`Kind`] `T`.
    #[deprecated(note = "use `Instance::<T>::from_entity_unchecked` instead")]
    pub unsafe fn as_entity_mut(&mut self) -> &mut Entity {
        &mut self.0
    }
}

impl<T: Component> Instance<T> {
    /// Creates a new instance of kind `T` from some [`EntityRef`] if the entity has a [`Component`] of type `T`.
    pub fn from_entity(entity: EntityRef) -> Option<Self> {
        if entity.contains::<T>() {
            // SAFE: `entity` must be of kind `T`.
            Some(unsafe { Self::from_entity_unchecked(entity.id()) })
        } else {
            None
        }
    }
}

impl<T: Kind> Clone for Instance<T> {
    fn clone(&self) -> Self {
        *self
    }
}

impl<T: Kind> Copy for Instance<T> {}

impl<T: Kind> fmt::Debug for Instance<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{}({:?})", T::debug_name(), self.0)
    }
}

impl<T: Kind> fmt::Display for Instance<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(
            f,
            "{}({}v{})",
            T::debug_name(),
            self.0.index(),
            self.0.generation()
        )
    }
}

impl<T: Kind> Hash for Instance<T> {
    fn hash<H: Hasher>(&self, state: &mut H) {
        self.0.hash(state);
    }
}

impl<T: Kind, U: Kind> PartialEq<Instance<U>> for Instance<T> {
    fn eq(&self, other: &Instance<U>) -> bool {
        self.0 == other.0
    }
}

impl<T: Kind> PartialEq<Entity> for Instance<T> {
    fn eq(&self, other: &Entity) -> bool {
        self.0 == *other
    }
}

impl<T: Kind> PartialEq<Instance<T>> for Entity {
    fn eq(&self, other: &Instance<T>) -> bool {
        other == self
    }
}

impl<T: Kind> Eq for Instance<T> {}

impl<T: Kind> PartialOrd for Instance<T> {
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        Some(self.cmp(other))
    }
}

impl<T: Kind> Ord for Instance<T> {
    fn cmp(&self, other: &Self) -> Ordering {
        self.0.cmp(&other.0)
    }
}

impl<T: Kind> Deref for Instance<T> {
    type Target = Entity;

    fn deref(&self) -> &Self::Target {
        &self.0
    }
}

impl<T: Kind> Borrow<Entity> for Instance<T> {
    fn borrow(&self) -> &Entity {
        &self.0
    }
}

unsafe impl<T: Kind> WorldQuery for Instance<T> {
    type Fetch<'a> = <T::Filter as WorldQuery>::Fetch<'a>;

    type State = <T::Filter as WorldQuery>::State;

    fn shrink_fetch<'wlong: 'wshort, 'wshort>(fetch: Self::Fetch<'wlong>) -> Self::Fetch<'wshort> {
        <T::Filter as WorldQuery>::shrink_fetch(fetch)
    }

    unsafe fn init_fetch<'w>(
        world: UnsafeWorldCell<'w>,
        state: &Self::State,
        last_change_tick: Tick,
        change_tick: Tick,
    ) -> Self::Fetch<'w> {
        <T::Filter as WorldQuery>::init_fetch(world, state, last_change_tick, change_tick)
    }

    const IS_DENSE: bool = <T::Filter as WorldQuery>::IS_DENSE;

    unsafe fn set_archetype<'w>(
        fetch: &mut Self::Fetch<'w>,
        state: &Self::State,
        archetype: &'w Archetype,
        table: &'w Table,
    ) {
        <T::Filter as WorldQuery>::set_archetype(fetch, state, archetype, table)
    }

    unsafe fn set_table<'w>(fetch: &mut Self::Fetch<'w>, state: &Self::State, table: &'w Table) {
        <T::Filter as WorldQuery>::set_table(fetch, state, table)
    }

    fn update_component_access(state: &Self::State, access: &mut FilteredAccess) {
        <T::Filter as WorldQuery>::update_component_access(state, access)
    }

    fn get_state(components: &Components) -> Option<Self::State> {
        <T::Filter as WorldQuery>::get_state(components)
    }

    fn init_state(world: &mut World) -> Self::State {
        <T::Filter as WorldQuery>::init_state(world)
    }

    fn matches_component_set(
        state: &Self::State,
        set_contains_id: &impl Fn(ComponentId) -> bool,
    ) -> bool {
        <T::Filter as WorldQuery>::matches_component_set(state, set_contains_id)
    }
}

unsafe impl<T: Kind> IterQueryData for Instance<T> {}

unsafe impl<T: Kind> ReadOnlyQueryData for Instance<T> {}

unsafe impl<T: Kind> QueryData for Instance<T> {
    type ReadOnly = Self;

    const IS_READ_ONLY: bool = <Entity as QueryData>::IS_READ_ONLY;

    const IS_ARCHETYPAL: bool = <Entity as QueryData>::IS_ARCHETYPAL;

    type Item<'w, 's> = Self;

    fn shrink<'wlong: 'wshort, 'wshort, 's>(
        item: Self::Item<'wlong, 's>,
    ) -> Self::Item<'wshort, 's> {
        item
    }

    unsafe fn fetch<'w, 's>(
        _state: &'s Self::State,
        _fetch: &mut Self::Fetch<'w>,
        entity: Entity,
        _table_row: TableRow,
    ) -> Option<Self::Item<'w, 's>> {
        Some(Instance::from_entity_unchecked(entity))
    }

    fn iter_access(_state: &Self::State) -> impl Iterator<Item = EcsAccessType<'_>> {
        // Based on impl for `Entity`
        std::iter::empty()
    }
}

impl<T: Kind> MapEntities for Instance<T> {
    fn map_entities<M: EntityMapper>(&mut self, entity_mapper: &mut M) {
        self.0 = entity_mapper.get_mapped(self.0);
    }
}

impl<T: Kind> From<Instance<T>> for Entity {
    fn from(instance: Instance<T>) -> Self {
        instance.entity()
    }
}

impl<T: Kind> RelationshipSourceCollection for Instance<T> {
    type SourceIter<'a> = <Entity as RelationshipSourceCollection>::SourceIter<'a>;

    fn new() -> Self {
        Self::PLACEHOLDER
    }

    fn with_capacity(_capacity: usize) -> Self {
        Self::new()
    }

    fn reserve(&mut self, additional: usize) {
        self.0.reserve(additional);
    }

    fn add(&mut self, entity: Entity) -> bool {
        self.0.add(entity)
    }

    fn remove(&mut self, entity: Entity) -> bool {
        self.0.remove(entity)
    }

    fn iter(&self) -> Self::SourceIter<'_> {
        self.0.iter()
    }

    fn len(&self) -> usize {
        self.0.len()
    }

    fn clear(&mut self) {
        self.0.clear();
    }

    fn shrink_to_fit(&mut self) {
        self.0.shrink_to_fit();
    }

    fn extend_from_iter(&mut self, entities: impl IntoIterator<Item = Entity>) {
        self.0.extend_from_iter(entities)
    }
}

impl From<Entity> for Instance<Any> {
    fn from(entity: Entity) -> Self {
        Self(entity, PhantomData)
    }
}

impl<T: Kind> ContainsEntity for Instance<T> {
    fn entity(&self) -> Entity {
        self.entity()
    }
}

/// Similar to [`ContainsEntity`], but for [`Instance<T>`].
pub trait ContainsInstance<T: Kind> {
    /// Returns the associated [`Instance<T>`].
    fn instance(&self) -> Instance<T>;

    /// Returns the [`Entity`] of the associated [`Instance<T>`].
    fn entity(&self) -> Entity {
        self.instance().entity()
    }
}

/// A [`QueryData`] item which represents a reference to an [`Instance<T>`] and its associated [`Component`].
///
/// This is analogous to a `(Instance<T>, &T)` query.
///
/// # Usage
/// If a [`Kind`] is also a component, it is often convenient to access the instance and component data together.
/// This type is designed to make these queries more ergonomic.
///
/// You may use this type as either a [`Query`] parameter, or access it from an [`EntityRef`].
///
/// # Example
/// ```
/// # use bevy::prelude::*;
/// # use moonshine_kind::prelude::*;
///
/// #[derive(Component)]
/// struct Apple {
///     freshness: f32,
/// }
///
/// impl Apple {
///     fn is_fresh(&self) -> bool {
///         self.freshness >= 0.5
///     }
/// }
///
/// // Query Access:
/// fn fresh_apples(query: Query<InstanceRef<Apple>>) -> Vec<Instance<Apple>> {
///     query.iter()
///         .filter_map(|apple| apple.is_fresh().then_some(apple.instance()))
///         .collect()
/// }
///
/// // Entity Access:
/// fn fresh_apples_world<'a>(world: &'a World) -> Vec<InstanceRef<'a, Apple>> {
///    world.try_query::<EntityRef>()
///         .unwrap()
///         .iter(&world)
///         .filter_map(|entity| InstanceRef::from_entity(entity))
///         .collect()
/// }
///
/// # bevy_ecs::system::assert_is_system(fresh_apples);
/// ```
pub struct InstanceRef<'a, T: Component>(Instance<T>, &'a T);

unsafe impl<T: Component> WorldQuery for InstanceRef<'_, T> {
    type Fetch<'w> = <(Instance<T>, &'static T) as WorldQuery>::Fetch<'w>;

    type State = <(Instance<T>, &'static T) as WorldQuery>::State;

    fn shrink_fetch<'wlong: 'wshort, 'wshort>(fetch: Self::Fetch<'wlong>) -> Self::Fetch<'wshort> {
        <(Instance<T>, &T) as WorldQuery>::shrink_fetch(fetch)
    }

    unsafe fn init_fetch<'w>(
        world: UnsafeWorldCell<'w>,
        state: &Self::State,
        last_run: Tick,
        this_run: Tick,
    ) -> Self::Fetch<'w> {
        <(Instance<T>, &T) as WorldQuery>::init_fetch(world, state, last_run, this_run)
    }

    const IS_DENSE: bool = <(Instance<T>, &T) as WorldQuery>::IS_DENSE;

    unsafe fn set_archetype<'w>(
        fetch: &mut Self::Fetch<'w>,
        state: &Self::State,
        archetype: &'w Archetype,
        table: &'w Table,
    ) {
        <(Instance<T>, &T) as WorldQuery>::set_archetype(fetch, state, archetype, table)
    }

    unsafe fn set_table<'w>(fetch: &mut Self::Fetch<'w>, state: &Self::State, table: &'w Table) {
        <(Instance<T>, &T) as WorldQuery>::set_table(fetch, state, table)
    }

    fn update_component_access(state: &Self::State, access: &mut FilteredAccess) {
        <(Instance<T>, &T) as WorldQuery>::update_component_access(state, access)
    }

    fn init_state(world: &mut World) -> Self::State {
        <(Instance<T>, &T) as WorldQuery>::init_state(world)
    }

    fn get_state(components: &Components) -> Option<Self::State> {
        <(Instance<T>, &T) as WorldQuery>::get_state(components)
    }

    fn matches_component_set(
        state: &Self::State,
        set_contains_id: &impl Fn(ComponentId) -> bool,
    ) -> bool {
        <(Instance<T>, &T) as WorldQuery>::matches_component_set(state, set_contains_id)
    }
}

unsafe impl<T: Component> QueryData for InstanceRef<'_, T> {
    type ReadOnly = Self;

    const IS_READ_ONLY: bool = true;

    const IS_ARCHETYPAL: bool = <&'static T as QueryData>::IS_ARCHETYPAL;

    type Item<'w, 's> = InstanceRef<'w, T>;

    fn shrink<'wlong: 'wshort, 'wshort, 's>(
        item: Self::Item<'wlong, 's>,
    ) -> Self::Item<'wshort, 's> {
        InstanceRef(item.0, item.1)
    }

    unsafe fn fetch<'w, 's>(
        state: &'s Self::State,
        fetch: &mut Self::Fetch<'w>,
        entity: Entity,
        table_row: TableRow,
    ) -> Option<Self::Item<'w, 's>> {
        <(Instance<T>, &T) as QueryData>::fetch(state, fetch, entity, table_row)
            .map(|(instance, data)| InstanceRef(instance, data))
    }

    fn iter_access(state: &Self::State) -> impl Iterator<Item = EcsAccessType<'_>> {
        <(Instance<T>, &T) as QueryData>::iter_access(state)
    }
}

unsafe impl<T: Component> IterQueryData for InstanceRef<'_, T> {}

unsafe impl<T: Component> ReadOnlyQueryData for InstanceRef<'_, T> {}

impl<'a, T: Component> InstanceRef<'a, T> {
    /// Creates a new [`InstanceRef<T>`] from an [`EntityRef`] if it contains a given [`Component`] of type `T`.
    pub fn from_entity(entity: EntityRef<'a>) -> Option<Self> {
        Some(Self(
            // SAFE: Kind is validated by `entity.get()` above.
            unsafe { Instance::from_entity_unchecked(entity.id()) },
            entity.get()?,
        ))
    }

    /// Creates a new [`InstanceRef<T>`] from [`EntityRef`] without any validation.
    ///
    /// # Safety
    /// Assumes `entity` is a valid instance of kind `T`.
    pub unsafe fn from_entity_unchecked(entity: EntityRef<'a>) -> Self {
        Self(
            Instance::from_entity_unchecked(entity.id()),
            entity.get().unwrap(),
        )
    }
}

impl<T: Component> Clone for InstanceRef<'_, T> {
    fn clone(&self) -> Self {
        *self
    }
}

impl<T: Component> Copy for InstanceRef<'_, T> {}

impl<T: Component> From<InstanceRef<'_, T>> for Instance<T> {
    fn from(item: InstanceRef<T>) -> Self {
        item.instance()
    }
}

impl<T: Component> From<&InstanceRef<'_, T>> for Instance<T> {
    fn from(item: &InstanceRef<T>) -> Self {
        item.instance()
    }
}

impl<T: Component> PartialEq for InstanceRef<'_, T> {
    fn eq(&self, other: &Self) -> bool {
        self.0 == other.0
    }
}

impl<T: Component> PartialEq<Entity> for InstanceRef<'_, T> {
    fn eq(&self, other: &Entity) -> bool {
        self.0 == *other
    }
}

impl<T: Component> PartialEq<InstanceRef<'_, T>> for Entity {
    fn eq(&self, other: &InstanceRef<'_, T>) -> bool {
        other == self
    }
}

impl<T: Component, U: Component> PartialEq<Instance<U>> for InstanceRef<'_, T>
where
    U: CastInto<T>,
{
    fn eq(&self, other: &Instance<U>) -> bool {
        *self.0 == *other
    }
}

impl<T: Component, U: Component> PartialEq<InstanceRef<'_, U>> for Instance<T>
where
    U: CastInto<T>,
{
    fn eq(&self, other: &InstanceRef<'_, U>) -> bool {
        *self == other.instance()
    }
}

impl<T: Component> Eq for InstanceRef<'_, T> {}

impl<T: Component> Deref for InstanceRef<'_, T> {
    type Target = T;

    fn deref(&self) -> &Self::Target {
        self.1
    }
}

impl<T: Component> AsRef<Instance<T>> for InstanceRef<'_, T> {
    fn as_ref(&self) -> &Instance<T> {
        &self.0
    }
}

impl<T: Component> AsRef<T> for InstanceRef<'_, T> {
    fn as_ref(&self) -> &T {
        self.1
    }
}

impl<T: Component> ContainsInstance<T> for InstanceRef<'_, T> {
    fn instance(&self) -> Instance<T> {
        self.0
    }
}

/// A [`QueryData`] item which represents a mutable reference to an [`Instance<T>`] and its associated [`Component`].
///
/// This is analogous to a `(Instance<T>, &mut T)` query.
///
/// # Usage
/// This type behaves similar like [`InstanceRef<T>`] but allows mutable access to its associated [`Component`].
///
/// The main difference is that you cannot create an [`InstanceMut<T>`] from an [`EntityMut`].
/// See [`InstanceMut::from_entity`] for more details.
///
/// See [`InstanceRef<T>`] for more information and examples.
pub struct InstanceMut<'a, T: Component>(Instance<T>, Mut<'a, T>);

unsafe impl<T: Component> WorldQuery for InstanceMut<'_, T> {
    type Fetch<'w> = <(Instance<T>, &'static mut T) as WorldQuery>::Fetch<'w>;

    type State = <(Instance<T>, &'static mut T) as WorldQuery>::State;

    fn shrink_fetch<'wlong: 'wshort, 'wshort>(fetch: Self::Fetch<'wlong>) -> Self::Fetch<'wshort> {
        <(Instance<T>, &mut T) as WorldQuery>::shrink_fetch(fetch)
    }

    unsafe fn init_fetch<'w>(
        world: UnsafeWorldCell<'w>,
        state: &Self::State,
        last_run: Tick,
        this_run: Tick,
    ) -> Self::Fetch<'w> {
        <(Instance<T>, &mut T) as WorldQuery>::init_fetch(world, state, last_run, this_run)
    }

    const IS_DENSE: bool = <(Instance<T>, &T) as WorldQuery>::IS_DENSE;

    unsafe fn set_archetype<'w>(
        fetch: &mut Self::Fetch<'w>,
        state: &Self::State,
        archetype: &'w Archetype,
        table: &'w Table,
    ) {
        <(Instance<T>, &mut T) as WorldQuery>::set_archetype(fetch, state, archetype, table)
    }

    unsafe fn set_table<'w>(fetch: &mut Self::Fetch<'w>, state: &Self::State, table: &'w Table) {
        <(Instance<T>, &mut T) as WorldQuery>::set_table(fetch, state, table)
    }

    fn update_component_access(state: &Self::State, access: &mut FilteredAccess) {
        <(Instance<T>, &T) as WorldQuery>::update_component_access(state, access)
    }

    fn init_state(world: &mut World) -> Self::State {
        <(Instance<T>, &T) as WorldQuery>::init_state(world)
    }

    fn get_state(components: &Components) -> Option<Self::State> {
        <(Instance<T>, &T) as WorldQuery>::get_state(components)
    }

    fn matches_component_set(
        state: &Self::State,
        set_contains_id: &impl Fn(ComponentId) -> bool,
    ) -> bool {
        <(Instance<T>, &T) as WorldQuery>::matches_component_set(state, set_contains_id)
    }
}

unsafe impl<'b, T: Component<Mutability = Mutable>> QueryData for InstanceMut<'b, T> {
    type ReadOnly = InstanceRef<'b, T>;

    const IS_READ_ONLY: bool = false;

    const IS_ARCHETYPAL: bool = <&'static mut T as QueryData>::IS_ARCHETYPAL;

    type Item<'w, 's> = InstanceMut<'w, T>;

    fn shrink<'wlong: 'wshort, 'wshort, 's>(
        item: Self::Item<'wlong, 's>,
    ) -> Self::Item<'wshort, 's> {
        InstanceMut(item.0, item.1)
    }

    unsafe fn fetch<'w, 's>(
        state: &'s Self::State,
        fetch: &mut Self::Fetch<'w>,
        entity: Entity,
        table_row: TableRow,
    ) -> Option<Self::Item<'w, 's>> {
        <(Instance<T>, &mut T) as QueryData>::fetch(state, fetch, entity, table_row)
            .map(|(instance, data)| InstanceMut(instance, data))
    }

    fn iter_access(state: &Self::State) -> impl Iterator<Item = EcsAccessType<'_>> {
        <(Instance<T>, &mut T) as QueryData>::iter_access(state)
    }
}

impl<'a, T: Component<Mutability = Mutable>> InstanceMut<'a, T> {
    /// Creates a new [`InstanceMut<T>`] from an [`EntityWorldMut`] if it contains a given [`Component`] of type `T`.
    pub fn from_entity(entity: EntityMut<'a>) -> Option<Self> {
        let id = entity.id();
        let data = entity.into_mut()?;
        Some(Self(
            // SAFE: Kind is validated by `entity.get_mut()` above.
            unsafe { Instance::from_entity_unchecked(id) },
            data,
        ))
    }

    /// Creates a new [`InstanceMut<T>`] from an [`EntityMut`] without any validation.
    ///
    /// # Safety
    /// Assumes `entity` is a valid instance of kind `T`.
    pub unsafe fn from_entity_unchecked(entity: EntityMut<'a>) -> Self {
        let id = entity.id();
        let data = entity.into_mut().unwrap();
        Self(Instance::from_entity_unchecked(id), data)
    }
}

impl<T: Component> From<InstanceMut<'_, T>> for Instance<T> {
    fn from(item: InstanceMut<T>) -> Self {
        item.instance()
    }
}

impl<T: Component> From<&InstanceMut<'_, T>> for Instance<T> {
    fn from(item: &InstanceMut<T>) -> Self {
        item.instance()
    }
}

impl<T: Component> PartialEq for InstanceMut<'_, T> {
    fn eq(&self, other: &Self) -> bool {
        self.0 == other.0
    }
}

impl<T: Component> PartialEq<Entity> for InstanceMut<'_, T> {
    fn eq(&self, other: &Entity) -> bool {
        self.0 == *other
    }
}

impl<T: Component> PartialEq<InstanceMut<'_, T>> for Entity {
    fn eq(&self, other: &InstanceMut<'_, T>) -> bool {
        other == self
    }
}

impl<T: Component, U: Component> PartialEq<Instance<U>> for InstanceMut<'_, T>
where
    U: CastInto<T>,
{
    fn eq(&self, other: &Instance<U>) -> bool {
        *self.0 == *other
    }
}

impl<T: Component, U: Component> PartialEq<InstanceMut<'_, U>> for Instance<T>
where
    U: CastInto<T>,
{
    fn eq(&self, other: &InstanceMut<'_, U>) -> bool {
        *self == other.instance()
    }
}

impl<T: Component> Eq for InstanceMut<'_, T> {}

impl<T: Component> Deref for InstanceMut<'_, T> {
    type Target = T;

    fn deref(&self) -> &Self::Target {
        self.1.as_ref()
    }
}

impl<T: Component> DerefMut for InstanceMut<'_, T> {
    fn deref_mut(&mut self) -> &mut Self::Target {
        self.1.as_mut()
    }
}

impl<T: Component> AsRef<Instance<T>> for InstanceMut<'_, T> {
    fn as_ref(&self) -> &Instance<T> {
        &self.0
    }
}

impl<T: Component> AsRef<T> for InstanceMut<'_, T> {
    fn as_ref(&self) -> &T {
        self.1.as_ref()
    }
}

impl<T: Component> AsMut<T> for InstanceMut<'_, T> {
    fn as_mut(&mut self) -> &mut T {
        self.1.as_mut()
    }
}

impl<T: Component> DetectChanges for InstanceMut<'_, T> {
    fn is_added(&self) -> bool {
        self.1.is_added()
    }

    fn is_changed(&self) -> bool {
        self.1.is_changed()
    }

    fn last_changed(&self) -> Tick {
        self.1.last_changed()
    }

    fn added(&self) -> Tick {
        self.1.added()
    }

    fn changed_by(&self) -> MaybeLocation {
        self.1.changed_by()
    }

    fn is_added_after(&self, other: Tick) -> bool {
        self.1.is_added_after(other)
    }

    fn is_changed_after(&self, other: Tick) -> bool {
        self.1.is_changed_after(other)
    }
}

impl<T: Component> DetectChangesMut for InstanceMut<'_, T> {
    type Inner = T;

    fn set_changed(&mut self) {
        self.1.set_changed();
    }

    fn set_last_changed(&mut self, last_changed: Tick) {
        self.1.set_last_changed(last_changed);
    }

    fn bypass_change_detection(&mut self) -> &mut Self::Inner {
        self.1.bypass_change_detection()
    }

    fn set_added(&mut self) {
        self.1.set_added();
    }

    fn set_last_added(&mut self, last_added: Tick) {
        self.1.set_last_added(last_added);
    }
}

impl<T: Component> ContainsInstance<T> for InstanceMut<'_, T> {
    fn instance(&self) -> Instance<T> {
        self.0
    }
}

/// Extension trait to access [`InstanceCommands<T>`] from [`Commands`].
///
/// See [`InstanceCommands`] for more information.
pub trait GetInstanceCommands<T: Kind> {
    /// Returns the [`InstanceCommands<T>`] for an [`Instance<T>`].
    fn instance(&mut self, instance: Instance<T>) -> InstanceCommands<'_, T>;
}

impl<T: Kind> GetInstanceCommands<T> for Commands<'_, '_> {
    fn instance(&mut self, instance: Instance<T>) -> InstanceCommands<'_, T> {
        InstanceCommands(self.entity(instance.entity()), PhantomData)
    }
}

/// [`EntityCommands`] with kind semantics.
///
/// # Usage
/// On its own, this type is not very useful. Instead, it is designed to be extended using traits.
/// This allows you to design commands for a specific kind of an entity in a type-safe manner.
///
/// # Example
/// ```
/// # use bevy::prelude::*;
/// # use moonshine_kind::prelude::*;
///
/// #[derive(Component)]
/// struct Apple;
///
/// #[derive(Component)]
/// struct Eat;
///
/// trait EatApple {
///     fn eat(&mut self);
/// }
///
/// impl EatApple for InstanceCommands<'_, Apple> {
///     fn eat(&mut self) {
///         info!("Crunch!");
///         self.despawn();
///     }
/// }
///
/// fn eat_apples(apples: Query<Instance<Apple>, With<Eat>>, mut commands: Commands) {
///     for apple in apples.iter() {
///         commands.instance(apple).eat();
///     }
/// }
///
/// # bevy_ecs::system::assert_is_system(eat_apples);
pub struct InstanceCommands<'a, T: Kind>(EntityCommands<'a>, PhantomData<T>);

impl<'a, T: Kind> InstanceCommands<'a, T> {
    /// Creates a new [`InstanceCommands<T>`] from [`EntityCommands`] without any validation.
    ///
    /// # Safety
    /// Assumes `entity` is a valid instance of kind `T`.
    pub unsafe fn from_entity_unchecked(entity: EntityCommands<'a>) -> Self {
        Self(entity, PhantomData)
    }

    /// Creates a new [`InstanceCommands<T>`] from [`EntityRef`] if it contains a [`Component`] of type `T`.
    pub fn from_entity(entity: EntityRef, commands: &'a mut Commands) -> Option<Self>
    where
        T: Component,
    {
        if entity.contains::<T>() {
            Some(Self(commands.entity(entity.id()), PhantomData))
        } else {
            None
        }
    }

    /// Returns the associated [`Instance<T>`].
    pub fn instance(&self) -> Instance<T> {
        // SAFE: `self.entity()` must be a valid instance of kind `T`.
        unsafe { Instance::from_entity_unchecked(self.id()) }
    }

    /// Returns the associated [`EntityCommands`].
    pub fn as_entity(&mut self) -> &mut EntityCommands<'a> {
        &mut self.0
    }

    /// Equivalent to [`EntityCommands::insert`], but it returns `self` to maintain kind semantics.
    pub fn insert(&mut self, bundle: impl Bundle) -> &mut Self {
        self.0.insert(bundle);
        self
    }

    /// Equivalent to [`EntityCommands::insert`], but it returns `self` to maintain kind semantics.
    pub fn remove<U: Component>(&mut self) -> &mut Self {
        self.0.remove::<U>();
        self
    }

    /// Equivalent to [`EntityCommands::try_insert`], but it returns `self` to maintain kind semantics.
    pub fn try_remove<U: Component>(&mut self) -> &mut Self {
        self.0.try_remove::<U>();
        self
    }

    /// Returns an [`InstanceCommands`] with a smaller lifetime.
    ///
    /// This is useful if you have `&mut InstanceCommands` but you need `InstanceCommands`.
    pub fn reborrow(&mut self) -> InstanceCommands<'_, T> {
        InstanceCommands(self.0.reborrow(), PhantomData)
    }

    /// Converts this [`InstanceCommands<T>`] into an [`InstanceCommands<U>`], given that `T` implements [`CastInto<U>`].
    pub fn cast_into<U: Kind>(self) -> InstanceCommands<'a, U>
    where
        T: CastInto<U>,
    {
        // SAFE: `CastInto<U>` is implemented for `T`.
        unsafe { InstanceCommands::from_entity_unchecked(self.0) }
    }
}

impl<'a, T: Kind> From<InstanceCommands<'a, T>> for Instance<T> {
    fn from(commands: InstanceCommands<'a, T>) -> Self {
        commands.instance()
    }
}

impl<'a, T: Kind> From<&InstanceCommands<'a, T>> for Instance<T> {
    fn from(commands: &InstanceCommands<'a, T>) -> Self {
        commands.instance()
    }
}

impl<'a, T: Kind> Deref for InstanceCommands<'a, T> {
    type Target = EntityCommands<'a>;

    fn deref(&self) -> &Self::Target {
        &self.0
    }
}

impl<T: Kind> DerefMut for InstanceCommands<'_, T> {
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.0
    }
}

impl<T: Kind> ContainsInstance<T> for InstanceCommands<'_, T> {
    fn instance(&self) -> Instance<T> {
        self.instance()
    }
}

/// A macro which implements [`EntityEvent`] from an [`Instance<T>`].
///
/// This is useful when you have an [`EntityEvent`] which refers to its target by [`Instance<T>`].
///
/// ```
/// use bevy::prelude::*;
/// use bevy::ecs::event::EntityTrigger;
/// use moonshine_kind::prelude::*;
/// use moonshine_kind::impl_entity_event_from_instance;
///
/// #[derive(Component)]
/// struct Fruit;
///
/// #[derive(Event)]
/// #[event(trigger = EntityTrigger)]
/// struct Eat {
///     target: Instance<Fruit>
/// }
///
/// impl_entity_event_from_instance!(Eat { .target, .. });
///
/// let mut world = World::new();
/// let fruit = world.spawn_instance(Fruit).instance();
/// world.trigger_with(Eat { target: fruit }, EntityTrigger);
/// ```
#[macro_export]
macro_rules! impl_entity_event_from_instance {
    ($name:ident < $($gen:tt),+ $(,)? > $(where $($where:tt)+)? ) => {
        $crate::impl_entity_event_from_instance!($name<$($gen),+> { .instance, .. } $(where $($where)+)?);
    };

    ($name:ident < $($gen:tt),+ $(,)? > { .$field:ident, .. } $(where $($where:tt)+)? ) => {
        impl<$($gen),+> EntityEvent for $name<$($gen),+>
        $(where $($where)+)? {
            fn event_target(&self) -> Entity {
                self.$field.entity()
            }
        }
    };

    ($name:ident) => {
        $crate::impl_entity_event_from_instance!($name { .instance, .. });
    };

    ($name:ident { .$field:ident, .. }) => {
        impl EntityEvent for $name {
            fn event_target(&self) -> Entity {
                self.$field.entity()
            }
        }

    }
}

#[test]
fn test_impl_entity_event_from_instance() {
    #![allow(unused)]

    #[derive(Event)]
    struct Foo {
        instance: Instance<Any>,
    }

    #[derive(Event)]
    struct Bar {
        inst: Instance<Any>,
    }

    #[derive(Event)]
    struct Baz<T: Kind> {
        instance: Instance<T>,
    }

    #[derive(Event)]
    struct Bat<T: Kind> {
        inst: Instance<T>,
    }

    impl_entity_event_from_instance!(Foo);
    impl_entity_event_from_instance!(Bar { .inst, .. });
    impl_entity_event_from_instance!(Baz<T> where T: Kind);
    impl_entity_event_from_instance!(Bat<T> { .inst, .. } where T: Kind);
}

// Experimental
#[doc(hidden)]
#[derive(Deref, DerefMut, Reflect)]
pub struct InstanceVec<T: Kind>(Vec<Instance<T>>);

impl<T: Kind> Default for InstanceVec<T> {
    fn default() -> Self {
        Self(Vec::new())
    }
}

impl<T: Kind> InstanceVec<T> {
    #[doc(hidden)]
    pub fn with_capacity(capacity: usize) -> Self {
        Self(Vec::with_capacity(capacity))
    }
}

impl<T: Kind> MapEntities for InstanceVec<T> {
    fn map_entities<M: EntityMapper>(&mut self, entity_mapper: &mut M) {
        let olds: Vec<_> = self.0.drain(..).collect();
        for old in olds {
            let new_entity = entity_mapper.get_mapped(old.entity());
            // SAFE: In Deserializer, we trust.
            let new = unsafe { Instance::from_entity_unchecked(new_entity) };
            self.0.push(new);
        }
    }
}

impl<T: Kind> RelationshipSourceCollection for InstanceVec<T> {
    type SourceIter<'a>
        = std::iter::Map<std::slice::Iter<'a, Instance<T>>, fn(&Instance<T>) -> Entity>
    where
        Self: 'a;

    fn new() -> Self {
        Self::default()
    }

    fn with_capacity(capacity: usize) -> Self {
        Self::with_capacity(capacity)
    }

    fn reserve(&mut self, additional: usize) {
        self.0.reserve(additional);
    }

    fn add(&mut self, entity: Entity) -> bool {
        self.0
            .push(unsafe { Instance::from_entity_unchecked(entity) });
        true
    }

    fn remove(&mut self, entity: Entity) -> bool {
        let Some(index) = self.0.iter().position(|i| *i == entity) else {
            return false;
        };
        self.0.swap_remove(index);
        true
    }

    fn iter(&self) -> Self::SourceIter<'_> {
        self.0.iter().map(|i| i.entity())
    }

    fn len(&self) -> usize {
        self.0.len()
    }

    fn clear(&mut self) {
        self.0.clear()
    }

    fn shrink_to_fit(&mut self) {
        self.0.shrink_to_fit()
    }

    fn extend_from_iter(&mut self, entities: impl IntoIterator<Item = Entity>) {
        self.0.extend(
            entities
                .into_iter()
                .map(|entity| unsafe { Instance::from_entity_unchecked(entity) }),
        )
    }
}

#[cfg(test)]
mod test_instance_vec {
    use super::*;

    // In a world, where the relationship:
    //      Friends(Vec<Instance<Person>>) <-> FriendOf(Instance<Person>)
    // Cannot exist due to Bevy derive requirements and Rust trait implementation restrictions...
    // This is a workaround to sort of guarantee kind safety:

    use bevy::prelude::*;
    use moonshine_util::expect::Expect; // <--- Required for this pattern to work.

    use crate::prelude::*;

    // Marker to "guarantee" kind safety across related components:
    #[derive(Component)]
    struct Person;

    // A potato is not a person. It doesn't have friends. :(
    #[derive(Component)]
    struct Potato;

    #[derive(Component, Deref)]
    #[require(Expect<Person>)] // <--- Only People can have Friends
    #[relationship_target(relationship = FriendOf)]
    struct Friends(
        // Wrapper around `Vec<Instance<Person>>` to implement `RelationshipSourceCollection`
        InstanceVec<Person>,
    );

    #[derive(Component)]
    #[require(Expect<Person>)] // <--- Only People become Friends
    #[relationship(relationship_target = Friends)]
    struct FriendOf(
        // This entity can point to anything, but if it points to anything other
        // than a Person, it would cause a panic because both `Friends` and `FriendOf` expect `Person`.
        pub Entity,
    );

    #[test]
    fn test_instance_vec_relationship() {
        let mut app = App::new();
        app.add_plugins(MinimalPlugins);
        let w = app.world_mut();
        let p0 = w.spawn_instance(Person).instance();
        let p1 = w.spawn_instance(Person).instance();
        w.entity_mut(*p1).insert(FriendOf(*p0));

        let fs = w.get::<Friends>(*p0).expect("Person 0 must have Friends");
        let f: Instance<Person> = *fs.first().expect("Person 0 must have at least 1 friend");
        assert_eq!(f, p1, "Person 1 must be a friend of Person 0");
    }

    #[test]
    #[should_panic]
    fn test_instance_vec_relationship_panic_1() {
        let mut app = App::new();
        app.add_plugins(MinimalPlugins);
        let w = app.world_mut();
        let potato = w.spawn_instance(Potato).instance();
        let person = w.spawn_instance(Person).instance();

        // PANIC: Person cannot be friends with Potato. :(
        w.entity_mut(*person).insert(FriendOf(*potato));
    }

    #[test]
    #[should_panic]
    fn test_instance_vec_relationship_panic_2() {
        let mut app = App::new();
        app.add_plugins(MinimalPlugins);
        let w = app.world_mut();
        let potato = w.spawn_instance(Potato).instance();
        let person = w.spawn_instance(Person).instance();

        // PANIC: Potato cannot be friends with Person. :(
        w.entity_mut(*potato).insert(FriendOf(*person));
    }
}