bevy_ecs/

component.rs

//! Types for declaring and storing [`Component`]s.

use crate::{
    self as bevy_ecs,
    archetype::ArchetypeFlags,
    bundle::BundleInfo,
    change_detection::MAX_CHANGE_AGE,
    entity::Entity,
    query::DebugCheckedUnwrap,
    storage::{SparseSetIndex, SparseSets, Storages, Table, TableRow},
    system::{Local, Resource, SystemParam},
    world::{DeferredWorld, FromWorld, World},
};
use alloc::{borrow::Cow, sync::Arc};
pub use bevy_ecs_macros::Component;
use bevy_ptr::{OwningPtr, UnsafeCellDeref};
#[cfg(feature = "bevy_reflect")]
use bevy_reflect::Reflect;
use bevy_utils::{HashMap, HashSet, TypeIdMap};
#[cfg(feature = "track_change_detection")]
use core::panic::Location;
use core::{
    alloc::Layout,
    any::{Any, TypeId},
    cell::UnsafeCell,
    fmt::Debug,
    marker::PhantomData,
    mem::needs_drop,
};
use derive_more::derive::{Display, Error};

/// A data type that can be used to store data for an [entity].
///
/// `Component` is a [derivable trait]: this means that a data type can implement it by applying a `#[derive(Component)]` attribute to it.
/// However, components must always satisfy the `Send + Sync + 'static` trait bounds.
///
/// [entity]: crate::entity
/// [derivable trait]: https://doc.rust-lang.org/book/appendix-03-derivable-traits.html
///
/// # Examples
///
/// Components can take many forms: they are usually structs, but can also be of every other kind of data type, like enums or zero sized types.
/// The following examples show how components are laid out in code.
///
/// ```
/// # use bevy_ecs::component::Component;
/// # struct Color;
/// #
/// // A component can contain data...
/// #[derive(Component)]
/// struct LicensePlate(String);
///
/// // ... but it can also be a zero-sized marker.
/// #[derive(Component)]
/// struct Car;
///
/// // Components can also be structs with named fields...
/// #[derive(Component)]
/// struct VehiclePerformance {
///     acceleration: f32,
///     top_speed: f32,
///     handling: f32,
/// }
///
/// // ... or enums.
/// #[derive(Component)]
/// enum WheelCount {
///     Two,
///     Three,
///     Four,
/// }
/// ```
///
/// # Component and data access
///
/// See the [`entity`] module level documentation to learn how to add or remove components from an entity.
///
/// See the documentation for [`Query`] to learn how to access component data from a system.
///
/// [`entity`]: crate::entity#usage
/// [`Query`]: crate::system::Query
///
/// # Choosing a storage type
///
/// Components can be stored in the world using different strategies with their own performance implications.
/// By default, components are added to the [`Table`] storage, which is optimized for query iteration.
///
/// Alternatively, components can be added to the [`SparseSet`] storage, which is optimized for component insertion and removal.
/// This is achieved by adding an additional `#[component(storage = "SparseSet")]` attribute to the derive one:
///
/// ```
/// # use bevy_ecs::component::Component;
/// #
/// #[derive(Component)]
/// #[component(storage = "SparseSet")]
/// struct ComponentA;
/// ```
///
/// [`Table`]: crate::storage::Table
/// [`SparseSet`]: crate::storage::SparseSet
///
/// # Required Components
///
/// Components can specify Required Components. If some [`Component`] `A` requires [`Component`] `B`,  then when `A` is inserted,
/// `B` will _also_ be initialized and inserted (if it was not manually specified).
///
/// The [`Default`] constructor will be used to initialize the component, by default:
///
/// ```
/// # use bevy_ecs::prelude::*;
/// #[derive(Component)]
/// #[require(B)]
/// struct A;
///
/// #[derive(Component, Default, PartialEq, Eq, Debug)]
/// struct B(usize);
///
/// # let mut world = World::default();
/// // This will implicitly also insert B with the Default constructor
/// let id = world.spawn(A).id();
/// assert_eq!(&B(0), world.entity(id).get::<B>().unwrap());
///
/// // This will _not_ implicitly insert B, because it was already provided
/// world.spawn((A, B(11)));
/// ```
///
/// Components can have more than one required component:
///
/// ```
/// # use bevy_ecs::prelude::*;
/// #[derive(Component)]
/// #[require(B, C)]
/// struct A;
///
/// #[derive(Component, Default, PartialEq, Eq, Debug)]
/// #[require(C)]
/// struct B(usize);
///
/// #[derive(Component, Default, PartialEq, Eq, Debug)]
/// struct C(u32);
///
/// # let mut world = World::default();
/// // This will implicitly also insert B and C with their Default constructors
/// let id = world.spawn(A).id();
/// assert_eq!(&B(0), world.entity(id).get::<B>().unwrap());
/// assert_eq!(&C(0), world.entity(id).get::<C>().unwrap());
/// ```
///
/// You can also define a custom constructor function or closure:
///
/// ```
/// # use bevy_ecs::prelude::*;
/// #[derive(Component)]
/// #[require(C(init_c))]
/// struct A;
///
/// #[derive(Component, PartialEq, Eq, Debug)]
/// #[require(C(|| C(20)))]
/// struct B;
///
/// #[derive(Component, PartialEq, Eq, Debug)]
/// struct C(usize);
///
/// fn init_c() -> C {
///     C(10)
/// }
///
/// # let mut world = World::default();
/// // This will implicitly also insert C with the init_c() constructor
/// let id = world.spawn(A).id();
/// assert_eq!(&C(10), world.entity(id).get::<C>().unwrap());
///
/// // This will implicitly also insert C with the `|| C(20)` constructor closure
/// let id = world.spawn(B).id();
/// assert_eq!(&C(20), world.entity(id).get::<C>().unwrap());
/// ```
///
/// Required components are _recursive_. This means, if a Required Component has required components,
/// those components will _also_ be inserted if they are missing:
///
/// ```
/// # use bevy_ecs::prelude::*;
/// #[derive(Component)]
/// #[require(B)]
/// struct A;
///
/// #[derive(Component, Default, PartialEq, Eq, Debug)]
/// #[require(C)]
/// struct B(usize);
///
/// #[derive(Component, Default, PartialEq, Eq, Debug)]
/// struct C(u32);
///
/// # let mut world = World::default();
/// // This will implicitly also insert B and C with their Default constructors
/// let id = world.spawn(A).id();
/// assert_eq!(&B(0), world.entity(id).get::<B>().unwrap());
/// assert_eq!(&C(0), world.entity(id).get::<C>().unwrap());
/// ```
///
/// Note that cycles in the "component require tree" will result in stack overflows when attempting to
/// insert a component.
///
/// This "multiple inheritance" pattern does mean that it is possible to have duplicate requires for a given type
/// at different levels of the inheritance tree:
///
/// ```
/// # use bevy_ecs::prelude::*;
/// #[derive(Component)]
/// struct X(usize);
///
/// #[derive(Component, Default)]
/// #[require(X(|| X(1)))]
/// struct Y;
///
/// #[derive(Component)]
/// #[require(
///     Y,
///     X(|| X(2)),
/// )]
/// struct Z;
///
/// # let mut world = World::default();
/// // In this case, the x2 constructor is used for X
/// let id = world.spawn(Z).id();
/// assert_eq!(2, world.entity(id).get::<X>().unwrap().0);
/// ```
///
/// In general, this shouldn't happen often, but when it does the algorithm for choosing the constructor from the tree is simple and predictable:
/// 1. A constructor from a direct `#[require()]`, if one exists, is selected with priority.
/// 2. Otherwise, perform a Depth First Search on the tree of requirements and select the first one found.
///
/// From a user perspective, just think about this as the following:
/// 1. Specifying a required component constructor for Foo directly on a spawned component Bar will result in that constructor being used (and overriding existing constructors lower in the inheritance tree). This is the classic "inheritance override" behavior people expect.
/// 2. For cases where "multiple inheritance" results in constructor clashes, Components should be listed in "importance order". List a component earlier in the requirement list to initialize its inheritance tree earlier.
///
/// ## Registering required components at runtime
///
/// In most cases, required components should be registered using the `require` attribute as shown above.
/// However, in some cases, it may be useful to register required components at runtime.
///
/// This can be done through [`World::register_required_components`] or  [`World::register_required_components_with`]
/// for the [`Default`] and custom constructors respectively:
///
/// ```
/// # use bevy_ecs::prelude::*;
/// #[derive(Component)]
/// struct A;
///
/// #[derive(Component, Default, PartialEq, Eq, Debug)]
/// struct B(usize);
///
/// #[derive(Component, PartialEq, Eq, Debug)]
/// struct C(u32);
///
/// # let mut world = World::default();
/// // Register B as required by A and C as required by B.
/// world.register_required_components::<A, B>();
/// world.register_required_components_with::<B, C>(|| C(2));
///
/// // This will implicitly also insert B with its Default constructor
/// // and C with the custom constructor defined by B.
/// let id = world.spawn(A).id();
/// assert_eq!(&B(0), world.entity(id).get::<B>().unwrap());
/// assert_eq!(&C(2), world.entity(id).get::<C>().unwrap());
/// ```
///
/// Similar rules as before apply to duplicate requires fer a given type at different levels
/// of the inheritance tree. `A` requiring `C` directly would take precedence over indirectly
/// requiring it through `A` requiring `B` and `B` requiring `C`.
///
/// Unlike with the `require` attribute, directly requiring the same component multiple times
/// for the same component will result in a panic. This is done to prevent conflicting constructors
/// and confusing ordering dependencies.
///
/// Note that requirements must currently be registered before the requiring component is inserted
/// into the world for the first time. Registering requirements after this will lead to a panic.
///
/// # Adding component's hooks
///
/// See [`ComponentHooks`] for a detailed explanation of component's hooks.
///
/// Alternatively to the example shown in [`ComponentHooks`]' documentation, hooks can be configured using following attributes:
/// - `#[component(on_add = on_add_function)]`
/// - `#[component(on_insert = on_insert_function)]`
/// - `#[component(on_replace = on_replace_function)]`
/// - `#[component(on_remove = on_remove_function)]`
///
/// ```
/// # use bevy_ecs::component::Component;
/// # use bevy_ecs::world::DeferredWorld;
/// # use bevy_ecs::entity::Entity;
/// # use bevy_ecs::component::ComponentId;
/// #
/// #[derive(Component)]
/// #[component(on_add = my_on_add_hook)]
/// #[component(on_insert = my_on_insert_hook)]
/// // Another possible way of configuring hooks:
/// // #[component(on_add = my_on_add_hook, on_insert = my_on_insert_hook)]
/// //
/// // We don't have a replace or remove hook, so we can leave them out:
/// // #[component(on_replace = my_on_replace_hook, on_remove = my_on_remove_hook)]
/// struct ComponentA;
///
/// fn my_on_add_hook(world: DeferredWorld, entity: Entity, id: ComponentId) {
///     // ...
/// }
///
/// // You can also omit writing some types using generics.
/// fn my_on_insert_hook<T1, T2>(world: DeferredWorld, _: T1, _: T2) {
///     // ...
/// }
/// ```
///
/// # Implementing the trait for foreign types
///
/// As a consequence of the [orphan rule], it is not possible to separate into two different crates the implementation of `Component` from the definition of a type.
/// This means that it is not possible to directly have a type defined in a third party library as a component.
/// This important limitation can be easily worked around using the [newtype pattern]:
/// this makes it possible to locally define and implement `Component` for a tuple struct that wraps the foreign type.
/// The following example gives a demonstration of this pattern.
///
/// ```
/// // `Component` is defined in the `bevy_ecs` crate.
/// use bevy_ecs::component::Component;
///
/// // `Duration` is defined in the `std` crate.
/// use std::time::Duration;
///
/// // It is not possible to implement `Component` for `Duration` from this position, as they are
/// // both foreign items, defined in an external crate. However, nothing prevents to define a new
/// // `Cooldown` type that wraps `Duration`. As `Cooldown` is defined in a local crate, it is
/// // possible to implement `Component` for it.
/// #[derive(Component)]
/// struct Cooldown(Duration);
/// ```
///
/// [orphan rule]: https://doc.rust-lang.org/book/ch10-02-traits.html#implementing-a-trait-on-a-type
/// [newtype pattern]: https://doc.rust-lang.org/book/ch19-03-advanced-traits.html#using-the-newtype-pattern-to-implement-external-traits-on-external-types
///
/// # `!Sync` Components
/// A `!Sync` type cannot implement `Component`. However, it is possible to wrap a `Send` but not `Sync`
/// type in [`SyncCell`] or the currently unstable [`Exclusive`] to make it `Sync`. This forces only
/// having mutable access (`&mut T` only, never `&T`), but makes it safe to reference across multiple
/// threads.
///
/// This will fail to compile since `RefCell` is `!Sync`.
/// ```compile_fail
/// # use std::cell::RefCell;
/// # use bevy_ecs::component::Component;
/// #[derive(Component)]
/// struct NotSync {
///    counter: RefCell<usize>,
/// }
/// ```
///
/// This will compile since the `RefCell` is wrapped with `SyncCell`.
/// ```
/// # use std::cell::RefCell;
/// # use bevy_ecs::component::Component;
/// use bevy_utils::synccell::SyncCell;
///
/// // This will compile.
/// #[derive(Component)]
/// struct ActuallySync {
///    counter: SyncCell<RefCell<usize>>,
/// }
/// ```
///
/// [`SyncCell`]: bevy_utils::synccell::SyncCell
/// [`Exclusive`]: https://doc.rust-lang.org/nightly/std/sync/struct.Exclusive.html
#[diagnostic::on_unimplemented(
    message = "`{Self}` is not a `Component`",
    label = "invalid `Component`",
    note = "consider annotating `{Self}` with `#[derive(Component)]`"
)]
pub trait Component: Send + Sync + 'static {
    /// A constant indicating the storage type used for this component.
    const STORAGE_TYPE: StorageType;

    /// Called when registering this component, allowing mutable access to its [`ComponentHooks`].
    fn register_component_hooks(_hooks: &mut ComponentHooks) {}

    /// Registers required components.
    fn register_required_components(
        _component_id: ComponentId,
        _components: &mut Components,
        _storages: &mut Storages,
        _required_components: &mut RequiredComponents,
        _inheritance_depth: u16,
    ) {
    }
}

/// The storage used for a specific component type.
///
/// # Examples
/// The [`StorageType`] for a component is configured via the derive attribute
///
/// ```
/// # use bevy_ecs::{prelude::*, component::*};
/// #[derive(Component)]
/// #[component(storage = "SparseSet")]
/// struct A;
/// ```
#[derive(Debug, Copy, Clone, Default, Eq, PartialEq)]
pub enum StorageType {
    /// Provides fast and cache-friendly iteration, but slower addition and removal of components.
    /// This is the default storage type.
    #[default]
    Table,
    /// Provides fast addition and removal of components, but slower iteration.
    SparseSet,
}

/// The type used for [`Component`] lifecycle hooks such as `on_add`, `on_insert` or `on_remove`
pub type ComponentHook = for<'w> fn(DeferredWorld<'w>, Entity, ComponentId);

/// [`World`]-mutating functions that run as part of lifecycle events of a [`Component`].
///
/// Hooks are functions that run when a component is added, overwritten, or removed from an entity.
/// These are intended to be used for structural side effects that need to happen when a component is added or removed,
/// and are not intended for general-purpose logic.
///
/// For example, you might use a hook to update a cached index when a component is added,
/// to clean up resources when a component is removed,
/// or to keep hierarchical data structures across entities in sync.
///
/// This information is stored in the [`ComponentInfo`] of the associated component.
///
/// There is two ways of configuring hooks for a component:
/// 1. Defining the [`Component::register_component_hooks`] method (see [`Component`])
/// 2. Using the [`World::register_component_hooks`] method
///
/// # Example 2
///
/// ```
/// use bevy_ecs::prelude::*;
/// use bevy_utils::HashSet;
///
/// #[derive(Component)]
/// struct MyTrackedComponent;
///
/// #[derive(Resource, Default)]
/// struct TrackedEntities(HashSet<Entity>);
///
/// let mut world = World::new();
/// world.init_resource::<TrackedEntities>();
///
/// // No entities with `MyTrackedComponent` have been added yet, so we can safely add component hooks
/// let mut tracked_component_query = world.query::<&MyTrackedComponent>();
/// assert!(tracked_component_query.iter(&world).next().is_none());
///
/// world.register_component_hooks::<MyTrackedComponent>().on_add(|mut world, entity, _component_id| {
///    let mut tracked_entities = world.resource_mut::<TrackedEntities>();
///   tracked_entities.0.insert(entity);
/// });
///
/// world.register_component_hooks::<MyTrackedComponent>().on_remove(|mut world, entity, _component_id| {
///   let mut tracked_entities = world.resource_mut::<TrackedEntities>();
///   tracked_entities.0.remove(&entity);
/// });
///
/// let entity = world.spawn(MyTrackedComponent).id();
/// let tracked_entities = world.resource::<TrackedEntities>();
/// assert!(tracked_entities.0.contains(&entity));
///
/// world.despawn(entity);
/// let tracked_entities = world.resource::<TrackedEntities>();
/// assert!(!tracked_entities.0.contains(&entity));
/// ```
#[derive(Debug, Clone, Default)]
pub struct ComponentHooks {
    pub(crate) on_add: Option<ComponentHook>,
    pub(crate) on_insert: Option<ComponentHook>,
    pub(crate) on_replace: Option<ComponentHook>,
    pub(crate) on_remove: Option<ComponentHook>,
}

impl ComponentHooks {
    /// Register a [`ComponentHook`] that will be run when this component is added to an entity.
    /// An `on_add` hook will always run before `on_insert` hooks. Spawning an entity counts as
    /// adding all of its components.
    ///
    /// # Panics
    ///
    /// Will panic if the component already has an `on_add` hook
    pub fn on_add(&mut self, hook: ComponentHook) -> &mut Self {
        self.try_on_add(hook)
            .expect("Component already has an on_add hook")
    }

    /// Register a [`ComponentHook`] that will be run when this component is added (with `.insert`)
    /// or replaced.
    ///
    /// An `on_insert` hook always runs after any `on_add` hooks (if the entity didn't already have the component).
    ///
    /// # Warning
    ///
    /// The hook won't run if the component is already present and is only mutated, such as in a system via a query.
    /// As a result, this is *not* an appropriate mechanism for reliably updating indexes and other caches.
    ///
    /// # Panics
    ///
    /// Will panic if the component already has an `on_insert` hook
    pub fn on_insert(&mut self, hook: ComponentHook) -> &mut Self {
        self.try_on_insert(hook)
            .expect("Component already has an on_insert hook")
    }

    /// Register a [`ComponentHook`] that will be run when this component is about to be dropped,
    /// such as being replaced (with `.insert`) or removed.
    ///
    /// If this component is inserted onto an entity that already has it, this hook will run before the value is replaced,
    /// allowing access to the previous data just before it is dropped.
    /// This hook does *not* run if the entity did not already have this component.
    ///
    /// An `on_replace` hook always runs before any `on_remove` hooks (if the component is being removed from the entity).
    ///
    /// # Warning
    ///
    /// The hook won't run if the component is already present and is only mutated, such as in a system via a query.
    /// As a result, this is *not* an appropriate mechanism for reliably updating indexes and other caches.
    ///
    /// # Panics
    ///
    /// Will panic if the component already has an `on_replace` hook
    pub fn on_replace(&mut self, hook: ComponentHook) -> &mut Self {
        self.try_on_replace(hook)
            .expect("Component already has an on_replace hook")
    }

    /// Register a [`ComponentHook`] that will be run when this component is removed from an entity.
    /// Despawning an entity counts as removing all of its components.
    ///
    /// # Panics
    ///
    /// Will panic if the component already has an `on_remove` hook
    pub fn on_remove(&mut self, hook: ComponentHook) -> &mut Self {
        self.try_on_remove(hook)
            .expect("Component already has an on_remove hook")
    }

    /// Attempt to register a [`ComponentHook`] that will be run when this component is added to an entity.
    ///
    /// This is a fallible version of [`Self::on_add`].
    ///
    /// Returns `None` if the component already has an `on_add` hook.
    pub fn try_on_add(&mut self, hook: ComponentHook) -> Option<&mut Self> {
        if self.on_add.is_some() {
            return None;
        }
        self.on_add = Some(hook);
        Some(self)
    }

    /// Attempt to register a [`ComponentHook`] that will be run when this component is added (with `.insert`)
    ///
    /// This is a fallible version of [`Self::on_insert`].
    ///
    /// Returns `None` if the component already has an `on_insert` hook.
    pub fn try_on_insert(&mut self, hook: ComponentHook) -> Option<&mut Self> {
        if self.on_insert.is_some() {
            return None;
        }
        self.on_insert = Some(hook);
        Some(self)
    }

    /// Attempt to register a [`ComponentHook`] that will be run when this component is replaced (with `.insert`) or removed
    ///
    /// This is a fallible version of [`Self::on_replace`].
    ///
    /// Returns `None` if the component already has an `on_replace` hook.
    pub fn try_on_replace(&mut self, hook: ComponentHook) -> Option<&mut Self> {
        if self.on_replace.is_some() {
            return None;
        }
        self.on_replace = Some(hook);
        Some(self)
    }

    /// Attempt to register a [`ComponentHook`] that will be run when this component is removed from an entity.
    ///
    /// This is a fallible version of [`Self::on_remove`].
    ///
    /// Returns `None` if the component already has an `on_remove` hook.
    pub fn try_on_remove(&mut self, hook: ComponentHook) -> Option<&mut Self> {
        if self.on_remove.is_some() {
            return None;
        }
        self.on_remove = Some(hook);
        Some(self)
    }
}

/// Stores metadata for a type of component or resource stored in a specific [`World`].
#[derive(Debug, Clone)]
pub struct ComponentInfo {
    id: ComponentId,
    descriptor: ComponentDescriptor,
    hooks: ComponentHooks,
    required_components: RequiredComponents,
    required_by: HashSet<ComponentId>,
}

impl ComponentInfo {
    /// Returns a value uniquely identifying the current component.
    #[inline]
    pub fn id(&self) -> ComponentId {
        self.id
    }

    /// Returns the name of the current component.
    #[inline]
    pub fn name(&self) -> &str {
        &self.descriptor.name
    }

    /// Returns the [`TypeId`] of the underlying component type.
    /// Returns `None` if the component does not correspond to a Rust type.
    #[inline]
    pub fn type_id(&self) -> Option<TypeId> {
        self.descriptor.type_id
    }

    /// Returns the layout used to store values of this component in memory.
    #[inline]
    pub fn layout(&self) -> Layout {
        self.descriptor.layout
    }

    #[inline]
    /// Get the function which should be called to clean up values of
    /// the underlying component type. This maps to the
    /// [`Drop`] implementation for 'normal' Rust components
    ///
    /// Returns `None` if values of the underlying component type don't
    /// need to be dropped, e.g. as reported by [`needs_drop`].
    pub fn drop(&self) -> Option<unsafe fn(OwningPtr<'_>)> {
        self.descriptor.drop
    }

    /// Returns a value indicating the storage strategy for the current component.
    #[inline]
    pub fn storage_type(&self) -> StorageType {
        self.descriptor.storage_type
    }

    /// Returns `true` if the underlying component type can be freely shared between threads.
    /// If this returns `false`, then extra care must be taken to ensure that components
    /// are not accessed from the wrong thread.
    #[inline]
    pub fn is_send_and_sync(&self) -> bool {
        self.descriptor.is_send_and_sync
    }

    /// Create a new [`ComponentInfo`].
    pub(crate) fn new(id: ComponentId, descriptor: ComponentDescriptor) -> Self {
        ComponentInfo {
            id,
            descriptor,
            hooks: Default::default(),
            required_components: Default::default(),
            required_by: Default::default(),
        }
    }

    /// Update the given flags to include any [`ComponentHook`] registered to self
    #[inline]
    pub(crate) fn update_archetype_flags(&self, flags: &mut ArchetypeFlags) {
        if self.hooks().on_add.is_some() {
            flags.insert(ArchetypeFlags::ON_ADD_HOOK);
        }
        if self.hooks().on_insert.is_some() {
            flags.insert(ArchetypeFlags::ON_INSERT_HOOK);
        }
        if self.hooks().on_replace.is_some() {
            flags.insert(ArchetypeFlags::ON_REPLACE_HOOK);
        }
        if self.hooks().on_remove.is_some() {
            flags.insert(ArchetypeFlags::ON_REMOVE_HOOK);
        }
    }

    /// Provides a reference to the collection of hooks associated with this [`Component`]
    pub fn hooks(&self) -> &ComponentHooks {
        &self.hooks
    }

    /// Retrieves the [`RequiredComponents`] collection, which contains all required components (and their constructors)
    /// needed by this component. This includes _recursive_ required components.
    pub fn required_components(&self) -> &RequiredComponents {
        &self.required_components
    }
}

/// A value which uniquely identifies the type of a [`Component`] or [`Resource`] within a
/// [`World`].
///
/// Each time a new `Component` type is registered within a `World` using
/// e.g. [`World::register_component`] or [`World::register_component_with_descriptor`]
/// or a Resource with e.g. [`World::init_resource`],
/// a corresponding `ComponentId` is created to track it.
///
/// While the distinction between `ComponentId` and [`TypeId`] may seem superficial, breaking them
/// into two separate but related concepts allows components to exist outside of Rust's type system.
/// Each Rust type registered as a `Component` will have a corresponding `ComponentId`, but additional
/// `ComponentId`s may exist in a `World` to track components which cannot be
/// represented as Rust types for scripting or other advanced use-cases.
///
/// A `ComponentId` is tightly coupled to its parent `World`. Attempting to use a `ComponentId` from
/// one `World` to access the metadata of a `Component` in a different `World` is undefined behavior
/// and must not be attempted.
///
/// Given a type `T` which implements [`Component`], the `ComponentId` for `T` can be retrieved
/// from a `World` using [`World::component_id()`] or via [`Components::component_id()`]. Access
/// to the `ComponentId` for a [`Resource`] is available via [`Components::resource_id()`].
#[derive(Debug, Copy, Clone, Hash, Ord, PartialOrd, Eq, PartialEq)]
#[cfg_attr(
    feature = "bevy_reflect",
    derive(Reflect),
    reflect(Debug, Hash, PartialEq)
)]
pub struct ComponentId(usize);

impl ComponentId {
    /// Creates a new [`ComponentId`].
    ///
    /// The `index` is a unique value associated with each type of component in a given world.
    /// Usually, this value is taken from a counter incremented for each type of component registered with the world.
    #[inline]
    pub const fn new(index: usize) -> ComponentId {
        ComponentId(index)
    }

    /// Returns the index of the current component.
    #[inline]
    pub fn index(self) -> usize {
        self.0
    }
}

impl SparseSetIndex for ComponentId {
    #[inline]
    fn sparse_set_index(&self) -> usize {
        self.index()
    }

    #[inline]
    fn get_sparse_set_index(value: usize) -> Self {
        Self(value)
    }
}

/// A value describing a component or resource, which may or may not correspond to a Rust type.
#[derive(Clone)]
pub struct ComponentDescriptor {
    name: Cow<'static, str>,
    // SAFETY: This must remain private. It must match the statically known StorageType of the
    // associated rust component type if one exists.
    storage_type: StorageType,
    // SAFETY: This must remain private. It must only be set to "true" if this component is
    // actually Send + Sync
    is_send_and_sync: bool,
    type_id: Option<TypeId>,
    layout: Layout,
    // SAFETY: this function must be safe to call with pointers pointing to items of the type
    // this descriptor describes.
    // None if the underlying type doesn't need to be dropped
    drop: Option<for<'a> unsafe fn(OwningPtr<'a>)>,
}

// We need to ignore the `drop` field in our `Debug` impl
impl Debug for ComponentDescriptor {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        f.debug_struct("ComponentDescriptor")
            .field("name", &self.name)
            .field("storage_type", &self.storage_type)
            .field("is_send_and_sync", &self.is_send_and_sync)
            .field("type_id", &self.type_id)
            .field("layout", &self.layout)
            .finish()
    }
}

impl ComponentDescriptor {
    /// # Safety
    ///
    /// `x` must point to a valid value of type `T`.
    unsafe fn drop_ptr<T>(x: OwningPtr<'_>) {
        // SAFETY: Contract is required to be upheld by the caller.
        unsafe {
            x.drop_as::<T>();
        }
    }

    /// Create a new `ComponentDescriptor` for the type `T`.
    pub fn new<T: Component>() -> Self {
        Self {
            name: Cow::Borrowed(core::any::type_name::<T>()),
            storage_type: T::STORAGE_TYPE,
            is_send_and_sync: true,
            type_id: Some(TypeId::of::<T>()),
            layout: Layout::new::<T>(),
            drop: needs_drop::<T>().then_some(Self::drop_ptr::<T> as _),
        }
    }

    /// Create a new `ComponentDescriptor`.
    ///
    /// # Safety
    /// - the `drop` fn must be usable on a pointer with a value of the layout `layout`
    /// - the component type must be safe to access from any thread (Send + Sync in rust terms)
    pub unsafe fn new_with_layout(
        name: impl Into<Cow<'static, str>>,
        storage_type: StorageType,
        layout: Layout,
        drop: Option<for<'a> unsafe fn(OwningPtr<'a>)>,
    ) -> Self {
        Self {
            name: name.into(),
            storage_type,
            is_send_and_sync: true,
            type_id: None,
            layout,
            drop,
        }
    }

    /// Create a new `ComponentDescriptor` for a resource.
    ///
    /// The [`StorageType`] for resources is always [`StorageType::Table`].
    pub fn new_resource<T: Resource>() -> Self {
        Self {
            name: Cow::Borrowed(core::any::type_name::<T>()),
            // PERF: `SparseStorage` may actually be a more
            // reasonable choice as `storage_type` for resources.
            storage_type: StorageType::Table,
            is_send_and_sync: true,
            type_id: Some(TypeId::of::<T>()),
            layout: Layout::new::<T>(),
            drop: needs_drop::<T>().then_some(Self::drop_ptr::<T> as _),
        }
    }

    fn new_non_send<T: Any>(storage_type: StorageType) -> Self {
        Self {
            name: Cow::Borrowed(core::any::type_name::<T>()),
            storage_type,
            is_send_and_sync: false,
            type_id: Some(TypeId::of::<T>()),
            layout: Layout::new::<T>(),
            drop: needs_drop::<T>().then_some(Self::drop_ptr::<T> as _),
        }
    }

    /// Returns a value indicating the storage strategy for the current component.
    #[inline]
    pub fn storage_type(&self) -> StorageType {
        self.storage_type
    }

    /// Returns the [`TypeId`] of the underlying component type.
    /// Returns `None` if the component does not correspond to a Rust type.
    #[inline]
    pub fn type_id(&self) -> Option<TypeId> {
        self.type_id
    }

    /// Returns the name of the current component.
    #[inline]
    pub fn name(&self) -> &str {
        self.name.as_ref()
    }
}

/// Stores metadata associated with each kind of [`Component`] in a given [`World`].
#[derive(Debug, Default)]
pub struct Components {
    components: Vec<ComponentInfo>,
    indices: TypeIdMap<ComponentId>,
    resource_indices: TypeIdMap<ComponentId>,
}

impl Components {
    /// Registers a [`Component`] of type `T` with this instance.
    /// If a component of this type has already been registered, this will return
    /// the ID of the pre-existing component.
    ///
    /// # See also
    ///
    /// * [`Components::component_id()`]
    /// * [`Components::register_component_with_descriptor()`]
    #[inline]
    pub fn register_component<T: Component>(&mut self, storages: &mut Storages) -> ComponentId {
        let mut registered = false;
        let id = {
            let Components {
                indices,
                components,
                ..
            } = self;
            let type_id = TypeId::of::<T>();
            *indices.entry(type_id).or_insert_with(|| {
                let id = Components::register_component_inner(
                    components,
                    storages,
                    ComponentDescriptor::new::<T>(),
                );
                registered = true;
                id
            })
        };
        if registered {
            let mut required_components = RequiredComponents::default();
            T::register_required_components(id, self, storages, &mut required_components, 0);
            let info = &mut self.components[id.index()];
            T::register_component_hooks(&mut info.hooks);
            info.required_components = required_components;
        }
        id
    }

    /// Registers a component described by `descriptor`.
    ///
    /// # Note
    ///
    /// If this method is called multiple times with identical descriptors, a distinct [`ComponentId`]
    /// will be created for each one.
    ///
    /// # See also
    ///
    /// * [`Components::component_id()`]
    /// * [`Components::register_component()`]
    pub fn register_component_with_descriptor(
        &mut self,
        storages: &mut Storages,
        descriptor: ComponentDescriptor,
    ) -> ComponentId {
        Components::register_component_inner(&mut self.components, storages, descriptor)
    }

    #[inline]
    fn register_component_inner(
        components: &mut Vec<ComponentInfo>,
        storages: &mut Storages,
        descriptor: ComponentDescriptor,
    ) -> ComponentId {
        let component_id = ComponentId(components.len());
        let info = ComponentInfo::new(component_id, descriptor);
        if info.descriptor.storage_type == StorageType::SparseSet {
            storages.sparse_sets.get_or_insert(&info);
        }
        components.push(info);
        component_id
    }

    /// Returns the number of components registered with this instance.
    #[inline]
    pub fn len(&self) -> usize {
        self.components.len()
    }

    /// Returns `true` if there are no components registered with this instance. Otherwise, this returns `false`.
    #[inline]
    pub fn is_empty(&self) -> bool {
        self.components.len() == 0
    }

    /// Gets the metadata associated with the given component.
    ///
    /// This will return an incorrect result if `id` did not come from the same world as `self`. It may return `None` or a garbage value.
    #[inline]
    pub fn get_info(&self, id: ComponentId) -> Option<&ComponentInfo> {
        self.components.get(id.0)
    }

    /// Returns the name associated with the given component.
    ///
    /// This will return an incorrect result if `id` did not come from the same world as `self`. It may return `None` or a garbage value.
    #[inline]
    pub fn get_name(&self, id: ComponentId) -> Option<&str> {
        self.get_info(id).map(ComponentInfo::name)
    }

    /// Gets the metadata associated with the given component.
    /// # Safety
    ///
    /// `id` must be a valid [`ComponentId`]
    #[inline]
    pub unsafe fn get_info_unchecked(&self, id: ComponentId) -> &ComponentInfo {
        debug_assert!(id.index() < self.components.len());
        // SAFETY: The caller ensures `id` is valid.
        unsafe { self.components.get_unchecked(id.0) }
    }

    #[inline]
    pub(crate) fn get_hooks_mut(&mut self, id: ComponentId) -> Option<&mut ComponentHooks> {
        self.components.get_mut(id.0).map(|info| &mut info.hooks)
    }

    #[inline]
    pub(crate) fn get_required_components_mut(
        &mut self,
        id: ComponentId,
    ) -> Option<&mut RequiredComponents> {
        self.components
            .get_mut(id.0)
            .map(|info| &mut info.required_components)
    }

    /// Registers the given component `R` and [required components] inherited from it as required by `T`.
    ///
    /// When `T` is added to an entity, `R` will also be added if it was not already provided.
    /// The given `constructor` will be used for the creation of `R`.
    ///
    /// [required components]: Component#required-components
    ///
    /// # Safety
    ///
    /// The given component IDs `required` and `requiree` must be valid.
    ///
    /// # Errors
    ///
    /// Returns a [`RequiredComponentsError`] if the `required` component is already a directly required component for the `requiree`.
    ///
    /// Indirect requirements through other components are allowed. In those cases, the more specific
    /// registration will be used.
    pub(crate) unsafe fn register_required_components<R: Component>(
        &mut self,
        requiree: ComponentId,
        required: ComponentId,
        constructor: fn() -> R,
    ) -> Result<(), RequiredComponentsError> {
        // SAFETY: The caller ensures that the `requiree` is valid.
        let required_components = unsafe {
            self.get_required_components_mut(requiree)
                .debug_checked_unwrap()
        };

        // Cannot directly require the same component twice.
        if required_components
            .0
            .get(&required)
            .is_some_and(|c| c.inheritance_depth == 0)
        {
            return Err(RequiredComponentsError::DuplicateRegistration(
                requiree, required,
            ));
        }

        // Register the required component for the requiree.
        // This is a direct requirement with a depth of `0`.
        required_components.register_by_id(required, constructor, 0);

        // Add the requiree to the list of components that require the required component.
        // SAFETY: The component is in the list of required components, so it must exist already.
        let required_by = unsafe { self.get_required_by_mut(required).debug_checked_unwrap() };
        required_by.insert(requiree);

        // SAFETY: The caller ensures that the `requiree` and `required` components are valid.
        let inherited_requirements =
            unsafe { self.register_inherited_required_components(requiree, required) };

        // Propagate the new required components up the chain to all components that require the requiree.
        if let Some(required_by) = self.get_required_by(requiree).cloned() {
            // `required` is now required by anything that `requiree` was required by.
            self.get_required_by_mut(required)
                .unwrap()
                .extend(required_by.iter().copied());
            for &required_by_id in required_by.iter() {
                // SAFETY: The component is in the list of required components, so it must exist already.
                let required_components = unsafe {
                    self.get_required_components_mut(required_by_id)
                        .debug_checked_unwrap()
                };

                // Register the original required component in the "parent" of the requiree.
                // The inheritance depth is 1 deeper than the `requiree` wrt `required_by_id`.
                let depth = required_components.0.get(&requiree).expect("requiree is required by required_by_id, so its required_components must include requiree").inheritance_depth;
                required_components.register_by_id(required, constructor, depth + 1);

                for (component_id, component) in inherited_requirements.iter() {
                    // Register the required component.
                    // The inheritance depth of inherited components is whatever the requiree's
                    // depth is relative to `required_by_id`, plus the inheritance depth of the
                    // inherited component relative to the requiree, plus 1 to account for the
                    // requiree in between.
                    // SAFETY: Component ID and constructor match the ones on the original requiree.
                    //         The original requiree is responsible for making sure the registration is safe.
                    unsafe {
                        required_components.register_dynamic(
                            *component_id,
                            component.constructor.clone(),
                            component.inheritance_depth + depth + 1,
                        );
                    };
                }
            }
        }

        Ok(())
    }

    /// Registers the components inherited from `required` for the given `requiree`,
    /// returning the requirements in a list.
    ///
    /// # Safety
    ///
    /// The given component IDs `requiree` and `required` must be valid.
    unsafe fn register_inherited_required_components(
        &mut self,
        requiree: ComponentId,
        required: ComponentId,
    ) -> Vec<(ComponentId, RequiredComponent)> {
        // Get required components inherited from the `required` component.
        // SAFETY: The caller ensures that the `required` component is valid.
        let required_component_info = unsafe { self.get_info(required).debug_checked_unwrap() };
        let inherited_requirements: Vec<(ComponentId, RequiredComponent)> = required_component_info
            .required_components()
            .0
            .iter()
            .map(|(component_id, required_component)| {
                (
                    *component_id,
                    RequiredComponent {
                        constructor: required_component.constructor.clone(),
                        // Add `1` to the inheritance depth since this will be registered
                        // for the component that requires `required`.
                        inheritance_depth: required_component.inheritance_depth + 1,
                    },
                )
            })
            .collect();

        // Register the new required components.
        for (component_id, component) in inherited_requirements.iter().cloned() {
            // SAFETY: The caller ensures that the `requiree` is valid.
            let required_components = unsafe {
                self.get_required_components_mut(requiree)
                    .debug_checked_unwrap()
            };

            // Register the required component for the requiree.
            // SAFETY: Component ID and constructor match the ones on the original requiree.
            unsafe {
                required_components.register_dynamic(
                    component_id,
                    component.constructor,
                    component.inheritance_depth,
                );
            };

            // Add the requiree to the list of components that require the required component.
            // SAFETY: The caller ensures that the required components are valid.
            let required_by = unsafe {
                self.get_required_by_mut(component_id)
                    .debug_checked_unwrap()
            };
            required_by.insert(requiree);
        }

        inherited_requirements
    }

    // NOTE: This should maybe be private, but it is currently public so that `bevy_ecs_macros` can use it.
    //       We can't directly move this there either, because this uses `Components::get_required_by_mut`,
    //       which is private, and could be equally risky to expose to users.
    /// Registers the given component `R` and [required components] inherited from it as required by `T`,
    /// and adds `T` to their lists of requirees.
    ///
    /// The given `inheritance_depth` determines how many levels of inheritance deep the requirement is.
    /// A direct requirement has a depth of `0`, and each level of inheritance increases the depth by `1`.
    /// Lower depths are more specific requirements, and can override existing less specific registrations.
    ///
    /// This method does *not* register any components as required by components that require `T`.
    ///
    /// Only use this method if you know what you are doing. In most cases, you should instead use [`World::register_required_components`],
    /// or the equivalent method in `bevy_app::App`.
    ///
    /// [required component]: Component#required-components
    #[doc(hidden)]
    pub fn register_required_components_manual<T: Component, R: Component>(
        &mut self,
        storages: &mut Storages,
        required_components: &mut RequiredComponents,
        constructor: fn() -> R,
        inheritance_depth: u16,
    ) {
        let requiree = self.register_component::<T>(storages);
        let required = self.register_component::<R>(storages);

        // SAFETY: We just created the components.
        unsafe {
            self.register_required_components_manual_unchecked::<R>(
                requiree,
                required,
                required_components,
                constructor,
                inheritance_depth,
            );
        }
    }

    /// Registers the given component `R` and [required components] inherited from it as required by `T`,
    /// and adds `T` to their lists of requirees.
    ///
    /// The given `inheritance_depth` determines how many levels of inheritance deep the requirement is.
    /// A direct requirement has a depth of `0`, and each level of inheritance increases the depth by `1`.
    /// Lower depths are more specific requirements, and can override existing less specific registrations.
    ///
    /// This method does *not* register any components as required by components that require `T`.
    ///
    /// [required component]: Component#required-components
    ///
    /// # Safety
    ///
    /// The given component IDs `required` and `requiree` must be valid.
    pub(crate) unsafe fn register_required_components_manual_unchecked<R: Component>(
        &mut self,
        requiree: ComponentId,
        required: ComponentId,
        required_components: &mut RequiredComponents,
        constructor: fn() -> R,
        inheritance_depth: u16,
    ) {
        // Components cannot require themselves.
        if required == requiree {
            return;
        }

        // Register the required component `R` for the requiree.
        required_components.register_by_id(required, constructor, inheritance_depth);

        // Add the requiree to the list of components that require `R`.
        // SAFETY: The caller ensures that the component ID is valid.
        //         Assuming it is valid, the component is in the list of required components, so it must exist already.
        let required_by = unsafe { self.get_required_by_mut(required).debug_checked_unwrap() };
        required_by.insert(requiree);

        // Register the inherited required components for the requiree.
        let required: Vec<(ComponentId, RequiredComponent)> = self
            .get_info(required)
            .unwrap()
            .required_components()
            .0
            .iter()
            .map(|(id, component)| (*id, component.clone()))
            .collect();

        for (id, component) in required {
            // Register the inherited required components for the requiree.
            // The inheritance depth is increased by `1` since this is a component required by the original required component.
            required_components.register_dynamic(
                id,
                component.constructor.clone(),
                component.inheritance_depth + 1,
            );
            self.get_required_by_mut(id).unwrap().insert(requiree);
        }
    }

    #[inline]
    pub(crate) fn get_required_by(&self, id: ComponentId) -> Option<&HashSet<ComponentId>> {
        self.components.get(id.0).map(|info| &info.required_by)
    }

    #[inline]
    pub(crate) fn get_required_by_mut(
        &mut self,
        id: ComponentId,
    ) -> Option<&mut HashSet<ComponentId>> {
        self.components
            .get_mut(id.0)
            .map(|info| &mut info.required_by)
    }

    /// Type-erased equivalent of [`Components::component_id()`].
    #[inline]
    pub fn get_id(&self, type_id: TypeId) -> Option<ComponentId> {
        self.indices.get(&type_id).copied()
    }

    /// Returns the [`ComponentId`] of the given [`Component`] type `T`.
    ///
    /// The returned `ComponentId` is specific to the `Components` instance
    /// it was retrieved from and should not be used with another `Components`
    /// instance.
    ///
    /// Returns [`None`] if the `Component` type has not
    /// yet been initialized using [`Components::register_component()`].
    ///
    /// ```
    /// use bevy_ecs::prelude::*;
    ///
    /// let mut world = World::new();
    ///
    /// #[derive(Component)]
    /// struct ComponentA;
    ///
    /// let component_a_id = world.register_component::<ComponentA>();
    ///
    /// assert_eq!(component_a_id, world.components().component_id::<ComponentA>().unwrap())
    /// ```
    ///
    /// # See also
    ///
    /// * [`Components::get_id()`]
    /// * [`Components::resource_id()`]
    /// * [`World::component_id()`]
    #[inline]
    pub fn component_id<T: Component>(&self) -> Option<ComponentId> {
        self.get_id(TypeId::of::<T>())
    }

    /// Type-erased equivalent of [`Components::resource_id()`].
    #[inline]
    pub fn get_resource_id(&self, type_id: TypeId) -> Option<ComponentId> {
        self.resource_indices.get(&type_id).copied()
    }

    /// Returns the [`ComponentId`] of the given [`Resource`] type `T`.
    ///
    /// The returned `ComponentId` is specific to the `Components` instance
    /// it was retrieved from and should not be used with another `Components`
    /// instance.
    ///
    /// Returns [`None`] if the `Resource` type has not
    /// yet been initialized using [`Components::register_resource()`].
    ///
    /// ```
    /// use bevy_ecs::prelude::*;
    ///
    /// let mut world = World::new();
    ///
    /// #[derive(Resource, Default)]
    /// struct ResourceA;
    ///
    /// let resource_a_id = world.init_resource::<ResourceA>();
    ///
    /// assert_eq!(resource_a_id, world.components().resource_id::<ResourceA>().unwrap())
    /// ```
    ///
    /// # See also
    ///
    /// * [`Components::component_id()`]
    /// * [`Components::get_resource_id()`]
    #[inline]
    pub fn resource_id<T: Resource>(&self) -> Option<ComponentId> {
        self.get_resource_id(TypeId::of::<T>())
    }

    /// Registers a [`Resource`] of type `T` with this instance.
    /// If a resource of this type has already been registered, this will return
    /// the ID of the pre-existing resource.
    ///
    /// # See also
    ///
    /// * [`Components::resource_id()`]
    /// * [`Components::register_resource_with_descriptor()`]
    #[inline]
    pub fn register_resource<T: Resource>(&mut self) -> ComponentId {
        // SAFETY: The [`ComponentDescriptor`] matches the [`TypeId`]
        unsafe {
            self.get_or_register_resource_with(TypeId::of::<T>(), || {
                ComponentDescriptor::new_resource::<T>()
            })
        }
    }

    /// Registers a [`Resource`] described by `descriptor`.
    ///
    /// # Note
    ///
    /// If this method is called multiple times with identical descriptors, a distinct [`ComponentId`]
    /// will be created for each one.
    ///
    /// # See also
    ///
    /// * [`Components::resource_id()`]
    /// * [`Components::register_resource()`]
    pub fn register_resource_with_descriptor(
        &mut self,
        descriptor: ComponentDescriptor,
    ) -> ComponentId {
        Components::register_resource_inner(&mut self.components, descriptor)
    }

    /// Registers a [non-send resource](crate::system::NonSend) of type `T` with this instance.
    /// If a resource of this type has already been registered, this will return
    /// the ID of the pre-existing resource.
    #[inline]
    pub fn register_non_send<T: Any>(&mut self) -> ComponentId {
        // SAFETY: The [`ComponentDescriptor`] matches the [`TypeId`]
        unsafe {
            self.get_or_register_resource_with(TypeId::of::<T>(), || {
                ComponentDescriptor::new_non_send::<T>(StorageType::default())
            })
        }
    }

    /// # Safety
    ///
    /// The [`ComponentDescriptor`] must match the [`TypeId`]
    #[inline]
    unsafe fn get_or_register_resource_with(
        &mut self,
        type_id: TypeId,
        func: impl FnOnce() -> ComponentDescriptor,
    ) -> ComponentId {
        let components = &mut self.components;
        *self.resource_indices.entry(type_id).or_insert_with(|| {
            let descriptor = func();
            Components::register_resource_inner(components, descriptor)
        })
    }

    #[inline]
    fn register_resource_inner(
        components: &mut Vec<ComponentInfo>,
        descriptor: ComponentDescriptor,
    ) -> ComponentId {
        let component_id = ComponentId(components.len());
        components.push(ComponentInfo::new(component_id, descriptor));
        component_id
    }

    /// Gets an iterator over all components registered with this instance.
    pub fn iter(&self) -> impl Iterator<Item = &ComponentInfo> + '_ {
        self.components.iter()
    }
}

/// A value that tracks when a system ran relative to other systems.
/// This is used to power change detection.
///
/// *Note* that a system that hasn't been run yet has a `Tick` of 0.
#[derive(Copy, Clone, Default, Debug, Eq, Hash, PartialEq)]
#[cfg_attr(
    feature = "bevy_reflect",
    derive(Reflect),
    reflect(Debug, Hash, PartialEq)
)]
pub struct Tick {
    tick: u32,
}

impl Tick {
    /// The maximum relative age for a change tick.
    /// The value of this is equal to [`MAX_CHANGE_AGE`].
    ///
    /// Since change detection will not work for any ticks older than this,
    /// ticks are periodically scanned to ensure their relative values are below this.
    pub const MAX: Self = Self::new(MAX_CHANGE_AGE);

    /// Creates a new [`Tick`] wrapping the given value.
    #[inline]
    pub const fn new(tick: u32) -> Self {
        Self { tick }
    }

    /// Gets the value of this change tick.
    #[inline]
    pub const fn get(self) -> u32 {
        self.tick
    }

    /// Sets the value of this change tick.
    #[inline]
    pub fn set(&mut self, tick: u32) {
        self.tick = tick;
    }

    /// Returns `true` if this `Tick` occurred since the system's `last_run`.
    ///
    /// `this_run` is the current tick of the system, used as a reference to help deal with wraparound.
    #[inline]
    pub fn is_newer_than(self, last_run: Tick, this_run: Tick) -> bool {
        // This works even with wraparound because the world tick (`this_run`) is always "newer" than
        // `last_run` and `self.tick`, and we scan periodically to clamp `ComponentTicks` values
        // so they never get older than `u32::MAX` (the difference would overflow).
        //
        // The clamp here ensures determinism (since scans could differ between app runs).
        let ticks_since_insert = this_run.relative_to(self).tick.min(MAX_CHANGE_AGE);
        let ticks_since_system = this_run.relative_to(last_run).tick.min(MAX_CHANGE_AGE);

        ticks_since_system > ticks_since_insert
    }

    /// Returns a change tick representing the relationship between `self` and `other`.
    #[inline]
    pub(crate) fn relative_to(self, other: Self) -> Self {
        let tick = self.tick.wrapping_sub(other.tick);
        Self { tick }
    }

    /// Wraps this change tick's value if it exceeds [`Tick::MAX`].
    ///
    /// Returns `true` if wrapping was performed. Otherwise, returns `false`.
    #[inline]
    pub(crate) fn check_tick(&mut self, tick: Tick) -> bool {
        let age = tick.relative_to(*self);
        // This comparison assumes that `age` has not overflowed `u32::MAX` before, which will be true
        // so long as this check always runs before that can happen.
        if age.get() > Self::MAX.get() {
            *self = tick.relative_to(Self::MAX);
            true
        } else {
            false
        }
    }
}

/// Interior-mutable access to the [`Tick`]s for a single component or resource.
#[derive(Copy, Clone, Debug)]
pub struct TickCells<'a> {
    /// The tick indicating when the value was added to the world.
    pub added: &'a UnsafeCell<Tick>,
    /// The tick indicating the last time the value was modified.
    pub changed: &'a UnsafeCell<Tick>,
}

impl<'a> TickCells<'a> {
    /// # Safety
    /// All cells contained within must uphold the safety invariants of [`UnsafeCellDeref::read`].
    #[inline]
    pub(crate) unsafe fn read(&self) -> ComponentTicks {
        ComponentTicks {
            // SAFETY: The callers uphold the invariants for `read`.
            added: unsafe { self.added.read() },
            // SAFETY: The callers uphold the invariants for `read`.
            changed: unsafe { self.changed.read() },
        }
    }
}

/// Records when a component or resource was added and when it was last mutably dereferenced (or added).
#[derive(Copy, Clone, Debug)]
#[cfg_attr(feature = "bevy_reflect", derive(Reflect), reflect(Debug))]
pub struct ComponentTicks {
    /// Tick recording the time this component or resource was added.
    pub added: Tick,

    /// Tick recording the time this component or resource was most recently changed.
    pub changed: Tick,
}

impl ComponentTicks {
    /// Returns `true` if the component or resource was added after the system last ran
    /// (or the system is running for the first time).
    #[inline]
    pub fn is_added(&self, last_run: Tick, this_run: Tick) -> bool {
        self.added.is_newer_than(last_run, this_run)
    }

    /// Returns `true` if the component or resource was added or mutably dereferenced after the system last ran
    /// (or the system is running for the first time).
    #[inline]
    pub fn is_changed(&self, last_run: Tick, this_run: Tick) -> bool {
        self.changed.is_newer_than(last_run, this_run)
    }

    /// Creates a new instance with the same change tick for `added` and `changed`.
    pub fn new(change_tick: Tick) -> Self {
        Self {
            added: change_tick,
            changed: change_tick,
        }
    }

    /// Manually sets the change tick.
    ///
    /// This is normally done automatically via the [`DerefMut`](std::ops::DerefMut) implementation
    /// on [`Mut<T>`](crate::change_detection::Mut), [`ResMut<T>`](crate::change_detection::ResMut), etc.
    /// However, components and resources that make use of interior mutability might require manual updates.
    ///
    /// # Example
    /// ```no_run
    /// # use bevy_ecs::{world::World, component::ComponentTicks};
    /// let world: World = unimplemented!();
    /// let component_ticks: ComponentTicks = unimplemented!();
    ///
    /// component_ticks.set_changed(world.read_change_tick());
    /// ```
    #[inline]
    pub fn set_changed(&mut self, change_tick: Tick) {
        self.changed = change_tick;
    }
}

/// A [`SystemParam`] that provides access to the [`ComponentId`] for a specific component type.
///
/// # Example
/// ```
/// # use bevy_ecs::{system::Local, component::{Component, ComponentId, ComponentIdFor}};
/// #[derive(Component)]
/// struct Player;
/// fn my_system(component_id: ComponentIdFor<Player>) {
///     let component_id: ComponentId = component_id.get();
///     // ...
/// }
/// ```
#[derive(SystemParam)]
pub struct ComponentIdFor<'s, T: Component>(Local<'s, InitComponentId<T>>);

impl<T: Component> ComponentIdFor<'_, T> {
    /// Gets the [`ComponentId`] for the type `T`.
    #[inline]
    pub fn get(&self) -> ComponentId {
        **self
    }
}

impl<T: Component> core::ops::Deref for ComponentIdFor<'_, T> {
    type Target = ComponentId;
    fn deref(&self) -> &Self::Target {
        &self.0.component_id
    }
}

impl<T: Component> From<ComponentIdFor<'_, T>> for ComponentId {
    #[inline]
    fn from(to_component_id: ComponentIdFor<T>) -> ComponentId {
        *to_component_id
    }
}

/// Initializes the [`ComponentId`] for a specific type when used with [`FromWorld`].
struct InitComponentId<T: Component> {
    component_id: ComponentId,
    marker: PhantomData<T>,
}

impl<T: Component> FromWorld for InitComponentId<T> {
    fn from_world(world: &mut World) -> Self {
        Self {
            component_id: world.register_component::<T>(),
            marker: PhantomData,
        }
    }
}

/// An error returned when the registration of a required component fails.
#[derive(Error, Display, Debug)]
#[non_exhaustive]
pub enum RequiredComponentsError {
    /// The component is already a directly required component for the requiree.
    #[display("Component {0:?} already directly requires component {_1:?}")]
    #[error(ignore)]
    DuplicateRegistration(ComponentId, ComponentId),
    /// An archetype with the component that requires other components already exists
    #[display(
        "An archetype with the component {_0:?} that requires other components already exists"
    )]
    #[error(ignore)]
    ArchetypeExists(ComponentId),
}

/// A Required Component constructor. See [`Component`] for details.
#[cfg(feature = "track_change_detection")]
#[derive(Clone)]
pub struct RequiredComponentConstructor(
    pub Arc<dyn Fn(&mut Table, &mut SparseSets, Tick, TableRow, Entity, &'static Location<'static>)>,
);

/// A Required Component constructor. See [`Component`] for details.
#[cfg(not(feature = "track_change_detection"))]
#[derive(Clone)]
pub struct RequiredComponentConstructor(
    pub Arc<dyn Fn(&mut Table, &mut SparseSets, Tick, TableRow, Entity)>,
);

impl RequiredComponentConstructor {
    /// # Safety
    /// This is intended to only be called in the context of [`BundleInfo::write_components`] to initialized required components.
    /// Calling it _anywhere else_ should be considered unsafe.
    ///
    /// `table_row` and `entity` must correspond to a valid entity that currently needs a component initialized via the constructor stored
    /// on this [`RequiredComponentConstructor`]. The stored constructor must correspond to a component on `entity` that needs initialization.
    /// `table` and `sparse_sets` must correspond to storages on a world where `entity` needs this required component initialized.
    ///
    /// Again, don't call this anywhere but [`BundleInfo::write_components`].
    pub(crate) unsafe fn initialize(
        &self,
        table: &mut Table,
        sparse_sets: &mut SparseSets,
        change_tick: Tick,
        table_row: TableRow,
        entity: Entity,
        #[cfg(feature = "track_change_detection")] caller: &'static Location<'static>,
    ) {
        (self.0)(
            table,
            sparse_sets,
            change_tick,
            table_row,
            entity,
            #[cfg(feature = "track_change_detection")]
            caller,
        );
    }
}

/// Metadata associated with a required component. See [`Component`] for details.
#[derive(Clone)]
pub struct RequiredComponent {
    /// The constructor used for the required component.
    pub constructor: RequiredComponentConstructor,

    /// The depth of the component requirement in the requirement hierarchy for this component.
    /// This is used for determining which constructor is used in cases where there are duplicate requires.
    ///
    /// For example, consider the inheritance tree `X -> Y -> Z`, where `->` indicates a requirement.
    /// `X -> Y` and `Y -> Z` are direct requirements with a depth of 0, while `Z` is only indirectly
    /// required for `X` with a depth of `1`.
    ///
    /// In cases where there are multiple conflicting requirements with the same depth, a higher priority
    /// will be given to components listed earlier in the `require` attribute, or to the latest added requirement
    /// if registered at runtime.
    pub inheritance_depth: u16,
}

/// The collection of metadata for components that are required for a given component.
///
/// For more information, see the "Required Components" section of [`Component`].
#[derive(Default, Clone)]
pub struct RequiredComponents(pub(crate) HashMap<ComponentId, RequiredComponent>);

impl Debug for RequiredComponents {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        f.debug_tuple("RequiredComponents")
            .field(&self.0.keys())
            .finish()
    }
}

impl RequiredComponents {
    /// Registers a required component.
    ///
    /// If the component is already registered, it will be overwritten if the given inheritance depth
    /// is smaller than the depth of the existing registration. Otherwise, the new registration will be ignored.
    ///
    /// # Safety
    ///
    /// `component_id` must match the type initialized by `constructor`.
    /// `constructor` _must_ initialize a component for `component_id` in such a way that
    /// matches the storage type of the component. It must only use the given `table_row` or `Entity` to
    /// initialize the storage for `component_id` corresponding to the given entity.
    pub unsafe fn register_dynamic(
        &mut self,
        component_id: ComponentId,
        constructor: RequiredComponentConstructor,
        inheritance_depth: u16,
    ) {
        self.0
            .entry(component_id)
            .and_modify(|component| {
                if component.inheritance_depth > inheritance_depth {
                    // New registration is more specific than existing requirement
                    component.constructor = constructor.clone();
                    component.inheritance_depth = inheritance_depth;
                }
            })
            .or_insert(RequiredComponent {
                constructor,
                inheritance_depth,
            });
    }

    /// Registers a required component.
    ///
    /// If the component is already registered, it will be overwritten if the given inheritance depth
    /// is smaller than the depth of the existing registration. Otherwise, the new registration will be ignored.
    pub fn register<C: Component>(
        &mut self,
        components: &mut Components,
        storages: &mut Storages,
        constructor: fn() -> C,
        inheritance_depth: u16,
    ) {
        let component_id = components.register_component::<C>(storages);
        self.register_by_id(component_id, constructor, inheritance_depth);
    }

    /// Registers the [`Component`] with the given ID as required if it exists.
    ///
    /// If the component is already registered, it will be overwritten if the given inheritance depth
    /// is smaller than the depth of the existing registration. Otherwise, the new registration will be ignored.
    pub fn register_by_id<C: Component>(
        &mut self,
        component_id: ComponentId,
        constructor: fn() -> C,
        inheritance_depth: u16,
    ) {
        let erased: RequiredComponentConstructor = RequiredComponentConstructor(Arc::new(
            move |table,
                  sparse_sets,
                  change_tick,
                  table_row,
                  entity,
                  #[cfg(feature = "track_change_detection")] caller| {
                OwningPtr::make(constructor(), |ptr| {
                    // SAFETY: This will only be called in the context of `BundleInfo::write_components`, which will
                    // pass in a valid table_row and entity requiring a C constructor
                    // C::STORAGE_TYPE is the storage type associated with `component_id` / `C`
                    // `ptr` points to valid `C` data, which matches the type associated with `component_id`
                    unsafe {
                        BundleInfo::initialize_required_component(
                            table,
                            sparse_sets,
                            change_tick,
                            table_row,
                            entity,
                            component_id,
                            C::STORAGE_TYPE,
                            ptr,
                            #[cfg(feature = "track_change_detection")]
                            caller,
                        );
                    }
                });
            },
        ));
        // SAFETY:
        // `component_id` matches the type initialized by the `erased` constructor above.
        // `erased` initializes a component for `component_id` in such a way that
        // matches the storage type of the component. It only uses the given `table_row` or `Entity` to
        // initialize the storage corresponding to the given entity.
        unsafe { self.register_dynamic(component_id, erased, inheritance_depth) };
    }

    /// Iterates the ids of all required components. This includes recursive required components.
    pub fn iter_ids(&self) -> impl Iterator<Item = ComponentId> + '_ {
        self.0.keys().copied()
    }

    /// Removes components that are explicitly provided in a given [`Bundle`]. These components should
    /// be logically treated as normal components, not "required components".
    ///
    /// [`Bundle`]: crate::bundle::Bundle
    pub(crate) fn remove_explicit_components(&mut self, components: &[ComponentId]) {
        for component in components {
            self.0.remove(component);
        }
    }

    // Merges `required_components` into this collection. This only inserts a required component
    // if it _did not already exist_.
    pub(crate) fn merge(&mut self, required_components: &RequiredComponents) {
        for (id, constructor) in &required_components.0 {
            self.0.entry(*id).or_insert_with(|| constructor.clone());
        }
    }
}