#[cfg(feature = "reflect")]
use bevy_ecs::reflect::{ReflectComponent, ReflectMapEntities};
use bevy_ecs::{
    component::Component,
    entity::{Entity, EntityMapper, MapEntities},
    prelude::FromWorld,
    world::World,
};
use core::slice;
use smallvec::SmallVec;
use std::ops::Deref;

/// Contains references to the child entities of this entity.
///
/// Each child must contain a [`Parent`] component that points back to this entity.
/// This component rarely needs to be created manually,
/// consider using higher level utilities like [`BuildChildren::with_children`]
/// which are safer and easier to use.
///
/// See [`HierarchyQueryExt`] for hierarchy related methods on [`Query`].
///
/// [`HierarchyQueryExt`]: crate::query_extension::HierarchyQueryExt
/// [`Query`]: bevy_ecs::system::Query
/// [`Parent`]: crate::components::parent::Parent
/// [`BuildChildren::with_children`]: crate::child_builder::BuildChildren::with_children
#[derive(Component, Debug)]
#[cfg_attr(feature = "reflect", derive(bevy_reflect::Reflect))]
#[cfg_attr(feature = "reflect", reflect(Component, MapEntities))]
pub struct Children(pub(crate) SmallVec<[Entity; 8]>);

impl MapEntities for Children {
    fn map_entities<M: EntityMapper>(&mut self, entity_mapper: &mut M) {
        for entity in &mut self.0 {
            *entity = entity_mapper.map_entity(*entity);
        }
    }
}

// TODO: We need to impl either FromWorld or Default so Children can be registered as Reflect.
// This is because Reflect deserialize by creating an instance and apply a patch on top.
// However Children should only ever be set with a real user-defined entities. Its worth looking
// into better ways to handle cases like this.
impl FromWorld for Children {
    #[inline]
    fn from_world(_world: &mut World) -> Self {
        Children(SmallVec::new())
    }
}

impl Children {
    /// Constructs a [`Children`] component with the given entities.
    #[inline]
    pub(crate) fn from_entities(entities: &[Entity]) -> Self {
        Self(SmallVec::from_slice(entities))
    }

    /// Swaps the child at `a_index` with the child at `b_index`.
    #[inline]
    pub fn swap(&mut self, a_index: usize, b_index: usize) {
        self.0.swap(a_index, b_index);
    }

    /// Sorts children [stably](https://en.wikipedia.org/wiki/Sorting_algorithm#Stability)
    /// in place using the provided comparator function.
    ///
    /// For the underlying implementation, see [`slice::sort_by`].
    ///
    /// For the unstable version, see [`sort_unstable_by`](Children::sort_unstable_by).
    ///
    /// See also [`sort_by_key`](Children::sort_by_key), [`sort_by_cached_key`](Children::sort_by_cached_key).
    #[inline]
    pub fn sort_by<F>(&mut self, compare: F)
    where
        F: FnMut(&Entity, &Entity) -> std::cmp::Ordering,
    {
        self.0.sort_by(compare);
    }

    /// Sorts children [stably](https://en.wikipedia.org/wiki/Sorting_algorithm#Stability)
    /// in place using the provided key extraction function.
    ///
    /// For the underlying implementation, see [`slice::sort_by_key`].
    ///
    /// For the unstable version, see [`sort_unstable_by_key`](Children::sort_unstable_by_key).
    ///
    /// See also [`sort_by`](Children::sort_by), [`sort_by_cached_key`](Children::sort_by_cached_key).
    #[inline]
    pub fn sort_by_key<K, F>(&mut self, compare: F)
    where
        F: FnMut(&Entity) -> K,
        K: Ord,
    {
        self.0.sort_by_key(compare);
    }

    /// Sorts children [stably](https://en.wikipedia.org/wiki/Sorting_algorithm#Stability)
    /// in place using the provided key extraction function. Only evaluates each key at most
    /// once per sort, caching the intermediate results in memory.
    ///
    /// For the underlying implementation, see [`slice::sort_by_cached_key`].
    ///
    /// See also [`sort_by`](Children::sort_by), [`sort_by_key`](Children::sort_by_key).
    #[inline]
    pub fn sort_by_cached_key<K, F>(&mut self, compare: F)
    where
        F: FnMut(&Entity) -> K,
        K: Ord,
    {
        self.0.sort_by_cached_key(compare);
    }

    /// Sorts children [unstably](https://en.wikipedia.org/wiki/Sorting_algorithm#Stability)
    /// in place using the provided comparator function.
    ///
    /// For the underlying implementation, see [`slice::sort_unstable_by`].
    ///
    /// For the stable version, see [`sort_by`](Children::sort_by).
    ///
    /// See also [`sort_unstable_by_key`](Children::sort_unstable_by_key).
    #[inline]
    pub fn sort_unstable_by<F>(&mut self, compare: F)
    where
        F: FnMut(&Entity, &Entity) -> std::cmp::Ordering,
    {
        self.0.sort_unstable_by(compare);
    }

    /// Sorts children [unstably](https://en.wikipedia.org/wiki/Sorting_algorithm#Stability)
    /// in place using the provided key extraction function.
    ///
    /// For the underlying implementation, see [`slice::sort_unstable_by_key`].
    ///
    /// For the stable version, see [`sort_by_key`](Children::sort_by_key).
    ///
    /// See also [`sort_unstable_by`](Children::sort_unstable_by).
    #[inline]
    pub fn sort_unstable_by_key<K, F>(&mut self, compare: F)
    where
        F: FnMut(&Entity) -> K,
        K: Ord,
    {
        self.0.sort_unstable_by_key(compare);
    }
}

impl Deref for Children {
    type Target = [Entity];

    #[inline(always)]
    fn deref(&self) -> &Self::Target {
        &self.0[..]
    }
}

impl<'a> IntoIterator for &'a Children {
    type Item = <Self::IntoIter as Iterator>::Item;

    type IntoIter = slice::Iter<'a, Entity>;

    #[inline(always)]
    fn into_iter(self) -> Self::IntoIter {
        self.0.iter()
    }
}