bevy/crates/bevy_animation/src/animation_curves.rs

//! The [`AnimationCurve`] trait and adaptors that allow curves to implement it.
//!
//! # Overview
//!
//! The flow of curves into the animation system generally begins with something that
//! implements the [`Curve`] trait. Let's imagine, for example, that we have some
//! `Curve<Vec3>` that we want to use to animate something. That could be defined in
//! a number of different ways, but let's imagine that we've defined it [using a function]:
//!
//!     # use bevy_math::curve::{Curve, Interval, FunctionCurve};
//!     # use bevy_math::vec3;
//!     let wobble_curve = FunctionCurve::new(
//!         Interval::UNIT,
//!         |t| { vec3(t.cos(), 0.0, 0.0) },
//!     );
//!
//! Okay, so we have a curve, but the animation system also needs to know, in some way,
//! how the values from this curve should actually be used. That is, it needs to know what
//! to animate! That's what [`AnimationCurve`] is for. In particular, what we need to do
//! is take our curve and turn it into an `AnimationCurve` which will be usable by the
//! animation system.
//!
//! For instance, let's imagine that we want to use the `Vec3` output
//! from our curve to animate the [translation component of a `Transform`]. For this, there is
//! the adaptor [`TranslationCurve`], which wraps any `Curve<Vec3>` and turns it into an
//! [`AnimationCurve`] that will use the given curve to animate the entity's translation:
//!
//!     # use bevy_math::curve::{Curve, Interval, FunctionCurve};
//!     # use bevy_math::vec3;
//!     # use bevy_animation::animation_curves::*;
//!     # let wobble_curve = FunctionCurve::new(
//!     #     Interval::UNIT,
//!     #     |t| vec3(t.cos(), 0.0, 0.0)
//!     # );
//!     let wobble_animation = TranslationCurve(wobble_curve);
//!
//! And finally, this `AnimationCurve` needs to be added to an [`AnimationClip`] in order to
//! actually animate something. This is what that looks like:
//!
//!     # use bevy_math::curve::{Curve, Interval, FunctionCurve};
//!     # use bevy_animation::{AnimationClip, AnimationTargetId, animation_curves::*};
//!     # use bevy_core::Name;
//!     # use bevy_math::vec3;
//!     # let wobble_curve = FunctionCurve::new(
//!     #     Interval::UNIT,
//!     #     |t| { vec3(t.cos(), 0.0, 0.0) },
//!     # );
//!     # let wobble_animation = TranslationCurve(wobble_curve);
//!     # let animation_target_id = AnimationTargetId::from(&Name::new("Test"));
//!     let mut animation_clip = AnimationClip::default();
//!     animation_clip.add_curve_to_target(
//!         animation_target_id,
//!         wobble_animation,
//!     );
//!
//! # Making animation curves
//!
//! The overview showed one example, but in general there are a few different ways of going from
//! a [`Curve`], which produces time-related data of some kind, to an [`AnimationCurve`], which
//! knows how to apply that data to an entity.
//!
//! ## `Transform`
//!
//! [`Transform`] is special and has its own adaptors:
//!     - [`TranslationCurve`], which uses `Vec3` output to animate [`Transform::translation`]
//!     - [`RotationCurve`], which uses `Quat` output to animate [`Transform::rotation`]
//!     - [`ScaleCurve`], which uses `Vec3` output to animate [`Transform::scale`]
//!
//! ## Animatable properties
//!
//! Animation of arbitrary components can be accomplished using [`AnimatableProperty`] in
//! conjunction with [`AnimatableCurve`]. See the documentation [there] for details.
//!
//! [using a function]: bevy_math::curve::FunctionCurve
//! [translation component of a `Transform`]: bevy_transform::prelude::Transform::translation
//! [`AnimationClip`]: crate::AnimationClip
//! [there]: AnimatableProperty

use core::{
    any::TypeId,
    fmt::{self, Debug, Formatter},
    marker::PhantomData,
};

use bevy_ecs::{component::Component, world::Mut};
use bevy_math::{
    curve::{
        cores::{UnevenCore, UnevenCoreError},
        iterable::IterableCurve,
        Curve, Interval,
    },
    Quat, Vec3,
};
use bevy_reflect::{FromReflect, Reflect, Reflectable, TypePath};
use bevy_render::mesh::morph::MorphWeights;
use bevy_transform::prelude::Transform;

use crate::{
    graph::AnimationNodeIndex,
    prelude::{Animatable, BlendInput},
    AnimationEntityMut, AnimationEvaluationError,
};

/// A value on a component that Bevy can animate.
///
/// You can implement this trait on a unit struct in order to support animating
/// custom components other than transforms and morph weights. Use that type in
/// conjunction with [`AnimatableCurve`] (and perhaps [`AnimatableKeyframeCurve`]
/// to define the animation itself).
/// For example, in order to animate field of view, you might use:
///
///     # use bevy_animation::prelude::AnimatableProperty;
///     # use bevy_reflect::Reflect;
///     # use bevy_render::camera::PerspectiveProjection;
///     #[derive(Reflect)]
///     struct FieldOfViewProperty;
///
///     impl AnimatableProperty for FieldOfViewProperty {
///         type Component = PerspectiveProjection;
///         type Property = f32;
///         fn get_mut(component: &mut Self::Component) -> Option<&mut Self::Property> {
///             Some(&mut component.fov)
///         }
///     }
///
/// You can then create an [`AnimationClip`] to animate this property like so:
///
///     # use bevy_animation::{AnimationClip, AnimationTargetId, VariableCurve};
///     # use bevy_animation::prelude::{AnimatableProperty, AnimatableKeyframeCurve, AnimatableCurve};
///     # use bevy_core::Name;
///     # use bevy_reflect::Reflect;
///     # use bevy_render::camera::PerspectiveProjection;
///     # let animation_target_id = AnimationTargetId::from(&Name::new("Test"));
///     # #[derive(Reflect)]
///     # struct FieldOfViewProperty;
///     # impl AnimatableProperty for FieldOfViewProperty {
///     #     type Component = PerspectiveProjection;
///     #     type Property = f32;
///     #     fn get_mut(component: &mut Self::Component) -> Option<&mut Self::Property> {
///     #         Some(&mut component.fov)
///     #     }
///     # }
///     let mut animation_clip = AnimationClip::default();
///     animation_clip.add_curve_to_target(
///         animation_target_id,
///         AnimatableKeyframeCurve::new(
///             [
///                 (0.0, core::f32::consts::PI / 4.0),
///                 (1.0, core::f32::consts::PI / 3.0),
///             ]
///         )
///         .map(AnimatableCurve::<FieldOfViewProperty, _>::from_curve)
///         .expect("Failed to create font size curve")
///     );
///
/// Here, the use of [`AnimatableKeyframeCurve`] creates a curve out of the given keyframe time-value
/// pairs, using the [`Animatable`] implementation of `f32` to interpolate between them. The
/// invocation of [`AnimatableCurve::from_curve`] with `FieldOfViewProperty` indicates that the `f32`
/// output from that curve is to be used to animate the font size of a `PerspectiveProjection` component (as
/// configured above).
///
/// [`AnimationClip`]: crate::AnimationClip
pub trait AnimatableProperty: Reflect + TypePath {
    /// The type of the component that the property lives on.
    type Component: Component;

    /// The type of the property to be animated.
    type Property: Animatable + FromReflect + Reflectable + Clone + Sync + Debug;

    /// Given a reference to the component, returns a reference to the property.
    ///
    /// If the property couldn't be found, returns `None`.
    fn get_mut(component: &mut Self::Component) -> Option<&mut Self::Property>;
}

/// This trait collects the additional requirements on top of [`Curve<T>`] needed for a
/// curve to be used as an [`AnimationCurve`].
pub trait AnimationCompatibleCurve<T>: Curve<T> + Debug + Clone + Reflectable {}

impl<T, C> AnimationCompatibleCurve<T> for C where C: Curve<T> + Debug + Clone + Reflectable {}

/// This type allows the conversion of a [curve] valued in the [property type] of an
/// [`AnimatableProperty`] into an [`AnimationCurve`] which animates that property.
///
/// [curve]: Curve
/// [property type]: AnimatableProperty::Property
#[derive(Reflect, FromReflect)]
#[reflect(from_reflect = false)]
pub struct AnimatableCurve<P, C> {
    /// The inner [curve] whose values are used to animate the property.
    ///
    /// [curve]: Curve
    pub curve: C,
    #[reflect(ignore)]
    _phantom: PhantomData<P>,
}

/// An [`AnimatableCurveEvaluator`] for [`AnimatableProperty`] instances.
///
/// You shouldn't ordinarily need to instantiate one of these manually. Bevy
/// will automatically do so when you use an [`AnimatableCurve`] instance.
#[derive(Reflect)]
pub struct AnimatableCurveEvaluator<P>
where
    P: AnimatableProperty,
{
    evaluator: BasicAnimationCurveEvaluator<P::Property>,
    #[reflect(ignore)]
    phantom: PhantomData<P>,
}

impl<P, C> AnimatableCurve<P, C>
where
    P: AnimatableProperty,
    C: AnimationCompatibleCurve<P::Property>,
{
    /// Create an [`AnimatableCurve`] (and thus an [`AnimationCurve`]) from a curve
    /// valued in an [animatable property].
    ///
    /// [animatable property]: AnimatableProperty::Property
    pub fn from_curve(curve: C) -> Self {
        Self {
            curve,
            _phantom: PhantomData,
        }
    }
}

impl<P, C> Clone for AnimatableCurve<P, C>
where
    C: Clone,
{
    fn clone(&self) -> Self {
        Self {
            curve: self.curve.clone(),
            _phantom: PhantomData,
        }
    }
}

impl<P, C> Debug for AnimatableCurve<P, C>
where
    C: Debug,
{
    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
        f.debug_struct("AnimatableCurve")
            .field("curve", &self.curve)
            .finish()
    }
}

impl<P, C> AnimationCurve for AnimatableCurve<P, C>
where
    P: AnimatableProperty,
    C: AnimationCompatibleCurve<P::Property>,
{
    fn clone_value(&self) -> Box<dyn AnimationCurve> {
        Box::new(self.clone())
    }

    fn domain(&self) -> Interval {
        self.curve.domain()
    }

    fn evaluator_type(&self) -> TypeId {
        TypeId::of::<AnimatableCurveEvaluator<P>>()
    }

    fn create_evaluator(&self) -> Box<dyn AnimationCurveEvaluator> {
        Box::new(AnimatableCurveEvaluator {
            evaluator: BasicAnimationCurveEvaluator::default(),
            phantom: PhantomData::<P>,
        })
    }

    fn apply(
        &self,
        curve_evaluator: &mut dyn AnimationCurveEvaluator,
        t: f32,
        weight: f32,
        graph_node: AnimationNodeIndex,
    ) -> Result<(), AnimationEvaluationError> {
        let curve_evaluator = (*Reflect::as_any_mut(curve_evaluator))
            .downcast_mut::<AnimatableCurveEvaluator<P>>()
            .unwrap();
        let value = self.curve.sample_clamped(t);
        curve_evaluator
            .evaluator
            .stack
            .push(BasicAnimationCurveEvaluatorStackElement {
                value,
                weight,
                graph_node,
            });
        Ok(())
    }
}

impl<P> AnimationCurveEvaluator for AnimatableCurveEvaluator<P>
where
    P: AnimatableProperty,
{
    fn blend(&mut self, graph_node: AnimationNodeIndex) -> Result<(), AnimationEvaluationError> {
        self.evaluator.combine(graph_node, /*additive=*/ false)
    }

    fn add(&mut self, graph_node: AnimationNodeIndex) -> Result<(), AnimationEvaluationError> {
        self.evaluator.combine(graph_node, /*additive=*/ true)
    }

    fn push_blend_register(
        &mut self,
        weight: f32,
        graph_node: AnimationNodeIndex,
    ) -> Result<(), AnimationEvaluationError> {
        self.evaluator.push_blend_register(weight, graph_node)
    }

    fn commit<'a>(
        &mut self,
        _: Option<Mut<'a, Transform>>,
        mut entity: AnimationEntityMut<'a>,
    ) -> Result<(), AnimationEvaluationError> {
        let mut component = entity.get_mut::<P::Component>().ok_or_else(|| {
            AnimationEvaluationError::ComponentNotPresent(TypeId::of::<P::Component>())
        })?;
        let property = P::get_mut(&mut component)
            .ok_or_else(|| AnimationEvaluationError::PropertyNotPresent(TypeId::of::<P>()))?;
        *property = self
            .evaluator
            .stack
            .pop()
            .ok_or_else(inconsistent::<AnimatableCurveEvaluator<P>>)?
            .value;
        Ok(())
    }
}

/// This type allows a [curve] valued in `Vec3` to become an [`AnimationCurve`] that animates
/// the translation component of a transform.
///
/// [curve]: Curve
#[derive(Debug, Clone, Reflect, FromReflect)]
#[reflect(from_reflect = false)]
pub struct TranslationCurve<C>(pub C);

/// An [`AnimationCurveEvaluator`] for use with [`TranslationCurve`]s.
///
/// You shouldn't need to instantiate this manually; Bevy will automatically do
/// so.
#[derive(Reflect)]
pub struct TranslationCurveEvaluator {
    evaluator: BasicAnimationCurveEvaluator<Vec3>,
}

impl<C> AnimationCurve for TranslationCurve<C>
where
    C: AnimationCompatibleCurve<Vec3>,
{
    fn clone_value(&self) -> Box<dyn AnimationCurve> {
        Box::new(self.clone())
    }

    fn domain(&self) -> Interval {
        self.0.domain()
    }

    fn evaluator_type(&self) -> TypeId {
        TypeId::of::<TranslationCurveEvaluator>()
    }

    fn create_evaluator(&self) -> Box<dyn AnimationCurveEvaluator> {
        Box::new(TranslationCurveEvaluator {
            evaluator: BasicAnimationCurveEvaluator::default(),
        })
    }

    fn apply(
        &self,
        curve_evaluator: &mut dyn AnimationCurveEvaluator,
        t: f32,
        weight: f32,
        graph_node: AnimationNodeIndex,
    ) -> Result<(), AnimationEvaluationError> {
        let curve_evaluator = (*Reflect::as_any_mut(curve_evaluator))
            .downcast_mut::<TranslationCurveEvaluator>()
            .unwrap();
        let value = self.0.sample_clamped(t);
        curve_evaluator
            .evaluator
            .stack
            .push(BasicAnimationCurveEvaluatorStackElement {
                value,
                weight,
                graph_node,
            });
        Ok(())
    }
}

impl AnimationCurveEvaluator for TranslationCurveEvaluator {
    fn blend(&mut self, graph_node: AnimationNodeIndex) -> Result<(), AnimationEvaluationError> {
        self.evaluator.combine(graph_node, /*additive=*/ false)
    }

    fn add(&mut self, graph_node: AnimationNodeIndex) -> Result<(), AnimationEvaluationError> {
        self.evaluator.combine(graph_node, /*additive=*/ true)
    }

    fn push_blend_register(
        &mut self,
        weight: f32,
        graph_node: AnimationNodeIndex,
    ) -> Result<(), AnimationEvaluationError> {
        self.evaluator.push_blend_register(weight, graph_node)
    }

    fn commit<'a>(
        &mut self,
        transform: Option<Mut<'a, Transform>>,
        _: AnimationEntityMut<'a>,
    ) -> Result<(), AnimationEvaluationError> {
        let mut component = transform.ok_or_else(|| {
            AnimationEvaluationError::ComponentNotPresent(TypeId::of::<Transform>())
        })?;
        component.translation = self
            .evaluator
            .stack
            .pop()
            .ok_or_else(inconsistent::<TranslationCurveEvaluator>)?
            .value;
        Ok(())
    }
}

/// This type allows a [curve] valued in `Quat` to become an [`AnimationCurve`] that animates
/// the rotation component of a transform.
///
/// [curve]: Curve
#[derive(Debug, Clone, Reflect, FromReflect)]
#[reflect(from_reflect = false)]
pub struct RotationCurve<C>(pub C);

/// An [`AnimationCurveEvaluator`] for use with [`RotationCurve`]s.
///
/// You shouldn't need to instantiate this manually; Bevy will automatically do
/// so.
#[derive(Reflect)]
pub struct RotationCurveEvaluator {
    evaluator: BasicAnimationCurveEvaluator<Quat>,
}

impl<C> AnimationCurve for RotationCurve<C>
where
    C: AnimationCompatibleCurve<Quat>,
{
    fn clone_value(&self) -> Box<dyn AnimationCurve> {
        Box::new(self.clone())
    }

    fn domain(&self) -> Interval {
        self.0.domain()
    }

    fn evaluator_type(&self) -> TypeId {
        TypeId::of::<RotationCurveEvaluator>()
    }

    fn create_evaluator(&self) -> Box<dyn AnimationCurveEvaluator> {
        Box::new(RotationCurveEvaluator {
            evaluator: BasicAnimationCurveEvaluator::default(),
        })
    }

    fn apply(
        &self,
        curve_evaluator: &mut dyn AnimationCurveEvaluator,
        t: f32,
        weight: f32,
        graph_node: AnimationNodeIndex,
    ) -> Result<(), AnimationEvaluationError> {
        let curve_evaluator = (*Reflect::as_any_mut(curve_evaluator))
            .downcast_mut::<RotationCurveEvaluator>()
            .unwrap();
        let value = self.0.sample_clamped(t);
        curve_evaluator
            .evaluator
            .stack
            .push(BasicAnimationCurveEvaluatorStackElement {
                value,
                weight,
                graph_node,
            });
        Ok(())
    }
}

impl AnimationCurveEvaluator for RotationCurveEvaluator {
    fn blend(&mut self, graph_node: AnimationNodeIndex) -> Result<(), AnimationEvaluationError> {
        self.evaluator.combine(graph_node, /*additive=*/ false)
    }

    fn add(&mut self, graph_node: AnimationNodeIndex) -> Result<(), AnimationEvaluationError> {
        self.evaluator.combine(graph_node, /*additive=*/ true)
    }

    fn push_blend_register(
        &mut self,
        weight: f32,
        graph_node: AnimationNodeIndex,
    ) -> Result<(), AnimationEvaluationError> {
        self.evaluator.push_blend_register(weight, graph_node)
    }

    fn commit<'a>(
        &mut self,
        transform: Option<Mut<'a, Transform>>,
        _: AnimationEntityMut<'a>,
    ) -> Result<(), AnimationEvaluationError> {
        let mut component = transform.ok_or_else(|| {
            AnimationEvaluationError::ComponentNotPresent(TypeId::of::<Transform>())
        })?;
        component.rotation = self
            .evaluator
            .stack
            .pop()
            .ok_or_else(inconsistent::<RotationCurveEvaluator>)?
            .value;
        Ok(())
    }
}

/// This type allows a [curve] valued in `Vec3` to become an [`AnimationCurve`] that animates
/// the scale component of a transform.
///
/// [curve]: Curve
#[derive(Debug, Clone, Reflect, FromReflect)]
#[reflect(from_reflect = false)]
pub struct ScaleCurve<C>(pub C);

/// An [`AnimationCurveEvaluator`] for use with [`ScaleCurve`]s.
///
/// You shouldn't need to instantiate this manually; Bevy will automatically do
/// so.
#[derive(Reflect)]
pub struct ScaleCurveEvaluator {
    evaluator: BasicAnimationCurveEvaluator<Vec3>,
}

impl<C> AnimationCurve for ScaleCurve<C>
where
    C: AnimationCompatibleCurve<Vec3>,
{
    fn clone_value(&self) -> Box<dyn AnimationCurve> {
        Box::new(self.clone())
    }

    fn domain(&self) -> Interval {
        self.0.domain()
    }

    fn evaluator_type(&self) -> TypeId {
        TypeId::of::<ScaleCurveEvaluator>()
    }

    fn create_evaluator(&self) -> Box<dyn AnimationCurveEvaluator> {
        Box::new(ScaleCurveEvaluator {
            evaluator: BasicAnimationCurveEvaluator::default(),
        })
    }

    fn apply(
        &self,
        curve_evaluator: &mut dyn AnimationCurveEvaluator,
        t: f32,
        weight: f32,
        graph_node: AnimationNodeIndex,
    ) -> Result<(), AnimationEvaluationError> {
        let curve_evaluator = (*Reflect::as_any_mut(curve_evaluator))
            .downcast_mut::<ScaleCurveEvaluator>()
            .unwrap();
        let value = self.0.sample_clamped(t);
        curve_evaluator
            .evaluator
            .stack
            .push(BasicAnimationCurveEvaluatorStackElement {
                value,
                weight,
                graph_node,
            });
        Ok(())
    }
}

impl AnimationCurveEvaluator for ScaleCurveEvaluator {
    fn blend(&mut self, graph_node: AnimationNodeIndex) -> Result<(), AnimationEvaluationError> {
        self.evaluator.combine(graph_node, /*additive=*/ false)
    }

    fn add(&mut self, graph_node: AnimationNodeIndex) -> Result<(), AnimationEvaluationError> {
        self.evaluator.combine(graph_node, /*additive=*/ true)
    }

    fn push_blend_register(
        &mut self,
        weight: f32,
        graph_node: AnimationNodeIndex,
    ) -> Result<(), AnimationEvaluationError> {
        self.evaluator.push_blend_register(weight, graph_node)
    }

    fn commit<'a>(
        &mut self,
        transform: Option<Mut<'a, Transform>>,
        _: AnimationEntityMut<'a>,
    ) -> Result<(), AnimationEvaluationError> {
        let mut component = transform.ok_or_else(|| {
            AnimationEvaluationError::ComponentNotPresent(TypeId::of::<Transform>())
        })?;
        component.scale = self
            .evaluator
            .stack
            .pop()
            .ok_or_else(inconsistent::<ScaleCurveEvaluator>)?
            .value;
        Ok(())
    }
}

/// This type allows an [`IterableCurve`] valued in `f32` to be used as an [`AnimationCurve`]
/// that animates [morph weights].
///
/// [morph weights]: MorphWeights
#[derive(Debug, Clone, Reflect, FromReflect)]
#[reflect(from_reflect = false)]
pub struct WeightsCurve<C>(pub C);

#[derive(Reflect)]
struct WeightsCurveEvaluator {
    /// The values of the stack, in which each element is a list of morph target
    /// weights.
    ///
    /// The stack elements are concatenated and tightly packed together.
    ///
    /// The number of elements in this stack will always be a multiple of
    /// [`Self::morph_target_count`].
    stack_morph_target_weights: Vec<f32>,

    /// The blend weights and graph node indices for each element of the stack.
    ///
    /// This should have as many elements as there are stack nodes. In other
    /// words, `Self::stack_morph_target_weights.len() *
    /// Self::morph_target_counts as usize ==
    /// Self::stack_blend_weights_and_graph_nodes`.
    stack_blend_weights_and_graph_nodes: Vec<(f32, AnimationNodeIndex)>,

    /// The morph target weights in the blend register, if any.
    ///
    /// This field should be ignored if [`Self::blend_register_blend_weight`] is
    /// `None`. If non-empty, it will always have [`Self::morph_target_count`]
    /// elements in it.
    blend_register_morph_target_weights: Vec<f32>,

    /// The weight in the blend register.
    ///
    /// This will be `None` if the blend register is empty. In that case,
    /// [`Self::blend_register_morph_target_weights`] will be empty.
    blend_register_blend_weight: Option<f32>,

    /// The number of morph targets that are to be animated.
    morph_target_count: Option<u32>,
}

impl<C> AnimationCurve for WeightsCurve<C>
where
    C: IterableCurve<f32> + Debug + Clone + Reflectable,
{
    fn clone_value(&self) -> Box<dyn AnimationCurve> {
        Box::new(self.clone())
    }

    fn domain(&self) -> Interval {
        self.0.domain()
    }

    fn evaluator_type(&self) -> TypeId {
        TypeId::of::<WeightsCurveEvaluator>()
    }

    fn create_evaluator(&self) -> Box<dyn AnimationCurveEvaluator> {
        Box::new(WeightsCurveEvaluator {
            stack_morph_target_weights: vec![],
            stack_blend_weights_and_graph_nodes: vec![],
            blend_register_morph_target_weights: vec![],
            blend_register_blend_weight: None,
            morph_target_count: None,
        })
    }

    fn apply(
        &self,
        curve_evaluator: &mut dyn AnimationCurveEvaluator,
        t: f32,
        weight: f32,
        graph_node: AnimationNodeIndex,
    ) -> Result<(), AnimationEvaluationError> {
        let curve_evaluator = (*Reflect::as_any_mut(curve_evaluator))
            .downcast_mut::<WeightsCurveEvaluator>()
            .unwrap();

        let prev_morph_target_weights_len = curve_evaluator.stack_morph_target_weights.len();
        curve_evaluator
            .stack_morph_target_weights
            .extend(self.0.sample_iter_clamped(t));
        curve_evaluator.morph_target_count = Some(
            (curve_evaluator.stack_morph_target_weights.len() - prev_morph_target_weights_len)
                as u32,
        );

        curve_evaluator
            .stack_blend_weights_and_graph_nodes
            .push((weight, graph_node));
        Ok(())
    }
}

impl WeightsCurveEvaluator {
    fn combine(
        &mut self,
        graph_node: AnimationNodeIndex,
        additive: bool,
    ) -> Result<(), AnimationEvaluationError> {
        let Some(&(_, top_graph_node)) = self.stack_blend_weights_and_graph_nodes.last() else {
            return Ok(());
        };
        if top_graph_node != graph_node {
            return Ok(());
        }

        let (weight_to_blend, _) = self.stack_blend_weights_and_graph_nodes.pop().unwrap();
        let stack_iter = self.stack_morph_target_weights.drain(
            (self.stack_morph_target_weights.len() - self.morph_target_count.unwrap() as usize)..,
        );

        match self.blend_register_blend_weight {
            None => {
                self.blend_register_blend_weight = Some(weight_to_blend);
                self.blend_register_morph_target_weights.clear();

                // In the additive case, the values pushed onto the blend register need
                // to be scaled by the weight.
                if additive {
                    self.blend_register_morph_target_weights
                        .extend(stack_iter.map(|m| m * weight_to_blend));
                } else {
                    self.blend_register_morph_target_weights.extend(stack_iter);
                }
            }

            Some(ref mut current_weight) => {
                *current_weight += weight_to_blend;
                for (dest, src) in self
                    .blend_register_morph_target_weights
                    .iter_mut()
                    .zip(stack_iter)
                {
                    if additive {
                        *dest += src * weight_to_blend;
                    } else {
                        *dest = f32::interpolate(dest, &src, weight_to_blend / *current_weight);
                    }
                }
            }
        }

        Ok(())
    }
}

impl AnimationCurveEvaluator for WeightsCurveEvaluator {
    fn blend(&mut self, graph_node: AnimationNodeIndex) -> Result<(), AnimationEvaluationError> {
        self.combine(graph_node, /*additive=*/ false)
    }

    fn add(&mut self, graph_node: AnimationNodeIndex) -> Result<(), AnimationEvaluationError> {
        self.combine(graph_node, /*additive=*/ true)
    }

    fn push_blend_register(
        &mut self,
        weight: f32,
        graph_node: AnimationNodeIndex,
    ) -> Result<(), AnimationEvaluationError> {
        if self.blend_register_blend_weight.take().is_some() {
            self.stack_morph_target_weights
                .append(&mut self.blend_register_morph_target_weights);
            self.stack_blend_weights_and_graph_nodes
                .push((weight, graph_node));
        }
        Ok(())
    }

    fn commit<'a>(
        &mut self,
        _: Option<Mut<'a, Transform>>,
        mut entity: AnimationEntityMut<'a>,
    ) -> Result<(), AnimationEvaluationError> {
        if self.stack_morph_target_weights.is_empty() {
            return Ok(());
        }

        // Compute the index of the first morph target in the last element of
        // the stack.
        let index_of_first_morph_target =
            self.stack_morph_target_weights.len() - self.morph_target_count.unwrap() as usize;

        for (dest, src) in entity
            .get_mut::<MorphWeights>()
            .ok_or_else(|| {
                AnimationEvaluationError::ComponentNotPresent(TypeId::of::<MorphWeights>())
            })?
            .weights_mut()
            .iter_mut()
            .zip(self.stack_morph_target_weights[index_of_first_morph_target..].iter())
        {
            *dest = *src;
        }
        self.stack_morph_target_weights.clear();
        self.stack_blend_weights_and_graph_nodes.clear();
        Ok(())
    }
}

#[derive(Reflect)]
struct BasicAnimationCurveEvaluator<A>
where
    A: Animatable,
{
    stack: Vec<BasicAnimationCurveEvaluatorStackElement<A>>,
    blend_register: Option<(A, f32)>,
}

#[derive(Reflect)]
struct BasicAnimationCurveEvaluatorStackElement<A>
where
    A: Animatable,
{
    value: A,
    weight: f32,
    graph_node: AnimationNodeIndex,
}

impl<A> Default for BasicAnimationCurveEvaluator<A>
where
    A: Animatable,
{
    fn default() -> Self {
        BasicAnimationCurveEvaluator {
            stack: vec![],
            blend_register: None,
        }
    }
}

impl<A> BasicAnimationCurveEvaluator<A>
where
    A: Animatable,
{
    fn combine(
        &mut self,
        graph_node: AnimationNodeIndex,
        additive: bool,
    ) -> Result<(), AnimationEvaluationError> {
        let Some(top) = self.stack.last() else {
            return Ok(());
        };
        if top.graph_node != graph_node {
            return Ok(());
        }

        let BasicAnimationCurveEvaluatorStackElement {
            value: value_to_blend,
            weight: weight_to_blend,
            graph_node: _,
        } = self.stack.pop().unwrap();

        match self.blend_register.take() {
            None => {
                self.initialize_blend_register(value_to_blend, weight_to_blend, additive);
            }
            Some((mut current_value, mut current_weight)) => {
                current_weight += weight_to_blend;

                if additive {
                    current_value = A::blend(
                        [
                            BlendInput {
                                weight: 1.0,
                                value: current_value,
                                additive: true,
                            },
                            BlendInput {
                                weight: weight_to_blend,
                                value: value_to_blend,
                                additive: true,
                            },
                        ]
                        .into_iter(),
                    );
                } else {
                    current_value = A::interpolate(
                        &current_value,
                        &value_to_blend,
                        weight_to_blend / current_weight,
                    );
                }

                self.blend_register = Some((current_value, current_weight));
            }
        }

        Ok(())
    }

    fn initialize_blend_register(&mut self, value: A, weight: f32, additive: bool) {
        if additive {
            let scaled_value = A::blend(
                [BlendInput {
                    weight,
                    value,
                    additive: true,
                }]
                .into_iter(),
            );
            self.blend_register = Some((scaled_value, weight));
        } else {
            self.blend_register = Some((value, weight));
        }
    }

    fn push_blend_register(
        &mut self,
        weight: f32,
        graph_node: AnimationNodeIndex,
    ) -> Result<(), AnimationEvaluationError> {
        if let Some((value, _)) = self.blend_register.take() {
            self.stack.push(BasicAnimationCurveEvaluatorStackElement {
                value,
                weight,
                graph_node,
            });
        }
        Ok(())
    }
}

/// A low-level trait that provides control over how curves are actually applied
/// to entities by the animation system.
///
/// Typically, this will not need to be implemented manually, since it is
/// automatically implemented by [`AnimatableCurve`] and other curves used by
/// the animation system (e.g. those that animate parts of transforms or morph
/// weights). However, this can be implemented manually when `AnimatableCurve`
/// is not sufficiently expressive.
///
/// In many respects, this behaves like a type-erased form of [`Curve`], where
/// the output type of the curve is remembered only in the components that are
/// mutated in the implementation of [`apply`].
///
/// [`apply`]: AnimationCurve::apply
pub trait AnimationCurve: Reflect + Debug + Send + Sync {
    /// Returns a boxed clone of this value.
    fn clone_value(&self) -> Box<dyn AnimationCurve>;

    /// The range of times for which this animation is defined.
    fn domain(&self) -> Interval;

    /// Returns the type ID of the [`AnimationCurveEvaluator`].
    ///
    /// This must match the type returned by [`Self::create_evaluator`]. It must
    /// be a single type that doesn't depend on the type of the curve.
    fn evaluator_type(&self) -> TypeId;

    /// Returns a newly-instantiated [`AnimationCurveEvaluator`] for use with
    /// this curve.
    ///
    /// All curve types must return the same type of
    /// [`AnimationCurveEvaluator`]. The returned value must match the type
    /// returned by [`Self::evaluator_type`].
    fn create_evaluator(&self) -> Box<dyn AnimationCurveEvaluator>;

    /// Samples the curve at the given time `t`, and pushes the sampled value
    /// onto the evaluation stack of the `curve_evaluator`.
    ///
    /// The `curve_evaluator` parameter points to the value returned by
    /// [`Self::create_evaluator`], upcast to an `&mut dyn
    /// AnimationCurveEvaluator`. Typically, implementations of [`Self::apply`]
    /// will want to downcast the `curve_evaluator` parameter to the concrete
    /// type [`Self::evaluator_type`] in order to push values of the appropriate
    /// type onto its evaluation stack.
    ///
    /// Be sure not to confuse the `t` and `weight` values. The former
    /// determines the position at which the *curve* is sampled, while `weight`
    /// ultimately determines how much the *stack values* will be blended
    /// together (see the definition of [`AnimationCurveEvaluator::blend`]).
    fn apply(
        &self,
        curve_evaluator: &mut dyn AnimationCurveEvaluator,
        t: f32,
        weight: f32,
        graph_node: AnimationNodeIndex,
    ) -> Result<(), AnimationEvaluationError>;
}

/// A low-level trait for use in [`crate::VariableCurve`] that provides fine
/// control over how animations are evaluated.
///
/// You can implement this trait when the generic [`AnimatableCurveEvaluator`]
/// isn't sufficiently-expressive for your needs. For example, [`MorphWeights`]
/// implements this trait instead of using [`AnimatableCurveEvaluator`] because
/// it needs to animate arbitrarily many weights at once, which can't be done
/// with [`Animatable`] as that works on fixed-size values only.
///
/// If you implement this trait, you should also implement [`AnimationCurve`] on
/// your curve type, as that trait allows creating instances of this one.
///
/// Implementations of [`AnimatableCurveEvaluator`] should maintain a *stack* of
/// (value, weight, node index) triples, as well as a *blend register*, which is
/// either a (value, weight) pair or empty. *Value* here refers to an instance
/// of the value being animated: for example, [`Vec3`] in the case of
/// translation keyframes.  The stack stores intermediate values generated while
/// evaluating the [`crate::graph::AnimationGraph`], while the blend register
/// stores the result of a blend operation.
pub trait AnimationCurveEvaluator: Reflect {
    /// Blends the top element of the stack with the blend register.
    ///
    /// The semantics of this method are as follows:
    ///
    /// 1. Pop the top element of the stack. Call its value vₘ and its weight
    ///    wₘ. If the stack was empty, return success.
    ///
    /// 2. If the blend register is empty, set the blend register value to vₘ
    ///    and the blend register weight to wₘ; then, return success.
    ///
    /// 3. If the blend register is nonempty, call its current value vₙ and its
    ///    current weight wₙ. Then, set the value of the blend register to
    ///    `interpolate(vₙ, vₘ, wₘ / (wₘ + wₙ))`, and set the weight of the blend
    ///    register to wₘ + wₙ.
    ///
    /// 4. Return success.
    fn blend(&mut self, graph_node: AnimationNodeIndex) -> Result<(), AnimationEvaluationError>;

    /// Additively blends the top element of the stack with the blend register.
    ///
    /// The semantics of this method are as follows:
    ///
    /// 1. Pop the top element of the stack. Call its value vₘ and its weight
    ///    wₘ. If the stack was empty, return success.
    ///
    /// 2. If the blend register is empty, set the blend register value to vₘ
    ///    and the blend register weight to wₘ; then, return success.
    ///
    /// 3. If the blend register is nonempty, call its current value vₙ.
    ///    Then, set the value of the blend register to vₙ + vₘwₘ.
    ///
    /// 4. Return success.
    fn add(&mut self, graph_node: AnimationNodeIndex) -> Result<(), AnimationEvaluationError>;

    /// Pushes the current value of the blend register onto the stack.
    ///
    /// If the blend register is empty, this method does nothing successfully.
    /// Otherwise, this method pushes the current value of the blend register
    /// onto the stack, alongside the weight and graph node supplied to this
    /// function. The weight present in the blend register is discarded; only
    /// the weight parameter to this function is pushed onto the stack. The
    /// blend register is emptied after this process.
    fn push_blend_register(
        &mut self,
        weight: f32,
        graph_node: AnimationNodeIndex,
    ) -> Result<(), AnimationEvaluationError>;

    /// Pops the top value off the stack and writes it into the appropriate
    /// component.
    ///
    /// If the stack is empty, this method does nothing successfully. Otherwise,
    /// it pops the top value off the stack, fetches the associated component
    /// from either the `transform` or `entity` values as appropriate, and
    /// updates the appropriate property with the value popped from the stack.
    /// The weight and node index associated with the popped stack element are
    /// discarded. After doing this, the stack is emptied.
    ///
    /// The property on the component must be overwritten with the value from
    /// the stack, not blended with it.
    fn commit<'a>(
        &mut self,
        transform: Option<Mut<'a, Transform>>,
        entity: AnimationEntityMut<'a>,
    ) -> Result<(), AnimationEvaluationError>;
}

/// A [curve] defined by keyframes with values in an [animatable] type.
///
/// The keyframes are interpolated using the type's [`Animatable::interpolate`] implementation.
///
/// [curve]: Curve
/// [animatable]: Animatable
#[derive(Debug, Clone, Reflect)]
pub struct AnimatableKeyframeCurve<T> {
    core: UnevenCore<T>,
}

impl<T> Curve<T> for AnimatableKeyframeCurve<T>
where
    T: Animatable + Clone,
{
    #[inline]
    fn domain(&self) -> Interval {
        self.core.domain()
    }

    #[inline]
    fn sample_clamped(&self, t: f32) -> T {
        // `UnevenCore::sample_with` is implicitly clamped.
        self.core.sample_with(t, <T as Animatable>::interpolate)
    }

    #[inline]
    fn sample_unchecked(&self, t: f32) -> T {
        self.sample_clamped(t)
    }
}

impl<T> AnimatableKeyframeCurve<T>
where
    T: Animatable,
{
    /// Create a new [`AnimatableKeyframeCurve`] from the given `keyframes`. The values of this
    /// curve are interpolated from the keyframes using the output type's implementation of
    /// [`Animatable::interpolate`].
    ///
    /// There must be at least two samples in order for this method to succeed.
    pub fn new(keyframes: impl IntoIterator<Item = (f32, T)>) -> Result<Self, UnevenCoreError> {
        Ok(Self {
            core: UnevenCore::new(keyframes)?,
        })
    }
}

fn inconsistent<P>() -> AnimationEvaluationError
where
    P: 'static + ?Sized,
{
    AnimationEvaluationError::InconsistentEvaluatorImplementation(TypeId::of::<P>())
}