style/servo/

animation.rs

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at https://mozilla.org/MPL/2.0/. */

//! CSS transitions and animations.

// NOTE(emilio): This code isn't really executed in Gecko, but we don't want to
// compile it out so that people remember it exists.

use crate::context::{CascadeInputs, SharedStyleContext};
use crate::derives::*;
use crate::dom::{OpaqueNode, TDocument, TElement, TNode};
use crate::properties::animated_properties::{AnimationValue, AnimationValueMap};
use crate::properties::longhands::animation_composition::computed_value::single_value::T as AnimationComposition;
use crate::properties::longhands::animation_direction::computed_value::single_value::T as AnimationDirection;
use crate::properties::longhands::animation_fill_mode::computed_value::single_value::T as AnimationFillMode;
use crate::properties::longhands::animation_play_state::computed_value::single_value::T as AnimationPlayState;
use crate::properties::AnimationDeclarations;
use crate::properties::{
    ComputedValues, Importance, LonghandId, PropertyDeclarationBlock, PropertyDeclarationId,
    PropertyDeclarationIdSet,
};
use crate::rule_tree::{CascadeLevel, CascadeOrigin, RuleCascadeFlags};
use crate::selector_parser::PseudoElement;
use crate::shared_lock::{Locked, SharedRwLock};
use crate::style_resolver::StyleResolverForElement;
use crate::stylesheets::keyframes_rule::{KeyframesAnimation, KeyframesStep, KeyframesStepValue};
use crate::stylesheets::layer_rule::LayerOrder;
use crate::values::animated::{Animate, Procedure};
use crate::values::computed::TimingFunction;
use crate::values::generics::easing::BeforeFlag;
use crate::values::specified::TransitionBehavior;
use crate::Atom;
use debug_unreachable::debug_unreachable;
use parking_lot::RwLock;
use rustc_hash::FxHashMap;
use servo_arc::Arc;
use std::fmt;

/// Represents an animation for a given property.
#[derive(Clone, Debug, MallocSizeOf)]
pub struct PropertyAnimation {
    /// The value we are animating from.
    from: AnimationValue,

    /// The value we are animating to.
    to: AnimationValue,

    /// The timing function of this `PropertyAnimation`.
    timing_function: TimingFunction,

    /// The duration of this `PropertyAnimation` in seconds.
    pub duration: f64,
}

impl PropertyAnimation {
    /// Returns the given property longhand id.
    pub fn property_id(&self) -> PropertyDeclarationId<'_> {
        debug_assert_eq!(self.from.id(), self.to.id());
        self.from.id()
    }

    /// The output of the timing function given the progress ration of this animation.
    fn timing_function_output(&self, progress: f64) -> f64 {
        let epsilon = 1. / (200. * self.duration);
        // FIXME: Need to set the before flag correctly.
        // In order to get the before flag, we have to know the current animation phase
        // and whether the iteration is reversed. For now, we skip this calculation
        // by treating as if the flag is unset at all times.
        // https://drafts.csswg.org/css-easing/#step-timing-function-algo
        self.timing_function
            .calculate_output(progress, BeforeFlag::Unset, epsilon)
    }

    /// Update the given animation at a given point of progress.
    fn calculate_value(&self, progress: f64) -> AnimationValue {
        let progress = self.timing_function_output(progress);
        let procedure = Procedure::Interpolate { progress };
        self.from.animate(&self.to, procedure).unwrap_or_else(|()| {
            // Fall back to discrete interpolation
            if progress < 0.5 {
                self.from.clone()
            } else {
                self.to.clone()
            }
        })
    }
}

/// This structure represents the state of an animation.
#[derive(Clone, Debug, MallocSizeOf, PartialEq)]
pub enum AnimationState {
    /// The animation has been created, but is not running yet. This state
    /// is also used when an animation is still in the first delay phase.
    Pending,
    /// This animation is currently running.
    Running,
    /// This animation is paused. The inner field is the percentage of progress
    /// when it was paused, from 0 to 1.
    Paused(f64),
    /// This animation has finished.
    Finished,
    /// This animation has been canceled.
    Canceled,
}

impl AnimationState {
    /// Whether or not this state requires its owning animation to be ticked.
    fn needs_to_be_ticked(&self) -> bool {
        *self == AnimationState::Running || *self == AnimationState::Pending
    }
}

enum IgnoreTransitions {
    Canceled,
    CanceledAndFinished,
}

/// This structure represents a keyframes animation current iteration state.
///
/// If the iteration count is infinite, there's no other state, otherwise we
/// have to keep track the current iteration and the max iteration count.
#[derive(Clone, Debug, MallocSizeOf)]
pub enum KeyframesIterationState {
    /// Infinite iterations with the current iteration count.
    Infinite(f64),
    /// Current and max iterations.
    Finite(f64, f64),
}

/// A temporary data structure used when calculating ComputedKeyframes for an
/// animation. This data structure is used to collapse information for steps
/// which may be spread across multiple keyframe declarations into a single
/// instance per `start_percentage`.
#[derive(Debug)]
struct IntermediateComputedKeyframe {
    declarations: PropertyDeclarationBlock,
    timing_function: Option<TimingFunction>,
    composition: Option<AnimationComposition>,
    start_percentage: f64,
}

impl IntermediateComputedKeyframe {
    fn new(start_percentage: f64) -> Self {
        IntermediateComputedKeyframe {
            declarations: PropertyDeclarationBlock::new(),
            timing_function: None,
            composition: None,
            start_percentage,
        }
    }

    /// Walk through all keyframe declarations and combine all declarations with the
    /// same `start_percentage` into individual `IntermediateComputedKeyframe`s.
    fn generate_for_keyframes(
        animation: &KeyframesAnimation,
        context: &SharedStyleContext,
        base_style: &ComputedValues,
    ) -> Vec<Self> {
        if animation.steps.is_empty() {
            return vec![];
        }

        let mut intermediate_steps: Vec<Self> = Vec::with_capacity(animation.steps.len());
        let mut current_step = IntermediateComputedKeyframe::new(0.);
        for step in animation.steps.iter() {
            let start_percentage = step.start_offset.percentage.0 as f64;
            if start_percentage != current_step.start_percentage {
                let new_step = IntermediateComputedKeyframe::new(start_percentage);
                intermediate_steps.push(std::mem::replace(&mut current_step, new_step));
            }

            current_step.update_from_step(step, context, base_style);
        }
        intermediate_steps.push(current_step);

        // We should always have a first and a last step, even if these are just
        // generated by KeyframesStepValue::ComputedValues.
        debug_assert!(intermediate_steps.first().unwrap().start_percentage == 0.);
        debug_assert!(intermediate_steps.last().unwrap().start_percentage == 1.);

        intermediate_steps
    }

    fn update_from_step(
        &mut self,
        step: &KeyframesStep,
        context: &SharedStyleContext,
        base_style: &ComputedValues,
    ) {
        // Each keyframe declaration may optionally specify a timing function, falling
        // back to the one defined global for the animation.
        let guard = &context.guards.author;
        if let Some(timing_function) = step.get_animation_timing_function(&guard) {
            self.timing_function = Some(timing_function.to_computed_value_without_context());
        }

        // Each keyframe declaration may optionally specify a composite operation,
        // falling back to the one defined globally for the animation.
        if let Some(composition) = step.get_animation_composition(&guard) {
            self.composition = Some(composition);
        }

        let block = match step.value {
            KeyframesStepValue::ComputedValues => return,
            KeyframesStepValue::Declarations { ref block } => block,
        };

        // Filter out !important, non-animatable properties, and the
        // 'display' property (which is only animatable from SMIL).
        let guard = block.read_with(&guard);
        for declaration in guard.normal_declaration_iter() {
            if let PropertyDeclarationId::Longhand(id) = declaration.id() {
                if id == LonghandId::Display {
                    continue;
                }

                if !id.is_animatable() {
                    continue;
                }
            }

            self.declarations.push(
                declaration.to_physical(base_style.writing_mode),
                Importance::Normal,
            );
        }
    }

    fn resolve_style<E>(
        self,
        element: E,
        context: &SharedStyleContext,
        base_style: &Arc<ComputedValues>,
        resolver: &mut StyleResolverForElement<E>,
    ) -> Arc<ComputedValues>
    where
        E: TElement,
    {
        if !self.declarations.any_normal() {
            return base_style.clone();
        }

        let document = element.as_node().owner_doc();
        let locked_block = Arc::new(document.shared_lock().wrap(self.declarations));
        let mut important_rules_changed = false;
        let rule_node = base_style.rules().clone();
        let new_node = context.stylist.rule_tree().update_rule_at_level(
            CascadeLevel::new(CascadeOrigin::Animations),
            LayerOrder::root(),
            Some(locked_block.borrow_arc()),
            &rule_node,
            &context.guards,
            &mut important_rules_changed,
        );

        if new_node.is_none() {
            return base_style.clone();
        }

        let inputs = CascadeInputs {
            rules: new_node,
            visited_rules: base_style.visited_rules().cloned(),
            flags: base_style.flags.for_cascade_inputs(),
            included_cascade_flags: RuleCascadeFlags::empty(),
        };
        resolver
            .cascade_style_and_visited_with_default_parents(inputs)
            .0
    }
}

#[derive(Clone, Debug, MallocSizeOf)]
struct PropertyDeclarationOffsets {
    /// The absolute index of the most recent preceding keyframe that declared
    /// the given property.
    preceding_declaration: usize,
    /// The absolute index of the next keyframe that will declare the given
    /// property.
    following_declaration: usize,
}

#[derive(Clone, Debug, MallocSizeOf)]
enum AnimationValueOrReference {
    /// This keyframe declares the property with the given value.
    AnimationValue(AnimationValue),
    /// This keyframe does not declare the property.
    NotDefinedHere(PropertyDeclarationOffsets),
}

/// A single computed keyframe for a CSS Animation.
#[derive(Clone, Debug, MallocSizeOf)]
struct ComputedKeyframe {
    /// The timing function to use for transitions between this step
    /// and the next one.
    timing_function: TimingFunction,

    /// The starting percentage (a number between 0 and 1) which represents
    /// at what point in an animation iteration this step is.
    start_percentage: f64,

    /// The animation values to transition to and from when processing this
    /// keyframe animation step.
    values: Box<[AnimationValueOrReference]>,
}

/// Composite a keyframe value with the underlying value according to the
/// given composite operation.
///
/// <https://drafts.csswg.org/web-animations-1/#applying-the-composite-operation>
fn composite_animation_value(
    underlying_value: &AnimationValue,
    keyframe_value: AnimationValue,
    composition: AnimationComposition,
) -> AnimationValue {
    let procedure = match composition {
        AnimationComposition::Replace => return keyframe_value,
        AnimationComposition::Add => Procedure::Add,
        AnimationComposition::Accumulate => Procedure::Accumulate { count: 1 },
    };
    underlying_value
        .animate(&keyframe_value, procedure)
        .unwrap_or(keyframe_value)
}

/// Caches the indices of keyframes that declare a specific property.
///
/// While traversing the list of keyframes, this is used to avoid repeatedly
/// searching for the next or last keyframe that declares the property. That
/// would result in quadratic runtime with respect to the number of keyframes.
#[derive(Clone, Copy, Debug, Default)]
struct KeyframeOffsetCacheForProperty {
    /// The index of a previous keyframe that declares the property.
    ///
    /// Note that if the first keyframe does not declare a property, then it implicitly
    /// uses the computed value of that property. That's why there's always a preceding keyframe
    /// with the property.
    last_keyframe_that_defined_property: usize,

    /// The index of a future keyframe or `None` if we have not yet walked the list of keyframes
    /// to find the next index.
    ///
    /// There will always be a next keyframe because the last keyframe (like the first keyframe)
    /// declares *all* animating properties.
    next_keyframe_that_defines_property: Option<usize>,
}

struct KeyframeDataForProperty<'a> {
    /// The timing function to use for transitions between this step
    /// and the next one.
    timing_function: &'a TimingFunction,

    /// The starting percentage (a number between 0 and 1) which represents
    /// at what point in an animation iteration this step is.
    start_percentage: f64,

    value: &'a AnimationValue,
}

#[derive(Clone, Copy, Debug)]
enum Direction {
    Forward,
    Backward,
}

impl Direction {
    fn relative_to_animation_direction(&self, reverse: bool) -> Self {
        match self {
            Self::Forward if reverse => Self::Backward,
            Self::Backward if reverse => Self::Forward,
            _ => *self,
        }
    }
}

impl Animation {
    /// Starting from the keyframe at `keyframe_index`, returns the contents of the next keyframe in `direction`
    /// that sets the property at `property_index`.
    ///
    /// Returns `None` if there is no keyframe in the specified direction that sets the property.
    fn next_relevant_keyframe_for_property_in_direction(
        &self,
        property_index: usize,
        keyframe_index: usize,
        direction: Direction,
    ) -> Option<KeyframeDataForProperty<'_>> {
        let relevant_keyframe = &self.computed_steps[keyframe_index];
        let parameters = match &relevant_keyframe.values[property_index] {
            AnimationValueOrReference::AnimationValue(animation_value) => KeyframeDataForProperty {
                timing_function: &relevant_keyframe.timing_function,
                start_percentage: relevant_keyframe.start_percentage,
                value: animation_value,
            },
            AnimationValueOrReference::NotDefinedHere(offsets) => {
                let next_relevant_keyframe_index = match direction {
                    Direction::Forward => offsets.following_declaration,
                    Direction::Backward => offsets.preceding_declaration,
                };
                let next_relevant_keyframe = &self.computed_steps[next_relevant_keyframe_index];
                let AnimationValueOrReference::AnimationValue(animation_value) =
                    &next_relevant_keyframe.values[property_index]
                else {
                    panic!("Referenced keyframe does not set property");
                };

                KeyframeDataForProperty {
                    timing_function: &next_relevant_keyframe.timing_function,
                    start_percentage: next_relevant_keyframe.start_percentage,
                    value: &animation_value,
                }
            },
        };

        Some(parameters)
    }
}
impl ComputedKeyframe {
    fn generate_for_keyframes<E>(
        element: E,
        animation: &KeyframesAnimation,
        context: &SharedStyleContext,
        base_style: &Arc<ComputedValues>,
        default_timing_function: TimingFunction,
        default_composition: AnimationComposition,
        resolver: &mut StyleResolverForElement<E>,
        animating_properties: PropertyDeclarationIdSet,
        number_of_animating_properties: usize,
    ) -> Box<[Self]>
    where
        E: TElement,
    {
        let animation_values_from_style: Vec<AnimationValue> = animating_properties
            .iter()
            .map(|property| {
                AnimationValue::from_computed_values(property, &**base_style)
                    .expect("Unexpected non-animatable property.")
            })
            .collect();

        let intermediate_steps =
            IntermediateComputedKeyframe::generate_for_keyframes(animation, context, base_style);

        // Used while iterating over the keyframes to, for each property, remember the most recent and
        // next keyframe that declares the property. That avoids a quadratic number of traversals per
        // property.
        let mut keyframe_offset_caches: Vec<KeyframeOffsetCacheForProperty> =
            vec![Default::default(); number_of_animating_properties];

        let mut computed_steps: Vec<Self> = Vec::with_capacity(intermediate_steps.len());
        let mut remaining_steps = intermediate_steps.into_iter();
        let mut step_index = 0;
        while let Some(step) = remaining_steps.next() {
            let start_percentage = step.start_percentage;
            let properties_changed_in_step = step.declarations.property_ids().clone();
            let timing_function = step
                .timing_function
                .clone()
                .unwrap_or_else(|| default_timing_function.clone());
            let composition = step.composition.unwrap_or(default_composition);
            let step_style = step.resolve_style(element, context, base_style, resolver);

            let values: Box<[_]> = {
                // For each property that is animating, pull the value from the resolved
                // style for this step if it's in one of the declarations.
                animating_properties
                    .iter()
                    .enumerate()
                    .map(|(property_index, property_declaration)| {
                        let keyframe_offset_cache = &mut keyframe_offset_caches[property_index];
                        if properties_changed_in_step.contains(property_declaration) {
                            keyframe_offset_cache.last_keyframe_that_defined_property = step_index;
                            let animation_value = AnimationValue::from_computed_values(
                                property_declaration,
                                &step_style,
                            )
                            .unwrap();
                            let animation_value = composite_animation_value(
                                &animation_values_from_style[property_index],
                                animation_value,
                                composition,
                            );
                            return AnimationValueOrReference::AnimationValue(animation_value);
                        }

                        // https://drafts.csswg.org/css-animations/#keyframes
                        // > If a 0% or from keyframe is not specified, then the user agent constructs a 0% keyframe
                        // > using the computed values of the properties being animated. If a 100% or to keyframe is
                        // > not specified, then the user agent constructs a 100% keyframe using the computed values
                        // > of the properties being animated.
                        if step_index == 0 || remaining_steps.as_slice().is_empty() {
                            return AnimationValueOrReference::AnimationValue(
                                animation_values_from_style[property_index].clone(),
                            );
                        }

                        // This animating property is not defined on this keyframe - we should act as if this keyframe
                        // didn't exist for this property, so we calculate an interpolated value.
                        // (https://drafts.csswg.org/css-animations/#keyframes)
                        //
                        // If the property was not defined on any previous keyframe then we use the value from style.
                        // and if it's not defined on any following keyframe then we've already finished animating it.
                        let preceding_declaration =
                            keyframe_offset_cache.last_keyframe_that_defined_property;
                        let following_declaration = keyframe_offset_cache
                            .next_keyframe_that_defines_property
                            .filter(|offset| *offset > step_index)
                            .unwrap_or_else(|| {
                                let relative_offset = remaining_steps
                                    .as_slice()
                                    .iter()
                                    .position(|step| {
                                        step.declarations.contains(property_declaration)
                                    })
                                    .unwrap_or(remaining_steps.as_slice().len() - 1);
                                let absolute_offset = step_index + 1 + relative_offset;

                                keyframe_offset_cache.next_keyframe_that_defines_property =
                                    Some(absolute_offset);
                                absolute_offset
                            });

                        AnimationValueOrReference::NotDefinedHere(PropertyDeclarationOffsets {
                            preceding_declaration,
                            following_declaration,
                        })
                    })
                    .collect()
            };
            debug_assert_eq!(values.len(), number_of_animating_properties);

            computed_steps.push(ComputedKeyframe {
                timing_function,
                start_percentage,
                values,
            });

            step_index += 1;
        }

        // The first and last steps (at 0% and 100% respectively) should declare all animating properties.
        // If they don't then we should have filled the missing properties with the computed values.
        debug_assert!(computed_steps.first().is_none_or(|first_step| {
            first_step
                .values
                .iter()
                .all(|value| matches!(value, AnimationValueOrReference::AnimationValue(_)))
        }));
        debug_assert!(computed_steps.last().is_none_or(|first_step| {
            first_step
                .values
                .iter()
                .all(|value| matches!(value, AnimationValueOrReference::AnimationValue(_)))
        }));

        computed_steps.into_boxed_slice()
    }
}

/// A CSS Animation
#[derive(Clone, MallocSizeOf)]
pub struct Animation {
    /// The name of this animation as defined by the style.
    pub name: Atom,

    /// The properties that change in this animation.
    properties_changed: PropertyDeclarationIdSet,

    /// The computed style for each keyframe of this animation.
    computed_steps: Box<[ComputedKeyframe]>,

    /// The time this animation started at, which is the current value of the animation
    /// timeline when this animation was created plus any animation delay.
    pub started_at: f64,

    /// The duration of this animation.
    pub duration: f64,

    /// The delay of the animation.
    pub delay: f64,

    /// The `animation-fill-mode` property of this animation.
    pub fill_mode: AnimationFillMode,

    /// The current iteration state for the animation.
    pub iteration_state: KeyframesIterationState,

    /// Whether this animation is paused.
    pub state: AnimationState,

    /// The declared animation direction of this animation.
    pub direction: AnimationDirection,

    /// The current animation direction. This can only be `normal` or `reverse`.
    pub current_direction: AnimationDirection,

    /// The number of properties that are affected by this animation.
    pub number_of_animating_properties: usize,

    /// Whether or not this animation is new and or has already been tracked
    /// by the script thread.
    pub is_new: bool,
}

impl Animation {
    /// Whether or not this animation is cancelled by changes from a new style.
    fn is_cancelled_in_new_style(&self, new_style: &Arc<ComputedValues>) -> bool {
        let new_ui = new_style.get_ui();
        let index = new_ui
            .animation_name_iter()
            .position(|animation_name| Some(&self.name) == animation_name.as_atom());
        let index = match index {
            Some(index) => index,
            None => return true,
        };

        new_ui.animation_duration_mod(index).seconds() == 0.
    }

    /// Given the current time, advances this animation to the next iteration,
    /// updates times, and then toggles the direction if appropriate. Otherwise
    /// does nothing. Returns true if this animation has iterated.
    pub fn iterate_if_necessary(&mut self, time: f64) -> bool {
        if !self.iteration_over(time) {
            return false;
        }

        // Only iterate animations that are currently running.
        if self.state != AnimationState::Running {
            return false;
        }

        self.iterate_by(1.) == 1.
    }

    /// Attempts to advance this animation by `n` iterations, but stops when reaching
    /// the last iteration, and doesn't perform fractional iterations.
    /// Returns the actual number of iterations that happened.
    fn iterate_by(&mut self, n: f64) -> f64 {
        let n = n.trunc().min(self.remaining_iterations().ceil() - 1.0);
        if n < 1. {
            return 0.;
        }

        match self.iteration_state {
            KeyframesIterationState::Finite(ref mut current, max) => {
                *current = (*current + n).min(max);
            },
            KeyframesIterationState::Infinite(ref mut current) => {
                *current += n;
            },
        }

        if let AnimationState::Paused(ref mut progress) = self.state {
            debug_assert!(*progress >= n);
            *progress -= n;
        }

        // Update the next iteration direction if applicable.
        self.started_at += self.duration * n;
        match self.direction {
            AnimationDirection::Alternate | AnimationDirection::AlternateReverse
                if n % 2. == 1.0 =>
            {
                self.current_direction = match self.current_direction {
                    AnimationDirection::Normal => AnimationDirection::Reverse,
                    AnimationDirection::Reverse => AnimationDirection::Normal,
                    _ => unreachable!(
                        "Current animation direction can only be `normal` or `reverse`."
                    ),
                };
            },
            _ => {},
        }

        n
    }

    fn remaining_iterations(&self) -> f64 {
        match self.iteration_state {
            KeyframesIterationState::Finite(current, max) => max - current,
            KeyframesIterationState::Infinite(_) => f64::INFINITY,
        }
    }

    /// A number (> 0 and <= 1) which represents the fraction of a full iteration
    /// that the current iteration of the animation lasts. This will be less than 1
    /// if the current iteration is the fractional remainder of a non-integral
    /// iteration count.
    pub fn current_iteration_end_progress(&self) -> f64 {
        self.remaining_iterations().min(1.)
    }

    /// The duration of the current iteration of this animation which may be less
    /// than the animation duration if it has a non-integral iteration count.
    pub fn current_iteration_duration(&self) -> f64 {
        self.current_iteration_end_progress() * self.duration
    }

    /// Whether or not the current iteration is over. Note that this method assumes that
    /// the animation is still running.
    fn iteration_over(&self, time: f64) -> bool {
        time > (self.started_at + self.current_iteration_duration())
    }

    /// Assuming this animation is running, whether or not it is on the last iteration.
    fn on_last_iteration(&self) -> bool {
        self.remaining_iterations() <= 1.
    }

    /// Whether or not this animation has finished at the provided time. This does
    /// not take into account canceling i.e. when an animation or transition is
    /// canceled due to changes in the style.
    pub fn has_ended(&self, time: f64) -> bool {
        if !self.on_last_iteration() {
            return false;
        }

        let progress = match self.state {
            AnimationState::Finished => return true,
            AnimationState::Paused(progress) => progress,
            AnimationState::Running => (time - self.started_at) / self.duration,
            AnimationState::Pending | AnimationState::Canceled => return false,
        };

        progress >= self.current_iteration_end_progress()
    }

    /// Updates the appropiate state from other animation.
    ///
    /// This happens when an animation is re-submitted to layout, presumably
    /// because of an state change.
    ///
    /// There are some bits of state we can't just replace, over all taking in
    /// account times, so here's that logic.
    pub fn update_from_other(&mut self, other: &Self, now: f64) {
        use self::AnimationState::*;

        debug!(
            "KeyframesAnimationState::update_from_other({:?}, {:?})",
            self, other
        );

        // NB: We shall not touch the started_at field, since we don't want to
        // restart the animation.
        let old_started_at = self.started_at;
        let old_delay = self.delay;
        let old_duration = self.duration;
        let old_direction = self.current_direction;
        let old_state = self.state.clone();
        let old_iteration_state = self.iteration_state.clone();

        *self = other.clone();
        self.current_direction = old_direction;

        if self.delay != old_delay {
            // `started_at` incorporates the delay, so changing the delay necessarily changes `started_at`.
            // Note: `started_at` may actually be in the future.
            self.started_at = old_started_at + (self.delay - old_delay);

            match old_state {
                Paused(old_progress) => {
                    let mut progress = old_progress + (old_delay - self.delay) / self.duration;
                    progress -= self.iterate_by(progress);
                    self.state = Paused(progress);
                },
                Finished => {
                    if self.has_ended(now) {
                        self.state = Finished;
                    } else if self.started_at <= now {
                        self.state = Running;
                    } else {
                        self.state = Pending;
                    }
                },
                Canceled | Pending | Running => {
                    // Re-advance iterations from a fresh iteration state.
                    let new_starting_progress = (now - self.started_at) / self.duration;
                    match self.iteration_state {
                        KeyframesIterationState::Finite(ref mut current, _) => *current = 0.0,
                        _ => {},
                    }
                    if let AnimationState::Paused(starting_progress) = &mut self.state {
                        *starting_progress = new_starting_progress;
                    }
                    self.iterate_by(new_starting_progress);
                },
            }

            // Don't check old_state when delay changed.
            if self.state == Pending && self.started_at <= now {
                self.state = Running;
            }
        } else {
            self.started_at = old_started_at;

            // Don't update the iteration count, just the iteration limit.
            // TODO: see how changing the limit affects rendering in other browsers.
            // We might need to keep the iteration count even when it's infinite.
            match (&mut self.iteration_state, old_iteration_state) {
                (
                    &mut KeyframesIterationState::Finite(ref mut iters, _),
                    KeyframesIterationState::Finite(old_iters, _),
                ) => *iters = old_iters,
                _ => {},
            }

            // Don't pause or restart animations that should remain finished.
            // We call mem::replace because `has_ended(...)` looks at `Animation::state`.
            let new_state = std::mem::replace(&mut self.state, Running);
            if old_state == Finished && self.has_ended(now) {
                self.state = Finished;
            } else {
                self.state = new_state;
            }

            // If we're unpausing the animation, fake the start time so we seem to
            // restore it.
            //
            // If the animation keeps paused, keep the old value.
            //
            // If we're pausing the animation, compute the progress value.
            match (&mut self.state, &old_state) {
                (&mut Pending, &Paused(progress)) => {
                    self.started_at = now - (self.duration * progress);
                },
                (&mut Paused(ref mut new), &Paused(old)) => *new = old,
                (&mut Paused(ref mut progress), &Running) => {
                    *progress = (now - old_started_at) / old_duration
                },
                _ => {},
            }

            // Try to detect when we should skip straight to the running phase to
            // avoid sending multiple animationstart events.
            if self.state == Pending && self.started_at <= now && old_state != Pending {
                self.state = Running;
            }
        }
    }

    /// Fill in an `AnimationValueMap` with values calculated from this animation at
    /// the given time value.
    fn get_property_declaration_at_time(&self, now: f64, map: &mut AnimationValueMap) {
        if self.computed_steps.is_empty() {
            // Nothing to do.
            return;
        }

        // Raw progress ratio of the animation: can be negative (before start) or
        // >1.0 (after end or during multiple iterations).
        let progress = match self.state {
            AnimationState::Running | AnimationState::Pending | AnimationState::Finished => {
                (now - self.started_at) / self.duration
            },
            AnimationState::Paused(progress) => progress,
            AnimationState::Canceled => return,
        };

        if progress < 0.
            && self.fill_mode != AnimationFillMode::Backwards
            && self.fill_mode != AnimationFillMode::Both
        {
            return;
        }
        if self.has_ended(now)
            && self.fill_mode != AnimationFillMode::Forwards
            && self.fill_mode != AnimationFillMode::Both
        {
            return;
        }

        // If we only need to take into account one keyframe, then exit early
        // in order to avoid doing more work.
        let mut add_declarations_to_map = |keyframe: &ComputedKeyframe| {
            for value_or_reference in keyframe.values.iter() {
                let AnimationValueOrReference::AnimationValue(value) = value_or_reference else {
                    unreachable!("First or last keyframes define all properties");
                };
                map.insert(value.id().to_owned(), value.clone());
            }
        };

        // Handle negative progress (before animation start) with backwards/both fill mode
        if progress < 0.0 {
            if let Some(keyframe) = match self.current_direction {
                AnimationDirection::Normal => self.computed_steps.first(),
                AnimationDirection::Reverse => self.computed_steps.last(),
                _ => unreachable!("Current animation direction can only be `normal` or `reverse`."),
            } {
                add_declarations_to_map(keyframe);
            }
            return;
        }

        // Progress clamped to the current iteration [0.0, 1.0].
        let total_progress = progress.min(self.current_iteration_end_progress()).max(0.0);

        // At 1.0 there is nothing left to interpolate. Return end keyframe.
        if total_progress == 1.0 {
            let keyframe = match self.current_direction {
                AnimationDirection::Normal => self.computed_steps.last().unwrap(),
                AnimationDirection::Reverse => self.computed_steps.first().unwrap(),
                _ => unreachable!("Current animation direction can only be `normal` or `reverse`."),
            };
            add_declarations_to_map(keyframe);
            return;
        }

        // Get the indices of the previous (from) keyframe and the next (to) keyframe.
        let next_keyframe_index;
        let prev_keyframe_index;
        let num_steps = self.computed_steps.len();
        match self.current_direction {
            AnimationDirection::Normal => {
                next_keyframe_index = self
                    .computed_steps
                    .iter()
                    .position(|step| total_progress < step.start_percentage);
                prev_keyframe_index = next_keyframe_index
                    .and_then(|pos| if pos != 0 { Some(pos - 1) } else { None })
                    .unwrap_or(0);
            },
            AnimationDirection::Reverse => {
                next_keyframe_index = self
                    .computed_steps
                    .iter()
                    .rev()
                    .position(|step| total_progress <= 1. - step.start_percentage)
                    .map(|pos| num_steps - pos - 1);
                prev_keyframe_index = next_keyframe_index
                    .and_then(|pos| {
                        if pos != num_steps - 1 {
                            Some(pos + 1)
                        } else {
                            None
                        }
                    })
                    .unwrap_or(num_steps - 1)
            },
            _ => unreachable!(),
        }

        debug!(
            "Animation::get_property_declaration_at_time: keyframe from {:?} to {:?}",
            prev_keyframe_index, next_keyframe_index
        );

        let prev_keyframe = &self.computed_steps[prev_keyframe_index];
        let Some(next_keyframe_index) = next_keyframe_index else {
            unsafe {
                debug_unreachable!(
                    "next_keyframe_index should always be Some: \
                     total_progress is in [0, 1) at this point. \
                     Normal direction: keyframe with start_percentage 1.0 always satisfies. \
                     Reverse direction: keyframe with start_percentage 0.0 always satisfies."
                );
            }
        };

        // Prevent division by zero from percentage_between_keyframes.
        // This can happen for reverse direction at total_progress == 0.0.
        if prev_keyframe_index == next_keyframe_index {
            add_declarations_to_map(&prev_keyframe);
            return;
        }

        // Interpolate a new value for each animating property
        let reversed = self.current_direction != AnimationDirection::Normal;
        for property_index in 0..self.number_of_animating_properties {
            let Some(previous_keyframe) = self.next_relevant_keyframe_for_property_in_direction(
                property_index,
                prev_keyframe_index,
                Direction::Backward.relative_to_animation_direction(reversed),
            ) else {
                // Animation of this property has not started yet
                continue;
            };

            let Some(next_keyframe) = self.next_relevant_keyframe_for_property_in_direction(
                property_index,
                next_keyframe_index,
                Direction::Forward.relative_to_animation_direction(reversed),
            ) else {
                // This property has finished animating, just use the previous data
                map.insert(
                    previous_keyframe.value.id().to_owned(),
                    previous_keyframe.value.clone(),
                );
                continue;
            };

            let percentage_between_keyframes =
                (next_keyframe.start_percentage - previous_keyframe.start_percentage).abs();
            let duration_between_keyframes = percentage_between_keyframes * self.duration;
            let direction_aware_prev_keyframe_start_percentage = match self.current_direction {
                AnimationDirection::Normal => previous_keyframe.start_percentage,
                AnimationDirection::Reverse => 1. - previous_keyframe.start_percentage,
                _ => unreachable!(),
            };
            let progress_between_keyframes = (total_progress
                - direction_aware_prev_keyframe_start_percentage)
                / percentage_between_keyframes;
            let animation = PropertyAnimation {
                from: previous_keyframe.value.clone(),
                to: next_keyframe.value.clone(),
                timing_function: previous_keyframe.timing_function.clone(),
                duration: duration_between_keyframes,
            };

            let value = animation.calculate_value(progress_between_keyframes);
            map.insert(value.id().to_owned(), value);
        }
    }
}

impl fmt::Debug for Animation {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("Animation")
            .field("name", &self.name)
            .field("started_at", &self.started_at)
            .field("duration", &self.duration)
            .field("delay", &self.delay)
            .field("iteration_state", &self.iteration_state)
            .field("state", &self.state)
            .field("direction", &self.direction)
            .field("current_direction", &self.current_direction)
            .field("cascade_style", &())
            .finish()
    }
}

/// A CSS Transition
#[derive(Clone, Debug, MallocSizeOf)]
pub struct Transition {
    /// The start time of this transition, which is the current value of the animation
    /// timeline when this transition was created plus any animation delay.
    pub start_time: f64,

    /// The delay used for this transition.
    pub delay: f64,

    /// The internal style `PropertyAnimation` for this transition.
    pub property_animation: PropertyAnimation,

    /// The state of this transition.
    pub state: AnimationState,

    /// Whether or not this transition is new and or has already been tracked
    /// by the script thread.
    pub is_new: bool,

    /// If this `Transition` has been replaced by a new one this field is
    /// used to help produce better reversed transitions.
    pub reversing_adjusted_start_value: AnimationValue,

    /// If this `Transition` has been replaced by a new one this field is
    /// used to help produce better reversed transitions.
    pub reversing_shortening_factor: f64,
}

impl Transition {
    fn new(
        start_time: f64,
        delay: f64,
        duration: f64,
        from: AnimationValue,
        to: AnimationValue,
        timing_function: &TimingFunction,
    ) -> Self {
        let property_animation = PropertyAnimation {
            from: from.clone(),
            to,
            timing_function: timing_function.clone(),
            duration,
        };
        Self {
            start_time,
            delay,
            property_animation,
            state: AnimationState::Pending,
            is_new: true,
            reversing_adjusted_start_value: from,
            reversing_shortening_factor: 1.0,
        }
    }

    fn update_for_possibly_reversed_transition(
        &mut self,
        replaced_transition: &Transition,
        delay: f64,
        now: f64,
    ) {
        // If we reach here, we need to calculate a reversed transition according to
        // https://drafts.csswg.org/css-transitions/#starting
        //
        //  "...if the reversing-adjusted start value of the running transition
        //  is the same as the value of the property in the after-change style (see
        //  the section on reversing of transitions for why these case exists),
        //  implementations must cancel the running transition and start
        //  a new transition..."
        if replaced_transition.reversing_adjusted_start_value != self.property_animation.to {
            return;
        }

        // "* reversing-adjusted start value is the end value of the running transition"
        let replaced_animation = &replaced_transition.property_animation;
        self.reversing_adjusted_start_value = replaced_animation.to.clone();

        // "* reversing shortening factor is the absolute value, clamped to the
        //    range [0, 1], of the sum of:
        //    1. the output of the timing function of the old transition at the
        //      time of the style change event, times the reversing shortening
        //      factor of the old transition
        //    2.  1 minus the reversing shortening factor of the old transition."
        let transition_progress = ((now - replaced_transition.start_time)
            / (replaced_transition.property_animation.duration))
            .min(1.0)
            .max(0.0);
        let timing_function_output = replaced_animation.timing_function_output(transition_progress);
        let old_reversing_shortening_factor = replaced_transition.reversing_shortening_factor;
        self.reversing_shortening_factor = ((timing_function_output
            * old_reversing_shortening_factor)
            + (1.0 - old_reversing_shortening_factor))
            .abs()
            .min(1.0)
            .max(0.0);

        // "* start time is the time of the style change event plus:
        //    1. if the matching transition delay is nonnegative, the matching
        //       transition delay, or.
        //    2. if the matching transition delay is negative, the product of the new
        //       transition’s reversing shortening factor and the matching transition delay,"
        self.start_time = if delay >= 0. {
            now + delay
        } else {
            now + (self.reversing_shortening_factor * delay)
        };

        // "* end time is the start time plus the product of the matching transition
        //    duration and the new transition’s reversing shortening factor,"
        self.property_animation.duration *= self.reversing_shortening_factor;

        // "* start value is the current value of the property in the running transition,
        //  * end value is the value of the property in the after-change style,"
        let procedure = Procedure::Interpolate {
            progress: timing_function_output,
        };
        match replaced_animation
            .from
            .animate(&replaced_animation.to, procedure)
        {
            Ok(new_start) => self.property_animation.from = new_start,
            Err(..) => {},
        }
    }

    /// Whether or not this animation has ended at the provided time. This does
    /// not take into account canceling i.e. when an animation or transition is
    /// canceled due to changes in the style.
    pub fn has_ended(&self, time: f64) -> bool {
        time >= self.start_time + (self.property_animation.duration)
    }

    /// Update the given animation at a given point of progress.
    pub fn calculate_value(&self, time: f64) -> AnimationValue {
        let progress = if time < self.start_time {
            0.0
        } else if self.property_animation.duration == 0.0 {
            1.0
        } else {
            ((time - self.start_time) / self.property_animation.duration).clamp(0.0, 1.0)
        };

        self.property_animation.calculate_value(progress)
    }
}

/// Holds the animation state for a particular element.
#[derive(Debug, Default, MallocSizeOf)]
pub struct ElementAnimationSet {
    /// The animations for this element.
    pub animations: Vec<Animation>,

    /// The transitions for this element.
    pub transitions: Vec<Transition>,

    /// Whether or not this ElementAnimationSet has had animations or transitions
    /// which have been added, removed, or had their state changed.
    pub dirty: bool,
}

impl ElementAnimationSet {
    /// Cancel all animations in this `ElementAnimationSet`. This is typically called
    /// when the element has been removed from the DOM.
    pub fn cancel_all_animations(&mut self) {
        self.dirty = !self.animations.is_empty();
        for animation in self.animations.iter_mut() {
            animation.state = AnimationState::Canceled;
        }
        self.cancel_active_transitions();
    }

    fn cancel_active_transitions(&mut self) {
        for transition in self.transitions.iter_mut() {
            if transition.state != AnimationState::Finished {
                self.dirty = true;
                transition.state = AnimationState::Canceled;
            }
        }
    }

    /// Apply all active animations.
    pub fn apply_active_animations(
        &self,
        context: &SharedStyleContext,
        style: &mut Arc<ComputedValues>,
    ) {
        let now = context.current_time_for_animations;
        let mutable_style = Arc::make_mut(style);
        if let Some(map) = self.get_value_map_for_active_animations(now) {
            for value in map.values() {
                value.set_in_style_for_servo(mutable_style, context);
            }
        }

        if let Some(map) = self.get_value_map_for_transitions(now, IgnoreTransitions::Canceled) {
            for value in map.values() {
                value.set_in_style_for_servo(mutable_style, context);
            }
        }
    }

    /// Clear all canceled animations and transitions from this `ElementAnimationSet`.
    pub fn clear_canceled_animations(&mut self) {
        self.animations
            .retain(|animation| animation.state != AnimationState::Canceled);
        self.transitions
            .retain(|animation| animation.state != AnimationState::Canceled);
    }

    /// Whether this `ElementAnimationSet` is empty, which means it doesn't
    /// hold any animations in any state.
    pub fn is_empty(&self) -> bool {
        self.animations.is_empty() && self.transitions.is_empty()
    }

    /// Whether or not this state needs animation ticks for its transitions
    /// or animations.
    pub fn needs_animation_ticks(&self) -> bool {
        self.animations
            .iter()
            .any(|animation| animation.state.needs_to_be_ticked())
            || self
                .transitions
                .iter()
                .any(|transition| transition.state.needs_to_be_ticked())
    }

    /// The number of running animations and transitions for this `ElementAnimationSet`.
    pub fn running_animation_and_transition_count(&self) -> usize {
        self.animations
            .iter()
            .filter(|animation| animation.state.needs_to_be_ticked())
            .count()
            + self
                .transitions
                .iter()
                .filter(|transition| transition.state.needs_to_be_ticked())
                .count()
    }

    /// If this `ElementAnimationSet` has any any active animations.
    pub fn has_active_animation(&self) -> bool {
        self.animations
            .iter()
            .any(|animation| animation.state != AnimationState::Canceled)
    }

    /// If this `ElementAnimationSet` has any any active transitions.
    pub fn has_active_transition(&self) -> bool {
        self.transitions
            .iter()
            .any(|transition| transition.state != AnimationState::Canceled)
    }

    /// Update our animations given a new style, canceling or starting new animations
    /// when appropriate.
    pub fn update_animations_for_new_style<E>(
        &mut self,
        element: E,
        context: &SharedStyleContext,
        new_style: &Arc<ComputedValues>,
        resolver: &mut StyleResolverForElement<E>,
    ) where
        E: TElement,
    {
        for animation in self.animations.iter_mut() {
            if animation.is_cancelled_in_new_style(new_style) {
                animation.state = AnimationState::Canceled;
            }
        }

        maybe_start_animations(element, &context, &new_style, self, resolver);
    }

    /// Update our transitions given a new style, canceling or starting new animations
    /// when appropriate.
    pub fn update_transitions_for_new_style(
        &mut self,
        might_need_transitions_update: bool,
        context: &SharedStyleContext,
        old_style: Option<&Arc<ComputedValues>>,
        after_change_style: &Arc<ComputedValues>,
    ) {
        // If this is the first style, we don't trigger any transitions and we assume
        // there were no previously triggered transitions.
        let mut before_change_style = match old_style {
            Some(old_style) => Arc::clone(old_style),
            None => return,
        };

        // If the style of this element is display:none, then cancel all active transitions.
        if after_change_style.get_box().clone_display().is_none() {
            self.cancel_active_transitions();
            return;
        }

        if !might_need_transitions_update {
            return;
        }

        // We convert old values into `before-change-style` here.
        if self.has_active_transition() || self.has_active_animation() {
            self.apply_active_animations(context, &mut before_change_style);
        }

        let transitioning_properties = start_transitions_if_applicable(
            context,
            &before_change_style,
            after_change_style,
            self,
        );

        // Cancel any non-finished transitions that have properties which no
        // longer transition.
        //
        // Step 3 in https://drafts.csswg.org/css-transitions/#starting:
        // > If the element has a running transition or completed transition for
        // > the property, and there is not a matching transition-property value,
        // > then implementations must cancel the running transition or remove the
        // > completed transition from the set of completed transitions.
        //
        // TODO: This is happening here as opposed to in
        // `start_transition_if_applicable` as an optimization, but maybe this
        // code should be reworked to be more like the specification.
        for transition in self.transitions.iter_mut() {
            if transition.state == AnimationState::Finished
                || transition.state == AnimationState::Canceled
            {
                continue;
            }
            if transitioning_properties.contains(transition.property_animation.property_id()) {
                continue;
            }
            transition.state = AnimationState::Canceled;
            self.dirty = true;
        }
    }

    fn start_transition_if_applicable(
        &mut self,
        context: &SharedStyleContext,
        property_declaration_id: &PropertyDeclarationId,
        index: usize,
        old_style: &ComputedValues,
        new_style: &Arc<ComputedValues>,
    ) {
        let style = new_style.get_ui();
        let allow_discrete =
            style.transition_behavior_mod(index) == TransitionBehavior::AllowDiscrete;

        // FIXME(emilio): Handle the case where old_style and new_style's writing mode differ.
        let Some(from) = AnimationValue::from_computed_values(*property_declaration_id, old_style)
        else {
            return;
        };
        let Some(to) = AnimationValue::from_computed_values(*property_declaration_id, new_style)
        else {
            return;
        };

        let timing_function = style.transition_timing_function_mod(index);
        let duration = style.transition_duration_mod(index).seconds() as f64;
        let delay = style.transition_delay_mod(index).seconds() as f64;
        let now = context.current_time_for_animations;
        let transitionable = property_declaration_id.is_animatable()
            && (allow_discrete || !property_declaration_id.is_discrete_animatable())
            && (allow_discrete || from.interpolable_with(&to));

        let mut existing_transition = self.transitions.iter_mut().find(|transition| {
            transition.property_animation.property_id() == *property_declaration_id
        });

        // Step 1:
        // > If all of the following are true:
        // >  - the element does not have a running transition for the property,
        // >  - the before-change style is different from the after-change style
        // >    for that property, and the values for the property are
        // >    transitionable,
        // >  - the element does not have a completed transition for the property
        // >    or the end value of the completed transition is different from the
        // >    after-change style for the property,
        // >  - there is a matching transition-property value, and
        // >  - the combined duration is greater than 0s,
        //
        // This function is only run if there is a matching transition-property
        // value, so that check is skipped here.
        let has_running_transition = existing_transition.as_ref().is_some_and(|transition| {
            transition.state != AnimationState::Finished
                && transition.state != AnimationState::Canceled
        });
        let no_completed_transition_or_end_values_differ =
            existing_transition.as_ref().is_none_or(|transition| {
                transition.state != AnimationState::Finished
                    || transition.property_animation.to != to
            });
        if !has_running_transition
            && from != to
            && transitionable
            && no_completed_transition_or_end_values_differ
            && (duration + delay > 0.0)
        {
            // > then implementations must remove the completed transition (if
            // > present) from the set of completed transitions and start a
            // > transition whose:
            // >
            // > - start time is the time of the style change event plus the matching transition delay,
            // > - end time is the start time plus the matching transition duration,
            // > - start value is the value of the transitioning property in the before-change style,
            // > - end value is the value of the transitioning property in the after-change style,
            // > - reversing-adjusted start value is the same as the start value, and
            // > - reversing shortening factor is 1.
            self.transitions.push(Transition::new(
                now + delay, /* start_time */
                delay,
                duration,
                from,
                to,
                &timing_function,
            ));
            self.dirty = true;
            return;
        }

        // > Step 2: Otherwise, if the element has a completed transition for the
        // > property and the end value of the completed transition is different
        // > from the after-change style for the property, then implementations
        // > must remove the completed transition from the set of completed
        // > transitions.
        //
        // All completed transitions will be cleared from the `AnimationSet` in
        // `process_animations_for_style in `matching.rs`.

        // > Step 3: If the element has a running transition or completed
        // > transition for the property, and there is not a matching
        // > transition-property value, then implementations must cancel the
        // > running transition or remove the completed transition from the set
        // > of completed transitions.
        //
        // - All completed transitions will be cleared cleared from the `AnimationSet` in
        //   `process_animations_for_style in `matching.rs`.
        // - Transitions for properties that don't have a matching transition-property
        //   value will be canceled in `Self::update_transitions_for_new_style`. In addition,
        //   this method is only called for properties that do ahave a matching
        //   transition-property value.

        let Some(existing_transition) = existing_transition.as_mut() else {
            return;
        };

        // > Step 4: If the element has a running transition for the property,
        // > there is a matching transition-property value, and the end value of
        // > the running transition is not equal to the value of the property in
        // > the after-change style, then:
        if has_running_transition && existing_transition.property_animation.to != to {
            // > Step 4.1: If the current value of the property in the running transition is
            // > equal to the value of the property in the after-change style, or
            // > if these two values are not transitionable, then implementations
            // > must cancel the running transition.
            let current_value = existing_transition.calculate_value(now);
            let transitionable_from_current_value =
                transitionable && (allow_discrete || current_value.interpolable_with(&to));
            if current_value == to || !transitionable_from_current_value {
                existing_transition.state = AnimationState::Canceled;
                self.dirty = true;
                return;
            }

            // > Step 4.2: Otherwise, if the combined duration is less than or
            // > equal to 0s, or if the current value of the property in the
            // > running transition is not transitionable with the value of the
            // > property in the after-change style, then implementations must
            // > cancel the running transition.
            if duration + delay <= 0.0 {
                existing_transition.state = AnimationState::Canceled;
                self.dirty = true;
                return;
            }

            // > Step 4.3: Otherwise, if the reversing-adjusted start value of the
            // > running transition is the same as the value of the property in
            // > the after-change style (see the section on reversing of
            // > transitions for why these case exists), implementations must
            // > cancel the running transition and start a new transition whose:
            if existing_transition.reversing_adjusted_start_value == to {
                existing_transition.state = AnimationState::Canceled;

                let mut transition = Transition::new(
                    now + delay, /* start_time */
                    delay,
                    duration,
                    from,
                    to,
                    &timing_function,
                );

                // This function takes care of applying all of the modifications to the transition
                // after "whose:" above.
                transition.update_for_possibly_reversed_transition(
                    &existing_transition,
                    delay,
                    now,
                );

                self.transitions.push(transition);
                self.dirty = true;
                return;
            }

            // > Step 4.4: Otherwise, implementations must cancel the running
            // > transition and start a new transition whose:
            // >  - start time is the time of the style change event plus the matching transition delay,
            // >  - end time is the start time plus the matching transition duration,
            // >  - start value is the current value of the property in the running transition,
            // >  - end value is the value of the property in the after-change style,
            // >  - reversing-adjusted start value is the same as the start value, and
            // >  - reversing shortening factor is 1.
            existing_transition.state = AnimationState::Canceled;
            self.transitions.push(Transition::new(
                now + delay, /* start_time */
                delay,
                duration,
                current_value,
                to,
                &timing_function,
            ));
            self.dirty = true;
        }
    }

    /// Generate a `AnimationValueMap` for this `ElementAnimationSet`'s
    /// transitions, ignoring those specified by the `ignore_transitions`
    /// argument.
    fn get_value_map_for_transitions(
        &self,
        now: f64,
        ignore_transitions: IgnoreTransitions,
    ) -> Option<AnimationValueMap> {
        if !self.has_active_transition() {
            return None;
        }

        let mut map =
            AnimationValueMap::with_capacity_and_hasher(self.transitions.len(), Default::default());
        for transition in &self.transitions {
            match ignore_transitions {
                IgnoreTransitions::Canceled => {
                    if transition.state == AnimationState::Canceled {
                        continue;
                    }
                },
                IgnoreTransitions::CanceledAndFinished => {
                    if transition.state == AnimationState::Canceled
                        || transition.state == AnimationState::Finished
                    {
                        continue;
                    }
                },
            }

            let value = transition.calculate_value(now);
            map.insert(value.id().to_owned(), value);
        }

        Some(map)
    }

    /// Generate a `AnimationValueMap` for this `ElementAnimationSet`'s
    /// active animations at the given time value.
    pub fn get_value_map_for_active_animations(&self, now: f64) -> Option<AnimationValueMap> {
        if !self.has_active_animation() {
            return None;
        }

        let mut map = Default::default();
        for animation in &self.animations {
            animation.get_property_declaration_at_time(now, &mut map);
        }

        Some(map)
    }
}

#[derive(Clone, Debug, Eq, Hash, MallocSizeOf, PartialEq)]
/// A key that is used to identify nodes in the `DocumentAnimationSet`.
pub struct AnimationSetKey {
    /// The node for this `AnimationSetKey`.
    pub node: OpaqueNode,
    /// The pseudo element for this `AnimationSetKey`. If `None` this key will
    /// refer to the main content for its node.
    pub pseudo_element: Option<PseudoElement>,
}

impl AnimationSetKey {
    /// Create a new key given a node and optional pseudo element.
    pub fn new(node: OpaqueNode, pseudo_element: Option<PseudoElement>) -> Self {
        AnimationSetKey {
            node,
            pseudo_element,
        }
    }

    /// Create a new key for the main content of this node.
    pub fn new_for_non_pseudo(node: OpaqueNode) -> Self {
        AnimationSetKey {
            node,
            pseudo_element: None,
        }
    }

    /// Create a new key for given node and pseudo element.
    pub fn new_for_pseudo(node: OpaqueNode, pseudo_element: PseudoElement) -> Self {
        AnimationSetKey {
            node,
            pseudo_element: Some(pseudo_element),
        }
    }
}

#[derive(Clone, Debug, Default, MallocSizeOf)]
/// A set of animations for a document.
pub struct DocumentAnimationSet {
    /// The `ElementAnimationSet`s that this set contains.
    #[ignore_malloc_size_of = "Arc is hard"]
    pub sets: Arc<RwLock<FxHashMap<AnimationSetKey, ElementAnimationSet>>>,
}

impl DocumentAnimationSet {
    /// Return whether or not the provided node has active CSS animations.
    pub fn has_active_animations(&self, key: &AnimationSetKey) -> bool {
        self.sets
            .read()
            .get(key)
            .map_or(false, |set| set.has_active_animation())
    }

    /// Return whether or not the provided node has active CSS transitions.
    pub fn has_active_transitions(&self, key: &AnimationSetKey) -> bool {
        self.sets
            .read()
            .get(key)
            .map_or(false, |set| set.has_active_transition())
    }

    /// Return a locked PropertyDeclarationBlock with animation values for the given
    /// key and time.
    pub fn get_animation_declarations(
        &self,
        key: &AnimationSetKey,
        time: f64,
        shared_lock: &SharedRwLock,
    ) -> Option<Arc<Locked<PropertyDeclarationBlock>>> {
        self.sets
            .read()
            .get(key)
            .and_then(|set| set.get_value_map_for_active_animations(time))
            .map(|map| {
                let block = PropertyDeclarationBlock::from_animation_value_map(&map);
                Arc::new(shared_lock.wrap(block))
            })
    }

    /// Return a locked PropertyDeclarationBlock with transition values for the given
    /// key and time.
    pub fn get_transition_declarations(
        &self,
        key: &AnimationSetKey,
        time: f64,
        shared_lock: &SharedRwLock,
    ) -> Option<Arc<Locked<PropertyDeclarationBlock>>> {
        self.sets
            .read()
            .get(key)
            .and_then(|set| {
                set.get_value_map_for_transitions(time, IgnoreTransitions::CanceledAndFinished)
            })
            .map(|map| {
                let block = PropertyDeclarationBlock::from_animation_value_map(&map);
                Arc::new(shared_lock.wrap(block))
            })
    }

    /// Get all the animation declarations for the given key, returning an empty
    /// `AnimationDeclarations` if there are no animations.
    pub fn get_all_declarations(
        &self,
        key: &AnimationSetKey,
        time: f64,
        shared_lock: &SharedRwLock,
    ) -> AnimationDeclarations {
        let sets = self.sets.read();
        let set = match sets.get(key) {
            Some(set) => set,
            None => return Default::default(),
        };

        let animations = set.get_value_map_for_active_animations(time).map(|map| {
            let block = PropertyDeclarationBlock::from_animation_value_map(&map);
            Arc::new(shared_lock.wrap(block))
        });
        let transitions = set
            .get_value_map_for_transitions(time, IgnoreTransitions::CanceledAndFinished)
            .map(|map| {
                let block = PropertyDeclarationBlock::from_animation_value_map(&map);
                Arc::new(shared_lock.wrap(block))
            });
        AnimationDeclarations {
            animations,
            transitions,
        }
    }

    /// Cancel all animations for set at the given key.
    pub fn cancel_all_animations_for_key(&self, key: &AnimationSetKey) {
        if let Some(set) = self.sets.write().get_mut(key) {
            set.cancel_all_animations();
        }
    }
}

/// Kick off any new transitions for this node and return all of the properties that are
/// transitioning. This is at the end of calculating style for a single node.
pub fn start_transitions_if_applicable(
    context: &SharedStyleContext,
    old_style: &ComputedValues,
    new_style: &Arc<ComputedValues>,
    animation_state: &mut ElementAnimationSet,
) -> PropertyDeclarationIdSet {
    // See <https://www.w3.org/TR/css-transitions-1/#transitions>
    // "If a property is specified multiple times in the value of transition-property
    // (either on its own, via a shorthand that contains it, or via the all value),
    // then the transition that starts uses the duration, delay, and timing function
    // at the index corresponding to the last item in the value of transition-property
    // that calls for animating that property."
    // See Example 3 of <https://www.w3.org/TR/css-transitions-1/#transitions>
    //
    // Reversing the transition order here means that transitions defined later in the list
    // have preference, in accordance with the specification.
    //
    // TODO: It would be better to be able to do this without having to allocate an array.
    // We should restructure the code or make `transition_properties()` return a reversible
    // iterator in order to avoid the allocation.
    let mut transition_properties = new_style.transition_properties().collect::<Vec<_>>();
    transition_properties.reverse();

    let mut properties_that_transition = PropertyDeclarationIdSet::default();
    for transition in transition_properties {
        let physical_property = transition
            .property
            .as_borrowed()
            .to_physical(new_style.writing_mode);
        if properties_that_transition.contains(physical_property) {
            continue;
        }

        properties_that_transition.insert(physical_property);
        animation_state.start_transition_if_applicable(
            context,
            &physical_property,
            transition.index,
            old_style,
            new_style,
        );
    }

    properties_that_transition
}

/// Triggers animations for a given node looking at the animation property
/// values.
pub fn maybe_start_animations<E>(
    element: E,
    context: &SharedStyleContext,
    new_style: &Arc<ComputedValues>,
    animation_state: &mut ElementAnimationSet,
    resolver: &mut StyleResolverForElement<E>,
) where
    E: TElement,
{
    let style = new_style.get_ui();
    for (i, name) in style.animation_name_iter().enumerate() {
        let name = match name.as_atom() {
            Some(atom) => atom,
            None => continue,
        };

        debug!("maybe_start_animations: name={}", name);
        let duration = style.animation_duration_mod(i).seconds() as f64;
        if duration == 0. {
            continue;
        }

        let Some(keyframe_animation) = context.stylist.lookup_keyframes(name, element) else {
            continue;
        };

        debug!("maybe_start_animations: animation {} found", name);

        // NB: This delay may be negative, meaning that the animation may be created
        // in a state where we have advanced one or more iterations or even that the
        // animation begins in a finished state.
        let delay = style.animation_delay_mod(i).seconds() as f64;

        let iteration_count = style.animation_iteration_count_mod(i);
        let iteration_state = if iteration_count.0.is_infinite() {
            KeyframesIterationState::Infinite(0.0)
        } else {
            KeyframesIterationState::Finite(0.0, iteration_count.0 as f64)
        };

        let animation_direction = style.animation_direction_mod(i);

        let initial_direction = match animation_direction {
            AnimationDirection::Normal | AnimationDirection::Alternate => {
                AnimationDirection::Normal
            },
            AnimationDirection::Reverse | AnimationDirection::AlternateReverse => {
                AnimationDirection::Reverse
            },
        };

        let now = context.current_time_for_animations;
        let started_at = now + delay;
        let starting_progress = (now - started_at) / duration;
        let state = match style.animation_play_state_mod(i) {
            AnimationPlayState::Paused => AnimationState::Paused(starting_progress),
            AnimationPlayState::Running => AnimationState::Pending,
        };

        // Determine the set of animating properties. This is not equivalent to the set of changed properties
        // when one changed property overrides another. (For example, "block-size" with writing-mode: initial
        // is the same as "height")
        let mut animating_properties = PropertyDeclarationIdSet::default();
        let mut number_of_animating_properties = 0;
        for property in keyframe_animation.properties_changed.iter() {
            debug_assert!(property.is_animatable());

            if animating_properties.insert(property.to_physical(new_style.writing_mode)) {
                number_of_animating_properties += 1;
            }
        }

        let computed_steps = ComputedKeyframe::generate_for_keyframes(
            element,
            &keyframe_animation,
            context,
            new_style,
            style.animation_timing_function_mod(i),
            style.animation_composition_mod(i),
            resolver,
            animating_properties,
            number_of_animating_properties,
        );

        let mut new_animation = Animation {
            name: name.clone(),
            properties_changed: keyframe_animation.properties_changed.clone(),
            computed_steps,
            started_at,
            duration,
            fill_mode: style.animation_fill_mode_mod(i),
            delay,
            iteration_state,
            state,
            direction: animation_direction,
            current_direction: initial_direction,
            number_of_animating_properties,
            is_new: true,
        };

        // If we started with a negative delay, make sure we iterate the animation if
        // the delay moves us past the first iteration.
        new_animation.iterate_by(starting_progress);

        animation_state.dirty = true;

        // If the animation was already present in the list for the node, just update its state.
        for existing_animation in animation_state.animations.iter_mut() {
            if existing_animation.state == AnimationState::Canceled {
                continue;
            }

            if new_animation.name == existing_animation.name {
                existing_animation
                    .update_from_other(&new_animation, context.current_time_for_animations);
                return;
            }
        }

        animation_state.animations.push(new_animation);
    }
}