zed/crates/gpui/src/taffy.rs

use crate::{
    AbsoluteLength, Bounds, DefiniteLength, Edges, Length, Pixels, Point, Size, Style,
    WindowContext,
};
use collections::{FxHashMap, FxHashSet};
use smallvec::SmallVec;
use std::fmt::Debug;
use taffy::{
    geometry::{Point as TaffyPoint, Rect as TaffyRect, Size as TaffySize},
    style::AvailableSpace as TaffyAvailableSpace,
    tree::NodeId,
    TaffyTree, TraversePartialTree as _,
};

type NodeMeasureFn =
    Box<dyn FnMut(Size<Option<Pixels>>, Size<AvailableSpace>, &mut WindowContext) -> Size<Pixels>>;

pub struct TaffyLayoutEngine {
    taffy: TaffyTree<()>,
    styles: FxHashMap<LayoutId, Style>,
    children_to_parents: FxHashMap<LayoutId, LayoutId>,
    absolute_layout_bounds: FxHashMap<LayoutId, Bounds<Pixels>>,
    computed_layouts: FxHashSet<LayoutId>,
    nodes_to_measure: FxHashMap<LayoutId, NodeMeasureFn>,
}

static EXPECT_MESSAGE: &str = "we should avoid taffy layout errors by construction if possible";

impl TaffyLayoutEngine {
    pub fn new() -> Self {
        TaffyLayoutEngine {
            taffy: TaffyTree::new(),
            styles: FxHashMap::default(),
            children_to_parents: FxHashMap::default(),
            absolute_layout_bounds: FxHashMap::default(),
            computed_layouts: FxHashSet::default(),
            nodes_to_measure: FxHashMap::default(),
        }
    }

    pub fn clear(&mut self) {
        self.taffy.clear();
        self.children_to_parents.clear();
        self.absolute_layout_bounds.clear();
        self.computed_layouts.clear();
        self.nodes_to_measure.clear();
        self.styles.clear();
    }

    pub fn request_layout(
        &mut self,
        style: Style,
        rem_size: Pixels,
        children: &[LayoutId],
    ) -> LayoutId {
        let taffy_style = style.to_taffy(rem_size);
        let layout_id = if children.is_empty() {
            self.taffy
                .new_leaf(taffy_style)
                .expect(EXPECT_MESSAGE)
                .into()
        } else {
            let parent_id = self
                .taffy
                // This is safe because LayoutId is repr(transparent) to taffy::tree::NodeId.
                .new_with_children(taffy_style, unsafe { std::mem::transmute(children) })
                .expect(EXPECT_MESSAGE)
                .into();
            self.children_to_parents
                .extend(children.into_iter().map(|child_id| (*child_id, parent_id)));
            parent_id
        };
        self.styles.insert(layout_id, style);
        layout_id
    }

    pub fn request_measured_layout(
        &mut self,
        style: Style,
        rem_size: Pixels,
        measure: impl FnMut(Size<Option<Pixels>>, Size<AvailableSpace>, &mut WindowContext) -> Size<Pixels>
            + 'static,
    ) -> LayoutId {
        let taffy_style = style.to_taffy(rem_size);

        let layout_id = self
            .taffy
            .new_leaf_with_context(taffy_style, ())
            .expect(EXPECT_MESSAGE)
            .into();
        self.nodes_to_measure.insert(layout_id, Box::new(measure));
        self.styles.insert(layout_id, style);
        layout_id
    }

    // Used to understand performance
    #[allow(dead_code)]
    fn count_all_children(&self, parent: LayoutId) -> anyhow::Result<u32> {
        let mut count = 0;

        for child in self.taffy.children(parent.0)? {
            // Count this child.
            count += 1;

            // Count all of this child's children.
            count += self.count_all_children(LayoutId(child))?
        }

        Ok(count)
    }

    // Used to understand performance
    #[allow(dead_code)]
    fn max_depth(&self, depth: u32, parent: LayoutId) -> anyhow::Result<u32> {
        println!(
            "{parent:?} at depth {depth} has {} children",
            self.taffy.child_count(parent.0)
        );

        let mut max_child_depth = 0;

        for child in self.taffy.children(parent.0)? {
            max_child_depth = std::cmp::max(max_child_depth, self.max_depth(0, LayoutId(child))?);
        }

        Ok(depth + 1 + max_child_depth)
    }

    // Used to understand performance
    #[allow(dead_code)]
    fn get_edges(&self, parent: LayoutId) -> anyhow::Result<Vec<(LayoutId, LayoutId)>> {
        let mut edges = Vec::new();

        for child in self.taffy.children(parent.0)? {
            edges.push((parent, LayoutId(child)));

            edges.extend(self.get_edges(LayoutId(child))?);
        }

        Ok(edges)
    }

    pub fn compute_layout(
        &mut self,
        id: LayoutId,
        available_space: Size<AvailableSpace>,
        cx: &mut WindowContext,
    ) {
        // Leaving this here until we have a better instrumentation approach.
        // println!("Laying out {} children", self.count_all_children(id)?);
        // println!("Max layout depth: {}", self.max_depth(0, id)?);

        // Output the edges (branches) of the tree in Mermaid format for visualization.
        // println!("Edges:");
        // for (a, b) in self.get_edges(id)? {
        //     println!("N{} --> N{}", u64::from(a), u64::from(b));
        // }
        // println!("");
        //

        if !self.computed_layouts.insert(id) {
            let mut stack = SmallVec::<[LayoutId; 64]>::new();
            stack.push(id);
            while let Some(id) = stack.pop() {
                self.absolute_layout_bounds.remove(&id);
                stack.extend(
                    self.taffy
                        .children(id.into())
                        .expect(EXPECT_MESSAGE)
                        .into_iter()
                        .map(Into::into),
                );
            }
        }

        // let started_at = std::time::Instant::now();
        self.taffy
            .compute_layout_with_measure(
                id.into(),
                available_space.into(),
                |known_dimensions, available_space, node_id, _context| {
                    let Some(measure) = self.nodes_to_measure.get_mut(&node_id.into()) else {
                        return taffy::geometry::Size::default();
                    };

                    let known_dimensions = Size {
                        width: known_dimensions.width.map(Pixels),
                        height: known_dimensions.height.map(Pixels),
                    };

                    measure(known_dimensions, available_space.into(), cx).into()
                },
            )
            .expect(EXPECT_MESSAGE);

        // println!("compute_layout took {:?}", started_at.elapsed());
    }

    pub fn layout_bounds(&mut self, id: LayoutId) -> Bounds<Pixels> {
        if let Some(layout) = self.absolute_layout_bounds.get(&id).cloned() {
            return layout;
        }

        let layout = self.taffy.layout(id.into()).expect(EXPECT_MESSAGE);
        let mut bounds = Bounds {
            origin: layout.location.into(),
            size: layout.size.into(),
        };

        if let Some(parent_id) = self.children_to_parents.get(&id).copied() {
            let parent_bounds = self.layout_bounds(parent_id);
            bounds.origin += parent_bounds.origin;
        }
        self.absolute_layout_bounds.insert(id, bounds);

        bounds
    }
}

/// A unique identifier for a layout node, generated when requesting a layout from Taffy
#[derive(Copy, Clone, Eq, PartialEq, Debug)]
#[repr(transparent)]
pub struct LayoutId(NodeId);

impl std::hash::Hash for LayoutId {
    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
        u64::from(self.0).hash(state);
    }
}

impl From<NodeId> for LayoutId {
    fn from(node_id: NodeId) -> Self {
        Self(node_id)
    }
}

impl From<LayoutId> for NodeId {
    fn from(layout_id: LayoutId) -> NodeId {
        layout_id.0
    }
}

trait ToTaffy<Output> {
    fn to_taffy(&self, rem_size: Pixels) -> Output;
}

impl ToTaffy<taffy::style::Style> for Style {
    fn to_taffy(&self, rem_size: Pixels) -> taffy::style::Style {
        taffy::style::Style {
            display: self.display,
            overflow: self.overflow.into(),
            scrollbar_width: self.scrollbar_width,
            position: self.position,
            inset: self.inset.to_taffy(rem_size),
            size: self.size.to_taffy(rem_size),
            min_size: self.min_size.to_taffy(rem_size),
            max_size: self.max_size.to_taffy(rem_size),
            aspect_ratio: self.aspect_ratio,
            margin: self.margin.to_taffy(rem_size),
            padding: self.padding.to_taffy(rem_size),
            border: self.border_widths.to_taffy(rem_size),
            align_items: self.align_items,
            align_self: self.align_self,
            align_content: self.align_content,
            justify_content: self.justify_content,
            gap: self.gap.to_taffy(rem_size),
            flex_direction: self.flex_direction,
            flex_wrap: self.flex_wrap,
            flex_basis: self.flex_basis.to_taffy(rem_size),
            flex_grow: self.flex_grow,
            flex_shrink: self.flex_shrink,
            ..Default::default() // Ignore grid properties for now
        }
    }
}

impl ToTaffy<taffy::style::LengthPercentageAuto> for Length {
    fn to_taffy(&self, rem_size: Pixels) -> taffy::prelude::LengthPercentageAuto {
        match self {
            Length::Definite(length) => length.to_taffy(rem_size),
            Length::Auto => taffy::prelude::LengthPercentageAuto::Auto,
        }
    }
}

impl ToTaffy<taffy::style::Dimension> for Length {
    fn to_taffy(&self, rem_size: Pixels) -> taffy::prelude::Dimension {
        match self {
            Length::Definite(length) => length.to_taffy(rem_size),
            Length::Auto => taffy::prelude::Dimension::Auto,
        }
    }
}

impl ToTaffy<taffy::style::LengthPercentage> for DefiniteLength {
    fn to_taffy(&self, rem_size: Pixels) -> taffy::style::LengthPercentage {
        match self {
            DefiniteLength::Absolute(length) => match length {
                AbsoluteLength::Pixels(pixels) => {
                    taffy::style::LengthPercentage::Length(pixels.into())
                }
                AbsoluteLength::Rems(rems) => {
                    taffy::style::LengthPercentage::Length((*rems * rem_size).into())
                }
            },
            DefiniteLength::Fraction(fraction) => {
                taffy::style::LengthPercentage::Percent(*fraction)
            }
        }
    }
}

impl ToTaffy<taffy::style::LengthPercentageAuto> for DefiniteLength {
    fn to_taffy(&self, rem_size: Pixels) -> taffy::style::LengthPercentageAuto {
        match self {
            DefiniteLength::Absolute(length) => match length {
                AbsoluteLength::Pixels(pixels) => {
                    taffy::style::LengthPercentageAuto::Length(pixels.into())
                }
                AbsoluteLength::Rems(rems) => {
                    taffy::style::LengthPercentageAuto::Length((*rems * rem_size).into())
                }
            },
            DefiniteLength::Fraction(fraction) => {
                taffy::style::LengthPercentageAuto::Percent(*fraction)
            }
        }
    }
}

impl ToTaffy<taffy::style::Dimension> for DefiniteLength {
    fn to_taffy(&self, rem_size: Pixels) -> taffy::style::Dimension {
        match self {
            DefiniteLength::Absolute(length) => match length {
                AbsoluteLength::Pixels(pixels) => taffy::style::Dimension::Length(pixels.into()),
                AbsoluteLength::Rems(rems) => {
                    taffy::style::Dimension::Length((*rems * rem_size).into())
                }
            },
            DefiniteLength::Fraction(fraction) => taffy::style::Dimension::Percent(*fraction),
        }
    }
}

impl ToTaffy<taffy::style::LengthPercentage> for AbsoluteLength {
    fn to_taffy(&self, rem_size: Pixels) -> taffy::style::LengthPercentage {
        match self {
            AbsoluteLength::Pixels(pixels) => taffy::style::LengthPercentage::Length(pixels.into()),
            AbsoluteLength::Rems(rems) => {
                taffy::style::LengthPercentage::Length((*rems * rem_size).into())
            }
        }
    }
}

impl<T, T2> From<TaffyPoint<T>> for Point<T2>
where
    T: Into<T2>,
    T2: Clone + Default + Debug,
{
    fn from(point: TaffyPoint<T>) -> Point<T2> {
        Point {
            x: point.x.into(),
            y: point.y.into(),
        }
    }
}

impl<T, T2> From<Point<T>> for TaffyPoint<T2>
where
    T: Into<T2> + Clone + Default + Debug,
{
    fn from(val: Point<T>) -> Self {
        TaffyPoint {
            x: val.x.into(),
            y: val.y.into(),
        }
    }
}

impl<T, U> ToTaffy<TaffySize<U>> for Size<T>
where
    T: ToTaffy<U> + Clone + Default + Debug,
{
    fn to_taffy(&self, rem_size: Pixels) -> TaffySize<U> {
        TaffySize {
            width: self.width.to_taffy(rem_size),
            height: self.height.to_taffy(rem_size),
        }
    }
}

impl<T, U> ToTaffy<TaffyRect<U>> for Edges<T>
where
    T: ToTaffy<U> + Clone + Default + Debug,
{
    fn to_taffy(&self, rem_size: Pixels) -> TaffyRect<U> {
        TaffyRect {
            top: self.top.to_taffy(rem_size),
            right: self.right.to_taffy(rem_size),
            bottom: self.bottom.to_taffy(rem_size),
            left: self.left.to_taffy(rem_size),
        }
    }
}

impl<T, U> From<TaffySize<T>> for Size<U>
where
    T: Into<U>,
    U: Clone + Default + Debug,
{
    fn from(taffy_size: TaffySize<T>) -> Self {
        Size {
            width: taffy_size.width.into(),
            height: taffy_size.height.into(),
        }
    }
}

impl<T, U> From<Size<T>> for TaffySize<U>
where
    T: Into<U> + Clone + Default + Debug,
{
    fn from(size: Size<T>) -> Self {
        TaffySize {
            width: size.width.into(),
            height: size.height.into(),
        }
    }
}

/// The space available for an element to be laid out in
#[derive(Copy, Clone, Default, Debug, Eq, PartialEq)]
pub enum AvailableSpace {
    /// The amount of space available is the specified number of pixels
    Definite(Pixels),
    /// The amount of space available is indefinite and the node should be laid out under a min-content constraint
    #[default]
    MinContent,
    /// The amount of space available is indefinite and the node should be laid out under a max-content constraint
    MaxContent,
}

impl AvailableSpace {
    /// Returns a `Size` with both width and height set to `AvailableSpace::MinContent`.
    ///
    /// This function is useful when you want to create a `Size` with the minimum content constraints
    /// for both dimensions.
    ///
    /// # Examples
    ///
    /// ```
    /// let min_content_size = AvailableSpace::min_size();
    /// assert_eq!(min_content_size.width, AvailableSpace::MinContent);
    /// assert_eq!(min_content_size.height, AvailableSpace::MinContent);
    /// ```
    pub const fn min_size() -> Size<Self> {
        Size {
            width: Self::MinContent,
            height: Self::MinContent,
        }
    }
}

impl From<AvailableSpace> for TaffyAvailableSpace {
    fn from(space: AvailableSpace) -> TaffyAvailableSpace {
        match space {
            AvailableSpace::Definite(Pixels(value)) => TaffyAvailableSpace::Definite(value),
            AvailableSpace::MinContent => TaffyAvailableSpace::MinContent,
            AvailableSpace::MaxContent => TaffyAvailableSpace::MaxContent,
        }
    }
}

impl From<TaffyAvailableSpace> for AvailableSpace {
    fn from(space: TaffyAvailableSpace) -> AvailableSpace {
        match space {
            TaffyAvailableSpace::Definite(value) => AvailableSpace::Definite(Pixels(value)),
            TaffyAvailableSpace::MinContent => AvailableSpace::MinContent,
            TaffyAvailableSpace::MaxContent => AvailableSpace::MaxContent,
        }
    }
}

impl From<Pixels> for AvailableSpace {
    fn from(pixels: Pixels) -> Self {
        AvailableSpace::Definite(pixels)
    }
}

impl From<Size<Pixels>> for Size<AvailableSpace> {
    fn from(size: Size<Pixels>) -> Self {
        Size {
            width: AvailableSpace::Definite(size.width),
            height: AvailableSpace::Definite(size.height),
        }
    }
}