crates/taffy/src/tree/cache.rs

//! A cache for storing the results of layout computation
use crate::geometry::Size;
use crate::style::AvailableSpace;
use crate::tree::{LayoutOutput, RunMode};

/// The number of cache entries for each node in the tree
const CACHE_SIZE: usize = 9;

/// Cached intermediate layout results
#[derive(Debug, Clone, Copy, PartialEq)]
#[cfg_attr(feature = "serde", derive(Serialize))]
pub(crate) struct CacheEntry<T> {
    /// The initial cached size of the node itself
    known_dimensions: Size<Option<f32>>,
    /// The initial cached size of the parent's node
    available_space: Size<AvailableSpace>,
    /// The cached size and baselines of the item
    content: T,
}

/// A cache for caching the results of a sizing a Grid Item or Flexbox Item
#[derive(Debug, Clone, PartialEq)]
#[cfg_attr(feature = "serde", derive(Serialize))]
pub struct Cache {
    /// The cache entry for the node's final layout
    final_layout_entry: Option<CacheEntry<LayoutOutput>>,
    /// The cache entries for the node's preliminary size measurements
    measure_entries: [Option<CacheEntry<Size<f32>>>; CACHE_SIZE],
}

impl Default for Cache {
    fn default() -> Self {
        Self::new()
    }
}

impl Cache {
    /// Create a new empty cache
    pub const fn new() -> Self {
        Self { final_layout_entry: None, measure_entries: [None; CACHE_SIZE] }
    }

    /// Return the cache slot to cache the current computed result in
    ///
    /// ## Caching Strategy
    ///
    /// We need multiple cache slots, because a node's size is often queried by it's parent multiple times in the course of the layout
    /// process, and we don't want later results to clobber earlier ones.
    ///
    /// The two variables that we care about when determining cache slot are:
    ///
    ///   - How many "known_dimensions" are set. In the worst case, a node may be called first with neither dimension known, then with one
    ///     dimension known (either width of height - which doesn't matter for our purposes here), and then with both dimensions known.
    ///   - Whether unknown dimensions are being sized under a min-content or a max-content available space constraint (definite available space
    ///     shares a cache slot with max-content because a node will generally be sized under one or the other but not both).
    ///
    /// ## Cache slots:
    ///
    /// - Slot 0: Both known_dimensions were set
    /// - Slots 1-4: 1 of 2 known_dimensions were set and:
    ///   - Slot 1: width but not height known_dimension was set and the other dimension was either a MaxContent or Definite available space constraintraint
    ///   - Slot 2: width but not height known_dimension was set and the other dimension was a MinContent constraint
    ///   - Slot 3: height but not width known_dimension was set and the other dimension was either a MaxContent or Definite available space constraintable space constraint
    ///   - Slot 4: height but not width known_dimension was set and the other dimension was a MinContent constraint
    /// - Slots 5-8: Neither known_dimensions were set and:
    ///   - Slot 5: x-axis available space is MaxContent or Definite and y-axis available space is MaxContent or Definite
    ///   - Slot 6: x-axis available space is MaxContent or Definite and y-axis available space is MinContent
    ///   - Slot 7: x-axis available space is MinContent and y-axis available space is MaxContent or Definite
    ///   - Slot 8: x-axis available space is MinContent and y-axis available space is MinContent
    #[inline]
    fn compute_cache_slot(known_dimensions: Size<Option<f32>>, available_space: Size<AvailableSpace>) -> usize {
        use AvailableSpace::{Definite, MaxContent, MinContent};

        let has_known_width = known_dimensions.width.is_some();
        let has_known_height = known_dimensions.height.is_some();

        // Slot 0: Both known_dimensions were set
        if has_known_width && has_known_height {
            return 0;
        }

        // Slot 1: width but not height known_dimension was set and the other dimension was either a MaxContent or Definite available space constraint
        // Slot 2: width but not height known_dimension was set and the other dimension was a MinContent constraint
        if has_known_width && !has_known_height {
            return 1 + (available_space.height == MinContent) as usize;
        }

        // Slot 3: height but not width known_dimension was set and the other dimension was either a MaxContent or Definite available space constraint
        // Slot 4: height but not width known_dimension was set and the other dimension was a MinContent constraint
        if has_known_height && !has_known_width {
            return 3 + (available_space.width == MinContent) as usize;
        }

        // Slots 5-8: Neither known_dimensions were set and:
        match (available_space.width, available_space.height) {
            // Slot 5: x-axis available space is MaxContent or Definite and y-axis available space is MaxContent or Definite
            (MaxContent | Definite(_), MaxContent | Definite(_)) => 5,
            // Slot 6: x-axis available space is MaxContent or Definite and y-axis available space is MinContent
            (MaxContent | Definite(_), MinContent) => 6,
            // Slot 7: x-axis available space is MinContent and y-axis available space is MaxContent or Definite
            (MinContent, MaxContent | Definite(_)) => 7,
            // Slot 8: x-axis available space is MinContent and y-axis available space is MinContent
            (MinContent, MinContent) => 8,
        }
    }

    /// Try to retrieve a cached result from the cache
    #[inline]
    pub fn get(
        &self,
        known_dimensions: Size<Option<f32>>,
        available_space: Size<AvailableSpace>,
        run_mode: RunMode,
    ) -> Option<LayoutOutput> {
        match run_mode {
            RunMode::PerformLayout => self
                .final_layout_entry
                .filter(|entry| {
                    let cached_size = entry.content.size;
                    (known_dimensions.width == entry.known_dimensions.width
                        || known_dimensions.width == Some(cached_size.width))
                        && (known_dimensions.height == entry.known_dimensions.height
                            || known_dimensions.height == Some(cached_size.height))
                        && (known_dimensions.width.is_some()
                            || entry.available_space.width.is_roughly_equal(available_space.width))
                        && (known_dimensions.height.is_some()
                            || entry.available_space.height.is_roughly_equal(available_space.height))
                })
                .map(|e| e.content),
            RunMode::ComputeSize => {
                for entry in self.measure_entries.iter().flatten() {
                    let cached_size = entry.content;

                    if (known_dimensions.width == entry.known_dimensions.width
                        || known_dimensions.width == Some(cached_size.width))
                        && (known_dimensions.height == entry.known_dimensions.height
                            || known_dimensions.height == Some(cached_size.height))
                        && (known_dimensions.width.is_some()
                            || entry.available_space.width.is_roughly_equal(available_space.width))
                        && (known_dimensions.height.is_some()
                            || entry.available_space.height.is_roughly_equal(available_space.height))
                    {
                        return Some(LayoutOutput::from_outer_size(cached_size));
                    }
                }

                None
            }
            RunMode::PerformHiddenLayout => None,
        }
    }

    /// Store a computed size in the cache
    pub fn store(
        &mut self,
        known_dimensions: Size<Option<f32>>,
        available_space: Size<AvailableSpace>,
        run_mode: RunMode,
        layout_output: LayoutOutput,
    ) {
        match run_mode {
            RunMode::PerformLayout => {
                self.final_layout_entry = Some(CacheEntry { known_dimensions, available_space, content: layout_output })
            }
            RunMode::ComputeSize => {
                let cache_slot = Self::compute_cache_slot(known_dimensions, available_space);
                self.measure_entries[cache_slot] =
                    Some(CacheEntry { known_dimensions, available_space, content: layout_output.size });
            }
            RunMode::PerformHiddenLayout => {}
        }
    }

    /// Clear all cache entries
    pub fn clear(&mut self) {
        self.final_layout_entry = None;
        self.measure_entries = [None; CACHE_SIZE];
    }

    /// Returns true if all cache entries are None, else false
    pub fn is_empty(&self) -> bool {
        self.final_layout_entry.is_none() && !self.measure_entries.iter().any(|entry| entry.is_some())
    }
}