taffy/compute/

flexbox.rs

//! Computes the [flexbox](https://css-tricks.com/snippets/css/a-guide-to-flexbox/) layout algorithm on [`TaffyTree`](crate::TaffyTree) according to the [spec](https://www.w3.org/TR/css-flexbox-1/)
use crate::compute::common::alignment::compute_alignment_offset;
use crate::geometry::{Line, Point, Rect, Size};
use crate::style::{
    AlignContent, AlignItems, AlignSelf, AvailableSpace, FlexWrap, JustifyContent, LengthPercentageAuto, Overflow,
    Position,
};
use crate::style::{CoreStyle, FlexDirection, FlexboxContainerStyle, FlexboxItemStyle};
use crate::style_helpers::{TaffyMaxContent, TaffyMinContent};
use crate::tree::{Layout, LayoutInput, LayoutOutput, RunMode, SizingMode};
use crate::tree::{LayoutFlexboxContainer, LayoutPartialTreeExt, NodeId};
use crate::util::debug::debug_log;
use crate::util::sys::{f32_max, new_vec_with_capacity, Vec};
use crate::util::MaybeMath;
use crate::util::{MaybeResolve, ResolveOrZero};
use crate::{BoxGenerationMode, BoxSizing};

use super::common::alignment::apply_alignment_fallback;
#[cfg(feature = "content_size")]
use super::common::content_size::compute_content_size_contribution;

/// The intermediate results of a flexbox calculation for a single item
struct FlexItem {
    /// The identifier for the associated node
    node: NodeId,

    /// The order of the node relative to it's siblings
    order: u32,

    /// The base size of this item
    size: Size<Option<f32>>,
    /// The minimum allowable size of this item
    min_size: Size<Option<f32>>,
    /// The maximum allowable size of this item
    max_size: Size<Option<f32>>,
    /// The cross-alignment of this item
    align_self: AlignSelf,

    /// The overflow style of the item
    overflow: Point<Overflow>,
    /// The width of the scrollbars (if it has any)
    scrollbar_width: f32,
    /// The flex shrink style of the item
    flex_shrink: f32,
    /// The flex grow style of the item
    flex_grow: f32,

    /// The minimum size of the item. This differs from min_size above because it also
    /// takes into account content based automatic minimum sizes
    resolved_minimum_main_size: f32,

    /// The final offset of this item
    inset: Rect<Option<f32>>,
    /// The margin of this item
    margin: Rect<f32>,
    /// Whether each margin is an auto margin or not
    margin_is_auto: Rect<bool>,
    /// The padding of this item
    padding: Rect<f32>,
    /// The border of this item
    border: Rect<f32>,

    /// The default size of this item
    flex_basis: f32,
    /// The default size of this item, minus padding and border
    inner_flex_basis: f32,
    /// The amount by which this item has deviated from its target size
    violation: f32,
    /// Is the size of this item locked
    frozen: bool,

    /// Either the max- or min- content flex fraction
    /// See https://www.w3.org/TR/css-flexbox-1/#intrinsic-main-sizes
    content_flex_fraction: f32,

    /// The proposed inner size of this item
    hypothetical_inner_size: Size<f32>,
    /// The proposed outer size of this item
    hypothetical_outer_size: Size<f32>,
    /// The size that this item wants to be
    target_size: Size<f32>,
    /// The size that this item wants to be, plus any padding and border
    outer_target_size: Size<f32>,

    /// The position of the bottom edge of this item
    baseline: f32,

    /// A temporary value for the main offset
    ///
    /// Offset is the relative position from the item's natural flow position based on
    /// relative position values, alignment, and justification. Does not include margin/padding/border.
    offset_main: f32,
    /// A temporary value for the cross offset
    ///
    /// Offset is the relative position from the item's natural flow position based on
    /// relative position values, alignment, and justification. Does not include margin/padding/border.
    offset_cross: f32,
}

impl FlexItem {
    /// Returns true if the item is a <https://www.w3.org/TR/css-overflow-3/#scroll-container>
    fn is_scroll_container(&self) -> bool {
        self.overflow.x.is_scroll_container() | self.overflow.y.is_scroll_container()
    }
}

/// A line of [`FlexItem`] used for intermediate computation
struct FlexLine<'a> {
    /// The slice of items to iterate over during computation of this line
    items: &'a mut [FlexItem],
    /// The dimensions of the cross-axis
    cross_size: f32,
    /// The relative offset of the cross-axis
    offset_cross: f32,
}

/// Values that can be cached during the flexbox algorithm
struct AlgoConstants {
    /// The direction of the current segment being laid out
    dir: FlexDirection,
    /// Is this segment a row
    is_row: bool,
    /// Is this segment a column
    is_column: bool,
    /// Is wrapping enabled (in either direction)
    is_wrap: bool,
    /// Is the wrap direction inverted
    is_wrap_reverse: bool,

    /// The item's min_size style
    min_size: Size<Option<f32>>,
    /// The item's max_size style
    max_size: Size<Option<f32>>,
    /// The margin of this section
    margin: Rect<f32>,
    /// The border of this section
    border: Rect<f32>,
    /// The space between the content box and the border box.
    /// This consists of padding + border + scrollbar_gutter.
    content_box_inset: Rect<f32>,
    /// The size reserved for scrollbar gutters in each axis
    scrollbar_gutter: Point<f32>,
    /// The gap of this section
    gap: Size<f32>,
    /// The align_items property of this node
    align_items: AlignItems,
    /// The align_content property of this node
    align_content: AlignContent,
    /// The justify_content property of this node
    justify_content: Option<JustifyContent>,

    /// The border-box size of the node being laid out (if known)
    node_outer_size: Size<Option<f32>>,
    /// The content-box size of the node being laid out (if known)
    node_inner_size: Size<Option<f32>>,

    /// The size of the virtual container containing the flex items.
    container_size: Size<f32>,
    /// The size of the internal container
    inner_container_size: Size<f32>,
}

/// Computes the layout of a box according to the flexbox algorithm
pub fn compute_flexbox_layout(
    tree: &mut impl LayoutFlexboxContainer,
    node: NodeId,
    inputs: LayoutInput,
) -> LayoutOutput {
    let LayoutInput { known_dimensions, parent_size, run_mode, .. } = inputs;
    let style = tree.get_flexbox_container_style(node);

    // Pull these out earlier to avoid borrowing issues
    let aspect_ratio = style.aspect_ratio();
    let padding = style.padding().resolve_or_zero(parent_size.width, |val, basis| tree.calc(val, basis));
    let border = style.border().resolve_or_zero(parent_size.width, |val, basis| tree.calc(val, basis));
    let padding_border_sum = padding.sum_axes() + border.sum_axes();
    let box_sizing_adjustment =
        if style.box_sizing() == BoxSizing::ContentBox { padding_border_sum } else { Size::ZERO };

    let min_size = style
        .min_size()
        .maybe_resolve(parent_size, |val, basis| tree.calc(val, basis))
        .maybe_apply_aspect_ratio(aspect_ratio)
        .maybe_add(box_sizing_adjustment);
    let max_size = style
        .max_size()
        .maybe_resolve(parent_size, |val, basis| tree.calc(val, basis))
        .maybe_apply_aspect_ratio(aspect_ratio)
        .maybe_add(box_sizing_adjustment);
    let clamped_style_size = if inputs.sizing_mode == SizingMode::InherentSize {
        style
            .size()
            .maybe_resolve(parent_size, |val, basis| tree.calc(val, basis))
            .maybe_apply_aspect_ratio(aspect_ratio)
            .maybe_add(box_sizing_adjustment)
            .maybe_clamp(min_size, max_size)
    } else {
        Size::NONE
    };

    // If both min and max in a given axis are set and max <= min then this determines the size in that axis
    let min_max_definite_size = min_size.zip_map(max_size, |min, max| match (min, max) {
        (Some(min), Some(max)) if max <= min => Some(min),
        _ => None,
    });

    // The size of the container should be floored by the padding and border
    let styled_based_known_dimensions =
        known_dimensions.or(min_max_definite_size.or(clamped_style_size).maybe_max(padding_border_sum));

    // Short-circuit layout if the container's size is fully determined by the container's size and the run mode
    // is ComputeSize (and thus the container's size is all that we're interested in)
    if run_mode == RunMode::ComputeSize {
        if let Size { width: Some(width), height: Some(height) } = styled_based_known_dimensions {
            return LayoutOutput::from_outer_size(Size { width, height });
        }
    }

    debug_log!("FLEX:", dbg:style.flex_direction());
    drop(style);

    compute_preliminary(tree, node, LayoutInput { known_dimensions: styled_based_known_dimensions, ..inputs })
}

/// Compute a preliminary size for an item
fn compute_preliminary(tree: &mut impl LayoutFlexboxContainer, node: NodeId, inputs: LayoutInput) -> LayoutOutput {
    let LayoutInput { known_dimensions, parent_size, available_space, run_mode, .. } = inputs;

    // Define some general constants we will need for the remainder of the algorithm.
    let mut constants = compute_constants(tree, tree.get_flexbox_container_style(node), known_dimensions, parent_size);

    // 9. Flex Layout Algorithm

    // 9.1. Initial Setup

    // 1. Generate anonymous flex items as described in §4 Flex Items.
    debug_log!("generate_anonymous_flex_items");
    let mut flex_items = generate_anonymous_flex_items(tree, node, &constants);

    // 9.2. Line Length Determination

    // 2. Determine the available main and cross space for the flex items
    debug_log!("determine_available_space");
    let available_space = determine_available_space(known_dimensions, available_space, &constants);

    // 3. Determine the flex base size and hypothetical main size of each item.
    debug_log!("determine_flex_base_size");
    determine_flex_base_size(tree, &constants, available_space, &mut flex_items);

    #[cfg(feature = "debug")]
    for item in flex_items.iter() {
        debug_log!("item.flex_basis", item.flex_basis);
        debug_log!("item.inner_flex_basis", item.inner_flex_basis);
        debug_log!("item.hypothetical_outer_size", dbg:item.hypothetical_outer_size);
        debug_log!("item.hypothetical_inner_size", dbg:item.hypothetical_inner_size);
        debug_log!("item.resolved_minimum_main_size", dbg:item.resolved_minimum_main_size);
    }

    // 4. Determine the main size of the flex container
    // This has already been done as part of compute_constants. The inner size is exposed as constants.node_inner_size.

    // 9.3. Main Size Determination

    // 5. Collect flex items into flex lines.
    debug_log!("collect_flex_lines");
    let mut flex_lines = collect_flex_lines(&constants, available_space, &mut flex_items);

    // If container size is undefined, determine the container's main size
    // and then re-resolve gaps based on newly determined size
    debug_log!("determine_container_main_size");
    if let Some(inner_main_size) = constants.node_inner_size.main(constants.dir) {
        let outer_main_size = inner_main_size + constants.content_box_inset.main_axis_sum(constants.dir);
        constants.inner_container_size.set_main(constants.dir, inner_main_size);
        constants.container_size.set_main(constants.dir, outer_main_size);
    } else {
        // Sets constants.container_size and constants.outer_container_size
        determine_container_main_size(tree, available_space, &mut flex_lines, &mut constants);
        constants.node_inner_size.set_main(constants.dir, Some(constants.inner_container_size.main(constants.dir)));
        constants.node_outer_size.set_main(constants.dir, Some(constants.container_size.main(constants.dir)));

        debug_log!("constants.node_outer_size", dbg:constants.node_outer_size);
        debug_log!("constants.node_inner_size", dbg:constants.node_inner_size);

        // Re-resolve percentage gaps
        let style = tree.get_flexbox_container_style(node);
        let inner_container_size = constants.inner_container_size.main(constants.dir);
        let new_gap = style
            .gap()
            .main(constants.dir)
            .maybe_resolve(inner_container_size, |val, basis| tree.calc(val, basis))
            .unwrap_or(0.0);
        constants.gap.set_main(constants.dir, new_gap);
    }

    // 6. Resolve the flexible lengths of all the flex items to find their used main size.
    debug_log!("resolve_flexible_lengths");
    for line in &mut flex_lines {
        resolve_flexible_lengths(line, &constants);
    }

    // 9.4. Cross Size Determination

    // 7. Determine the hypothetical cross size of each item.
    debug_log!("determine_hypothetical_cross_size");
    for line in &mut flex_lines {
        determine_hypothetical_cross_size(tree, line, &constants, available_space);
    }

    // Calculate child baselines. This function is internally smart and only computes child baselines
    // if they are necessary.
    debug_log!("calculate_children_base_lines");
    calculate_children_base_lines(tree, known_dimensions, available_space, &mut flex_lines, &constants);

    // 8. Calculate the cross size of each flex line.
    debug_log!("calculate_cross_size");
    calculate_cross_size(&mut flex_lines, known_dimensions, &constants);

    // 9. Handle 'align-content: stretch'.
    debug_log!("handle_align_content_stretch");
    handle_align_content_stretch(&mut flex_lines, known_dimensions, &constants);

    // 10. Collapse visibility:collapse items. If any flex items have visibility: collapse,
    //     note the cross size of the line they’re in as the item’s strut size, and restart
    //     layout from the beginning.
    //
    //     In this second layout round, when collecting items into lines, treat the collapsed
    //     items as having zero main size. For the rest of the algorithm following that step,
    //     ignore the collapsed items entirely (as if they were display:none) except that after
    //     calculating the cross size of the lines, if any line’s cross size is less than the
    //     largest strut size among all the collapsed items in the line, set its cross size to
    //     that strut size.
    //
    //     Skip this step in the second layout round.

    // TODO implement once (if ever) we support visibility:collapse

    // 11. Determine the used cross size of each flex item.
    debug_log!("determine_used_cross_size");
    determine_used_cross_size(tree, &mut flex_lines, &constants);

    // 9.5. Main-Axis Alignment

    // 12. Distribute any remaining free space.
    debug_log!("distribute_remaining_free_space");
    distribute_remaining_free_space(&mut flex_lines, &constants);

    // 9.6. Cross-Axis Alignment

    // 13. Resolve cross-axis auto margins (also includes 14).
    debug_log!("resolve_cross_axis_auto_margins");
    resolve_cross_axis_auto_margins(&mut flex_lines, &constants);

    // 15. Determine the flex container’s used cross size.
    debug_log!("determine_container_cross_size");
    let total_line_cross_size = determine_container_cross_size(&flex_lines, known_dimensions, &mut constants);

    // We have the container size.
    // If our caller does not care about performing layout we are done now.
    if run_mode == RunMode::ComputeSize {
        return LayoutOutput::from_outer_size(constants.container_size);
    }

    // 16. Align all flex lines per align-content.
    debug_log!("align_flex_lines_per_align_content");
    align_flex_lines_per_align_content(&mut flex_lines, &constants, total_line_cross_size);

    // Do a final layout pass and gather the resulting layouts
    debug_log!("final_layout_pass");
    let inflow_content_size = final_layout_pass(tree, &mut flex_lines, &constants);

    // Before returning we perform absolute layout on all absolutely positioned children
    debug_log!("perform_absolute_layout_on_absolute_children");
    let absolute_content_size = perform_absolute_layout_on_absolute_children(tree, node, &constants);

    debug_log!("hidden_layout");
    let len = tree.child_count(node);
    for order in 0..len {
        let child = tree.get_child_id(node, order);
        if tree.get_flexbox_child_style(child).box_generation_mode() == BoxGenerationMode::None {
            tree.set_unrounded_layout(child, &Layout::with_order(order as u32));
            tree.perform_child_layout(
                child,
                Size::NONE,
                Size::NONE,
                Size::MAX_CONTENT,
                SizingMode::InherentSize,
                Line::FALSE,
            );
        }
    }

    // 8.5. Flex Container Baselines: calculate the flex container's first baseline
    // See https://www.w3.org/TR/css-flexbox-1/#flex-baselines
    let first_vertical_baseline = if flex_lines.is_empty() {
        None
    } else {
        flex_lines[0]
            .items
            .iter()
            .find(|item| constants.is_column || item.align_self == AlignSelf::Baseline)
            .or_else(|| flex_lines[0].items.iter().next())
            .map(|child| {
                let offset_vertical = if constants.is_row { child.offset_cross } else { child.offset_main };
                offset_vertical + child.baseline
            })
    };

    LayoutOutput::from_sizes_and_baselines(
        constants.container_size,
        inflow_content_size.f32_max(absolute_content_size),
        Point { x: None, y: first_vertical_baseline },
    )
}

/// Compute constants that can be reused during the flexbox algorithm.
#[inline]
fn compute_constants(
    tree: &impl LayoutFlexboxContainer,
    style: impl FlexboxContainerStyle,
    known_dimensions: Size<Option<f32>>,
    parent_size: Size<Option<f32>>,
) -> AlgoConstants {
    let dir = style.flex_direction();
    let is_row = dir.is_row();
    let is_column = dir.is_column();
    let is_wrap = matches!(style.flex_wrap(), FlexWrap::Wrap | FlexWrap::WrapReverse);
    let is_wrap_reverse = style.flex_wrap() == FlexWrap::WrapReverse;

    let aspect_ratio = style.aspect_ratio();
    let margin = style.margin().resolve_or_zero(parent_size.width, |val, basis| tree.calc(val, basis));
    let padding = style.padding().resolve_or_zero(parent_size.width, |val, basis| tree.calc(val, basis));
    let border = style.border().resolve_or_zero(parent_size.width, |val, basis| tree.calc(val, basis));
    let padding_border_sum = padding.sum_axes() + border.sum_axes();
    let box_sizing_adjustment =
        if style.box_sizing() == BoxSizing::ContentBox { padding_border_sum } else { Size::ZERO };

    let align_items = style.align_items().unwrap_or(AlignItems::Stretch);
    let align_content = style.align_content().unwrap_or(AlignContent::Stretch);
    let justify_content = style.justify_content();

    // Scrollbar gutters are reserved when the `overflow` property is set to `Overflow::Scroll`.
    // However, the axis are switched (transposed) because a node that scrolls vertically needs
    // *horizontal* space to be reserved for a scrollbar
    let scrollbar_gutter = style.overflow().transpose().map(|overflow| match overflow {
        Overflow::Scroll => style.scrollbar_width(),
        _ => 0.0,
    });
    // TODO: make side configurable based on the `direction` property
    let mut content_box_inset = padding + border;
    content_box_inset.right += scrollbar_gutter.x;
    content_box_inset.bottom += scrollbar_gutter.y;

    let node_outer_size = known_dimensions;
    let node_inner_size = node_outer_size.maybe_sub(content_box_inset.sum_axes());
    let gap = style.gap().resolve_or_zero(node_inner_size.or(Size::zero()), |val, basis| tree.calc(val, basis));

    let container_size = Size::zero();
    let inner_container_size = Size::zero();

    AlgoConstants {
        dir,
        is_row,
        is_column,
        is_wrap,
        is_wrap_reverse,
        min_size: style
            .min_size()
            .maybe_resolve(parent_size, |val, basis| tree.calc(val, basis))
            .maybe_apply_aspect_ratio(aspect_ratio)
            .maybe_add(box_sizing_adjustment),
        max_size: style
            .max_size()
            .maybe_resolve(parent_size, |val, basis| tree.calc(val, basis))
            .maybe_apply_aspect_ratio(aspect_ratio)
            .maybe_add(box_sizing_adjustment),
        margin,
        border,
        gap,
        content_box_inset,
        scrollbar_gutter,
        align_items,
        align_content,
        justify_content,
        node_outer_size,
        node_inner_size,
        container_size,
        inner_container_size,
    }
}

/// Generate anonymous flex items.
///
/// # [9.1. Initial Setup](https://www.w3.org/TR/css-flexbox-1/#box-manip)
///
/// - [**Generate anonymous flex items**](https://www.w3.org/TR/css-flexbox-1/#algo-anon-box) as described in [§4 Flex Items](https://www.w3.org/TR/css-flexbox-1/#flex-items).
#[inline]
fn generate_anonymous_flex_items(
    tree: &impl LayoutFlexboxContainer,
    node: NodeId,
    constants: &AlgoConstants,
) -> Vec<FlexItem> {
    tree.child_ids(node)
        .enumerate()
        .map(|(index, child)| (index, child, tree.get_flexbox_child_style(child)))
        .filter(|(_, _, style)| style.position() != Position::Absolute)
        .filter(|(_, _, style)| style.box_generation_mode() != BoxGenerationMode::None)
        .map(|(index, child, child_style)| {
            let aspect_ratio = child_style.aspect_ratio();
            let padding = child_style
                .padding()
                .resolve_or_zero(constants.node_inner_size.width, |val, basis| tree.calc(val, basis));
            let border = child_style
                .border()
                .resolve_or_zero(constants.node_inner_size.width, |val, basis| tree.calc(val, basis));
            let pb_sum = (padding + border).sum_axes();
            let box_sizing_adjustment =
                if child_style.box_sizing() == BoxSizing::ContentBox { pb_sum } else { Size::ZERO };
            FlexItem {
                node: child,
                order: index as u32,
                size: child_style
                    .size()
                    .maybe_resolve(constants.node_inner_size, |val, basis| tree.calc(val, basis))
                    .maybe_apply_aspect_ratio(aspect_ratio)
                    .maybe_add(box_sizing_adjustment),
                min_size: child_style
                    .min_size()
                    .maybe_resolve(constants.node_inner_size, |val, basis| tree.calc(val, basis))
                    .maybe_apply_aspect_ratio(aspect_ratio)
                    .maybe_add(box_sizing_adjustment),
                max_size: child_style
                    .max_size()
                    .maybe_resolve(constants.node_inner_size, |val, basis| tree.calc(val, basis))
                    .maybe_apply_aspect_ratio(aspect_ratio)
                    .maybe_add(box_sizing_adjustment),

                inset: child_style
                    .inset()
                    .zip_size(constants.node_inner_size, |p, s| p.maybe_resolve(s, |val, basis| tree.calc(val, basis))),
                margin: child_style
                    .margin()
                    .resolve_or_zero(constants.node_inner_size.width, |val, basis| tree.calc(val, basis)),
                margin_is_auto: child_style.margin().map(LengthPercentageAuto::is_auto),
                padding: child_style
                    .padding()
                    .resolve_or_zero(constants.node_inner_size.width, |val, basis| tree.calc(val, basis)),
                border: child_style
                    .border()
                    .resolve_or_zero(constants.node_inner_size.width, |val, basis| tree.calc(val, basis)),
                align_self: child_style.align_self().unwrap_or(constants.align_items),
                overflow: child_style.overflow(),
                scrollbar_width: child_style.scrollbar_width(),
                flex_grow: child_style.flex_grow(),
                flex_shrink: child_style.flex_shrink(),
                flex_basis: 0.0,
                inner_flex_basis: 0.0,
                violation: 0.0,
                frozen: false,

                resolved_minimum_main_size: 0.0,
                hypothetical_inner_size: Size::zero(),
                hypothetical_outer_size: Size::zero(),
                target_size: Size::zero(),
                outer_target_size: Size::zero(),
                content_flex_fraction: 0.0,

                baseline: 0.0,

                offset_main: 0.0,
                offset_cross: 0.0,
            }
        })
        .collect()
}

/// Determine the available main and cross space for the flex items.
///
/// # [9.2. Line Length Determination](https://www.w3.org/TR/css-flexbox-1/#line-sizing)
///
/// - [**Determine the available main and cross space for the flex items**](https://www.w3.org/TR/css-flexbox-1/#algo-available).
///
/// For each dimension, if that dimension of the flex container’s content box is a definite size, use that;
/// if that dimension of the flex container is being sized under a min or max-content constraint, the available space in that dimension is that constraint;
/// otherwise, subtract the flex container’s margin, border, and padding from the space available to the flex container in that dimension and use that value.
/// **This might result in an infinite value**.
#[inline]
#[must_use]
fn determine_available_space(
    known_dimensions: Size<Option<f32>>,
    outer_available_space: Size<AvailableSpace>,
    constants: &AlgoConstants,
) -> Size<AvailableSpace> {
    // Note: min/max/preferred size styles have already been applied to known_dimensions in the `compute` function above
    let width = match known_dimensions.width {
        Some(node_width) => AvailableSpace::Definite(node_width - constants.content_box_inset.horizontal_axis_sum()),
        None => outer_available_space
            .width
            .maybe_sub(constants.margin.horizontal_axis_sum())
            .maybe_sub(constants.content_box_inset.horizontal_axis_sum()),
    };

    let height = match known_dimensions.height {
        Some(node_height) => AvailableSpace::Definite(node_height - constants.content_box_inset.vertical_axis_sum()),
        None => outer_available_space
            .height
            .maybe_sub(constants.margin.vertical_axis_sum())
            .maybe_sub(constants.content_box_inset.vertical_axis_sum()),
    };

    Size { width, height }
}

/// Determine the flex base size and hypothetical main size of each item.
///
/// # [9.2. Line Length Determination](https://www.w3.org/TR/css-flexbox-1/#line-sizing)
///
/// - [**Determine the flex base size and hypothetical main size of each item:**](https://www.w3.org/TR/css-flexbox-1/#algo-main-item)
///
///     - A. If the item has a definite used flex basis, that’s the flex base size.
///
///     - B. If the flex item has ...
///
///         - an intrinsic aspect ratio,
///         - a used flex basis of content, and
///         - a definite cross size,
///
///       then the flex base size is calculated from its inner cross size and the flex item’s intrinsic aspect ratio.
///
///     - C. If the used flex basis is content or depends on its available space, and the flex container is being sized under a min-content
///       or max-content constraint (e.g. when performing automatic table layout \[CSS21\]), size the item under that constraint.
///       The flex base size is the item’s resulting main size.
///
///     - E. Otherwise, size the item into the available space using its used flex basis in place of its main size, treating a value of content as max-content.
///       If a cross size is needed to determine the main size (e.g. when the flex item’s main size is in its block axis) and the flex item’s cross size is auto and not definite,
///       in this calculation use fit-content as the flex item’s cross size. The flex base size is the item’s resulting main size.
///
///   When determining the flex base size, the item’s min and max main sizes are ignored (no clamping occurs).
///   Furthermore, the sizing calculations that floor the content box size at zero when applying box-sizing are also ignored.
///   (For example, an item with a specified size of zero, positive padding, and box-sizing: border-box will have an outer flex base size of zero—and hence a negative inner flex base size.)
#[inline]
fn determine_flex_base_size(
    tree: &mut impl LayoutFlexboxContainer,
    constants: &AlgoConstants,
    available_space: Size<AvailableSpace>,
    flex_items: &mut [FlexItem],
) {
    let dir = constants.dir;

    for child in flex_items.iter_mut() {
        let child_style = tree.get_flexbox_child_style(child.node);

        // Parent size for child sizing
        let cross_axis_parent_size = constants.node_inner_size.cross(dir);
        let child_parent_size = Size::from_cross(dir, cross_axis_parent_size);

        // Available space for child sizing
        let cross_axis_margin_sum = constants.margin.cross_axis_sum(dir);
        let child_min_cross = child.min_size.cross(dir).maybe_add(cross_axis_margin_sum);
        let child_max_cross = child.max_size.cross(dir).maybe_add(cross_axis_margin_sum);

        // Clamp available space by min- and max- size
        let cross_axis_available_space: AvailableSpace = match available_space.cross(dir) {
            AvailableSpace::Definite(val) => AvailableSpace::Definite(
                cross_axis_parent_size.unwrap_or(val).maybe_clamp(child_min_cross, child_max_cross),
            ),
            AvailableSpace::MinContent => match child_min_cross {
                Some(min) => AvailableSpace::Definite(min),
                None => AvailableSpace::MinContent,
            },
            AvailableSpace::MaxContent => match child_max_cross {
                Some(max) => AvailableSpace::Definite(max),
                None => AvailableSpace::MaxContent,
            },
        };

        // Known dimensions for child sizing
        let child_known_dimensions = {
            let mut ckd = child.size.with_main(dir, None);
            if child.align_self == AlignSelf::Stretch
                && !child.margin_is_auto.cross_start(constants.dir)
                && !child.margin_is_auto.cross_end(constants.dir)
                && ckd.cross(dir).is_none()
            {
                ckd.set_cross(
                    dir,
                    cross_axis_available_space.into_option().maybe_sub(child.margin.cross_axis_sum(dir)),
                );
            }
            ckd
        };

        let container_width = constants.node_inner_size.main(dir);
        let box_sizing_adjustment = if child_style.box_sizing() == BoxSizing::ContentBox {
            let padding = child_style.padding().resolve_or_zero(container_width, |val, basis| tree.calc(val, basis));
            let border = child_style.border().resolve_or_zero(container_width, |val, basis| tree.calc(val, basis));
            (padding + border).sum_axes()
        } else {
            Size::ZERO
        }
        .main(dir);
        let flex_basis = child_style
            .flex_basis()
            .maybe_resolve(container_width, |val, basis| tree.calc(val, basis))
            .maybe_add(box_sizing_adjustment);

        drop(child_style);

        child.flex_basis = 'flex_basis: {
            // A. If the item has a definite used flex basis, that’s the flex base size.

            // B. If the flex item has an intrinsic aspect ratio,
            //    a used flex basis of content, and a definite cross size,
            //    then the flex base size is calculated from its inner
            //    cross size and the flex item’s intrinsic aspect ratio.

            // Note: `child.size` has already been resolved against aspect_ratio in generate_anonymous_flex_items
            // So B will just work here by using main_size without special handling for aspect_ratio
            let main_size = child.size.main(dir);
            if let Some(flex_basis) = flex_basis.or(main_size) {
                break 'flex_basis flex_basis;
            };

            // C. If the used flex basis is content or depends on its available space,
            //    and the flex container is being sized under a min-content or max-content
            //    constraint (e.g. when performing automatic table layout [CSS21]),
            //    size the item under that constraint. The flex base size is the item’s
            //    resulting main size.

            // This is covered by the implementation of E below, which passes the available_space constraint
            // through to the child size computation. It may need a separate implementation if/when D is implemented.

            // D. Otherwise, if the used flex basis is content or depends on its
            //    available space, the available main size is infinite, and the flex item’s
            //    inline axis is parallel to the main axis, lay the item out using the rules
            //    for a box in an orthogonal flow [CSS3-WRITING-MODES]. The flex base size
            //    is the item’s max-content main size.

            // TODO if/when vertical writing modes are supported

            // E. Otherwise, size the item into the available space using its used flex basis
            //    in place of its main size, treating a value of content as max-content.
            //    If a cross size is needed to determine the main size (e.g. when the
            //    flex item’s main size is in its block axis) and the flex item’s cross size
            //    is auto and not definite, in this calculation use fit-content as the
            //    flex item’s cross size. The flex base size is the item’s resulting main size.

            let child_available_space = Size::MAX_CONTENT
                .with_main(
                    dir,
                    // Map AvailableSpace::Definite to AvailableSpace::MaxContent
                    if available_space.main(dir) == AvailableSpace::MinContent {
                        AvailableSpace::MinContent
                    } else {
                        AvailableSpace::MaxContent
                    },
                )
                .with_cross(dir, cross_axis_available_space);

            debug_log!("COMPUTE CHILD BASE SIZE:");
            break 'flex_basis tree.measure_child_size(
                child.node,
                child_known_dimensions,
                child_parent_size,
                child_available_space,
                SizingMode::ContentSize,
                dir.main_axis(),
                Line::FALSE,
            );
        };

        // Floor flex-basis by the padding_border_sum (floors inner_flex_basis at zero)
        // This seems to be in violation of the spec which explicitly states that the content box should not be floored at zero
        // (like it usually is) when calculating the flex-basis. But including this matches both Chrome and Firefox's behaviour.
        //
        // TODO: resolve spec violation
        // Spec: https://www.w3.org/TR/css-flexbox-1/#intrinsic-item-contributions
        // Spec: https://www.w3.org/TR/css-flexbox-1/#change-2016-max-contribution
        let padding_border_sum = child.padding.main_axis_sum(constants.dir) + child.border.main_axis_sum(constants.dir);
        child.flex_basis = child.flex_basis.max(padding_border_sum);

        // The hypothetical main size is the item’s flex base size clamped according to its
        // used min and max main sizes (and flooring the content box size at zero).

        child.inner_flex_basis =
            child.flex_basis - child.padding.main_axis_sum(constants.dir) - child.border.main_axis_sum(constants.dir);

        let padding_border_axes_sums = (child.padding + child.border).sum_axes().map(Some);

        // Note that it is important that the `parent_size` parameter in the main axis is not set for this
        // function call as it used for resolving percentages, and percentage size in an axis should not contribute
        // to a min-content contribution in that same axis. However the `parent_size` and `available_space` *should*
        // be set to their usual values in the cross axis so that wrapping content can wrap correctly.
        //
        // See https://drafts.csswg.org/css-sizing-3/#min-percentage-contribution
        let style_min_main_size =
            child.min_size.or(child.overflow.map(Overflow::maybe_into_automatic_min_size).into()).main(dir);

        child.resolved_minimum_main_size = style_min_main_size.unwrap_or({
            let min_content_main_size = {
                let child_available_space = Size::MIN_CONTENT.with_cross(dir, cross_axis_available_space);

                debug_log!("COMPUTE CHILD MIN SIZE:");
                tree.measure_child_size(
                    child.node,
                    child_known_dimensions,
                    child_parent_size,
                    child_available_space,
                    SizingMode::ContentSize,
                    dir.main_axis(),
                    Line::FALSE,
                )
            };

            // 4.5. Automatic Minimum Size of Flex Items
            // https://www.w3.org/TR/css-flexbox-1/#min-size-auto
            let clamped_min_content_size =
                min_content_main_size.maybe_min(child.size.main(dir)).maybe_min(child.max_size.main(dir));
            clamped_min_content_size.maybe_max(padding_border_axes_sums.main(dir))
        });

        let hypothetical_inner_min_main =
            child.resolved_minimum_main_size.maybe_max(padding_border_axes_sums.main(constants.dir));
        let hypothetical_inner_size =
            child.flex_basis.maybe_clamp(Some(hypothetical_inner_min_main), child.max_size.main(constants.dir));
        let hypothetical_outer_size = hypothetical_inner_size + child.margin.main_axis_sum(constants.dir);

        child.hypothetical_inner_size.set_main(constants.dir, hypothetical_inner_size);
        child.hypothetical_outer_size.set_main(constants.dir, hypothetical_outer_size);
    }
}

/// Collect flex items into flex lines.
///
/// # [9.3. Main Size Determination](https://www.w3.org/TR/css-flexbox-1/#main-sizing)
///
/// - [**Collect flex items into flex lines**](https://www.w3.org/TR/css-flexbox-1/#algo-line-break):
///
///     - If the flex container is single-line, collect all the flex items into a single flex line.
///
///     - Otherwise, starting from the first uncollected item, collect consecutive items one by one until the first time that the next collected item would not fit into the flex container’s inner main size
///       (or until a forced break is encountered, see [§10 Fragmenting Flex Layout](https://www.w3.org/TR/css-flexbox-1/#pagination)).
///       If the very first uncollected item wouldn't fit, collect just it into the line.
///
///       For this step, the size of a flex item is its outer hypothetical main size. (**Note: This can be negative**.)
///
///       Repeat until all flex items have been collected into flex lines.
///
///       **Note that the "collect as many" line will collect zero-sized flex items onto the end of the previous line even if the last non-zero item exactly "filled up" the line**.
#[inline]
fn collect_flex_lines<'a>(
    constants: &AlgoConstants,
    available_space: Size<AvailableSpace>,
    flex_items: &'a mut Vec<FlexItem>,
) -> Vec<FlexLine<'a>> {
    if !constants.is_wrap {
        let mut lines = new_vec_with_capacity(1);
        lines.push(FlexLine { items: flex_items.as_mut_slice(), cross_size: 0.0, offset_cross: 0.0 });
        lines
    } else {
        let main_axis_available_space = match constants.max_size.main(constants.dir) {
            Some(max_size) => AvailableSpace::Definite(
                available_space
                    .main(constants.dir)
                    .into_option()
                    .unwrap_or(max_size)
                    .maybe_max(constants.min_size.main(constants.dir)),
            ),
            None => available_space.main(constants.dir),
        };

        match main_axis_available_space {
            // If we're sizing under a max-content constraint then the flex items will never wrap
            // (at least for now - future extensions to the CSS spec may add provisions for forced wrap points)
            AvailableSpace::MaxContent => {
                let mut lines = new_vec_with_capacity(1);
                lines.push(FlexLine { items: flex_items.as_mut_slice(), cross_size: 0.0, offset_cross: 0.0 });
                lines
            }
            // If flex-wrap is Wrap and we're sizing under a min-content constraint, then we take every possible wrapping opportunity
            // and place each item in it's own line
            AvailableSpace::MinContent => {
                let mut lines = new_vec_with_capacity(flex_items.len());
                let mut items = &mut flex_items[..];
                while !items.is_empty() {
                    let (line_items, rest) = items.split_at_mut(1);
                    lines.push(FlexLine { items: line_items, cross_size: 0.0, offset_cross: 0.0 });
                    items = rest;
                }
                lines
            }
            AvailableSpace::Definite(main_axis_available_space) => {
                let mut lines = new_vec_with_capacity(1);
                let mut flex_items = &mut flex_items[..];
                let main_axis_gap = constants.gap.main(constants.dir);

                while !flex_items.is_empty() {
                    // Find index of the first item in the next line
                    // (or the last item if all remaining items are in the current line)
                    let mut line_length = 0.0;
                    let index = flex_items
                        .iter()
                        .enumerate()
                        .find(|&(idx, child)| {
                            // Gaps only occur between items (not before the first one or after the last one)
                            // So first item in the line does not contribute a gap to the line length
                            let gap_contribution = if idx == 0 { 0.0 } else { main_axis_gap };
                            line_length += child.hypothetical_outer_size.main(constants.dir) + gap_contribution;
                            line_length > main_axis_available_space && idx != 0
                        })
                        .map(|(idx, _)| idx)
                        .unwrap_or(flex_items.len());

                    let (items, rest) = flex_items.split_at_mut(index);
                    lines.push(FlexLine { items, cross_size: 0.0, offset_cross: 0.0 });
                    flex_items = rest;
                }
                lines
            }
        }
    }
}

/// Determine the container's main size (if not already known)
fn determine_container_main_size(
    tree: &mut impl LayoutFlexboxContainer,
    available_space: Size<AvailableSpace>,
    lines: &mut [FlexLine<'_>],
    constants: &mut AlgoConstants,
) {
    let dir = constants.dir;
    let main_content_box_inset = constants.content_box_inset.main_axis_sum(constants.dir);

    let outer_main_size: f32 = constants.node_outer_size.main(constants.dir).unwrap_or_else(|| {
        match available_space.main(dir) {
            AvailableSpace::Definite(main_axis_available_space) => {
                let longest_line_length: f32 = lines
                    .iter()
                    .map(|line| {
                        let line_main_axis_gap = sum_axis_gaps(constants.gap.main(constants.dir), line.items.len());
                        let total_target_size = line
                            .items
                            .iter()
                            .map(|child| {
                                let padding_border_sum = (child.padding + child.border).main_axis_sum(constants.dir);
                                (child.flex_basis.maybe_max(child.min_size.main(constants.dir))
                                    + child.margin.main_axis_sum(constants.dir))
                                .max(padding_border_sum)
                            })
                            .sum::<f32>();
                        total_target_size + line_main_axis_gap
                    })
                    .max_by(|a, b| a.total_cmp(b))
                    .unwrap_or(0.0);
                let size = longest_line_length + main_content_box_inset;
                if lines.len() > 1 {
                    f32_max(size, main_axis_available_space)
                } else {
                    size
                }
            }
            AvailableSpace::MinContent if constants.is_wrap => {
                let longest_line_length: f32 = lines
                    .iter()
                    .map(|line| {
                        let line_main_axis_gap = sum_axis_gaps(constants.gap.main(constants.dir), line.items.len());
                        let total_target_size = line
                            .items
                            .iter()
                            .map(|child| {
                                let padding_border_sum = (child.padding + child.border).main_axis_sum(constants.dir);
                                (child.flex_basis.maybe_max(child.min_size.main(constants.dir))
                                    + child.margin.main_axis_sum(constants.dir))
                                .max(padding_border_sum)
                            })
                            .sum::<f32>();
                        total_target_size + line_main_axis_gap
                    })
                    .max_by(|a, b| a.total_cmp(b))
                    .unwrap_or(0.0);
                longest_line_length + main_content_box_inset
            }
            AvailableSpace::MinContent | AvailableSpace::MaxContent => {
                // Define a base main_size variable. This is mutated once for iteration over the outer
                // loop over the flex lines as:
                //   "The flex container’s max-content size is the largest sum of the afore-calculated sizes of all items within a single line."
                let mut main_size = 0.0;

                for line in lines.iter_mut() {
                    for item in line.items.iter_mut() {
                        let style_min = item.min_size.main(constants.dir);
                        let style_preferred = item.size.main(constants.dir);
                        let style_max = item.max_size.main(constants.dir);

                        // The spec seems a bit unclear on this point (my initial reading was that the `.maybe_max(style_preferred)` should
                        // not be included here), however this matches both Chrome and Firefox as of 9th March 2023.
                        //
                        // Spec: https://www.w3.org/TR/css-flexbox-1/#intrinsic-item-contributions
                        // Spec modification: https://www.w3.org/TR/css-flexbox-1/#change-2016-max-contribution
                        // Issue: https://github.com/w3c/csswg-drafts/issues/1435
                        // Gentest: padding_border_overrides_size_flex_basis_0.html
                        let clamping_basis = Some(item.flex_basis).maybe_max(style_preferred);
                        let flex_basis_min = clamping_basis.filter(|_| item.flex_shrink == 0.0);
                        let flex_basis_max = clamping_basis.filter(|_| item.flex_grow == 0.0);

                        let min_main_size = style_min
                            .maybe_max(flex_basis_min)
                            .or(flex_basis_min)
                            .unwrap_or(item.resolved_minimum_main_size)
                            .max(item.resolved_minimum_main_size);
                        let max_main_size =
                            style_max.maybe_min(flex_basis_max).or(flex_basis_max).unwrap_or(f32::INFINITY);

                        let content_contribution = match (min_main_size, style_preferred, max_main_size) {
                            // If the clamping values are such that max <= min, then we can avoid the expensive step of computing the content size
                            // as we know that the clamping values will override it anyway
                            (min, Some(pref), max) if max <= min || max <= pref => {
                                pref.min(max).max(min) + item.margin.main_axis_sum(constants.dir)
                            }
                            (min, _, max) if max <= min => min + item.margin.main_axis_sum(constants.dir),

                            // Else compute the min- or -max content size and apply the full formula for computing the
                            // min- or max- content contribution
                            _ if item.is_scroll_container() => {
                                item.flex_basis + item.margin.main_axis_sum(constants.dir)
                            }
                            _ => {
                                // Parent size for child sizing
                                let cross_axis_parent_size = constants.node_inner_size.cross(dir);

                                // Available space for child sizing
                                let cross_axis_margin_sum = constants.margin.cross_axis_sum(dir);
                                let child_min_cross = item.min_size.cross(dir).maybe_add(cross_axis_margin_sum);
                                let child_max_cross = item.max_size.cross(dir).maybe_add(cross_axis_margin_sum);
                                let cross_axis_available_space: AvailableSpace = available_space
                                    .cross(dir)
                                    .map_definite_value(|val| cross_axis_parent_size.unwrap_or(val))
                                    .maybe_clamp(child_min_cross, child_max_cross);

                                let child_available_space = available_space.with_cross(dir, cross_axis_available_space);

                                // Known dimensions for child sizing
                                let child_known_dimensions = {
                                    let mut ckd = item.size.with_main(dir, None);
                                    if item.align_self == AlignSelf::Stretch && ckd.cross(dir).is_none() {
                                        ckd.set_cross(
                                            dir,
                                            cross_axis_available_space
                                                .into_option()
                                                .maybe_sub(item.margin.cross_axis_sum(dir)),
                                        );
                                    }
                                    ckd
                                };

                                // Either the min- or max- content size depending on which constraint we are sizing under.
                                // TODO: Optimise by using already computed values where available
                                debug_log!("COMPUTE CHILD BASE SIZE (for intrinsic main size):");
                                let content_main_size = tree.measure_child_size(
                                    item.node,
                                    child_known_dimensions,
                                    constants.node_inner_size,
                                    child_available_space,
                                    SizingMode::InherentSize,
                                    dir.main_axis(),
                                    Line::FALSE,
                                ) + item.margin.main_axis_sum(constants.dir);

                                // This is somewhat bizarre in that it's asymmetrical depending whether the flex container is a column or a row.
                                //
                                // I *think* this might relate to https://drafts.csswg.org/css-flexbox-1/#algo-main-container:
                                //
                                //    "The automatic block size of a block-level flex container is its max-content size."
                                //
                                // Which could suggest that flex-basis defining a vertical size does not shrink because it is in the block axis, and the automatic size
                                // in the block axis is a MAX content size. Whereas a flex-basis defining a horizontal size does shrink because the automatic size in
                                // inline axis is MIN content size (although I don't have a reference for that).
                                //
                                // Ultimately, this was not found by reading the spec, but by trial and error fixing tests to align with Webkit/Firefox output.
                                // (see the `flex_basis_unconstraint_row` and `flex_basis_uncontraint_column` generated tests which demonstrate this)
                                if constants.is_row {
                                    content_main_size.maybe_clamp(style_min, style_max).max(main_content_box_inset)
                                } else {
                                    content_main_size
                                        .max(item.flex_basis)
                                        .maybe_clamp(style_min, style_max)
                                        .max(main_content_box_inset)
                                }
                            }
                        };
                        item.content_flex_fraction = {
                            let diff = content_contribution - item.flex_basis;
                            if diff > 0.0 {
                                diff / f32_max(1.0, item.flex_grow)
                            } else if diff < 0.0 {
                                let scaled_shrink_factor = f32_max(1.0, item.flex_shrink * item.inner_flex_basis);
                                diff / scaled_shrink_factor
                            } else {
                                // We are assuming that diff is 0.0 here and that we haven't accidentally introduced a NaN
                                0.0
                            }
                        };
                    }

                    // TODO Spec says to scale everything by the line's max flex fraction. But neither Chrome nor firefox implement this
                    // so we don't either. But if we did want to, we'd need this computation here (and to use it below):
                    //
                    // Within each line, find the largest max-content flex fraction among all the flex items.
                    // let line_flex_fraction = line
                    //     .items
                    //     .iter()
                    //     .map(|item| item.content_flex_fraction)
                    //     .max_by(|a, b| a.total_cmp(b))
                    //     .unwrap_or(0.0); // Unwrap case never gets hit because there is always at least one item a line

                    // Add each item’s flex base size to the product of:
                    //   - its flex grow factor (or scaled flex shrink factor,if the chosen max-content flex fraction was negative)
                    //   - the chosen max-content flex fraction
                    // then clamp that result by the max main size floored by the min main size.
                    //
                    // The flex container’s max-content size is the largest sum of the afore-calculated sizes of all items within a single line.
                    let item_main_size_sum = line
                        .items
                        .iter_mut()
                        .map(|item| {
                            let flex_fraction = item.content_flex_fraction;
                            // let flex_fraction = line_flex_fraction;

                            let flex_contribution = if item.content_flex_fraction > 0.0 {
                                f32_max(1.0, item.flex_grow) * flex_fraction
                            } else if item.content_flex_fraction < 0.0 {
                                let scaled_shrink_factor = f32_max(1.0, item.flex_shrink) * item.inner_flex_basis;
                                scaled_shrink_factor * flex_fraction
                            } else {
                                0.0
                            };
                            let size = item.flex_basis + flex_contribution;
                            item.outer_target_size.set_main(constants.dir, size);
                            item.target_size.set_main(constants.dir, size);
                            size
                        })
                        .sum::<f32>();

                    let gap_sum = sum_axis_gaps(constants.gap.main(constants.dir), line.items.len());
                    main_size = f32_max(main_size, item_main_size_sum + gap_sum)
                }

                main_size + main_content_box_inset
            }
        }
    });

    let outer_main_size = outer_main_size
        .maybe_clamp(constants.min_size.main(constants.dir), constants.max_size.main(constants.dir))
        .max(main_content_box_inset - constants.scrollbar_gutter.main(constants.dir));

    // let outer_main_size = inner_main_size + constants.padding_border.main_axis_sum(constants.dir);
    let inner_main_size = f32_max(outer_main_size - main_content_box_inset, 0.0);
    constants.container_size.set_main(constants.dir, outer_main_size);
    constants.inner_container_size.set_main(constants.dir, inner_main_size);
    constants.node_inner_size.set_main(constants.dir, Some(inner_main_size));
}

/// Resolve the flexible lengths of the items within a flex line.
/// Sets the `main` component of each item's `target_size` and `outer_target_size`
///
/// # [9.7. Resolving Flexible Lengths](https://www.w3.org/TR/css-flexbox-1/#resolve-flexible-lengths)
#[inline]
fn resolve_flexible_lengths(line: &mut FlexLine, constants: &AlgoConstants) {
    let total_main_axis_gap = sum_axis_gaps(constants.gap.main(constants.dir), line.items.len());

    // 1. Determine the used flex factor. Sum the outer hypothetical main sizes of all
    //    items on the line. If the sum is less than the flex container’s inner main size,
    //    use the flex grow factor for the rest of this algorithm; otherwise, use the
    //    flex shrink factor.

    let total_hypothetical_outer_main_size =
        line.items.iter().map(|child| child.hypothetical_outer_size.main(constants.dir)).sum::<f32>();
    let used_flex_factor: f32 = total_main_axis_gap + total_hypothetical_outer_main_size;
    let growing = used_flex_factor < constants.node_inner_size.main(constants.dir).unwrap_or(0.0);
    let shrinking = used_flex_factor > constants.node_inner_size.main(constants.dir).unwrap_or(0.0);
    let exactly_sized = !growing & !shrinking;

    // 2. Size inflexible items. Freeze, setting its target main size to its hypothetical main size
    //    - Any item that has a flex factor of zero
    //    - If using the flex grow factor: any item that has a flex base size
    //      greater than its hypothetical main size
    //    - If using the flex shrink factor: any item that has a flex base size
    //      smaller than its hypothetical main size

    for child in line.items.iter_mut() {
        let inner_target_size = child.hypothetical_inner_size.main(constants.dir);
        child.target_size.set_main(constants.dir, inner_target_size);

        if exactly_sized
            || (child.flex_grow == 0.0 && child.flex_shrink == 0.0)
            || (growing && child.flex_basis > child.hypothetical_inner_size.main(constants.dir))
            || (shrinking && child.flex_basis < child.hypothetical_inner_size.main(constants.dir))
        {
            child.frozen = true;
            let outer_target_size = inner_target_size + child.margin.main_axis_sum(constants.dir);
            child.outer_target_size.set_main(constants.dir, outer_target_size);
        }
    }

    if exactly_sized {
        return;
    }

    // 3. Calculate initial free space. Sum the outer sizes of all items on the line,
    //    and subtract this from the flex container’s inner main size. For frozen items,
    //    use their outer target main size; for other items, use their outer flex base size.

    let used_space: f32 = total_main_axis_gap
        + line
            .items
            .iter()
            .map(|child| {
                if child.frozen {
                    child.outer_target_size.main(constants.dir)
                } else {
                    child.flex_basis + child.margin.main_axis_sum(constants.dir)
                }
            })
            .sum::<f32>();

    let initial_free_space = constants.node_inner_size.main(constants.dir).maybe_sub(used_space).unwrap_or(0.0);

    // 4. Loop

    loop {
        // a. Check for flexible items. If all the flex items on the line are frozen,
        //    free space has been distributed; exit this loop.

        if line.items.iter().all(|child| child.frozen) {
            break;
        }

        // b. Calculate the remaining free space as for initial free space, above.
        //    If the sum of the unfrozen flex items’ flex factors is less than one,
        //    multiply the initial free space by this sum. If the magnitude of this
        //    value is less than the magnitude of the remaining free space, use this
        //    as the remaining free space.

        let used_space: f32 = total_main_axis_gap
            + line
                .items
                .iter()
                .map(|child| {
                    if child.frozen {
                        child.outer_target_size.main(constants.dir)
                    } else {
                        child.flex_basis + child.margin.main_axis_sum(constants.dir)
                    }
                })
                .sum::<f32>();

        let mut unfrozen: Vec<&mut FlexItem> = line.items.iter_mut().filter(|child| !child.frozen).collect();

        let (sum_flex_grow, sum_flex_shrink): (f32, f32) =
            unfrozen.iter().fold((0.0, 0.0), |(flex_grow, flex_shrink), item| {
                (flex_grow + item.flex_grow, flex_shrink + item.flex_shrink)
            });

        let free_space = if growing && sum_flex_grow < 1.0 {
            (initial_free_space * sum_flex_grow - total_main_axis_gap)
                .maybe_min(constants.node_inner_size.main(constants.dir).maybe_sub(used_space))
        } else if shrinking && sum_flex_shrink < 1.0 {
            (initial_free_space * sum_flex_shrink - total_main_axis_gap)
                .maybe_max(constants.node_inner_size.main(constants.dir).maybe_sub(used_space))
        } else {
            (constants.node_inner_size.main(constants.dir).maybe_sub(used_space))
                .unwrap_or(used_flex_factor - used_space)
        };

        // c. Distribute free space proportional to the flex factors.
        //    - If the remaining free space is zero
        //        Do Nothing
        //    - If using the flex grow factor
        //        Find the ratio of the item’s flex grow factor to the sum of the
        //        flex grow factors of all unfrozen items on the line. Set the item’s
        //        target main size to its flex base size plus a fraction of the remaining
        //        free space proportional to the ratio.
        //    - If using the flex shrink factor
        //        For every unfrozen item on the line, multiply its flex shrink factor by
        //        its inner flex base size, and note this as its scaled flex shrink factor.
        //        Find the ratio of the item’s scaled flex shrink factor to the sum of the
        //        scaled flex shrink factors of all unfrozen items on the line. Set the item’s
        //        target main size to its flex base size minus a fraction of the absolute value
        //        of the remaining free space proportional to the ratio. Note this may result
        //        in a negative inner main size; it will be corrected in the next step.
        //    - Otherwise
        //        Do Nothing

        if free_space.is_normal() {
            if growing && sum_flex_grow > 0.0 {
                for child in &mut unfrozen {
                    child
                        .target_size
                        .set_main(constants.dir, child.flex_basis + free_space * (child.flex_grow / sum_flex_grow));
                }
            } else if shrinking && sum_flex_shrink > 0.0 {
                let sum_scaled_shrink_factor: f32 =
                    unfrozen.iter().map(|child| child.inner_flex_basis * child.flex_shrink).sum();

                if sum_scaled_shrink_factor > 0.0 {
                    for child in &mut unfrozen {
                        let scaled_shrink_factor = child.inner_flex_basis * child.flex_shrink;
                        child.target_size.set_main(
                            constants.dir,
                            child.flex_basis + free_space * (scaled_shrink_factor / sum_scaled_shrink_factor),
                        )
                    }
                }
            }
        }

        // d. Fix min/max violations. Clamp each non-frozen item’s target main size by its
        //    used min and max main sizes and floor its content-box size at zero. If the
        //    item’s target main size was made smaller by this, it’s a max violation.
        //    If the item’s target main size was made larger by this, it’s a min violation.

        let total_violation = unfrozen.iter_mut().fold(0.0, |acc, child| -> f32 {
            let resolved_min_main: Option<f32> = child.resolved_minimum_main_size.into();
            let max_main = child.max_size.main(constants.dir);
            let clamped = child.target_size.main(constants.dir).maybe_clamp(resolved_min_main, max_main).max(0.0);
            child.violation = clamped - child.target_size.main(constants.dir);
            child.target_size.set_main(constants.dir, clamped);
            child.outer_target_size.set_main(
                constants.dir,
                child.target_size.main(constants.dir) + child.margin.main_axis_sum(constants.dir),
            );

            acc + child.violation
        });

        // e. Freeze over-flexed items. The total violation is the sum of the adjustments
        //    from the previous step ∑(clamped size - unclamped size). If the total violation is:
        //    - Zero
        //        Freeze all items.
        //    - Positive
        //        Freeze all the items with min violations.
        //    - Negative
        //        Freeze all the items with max violations.

        for child in &mut unfrozen {
            match total_violation {
                v if v > 0.0 => child.frozen = child.violation > 0.0,
                v if v < 0.0 => child.frozen = child.violation < 0.0,
                _ => child.frozen = true,
            }
        }

        // f. Return to the start of this loop.
    }
}

/// Determine the hypothetical cross size of each item.
///
/// # [9.4. Cross Size Determination](https://www.w3.org/TR/css-flexbox-1/#cross-sizing)
///
/// - [**Determine the hypothetical cross size of each item**](https://www.w3.org/TR/css-flexbox-1/#algo-cross-item)
///   by performing layout with the used main size and the available space, treating auto as fit-content.
#[inline]
fn determine_hypothetical_cross_size(
    tree: &mut impl LayoutFlexboxContainer,
    line: &mut FlexLine,
    constants: &AlgoConstants,
    available_space: Size<AvailableSpace>,
) {
    for child in line.items.iter_mut() {
        let padding_border_sum = (child.padding + child.border).cross_axis_sum(constants.dir);

        let child_known_main = constants.container_size.main(constants.dir).into();

        let child_cross = child
            .size
            .cross(constants.dir)
            .maybe_clamp(child.min_size.cross(constants.dir), child.max_size.cross(constants.dir))
            .maybe_max(padding_border_sum);

        let child_available_cross = available_space
            .cross(constants.dir)
            .maybe_clamp(child.min_size.cross(constants.dir), child.max_size.cross(constants.dir))
            .maybe_max(padding_border_sum);

        let child_inner_cross = child_cross.unwrap_or_else(|| {
            tree.measure_child_size(
                child.node,
                Size {
                    width: if constants.is_row { child.target_size.width.into() } else { child_cross },
                    height: if constants.is_row { child_cross } else { child.target_size.height.into() },
                },
                constants.node_inner_size,
                Size {
                    width: if constants.is_row { child_known_main } else { child_available_cross },
                    height: if constants.is_row { child_available_cross } else { child_known_main },
                },
                SizingMode::ContentSize,
                constants.dir.cross_axis(),
                Line::FALSE,
            )
            .maybe_clamp(child.min_size.cross(constants.dir), child.max_size.cross(constants.dir))
            .max(padding_border_sum)
        });
        let child_outer_cross = child_inner_cross + child.margin.cross_axis_sum(constants.dir);

        child.hypothetical_inner_size.set_cross(constants.dir, child_inner_cross);
        child.hypothetical_outer_size.set_cross(constants.dir, child_outer_cross);
    }
}

/// Calculate the base lines of the children.
#[inline]
fn calculate_children_base_lines(
    tree: &mut impl LayoutFlexboxContainer,
    node_size: Size<Option<f32>>,
    available_space: Size<AvailableSpace>,
    flex_lines: &mut [FlexLine],
    constants: &AlgoConstants,
) {
    // Only compute baselines for flex rows because we only support baseline alignment in the cross axis
    // where that axis is also the inline axis
    // TODO: this may need revisiting if/when we support vertical writing modes
    if !constants.is_row {
        return;
    }

    for line in flex_lines {
        // If a flex line has one or zero items participating in baseline alignment then baseline alignment is a no-op so we skip
        let line_baseline_child_count =
            line.items.iter().filter(|child| child.align_self == AlignSelf::Baseline).count();
        if line_baseline_child_count <= 1 {
            continue;
        }

        for child in line.items.iter_mut() {
            // Only calculate baselines for children participating in baseline alignment
            if child.align_self != AlignSelf::Baseline {
                continue;
            }

            let measured_size_and_baselines = tree.perform_child_layout(
                child.node,
                Size {
                    width: if constants.is_row {
                        child.target_size.width.into()
                    } else {
                        child.hypothetical_inner_size.width.into()
                    },
                    height: if constants.is_row {
                        child.hypothetical_inner_size.height.into()
                    } else {
                        child.target_size.height.into()
                    },
                },
                constants.node_inner_size,
                Size {
                    width: if constants.is_row {
                        constants.container_size.width.into()
                    } else {
                        available_space.width.maybe_set(node_size.width)
                    },
                    height: if constants.is_row {
                        available_space.height.maybe_set(node_size.height)
                    } else {
                        constants.container_size.height.into()
                    },
                },
                SizingMode::ContentSize,
                Line::FALSE,
            );

            let baseline = measured_size_and_baselines.first_baselines.y;
            let height = measured_size_and_baselines.size.height;

            child.baseline = baseline.unwrap_or(height) + child.margin.top;
        }
    }
}

/// Calculate the cross size of each flex line.
///
/// # [9.4. Cross Size Determination](https://www.w3.org/TR/css-flexbox-1/#cross-sizing)
///
/// - [**Calculate the cross size of each flex line**](https://www.w3.org/TR/css-flexbox-1/#algo-cross-line).
#[inline]
fn calculate_cross_size(flex_lines: &mut [FlexLine], node_size: Size<Option<f32>>, constants: &AlgoConstants) {
    // If the flex container is single-line and has a definite cross size,
    // the cross size of the flex line is the flex container’s inner cross size.
    if !constants.is_wrap && node_size.cross(constants.dir).is_some() {
        let cross_axis_padding_border = constants.content_box_inset.cross_axis_sum(constants.dir);
        let cross_min_size = constants.min_size.cross(constants.dir);
        let cross_max_size = constants.max_size.cross(constants.dir);
        flex_lines[0].cross_size = node_size
            .cross(constants.dir)
            .maybe_clamp(cross_min_size, cross_max_size)
            .maybe_sub(cross_axis_padding_border)
            .maybe_max(0.0)
            .unwrap_or(0.0);
    } else {
        // Otherwise, for each flex line:
        //
        //    1. Collect all the flex items whose inline-axis is parallel to the main-axis, whose
        //       align-self is baseline, and whose cross-axis margins are both non-auto. Find the
        //       largest of the distances between each item’s baseline and its hypothetical outer
        //       cross-start edge, and the largest of the distances between each item’s baseline
        //       and its hypothetical outer cross-end edge, and sum these two values.

        //    2. Among all the items not collected by the previous step, find the largest
        //       outer hypothetical cross size.

        //    3. The used cross-size of the flex line is the largest of the numbers found in the
        //       previous two steps and zero.
        for line in flex_lines.iter_mut() {
            let max_baseline: f32 = line.items.iter().map(|child| child.baseline).fold(0.0, |acc, x| acc.max(x));
            line.cross_size = line
                .items
                .iter()
                .map(|child| {
                    if child.align_self == AlignSelf::Baseline
                        && !child.margin_is_auto.cross_start(constants.dir)
                        && !child.margin_is_auto.cross_end(constants.dir)
                    {
                        max_baseline - child.baseline + child.hypothetical_outer_size.cross(constants.dir)
                    } else {
                        child.hypothetical_outer_size.cross(constants.dir)
                    }
                })
                .fold(0.0, |acc, x| acc.max(x));
        }

        // If the flex container is single-line, then clamp the line’s cross-size to be within the container’s computed min and max cross sizes.
        // Note that if CSS 2.1’s definition of min/max-width/height applied more generally, this behavior would fall out automatically.
        if !constants.is_wrap {
            let cross_axis_padding_border = constants.content_box_inset.cross_axis_sum(constants.dir);
            let cross_min_size = constants.min_size.cross(constants.dir);
            let cross_max_size = constants.max_size.cross(constants.dir);
            flex_lines[0].cross_size = flex_lines[0].cross_size.maybe_clamp(
                cross_min_size.maybe_sub(cross_axis_padding_border),
                cross_max_size.maybe_sub(cross_axis_padding_border),
            );
        }
    }
}

/// Handle 'align-content: stretch'.
///
/// # [9.4. Cross Size Determination](https://www.w3.org/TR/css-flexbox-1/#cross-sizing)
///
/// - [**Handle 'align-content: stretch'**](https://www.w3.org/TR/css-flexbox-1/#algo-line-stretch). If the flex container has a definite cross size, align-content is stretch,
///   and the sum of the flex lines' cross sizes is less than the flex container’s inner cross size,
///   increase the cross size of each flex line by equal amounts such that the sum of their cross sizes exactly equals the flex container’s inner cross size.
#[inline]
fn handle_align_content_stretch(flex_lines: &mut [FlexLine], node_size: Size<Option<f32>>, constants: &AlgoConstants) {
    if constants.align_content == AlignContent::Stretch {
        let cross_axis_padding_border = constants.content_box_inset.cross_axis_sum(constants.dir);
        let cross_min_size = constants.min_size.cross(constants.dir);
        let cross_max_size = constants.max_size.cross(constants.dir);
        let container_min_inner_cross = node_size
            .cross(constants.dir)
            .or(cross_min_size)
            .maybe_clamp(cross_min_size, cross_max_size)
            .maybe_sub(cross_axis_padding_border)
            .maybe_max(0.0)
            .unwrap_or(0.0);

        let total_cross_axis_gap = sum_axis_gaps(constants.gap.cross(constants.dir), flex_lines.len());
        let lines_total_cross: f32 = flex_lines.iter().map(|line| line.cross_size).sum::<f32>() + total_cross_axis_gap;

        if lines_total_cross < container_min_inner_cross {
            let remaining = container_min_inner_cross - lines_total_cross;
            let addition = remaining / flex_lines.len() as f32;
            flex_lines.iter_mut().for_each(|line| line.cross_size += addition);
        }
    }
}

/// Determine the used cross size of each flex item.
///
/// # [9.4. Cross Size Determination](https://www.w3.org/TR/css-flexbox-1/#cross-sizing)
///
/// - [**Determine the used cross size of each flex item**](https://www.w3.org/TR/css-flexbox-1/#algo-stretch). If a flex item has align-self: stretch, its computed cross size property is auto,
///   and neither of its cross-axis margins are auto, the used outer cross size is the used cross size of its flex line, clamped according to the item’s used min and max cross sizes.
///   Otherwise, the used cross size is the item’s hypothetical cross size.
///
///   If the flex item has align-self: stretch, redo layout for its contents, treating this used size as its definite cross size so that percentage-sized children can be resolved.
///
///   **Note that this step does not affect the main size of the flex item, even if it has an intrinsic aspect ratio**.
#[inline]
fn determine_used_cross_size(
    tree: &impl LayoutFlexboxContainer,
    flex_lines: &mut [FlexLine],
    constants: &AlgoConstants,
) {
    for line in flex_lines {
        let line_cross_size = line.cross_size;

        for child in line.items.iter_mut() {
            let child_style = tree.get_flexbox_child_style(child.node);
            child.target_size.set_cross(
                constants.dir,
                if child.align_self == AlignSelf::Stretch
                    && !child.margin_is_auto.cross_start(constants.dir)
                    && !child.margin_is_auto.cross_end(constants.dir)
                    && child_style.size().cross(constants.dir).is_auto()
                {
                    // For some reason this particular usage of max_width is an exception to the rule that max_width's transfer
                    // using the aspect_ratio (if set). Both Chrome and Firefox agree on this. And reading the spec, it seems like
                    // a reasonable interpretation. Although it seems to me that the spec *should* apply aspect_ratio here.
                    let padding = child_style
                        .padding()
                        .resolve_or_zero(constants.node_inner_size, |val, basis| tree.calc(val, basis));
                    let border = child_style
                        .border()
                        .resolve_or_zero(constants.node_inner_size, |val, basis| tree.calc(val, basis));
                    let pb_sum = (padding + border).sum_axes();
                    let box_sizing_adjustment =
                        if child_style.box_sizing() == BoxSizing::ContentBox { pb_sum } else { Size::ZERO };

                    let max_size_ignoring_aspect_ratio = child_style
                        .max_size()
                        .maybe_resolve(constants.node_inner_size, |val, basis| tree.calc(val, basis))
                        .maybe_add(box_sizing_adjustment);

                    (line_cross_size - child.margin.cross_axis_sum(constants.dir)).maybe_clamp(
                        child.min_size.cross(constants.dir),
                        max_size_ignoring_aspect_ratio.cross(constants.dir),
                    )
                } else {
                    child.hypothetical_inner_size.cross(constants.dir)
                },
            );

            child.outer_target_size.set_cross(
                constants.dir,
                child.target_size.cross(constants.dir) + child.margin.cross_axis_sum(constants.dir),
            );
        }
    }
}

/// Distribute any remaining free space.
///
/// # [9.5. Main-Axis Alignment](https://www.w3.org/TR/css-flexbox-1/#main-alignment)
///
/// - [**Distribute any remaining free space**](https://www.w3.org/TR/css-flexbox-1/#algo-main-align). For each flex line:
///
///   1. If the remaining free space is positive and at least one main-axis margin on this line is `auto`, distribute the free space equally among these margins.
///      Otherwise, set all `auto` margins to zero.
///
///   2. Align the items along the main-axis per `justify-content`.
#[inline]
fn distribute_remaining_free_space(flex_lines: &mut [FlexLine], constants: &AlgoConstants) {
    for line in flex_lines {
        let total_main_axis_gap = sum_axis_gaps(constants.gap.main(constants.dir), line.items.len());
        let used_space: f32 = total_main_axis_gap
            + line.items.iter().map(|child| child.outer_target_size.main(constants.dir)).sum::<f32>();
        let free_space = constants.inner_container_size.main(constants.dir) - used_space;
        let mut num_auto_margins = 0;

        for child in line.items.iter_mut() {
            if child.margin_is_auto.main_start(constants.dir) {
                num_auto_margins += 1;
            }
            if child.margin_is_auto.main_end(constants.dir) {
                num_auto_margins += 1;
            }
        }

        if free_space > 0.0 && num_auto_margins > 0 {
            let margin = free_space / num_auto_margins as f32;

            for child in line.items.iter_mut() {
                if child.margin_is_auto.main_start(constants.dir) {
                    if constants.is_row {
                        child.margin.left = margin;
                    } else {
                        child.margin.top = margin;
                    }
                }
                if child.margin_is_auto.main_end(constants.dir) {
                    if constants.is_row {
                        child.margin.right = margin;
                    } else {
                        child.margin.bottom = margin;
                    }
                }
            }
        } else {
            let num_items = line.items.len();
            let layout_reverse = constants.dir.is_reverse();
            let gap = constants.gap.main(constants.dir);
            let is_safe = false; // TODO: Implement safe alignment
            let raw_justify_content_mode = constants.justify_content.unwrap_or(JustifyContent::FlexStart);
            let justify_content_mode =
                apply_alignment_fallback(free_space, num_items, raw_justify_content_mode, is_safe);

            let justify_item = |(i, child): (usize, &mut FlexItem)| {
                child.offset_main =
                    compute_alignment_offset(free_space, num_items, gap, justify_content_mode, layout_reverse, i == 0);
            };

            if layout_reverse {
                line.items.iter_mut().rev().enumerate().for_each(justify_item);
            } else {
                line.items.iter_mut().enumerate().for_each(justify_item);
            }
        }
    }
}

/// Resolve cross-axis `auto` margins.
///
/// # [9.6. Cross-Axis Alignment](https://www.w3.org/TR/css-flexbox-1/#cross-alignment)
///
/// - [**Resolve cross-axis `auto` margins**](https://www.w3.org/TR/css-flexbox-1/#algo-cross-margins).
///   If a flex item has auto cross-axis margins:
///
///   - If its outer cross size (treating those auto margins as zero) is less than the cross size of its flex line,
///     distribute the difference in those sizes equally to the auto margins.
///
///   - Otherwise, if the block-start or inline-start margin (whichever is in the cross axis) is auto, set it to zero.
///     Set the opposite margin so that the outer cross size of the item equals the cross size of its flex line.
#[inline]
fn resolve_cross_axis_auto_margins(flex_lines: &mut [FlexLine], constants: &AlgoConstants) {
    for line in flex_lines {
        let line_cross_size = line.cross_size;
        let max_baseline: f32 = line.items.iter_mut().map(|child| child.baseline).fold(0.0, |acc, x| acc.max(x));

        for child in line.items.iter_mut() {
            let free_space = line_cross_size - child.outer_target_size.cross(constants.dir);

            if child.margin_is_auto.cross_start(constants.dir) && child.margin_is_auto.cross_end(constants.dir) {
                if constants.is_row {
                    child.margin.top = free_space / 2.0;
                    child.margin.bottom = free_space / 2.0;
                } else {
                    child.margin.left = free_space / 2.0;
                    child.margin.right = free_space / 2.0;
                }
            } else if child.margin_is_auto.cross_start(constants.dir) {
                if constants.is_row {
                    child.margin.top = free_space;
                } else {
                    child.margin.left = free_space;
                }
            } else if child.margin_is_auto.cross_end(constants.dir) {
                if constants.is_row {
                    child.margin.bottom = free_space;
                } else {
                    child.margin.right = free_space;
                }
            } else {
                // 14. Align all flex items along the cross-axis.
                child.offset_cross = align_flex_items_along_cross_axis(child, free_space, max_baseline, constants);
            }
        }
    }
}

/// Align all flex items along the cross-axis.
///
/// # [9.6. Cross-Axis Alignment](https://www.w3.org/TR/css-flexbox-1/#cross-alignment)
///
/// - [**Align all flex items along the cross-axis**](https://www.w3.org/TR/css-flexbox-1/#algo-cross-align) per `align-self`,
///   if neither of the item's cross-axis margins are `auto`.
#[inline]
fn align_flex_items_along_cross_axis(
    child: &FlexItem,
    free_space: f32,
    max_baseline: f32,
    constants: &AlgoConstants,
) -> f32 {
    match child.align_self {
        AlignSelf::Start => 0.0,
        AlignSelf::FlexStart => {
            if constants.is_wrap_reverse {
                free_space
            } else {
                0.0
            }
        }
        AlignSelf::End => free_space,
        AlignSelf::FlexEnd => {
            if constants.is_wrap_reverse {
                0.0
            } else {
                free_space
            }
        }
        AlignSelf::Center => free_space / 2.0,
        AlignSelf::Baseline => {
            if constants.is_row {
                max_baseline - child.baseline
            } else {
                // Until we support vertical writing modes, baseline alignment only makes sense if
                // the constants.direction is row, so we treat it as flex-start alignment in columns.
                if constants.is_wrap_reverse {
                    free_space
                } else {
                    0.0
                }
            }
        }
        AlignSelf::Stretch => {
            if constants.is_wrap_reverse {
                free_space
            } else {
                0.0
            }
        }
    }
}

/// Determine the flex container’s used cross size.
///
/// # [9.6. Cross-Axis Alignment](https://www.w3.org/TR/css-flexbox-1/#cross-alignment)
///
/// - [**Determine the flex container’s used cross size**](https://www.w3.org/TR/css-flexbox-1/#algo-cross-container):
///
///     - If the cross size property is a definite size, use that, clamped by the used min and max cross sizes of the flex container.
///
///     - Otherwise, use the sum of the flex lines' cross sizes, clamped by the used min and max cross sizes of the flex container.
#[inline]
#[must_use]
fn determine_container_cross_size(
    flex_lines: &[FlexLine],
    node_size: Size<Option<f32>>,
    constants: &mut AlgoConstants,
) -> f32 {
    let total_cross_axis_gap = sum_axis_gaps(constants.gap.cross(constants.dir), flex_lines.len());
    let total_line_cross_size: f32 = flex_lines.iter().map(|line| line.cross_size).sum::<f32>();

    let padding_border_sum = constants.content_box_inset.cross_axis_sum(constants.dir);
    let cross_scrollbar_gutter = constants.scrollbar_gutter.cross(constants.dir);
    let min_cross_size = constants.min_size.cross(constants.dir);
    let max_cross_size = constants.max_size.cross(constants.dir);
    let outer_container_size = node_size
        .cross(constants.dir)
        .unwrap_or(total_line_cross_size + total_cross_axis_gap + padding_border_sum)
        .maybe_clamp(min_cross_size, max_cross_size)
        .max(padding_border_sum - cross_scrollbar_gutter);
    let inner_container_size = f32_max(outer_container_size - padding_border_sum, 0.0);

    constants.container_size.set_cross(constants.dir, outer_container_size);
    constants.inner_container_size.set_cross(constants.dir, inner_container_size);

    total_line_cross_size
}

/// Align all flex lines per `align-content`.
///
/// # [9.6. Cross-Axis Alignment](https://www.w3.org/TR/css-flexbox-1/#cross-alignment)
///
/// - [**Align all flex lines**](https://www.w3.org/TR/css-flexbox-1/#algo-line-align) per `align-content`.
#[inline]
fn align_flex_lines_per_align_content(flex_lines: &mut [FlexLine], constants: &AlgoConstants, total_cross_size: f32) {
    let num_lines = flex_lines.len();
    let gap = constants.gap.cross(constants.dir);
    let total_cross_axis_gap = sum_axis_gaps(gap, num_lines);
    let free_space = constants.inner_container_size.cross(constants.dir) - total_cross_size - total_cross_axis_gap;
    let is_safe = false; // TODO: Implement safe alignment

    let align_content_mode = apply_alignment_fallback(free_space, num_lines, constants.align_content, is_safe);

    let align_line = |(i, line): (usize, &mut FlexLine)| {
        line.offset_cross =
            compute_alignment_offset(free_space, num_lines, gap, align_content_mode, constants.is_wrap_reverse, i == 0);
    };

    if constants.is_wrap_reverse {
        flex_lines.iter_mut().rev().enumerate().for_each(align_line);
    } else {
        flex_lines.iter_mut().enumerate().for_each(align_line);
    }
}

/// Calculates the layout for a flex-item
#[allow(clippy::too_many_arguments)]
fn calculate_flex_item(
    tree: &mut impl LayoutFlexboxContainer,
    item: &mut FlexItem,
    total_offset_main: &mut f32,
    total_offset_cross: f32,
    line_offset_cross: f32,
    #[cfg(feature = "content_size")] total_content_size: &mut Size<f32>,
    container_size: Size<f32>,
    node_inner_size: Size<Option<f32>>,
    direction: FlexDirection,
) {
    let layout_output = tree.perform_child_layout(
        item.node,
        item.target_size.map(|s| s.into()),
        node_inner_size,
        container_size.map(|s| s.into()),
        SizingMode::ContentSize,
        Line::FALSE,
    );
    let LayoutOutput {
        size,
        #[cfg(feature = "content_size")]
        content_size,
        ..
    } = layout_output;

    let offset_main = *total_offset_main
        + item.offset_main
        + item.margin.main_start(direction)
        + (item.inset.main_start(direction).or(item.inset.main_end(direction).map(|pos| -pos)).unwrap_or(0.0));

    let offset_cross = total_offset_cross
        + item.offset_cross
        + line_offset_cross
        + item.margin.cross_start(direction)
        + (item.inset.cross_start(direction).or(item.inset.cross_end(direction).map(|pos| -pos)).unwrap_or(0.0));

    if direction.is_row() {
        let baseline_offset_cross = total_offset_cross + item.offset_cross + item.margin.cross_start(direction);
        let inner_baseline = layout_output.first_baselines.y.unwrap_or(size.height);
        item.baseline = baseline_offset_cross + inner_baseline;
    } else {
        let baseline_offset_main = *total_offset_main + item.offset_main + item.margin.main_start(direction);
        let inner_baseline = layout_output.first_baselines.y.unwrap_or(size.height);
        item.baseline = baseline_offset_main + inner_baseline;
    }

    let location = match direction.is_row() {
        true => Point { x: offset_main, y: offset_cross },
        false => Point { x: offset_cross, y: offset_main },
    };
    let scrollbar_size = Size {
        width: if item.overflow.y == Overflow::Scroll { item.scrollbar_width } else { 0.0 },
        height: if item.overflow.x == Overflow::Scroll { item.scrollbar_width } else { 0.0 },
    };

    tree.set_unrounded_layout(
        item.node,
        &Layout {
            order: item.order,
            size,
            #[cfg(feature = "content_size")]
            content_size,
            scrollbar_size,
            location,
            padding: item.padding,
            border: item.border,
            margin: item.margin,
        },
    );

    *total_offset_main += item.offset_main + item.margin.main_axis_sum(direction) + size.main(direction);

    #[cfg(feature = "content_size")]
    {
        *total_content_size =
            total_content_size.f32_max(compute_content_size_contribution(location, size, content_size, item.overflow));
    }
}

/// Calculates the layout line
#[allow(clippy::too_many_arguments)]
fn calculate_layout_line(
    tree: &mut impl LayoutFlexboxContainer,
    line: &mut FlexLine,
    total_offset_cross: &mut f32,
    #[cfg(feature = "content_size")] content_size: &mut Size<f32>,
    container_size: Size<f32>,
    node_inner_size: Size<Option<f32>>,
    padding_border: Rect<f32>,
    direction: FlexDirection,
) {
    let mut total_offset_main = padding_border.main_start(direction);
    let line_offset_cross = line.offset_cross;

    if direction.is_reverse() {
        for item in line.items.iter_mut().rev() {
            calculate_flex_item(
                tree,
                item,
                &mut total_offset_main,
                *total_offset_cross,
                line_offset_cross,
                #[cfg(feature = "content_size")]
                content_size,
                container_size,
                node_inner_size,
                direction,
            );
        }
    } else {
        for item in line.items.iter_mut() {
            calculate_flex_item(
                tree,
                item,
                &mut total_offset_main,
                *total_offset_cross,
                line_offset_cross,
                #[cfg(feature = "content_size")]
                content_size,
                container_size,
                node_inner_size,
                direction,
            );
        }
    }

    *total_offset_cross += line_offset_cross + line.cross_size;
}

/// Do a final layout pass and collect the resulting layouts.
#[inline]
fn final_layout_pass(
    tree: &mut impl LayoutFlexboxContainer,
    flex_lines: &mut [FlexLine],
    constants: &AlgoConstants,
) -> Size<f32> {
    let mut total_offset_cross = constants.content_box_inset.cross_start(constants.dir);

    #[cfg_attr(not(feature = "content_size"), allow(unused_mut))]
    let mut content_size = Size::ZERO;

    if constants.is_wrap_reverse {
        for line in flex_lines.iter_mut().rev() {
            calculate_layout_line(
                tree,
                line,
                &mut total_offset_cross,
                #[cfg(feature = "content_size")]
                &mut content_size,
                constants.container_size,
                constants.node_inner_size,
                constants.content_box_inset,
                constants.dir,
            );
        }
    } else {
        for line in flex_lines.iter_mut() {
            calculate_layout_line(
                tree,
                line,
                &mut total_offset_cross,
                #[cfg(feature = "content_size")]
                &mut content_size,
                constants.container_size,
                constants.node_inner_size,
                constants.content_box_inset,
                constants.dir,
            );
        }
    }

    content_size.width += constants.content_box_inset.right - constants.border.right - constants.scrollbar_gutter.x;
    content_size.height += constants.content_box_inset.bottom - constants.border.bottom - constants.scrollbar_gutter.y;

    content_size
}

/// Perform absolute layout on all absolutely positioned children.
#[inline]
fn perform_absolute_layout_on_absolute_children(
    tree: &mut impl LayoutFlexboxContainer,
    node: NodeId,
    constants: &AlgoConstants,
) -> Size<f32> {
    let container_width = constants.container_size.width;
    let container_height = constants.container_size.height;
    let inset_relative_size =
        constants.container_size - constants.border.sum_axes() - constants.scrollbar_gutter.into();

    #[cfg_attr(not(feature = "content_size"), allow(unused_mut))]
    let mut content_size = Size::ZERO;

    for order in 0..tree.child_count(node) {
        let child = tree.get_child_id(node, order);
        let child_style = tree.get_flexbox_child_style(child);

        // Skip items that are display:none or are not position:absolute
        if child_style.box_generation_mode() == BoxGenerationMode::None || child_style.position() != Position::Absolute
        {
            continue;
        }

        let overflow = child_style.overflow();
        let scrollbar_width = child_style.scrollbar_width();
        let aspect_ratio = child_style.aspect_ratio();
        let align_self = child_style.align_self().unwrap_or(constants.align_items);
        let margin = child_style
            .margin()
            .map(|margin| margin.resolve_to_option(inset_relative_size.width, |val, basis| tree.calc(val, basis)));
        let padding =
            child_style.padding().resolve_or_zero(Some(inset_relative_size.width), |val, basis| tree.calc(val, basis));
        let border =
            child_style.border().resolve_or_zero(Some(inset_relative_size.width), |val, basis| tree.calc(val, basis));
        let padding_border_sum = (padding + border).sum_axes();
        let box_sizing_adjustment =
            if child_style.box_sizing() == BoxSizing::ContentBox { padding_border_sum } else { Size::ZERO };

        // Resolve inset
        // Insets are resolved against the container size minus border
        let left =
            child_style.inset().left.maybe_resolve(inset_relative_size.width, |val, basis| tree.calc(val, basis));
        let right =
            child_style.inset().right.maybe_resolve(inset_relative_size.width, |val, basis| tree.calc(val, basis));
        let top = child_style.inset().top.maybe_resolve(inset_relative_size.height, |val, basis| tree.calc(val, basis));
        let bottom =
            child_style.inset().bottom.maybe_resolve(inset_relative_size.height, |val, basis| tree.calc(val, basis));

        // Compute known dimensions from min/max/inherent size styles
        let style_size = child_style
            .size()
            .maybe_resolve(inset_relative_size, |val, basis| tree.calc(val, basis))
            .maybe_apply_aspect_ratio(aspect_ratio)
            .maybe_add(box_sizing_adjustment);
        let min_size = child_style
            .min_size()
            .maybe_resolve(inset_relative_size, |val, basis| tree.calc(val, basis))
            .maybe_apply_aspect_ratio(aspect_ratio)
            .maybe_add(box_sizing_adjustment)
            .or(padding_border_sum.map(Some))
            .maybe_max(padding_border_sum);
        let max_size = child_style
            .max_size()
            .maybe_resolve(inset_relative_size, |val, basis| tree.calc(val, basis))
            .maybe_apply_aspect_ratio(aspect_ratio)
            .maybe_add(box_sizing_adjustment);
        let mut known_dimensions = style_size.maybe_clamp(min_size, max_size);

        drop(child_style);

        // Fill in width from left/right and reapply aspect ratio if:
        //   - Width is not already known
        //   - Item has both left and right inset properties set
        if let (None, Some(left), Some(right)) = (known_dimensions.width, left, right) {
            let new_width_raw = inset_relative_size.width.maybe_sub(margin.left).maybe_sub(margin.right) - left - right;
            known_dimensions.width = Some(f32_max(new_width_raw, 0.0));
            known_dimensions = known_dimensions.maybe_apply_aspect_ratio(aspect_ratio).maybe_clamp(min_size, max_size);
        }

        // Fill in height from top/bottom and reapply aspect ratio if:
        //   - Height is not already known
        //   - Item has both top and bottom inset properties set
        if let (None, Some(top), Some(bottom)) = (known_dimensions.height, top, bottom) {
            let new_height_raw =
                inset_relative_size.height.maybe_sub(margin.top).maybe_sub(margin.bottom) - top - bottom;
            known_dimensions.height = Some(f32_max(new_height_raw, 0.0));
            known_dimensions = known_dimensions.maybe_apply_aspect_ratio(aspect_ratio).maybe_clamp(min_size, max_size);
        }
        let layout_output = tree.perform_child_layout(
            child,
            known_dimensions,
            constants.node_inner_size,
            Size {
                width: AvailableSpace::Definite(container_width.maybe_clamp(min_size.width, max_size.width)),
                height: AvailableSpace::Definite(container_height.maybe_clamp(min_size.height, max_size.height)),
            },
            SizingMode::InherentSize,
            Line::FALSE,
        );
        let measured_size = layout_output.size;
        let final_size = known_dimensions.unwrap_or(measured_size).maybe_clamp(min_size, max_size);

        let non_auto_margin = margin.map(|m| m.unwrap_or(0.0));

        let free_space = Size {
            width: constants.container_size.width - final_size.width - non_auto_margin.horizontal_axis_sum(),
            height: constants.container_size.height - final_size.height - non_auto_margin.vertical_axis_sum(),
        }
        .f32_max(Size::ZERO);

        // Expand auto margins to fill available space
        let resolved_margin = {
            let auto_margin_size = Size {
                width: {
                    let auto_margin_count = margin.left.is_none() as u8 + margin.right.is_none() as u8;
                    if auto_margin_count > 0 {
                        free_space.width / auto_margin_count as f32
                    } else {
                        0.0
                    }
                },
                height: {
                    let auto_margin_count = margin.top.is_none() as u8 + margin.bottom.is_none() as u8;
                    if auto_margin_count > 0 {
                        free_space.height / auto_margin_count as f32
                    } else {
                        0.0
                    }
                },
            };

            Rect {
                left: margin.left.unwrap_or(auto_margin_size.width),
                right: margin.right.unwrap_or(auto_margin_size.width),
                top: margin.top.unwrap_or(auto_margin_size.height),
                bottom: margin.bottom.unwrap_or(auto_margin_size.height),
            }
        };

        // Determine flex-relative insets
        let (start_main, end_main) = if constants.is_row { (left, right) } else { (top, bottom) };
        let (start_cross, end_cross) = if constants.is_row { (top, bottom) } else { (left, right) };

        // Apply main-axis alignment
        // let free_main_space = free_space.main(constants.dir) - resolved_margin.main_axis_sum(constants.dir);
        let offset_main = if let Some(start) = start_main {
            start + constants.border.main_start(constants.dir) + resolved_margin.main_start(constants.dir)
        } else if let Some(end) = end_main {
            constants.container_size.main(constants.dir)
                - constants.border.main_end(constants.dir)
                - constants.scrollbar_gutter.main(constants.dir)
                - final_size.main(constants.dir)
                - end
                - resolved_margin.main_end(constants.dir)
        } else {
            // Stretch is an invalid value for justify_content in the flexbox algorithm, so we
            // treat it as if it wasn't set (and thus we default to FlexStart behaviour)
            match (constants.justify_content.unwrap_or(JustifyContent::Start), constants.is_wrap_reverse) {
                (JustifyContent::SpaceBetween, _)
                | (JustifyContent::Start, _)
                | (JustifyContent::Stretch, false)
                | (JustifyContent::FlexStart, false)
                | (JustifyContent::FlexEnd, true) => {
                    constants.content_box_inset.main_start(constants.dir) + resolved_margin.main_start(constants.dir)
                }
                (JustifyContent::End, _)
                | (JustifyContent::FlexEnd, false)
                | (JustifyContent::FlexStart, true)
                | (JustifyContent::Stretch, true) => {
                    constants.container_size.main(constants.dir)
                        - constants.content_box_inset.main_end(constants.dir)
                        - final_size.main(constants.dir)
                        - resolved_margin.main_end(constants.dir)
                }
                (JustifyContent::SpaceEvenly, _) | (JustifyContent::SpaceAround, _) | (JustifyContent::Center, _) => {
                    (constants.container_size.main(constants.dir)
                        + constants.content_box_inset.main_start(constants.dir)
                        - constants.content_box_inset.main_end(constants.dir)
                        - final_size.main(constants.dir)
                        + resolved_margin.main_start(constants.dir)
                        - resolved_margin.main_end(constants.dir))
                        / 2.0
                }
            }
        };

        // Apply cross-axis alignment
        // let free_cross_space = free_space.cross(constants.dir) - resolved_margin.cross_axis_sum(constants.dir);
        let offset_cross = if let Some(start) = start_cross {
            start + constants.border.cross_start(constants.dir) + resolved_margin.cross_start(constants.dir)
        } else if let Some(end) = end_cross {
            constants.container_size.cross(constants.dir)
                - constants.border.cross_end(constants.dir)
                - constants.scrollbar_gutter.cross(constants.dir)
                - final_size.cross(constants.dir)
                - end
                - resolved_margin.cross_end(constants.dir)
        } else {
            match (align_self, constants.is_wrap_reverse) {
                // Stretch alignment does not apply to absolutely positioned items
                // See "Example 3" at https://www.w3.org/TR/css-flexbox-1/#abspos-items
                // Note: Stretch should be FlexStart not Start when we support both
                (AlignSelf::Start, _)
                | (AlignSelf::Baseline | AlignSelf::Stretch | AlignSelf::FlexStart, false)
                | (AlignSelf::FlexEnd, true) => {
                    constants.content_box_inset.cross_start(constants.dir) + resolved_margin.cross_start(constants.dir)
                }
                (AlignSelf::End, _)
                | (AlignSelf::Baseline | AlignSelf::Stretch | AlignSelf::FlexStart, true)
                | (AlignSelf::FlexEnd, false) => {
                    constants.container_size.cross(constants.dir)
                        - constants.content_box_inset.cross_end(constants.dir)
                        - final_size.cross(constants.dir)
                        - resolved_margin.cross_end(constants.dir)
                }
                (AlignSelf::Center, _) => {
                    (constants.container_size.cross(constants.dir)
                        + constants.content_box_inset.cross_start(constants.dir)
                        - constants.content_box_inset.cross_end(constants.dir)
                        - final_size.cross(constants.dir)
                        + resolved_margin.cross_start(constants.dir)
                        - resolved_margin.cross_end(constants.dir))
                        / 2.0
                }
            }
        };

        let location = match constants.is_row {
            true => Point { x: offset_main, y: offset_cross },
            false => Point { x: offset_cross, y: offset_main },
        };
        let scrollbar_size = Size {
            width: if overflow.y == Overflow::Scroll { scrollbar_width } else { 0.0 },
            height: if overflow.x == Overflow::Scroll { scrollbar_width } else { 0.0 },
        };
        tree.set_unrounded_layout(
            child,
            &Layout {
                order: order as u32,
                size: final_size,
                #[cfg(feature = "content_size")]
                content_size: layout_output.content_size,
                scrollbar_size,
                location,
                padding,
                border,
                margin: resolved_margin,
            },
        );

        #[cfg(feature = "content_size")]
        {
            let size_content_size_contribution = Size {
                width: match overflow.x {
                    Overflow::Visible => f32_max(final_size.width, layout_output.content_size.width),
                    _ => final_size.width,
                },
                height: match overflow.y {
                    Overflow::Visible => f32_max(final_size.height, layout_output.content_size.height),
                    _ => final_size.height,
                },
            };
            if size_content_size_contribution.has_non_zero_area() {
                let content_size_contribution = Size {
                    width: location.x + size_content_size_contribution.width,
                    height: location.y + size_content_size_contribution.height,
                };
                content_size = content_size.f32_max(content_size_contribution);
            }
        }
    }

    content_size
}

/// Computes the total space taken up by gaps in an axis given:
///   - The size of each gap
///   - The number of items (children or flex-lines) between which there are gaps
#[inline(always)]
fn sum_axis_gaps(gap: f32, num_items: usize) -> f32 {
    // Gaps only exist between items, so...
    if num_items <= 1 {
        // ...if there are less than 2 items then there are no gaps
        0.0
    } else {
        // ...otherwise there are (num_items - 1) gaps
        gap * (num_items - 1) as f32
    }
}