layout/flow/inline/

line.rs

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

use std::ops::Range;
use std::sync::Arc;

use app_units::Au;
use bitflags::bitflags;
use fonts::ShapedTextSlice;
use itertools::Either;
use layout_api::SharedSelection;
use malloc_size_of_derive::MallocSizeOf;
use style::Zero;
use style::computed_values::position::T as Position;
use style::computed_values::white_space_collapse::T as WhiteSpaceCollapse;
use style::values::computed::BaselineShift;
use style::values::generics::box_::BaselineShiftKeyword;
use style::values::specified::align::AlignFlags;
use style::values::specified::box_::DisplayOutside;
use unicode_bidi::{BidiInfo, Level};

use super::inline_box::{InlineBoxContainerState, InlineBoxIdentifier, InlineBoxTreePathToken};
use super::{InlineFormattingContextLayout, LineBlockSizes, SharedInlineStyles, line_height};
use crate::cell::ArcRefCell;
use crate::flow::inline::text_run::FontAndScriptInfo;
use crate::fragment_tree::{BaseFragment, BaseFragmentInfo, BoxFragment, Fragment, TextFragment};
use crate::geom::{
    LogicalRect, LogicalVec2, PhysicalRect, ToLogical, ToLogicalWithContainingBlock,
};
use crate::positioned::{
    AbsolutelyPositionedBox, PositioningContext, PositioningContextLength, relative_adjustement,
};
use crate::{ContainingBlock, ContainingBlockSize};

pub(super) struct LineMetrics {
    /// The block offset of the line start in the containing
    /// [`crate::flow::InlineFormattingContext`].
    pub block_offset: Au,

    /// The block size of this line.
    pub block_size: Au,

    /// The block offset of this line's baseline from [`Self::block_offset`].
    pub baseline_block_offset: Au,
}

bitflags! {
    struct LineLayoutInlineContainerFlags: u8 {
        /// Whether or not any line items were processed for this inline box, this includes
        /// any child inline boxes.
        const HAD_ANY_LINE_ITEMS = 1 << 0;
        /// Whether or not the starting inline border, padding, or margin of the inline box
        /// was encountered.
        const HAD_INLINE_START_PBM = 1 << 2;
        /// Whether or not the ending inline border, padding, or margin of the inline box
        /// was encountered.
        const HAD_INLINE_END_PBM = 1 << 3;
        /// Whether or not any floats were encountered while laying out this inline box.
        const HAD_ANY_FLOATS = 1 << 4;
    }
}

/// The state used when laying out a collection of [`LineItem`]s into a line. This state is stored
/// per-inline container. For instance, when laying out the conents of a `<span>` a fresh
/// [`LineItemLayoutInlineContainerState`] is pushed onto [`LineItemLayout`]'s stack of states.
pub(super) struct LineItemLayoutInlineContainerState {
    /// If this inline container is not the root inline container, the identifier of the [`super::InlineBox`]
    /// that is currently being laid out.
    pub identifier: Option<InlineBoxIdentifier>,

    /// The fragments and their logical rectangle relative within the current inline box (or
    /// line). These logical rectangles will be converted into physical ones and the Fragment's
    /// `content_rect` will be updated once the inline box's final size is known in
    /// [`LineItemLayout::end_inline_box`].
    pub fragments: Vec<(Fragment, LogicalRect<Au>)>,

    /// The current inline advance of the layout in the coordinates of this inline box.
    pub inline_advance: Au,

    /// Flags which track various features during layout.
    flags: LineLayoutInlineContainerFlags,

    /// The offset of the parent, relative to the start position of the line, not including
    /// any inline start and end borders which are only processed when the inline box is
    /// finished.
    pub parent_offset: LogicalVec2<Au>,

    /// The block offset of the parent's baseline relative to the block start of the line. This
    /// is often the same as [`Self::parent_offset`], but can be different for the root
    /// element.
    pub baseline_offset: Au,

    /// If this inline box establishes a containing block for positioned elements, this
    /// is a fresh positioning context to contain them. Otherwise, this holds the starting
    /// offset in the *parent* positioning context so that static positions can be updated
    /// at the end of layout.
    pub positioning_context_or_start_offset_in_parent:
        Either<PositioningContext, PositioningContextLength>,
}

impl LineItemLayoutInlineContainerState {
    fn new(
        identifier: Option<InlineBoxIdentifier>,
        parent_offset: LogicalVec2<Au>,
        baseline_offset: Au,
        positioning_context_or_start_offset_in_parent: Either<
            PositioningContext,
            PositioningContextLength,
        >,
    ) -> Self {
        Self {
            identifier,
            fragments: Vec::new(),
            inline_advance: Au::zero(),
            flags: LineLayoutInlineContainerFlags::empty(),
            parent_offset,
            baseline_offset,
            positioning_context_or_start_offset_in_parent,
        }
    }

    fn root(starting_inline_advance: Au, baseline_offset: Au) -> Self {
        let mut state = Self::new(
            None,
            LogicalVec2::zero(),
            baseline_offset,
            Either::Right(PositioningContextLength::zero()),
        );
        state.inline_advance = starting_inline_advance;
        state
    }
}

/// The second phase of [`super::InlineFormattingContext`] layout: once items are gathered
/// for a line, we must lay them out and create fragments for them, properly positioning them
/// according to their baselines and also handling absolutely positioned children.
pub(super) struct LineItemLayout<'layout_data, 'layout> {
    /// The state of the overall [`super::InlineFormattingContext`] layout.
    layout: &'layout mut InlineFormattingContextLayout<'layout_data>,

    /// The set of [`LineItemLayoutInlineContainerState`] created while laying out items
    /// on this line. This does not include the current level of recursion.
    pub state_stack: Vec<LineItemLayoutInlineContainerState>,

    /// The current [`LineItemLayoutInlineContainerState`].
    pub current_state: LineItemLayoutInlineContainerState,

    /// The metrics of this line, which should remain constant throughout the
    /// layout process.
    pub line_metrics: LineMetrics,

    /// The amount of space to add to each justification opportunity in order to implement
    /// `text-align: justify`.
    pub justification_adjustment: Au,

    /// Whether this is a phantom line box.
    /// <https://drafts.csswg.org/css-inline-3/#invisible-line-boxes>
    is_phantom_line: bool,
}

impl LineItemLayout<'_, '_> {
    pub(super) fn layout_line_items(
        layout: &mut InlineFormattingContextLayout,
        line_items: Vec<LineItem>,
        start_position: LogicalVec2<Au>,
        effective_block_advance: &LineBlockSizes,
        justification_adjustment: Au,
        is_phantom_line: bool,
    ) -> Vec<Fragment> {
        let baseline_offset = effective_block_advance.find_baseline_offset();
        LineItemLayout {
            layout,
            state_stack: Vec::new(),
            current_state: LineItemLayoutInlineContainerState::root(
                start_position.inline,
                baseline_offset,
            ),
            line_metrics: LineMetrics {
                block_offset: start_position.block,
                block_size: effective_block_advance.resolve(),
                baseline_block_offset: baseline_offset,
            },
            justification_adjustment,
            is_phantom_line,
        }
        .layout(line_items)
    }

    /// Start and end inline boxes in tree order, so that it reflects the given inline box.
    fn prepare_layout_for_inline_box(&mut self, new_inline_box: Option<InlineBoxIdentifier>) {
        // Optimize the case where we are moving to the root of the inline box stack.
        let Some(new_inline_box) = new_inline_box else {
            while !self.state_stack.is_empty() {
                self.end_inline_box();
            }
            return;
        };

        // Otherwise, follow the path given to us by our collection of inline boxes, so we know which
        // inline boxes to start and end.
        let path = self
            .layout
            .ifc
            .inline_boxes
            .get_path(self.current_state.identifier, new_inline_box);
        for token in path {
            match token {
                InlineBoxTreePathToken::Start(ref identifier) => self.start_inline_box(identifier),
                InlineBoxTreePathToken::End(_) => self.end_inline_box(),
            }
        }
    }

    pub(super) fn layout(&mut self, mut line_items: Vec<LineItem>) -> Vec<Fragment> {
        let mut last_level = Level::ltr();
        let levels: Vec<_> = line_items
            .iter()
            .map(|item| {
                let level = match item {
                    LineItem::TextRun(_, text_run) => text_run.info.bidi_level,
                    // TODO: This level needs either to be last_level, or if there were
                    // unicode characters inserted for the inline box, we need to get the
                    // level from them.
                    LineItem::InlineStartBoxPaddingBorderMargin(_) => last_level,
                    LineItem::InlineEndBoxPaddingBorderMargin(_) => last_level,
                    LineItem::Atomic(_, atomic) => atomic.bidi_level,
                    LineItem::AbsolutelyPositioned(..) => last_level,
                    LineItem::Float(..) => {
                        // At this point the float is already positioned, so it doesn't really matter what
                        // position it's fragment has in the order of line items.
                        last_level
                    },
                    LineItem::BlockLevel(..) => last_level,
                    LineItem::Tab { bidi_level, .. } => *bidi_level,
                };
                last_level = level;
                level
            })
            .collect();

        if self.layout.ifc.has_right_to_left_content {
            sort_by_indices_in_place(&mut line_items, BidiInfo::reorder_visual(&levels));
        }

        // `BidiInfo::reorder_visual` will reorder the contents of the line so that they
        // are in the correct order as if one was looking at the line from left-to-right.
        // During this layout we do not lay out from left to right. Instead we lay out
        // from inline-start to inline-end. If the overall line contents have been flipped
        // for BiDi, flip them again so that they are in line start-to-end order rather
        // than left-to-right order.
        let containing_block = self.containing_block();
        let line_item_iterator = if containing_block.style.writing_mode.is_bidi_ltr() {
            Either::Left(line_items.into_iter())
        } else {
            Either::Right(line_items.into_iter().rev())
        };

        for item in line_item_iterator.into_iter().by_ref() {
            // When preparing to lay out a new line item, start and end inline boxes, so that the current
            // inline box state reflects the item's parent. Items in the line are not necessarily in tree
            // order due to BiDi and other reordering so the inline box of the item could potentially be
            // any in the inline formatting context.
            self.prepare_layout_for_inline_box(item.inline_box_identifier());

            self.current_state
                .flags
                .insert(LineLayoutInlineContainerFlags::HAD_ANY_LINE_ITEMS);
            match item {
                LineItem::InlineStartBoxPaddingBorderMargin(_) => {
                    self.current_state
                        .flags
                        .insert(LineLayoutInlineContainerFlags::HAD_INLINE_START_PBM);
                },
                LineItem::InlineEndBoxPaddingBorderMargin(_) => {
                    self.current_state
                        .flags
                        .insert(LineLayoutInlineContainerFlags::HAD_INLINE_END_PBM);
                },
                LineItem::TextRun(_, text_run) => self.layout_text_run(text_run),
                LineItem::Atomic(_, atomic) => self.layout_atomic(atomic),
                LineItem::AbsolutelyPositioned(_, absolute) => self.layout_absolute(absolute),
                LineItem::Float(_, float) => self.layout_float(float),
                LineItem::BlockLevel(_, block_level) => self.layout_block_level(block_level),
                LineItem::Tab { advance, .. } => self.layout_tab(advance),
            }
        }

        // Move back to the root of the inline box tree, so that all boxes are ended.
        self.prepare_layout_for_inline_box(None);

        let fragments_and_rectangles = std::mem::take(&mut self.current_state.fragments);
        let containing_block = self.containing_block();
        fragments_and_rectangles
            .into_iter()
            .map(|(fragment, logical_rect)| {
                if matches!(fragment, Fragment::Float(_)) {
                    return fragment;
                }

                // We do not know the actual physical position of a logically laid out inline element, until
                // we know the width of the containing inline block. This step converts the logical rectangle
                // into a physical one based on the inline formatting context width.
                if let Some(mut base) = fragment.base_mut() {
                    base.rect = logical_rect.as_physical(Some(containing_block));
                }

                fragment
            })
            .collect()
    }

    fn current_positioning_context_mut(&mut self) -> &mut PositioningContext {
        if let Either::Left(ref mut positioning_context) = self
            .current_state
            .positioning_context_or_start_offset_in_parent
        {
            return positioning_context;
        }
        self.state_stack
            .iter_mut()
            .rev()
            .find_map(
                |state| match state.positioning_context_or_start_offset_in_parent {
                    Either::Left(ref mut positioning_context) => Some(positioning_context),
                    Either::Right(_) => None,
                },
            )
            .unwrap_or(self.layout.positioning_context)
    }

    fn start_inline_box(&mut self, identifier: &InlineBoxIdentifier) {
        let inline_box_state =
            &*self.layout.inline_box_states[identifier.index_in_inline_boxes as usize];
        let inline_box = self.layout.ifc.inline_boxes.get(identifier);
        let inline_box = &*(inline_box.borrow());

        let space_above_baseline = inline_box_state.calculate_space_above_baseline();
        let block_start_offset =
            self.calculate_inline_box_block_start(inline_box_state, space_above_baseline);

        let positioning_context_or_start_offset_in_parent =
            match PositioningContext::new_for_layout_box_base(&inline_box.base) {
                Some(positioning_context) => Either::Left(positioning_context),
                None => Either::Right(self.current_positioning_context_mut().len()),
            };

        let parent_offset = LogicalVec2 {
            inline: self.current_state.inline_advance + self.current_state.parent_offset.inline,
            block: block_start_offset,
        };

        let outer_state = std::mem::replace(
            &mut self.current_state,
            LineItemLayoutInlineContainerState::new(
                Some(*identifier),
                parent_offset,
                block_start_offset + space_above_baseline,
                positioning_context_or_start_offset_in_parent,
            ),
        );

        self.state_stack.push(outer_state);
    }

    fn end_inline_box(&mut self) {
        let outer_state = self.state_stack.pop().expect("Ended unknown inline box");
        let inner_state = std::mem::replace(&mut self.current_state, outer_state);

        let identifier = inner_state.identifier.expect("Ended unknown inline box");
        let inline_box_state =
            &*self.layout.inline_box_states[identifier.index_in_inline_boxes as usize];
        let inline_box = self.layout.ifc.inline_boxes.get(&identifier);
        let inline_box = &*(inline_box.borrow());

        let mut had_start = inner_state
            .flags
            .contains(LineLayoutInlineContainerFlags::HAD_INLINE_START_PBM);
        let mut had_end = inner_state
            .flags
            .contains(LineLayoutInlineContainerFlags::HAD_INLINE_END_PBM);

        let containing_block = self.containing_block();
        let containing_block_writing_mode = containing_block.style.writing_mode;
        if containing_block_writing_mode.is_bidi_ltr() !=
            inline_box.base.style.writing_mode.is_bidi_ltr()
        {
            std::mem::swap(&mut had_start, &mut had_end)
        }

        let mut padding = inline_box_state.pbm.padding;
        let mut border = inline_box_state.pbm.border;
        let mut margin = inline_box_state.pbm.margin.auto_is(Au::zero);
        if !had_start {
            padding.inline_start = Au::zero();
            border.inline_start = Au::zero();
            margin.inline_start = Au::zero();
        }
        if !had_end {
            padding.inline_end = Au::zero();
            border.inline_end = Au::zero();
            margin.inline_end = Au::zero();
        }
        let pbm_sums = padding + border + margin;

        // Make `content_rect` relative to the parent Fragment.
        let mut content_rect = LogicalRect {
            start_corner: LogicalVec2 {
                inline: self.current_state.inline_advance + pbm_sums.inline_start,
                block: inner_state.parent_offset.block - self.current_state.parent_offset.block,
            },
            size: LogicalVec2 {
                inline: inner_state.inline_advance,
                block: if self.is_phantom_line {
                    Au::zero()
                } else {
                    inline_box_state.base.font_metrics.line_gap
                },
            },
        };

        // Relative adjustment should not affect the rest of line layout, so we can
        // do it right before creating the Fragment.
        let style = &inline_box.base.style;
        if style.get_box().position == Position::Relative {
            content_rect.start_corner += relative_adjustement(style, containing_block);
        }

        let inline_box_containing_block = ContainingBlock {
            size: ContainingBlockSize {
                inline: content_rect.size.inline,
                block: Default::default(),
            },
            style: containing_block.style,
        };
        let fragments = inner_state
            .fragments
            .into_iter()
            .map(|(fragment, logical_rect)| {
                let is_float = matches!(fragment, Fragment::Float(_));
                if let Some(mut base) = fragment.base_mut() {
                    if is_float {
                        base.rect.origin -= pbm_sums
                            .start_offset()
                            .to_physical_size(containing_block_writing_mode);
                    } else {
                        // We do not know the actual physical position of a logically laid out inline element, until
                        // we know the width of the containing inline block. This step converts the logical rectangle
                        // into a physical one now that we've computed inline size of the containing inline block above.
                        base.rect = logical_rect.as_physical(Some(&inline_box_containing_block))
                    }
                }
                fragment
            })
            .collect();

        // Previously all the fragment's children were positioned relative to the linebox,
        // but they need to be made relative to this fragment.
        let physical_content_rect = content_rect.as_physical(Some(containing_block));
        let mut fragment = BoxFragment::new(
            inline_box.base.base_fragment_info,
            style.clone(),
            fragments,
            physical_content_rect,
            padding.to_physical(containing_block_writing_mode),
            border.to_physical(containing_block_writing_mode),
            margin.to_physical(containing_block_writing_mode),
            None, /* specific_layout_info */
        );

        let offset_from_parent_ifc = LogicalVec2 {
            inline: pbm_sums.inline_start + self.current_state.inline_advance,
            block: content_rect.start_corner.block,
        }
        .to_physical_vector(containing_block_writing_mode);

        match inner_state.positioning_context_or_start_offset_in_parent {
            Either::Left(mut positioning_context) => {
                positioning_context
                    .layout_collected_children(self.layout.layout_context, &mut fragment);
                positioning_context.adjust_static_position_of_hoisted_fragments_with_offset(
                    &offset_from_parent_ifc,
                    PositioningContextLength::zero(),
                );
                self.current_positioning_context_mut()
                    .append(positioning_context);
            },
            Either::Right(start_offset) => {
                self.current_positioning_context_mut()
                    .adjust_static_position_of_hoisted_fragments_with_offset(
                        &offset_from_parent_ifc,
                        start_offset,
                    );
            },
        }

        self.current_state.inline_advance += inner_state.inline_advance + pbm_sums.inline_sum();

        let fragment = Fragment::Box(ArcRefCell::new(fragment));
        inline_box.base.add_fragment(fragment.clone());

        self.current_state.fragments.push((fragment, content_rect));
    }

    fn calculate_inline_box_block_start(
        &self,
        inline_box_state: &InlineBoxContainerState,
        space_above_baseline: Au,
    ) -> Au {
        if self.is_phantom_line {
            return Au::zero();
        };
        let font_metrics = &inline_box_state.base.font_metrics;
        let style = &inline_box_state.base.style;
        let line_gap = font_metrics.line_gap;

        // The baseline offset that we have in `Self::baseline_offset` is relative to the line
        // baseline, so we need to make it relative to the line block start.
        match inline_box_state.base.style.clone_baseline_shift() {
            BaselineShift::Keyword(BaselineShiftKeyword::Top) => {
                let line_height = line_height(style, font_metrics, &inline_box_state.base.flags);
                (line_height - line_gap).scale_by(0.5)
            },
            BaselineShift::Keyword(BaselineShiftKeyword::Center) => {
                (self.line_metrics.block_size - line_gap).scale_by(0.5)
            },
            BaselineShift::Keyword(BaselineShiftKeyword::Bottom) => {
                let line_height = line_height(style, font_metrics, &inline_box_state.base.flags);
                let half_leading = (line_height - line_gap).scale_by(0.5);
                self.line_metrics.block_size - line_height + half_leading
            },
            _ => {
                self.line_metrics.baseline_block_offset + inline_box_state.base.baseline_offset -
                    space_above_baseline
            },
        }
    }

    fn layout_text_run(&mut self, text_item: TextRunLineItem) {
        if text_item.text.is_empty() && !text_item.is_empty_for_text_cursor {
            return;
        }

        let mut number_of_justification_opportunities = 0;
        let mut inline_advance = text_item
            .text
            .iter()
            .map(|shaped_text_slice| {
                number_of_justification_opportunities += shaped_text_slice.total_word_separators();
                shaped_text_slice.total_advance()
            })
            .sum();

        if !self.justification_adjustment.is_zero() {
            inline_advance += self
                .justification_adjustment
                .scale_by(number_of_justification_opportunities as f32);
        }

        // The block start of the TextRun is often zero (meaning it has the same font metrics as the
        // inline box's strut), but for children of the inline formatting context root or for
        // fallback fonts that use baseline relative alignment, it might be different.
        let font_metrics = &text_item.info.font.metrics;
        let start_corner = LogicalVec2 {
            inline: self.current_state.inline_advance,
            block: self.current_state.baseline_offset -
                font_metrics.ascent -
                self.current_state.parent_offset.block,
        };
        let content_rect = LogicalRect {
            start_corner,
            size: LogicalVec2 {
                block: font_metrics.line_gap,
                inline: inline_advance,
            },
        };

        let font_key = text_item.info.font.key(
            self.layout.layout_context.painter_id,
            &self.layout.layout_context.font_context,
        );

        self.current_state.inline_advance += inline_advance;
        self.current_state.fragments.push((
            Fragment::Text(ArcRefCell::new(TextFragment {
                base: BaseFragment::new(
                    text_item.base_fragment_info,
                    text_item.inline_styles.style.clone().into(),
                    PhysicalRect::zero(),
                ),
                selected_style: text_item.inline_styles.selected.clone(),
                font_metrics: font_metrics.clone(),
                font_key,
                glyphs: text_item.text,
                justification_adjustment: self.justification_adjustment,
                offsets: text_item.offsets,
                is_empty_for_text_cursor: text_item.is_empty_for_text_cursor,
            })),
            content_rect,
        ));
    }

    fn layout_atomic(&mut self, atomic: AtomicLineItem) {
        // The initial `start_corner` of the Fragment is only the PaddingBorderMargin sum start
        // offset, which is the sum of the start component of the padding, border, and margin.
        // This needs to be added to the calculated block and inline positions.
        // Make the final result relative to the parent box.
        let containing_block = self.containing_block();
        let ifc_writing_mode = containing_block.style.writing_mode;
        let content_rect = {
            let block_start = atomic.calculate_block_start(&self.line_metrics);
            let atomic_fragment = atomic.fragment.borrow_mut();
            let padding_border_margin_sides = atomic_fragment
                .padding_border_margin()
                .to_logical(ifc_writing_mode);

            let mut atomic_offset = LogicalVec2 {
                inline: self.current_state.inline_advance +
                    padding_border_margin_sides.inline_start,
                block: block_start - self.current_state.parent_offset.block +
                    padding_border_margin_sides.block_start,
            };

            let style = atomic_fragment.style();
            if style.get_box().position == Position::Relative {
                atomic_offset += relative_adjustement(&style, containing_block);
            }

            // Reconstruct a logical rectangle relative to the inline box container that will be used
            // after the inline box is procesed to find a final physical rectangle.
            LogicalRect {
                start_corner: atomic_offset,
                size: atomic_fragment
                    .content_rect()
                    .size
                    .to_logical(ifc_writing_mode),
            }
        };

        if let Some(mut positioning_context) = atomic.positioning_context {
            let physical_rect_as_if_in_root = content_rect.as_physical(Some(containing_block));
            positioning_context.adjust_static_position_of_hoisted_fragments_with_offset(
                &physical_rect_as_if_in_root.origin.to_vector(),
                PositioningContextLength::zero(),
            );

            self.current_positioning_context_mut()
                .append(positioning_context);
        }

        self.current_state.inline_advance += atomic.size.inline;

        self.current_state
            .fragments
            .push((Fragment::Box(atomic.fragment), content_rect));
    }

    fn layout_absolute(&mut self, absolute: AbsolutelyPositionedLineItem) {
        let absolutely_positioned_box = (*absolute.absolutely_positioned_box).borrow();
        let style = absolutely_positioned_box.context.style();

        // From https://drafts.csswg.org/css2/#abs-non-replaced-width
        // > The static-position containing block is the containing block of a
        // > hypothetical box that would have been the first box of the element if its
        // > specified position value had been static and its specified float had been
        // > none. (Note that due to the rules in section 9.7 this hypothetical
        // > calculation might require also assuming a different computed value for
        // > display.)
        //
        // This box is different based on the original `display` value of the
        // absolutely positioned element. If it's `inline` it would be placed inline
        // at the top of the line, but if it's block it would be placed in a new
        // block position after the linebox established by this line.
        let block_position = self.layout.placement_state.current_margin.solve() -
            self.current_state.parent_offset.block;
        let initial_start_corner =
            if style.get_box().original_display.outside() == DisplayOutside::Inline {
                // Top of the line at the current inline position.
                LogicalVec2 {
                    inline: self.current_state.inline_advance,
                    block: block_position,
                }
            } else {
                // After the bottom of the line at the start of the inline formatting context.
                // Note that phantom lines are treated as being zero-height for this purpose.
                // <https://drafts.csswg.org/css-inline-3/#invisible-line-boxes>
                LogicalVec2 {
                    inline: -self.current_state.parent_offset.inline,
                    block: if absolute.preceding_line_content_would_produce_phantom_line {
                        block_position
                    } else {
                        block_position + self.line_metrics.block_size
                    },
                }
            };

        // Since alignment of absolutes in inlines is currently always `start`, the size of
        // of the static position rectangle does not matter.
        let containing_block = self.containing_block();
        let static_position_rect = LogicalRect {
            start_corner: initial_start_corner,
            size: LogicalVec2::zero(),
        }
        .as_physical(Some(containing_block));

        let hoisted_box = AbsolutelyPositionedBox::to_hoisted(
            absolute.absolutely_positioned_box.clone(),
            static_position_rect,
            LogicalVec2 {
                inline: AlignFlags::START,
                block: AlignFlags::START,
            },
            containing_block.style.writing_mode,
        );

        let hoisted_fragment = hoisted_box.fragment.clone();
        self.current_positioning_context_mut().push(hoisted_box);
        self.current_state.fragments.push((
            Fragment::AbsoluteOrFixedPositioned(hoisted_fragment),
            LogicalRect::zero(),
        ));
    }

    fn layout_float(&mut self, float: FloatLineItem) {
        self.current_state
            .flags
            .insert(LineLayoutInlineContainerFlags::HAD_ANY_FLOATS);

        // The `BoxFragment` for this float is positioned relative to the IFC, so we need
        // to move it to be positioned relative to our parent InlineBox line item. Float
        // fragments are children of these InlineBoxes and not children of the inline
        // formatting context, so that they are parented properly for StackingContext
        // properties such as opacity & filters.
        let distance_from_parent_to_ifc = LogicalVec2 {
            inline: self.current_state.parent_offset.inline,
            block: self.line_metrics.block_offset + self.current_state.parent_offset.block,
        };
        float.fragment.borrow_mut().base.rect.origin -= distance_from_parent_to_ifc
            .to_physical_size(self.containing_block().style.writing_mode);

        self.current_state
            .fragments
            .push((Fragment::Float(float.fragment), LogicalRect::zero()));
    }

    fn layout_block_level(&mut self, block_level: ArcRefCell<BoxFragment>) {
        let containing_block = self.containing_block();
        let mut content_rect = block_level
            .borrow()
            .content_rect()
            .to_logical(containing_block);
        // Block-level boxes are always placed at the logical origin of the line.
        content_rect.start_corner.inline -= self.current_state.parent_offset.inline;
        content_rect.start_corner.block -= self.line_metrics.block_offset;
        let fragment_and_rect = (Fragment::Box(block_level), content_rect);
        self.current_state.fragments.push(fragment_and_rect);
    }

    #[inline]
    fn containing_block(&self) -> &ContainingBlock<'_> {
        self.layout.containing_block()
    }

    fn layout_tab(&mut self, advance: Au) {
        self.current_state.inline_advance += advance;
    }
}

pub(super) enum LineItem {
    InlineStartBoxPaddingBorderMargin(InlineBoxIdentifier),
    InlineEndBoxPaddingBorderMargin(InlineBoxIdentifier),
    TextRun(Option<InlineBoxIdentifier>, TextRunLineItem),
    Atomic(Option<InlineBoxIdentifier>, AtomicLineItem),
    AbsolutelyPositioned(Option<InlineBoxIdentifier>, AbsolutelyPositionedLineItem),
    Float(Option<InlineBoxIdentifier>, FloatLineItem),
    BlockLevel(Option<InlineBoxIdentifier>, ArcRefCell<BoxFragment>),
    Tab {
        inline_box_identifier: Option<InlineBoxIdentifier>,
        advance: Au,
        bidi_level: Level,
    },
}

impl LineItem {
    pub(crate) fn is_in_flow_content(&self) -> bool {
        matches!(
            self,
            Self::TextRun(..) | Self::Atomic(..) | Self::BlockLevel(..)
        )
    }

    fn inline_box_identifier(&self) -> Option<InlineBoxIdentifier> {
        match self {
            LineItem::InlineStartBoxPaddingBorderMargin(identifier) => Some(*identifier),
            LineItem::InlineEndBoxPaddingBorderMargin(identifier) => Some(*identifier),
            LineItem::TextRun(identifier, _) => *identifier,
            LineItem::Atomic(identifier, _) => *identifier,
            LineItem::AbsolutelyPositioned(identifier, _) => *identifier,
            LineItem::Float(identifier, _) => *identifier,
            LineItem::BlockLevel(identifier, _) => *identifier,
            LineItem::Tab {
                inline_box_identifier,
                ..
            } => *inline_box_identifier,
        }
    }

    pub(super) fn trim_whitespace_at_end(&mut self, whitespace_trimmed: &mut Au) -> bool {
        match self {
            LineItem::InlineStartBoxPaddingBorderMargin(_) => true,
            LineItem::InlineEndBoxPaddingBorderMargin(_) => true,
            LineItem::TextRun(_, item) => item.trim_whitespace_at_end(whitespace_trimmed),
            LineItem::Atomic(..) => false,
            LineItem::AbsolutelyPositioned(..) => true,
            LineItem::Float(..) => true,
            LineItem::BlockLevel(..) => true,
            LineItem::Tab { .. } => false,
        }
    }

    pub(super) fn trim_whitespace_at_start(&mut self, whitespace_trimmed: &mut Au) -> bool {
        match self {
            LineItem::InlineStartBoxPaddingBorderMargin(_) => true,
            LineItem::InlineEndBoxPaddingBorderMargin(_) => true,
            LineItem::TextRun(_, item) => item.trim_whitespace_at_start(whitespace_trimmed),
            LineItem::Atomic(..) => false,
            LineItem::AbsolutelyPositioned(..) => true,
            LineItem::Float(..) => true,
            LineItem::BlockLevel(..) => true,
            LineItem::Tab { .. } => false,
        }
    }
}

#[derive(Debug, MallocSizeOf)]
pub(crate) struct TextRunOffsets {
    /// The selection range of the containing inline formatting context.
    #[ignore_malloc_size_of = "This is stored primarily in the DOM"]
    pub shared_selection: SharedSelection,
    /// The range of characters this [`TextRun`] represents within the entire text of its
    /// inline formatting context.
    pub character_range: Range<usize>,
}

pub(super) struct TextRunLineItem {
    pub info: Arc<FontAndScriptInfo>,
    pub base_fragment_info: BaseFragmentInfo,
    pub inline_styles: SharedInlineStyles,
    pub text: Vec<Arc<ShapedTextSlice>>,
    /// When necessary, this field store the [`TextRunOffsets`] for a particular
    /// [`TextRunLineItem`]. This is currently only used inside of text inputs.
    pub offsets: Option<Box<TextRunOffsets>>,
    /// Whether or not this [`TextFragment`] is an empty fragment added for the
    /// benefit of placing a text cursor on an otherwise empty editable line.
    pub is_empty_for_text_cursor: bool,
}

impl TextRunLineItem {
    fn trim_whitespace_at_end(&mut self, whitespace_trimmed: &mut Au) -> bool {
        if matches!(
            self.inline_styles
                .style
                .borrow()
                .get_inherited_text()
                .white_space_collapse,
            WhiteSpaceCollapse::Preserve | WhiteSpaceCollapse::BreakSpaces
        ) {
            return false;
        }

        let index_of_last_non_whitespace = self
            .text
            .iter()
            .rev()
            .position(|glyph| !glyph.is_whitespace())
            .map(|offset_from_end| self.text.len() - offset_from_end);

        let first_whitespace_index = index_of_last_non_whitespace.unwrap_or(0);
        *whitespace_trimmed += self
            .text
            .drain(first_whitespace_index..)
            .map(|glyph| glyph.total_advance())
            .sum();

        // Only keep going if we only encountered whitespace.
        index_of_last_non_whitespace.is_none()
    }

    fn trim_whitespace_at_start(&mut self, whitespace_trimmed: &mut Au) -> bool {
        if matches!(
            self.inline_styles
                .style
                .borrow()
                .get_inherited_text()
                .white_space_collapse,
            WhiteSpaceCollapse::Preserve | WhiteSpaceCollapse::BreakSpaces
        ) {
            return false;
        }

        let index_of_first_non_whitespace = self
            .text
            .iter()
            .position(|glyph| !glyph.is_whitespace())
            .unwrap_or(self.text.len());

        *whitespace_trimmed += self
            .text
            .drain(0..index_of_first_non_whitespace)
            .map(|glyph| glyph.total_advance())
            .sum();

        // Only keep going if we only encountered whitespace.
        self.text.is_empty()
    }

    pub(crate) fn merge_if_possible(
        &mut self,
        new_info: &Arc<FontAndScriptInfo>,
        new_glyph_store: &Arc<ShapedTextSlice>,
        new_offsets: &Option<TextRunOffsets>,
        new_inline_styles: &SharedInlineStyles,
    ) -> bool {
        if !Arc::ptr_eq(&self.info.font, &new_info.font) ||
            self.info.bidi_level != new_info.bidi_level ||
            !self.inline_styles.ptr_eq(new_inline_styles)
        {
            return false;
        }
        self.text.push(new_glyph_store.clone());

        assert_eq!(self.offsets.is_some(), new_offsets.is_some());
        if let (Some(new_offsets), Some(existing_offsets)) = (new_offsets, self.offsets.as_mut()) {
            existing_offsets.character_range.end = new_offsets.character_range.end;
        }

        true
    }
}

pub(super) struct AtomicLineItem {
    pub fragment: ArcRefCell<BoxFragment>,
    pub size: LogicalVec2<Au>,
    pub positioning_context: Option<PositioningContext>,

    /// The block offset of this items' baseline relative to the baseline of the line.
    /// This will be zero for boxes with `vertical-align: top` and `vertical-align:
    /// bottom` since their baselines are calculated late in layout.
    pub baseline_offset_in_parent: Au,

    /// The offset of the baseline inside this item.
    pub baseline_offset_in_item: Au,

    /// The BiDi level of this [`AtomicLineItem`] to enable reordering.
    pub bidi_level: Level,
}

impl AtomicLineItem {
    /// Given the metrics for a line, our vertical alignment, and our block size, find a block start
    /// position relative to the top of the line.
    fn calculate_block_start(&self, line_metrics: &LineMetrics) -> Au {
        match self.fragment.borrow().style().clone_baseline_shift() {
            BaselineShift::Keyword(BaselineShiftKeyword::Top) => Au::zero(),
            BaselineShift::Keyword(BaselineShiftKeyword::Center) => {
                (line_metrics.block_size - self.size.block).scale_by(0.5)
            },
            BaselineShift::Keyword(BaselineShiftKeyword::Bottom) => {
                line_metrics.block_size - self.size.block
            },

            // This covers all baseline-relative vertical alignment.
            _ => {
                let baseline = line_metrics.baseline_block_offset + self.baseline_offset_in_parent;
                baseline - self.baseline_offset_in_item
            },
        }
    }
}

pub(super) struct AbsolutelyPositionedLineItem {
    pub absolutely_positioned_box: ArcRefCell<AbsolutelyPositionedBox>,
    /// Whether the line would be phantom if it were to end before the abspos.
    /// This is used when computing the static position (in the block axis) of
    /// an abspos whose original display had a block outer display type.
    pub preceding_line_content_would_produce_phantom_line: bool,
}

pub(super) struct FloatLineItem {
    pub fragment: ArcRefCell<BoxFragment>,
    /// Whether or not this float Fragment has been placed yet. Fragments that
    /// do not fit on a line need to be placed after the hypothetical block start
    /// of the next line.
    pub needs_placement: bool,
}

/// Sort a mutable slice by the given indices array in place, reording the slice so that final
/// value of `slice[x]` is `slice[indices[x]]`.
fn sort_by_indices_in_place<T>(data: &mut [T], mut indices: Vec<usize>) {
    for idx in 0..data.len() {
        if indices[idx] == idx {
            continue;
        }

        let mut current_idx = idx;
        loop {
            let target_idx = indices[current_idx];
            indices[current_idx] = current_idx;
            if indices[target_idx] == target_idx {
                break;
            }
            data.swap(current_idx, target_idx);
            current_idx = target_idx;
        }
    }
}