ladybird/Libraries/LibWeb/Layout/LineBuilder.cpp

/*
 * Copyright (c) 2022, Andreas Kling <andreas@ladybird.org>
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#include <LibWeb/Layout/BlockFormattingContext.h>
#include <LibWeb/Layout/LineBuilder.h>
#include <LibWeb/Layout/TextNode.h>

namespace Web::Layout {

LineBuilder::LineBuilder(InlineFormattingContext& context, LayoutState& layout_state, LayoutState::UsedValues& containing_block_used_values, CSS::Direction direction, CSS::WritingMode writing_mode)
    : m_context(context)
    , m_layout_state(layout_state)
    , m_containing_block_used_values(containing_block_used_values)
    , m_direction(direction)
    , m_writing_mode(writing_mode)
{
    m_text_indent = m_context.containing_block().computed_values().text_indent().to_px(m_context.containing_block(), m_containing_block_used_values.content_width());
    begin_new_line(false);
}

void LineBuilder::break_line(ForcedBreak forced_break, Optional<CSSPixels> next_item_width)
{
    // FIXME: Respect inline direction.

    auto& last_line_box = ensure_last_line_box();
    last_line_box.m_has_break = true;
    last_line_box.m_has_forced_break = forced_break == ForcedBreak::Yes;

    m_last_line_needs_update = true;
    update_last_line();

    size_t break_count = 0;
    bool floats_intrude_at_current_y = false;
    do {
        m_containing_block_used_values.line_boxes.append(LineBox(m_direction, m_writing_mode));
        begin_new_line(true, break_count == 0);
        break_count++;
        floats_intrude_at_current_y = m_context.any_floats_intrude_at_block_offset(m_current_block_offset);
    } while (floats_intrude_at_current_y
        && (!m_context.can_fit_new_line_at_block_offset(m_current_block_offset)
            || (next_item_width.value_or(0) > m_available_width_for_current_line)));
}

void LineBuilder::begin_new_line(bool increment_y, bool is_first_break_in_sequence)
{
    if (increment_y) {
        if (is_first_break_in_sequence) {
            // First break is simple, just go to the start of the next line.
            m_current_block_offset += max(m_max_height_on_current_line, m_context.containing_block().computed_values().line_height());
        } else {
            // We're doing more than one break in a row.
            // This means we're trying to squeeze past intruding floats.
            // Scan 1px at a time until we find a Y value where a new line can fit.
            // FIXME: This is super dumb and inefficient.
            CSSPixels candidate_block_offset = m_current_block_offset + 1;
            while (true) {
                if (m_context.can_fit_new_line_at_block_offset(candidate_block_offset))
                    break;
                ++candidate_block_offset;
            }
            m_current_block_offset = candidate_block_offset;
        }
    }
    recalculate_available_space();
    ensure_last_line_box().m_original_available_width = m_available_width_for_current_line;
    m_max_height_on_current_line = 0;
    m_last_line_needs_update = true;

    // FIXME: Support text-indent with "each-line".
    if (m_containing_block_used_values.line_boxes.size() <= 1)
        ensure_last_line_box().m_inline_length += m_text_indent;
}

LineBox& LineBuilder::ensure_last_line_box()
{
    auto& line_boxes = m_containing_block_used_values.line_boxes;
    if (line_boxes.is_empty())
        line_boxes.append(LineBox(m_direction, m_writing_mode));
    return line_boxes.last();
}

void LineBuilder::append_box(Box const& box, CSSPixels leading_size, CSSPixels trailing_size, CSSPixels leading_margin, CSSPixels trailing_margin)
{
    auto& box_state = m_layout_state.get_mutable(box);
    auto& line_box = ensure_last_line_box();
    line_box.add_fragment(box, 0, 0, leading_size, trailing_size, leading_margin, trailing_margin,
        box_state.content_width(), box_state.content_height(), box_state.border_box_top(), box_state.border_box_bottom());
    m_max_height_on_current_line = max(m_max_height_on_current_line, box_state.margin_box_height());

    box_state.containing_line_box_fragment = LineBoxFragmentCoordinate {
        .line_box_index = m_containing_block_used_values.line_boxes.size() - 1,
        .fragment_index = line_box.fragments().size() - 1,
    };
}

void LineBuilder::append_text_chunk(TextNode const& text_node, size_t offset_in_node, size_t length_in_node, CSSPixels leading_size, CSSPixels trailing_size, CSSPixels leading_margin, CSSPixels trailing_margin, CSSPixels content_width, CSSPixels content_height, RefPtr<Gfx::GlyphRun> glyph_run)
{
    auto& line_box = ensure_last_line_box();
    line_box.add_fragment(text_node, offset_in_node, length_in_node, leading_size, trailing_size, leading_margin,
        trailing_margin, content_width, content_height, 0, 0, move(glyph_run));

    m_max_height_on_current_line = max(m_max_height_on_current_line, line_box.block_length());
}

CSSPixels LineBuilder::y_for_float_to_be_inserted_here(Box const& box)
{
    auto const& box_state = m_layout_state.get(box);
    CSSPixels const width = box_state.margin_box_width();
    CSSPixels const height = box_state.margin_box_height();

    CSSPixels candidate_block_offset = m_current_block_offset;

    // Determine the current line width and subtract trailing whitespace, since those have not yet been removed while
    // placing floating boxes.
    auto const& current_line = ensure_last_line_box();
    auto current_line_width = current_line.width() - current_line.get_trailing_whitespace_width();

    // If there's already inline content on the current line, check if the new float can fit
    // alongside the content. If not, place it on the next line.
    if (current_line_width > 0 && (current_line_width + width) > m_available_width_for_current_line)
        candidate_block_offset += current_line.height();

    // Then, look for the next Y position where we can fit the new float.
    auto box_in_root_rect = m_context.parent().content_box_rect_in_ancestor_coordinate_space(box_state, m_context.parent().root());

    // New floats will always be placed vertically at or below the lowest float.
    // This applies to all floats, so the last inserted float will always be the lowest.
    auto last_float = m_context.parent().last_inserted_float();
    if (last_float.has_value()) {
        auto float_box_top = last_float->margin_box_rect_in_root_coordinate_space.top() - box_in_root_rect.y();
        candidate_block_offset = max(candidate_block_offset, float_box_top);
    }

    HashMap<CSSPixels, AvailableSize> available_space_cache;
    for (;;) {
        Optional<CSSPixels> highest_intersection_bottom;
        auto candidate_block_bottom = candidate_block_offset + height;

        m_context.parent().for_each_floating_box([&](auto const& float_box) {
            auto float_box_top = float_box.margin_box_rect_in_root_coordinate_space.top() - box_in_root_rect.y();
            auto float_box_bottom = float_box.margin_box_rect_in_root_coordinate_space.bottom() - box_in_root_rect.y();
            if (float_box_bottom <= candidate_block_offset)
                return IterationDecision::Continue;

            auto intersection_test = [&](auto y_coordinate, auto top, auto bottom) {
                if (y_coordinate < top || y_coordinate > bottom)
                    return;
                auto available_space = available_space_cache.ensure(y_coordinate, [&]() {
                    return m_context.available_space_for_line(y_coordinate);
                });
                if (width > available_space)
                    highest_intersection_bottom = min(highest_intersection_bottom.value_or(float_box_bottom), float_box_bottom);
            };

            intersection_test(float_box_top, candidate_block_offset, candidate_block_bottom);
            intersection_test(float_box_bottom, candidate_block_offset, candidate_block_bottom);
            intersection_test(candidate_block_offset, float_box_top, float_box_bottom);
            intersection_test(candidate_block_bottom, float_box_top, float_box_bottom);

            return IterationDecision::Continue;
        });
        if (!highest_intersection_bottom.has_value() || highest_intersection_bottom.value() == candidate_block_offset)
            break;
        candidate_block_offset = highest_intersection_bottom.value();
    }

    return max(candidate_block_offset, m_context.vertical_float_clearance());
}

bool LineBuilder::should_break(CSSPixels next_item_width)
{
    if (m_available_width_for_current_line.is_max_content())
        return false;

    auto const& line_boxes = m_containing_block_used_values.line_boxes;
    if (line_boxes.is_empty() || line_boxes.last().is_empty()) {
        // If we don't have a single line box yet *and* there are no floats intruding
        // at this Y coordinate, we don't need to break before inserting anything.
        if (!m_context.any_floats_intrude_at_block_offset(m_current_block_offset))
            return false;
        if (!m_context.any_floats_intrude_at_block_offset(m_current_block_offset + m_context.containing_block().computed_values().line_height()))
            return false;
    }
    auto current_line_width = ensure_last_line_box().width();
    return (current_line_width + next_item_width) > m_available_width_for_current_line;
}

void LineBuilder::update_last_line()
{
    if (!m_last_line_needs_update)
        return;
    m_last_line_needs_update = false;

    auto& line_boxes = m_containing_block_used_values.line_boxes;
    if (line_boxes.is_empty())
        return;

    auto& line_box = line_boxes.last();

    auto text_align = m_context.containing_block().computed_values().text_align();
    auto direction = m_context.containing_block().computed_values().direction();

    auto current_line_height = max(m_max_height_on_current_line, m_context.containing_block().computed_values().line_height());
    CSSPixels inline_offset_top = m_context.leftmost_inline_offset_at(m_current_block_offset);
    CSSPixels inline_offset_bottom = m_context.leftmost_inline_offset_at(m_current_block_offset + current_line_height - 1);
    CSSPixels inline_offset = max(inline_offset_top, inline_offset_bottom);
    CSSPixels block_offset = 0;

    // FIXME: Respect inline direction.
    CSSPixels excess_inline_space = m_available_width_for_current_line.to_px_or_zero() - line_box.inline_length();

    if (m_writing_mode != CSS::WritingMode::HorizontalTb) {
        block_offset = m_available_width_for_current_line.to_px_or_zero() - line_box.block_length();
    }

    // If (after justification, if any) the inline contents of a line box are too long to fit within it,
    // then the contents are start-aligned: any content that doesn't fit overflows the line box’s end edge.
    if (excess_inline_space > 0) {
        switch (text_align) {
        case CSS::TextAlign::Center:
        case CSS::TextAlign::LibwebCenter:
        case CSS::TextAlign::LibwebInheritOrCenter:
            inline_offset += excess_inline_space / 2;
            break;
        case CSS::TextAlign::Start:
            if (direction == CSS::Direction::Rtl)
                inline_offset += excess_inline_space;
            break;
        case CSS::TextAlign::End:
            if (direction == CSS::Direction::Ltr)
                inline_offset += excess_inline_space;
            break;
        case CSS::TextAlign::Right:
        case CSS::TextAlign::LibwebRight:
            inline_offset += excess_inline_space;
            break;
        case CSS::TextAlign::MatchParent:
            // This should have been replaced before this point.
            VERIFY_NOT_REACHED();
        case CSS::TextAlign::Left:
        case CSS::TextAlign::LibwebLeft:
        case CSS::TextAlign::Justify:
        default:
            break;
        }
    }

    auto strut_baseline = [&] {
        auto& font = m_context.containing_block().first_available_font();
        auto const line_height = m_context.containing_block().computed_values().line_height();
        auto const font_metrics = font.pixel_metrics();
        auto const typographic_height = CSSPixels::nearest_value_for(font_metrics.ascent + font_metrics.descent);
        auto const leading = line_height - typographic_height;
        auto const half_leading = leading / 2;
        return CSSPixels::nearest_value_for(font_metrics.ascent) + half_leading;
    }();

    auto line_box_baseline = [&] {
        CSSPixels line_box_baseline = strut_baseline;
        for (auto& fragment : line_box.fragments()) {
            auto const& font = fragment.layout_node().first_available_font();
            auto const line_height = fragment.layout_node().computed_values().line_height();
            auto const font_metrics = font.pixel_metrics();
            auto const typographic_height = CSSPixels::nearest_value_for(font_metrics.ascent + font_metrics.descent);
            auto const leading = line_height - typographic_height;
            auto const half_leading = leading / 2;

            // The CSS specification calls this AD (A+D, Ascent + Descent).

            CSSPixels fragment_baseline = 0;
            if (fragment.layout_node().is_text_node()) {
                fragment_baseline = CSSPixels::nearest_value_for(font_metrics.ascent) + half_leading;
            } else {
                auto const& box = as<Layout::Box>(fragment.layout_node());
                fragment_baseline = m_context.box_baseline(box);
            }

            // Remember the baseline used for this fragment. This will be used when painting the fragment.
            fragment.set_baseline(fragment_baseline);

            // NOTE: For fragments with a <length> vertical-align, shift the line box baseline down by the length.
            //       This ensures that we make enough vertical space on the line for any manually-aligned fragments.
            if (auto const* length_percentage = fragment.layout_node().computed_values().vertical_align().get_pointer<CSS::LengthPercentage>()) {
                if (length_percentage->is_length())
                    fragment_baseline += length_percentage->length().to_px(fragment.layout_node());
                else if (length_percentage->is_percentage())
                    fragment_baseline += line_height.scaled(length_percentage->percentage().as_fraction());
            }

            line_box_baseline = max(line_box_baseline, fragment_baseline);
        }
        return line_box_baseline;
    }();

    // Start with the "strut", an imaginary zero-width box at the start of each line box.
    auto strut_top = m_current_block_offset;
    auto strut_bottom = m_current_block_offset + m_context.containing_block().computed_values().line_height();

    CSSPixels uppermost_box_top = strut_top;
    CSSPixels lowermost_box_bottom = strut_bottom;

    for (auto& fragment : line_box.fragments()) {
        CSSPixels new_fragment_inline_offset = inline_offset + fragment.inline_offset();
        CSSPixels new_fragment_block_offset = 0;

        auto block_offset_value_for_alignment = [&](CSS::VerticalAlign vertical_align) {
            CSSPixels effective_box_top_offset = fragment.border_box_top();
            CSSPixels effective_box_bottom_offset = fragment.border_box_top();
            if (fragment.is_atomic_inline()) {
                auto const& fragment_box_state = m_layout_state.get(static_cast<Box const&>(fragment.layout_node()));
                effective_box_top_offset = fragment_box_state.margin_box_top();
                effective_box_bottom_offset = fragment_box_state.margin_box_bottom();
            }

            auto alphabetic_baseline = m_current_block_offset + line_box_baseline - fragment.baseline() + effective_box_top_offset;

            switch (vertical_align) {
            case CSS::VerticalAlign::Baseline:
                return alphabetic_baseline;
            case CSS::VerticalAlign::Top:
                return m_current_block_offset + effective_box_top_offset;
            case CSS::VerticalAlign::Middle: {
                // Align the vertical midpoint of the box with the baseline of the parent box
                // plus half the x-height of the parent.
                auto const x_height = CSSPixels::nearest_value_for(m_context.containing_block().first_available_font().pixel_metrics().x_height);
                return m_current_block_offset + line_box_baseline + ((effective_box_top_offset - effective_box_bottom_offset - x_height - fragment.height()) / 2);
            }
            case CSS::VerticalAlign::Sub:
                // https://drafts.csswg.org/css-inline/#valdef-baseline-shift-sub
                // Lower by the offset appropriate for subscripts of the parent’s box.
                // The UA may use the parent’s font metrics to find this offset; otherwise it defaults to dropping by one fifth of the parent’s used font-size.
                // FIXME: Use font metrics to find a more appropriate offset, if possible
                return alphabetic_baseline + m_context.containing_block().computed_values().font_size() / 5;
            case CSS::VerticalAlign::Super:
                // https://drafts.csswg.org/css-inline/#valdef-baseline-shift-super
                // Raise by the offset appropriate for superscripts of the parent’s box.
                // The UA may use the parent’s font metrics to find this offset; otherwise it defaults to raising by one third of the parent’s used font-size.
                // FIXME: Use font metrics to find a more appropriate offset, if possible
                return alphabetic_baseline - m_context.containing_block().computed_values().font_size() / 3;
            case CSS::VerticalAlign::Bottom:
            case CSS::VerticalAlign::TextBottom:
            case CSS::VerticalAlign::TextTop:
                // FIXME: These are all 'baseline'
                return alphabetic_baseline;
            }
            VERIFY_NOT_REACHED();
        };

        auto const& vertical_align = fragment.layout_node().computed_values().vertical_align();
        if (vertical_align.has<CSS::VerticalAlign>()) {
            new_fragment_block_offset = block_offset_value_for_alignment(vertical_align.get<CSS::VerticalAlign>());
        } else {
            if (auto const* length_percentage = vertical_align.get_pointer<CSS::LengthPercentage>()) {
                if (length_percentage->is_length()) {
                    auto vertical_align_amount = length_percentage->length().to_px(fragment.layout_node());
                    new_fragment_block_offset = block_offset_value_for_alignment(CSS::VerticalAlign::Baseline) - vertical_align_amount;
                } else if (length_percentage->is_percentage()) {
                    auto vertical_align_amount = m_context.containing_block().computed_values().line_height().scaled(length_percentage->percentage().as_fraction());
                    new_fragment_block_offset = block_offset_value_for_alignment(CSS::VerticalAlign::Baseline) - vertical_align_amount;
                }
            }
        }

        fragment.set_inline_offset(new_fragment_inline_offset);
        fragment.set_block_offset(floor(new_fragment_block_offset) + block_offset);

        CSSPixels top_of_inline_box = 0;
        CSSPixels bottom_of_inline_box = 0;
        {
            // FIXME: Support inline-table elements.
            if (fragment.is_atomic_inline()) {
                auto const& fragment_box_state = m_layout_state.get(static_cast<Box const&>(fragment.layout_node()));
                top_of_inline_box = (fragment.block_offset() - fragment_box_state.margin_box_top());
                bottom_of_inline_box = (fragment.block_offset() + fragment_box_state.content_height() + fragment_box_state.margin_box_bottom());
            } else {
                auto font_metrics = fragment.layout_node().first_available_font().pixel_metrics();
                auto typographic_height = CSSPixels::nearest_value_for(font_metrics.ascent + font_metrics.descent);
                auto leading = fragment.layout_node().computed_values().line_height() - typographic_height;
                auto half_leading = leading / 2;
                top_of_inline_box = (fragment.block_offset() + fragment.baseline() - CSSPixels::nearest_value_for(font_metrics.ascent) - half_leading);
                bottom_of_inline_box = (fragment.block_offset() + fragment.baseline() + CSSPixels::nearest_value_for(font_metrics.descent) + half_leading);
            }
            if (auto const* length_percentage = fragment.layout_node().computed_values().vertical_align().get_pointer<CSS::LengthPercentage>()) {
                if (length_percentage->is_length())
                    bottom_of_inline_box += length_percentage->length().to_px(fragment.layout_node());
                else if (length_percentage->is_percentage())
                    bottom_of_inline_box += m_context.containing_block().computed_values().line_height().scaled(length_percentage->percentage().as_fraction());
            }
        }

        uppermost_box_top = min(uppermost_box_top, top_of_inline_box);
        lowermost_box_bottom = max(lowermost_box_bottom, bottom_of_inline_box);
    }

    // 3. The line box height is the distance between the uppermost box top and the lowermost box bottom.
    line_box.m_block_length = lowermost_box_bottom - uppermost_box_top;

    line_box.m_bottom = m_current_block_offset + line_box.m_block_length;
    line_box.m_baseline = line_box_baseline;
}

void LineBuilder::remove_last_line_if_empty()
{
    // If there's an empty line box at the bottom, just remove it instead of giving it height.
    auto& line_boxes = m_containing_block_used_values.line_boxes;
    if (!line_boxes.is_empty() && line_boxes.last().is_empty()) {
        line_boxes.take_last();
        m_last_line_needs_update = false;
    }
}

void LineBuilder::recalculate_available_space()
{
    auto current_line_height = max(m_max_height_on_current_line, m_context.containing_block().computed_values().line_height());
    auto available_at_top_of_line_box = m_context.available_space_for_line(m_current_block_offset);
    auto available_at_bottom_of_line_box = m_context.available_space_for_line(m_current_block_offset + current_line_height - 1);
    m_available_width_for_current_line = min(available_at_bottom_of_line_box, available_at_top_of_line_box);
    if (!m_containing_block_used_values.line_boxes.is_empty())
        m_containing_block_used_values.line_boxes.last().m_original_available_width = m_available_width_for_current_line;
}

void LineBuilder::did_introduce_clearance(CSSPixels clearance)
{
    // If clearance was introduced but our current line box starts beyond it, we don't need to do anything.
    if (clearance <= m_current_block_offset)
        return;

    // Increase the height of the previous line box so it matches the clearance, because the element's height is first
    // determined by the bottom of the last line box (after trimming empty/whitespace boxes).
    auto& line_boxes = m_containing_block_used_values.line_boxes;
    if (line_boxes.size() > 1) {
        auto& previous_line_box = line_boxes[line_boxes.size() - 2];
        previous_line_box.m_bottom = clearance;
    }

    // The current line box will start directly after any cleared floats.
    m_current_block_offset = clearance;
}

}