serenity/Userland/Libraries/LibWeb/Layout/LineBuilder.cpp

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

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

namespace Web::Layout {

LineBuilder::LineBuilder(InlineFormattingContext& context, FormattingState& formatting_state, LayoutMode layout_mode)
    : m_context(context)
    , m_formatting_state(formatting_state)
    , m_containing_block_state(formatting_state.get_mutable(context.containing_block()))
    , m_layout_mode(layout_mode)
{
    begin_new_line(false);
}

LineBuilder::~LineBuilder()
{
    if (m_last_line_needs_update)
        update_last_line();
}

void LineBuilder::break_line()
{
    update_last_line();
    m_containing_block_state.line_boxes.append(LineBox());
    begin_new_line(true);
}

void LineBuilder::begin_new_line(bool increment_y)
{
    if (increment_y)
        m_current_y += max(m_max_height_on_current_line, m_context.containing_block().line_height());

    switch (m_layout_mode) {
    case LayoutMode::Normal:
        m_available_width_for_current_line = m_context.available_space_for_line(m_current_y);
        break;
    case LayoutMode::MinContent:
        m_available_width_for_current_line = 0;
        break;
    case LayoutMode::MaxContent:
        m_available_width_for_current_line = INFINITY;
        break;
    }
    m_max_height_on_current_line = 0;

    m_last_line_needs_update = true;
}

LineBox& LineBuilder::ensure_last_line_box()
{
    auto& line_boxes = m_containing_block_state.line_boxes;
    if (line_boxes.is_empty())
        line_boxes.append(LineBox {});
    return line_boxes.last();
}

void LineBuilder::append_box(Box const& box, float leading_size, float trailing_size, float leading_margin, float trailing_margin)
{
    auto& box_state = m_formatting_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.border_box_height());

    box_state.containing_line_box_fragment = LineBoxFragmentCoordinate {
        .line_box_index = m_containing_block_state.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, float leading_size, float trailing_size, float leading_margin, float trailing_margin, float content_width, float content_height)
{
    ensure_last_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);
    m_max_height_on_current_line = max(m_max_height_on_current_line, content_height);
}

bool LineBuilder::should_break(LayoutMode layout_mode, float next_item_width, bool should_force_break)
{
    if (layout_mode == LayoutMode::MinContent)
        return true;
    if (should_force_break)
        return true;
    if (layout_mode == LayoutMode::MaxContent)
        return false;
    auto const& line_boxes = m_containing_block_state.line_boxes;
    if (line_boxes.is_empty() || line_boxes.last().is_empty())
        return false;
    auto current_line_width = line_boxes.last().width();
    return (current_line_width + next_item_width) > m_available_width_for_current_line;
}

static float box_baseline(FormattingState const& state, Box const& box)
{
    auto const& box_state = state.get(box);
    if (!box_state.line_boxes.is_empty())
        return box_state.offset.y() + box_state.line_boxes.last().baseline();
    if (box.has_children() && !box.children_are_inline()) {
        auto const* child_box = box.last_child_of_type<Box>();
        VERIFY(child_box);
        return box_baseline(state, *child_box);
    }
    return box_state.border_box_height();
}

void LineBuilder::update_last_line()
{
    m_last_line_needs_update = false;
    auto& line_boxes = m_containing_block_state.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();
    float x_offset = m_context.leftmost_x_offset_at(m_current_y);
    float bottom = m_current_y + m_context.containing_block().line_height();
    float excess_horizontal_space = m_containing_block_state.content_width - line_box.width();

    switch (text_align) {
    case CSS::TextAlign::Center:
    case CSS::TextAlign::LibwebCenter:
        x_offset += excess_horizontal_space / 2;
        break;
    case CSS::TextAlign::Right:
        x_offset += excess_horizontal_space;
        break;
    case CSS::TextAlign::Left:
    case CSS::TextAlign::Justify:
    default:
        break;
    }

    auto fragment_baseline = [&](auto const& fragment) -> float {
        if (fragment.layout_node().is_text_node())
            return fragment.layout_node().font().baseline();
        auto const& box = verify_cast<Layout::Box>(fragment.layout_node());
        return box_baseline(m_formatting_state, box);
    };

    auto line_box_baseline = [&] {
        float line_box_baseline = 0;
        for (auto const& fragment : line_box.fragments()) {
            auto baseline = fragment_baseline(fragment);
            // NOTE: For fragments with a <length> vertical-align, shift the fragment baseline down by the length.
            //       This ensures that we make enough vertical space on the line for any manually-aligned fragments.
            if (auto length_percentage = fragment.layout_node().computed_values().vertical_align().get_pointer<CSS::LengthPercentage>(); length_percentage && length_percentage->is_length())
                baseline += length_percentage->length().to_px(fragment.layout_node());
            line_box_baseline = max(line_box_baseline, baseline);
        }
        return line_box_baseline;
    }();

    for (size_t i = 0; i < line_box.fragments().size(); ++i) {
        auto& fragment = line_box.fragments()[i];

        float new_fragment_x = roundf(x_offset + fragment.offset().x());
        float new_fragment_y = 0;

        auto y_value_for_alignment = [&](CSS::VerticalAlign vertical_align) {
            switch (vertical_align) {
            case CSS::VerticalAlign::Baseline:
            case CSS::VerticalAlign::Bottom:
            case CSS::VerticalAlign::Middle:
            case CSS::VerticalAlign::Sub:
            case CSS::VerticalAlign::Super:
            case CSS::VerticalAlign::TextBottom:
            case CSS::VerticalAlign::TextTop:
            case CSS::VerticalAlign::Top:
                // FIXME: These are all 'baseline'
                return m_current_y + line_box_baseline - fragment_baseline(fragment) + fragment.border_box_top();
            }
            VERIFY_NOT_REACHED();
        };

        auto const& vertical_align = fragment.layout_node().computed_values().vertical_align();
        if (vertical_align.has<CSS::VerticalAlign>()) {
            new_fragment_y = y_value_for_alignment(vertical_align.get<CSS::VerticalAlign>());
        } else {
            if (auto length_percentage = vertical_align.get_pointer<CSS::LengthPercentage>(); length_percentage && length_percentage->is_length()) {
                auto vertical_align_amount = length_percentage->length().to_px(fragment.layout_node());
                new_fragment_y = y_value_for_alignment(CSS::VerticalAlign::Baseline) - vertical_align_amount;
            }
        }

        fragment.set_offset({ new_fragment_x, new_fragment_y });

        bottom = max(bottom, new_fragment_y + fragment.height() + fragment.border_box_bottom());
    }

    line_box.m_bottom = bottom;
    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_state.line_boxes;
    if (!line_boxes.is_empty() && line_boxes.last().fragments().is_empty()) {
        line_boxes.take_last();
        m_last_line_needs_update = false;
    }
}
}