LibWeb: Add Layout::LineBuilder class for incremental line box layout

This class will be used to place items on lines incrementally instead of
the current two-phase approach.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -221,6 +221,7 @@ set(SOURCES
     Layout/LayoutPosition.cpp
     Layout/LineBox.cpp
     Layout/LineBoxFragment.cpp
+    Layout/LineBuilder.cpp
     Layout/ListItemBox.cpp
     Layout/ListItemMarkerBox.cpp
     Layout/Node.cpp

Userland/Libraries/LibWeb/Layout/LineBox.h

@@ -32,6 +32,8 @@ public:
 private:
     friend class BlockContainer;
     friend class InlineFormattingContext;
+    friend class LineBuilder;
+
     NonnullOwnPtrVector<LineBoxFragment> m_fragments;
     float m_width { 0 };
 };

Userland/Libraries/LibWeb/Layout/LineBuilder.cpp

@@ -0,0 +1,124 @@
+/*
+ * 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)
+    : m_context(context)
+{
+}
+
+LineBuilder::~LineBuilder()
+{
+    update_last_line();
+}
+
+void LineBuilder::break_line()
+{
+    update_last_line();
+    m_context.containing_block().line_boxes().append(LineBox());
+    begin_new_line();
+}
+
+void LineBuilder::begin_new_line()
+{
+    m_current_y += m_max_height_on_current_line;
+    auto space = m_context.available_space_for_line(m_current_y);
+    m_available_width_for_current_line = space.right - space.left;
+    m_max_height_on_current_line = 0;
+}
+
+void LineBuilder::append_box(Box& box)
+{
+    m_context.containing_block().line_boxes().last().add_fragment(box, 0, 0, box.width(), box.height());
+    m_max_height_on_current_line = max(m_max_height_on_current_line, box.height());
+}
+
+void LineBuilder::append_text_chunk(TextNode& text_node, size_t offset_in_node, size_t length_in_node, float width, float height)
+{
+    m_context.containing_block().line_boxes().last().add_fragment(text_node, offset_in_node, length_in_node, width, height);
+    m_max_height_on_current_line = max(m_max_height_on_current_line, height);
+}
+
+void LineBuilder::break_if_needed(LayoutMode layout_mode, float next_item_width)
+{
+    if (layout_mode == LayoutMode::AllPossibleLineBreaks
+        || (m_context.containing_block().line_boxes().last().width() + next_item_width) > m_available_width_for_current_line)
+        break_line();
+}
+
+void LineBuilder::update_last_line()
+{
+    if (m_context.containing_block().line_boxes().is_empty())
+        return;
+
+    auto& line_box = m_context.containing_block().line_boxes().last();
+
+    auto text_align = m_context.containing_block().computed_values().text_align();
+    float x_offset = m_context.available_space_for_line(m_current_y).left;
+
+    float excess_horizontal_space = m_context.containing_block().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;
+    }
+
+    float excess_horizontal_space_including_whitespace = excess_horizontal_space;
+    size_t whitespace_count = 0;
+    if (text_align == CSS::TextAlign::Justify) {
+        for (auto& fragment : line_box.fragments()) {
+            if (fragment.is_justifiable_whitespace()) {
+                ++whitespace_count;
+                excess_horizontal_space_including_whitespace += fragment.width();
+            }
+        }
+    }
+
+    float justified_space_width = whitespace_count > 0 ? (excess_horizontal_space_including_whitespace / static_cast<float>(whitespace_count)) : 0;
+
+    for (size_t i = 0; i < line_box.fragments().size(); ++i) {
+        auto& fragment = line_box.fragments()[i];
+
+        // Vertically align everyone's bottom to the line.
+        // FIXME: Support other kinds of vertical alignment.
+        fragment.set_offset({ roundf(x_offset + fragment.offset().x()), m_current_y + (m_max_height_on_current_line - fragment.height()) });
+
+        if (text_align == CSS::TextAlign::Justify
+            && fragment.is_justifiable_whitespace()
+            && fragment.width() != justified_space_width) {
+            float diff = justified_space_width - fragment.width();
+            fragment.set_width(justified_space_width);
+            // Shift subsequent sibling fragments to the right to adjust for change in width.
+            for (size_t j = i + 1; j < line_box.fragments().size(); ++j) {
+                auto offset = line_box.fragments()[j].offset();
+                offset.translate_by(diff, 0);
+                line_box.fragments()[j].set_offset(offset);
+            }
+        }
+    }
+
+    if (!line_box.fragments().is_empty()) {
+        float left_edge = line_box.fragments().first().offset().x();
+        float right_edge = line_box.fragments().last().offset().x() + line_box.fragments().last().width();
+        float final_line_box_width = right_edge - left_edge;
+        line_box.m_width = final_line_box_width;
+    }
+}
+
+}

Userland/Libraries/LibWeb/Layout/LineBuilder.h

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2022, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/Layout/InlineFormattingContext.h>
+
+namespace Web::Layout {
+
+class LineBuilder {
+    AK_MAKE_NONCOPYABLE(LineBuilder);
+    AK_MAKE_NONMOVABLE(LineBuilder);
+
+public:
+    explicit LineBuilder(InlineFormattingContext&);
+    ~LineBuilder();
+
+    void break_line();
+    void begin_new_line();
+    void append_box(Box&);
+    void append_text_chunk(TextNode&, size_t offset_in_node, size_t length_in_node, float width, float height);
+
+    void break_if_needed(LayoutMode, float next_item_width);
+
+    float available_width_for_current_line() const { return m_available_width_for_current_line; }
+
+    void update_last_line();
+
+private:
+    InlineFormattingContext& m_context;
+    float m_available_width_for_current_line { 0 };
+    float m_current_y { 0 };
+    float m_max_height_on_current_line { 0 };
+};
+
+}