LibWeb: Start making our layout system "transactional"

This patch adds a map of Layout::Node to FormattingState::NodeState.
Instead of updating layout nodes incrementally as layout progresses
through the formatting contexts, all updates are now written to the
corresponding NodeState instead.

At the end of layout, FormattingState::commit() is called, which
transfers all the values from the NodeState objects to the Node.

This will soon allow us to perform completely non-destructive layouts
which don't affect the tree.

Note that there are many imperfections here, and still many places
where we assign to the NodeState, but later read directly from the Node
instead. I'm just committing at this stage to make subsequent diffs
easier to understand.

1 files changed, 26 insertions, 15 deletions

diff --git a/Userland/Libraries/LibWeb/Layout/LineBuilder.cpp b/Userland/Libraries/LibWeb/Layout/LineBuilder.cpp
index 73884f4e6e..bf25487309 100644
--- a/Userland/Libraries/LibWeb/Layout/LineBuilder.cpp
+++ b/Userland/Libraries/LibWeb/Layout/LineBuilder.cpp
@@ -9,8 +9,10 @@
 
 namespace Web::Layout {
 
-LineBuilder::LineBuilder(InlineFormattingContext& context)
+LineBuilder::LineBuilder(InlineFormattingContext& context, FormattingState& formatting_state)
     : m_context(context)
+    , m_formatting_state(formatting_state)
+    , m_containing_block_state(formatting_state.ensure(context.containing_block()))
 {
     begin_new_line(false);
 }
@@ -24,7 +26,7 @@ LineBuilder::~LineBuilder()
 void LineBuilder::break_line()
 {
     update_last_line();
-    m_context.containing_block().line_boxes().append(LineBox());
+    m_containing_block_state.line_boxes.append(LineBox());
     begin_new_line(true);
 }
 
@@ -39,15 +41,24 @@ void LineBuilder::begin_new_line(bool increment_y)
     m_last_line_needs_update = true;
 }
 
-void LineBuilder::append_box(Box& box, float leading_size, float trailing_size)
+LineBox& LineBuilder::ensure_last_line_box()
 {
-    m_context.containing_block().ensure_last_line_box().add_fragment(box, 0, 0, leading_size, trailing_size, box.content_width(), box.content_height());
-    m_max_height_on_current_line = max(m_max_height_on_current_line, box.content_height());
+    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_text_chunk(TextNode& text_node, size_t offset_in_node, size_t length_in_node, float leading_size, float trailing_size, float content_width, float content_height)
+void LineBuilder::append_box(Box const& box, float leading_size, float trailing_size)
 {
-    m_context.containing_block().ensure_last_line_box().add_fragment(text_node, offset_in_node, length_in_node, leading_size, trailing_size, content_width, content_height);
+    auto const& box_state = m_formatting_state.get(box);
+    ensure_last_line_box().add_fragment(box, 0, 0, leading_size, trailing_size, box_state.content_width, box_state.content_height);
+    m_max_height_on_current_line = max(m_max_height_on_current_line, box_state.content_height);
+}
+
+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 content_width, float content_height)
+{
+    ensure_last_line_box().add_fragment(text_node, offset_in_node, length_in_node, leading_size, trailing_size, content_width, content_height);
     m_max_height_on_current_line = max(m_max_height_on_current_line, content_height);
 }
 
@@ -59,26 +70,27 @@ bool LineBuilder::should_break(LayoutMode layout_mode, float next_item_width, bo
         return true;
     if (layout_mode == LayoutMode::OnlyRequiredLineBreaks)
         return false;
-    auto const& line_boxes = m_context.containing_block().line_boxes();
+    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 = m_context.containing_block().line_boxes().last().width();
+    auto current_line_width = line_boxes.last().width();
     return (current_line_width + next_item_width) > m_available_width_for_current_line;
 }
 
 void LineBuilder::update_last_line()
 {
     m_last_line_needs_update = false;
+    auto& line_boxes = m_containing_block_state.line_boxes;
 
-    if (m_context.containing_block().line_boxes().is_empty())
+    if (line_boxes.is_empty())
         return;
 
-    auto& line_box = m_context.containing_block().line_boxes().last();
+    auto& line_box = 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().content_width() - line_box.width();
+    float excess_horizontal_space = m_containing_block_state.content_width - line_box.width();
 
     switch (text_align) {
     case CSS::TextAlign::Center:
@@ -115,7 +127,7 @@ void LineBuilder::update_last_line()
     for (auto& fragment : line_box.fragments()) {
         float fragment_baseline;
         if (fragment.layout_node().is_box()) {
-            fragment_baseline = static_cast<Box const&>(fragment.layout_node()).content_height();
+            fragment_baseline = m_formatting_state.get(static_cast<Box const&>(fragment.layout_node())).content_height;
         } else {
             float font_baseline = fragment.layout_node().font().baseline();
             fragment_baseline = (max_height / 2.0f) + (font_baseline / 2.0f);
@@ -155,11 +167,10 @@ void LineBuilder::update_last_line()
 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_context.containing_block().line_boxes();
+    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;
     }
 }
-
 }