LibWeb: Make Layout::FormattingState copies shallow

Previously, each NodeState in a FormattingState was shared with the
parent FormattingState, but the HashMap of NodeState had to be copied
when making FormattingState copies.

This patch makes copying instant by keeping a pointer to the parent
FormattingState instead. When fetching immutable state via get(), we may
now return a reference to a NodeState owned by a parent FormattingState.

get_mutable() will copy any NodeState found in the ancestor chain before
making a brand new one.

Userland/Libraries/LibWeb/Layout/FlexFormattingContext.cpp

@@ -440,7 +440,7 @@ float FlexFormattingContext::calculate_indefinite_main_size(FlexItem const& item
 
     if (has_definite_cross_size(item.box)) {
         // For indefinite main sizes, we perform a throwaway layout and then measure it.
-        auto throwaway_state = m_state;
+        FormattingState throwaway_state(&m_state);
         auto& box_state = throwaway_state.get_mutable(item.box);
 
         // Item has definite cross size, layout with that as the used cross size.
@@ -470,7 +470,7 @@ float FlexFormattingContext::calculate_indefinite_main_size(FlexItem const& item
     // then layout with that and see what height comes out of it.
     float fit_content_cross_size = calculate_fit_content_width(item.box, m_available_space->cross);
 
-    auto throwaway_state = m_state;
+    FormattingState throwaway_state(&m_state);
     auto& box_state = throwaway_state.get_mutable(item.box);
 
     // Item has definite cross size, layout with that as the used cross size.
@@ -814,7 +814,7 @@ void FlexFormattingContext::determine_hypothetical_cross_size_of_item(FlexItem&
 
     if (has_definite_main_size(item.box)) {
         // For indefinite cross sizes, we perform a throwaway layout and then measure it.
-        auto throwaway_state = m_state;
+        FormattingState throwaway_state(&m_state);
         auto& box_state = throwaway_state.get_mutable(item.box);
 
         // Item has definite main size, layout with that as the used main size.
@@ -842,7 +842,7 @@ void FlexFormattingContext::determine_hypothetical_cross_size_of_item(FlexItem&
         else
             fit_content_main_size = calculate_fit_content_height(item.box, m_available_space->main);
 
-        auto throwaway_state = m_state;
+        FormattingState throwaway_state(&m_state);
         auto& box_state = throwaway_state.get_mutable(item.box);
 
         // Item has definite main size, layout with that as the used main size.

Userland/Libraries/LibWeb/Layout/FormattingContext.cpp

@@ -824,7 +824,7 @@ FormattingState::IntrinsicSizes FormattingContext::calculate_intrinsic_sizes(Lay
     auto& cached_box_sizes = m_state.intrinsic_sizes.ensure(&box);
     auto const& containing_block = *box.containing_block();
     {
-        auto throwaway_state = m_state;
+        FormattingState throwaway_state(&m_state);
         throwaway_state.get_mutable(containing_block).content_width = INFINITY;
         throwaway_state.get_mutable(containing_block).content_height = INFINITY;
         auto independent_formatting_context = const_cast<FormattingContext*>(this)->create_independent_formatting_context_if_needed(throwaway_state, box);
@@ -836,7 +836,7 @@ FormattingState::IntrinsicSizes FormattingContext::calculate_intrinsic_sizes(Lay
     }
 
     {
-        auto throwaway_state = m_state;
+        FormattingState throwaway_state(&m_state);
         throwaway_state.get_mutable(containing_block).content_width = 0;
         throwaway_state.get_mutable(containing_block).content_height = 0;
         auto independent_formatting_context = const_cast<FormattingContext*>(this)->create_independent_formatting_context_if_needed(throwaway_state, box);

Userland/Libraries/LibWeb/Layout/FormattingState.cpp

@@ -12,23 +12,39 @@ namespace Web::Layout {
 
 FormattingState::NodeState& FormattingState::get_mutable(NodeWithStyleAndBoxModelMetrics const& box)
 {
-    auto state = nodes.ensure(&box, [] { return adopt_ref(*new NodeState); });
-    // CoW if ref_count > 2 (1 for the entry in `this->nodes`, 1 for the `state` local in this function)
-    if (state->ref_count > 2) {
-        state = adopt_ref(*new NodeState { *state });
-        state->ref_count = 1;
-        nodes.set(&box, state);
+    if (auto it = nodes.find(&box); it != nodes.end())
+        return *it->value;
+
+    for (auto* ancestor = m_parent; ancestor; ancestor = ancestor->m_parent) {
+        if (auto it = ancestor->nodes.find(&box); it != ancestor->nodes.end()) {
+            auto cow_node_state = adopt_own(*new NodeState(*it->value));
+            auto* cow_node_state_ptr = cow_node_state.ptr();
+            nodes.set(&box, move(cow_node_state));
+            return *cow_node_state_ptr;
+        }
     }
-    return state;
+
+    return *nodes.ensure(&box, [] { return adopt_own(*new NodeState); });
 }
 
 FormattingState::NodeState const& FormattingState::get(NodeWithStyleAndBoxModelMetrics const& box) const
 {
-    return *const_cast<FormattingState&>(*this).nodes.ensure(&box, [] { return adopt_ref(*new NodeState); });
+    if (auto it = nodes.find(&box); it != nodes.end())
+        return *it->value;
+
+    for (auto* ancestor = m_parent; ancestor; ancestor = ancestor->m_parent) {
+        if (auto it = ancestor->nodes.find(&box); it != ancestor->nodes.end())
+            return *it->value;
+    }
+
+    return *const_cast<FormattingState&>(*this).nodes.ensure(&box, [] { return adopt_own(*new NodeState); });
 }
 
 void FormattingState::commit()
 {
+    // Only the top-level FormattingState should ever be committed.
+    VERIFY(!m_parent);
+
     HashTable<Layout::TextNode*> text_nodes;
 
     for (auto& it : nodes) {

Userland/Libraries/LibWeb/Layout/FormattingState.h

@@ -15,6 +15,12 @@
 namespace Web::Layout {
 
 struct FormattingState {
+    FormattingState() { }
+    explicit FormattingState(FormattingState const* parent)
+        : m_parent(parent)
+    {
+    }
+
     struct NodeState {
         float content_width { 0 };
         float content_height { 0 };
@@ -65,31 +71,17 @@ struct FormattingState {
         }
 
         Optional<LineBoxFragmentCoordinate> containing_line_box_fragment;
-
-        // NOTE: NodeState is ref-counted and accessed via copy-on-write helpers below.
-        size_t ref_count { 1 };
-        void ref()
-        {
-            VERIFY(ref_count);
-            ++ref_count;
-        }
-        void unref()
-        {
-            VERIFY(ref_count);
-            if (!--ref_count)
-                delete this;
-        }
     };
 
     void commit();
 
-    // NOTE: get_mutable() will CoW the NodeState if it's shared with another FormattingContext.
+    // NOTE: get_mutable() will CoW the NodeState if it's inherited from an ancestor state;
     NodeState& get_mutable(NodeWithStyleAndBoxModelMetrics const&);
 
     // NOTE: get() will not CoW the NodeState.
     NodeState const& get(NodeWithStyleAndBoxModelMetrics const&) const;
 
-    HashMap<NodeWithStyleAndBoxModelMetrics const*, NonnullRefPtr<NodeState>> nodes;
+    HashMap<NodeWithStyleAndBoxModelMetrics const*, NonnullOwnPtr<NodeState>> nodes;
 
     // We cache intrinsic sizes once determined, as they will not change over the course of a full layout.
     // This avoids computing them several times while performing flex layout.
@@ -98,6 +90,8 @@ struct FormattingState {
         Gfx::FloatSize max_content_size;
     };
     HashMap<NodeWithStyleAndBoxModelMetrics const*, IntrinsicSizes> mutable intrinsic_sizes;
+
+    FormattingState const* m_parent { nullptr };
 };
 
 Gfx::FloatRect absolute_content_rect(Box const&, FormattingState const&);