Implement collection of out-of-flow descendants

third_party/WebKit/Source/core/layout/ng/ng_fragment_builder.h

Index: third_party/WebKit/Source/core/layout/ng/ng_fragment_builder.h
diff --git a/third_party/WebKit/Source/core/layout/ng/ng_fragment_builder.h b/third_party/WebKit/Source/core/layout/ng/ng_fragment_builder.h
index 5b1a1f14a45407523f45b47e291a92e618ff25d8..ac703db1cbf49fe72e8cdd2c98ffad38dc98c911 100644
--- a/third_party/WebKit/Source/core/layout/ng/ng_fragment_builder.h
+++ b/third_party/WebKit/Source/core/layout/ng/ng_fragment_builder.h
@@ -28,8 +28,46 @@ class CORE_EXPORT NGFragmentBuilder final
 
   NGFragmentBuilder& AddChild(NGFragmentBase*, NGLogicalOffset);
 
-  NGFragmentBuilder& SetOutOfFlowDescendants(WeakBoxList&,
-                                             Vector<NGLogicalOffset>&);
+  // Builder has non-trivial out-of-flow descendant methods.
+  // These methods are building blocks for implementation of
+  // out-of-flow descendants by layout algorithms.
+  //
+  // They are intended to be used by layout algorithm like this:
+  //
+  // Part 1: layout algorithm positions in-flow children.
+  //   out-of-flow children, and out-of-flow descendants of fragments
+  //   are stored inside builder.
+  //
+  // for (child : children)
+  //   if (child->position == (Absolute or Fixed))
+  //     builder->AddOutOfFlowChildCandidate(child);
+  //   else
+  //     fragment = child->Layout()
+  //     builder->AddChild(fragment)
+  // end
+  //
+  // Part 2: layout algorithm positions out-of-flow descendants.
+  //
+  // builder->SetInlineSize/SetBlockSize
+  // builder->GetAndClearOutOfFlowDescendantCandidates(oof_candidates)
+  // while (oof_candidates.size() > 0)
+  // {
+  //   candidate = oof_candidates.shift()
+  //   if (CanPosition(candidate))
+  //     fragment = candidate->Layout();
+  //     builder->AddChild(fragment);
+  //     builder->GetAndClearOutOfFlowDescendantCandidates(child_oof_candidates)
+  //     oof_candidates.prepend(child_oof_candidates)
+  //   else
+  //     builder->AddOutOfFlowDescendant();
+  // }
+  NGFragmentBuilder& AddOutOfFlowChildCandidate(NGBlockNode*, NGLogicalOffset);
+
+  void GetAndClearOutOfFlowDescendantCandidates(WeakBoxList*,
+                                                Vector<NGStaticPosition>*);
+
+  NGFragmentBuilder& AddOutOfFlowDescendant(NGBlockNode*,
+                                            const NGStaticPosition&);
 
   // Sets MarginStrut for the resultant fragment.
   NGFragmentBuilder& SetMarginStrutBlockStart(const NGMarginStrut& from);
@@ -44,6 +82,24 @@ class CORE_EXPORT NGFragmentBuilder final
   DECLARE_VIRTUAL_TRACE();
 
  private:
+  // Out-of-flow descendant placement information.
+  // The generated fragment must compute NGStaticPosition for all
+  // out-of-flow descendants.
+  // The resulting NGStaticPosition gets derived from:
+  // 1. The offset of fragment's child.
+  // 2. The static position of descendant wrt child.
+  //
+  // A child can be:
+  // 1. A descendant itself. In this case, descendant position is (0,0).
+  // 2. A fragment containing a descendant.
+  //
+  // child_offset is stored as NGLogicalOffset because physical offset cannot
+  // be computed until we know fragment's size.
+  struct OutOfFlowPlacement {
+    NGLogicalOffset child_offset;
+    NGStaticPosition descendant_position;
+  };
+
   NGPhysicalFragmentBase::NGFragmentType type_;
   NGWritingMode writing_mode_;
   TextDirection direction_;
@@ -55,8 +111,12 @@ class CORE_EXPORT NGFragmentBuilder final
 
   HeapVector<Member<NGPhysicalFragmentBase>> children_;
   Vector<NGLogicalOffset> offsets_;
+
+  WeakBoxList out_of_flow_descendant_candidates_;
+  Vector<OutOfFlowPlacement> out_of_flow_candidate_placements_;
+
   WeakBoxList out_of_flow_descendants_;
-  Vector<NGLogicalOffset> out_of_flow_offsets_;
+  Vector<NGStaticPosition> out_of_flow_positions_;
 };
 
 }  // namespace blink