Bug 1803861 - Implement nsPageFrame::FirstContinuation and nsPageContentFrame::FirstContinuation with constant complexity r=dholbert

Differential Revision: https://phabricator.services.mozilla.com/D163759

layout/generic/nsPageContentFrame.cpp

@@ -421,6 +421,21 @@ void nsPageContentFrame::EnsurePageName() {
   SetComputedStyleWithoutNotification(pageContentPseudoStyle);
 }
 
+nsIFrame* nsPageContentFrame::FirstContinuation() const {
+  const nsContainerFrame* const parent = GetParent();
+  MOZ_ASSERT(parent && parent->IsPageFrame(),
+             "Parent of nsPageContentFrame should be nsPageFrame");
+  // static cast so the compiler has a chance to devirtualize the call.
+  const auto* const pageFrameParent = static_cast<const nsPageFrame*>(parent);
+  nsPageContentFrame* const pageContentFrame =
+      static_cast<const nsPageFrame*>(pageFrameParent->FirstContinuation())
+          ->PageContentFrame();
+  MOZ_ASSERT(pageContentFrame && !pageContentFrame->GetPrevContinuation(),
+             "First descendent of nsPageSequenceFrame should not have a "
+             "previous continuation");
+  return pageContentFrame;
+}
+
 #ifdef DEBUG_FRAME_DUMP
 nsresult nsPageContentFrame::GetFrameName(nsAString& aResult) const {
   return MakeFrameName(u"PageContent"_ns, aResult);

layout/generic/nsPageContentFrame.h

@@ -51,6 +51,13 @@ class nsPageContentFrame final : public mozilla::ViewportFrame {
 
   void EnsurePageName();
 
+  // The default implementation of FirstContinuation in nsSplittableFrame is
+  // implemented in linear time, walking back through the linked list of
+  // continuations via mPrevContinuation.
+  // For nsPageContentFrames, we can find the first continuation through the
+  // frame tree structure in constant time.
+  nsIFrame* FirstContinuation() const final;
+
 #ifdef DEBUG_FRAME_DUMP
   // Debugging
   nsresult GetFrameName(nsAString& aResult) const override;

layout/generic/nsPageFrame.cpp

@@ -656,6 +656,35 @@ float nsPageFrame::ComputePageSizeScale(const nsSize aContentPageSize) const {
   return scale;
 }
 
+nsIFrame* nsPageFrame::FirstContinuation() const {
+  // Walk up to our grandparent, and then to down the grandparent's first
+  // child, and then that frame's first child.
+  // At every step we assert the frames are the type we expect them to be.
+  const nsContainerFrame* const parent = GetParent();
+  MOZ_ASSERT(parent && parent->IsPrintedSheetFrame(),
+             "Parent of nsPageFrame should be PrintedSheetFrame");
+  const nsContainerFrame* const pageSequenceFrame = parent->GetParent();
+  MOZ_ASSERT(pageSequenceFrame && pageSequenceFrame->IsPageSequenceFrame(),
+             "Parent of PrintedSheetFrame should be nsPageSequenceFrame");
+  const nsIFrame* const firstPrintedSheetFrame =
+      pageSequenceFrame->PrincipalChildList().FirstChild();
+  MOZ_ASSERT(
+      firstPrintedSheetFrame && firstPrintedSheetFrame->IsPrintedSheetFrame(),
+      "Should have at least one child in nsPageSequenceFrame, and all "
+      "children of nsPageSequenceFrame should be PrintedSheetFrames");
+  nsIFrame* const firstPageFrame =
+      static_cast<const nsContainerFrame*>(firstPrintedSheetFrame)
+          ->PrincipalChildList()
+          .FirstChild();
+  MOZ_ASSERT(firstPageFrame && firstPageFrame->IsPageFrame(),
+             "Should have at least one child in PrintedSheetFrame, and all "
+             "children of PrintedSheetFrame should be nsPageFrames");
+  MOZ_ASSERT(!firstPageFrame->GetPrevContinuation(),
+             "First descendent of nsPageSequenceFrame should not have a "
+             "previous continuation");
+  return firstPageFrame;
+}
+
 void nsPageFrame::BuildDisplayList(nsDisplayListBuilder* aBuilder,
                                    const nsDisplayListSet& aLists) {
   nsDisplayList content(aBuilder);

layout/generic/nsPageFrame.h

@@ -81,6 +81,13 @@ class nsPageFrame final : public nsContainerFrame {
     return ComputePageSizeScale(ComputePageSize());
   }
 
+  // The default implementation of FirstContinuation in nsSplittableFrame is
+  // implemented in linear time, walking back through the linked list of
+  // continuations via mPrevContinuation.
+  // For nsPageFrames, we can find the first continuation through the frame
+  // tree structure in constant time.
+  nsIFrame* FirstContinuation() const final;
+
  protected:
   explicit nsPageFrame(ComputedStyle* aStyle, nsPresContext* aPresContext);
   virtual ~nsPageFrame();

layout/generic/nsSplittableFrame.h

@@ -50,7 +50,7 @@ class nsSplittableFrame : public nsIFrame {
   void SetNextContinuation(nsIFrame*) final;
 
   // Get the first/last continuation for this frame.
-  nsIFrame* FirstContinuation() const final;
+  nsIFrame* FirstContinuation() const override;
   nsIFrame* LastContinuation() const final;
 
 #ifdef DEBUG