Common up SVG transform "change detection" (classification)

third_party/WebKit/Source/core/layout/svg/SVGLayoutSupport.h

Index: third_party/WebKit/Source/core/layout/svg/SVGLayoutSupport.h
diff --git a/third_party/WebKit/Source/core/layout/svg/SVGLayoutSupport.h b/third_party/WebKit/Source/core/layout/svg/SVGLayoutSupport.h
index 9097278a4aae2b30118a8dc070a20c826b9f6d1a..43b7ddb4ea31193fc79ba4d8fd5965f5768e7625 100644
--- a/third_party/WebKit/Source/core/layout/svg/SVGLayoutSupport.h
+++ b/third_party/WebKit/Source/core/layout/svg/SVGLayoutSupport.h
@@ -130,6 +130,43 @@ private:
     AffineTransform m_savedContentTransformation;
 };
 
+// The following enumeration is used to optimize cases where the scale is known
+// to be invariant (see: LayoutSVGContainer::layout and LayoutSVGroot). The
+// value 'Full' can be used in the general case when the scale change is
+// unknown, or known to change.
+enum class SVGTransformChange {
+    None,
+    ScaleInvariant,
+    Full,
+};
+
+// Helper for computing ("classifying") a change to a transform using the
+// categoies defined above.
+class SVGTransformChangeDetector {
+    STACK_ALLOCATED();
+public:
+    explicit SVGTransformChangeDetector(const AffineTransform& previous)
+        : m_previousTransform(previous)
+    {
+    }
+
+    SVGTransformChange computeChange(const AffineTransform& current)
+    {
+        if (m_previousTransform == current)
+            return SVGTransformChange::None;
+        if (scaleReference(m_previousTransform) == scaleReference(current))
+            return SVGTransformChange::ScaleInvariant;
+        return SVGTransformChange::Full;
+    }
+
+private:
+    static std::pair<double, double> scaleReference(const AffineTransform& transform)
+    {
+        return std::make_pair(transform.xScaleSquared(), transform.yScaleSquared());
+    }
+    AffineTransform m_previousTransform;
+};
+
 template <typename LayoutObjectType>
 bool SVGLayoutSupport::computeHasNonIsolatedBlendingDescendants(const LayoutObjectType* object)
 {