Implemented additive animations for length

Source/core/animation/Interpolation.cpp

Index: Source/core/animation/Interpolation.cpp
diff --git a/Source/core/animation/Interpolation.cpp b/Source/core/animation/Interpolation.cpp
index b1aaee7cc40da8ffb9c617dc4173d0b7b1fba886..5ebc9dcac7ca5dfa324e51f94c14890d434075e1 100644
--- a/Source/core/animation/Interpolation.cpp
+++ b/Source/core/animation/Interpolation.cpp
@@ -5,6 +5,8 @@
 #include "config.h"
 #include "core/animation/Interpolation.h"
 
+#include "platform/animation/AnimationUtilities.h"
+
 namespace blink {
 
 namespace {
@@ -35,9 +37,12 @@ bool typesMatch(const InterpolableValue* start, const InterpolableValue* end)
 Interpolation::Interpolation(PassOwnPtrWillBeRawPtr<InterpolableValue> start, PassOwnPtrWillBeRawPtr<InterpolableValue> end)
     : m_start(start)
     , m_end(end)
+    , m_compositeStart(AnimationEffect::CompositeReplace)
+    , m_compositeEnd(AnimationEffect::CompositeReplace)
     , m_cachedFraction(0)
     , m_cachedIteration(0)
     , m_cachedValue(m_start->clone())
+    , m_cachedUnderlyingFraction(0)
 {
     RELEASE_ASSERT(typesMatch(m_start.get(), m_end.get()));
 }
@@ -46,12 +51,38 @@ Interpolation::~Interpolation()
 {
 }
 
+Interpolation::Interpolation(PassOwnPtrWillBeRawPtr<InterpolableValue> start, PassOwnPtrWillBeRawPtr<InterpolableValue> end,
+    AnimationEffect::CompositeOperation compositeStart, AnimationEffect::CompositeOperation compositeEnd)
+    : m_start(start)
+    , m_end(end)
+    , m_compositeStart(compositeStart)
+    , m_compositeEnd(compositeEnd)
+    , m_cachedFraction(0)
+    , m_cachedIteration(0)
+    , m_cachedValue(m_start->clone())
+    , m_cachedUnderlyingFraction(0)
+{
+    RELEASE_ASSERT(typesMatch(m_start.get(), m_end.get()));
+
+    if (m_compositeStart == AnimationEffect::CompositeAdd)
+        m_cachedUnderlyingFraction = 1;
+}
+
 void Interpolation::interpolate(int iteration, double fraction) const
 {
     if (m_cachedFraction != fraction || m_cachedIteration != iteration) {
         m_start->interpolate(*m_end, fraction, *m_cachedValue);
         m_cachedIteration = iteration;
         m_cachedFraction = fraction;
+
+        if (m_compositeStart == AnimationEffect::CompositeReplace && m_compositeEnd == AnimationEffect::CompositeReplace)
+            m_cachedUnderlyingFraction = 0;
+        else if (m_compositeStart == AnimationEffect::CompositeAdd && m_compositeEnd == AnimationEffect::CompositeAdd)
+            m_cachedUnderlyingFraction = 1;
+        else if (m_compositeStart == AnimationEffect::CompositeAdd && m_compositeEnd == AnimationEffect::CompositeReplace)
+            m_cachedUnderlyingFraction = 1 - fraction;
+        else if (m_compositeStart == AnimationEffect::CompositeReplace && m_compositeEnd == AnimationEffect::CompositeAdd)
+            m_cachedUnderlyingFraction = fraction;
     }
 }