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diff --git a/dom/smil/SMILCompositor.cpp b/dom/smil/SMILCompositor.cpp
new file mode 100644
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+++ b/dom/smil/SMILCompositor.cpp
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "SMILCompositor.h"
+
+#include "mozilla/dom/SVGSVGElement.h"
+#include "nsComputedDOMStyle.h"
+#include "nsCSSProps.h"
+#include "nsHashKeys.h"
+#include "SMILCSSProperty.h"
+
+namespace mozilla {
+
+// PLDHashEntryHdr methods
+bool SMILCompositor::KeyEquals(KeyTypePointer aKey) const {
+  return aKey && aKey->Equals(mKey);
+}
+
+/*static*/
+PLDHashNumber SMILCompositor::HashKey(KeyTypePointer aKey) {
+  // Combine the 3 values into one numeric value, which will be hashed.
+  // NOTE: We right-shift one of the pointers by 2 to get some randomness in
+  // its 2 lowest-order bits. (Those shifted-off bits will always be 0 since
+  // our pointers will be word-aligned.)
+  return (NS_PTR_TO_UINT32(aKey->mElement.get()) >> 2) +
+         NS_PTR_TO_UINT32(aKey->mAttributeName.get());
+}
+
+// Cycle-collection support
+void SMILCompositor::Traverse(nsCycleCollectionTraversalCallback* aCallback) {
+  if (!mKey.mElement) return;
+
+  NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(*aCallback, "Compositor mKey.mElement");
+  aCallback->NoteXPCOMChild(mKey.mElement);
+}
+
+// Other methods
+void SMILCompositor::AddAnimationFunction(SMILAnimationFunction* aFunc) {
+  if (aFunc) {
+    mAnimationFunctions.AppendElement(aFunc);
+  }
+}
+
+void SMILCompositor::ComposeAttribute(bool& aMightHavePendingStyleUpdates) {
+  if (!mKey.mElement) return;
+
+  // If we might need to resolve base styles, grab a suitable ComputedStyle
+  // for initializing our SMILAttr with.
+  RefPtr<const ComputedStyle> baseComputedStyle;
+  if (MightNeedBaseStyle()) {
+    baseComputedStyle = nsComputedDOMStyle::GetUnanimatedComputedStyleNoFlush(
+        mKey.mElement, PseudoStyleType::NotPseudo);
+  }
+
+  // FIRST: Get the SMILAttr (to grab base value from, and to eventually
+  // give animated value to)
+  UniquePtr<SMILAttr> smilAttr = CreateSMILAttr(baseComputedStyle);
+  if (!smilAttr) {
+    // Target attribute not found (or, out of memory)
+    return;
+  }
+  if (mAnimationFunctions.IsEmpty()) {
+    // No active animation functions. (We can still have a SMILCompositor in
+    // that case if an animation function has *just* become inactive)
+    smilAttr->ClearAnimValue();
+    // Removing the animation effect may require a style update.
+    aMightHavePendingStyleUpdates = true;
+    return;
+  }
+
+  // SECOND: Sort the animationFunctions, to prepare for compositing.
+  SMILAnimationFunction::Comparator comparator;
+  mAnimationFunctions.Sort(comparator);
+
+  // THIRD: Step backwards through animation functions to find out
+  // which ones we actually care about.
+  uint32_t firstFuncToCompose = GetFirstFuncToAffectSandwich();
+
+  // FOURTH: Get & cache base value
+  SMILValue sandwichResultValue;
+  if (!mAnimationFunctions[firstFuncToCompose]->WillReplace()) {
+    sandwichResultValue = smilAttr->GetBaseValue();
+  }
+  UpdateCachedBaseValue(sandwichResultValue);
+
+  if (!mForceCompositing) {
+    return;
+  }
+
+  // FIFTH: Compose animation functions
+  aMightHavePendingStyleUpdates = true;
+  uint32_t length = mAnimationFunctions.Length();
+  for (uint32_t i = firstFuncToCompose; i < length; ++i) {
+    mAnimationFunctions[i]->ComposeResult(*smilAttr, sandwichResultValue);
+  }
+  if (sandwichResultValue.IsNull()) {
+    smilAttr->ClearAnimValue();
+    return;
+  }
+
+  // SIXTH: Set the animated value to the final composited result.
+  nsresult rv = smilAttr->SetAnimValue(sandwichResultValue);
+  if (NS_FAILED(rv)) {
+    NS_WARNING("SMILAttr::SetAnimValue failed");
+  }
+}
+
+void SMILCompositor::ClearAnimationEffects() {
+  if (!mKey.mElement || !mKey.mAttributeName) return;
+
+  UniquePtr<SMILAttr> smilAttr = CreateSMILAttr(nullptr);
+  if (!smilAttr) {
+    // Target attribute not found (or, out of memory)
+    return;
+  }
+  smilAttr->ClearAnimValue();
+}
+
+// Protected Helper Functions
+// --------------------------
+UniquePtr<SMILAttr> SMILCompositor::CreateSMILAttr(
+    const ComputedStyle* aBaseComputedStyle) {
+  nsCSSPropertyID propID = GetCSSPropertyToAnimate();
+
+  if (propID != eCSSProperty_UNKNOWN) {
+    return MakeUnique<SMILCSSProperty>(propID, mKey.mElement.get(),
+                                       aBaseComputedStyle);
+  }
+
+  return mKey.mElement->GetAnimatedAttr(mKey.mAttributeNamespaceID,
+                                        mKey.mAttributeName);
+}
+
+nsCSSPropertyID SMILCompositor::GetCSSPropertyToAnimate() const {
+  if (mKey.mAttributeNamespaceID != kNameSpaceID_None) {
+    return eCSSProperty_UNKNOWN;
+  }
+
+  nsCSSPropertyID propID =
+      nsCSSProps::LookupProperty(nsAtomCString(mKey.mAttributeName));
+
+  if (!SMILCSSProperty::IsPropertyAnimatable(propID)) {
+    return eCSSProperty_UNKNOWN;
+  }
+
+  // If we are animating the 'width' or 'height' of an outer SVG
+  // element we should animate it as a CSS property, but for other elements
+  // in SVG namespace (e.g. <rect>) we should animate it as a length attribute.
+  if ((mKey.mAttributeName == nsGkAtoms::width ||
+       mKey.mAttributeName == nsGkAtoms::height) &&
+      mKey.mElement->GetNameSpaceID() == kNameSpaceID_SVG) {
+    // Not an <svg> element.
+    if (!mKey.mElement->IsSVGElement(nsGkAtoms::svg)) {
+      return eCSSProperty_UNKNOWN;
+    }
+
+    // An inner <svg> element
+    if (static_cast<dom::SVGSVGElement const&>(*mKey.mElement).IsInner()) {
+      return eCSSProperty_UNKNOWN;
+    }
+
+    // Indeed an outer <svg> element, fall through.
+  }
+
+  return propID;
+}
+
+bool SMILCompositor::MightNeedBaseStyle() const {
+  if (GetCSSPropertyToAnimate() == eCSSProperty_UNKNOWN) {
+    return false;
+  }
+
+  // We should return true if at least one animation function might build on
+  // the base value.
+  for (const SMILAnimationFunction* func : mAnimationFunctions) {
+    if (!func->WillReplace()) {
+      return true;
+    }
+  }
+
+  return false;
+}
+
+uint32_t SMILCompositor::GetFirstFuncToAffectSandwich() {
+  // For performance reasons, we throttle most animations on elements in
+  // display:none subtrees. (We can't throttle animations that target the
+  // "display" property itself, though -- if we did, display:none elements
+  // could never be dynamically displayed via animations.)
+  // To determine whether we're in a display:none subtree, we will check the
+  // element's primary frame since element in display:none subtree doesn't have
+  // a primary frame. Before this process, we will construct frame when we
+  // append an element to subtree. So we will not need to worry about pending
+  // frame construction in this step.
+  bool canThrottle = mKey.mAttributeName != nsGkAtoms::display &&
+                     !mKey.mElement->GetPrimaryFrame();
+
+  uint32_t i;
+  for (i = mAnimationFunctions.Length(); i > 0; --i) {
+    SMILAnimationFunction* curAnimFunc = mAnimationFunctions[i - 1];
+    // In the following, the lack of short-circuit behavior of |= means that we
+    // will ALWAYS run UpdateCachedTarget (even if mForceCompositing is true)
+    // but only call HasChanged and WasSkippedInPrevSample if necessary.  This
+    // is important since we need UpdateCachedTarget to run in order to detect
+    // changes to the target in subsequent samples.
+    mForceCompositing |= curAnimFunc->UpdateCachedTarget(mKey) ||
+                         (curAnimFunc->HasChanged() && !canThrottle) ||
+                         curAnimFunc->WasSkippedInPrevSample();
+
+    if (curAnimFunc->WillReplace()) {
+      --i;
+      break;
+    }
+  }
+
+  // Mark remaining animation functions as having been skipped so if we later
+  // use them we'll know to force compositing.
+  // Note that we only really need to do this if something has changed
+  // (otherwise we would have set the flag on a previous sample) and if
+  // something has changed mForceCompositing will be true.
+  if (mForceCompositing) {
+    for (uint32_t j = i; j > 0; --j) {
+      mAnimationFunctions[j - 1]->SetWasSkipped();
+    }
+  }
+  return i;
+}
+
+void SMILCompositor::UpdateCachedBaseValue(const SMILValue& aBaseValue) {
+  if (mCachedBaseValue != aBaseValue) {
+    // Base value has changed since last sample.
+    mCachedBaseValue = aBaseValue;
+    mForceCompositing = true;
+  }
+}
+
+}  // namespace mozilla