1 files changed, 133 insertions, 0 deletions
diff --git a/layout/generic/AsyncScrollBase.cpp b/layout/generic/AsyncScrollBase.cpp
new file mode 100644
index 000000000..e85444f0f
--- /dev/null
+++ b/layout/generic/AsyncScrollBase.cpp
@@ -0,0 +1,133 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* 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 "AsyncScrollBase.h"
+#include "gfxPrefs.h"
+
+using namespace mozilla;
+
+AsyncScrollBase::AsyncScrollBase(nsPoint aStartPos)
+ : mIsFirstIteration(true)
+ , mStartPos(aStartPos)
+{
+}
+
+void
+AsyncScrollBase::Update(TimeStamp aTime,
+                        nsPoint aDestination,
+                        const nsSize& aCurrentVelocity)
+{
+  TimeDuration duration = ComputeDuration(aTime);
+  nsSize currentVelocity = aCurrentVelocity;
+
+  if (!mIsFirstIteration) {
+    // If an additional event has not changed the destination, then do not let
+    // another minimum duration reset slow things down.  If it would then
+    // instead continue with the existing timing function.
+    if (aDestination == mDestination &&
+        aTime + duration > mStartTime + mDuration)
+    {
+      return;
+    }
+
+    currentVelocity = VelocityAt(aTime);
+    mStartPos = PositionAt(aTime);
+  }
+
+  mStartTime = aTime;
+  mDuration = duration;
+  mDestination = aDestination;
+  InitTimingFunction(mTimingFunctionX, mStartPos.x, currentVelocity.width,
+                     aDestination.x);
+  InitTimingFunction(mTimingFunctionY, mStartPos.y, currentVelocity.height,
+                     aDestination.y);
+  mIsFirstIteration = false;
+}
+
+TimeDuration
+AsyncScrollBase::ComputeDuration(TimeStamp aTime)
+{
+  // Average last 3 delta durations (rounding errors up to 2ms are negligible for us)
+  int32_t eventsDeltaMs = (aTime - mPrevEventTime[2]).ToMilliseconds() / 3;
+  mPrevEventTime[2] = mPrevEventTime[1];
+  mPrevEventTime[1] = mPrevEventTime[0];
+  mPrevEventTime[0] = aTime;
+
+  // Modulate duration according to events rate (quicker events -> shorter durations).
+  // The desired effect is to use longer duration when scrolling slowly, such that
+  // it's easier to follow, but reduce the duration to make it feel more snappy when
+  // scrolling quickly. To reduce fluctuations of the duration, we average event
+  // intervals using the recent 4 timestamps (now + three prev -> 3 intervals).
+  int32_t durationMS = clamped<int32_t>(eventsDeltaMs * mIntervalRatio, mOriginMinMS, mOriginMaxMS);
+
+  return TimeDuration::FromMilliseconds(durationMS);
+}
+
+void
+AsyncScrollBase::InitializeHistory(TimeStamp aTime)
+{
+  // Starting a new scroll (i.e. not when extending an existing scroll animation),
+  // create imaginary prev timestamps with maximum relevant intervals between them.
+
+  // Longest relevant interval (which results in maximum duration)
+  TimeDuration maxDelta = TimeDuration::FromMilliseconds(mOriginMaxMS / mIntervalRatio);
+  mPrevEventTime[0] = aTime              - maxDelta;
+  mPrevEventTime[1] = mPrevEventTime[0]  - maxDelta;
+  mPrevEventTime[2] = mPrevEventTime[1]  - maxDelta;
+}
+
+void
+AsyncScrollBase::InitTimingFunction(nsSMILKeySpline& aTimingFunction,
+                                    nscoord aCurrentPos,
+                                    nscoord aCurrentVelocity,
+                                    nscoord aDestination)
+{
+  if (aDestination == aCurrentPos || gfxPrefs::SmoothScrollCurrentVelocityWeighting() == 0) {
+    aTimingFunction.Init(0, 0, 1 - gfxPrefs::SmoothScrollStopDecelerationWeighting(), 1);
+    return;
+  }
+
+  const TimeDuration oneSecond = TimeDuration::FromSeconds(1);
+  double slope = aCurrentVelocity * (mDuration / oneSecond) / (aDestination - aCurrentPos);
+  double normalization = sqrt(1.0 + slope * slope);
+  double dt = 1.0 / normalization * gfxPrefs::SmoothScrollCurrentVelocityWeighting();
+  double dxy = slope / normalization * gfxPrefs::SmoothScrollCurrentVelocityWeighting();
+  aTimingFunction.Init(dt, dxy, 1 - gfxPrefs::SmoothScrollStopDecelerationWeighting(), 1);
+}
+
+nsPoint
+AsyncScrollBase::PositionAt(TimeStamp aTime) const
+{
+  double progressX = mTimingFunctionX.GetSplineValue(ProgressAt(aTime));
+  double progressY = mTimingFunctionY.GetSplineValue(ProgressAt(aTime));
+  return nsPoint(NSToCoordRound((1 - progressX) * mStartPos.x + progressX * mDestination.x),
+                 NSToCoordRound((1 - progressY) * mStartPos.y + progressY * mDestination.y));
+}
+
+nsSize
+AsyncScrollBase::VelocityAt(TimeStamp aTime) const
+{
+  double timeProgress = ProgressAt(aTime);
+  return nsSize(VelocityComponent(timeProgress, mTimingFunctionX,
+                                  mStartPos.x, mDestination.x),
+                VelocityComponent(timeProgress, mTimingFunctionY,
+                                  mStartPos.y, mDestination.y));
+}
+
+nscoord
+AsyncScrollBase::VelocityComponent(double aTimeProgress,
+                                   const nsSMILKeySpline& aTimingFunction,
+                                   nscoord aStart,
+                                   nscoord aDestination) const
+{
+  double dt, dxy;
+  aTimingFunction.GetSplineDerivativeValues(aTimeProgress, dt, dxy);
+  if (dt == 0)
+    return dxy >= 0 ? nscoord_MAX : nscoord_MIN;
+
+  const TimeDuration oneSecond = TimeDuration::FromSeconds(1);
+  double slope = dxy / dt;
+  return NSToCoordRound(slope * (aDestination - aStart) / (mDuration / oneSecond));
+}