1 files changed, 152 insertions, 0 deletions
diff --git a/layout/generic/ScrollAnimationBezierPhysics.cpp b/layout/generic/ScrollAnimationBezierPhysics.cpp
new file mode 100644
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+++ b/layout/generic/ScrollAnimationBezierPhysics.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 "ScrollAnimationBezierPhysics.h"
+#include "mozilla/StaticPrefs_general.h"
+
+using namespace mozilla;
+
+ScrollAnimationBezierPhysics::ScrollAnimationBezierPhysics(
+    const nsPoint& aStartPos,
+    const ScrollAnimationBezierPhysicsSettings& aSettings)
+    : mSettings(aSettings), mStartPos(aStartPos), mIsFirstIteration(true) {}
+
+void ScrollAnimationBezierPhysics::Update(const TimeStamp& aTime,
+                                          const nsPoint& aDestination,
+                                          const nsSize& aCurrentVelocity) {
+  if (mIsFirstIteration) {
+    InitializeHistory(aTime);
+  }
+
+  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;
+}
+
+void ScrollAnimationBezierPhysics::ApplyContentShift(
+    const CSSPoint& aShiftDelta) {
+  nsPoint shiftDelta = CSSPoint::ToAppUnits(aShiftDelta);
+  mStartPos += shiftDelta;
+  mDestination += shiftDelta;
+}
+
+TimeDuration ScrollAnimationBezierPhysics::ComputeDuration(
+    const 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 * mSettings.mIntervalRatio,
+                       mSettings.mMinMS, mSettings.mMaxMS);
+
+  return TimeDuration::FromMilliseconds(durationMS);
+}
+
+void ScrollAnimationBezierPhysics::InitializeHistory(const 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(
+      mSettings.mMaxMS / mSettings.mIntervalRatio);
+  mPrevEventTime[0] = aTime - maxDelta;
+  mPrevEventTime[1] = mPrevEventTime[0] - maxDelta;
+  mPrevEventTime[2] = mPrevEventTime[1] - maxDelta;
+}
+
+void ScrollAnimationBezierPhysics::InitTimingFunction(
+    SMILKeySpline& aTimingFunction, nscoord aCurrentPos,
+    nscoord aCurrentVelocity, nscoord aDestination) {
+  if (aDestination == aCurrentPos ||
+      StaticPrefs::general_smoothScroll_currentVelocityWeighting() == 0) {
+    aTimingFunction.Init(
+        0, 0, 1 - StaticPrefs::general_smoothScroll_stopDecelerationWeighting(),
+        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 *
+              StaticPrefs::general_smoothScroll_currentVelocityWeighting();
+  double dxy = slope / normalization *
+               StaticPrefs::general_smoothScroll_currentVelocityWeighting();
+  aTimingFunction.Init(
+      dt, dxy,
+      1 - StaticPrefs::general_smoothScroll_stopDecelerationWeighting(), 1);
+}
+
+nsPoint ScrollAnimationBezierPhysics::PositionAt(const TimeStamp& aTime) {
+  if (IsFinished(aTime)) {
+    return mDestination;
+  }
+
+  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 ScrollAnimationBezierPhysics::VelocityAt(const TimeStamp& aTime) {
+  if (IsFinished(aTime)) {
+    return nsSize(0, 0);
+  }
+
+  double timeProgress = ProgressAt(aTime);
+  return nsSize(VelocityComponent(timeProgress, mTimingFunctionX, mStartPos.x,
+                                  mDestination.x),
+                VelocityComponent(timeProgress, mTimingFunctionY, mStartPos.y,
+                                  mDestination.y));
+}
+
+nscoord ScrollAnimationBezierPhysics::VelocityComponent(
+    double aTimeProgress, const SMILKeySpline& 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));
+}