1 files changed, 128 insertions, 0 deletions
diff --git a/gfx/layers/apz/src/FlingAccelerator.cpp b/gfx/layers/apz/src/FlingAccelerator.cpp
new file mode 100644
index 0000000000..f37d04c77b
--- /dev/null
+++ b/gfx/layers/apz/src/FlingAccelerator.cpp
@@ -0,0 +1,128 @@
+/* -*- 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 "FlingAccelerator.h"
+
+#include "mozilla/StaticPrefs_apz.h"
+
+#include "GenericFlingAnimation.h"  // for FLING_LOG and FlingHandoffState
+
+namespace mozilla {
+namespace layers {
+
+void FlingAccelerator::Reset() {
+  mPreviousFlingStartingVelocity = ParentLayerPoint{};
+  mPreviousFlingCancelVelocity = ParentLayerPoint{};
+  mIsTracking = false;
+}
+
+static bool SameDirection(float aVelocity1, float aVelocity2) {
+  return (aVelocity1 == 0.0f) || (aVelocity2 == 0.0f) ||
+         (IsNegative(aVelocity1) == IsNegative(aVelocity2));
+}
+
+static float Accelerate(float aBase, float aSupplemental) {
+  return (aBase * StaticPrefs::apz_fling_accel_base_mult()) +
+         (aSupplemental * StaticPrefs::apz_fling_accel_supplemental_mult());
+}
+
+ParentLayerPoint FlingAccelerator::GetFlingStartingVelocity(
+    const SampleTime& aNow, const ParentLayerPoint& aVelocity,
+    const FlingHandoffState& aHandoffState) {
+  // If the fling should be accelerated and is in the same direction as the
+  // previous fling, boost the velocity to be the sum of the two. Check separate
+  // axes separately because we could have two vertical flings with small
+  // horizontal components on the opposite side of zero, and we still want the
+  // y-fling to get accelerated.
+  ParentLayerPoint velocity = aVelocity;
+  if (ShouldAccelerate(aNow, aVelocity, aHandoffState)) {
+    if (velocity.x != 0 &&
+        SameDirection(velocity.x, mPreviousFlingStartingVelocity.x)) {
+      velocity.x = Accelerate(velocity.x, mPreviousFlingStartingVelocity.x);
+      FLING_LOG("%p Applying fling x-acceleration from %f to %f (delta %f)\n",
+                this, aVelocity.x.value, velocity.x.value,
+                mPreviousFlingStartingVelocity.x.value);
+    }
+    if (velocity.y != 0 &&
+        SameDirection(velocity.y, mPreviousFlingStartingVelocity.y)) {
+      velocity.y = Accelerate(velocity.y, mPreviousFlingStartingVelocity.y);
+      FLING_LOG("%p Applying fling y-acceleration from %f to %f (delta %f)\n",
+                this, aVelocity.y.value, velocity.y.value,
+                mPreviousFlingStartingVelocity.y.value);
+    }
+  }
+
+  Reset();
+
+  mPreviousFlingStartingVelocity = velocity;
+  mIsTracking = true;
+
+  return velocity;
+}
+
+bool FlingAccelerator::ShouldAccelerate(
+    const SampleTime& aNow, const ParentLayerPoint& aVelocity,
+    const FlingHandoffState& aHandoffState) const {
+  if (!IsTracking()) {
+    FLING_LOG("%p Fling accelerator was reset, not accelerating.\n", this);
+    return false;
+  }
+
+  if (!aHandoffState.mTouchStartRestingTime) {
+    FLING_LOG("%p Don't have a touch start resting time, not accelerating.\n",
+              this);
+    return false;
+  }
+
+  double msBetweenTouchStartAndPanStart =
+      aHandoffState.mTouchStartRestingTime->ToMilliseconds();
+  FLING_LOG(
+      "%p ShouldAccelerate with pan velocity %f pixels/ms, min pan velocity %f "
+      "pixels/ms, previous fling cancel velocity %f pixels/ms, time elapsed "
+      "since starting previous time between touch start and pan "
+      "start %fms.\n",
+      this, float(aVelocity.Length()), float(aHandoffState.mMinPanVelocity),
+      float(mPreviousFlingCancelVelocity.Length()),
+      float(msBetweenTouchStartAndPanStart));
+
+  if (aVelocity.Length() < StaticPrefs::apz_fling_accel_min_fling_velocity()) {
+    FLING_LOG("%p Fling velocity too low (%f), not accelerating.\n", this,
+              float(aVelocity.Length()));
+    return false;
+  }
+
+  if (aHandoffState.mMinPanVelocity <
+      StaticPrefs::apz_fling_accel_min_pan_velocity()) {
+    FLING_LOG(
+        "%p Panning velocity was too slow at some point during the pan (%f), "
+        "not accelerating.\n",
+        this, float(aHandoffState.mMinPanVelocity));
+    return false;
+  }
+
+  if (mPreviousFlingCancelVelocity.Length() <
+      StaticPrefs::apz_fling_accel_min_fling_velocity()) {
+    FLING_LOG(
+        "%p The previous fling animation had slowed down too much when it was "
+        "interrupted (%f), not accelerating.\n",
+        this, float(mPreviousFlingCancelVelocity.Length()));
+    return false;
+  }
+
+  if (msBetweenTouchStartAndPanStart >=
+      StaticPrefs::apz_fling_accel_max_pause_interval_ms()) {
+    FLING_LOG(
+        "%p Too much time (%fms) elapsed between touch start and pan start, "
+        "not accelerating.\n",
+        this, msBetweenTouchStartAndPanStart);
+    return false;
+  }
+
+  return true;
+}
+
+}  // namespace layers
+}  // namespace mozilla