1 files changed, 69 insertions, 0 deletions
diff --git a/gfx/layers/apz/src/OvershootDetector.cpp b/gfx/layers/apz/src/OvershootDetector.cpp
new file mode 100644
index 0000000000..c713ce8d1a
--- /dev/null
+++ b/gfx/layers/apz/src/OvershootDetector.cpp
@@ -0,0 +1,69 @@
+/* -*- 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 "OvershootDetector.h"
+
+#include "InputData.h"
+#include "mozilla/Telemetry.h"  // for Telemetry
+
+namespace mozilla {
+namespace layers {
+
+const TimeDuration kOvershootInterval = TimeDuration::FromMilliseconds(500);
+
+static Maybe<Side> GetScrollDirection(const ScrollWheelInput& aInput) {
+  // If the wheel input is scrolling on both axes, we just take the y-axis.
+  // This is mostly out of laziness, because this code is for experiment
+  // purposes and just needs to handle the cases that will dominate in the data.
+  if (aInput.mDeltaY > 0) {
+    return Some(Side::eSideBottom);
+  }
+  if (aInput.mDeltaY < 0) {
+    return Some(Side::eSideTop);
+  }
+  if (aInput.mDeltaX > 0) {
+    return Some(Side::eSideRight);
+  }
+  if (aInput.mDeltaX < 0) {
+    return Some(Side::eSideLeft);
+  }
+  return Nothing();
+}
+
+void OvershootDetector::Update(const ScrollWheelInput& aInput) {
+  TimeStamp inputTime = aInput.mTimeStamp;
+  Maybe<Side> inputDirection = GetScrollDirection(aInput);
+  if (mLastTimeStamp && (inputTime - mLastTimeStamp) < kOvershootInterval &&
+      mLastDirection && inputDirection) {
+    // The new input we got happened within the kOvershootInterval of the last
+    // input. Let's see if the direction is reversed; if so, we accumulate to
+    // telemetry.
+    bool reversed = false;
+    switch (*mLastDirection) {
+      case Side::eSideBottom:
+        reversed = (*inputDirection == Side::eSideTop);
+        break;
+      case Side::eSideTop:
+        reversed = (*inputDirection == Side::eSideBottom);
+        break;
+      case Side::eSideRight:
+        reversed = (*inputDirection == Side::eSideLeft);
+        break;
+      case Side::eSideLeft:
+        reversed = (*inputDirection == Side::eSideRight);
+        break;
+    }
+    if (reversed) {
+      Telemetry::ScalarAdd(Telemetry::ScalarID::APZ_SCROLLWHEEL_OVERSHOOT, 1);
+    }
+  }
+
+  mLastTimeStamp = inputTime;
+  mLastDirection = inputDirection;
+}
+
+}  // namespace layers
+}  // namespace mozilla