1 files changed, 93 insertions, 0 deletions
diff --git a/layout/generic/ScrollVelocityQueue.cpp b/layout/generic/ScrollVelocityQueue.cpp
new file mode 100644
index 0000000000..44a8ae0033
--- /dev/null
+++ b/layout/generic/ScrollVelocityQueue.cpp
@@ -0,0 +1,93 @@
+/* -*- 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 "ScrollVelocityQueue.h"
+
+#include "mozilla/StaticPrefs_apz.h"
+#include "mozilla/StaticPrefs_layout.h"
+#include "nsPresContext.h"
+#include "nsRefreshDriver.h"
+
+namespace mozilla {
+namespace layout {
+
+void ScrollVelocityQueue::Sample(const nsPoint& aScrollPosition) {
+  float flingSensitivity =
+      StaticPrefs::layout_css_scroll_snap_prediction_sensitivity();
+  int maxVelocity =
+      StaticPrefs::layout_css_scroll_snap_prediction_max_velocity();
+  maxVelocity = nsPresContext::CSSPixelsToAppUnits(maxVelocity);
+  int maxOffset = maxVelocity * flingSensitivity;
+  TimeStamp currentRefreshTime =
+      mPresContext->RefreshDriver()->MostRecentRefresh();
+  if (mSampleTime.IsNull()) {
+    mAccumulator = nsPoint();
+  } else {
+    uint32_t durationMs = (currentRefreshTime - mSampleTime).ToMilliseconds();
+    if (durationMs > StaticPrefs::apz_velocity_relevance_time_ms()) {
+      mAccumulator = nsPoint();
+      mQueue.Clear();
+    } else if (durationMs == 0) {
+      mAccumulator += aScrollPosition - mLastPosition;
+    } else {
+      nsPoint velocity = mAccumulator * 1000 / durationMs;
+      velocity.Clamp(maxVelocity);
+      mQueue.AppendElement(std::make_pair(durationMs, velocity));
+      mAccumulator = aScrollPosition - mLastPosition;
+    }
+  }
+  mAccumulator.Clamp(maxOffset);
+  mSampleTime = currentRefreshTime;
+  mLastPosition = aScrollPosition;
+  TrimQueue();
+}
+
+void ScrollVelocityQueue::TrimQueue() {
+  if (mSampleTime.IsNull()) {
+    // There are no samples, nothing to do here.
+    return;
+  }
+
+  TimeStamp currentRefreshTime =
+      mPresContext->RefreshDriver()->MostRecentRefresh();
+  uint32_t timeDelta = (currentRefreshTime - mSampleTime).ToMilliseconds();
+  for (int i = mQueue.Length() - 1; i >= 0; i--) {
+    timeDelta += mQueue[i].first;
+    if (timeDelta >= StaticPrefs::apz_velocity_relevance_time_ms()) {
+      // The rest of the samples have expired and should be dropped
+      for (; i >= 0; i--) {
+        mQueue.RemoveElementAt(0);
+      }
+    }
+  }
+}
+
+void ScrollVelocityQueue::Reset() {
+  mAccumulator = nsPoint();
+  mSampleTime = TimeStamp();
+  mQueue.Clear();
+}
+
+/**
+  Calculate the velocity of the scroll frame, in appunits / second.
+*/
+nsPoint ScrollVelocityQueue::GetVelocity() {
+  TrimQueue();
+  if (mQueue.Length() == 0) {
+    // If getting the scroll velocity before any scrolling has occurred,
+    // the velocity must be (0, 0)
+    return nsPoint();
+  }
+  nsPoint velocity;
+  for (int i = mQueue.Length() - 1; i >= 0; i--) {
+    velocity += mQueue[i].second;
+  }
+  return velocity / mQueue.Length();
+  ;
+}
+
+}  // namespace layout
+}  // namespace mozilla