1 files changed, 97 insertions, 0 deletions
diff --git a/layout/generic/ScrollVelocityQueue.cpp b/layout/generic/ScrollVelocityQueue.cpp
new file mode 100644
index 000000000..820e6cb5c
--- /dev/null
+++ b/layout/generic/ScrollVelocityQueue.cpp
@@ -0,0 +1,97 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set shiftwidth=2 tabstop=8 autoindent cindent expandtab: */
+/* 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 "gfxPrefs.h"
+#include "nsPresContext.h"
+#include "nsRefreshDriver.h"
+
+namespace mozilla {
+namespace layout {
+
+void
+ScrollVelocityQueue::Sample(const nsPoint& aScrollPosition)
+{
+  float flingSensitivity = gfxPrefs::ScrollSnapPredictionSensitivity();
+  int maxVelocity = gfxPrefs::ScrollSnapPredictionMaxVelocity();
+  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 > gfxPrefs::APZVelocityRelevanceTime()) {
+      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();
+  nsPoint velocity;
+  uint32_t timeDelta = (currentRefreshTime - mSampleTime).ToMilliseconds();
+  for (int i = mQueue.Length() - 1; i >= 0; i--) {
+    timeDelta += mQueue[i].first;
+    if (timeDelta >= gfxPrefs::APZVelocityRelevanceTime()) {
+      // 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
+