Implement Centroid calculation using a Kahan sum. Makes calc O(1).

This modifies Centroid calculation to be O(1) when calculating a
centroid of a stroke. Centroid fetching/checking happens a lot, so
it's worthwhile to optimize.

Nevertheless this is still a heavy O(1) - so alternatives are happily
considered.

Bug: 1009290
Change-Id: Ia421502c1cdf5e37bd5c6f80dc4f866af2119b7f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1880237Reviewed-by: Michael Spang <spang@chromium.org>
Commit-Queue: Rob Schonberger <robsc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709777}

ui/events/ozone/evdev/touch_filter/neural_stylus_palm_detection_filter_util.cc

@@ -91,22 +91,27 @@ void PalmFilterStroke::AddSample(const PalmFilterSample& sample) {
   }
   DCHECK_EQ(tracking_id_, sample.tracking_id);
   samples_.push_back(sample);
+  AddToUnscaledCentroid(sample.point.OffsetFromOrigin());
   while (samples_.size() > max_length_) {
+    AddToUnscaledCentroid(-samples_.front().point.OffsetFromOrigin());
     samples_.pop_front();
   }
 }
 
+void PalmFilterStroke::AddToUnscaledCentroid(const gfx::Vector2dF point) {
+  const gfx::Vector2dF corrected_point = point - unscaled_centroid_sum_error_;
+  const gfx::PointF new_unscaled_centroid =
+      unscaled_centroid_ + corrected_point;
+  unscaled_centroid_sum_error_ =
+      (new_unscaled_centroid - unscaled_centroid_) - corrected_point;
+  unscaled_centroid_ = new_unscaled_centroid;
+}
+
 gfx::PointF PalmFilterStroke::GetCentroid() const {
-  // TODO(robsc): Implement a Kahan sum to accurately track running sum instead
-  // of brute force.
   if (samples_.size() == 0) {
     return gfx::PointF(0., 0.);
   }
-  gfx::PointF unscaled_centroid;
-  for (const auto& sample : samples_) {
-    unscaled_centroid += sample.point.OffsetFromOrigin();
-  }
-  return gfx::ScalePoint(unscaled_centroid, 1.f / samples_.size());
+  return gfx::ScalePoint(unscaled_centroid_, 1.f / samples_.size());
 }
 
 const std::deque<PalmFilterSample>& PalmFilterStroke::samples() const {

ui/events/ozone/evdev/touch_filter/neural_stylus_palm_detection_filter_util.h

@@ -65,10 +65,16 @@ class EVENTS_OZONE_EVDEV_EXPORT PalmFilterStroke {
   int tracking_id() const;
 
  private:
+  void AddToUnscaledCentroid(const gfx::Vector2dF point);
+
   std::deque<PalmFilterSample> samples_;
   int tracking_id_ = 0;
   uint64_t samples_seen_ = 0;
   uint64_t max_length_;
+  gfx::PointF unscaled_centroid_ = gfx::PointF(0., 0.);
+  // Used in part of the kahan summation.
+  gfx::Vector2dF unscaled_centroid_sum_error_ =
+      gfx::PointF(0., 0.).OffsetFromOrigin();
 };
 
 }  // namespace ui