NOT READY FOR REVIEW - switch to a subscriber model for rendering stats

cc/rendering_stats_subscriber.cc

Index: cc/rendering_stats_subscriber.cc
diff --git a/cc/rendering_stats_subscriber.cc b/cc/rendering_stats_subscriber.cc
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index 0000000000000000000000000000000000000000..557c7f8354c3bfdbeb66e86573b7808ba514afc7
--- /dev/null
+++ b/cc/rendering_stats_subscriber.cc
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+// Copyright 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "config.h"
+
+#include "rendering_stats_subscriber.h"
+
+#include <cmath>
+
+namespace {
+// If a frame takes less than 1/70 seconds, we will assume it is an error and
+// ignore it.
+static const double kFrameTooFast = 1.0 / 70.0;
+}  // namespace
+
+namespace cc {
+
+scoped_ptr<RenderingStatsSubscriber> RenderingStatsSubscriber::create()
+{
+  scoped_ptr<RenderingStatsSubscriber> subscriber(new RenderingStatsSubscriber);
+  return subscriber.Pass();
+}
+
+RenderingStatsSubscriber::RenderingStatsSubscriber()
+    : is_scrolling_(false),
+      has_scrolled_(false)
+{}
+
+RenderingStatsSubscriber::~RenderingStatsSubscriber() {}
+
+void RenderingStatsSubscriber::markBeginningOfFrame(base::TimeTicks timestamp) {
+  // We ignore timestamps that occur after the scroll.
+  if (has_scrolled_ && !is_scrolling_)
+    return;
+
+  bool should_add_timestamp = time_stamp_history_.empty();
+  if (!should_add_timestamp) {
+    base::TimeDelta delta = timestamp - time_stamp_history_.back();
+    should_add_timestamp = delta.InMillisecondsF() > kFrameTooFast;
+  }
+  if (should_add_timestamp)
+    time_stamp_history_.push_back(timestamp);
+}
+
+void RenderingStatsSubscriber::handledScrollEvent(bool content_did_move) {
+  if (!has_scrolled_ && content_did_move) {
+    // We're starting a scroll. We'll want to measure the framerate during the
+    // scroll, so we'll discard any timestamps we've gathered so far.
+    time_stamp_history_.clear();
+    has_scrolled_ = true;
+  }
+
+  is_scrolling_ = content_did_move;
+}
+
+void RenderingStatsSubscriber::getFPSAndStandardDeviation(
+    double& average_fps,
+    double& standard_deviation) const {
+
+  base::TimeDelta duration =
+      time_stamp_history_.front() - time_stamp_history_.back();
+
+  double average_frame_duration = duration.InSecondsF() /
+      static_cast<double>(time_stamp_history_.size());
+
+  if (average_frame_duration > 0.0)
+    average_fps = 1.0 / average_frame_duration;
+
+  if (time_stamp_history_.size() > 2) {
+    double sum_squared_distances_to_mean = 0.0;
+    for (size_t i = 1; i < time_stamp_history_.size(); ++i) {
+      base::TimeDelta frame_duration =
+          time_stamp_history_[i] - time_stamp_history_[i - 1];
+      double signed_distance_to_mean =
+          average_fps - (1.0 / frame_duration.InSecondsF());
+      sum_squared_distances_to_mean +=
+          signed_distance_to_mean * signed_distance_to_mean;
+    }
+    standard_deviation = std::sqrt(sum_squared_distances_to_mean /
+        static_cast<double>(time_stamp_history_.size() - 2));
+  }
+}
+
+}  // namespace cc