FeedbackSignalAccumulator utility class for averaging feedback signals

content/browser/media/capture/feedback_signal_accumulator.cc

Index: content/browser/media/capture/feedback_signal_accumulator.cc
diff --git a/content/browser/media/capture/feedback_signal_accumulator.cc b/content/browser/media/capture/feedback_signal_accumulator.cc
new file mode 100644
index 0000000000000000000000000000000000000000..43d6d833c71dcdc37038cba96f11f377aa3f8c33
--- /dev/null
+++ b/content/browser/media/capture/feedback_signal_accumulator.cc
@@ -0,0 +1,56 @@
+// Copyright (c) 2015 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 "content/browser/media/capture/feedback_signal_accumulator.h"
+
+#include <algorithm>
+#include <cmath>
+
+namespace content {
+
+FeedbackSignalAccumulator::FeedbackSignalAccumulator(base::TimeDelta half_life)
+    : half_life_(half_life) {
+  DCHECK(half_life_ > base::TimeDelta());
+}
+
+void FeedbackSignalAccumulator::Reset(double starting_value,
+                                      base::TimeTicks timestamp) {
+  DCHECK(!timestamp.is_null());
+  average_ = update_value_ = prior_average_ = starting_value;
+  reset_time_ = update_time_ = prior_update_time_ = timestamp;
+}
+
+bool FeedbackSignalAccumulator::Update(double value,
+                                       base::TimeTicks timestamp) {
+  DCHECK(!reset_time_.is_null());
+
+  if (timestamp < update_time_) {
+    return false;  // Not in chronological order.
+  } else if (timestamp == update_time_) {
+    if (timestamp == reset_time_) {
+      // Edge case: Multiple updates at reset timestamp.
+      average_ = update_value_ = prior_average_ =
+          std::max(value, update_value_);
+      return true;
+    }
+    if (value <= update_value_)
+      return true;
+    update_value_ = value;
+  } else {
+    prior_average_ = average_;

[hubbe, 2015/05/12 19:10:07]

Make prior_average_ and prior_update_time_ local variables?

[miu, 2015/05/12 20:58:37]

I can't.  They're needed for the timestamp == update_time_ case where the
average_ needs to be recomputed with the new, larger value for the same
timestamp.

+    prior_update_time_ = update_time_;
+    update_value_ = value;
+    update_time_ = timestamp;
+  }
+
+  const double elapsed_us =
+      static_cast<double>((update_time_ - prior_update_time_).InMicroseconds());
+  const double weight = elapsed_us / (elapsed_us + half_life_.InMicroseconds());
+  average_ = weight * update_value_ + (1.0 - weight) * prior_average_;
+  DCHECK(std::isfinite(average_));
+
+  return true;
+}
+
+}  // namespace content