Add SlidingAverage to FPSMeter, use for WebVR prediction time

chrome/browser/android/vr_shell/fps_meter.cc

 // Copyright 2017 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 "chrome/browser/android/vr_shell/fps_meter.h"
 
 #include <algorithm>
 
 namespace vr_shell {
 
 namespace {
 
-static constexpr size_t kNumFrameTimes = 10;
+static constexpr size_t kDefaultNumFrameTimes = 10;
 
-}  // namepsace
+}  // namespace
 
-FPSMeter::FPSMeter() : total_time_us_(0) {
-  frame_times_.reserve(kNumFrameTimes);
+SampleQueue::SampleQueue(size_t window_size) : window_size_(window_size) {
+  samples_.reserve(window_size);
 }
 
-FPSMeter::~FPSMeter() {}
+SampleQueue::~SampleQueue() = default;
+
+void SampleQueue::AddSample(int64_t value) {
+  sum_ += value;
+
+  if (samples_.size() < window_size_) {
+    samples_.push_back(value);
+  } else {
+    sum_ -= samples_[current_index_];
+    samples_[current_index_] = value;
+  }
+
+  ++current_index_;
+  if (current_index_ >= window_size_) {
+    current_index_ = 0;
+  }
+}
+
+FPSMeter::FPSMeter() : frame_times_(kDefaultNumFrameTimes) {}
+
+FPSMeter::FPSMeter(size_t window_size) : frame_times_(window_size) {}
+
+FPSMeter::~FPSMeter() = default;
 
 size_t FPSMeter::GetNumFrameTimes() {
-  return kNumFrameTimes;
+  return frame_times_.GetWindowSize();
 }
 
 void FPSMeter::AddFrame(const base::TimeTicks& time_stamp) {
   if (last_time_stamp_.is_null()) {
     last_time_stamp_ = time_stamp;
     return;
   }
 
-  base::TimeDelta delta = time_stamp - last_time_stamp_;
+  int64_t delta = (time_stamp - last_time_stamp_).InMicroseconds();
   last_time_stamp_ = time_stamp;
 
-  total_time_us_ += delta.InMicroseconds();
-
-  if (frame_times_.size() < kNumFrameTimes) {
-    frame_times_.push_back(delta);
-  } else {
-    total_time_us_ -= frame_times_[current_index_].InMicroseconds();
-    frame_times_[current_index_] = delta;
-  }
-
-  current_index_++;
-  if (current_index_ >= kNumFrameTimes)
-    current_index_ = 0;
+  frame_times_.AddSample(delta);
 }
 
 bool FPSMeter::CanComputeFPS() const {
-  return !frame_times_.empty();
+  return frame_times_.GetCount() > 0;
 }
 
 // Simply takes the average time delta.
 double FPSMeter::GetFPS() const {
   if (!CanComputeFPS())
     return 0.0;
 
-  return (frame_times_.size() * 1.0e6) / total_time_us_;
+  return (frame_times_.GetCount() * 1.0e6) / frame_times_.GetSum();
+}
+
+SlidingAverage::SlidingAverage(size_t window_size) : values_(window_size) {}
+
+SlidingAverage::~SlidingAverage() = default;
+
+void SlidingAverage::AddSample(int64_t value) {
+  values_.AddSample(value);
+}
+
+int64_t SlidingAverage::GetAverageOrDefault(int64_t default_value) const {
+  if (values_.GetCount() == 0)
+    return default_value;
+  return values_.GetSum() / values_.GetCount();
 }
 
 }  // namespace vr_shell