[vr] Add a frame rate counter for the vr UI

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 = 200;
+
+}  // namepsace
+
+FPSMeter::FPSMeter() {
+  frame_times_.reserve(kNumFrameTimes);
+}
+
+FPSMeter::~FPSMeter() {}
+
+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_;
+  last_time_stamp_ = time_stamp;
+
+  if (frame_times_.size() + 1 < kNumFrameTimes)
+    frame_times_.push_back(delta);
+  else
+    frame_times_[current_index_] = delta;
+
+  current_index_++;
+  if (current_index_ >= kNumFrameTimes)
+    current_index_ = 0;
+}
+
+bool FPSMeter::CanComputeFPS() const {
+  return !frame_times_.empty();
+}
+
+// Simply takes the average time delta.
+double FPSMeter::GetFPS() const {
+  if (!CanComputeFPS())
+    return 0.0;
+
+  double total_duration = 0.0;
+  for (const auto& delta : frame_times_)

[mthiesse, 2017/04/11 19:14:36]

Instead of recomputing the sum each time you read, why not update the sum as you
go?

eg.
sum -= frame_times_[current_index_].InSecondsF();
sum += delta.InSecondsF();
frame_times_[current_index_] = delta;

[billorr, 2017/04/12 00:40:03]

floating point error will accumulate

+    total_duration += delta.InSecondsF();
+
+  return frame_times_.size() / total_duration;
+}
+
+}  // namespace vr_shell