Injectable output rate calculater for AudioMixer.

webrtc/modules/audio_mixer/default_output_rate_calculator.cc

+/*
+ *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include "webrtc/modules/audio_mixer/default_output_rate_calculator.h"
+
+#include <algorithm>
+
+#include "webrtc/modules/audio_processing/include/audio_processing.h"
+
+namespace webrtc {
+
+int DefaultOutputRateCalculator::CalculateOutputRate(
+    const std::vector<int>& preferred_sample_rates) {
+  if (preferred_sample_rates.empty()) {
+    return DefaultOutputRateCalculator::kDefaultFrequency;
+  }
+  using NativeRate = AudioProcessing::NativeRate;
+  int maximal_frequency = 0;

[ivoc, 2016/12/08 10:09:47]

How about this:
int maximal_frequency = *std::max_element(preferred_sample_rates.begin(),
preferred_sample_rates.end());

+  for (const int rate : preferred_sample_rates) {
+    RTC_DCHECK_LE(NativeRate::kSampleRate8kHz, rate);
+    RTC_DCHECK_GE(NativeRate::kSampleRate48kHz, rate);
+    maximal_frequency = std::max(maximal_frequency, rate);
+  }
+
+  static constexpr NativeRate native_rates[] = {
+      NativeRate::kSampleRate8kHz, NativeRate::kSampleRate16kHz,
+      NativeRate::kSampleRate32kHz, NativeRate::kSampleRate48kHz};
+  const auto rounded_up_index = std::lower_bound(
+      std::begin(native_rates), std::end(native_rates), maximal_frequency);
+  RTC_DCHECK(rounded_up_index != std::end(native_rates));
+  return *rounded_up_index;
+}
+}  // namespace webrtc