First try on a fake analog volume control interface for use in audioproc_f.

webrtc/modules/audio_processing/test/analog_volume_mapper_unittest.cc

+/*
+ *  Copyright (c) 2017 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_processing/test/analog_volume_mapper.h"
+#include "webrtc/test/gtest.h"
+
+TEST(AnalogVolumeMapper, GainCurveShouldBeMonotone) {
+  using MappingKind = webrtc::AnalogLevelMapper::LevelToScalingMappingKind;
+
+  for (auto kind : {MappingKind::kIdentity, MappingKind::kLinear}) {
+    webrtc::AnalogLevelMapper level_mapper(kind);
+    float last_level = level_mapper.GetScalingFactor();
+    RTC_DCHECK_LE(0.f, last_level);
+
+    for (int i = 0; i < 255; ++i) {
+      level_mapper.set_analog_level(i);
+      const float current_level = level_mapper.GetScalingFactor();
+      RTC_DCHECK_LE(last_level, current_level);
+
+      last_level = current_level;
+    }
+  }
+}