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

webrtc/modules/audio_processing/test/analog_volume_mapper.h

+/*
+ *  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.
+ */
+
+#ifndef WEBRTC_MODULES_AUDIO_PROCESSING_TEST_ANALOG_VOLUME_MAPPER_H_
+#define WEBRTC_MODULES_AUDIO_PROCESSING_TEST_ANALOG_VOLUME_MAPPER_H_
+
+#include "webrtc/base/checks.h"
+
+namespace webrtc {
+
+// Class for simulating an analog gain controller controlled by
+// webrtc::GainControl. The class wraps a mapping from the current
+// level to a floating point scaling factor abstracting
+// non-linearities in the gain curve of real analog microphones. The
+// intended mode of operation is to use get_scaling_factor() to apply
+// a gain factor to a signal.
+class AnalogLevelMapper {
+ public:
+  enum class LevelToScalingMappingKind {
+    kIdentity,  // Any level produces a

[AleBzk, 2017/04/24 14:54:30]

The comment looks incomplete.

[aleloi, 2017/04/25 08:23:58]

Thanks, done.

+    kLinear     // A level within [0, 255] is linearly scaled. 0 produces
+                // 0.f, and 255 is 1.0f.
+  };
+
+  explicit AnalogLevelMapper(LevelToScalingMappingKind mapping_kind)
+      : mapping_kind_(mapping_kind) {}
+
+  bool AnalogLevelIsValid(int level) {

[AleBzk, 2017/04/24 14:54:30]

From this method, I understand that different mappings require different ranges
for level. If I'm right, to me it looks like we're adding a coupling with
existing code that works within a specific range - which may be unavoidable.
In fact, once we get the mappings from actual devices, it could happen that
different devices have different ranges.

If this is the case, I would explain everything when you define
LevelToScalingMappingKind, otherwise the code may look too hacky/unclear.

[aleloi, 2017/04/25 08:23:58]

I couldn't find a range outside 0, 255 except in tests. I've changed it to [0,
255]. We can make the range larger later if needed.

+    switch (mapping_kind_) {
+      case LevelToScalingMappingKind::kIdentity: {
+        return level_ <= 65535 && 0 <= level_;

[AleBzk, 2017/04/24 14:54:30]

You probably want to check level and not level_.

Just a personal taste, 0 <= level_ && level_ <= 65535 (which reads more like the
familiar 0 <= level_ <= 65535).

[aleloi, 2017/04/25 08:23:58]

Oh, good catch! Done.

+      }
+      case LevelToScalingMappingKind::kLinear: {
+        return level_ <= 255 && 0 <= level_;

[AleBzk, 2017/04/24 14:54:30]

Same here

[aleloi, 2017/04/25 08:23:58]

Done.

+        RTC_DCHECK_LE(0, level_);
+      }
+    }
+  }
+
+  // As the analog level limits in webrtc::GainControl, |level| must
+  // be within [0, 65535].
+  void set_analog_level(int level) {
+    RTC_DCHECK(AnalogLevelIsValid(level));
+    level_ = level;
+  }
+
+  int analog_level() const { return level_; }
+
+  float GetScalingFactor() const {
+    switch (mapping_kind_) {
+      case LevelToScalingMappingKind::kIdentity: {
+        return 1.0f;
+      }
+      case LevelToScalingMappingKind::kLinear: {
+        return static_cast<float>(level_) / 255.0f;
+      }
+    }
+  }
+
+ private:
+  int level_ = 0;
+  const LevelToScalingMappingKind mapping_kind_;
+};
+}  // namespace webrtc
+
+#endif  // WEBRTC_MODULES_AUDIO_PROCESSING_TEST_ANALOG_VOLUME_MAPPER_H_