Update to current webrtc library

This is from the upstream library commit id
3326535126e435f1ba647885ce43a8f0f3d317eb, corresponding to Chromium
88.0.4290.1.

webrtc/modules/audio_processing/agc2/limiter_db_gain_curve.h

@@ -0,0 +1,76 @@
+/*
+ *  Copyright (c) 2018 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 MODULES_AUDIO_PROCESSING_AGC2_LIMITER_DB_GAIN_CURVE_H_
+#define MODULES_AUDIO_PROCESSING_AGC2_LIMITER_DB_GAIN_CURVE_H_
+
+#include <array>
+
+#include "modules/audio_processing/agc2/agc2_testing_common.h"
+
+namespace webrtc {
+
+// A class for computing a limiter gain curve (in dB scale) given a set of
+// hard-coded parameters (namely, kLimiterDbGainCurveMaxInputLevelDbFs,
+// kLimiterDbGainCurveKneeSmoothnessDb, and
+// kLimiterDbGainCurveCompressionRatio). The generated curve consists of four
+// regions: identity (linear), knee (quadratic polynomial), compression
+// (linear), saturation (linear). The aforementioned constants are used to shape
+// the different regions.
+class LimiterDbGainCurve {
+ public:
+  LimiterDbGainCurve();
+
+  double max_input_level_db() const { return max_input_level_db_; }
+  double max_input_level_linear() const { return max_input_level_linear_; }
+  double knee_start_linear() const { return knee_start_linear_; }
+  double limiter_start_linear() const { return limiter_start_linear_; }
+
+  // These methods can be marked 'constexpr' in C++ 14.
+  double GetOutputLevelDbfs(double input_level_dbfs) const;
+  double GetGainLinear(double input_level_linear) const;
+  double GetGainFirstDerivativeLinear(double x) const;
+  double GetGainIntegralLinear(double x0, double x1) const;
+
+ private:
+  double GetKneeRegionOutputLevelDbfs(double input_level_dbfs) const;
+  double GetCompressorRegionOutputLevelDbfs(double input_level_dbfs) const;
+
+  static constexpr double max_input_level_db_ = test::kLimiterMaxInputLevelDbFs;
+  static constexpr double knee_smoothness_db_ = test::kLimiterKneeSmoothnessDb;
+  static constexpr double compression_ratio_ = test::kLimiterCompressionRatio;
+
+  const double max_input_level_linear_;
+
+  // Do not modify signal with level <= knee_start_dbfs_.
+  const double knee_start_dbfs_;
+  const double knee_start_linear_;
+
+  // The upper end of the knee region, which is between knee_start_dbfs_ and
+  // limiter_start_dbfs_.
+  const double limiter_start_dbfs_;
+  const double limiter_start_linear_;
+
+  // Coefficients {a, b, c} of the knee region polynomial
+  // ax^2 + bx + c in the DB scale.
+  const std::array<double, 3> knee_region_polynomial_;
+
+  // Parameters for the computation of the first derivative of GetGainLinear().
+  const double gain_curve_limiter_d1_;
+  const double gain_curve_limiter_d2_;
+
+  // Parameters for the computation of the integral of GetGainLinear().
+  const double gain_curve_limiter_i1_;
+  const double gain_curve_limiter_i2_;
+};
+
+}  // namespace webrtc
+
+#endif  // MODULES_AUDIO_PROCESSING_AGC2_LIMITER_DB_GAIN_CURVE_H_