text: /*
   *  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.
   */
  
  #include "modules/audio_processing/agc2/limiter.h"
  
  #include <cmath>
  
  #include "common_audio/include/audio_util.h"
  #include "modules/audio_processing/agc2/agc2_common.h"
  #include "rtc_base/checks.h"
  
  namespace webrtc {
  namespace {
  
  double ComputeKneeStart(double max_input_level_db,
                          double knee_smoothness_db,
                          double compression_ratio) {
    RTC_CHECK_LT((compression_ratio - 1.0) * knee_smoothness_db /
                     (2.0 * compression_ratio),
                 max_input_level_db);
    return -knee_smoothness_db / 2.0 -
           max_input_level_db / (compression_ratio - 1.0);
  }
  
  std::array<double, 3> ComputeKneeRegionPolynomial(double knee_start_dbfs,
                                                    double knee_smoothness_db,
                                                    double compression_ratio) {
    const double a = (1.0 - compression_ratio) /
                     (2.0 * knee_smoothness_db * compression_ratio);
    const double b = 1.0 - 2.0 * a * knee_start_dbfs;
    const double c = a * knee_start_dbfs * knee_start_dbfs;
    return {{a, b, c}};
  }
  
  double ComputeLimiterD1(double max_input_level_db, double compression_ratio) {
    return (std::pow(10.0, -max_input_level_db / (20.0 * compression_ratio)) *
            (1.0 - compression_ratio) / compression_ratio) /
           kMaxAbsFloatS16Value;
  }
  
  constexpr double ComputeLimiterD2(double compression_ratio) {
    return (1.0 - 2.0 * compression_ratio) / compression_ratio;
  }
  
  double ComputeLimiterI2(double max_input_level_db,
                          double compression_ratio,
                          double gain_curve_limiter_i1) {
    RTC_CHECK_NE(gain_curve_limiter_i1, 0.f);
    return std::pow(10.0, -max_input_level_db / (20.0 * compression_ratio)) /
           gain_curve_limiter_i1 /
           std::pow(kMaxAbsFloatS16Value, gain_curve_limiter_i1 - 1);
  }
  
  }  // namespace
  
  Limiter::Limiter()
      : max_input_level_linear_(DbfsToFloatS16(max_input_level_db_)),
        knee_start_dbfs_(ComputeKneeStart(max_input_level_db_,
                                          knee_smoothness_db_,
                                          compression_ratio_)),
        knee_start_linear_(DbfsToFloatS16(knee_start_dbfs_)),
        limiter_start_dbfs_(knee_start_dbfs_ + knee_smoothness_db_),
        limiter_start_linear_(DbfsToFloatS16(limiter_start_dbfs_)),
        knee_region_polynomial_(ComputeKneeRegionPolynomial(knee_start_dbfs_,
                                                            knee_smoothness_db_,
                                                            compression_ratio_)),
        gain_curve_limiter_d1_(
            ComputeLimiterD1(max_input_level_db_, compression_ratio_)),
        gain_curve_limiter_d2_(ComputeLimiterD2(compression_ratio_)),
        gain_curve_limiter_i1_(1.0 / compression_ratio_),
        gain_curve_limiter_i2_(ComputeLimiterI2(max_input_level_db_,
                                                compression_ratio_,
                                                gain_curve_limiter_i1_)) {
    static_assert(knee_smoothness_db_ > 0.0f, "");
    static_assert(compression_ratio_ > 1.0f, "");
    RTC_CHECK_GE(max_input_level_db_, knee_start_dbfs_ + knee_smoothness_db_);
  }
  
  constexpr double Limiter::max_input_level_db_;
  constexpr double Limiter::knee_smoothness_db_;
  constexpr double Limiter::compression_ratio_;
  
  double Limiter::GetOutputLevelDbfs(double input_level_dbfs) const {
    if (input_level_dbfs < knee_start_dbfs_) {
      return input_level_dbfs;
    } else if (input_level_dbfs < limiter_start_dbfs_) {
      return GetKneeRegionOutputLevelDbfs(input_level_dbfs);
    }
    return GetCompressorRegionOutputLevelDbfs(input_level_dbfs);
  }
  
  double Limiter::GetGainLinear(double input_level_linear) const {
    if (input_level_linear < knee_start_linear_) {
      return 1.0;
    }
    return DbfsToFloatS16(
               GetOutputLevelDbfs(FloatS16ToDbfs(input_level_linear))) /
           input_level_linear;
  }
  
  // Computes the first derivative of GetGainLinear() in |x|.
  double Limiter::GetGainFirstDerivativeLinear(double x) const {
    // Beyond-knee region only.
    RTC_CHECK_GE(x, limiter_start_linear_ - 1e-7 * kMaxAbsFloatS16Value);
    return gain_curve_limiter_d1_ *
           std::pow(x / kMaxAbsFloatS16Value, gain_curve_limiter_d2_);
  }
  
  // Computes the integral of GetGainLinear() in the range [x0, x1].
  double Limiter::GetGainIntegralLinear(double x0, double x1) const {
    RTC_CHECK_LE(x0, x1);                     // Valid interval.
    RTC_CHECK_GE(x0, limiter_start_linear_);  // Beyond-knee region only.
    auto limiter_integral = [this](const double& x) {
      return gain_curve_limiter_i2_ * std::pow(x, gain_curve_limiter_i1_);
    };
    return limiter_integral(x1) - limiter_integral(x0);
  }
  
  double Limiter::GetKneeRegionOutputLevelDbfs(double input_level_dbfs) const {
    return knee_region_polynomial_[0] * input_level_dbfs * input_level_dbfs +
           knee_region_polynomial_[1] * input_level_dbfs +
           knee_region_polynomial_[2];
  }
  
  double Limiter::GetCompressorRegionOutputLevelDbfs(
      double input_level_dbfs) const {
    return (input_level_dbfs - max_input_level_db_) / compression_ratio_;
  }
  
  }  // namespace webrtc
  