media/base/audio_sample_types.h

// Copyright 2016 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef MEDIA_BASE_AUDIO_SAMPLE_TYPES_H_
#define MEDIA_BASE_AUDIO_SAMPLE_TYPES_H_

#include <cstdint>
#include <limits>
#include <type_traits>

// To specify different sample formats, we provide a class for each sample
// format that knows certain things about it, such as the C++ data type used
// to store sample values, min and max values, as well as how to convert to
// and from floating point formats. Each class must satisfy a concept we call
// "SampleTypeTraits", which requires that the following publics are provided:
//   * A type |ValueType| specifying the C++ type for storing sample values
//   * A static constant kMinValue which specifies the minimum sample value
//   * A static constant kMaxValue which specifies the maximum sample value
//   * A static constant kZeroPointValue which specifies the sample value
//     representing an amplitude of zero
//   * A static method ConvertFromFloat() that takes a float sample value and
//     converts it to the corresponding ValueType
//   * A static method ConvertFromDouble() that takes a double sample value and
//     converts it to the corresponding ValueType
//   * A static method ConvertToFloat() that takes a ValueType sample value and
//     converts it to the corresponding float value
//   * A static method ConvertToDouble() that takes a ValueType sample value and
//     converts it to the corresponding double value

namespace media {

// For float or double.
// See also the aliases for commonly used types at the bottom of this file.
template <typename SampleType>
class FloatSampleTypeTraits {
  static_assert(std::is_floating_point<SampleType>::value,
                "Template is only valid for float types.");

 public:
  using ValueType = SampleType;

  static constexpr SampleType kMinValue = -1.0f;
  static constexpr SampleType kMaxValue = +1.0f;
  static constexpr SampleType kZeroPointValue = 0.0f;

  static SampleType FromFloat(float source_value) {
    return From<float>(source_value);
  }
  static float ToFloat(SampleType source_value) {
    return To<float>(source_value);
  }
  static SampleType FromDouble(double source_value) {
    return From<double>(source_value);
  }
  static double ToDouble(SampleType source_value) {
    return To<double>(source_value);
  }

 private:
  template <typename FloatType>
  static SampleType From(FloatType source_value) {
    return static_cast<SampleType>(source_value);
  }

  template <typename FloatType>
  static FloatType To(SampleType source_value) {
    return static_cast<FloatType>(source_value);
  }
};

// For uint8_t, int16_t, int32_t...
// See also the aliases for commonly used types at the bottom of this file.
template <typename SampleType>
class FixedSampleTypeTraits {
  static_assert(std::numeric_limits<SampleType>::is_integer,
                "Template is only valid for integer types.");

 public:
  using ValueType = SampleType;

  static constexpr SampleType kMinValue =
      std::numeric_limits<SampleType>::min();
  static constexpr SampleType kMaxValue =
      std::numeric_limits<SampleType>::max();
  static constexpr SampleType kZeroPointValue =
      (kMinValue == 0) ? (kMaxValue / 2 + 1) : 0;

  static SampleType FromFloat(float source_value) {
    return From<float>(source_value);
  }
  static float ToFloat(SampleType source_value) {
    return To<float>(source_value);
  }
  static SampleType FromDouble(double source_value) {
    return From<double>(source_value);
  }
  static double ToDouble(SampleType source_value) {
    return To<double>(source_value);
  }

 private:
  // We pre-compute the scaling factors for conversion at compile-time in order
  // to save computation time during runtime.
  template <typename FloatType>
  struct ScalingFactors {
    // Since zero_point_value() is not the exact center between
    // min_value() and max_value(), we apply a different scaling for positive
    // and negative values.
    // Note that due to the limited precision, the FloatType values may not
    // always be able to represent the max and min values of the integer
    // SampleType exactly. This is a concern when using these scale factors for
    // scaling input sample values for conversion. However, since the min value
    // of SampleType is usually of the form -2^N and the max value is usually of
    // the form (+2^N)-1, and due to the fact that the float types store a
    // significand value plus a binary exponent it just so happens that
    // FloatType can usually represent the min value exactly and its
    // representation of the max value is only off by 1, i.e. it quantizes to
    // (+2^N) instead of (+2^N-1).

    static constexpr FloatType kForPositiveInput =
        static_cast<FloatType>(kMaxValue) -
        static_cast<FloatType>(kZeroPointValue);

    // Note: In the below expression, it is important that we cast kMinValue to
    // FloatType _before_ taking the negative of it. For example, for SampleType
    // int32_t, the expression (- kMinValue) would evaluate to
    // -numeric_limits<int32_t>::min(), which falls outside the numeric
    // range, wraps around, and ends up being the same as
    // +numeric_limits<int32_t>::min().
    static constexpr FloatType kForNegativeInput =
        static_cast<FloatType>(kZeroPointValue) -
        static_cast<FloatType>(kMinValue);

    static constexpr FloatType kInverseForPositiveInput =
        1.0f / kForPositiveInput;

    static constexpr FloatType kInverseForNegativeInput =
        1.0f / kForNegativeInput;
  };

  template <typename FloatType>
  static SampleType From(FloatType source_value) {
    // Note, that the for the case of |source_value| == 1.0, the imprecision of
    // |kScalingFactorForPositive| can lead to a product that is larger than the
    // maximum possible value of SampleType. To ensure this does not happen, we
    // handle the case of |source_value| == 1.0 as part of the clipping check.
    // For all FloatType values smaller than 1.0, the imprecision of
    // |kScalingFactorForPositive| is small enough to not push the scaled
    // |source_value| outside the numeric range of SampleType.

    // The nested if/else structure appears to compile to a
    // better-performing release binary compared to handling the clipping for
    // both positive and negative values first.
    //
    // Inlining the computation formula for multiplication with the scaling
    // factor and addition of |kZeroPointValue| results in better performance
    // for the int16_t case on Arm when compared to storing the scaling factor
    // in a temporary variable and applying it outside of the if-else block.
    //
    // It is important to have the cast to SampleType take place _after_
    // adding |kZeroPointValue|, because the scaled source value may be negative
    // and SampleType may be an unsigned integer type. The result of casting a
    // negative float to an unsigned integer is undefined.
    if (source_value < 0) {
      // Apply clipping (aka. clamping).
      if (source_value <= FloatSampleTypeTraits<float>::kMinValue)
        return kMinValue;

      return static_cast<SampleType>(
          (source_value * ScalingFactors<FloatType>::kForNegativeInput) +
          static_cast<FloatType>(kZeroPointValue));
    } else {
      // Apply clipping (aka. clamping).
      // As mentioned above, here we must include the case |source_value| == 1.
      if (source_value >= FloatSampleTypeTraits<float>::kMaxValue)
        return kMaxValue;
      return static_cast<SampleType>(
          (source_value * ScalingFactors<FloatType>::kForPositiveInput) +
          static_cast<FloatType>(kZeroPointValue));
    }
  }

  template <typename FloatType>
  static FloatType To(SampleType source_value) {
    FloatType offset_value =
        static_cast<FloatType>(source_value - kZeroPointValue);

    // We multiply with the inverse scaling factor instead of dividing by the
    // scaling factor, because multiplication performs faster than division
    // on many platforms.
    return (offset_value < 0.0f)
               ? (offset_value *
                  ScalingFactors<FloatType>::kInverseForNegativeInput)
               : (offset_value *
                  ScalingFactors<FloatType>::kInverseForPositiveInput);
  }
};

// Aliases for commonly used sample formats.
using Float32SampleTypeTraits = FloatSampleTypeTraits<float>;
using Float64SampleTypeTraits = FloatSampleTypeTraits<double>;
using UnsignedInt8SampleTypeTraits = FixedSampleTypeTraits<uint8_t>;
using SignedInt16SampleTypeTraits = FixedSampleTypeTraits<int16_t>;
using SignedInt32SampleTypeTraits = FixedSampleTypeTraits<int32_t>;

}  // namespace media

#endif  // MEDIA_BASE_AUDIO_SAMPLE_TYPES_H_