RationalKnee.h

// DSPark - Professional Audio DSP Framework
// Copyright (c) 2026 Cristian Moresi - MIT License
 
#pragma once
 
#include "../DspMath.h"
#include <cmath>
 
namespace dspark::detail
{
template <FloatType T>
[[nodiscard]] inline T rationalKneeMagnitude(T magnitude, T knee, T ceiling) noexcept
{
    if (magnitude <= knee)
        return magnitude;
    const T excess = magnitude - knee;
    const T range = ceiling - knee;
    return knee + (range * excess) / (excess + range);
}
 
[[nodiscard]] inline double rationalKneeShape(double input, double positiveKnee,
                                              double negativeKnee) noexcept
{
    const double knee = input >= 0.0 ? positiveKnee : negativeKnee;
    return std::copysign(rationalKneeMagnitude(std::abs(input), knee, 1.0), input);
}
 
[[nodiscard]] inline double rationalKneeIntegral(double input, double positiveKnee,
                                                 double negativeKnee) noexcept
{
    const double magnitude = std::abs(input);
    const double knee = input >= 0.0 ? positiveKnee : negativeKnee;
    if (magnitude <= knee)
        return 0.5 * magnitude * magnitude;
    const double range = 1.0 - knee;
    const double excess = magnitude - knee;
    return 0.5 * knee * knee + excess - range * range * std::log((excess + range) / range);
}
 
} // namespace dspark::detail