dynamics_core.hpp

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#pragma once

 
#include <algorithm>
#include <cmath>
 
namespace pulp::signal::dynamics {

inline double retain_finite(double candidate, double current) noexcept {
    return std::isfinite(candidate) ? candidate : current;
}

inline constexpr double kLevelEpsilon = 1e-12;
 
// ── Level conversion ──────────────────────────────────────────────────────

inline double amplitude_db(double amplitude, double epsilon) {
    return 20.0 * std::log10(std::abs(amplitude) + epsilon);
}

inline double power_db(double mean_square, double epsilon) {
    return 10.0 * std::log10(mean_square + epsilon);
}
 
// ── Ballistics ────────────────────────────────────────────────────────────

inline double one_pole_retain(double tau_seconds, double sample_rate) {
    if (!(tau_seconds > 0.0) || !(sample_rate > 0.0)) return 0.0;
    return std::exp(-1.0 / (tau_seconds * sample_rate));
}
 
// ── The soft-knee static characteristic (the paper's eq. 4) ───────────────

inline double knee_term_db(double over_db, double knee_db, double slope) {
    const double t = over_db + knee_db * 0.5;
    return (slope - 1.0) * t * t / (2.0 * knee_db);
}

inline double soft_knee_output_db(double input_db, double threshold_db, double knee_db,
                                  double ratio) {
    const double over = input_db - threshold_db;
    if (2.0 * over < -knee_db) return input_db;
    if (knee_db > 0.0 && 2.0 * std::abs(over) <= knee_db)
        return input_db + knee_term_db(over, knee_db, 1.0 / ratio);
    return threshold_db + over / ratio;
}

inline double soft_knee_reduction_db(double over_db, double knee_db, double slope) {
    if (knee_db > 0.0) {
        if (2.0 * over_db <= -knee_db) return 0.0;
        if (2.0 * over_db < knee_db) return knee_term_db(over_db, knee_db, slope);
    } else if (over_db <= 0.0) {
        return 0.0;
    }
    return (slope - 1.0) * over_db;
}
 
}  // namespace pulp::signal::dynamics