spectral_equalizer.h

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#pragma once
 
#include "fl/stl/int.h"
#include "fl/stl/vector.h"
#include "fl/stl/span.h"
#include "fl/stl/noexcept.h"
 
namespace fl {
namespace audio {

enum class EqualizationCurve {
    Flat,        // No equalization (all gains = 1.0)
    AWeighting,  // A-weighting curve (emphasizes 1-6 kHz, de-emphasizes bass/treble)
    Custom       // User-defined per-band gains
};

struct SpectralEqualizerConfig {
    EqualizationCurve curve = EqualizationCurve::Flat;

    size numBands = 16;

    vector<float> customGains;

    bool applyMakeupGain = false;

    float makeupGainTarget = 0.5f;

    bool enableCompression = false;

    float compressionThreshold = 0.7f;

    float compressionRatio = 2.0f;
};

class SpectralEqualizer {
public:
    SpectralEqualizer() FL_NOEXCEPT;
    explicit SpectralEqualizer(const SpectralEqualizerConfig& config);
    ~SpectralEqualizer() FL_NOEXCEPT;

    void configure(const SpectralEqualizerConfig& config);

    void apply(span<const float> inputBins, span<float> outputBins) const;

    void setCustomGains(span<const float> gains);

    span<const float> getGains() const { return mGains; }

    const SpectralEqualizerConfig& getConfig() const { return mConfig; }

    struct Stats {
        u32 applicationsCount = 0;      // Total applications performed
        float lastInputPeak = 0.0f;     // Peak value in last input
        float lastOutputPeak = 0.0f;    // Peak value in last output
        float lastMakeupGain = 1.0f;    // Makeup gain applied in last call
        float avgInputLevel = 0.0f;     // Average input level (last call)
        float avgOutputLevel = 0.0f;    // Average output level (last call)
    };
 
    const Stats& getStats() const { return mStats; }

    void resetStats();
 
private:
    void calculateGains();

    void calculateAWeightingGains();

    void calculateFlatGains();

    float calculateMakeupGain(span<const float> inputBins, span<const float> outputBins) const;

    float applyCompression(float value) const;
 
    SpectralEqualizerConfig mConfig;
    mutable Stats mStats;

    vector<float> mGains;

    static constexpr float A_WEIGHTING_16BAND[16] = {
        0.5f,  // Bin 0: 20-40 Hz (bass rolloff)
        0.6f,  // Bin 1: 40-80 Hz (bass rolloff)
        0.8f,  // Bin 2: 80-160 Hz (gradual increase)
        1.0f,  // Bin 3: 160-320 Hz (flat)
        1.2f,  // Bin 4: 320-640 Hz (emphasis begins)
        1.3f,  // Bin 5: 640-1280 Hz (emphasis)
        1.4f,  // Bin 6: 1280-2560 Hz (peak emphasis)
        1.4f,  // Bin 7: 2560-5120 Hz (peak emphasis)
        1.3f,  // Bin 8: 5120-10240 Hz (gradual rolloff)
        1.2f,  // Bin 9: 10240-16000 Hz (rolloff continues)
        1.0f,  // Bin 10 (fallback)
        0.8f,  // Bin 11 (fallback)
        0.6f,  // Bin 12 (fallback)
        0.4f,  // Bin 13 (fallback)
        0.2f,  // Bin 14 (fallback)
        0.1f   // Bin 15 (fallback)
    };

    static constexpr float A_WEIGHTING_32BAND[32] = {
        0.3f, 0.4f, 0.5f, 0.6f, 0.7f, 0.8f, 0.9f, 1.0f,  // Bass rolloff (0-7)
        1.1f, 1.2f, 1.3f, 1.4f, 1.4f, 1.4f, 1.3f, 1.2f,  // Mid emphasis (8-15)
        1.1f, 1.0f, 0.9f, 0.8f, 0.7f, 0.6f, 0.5f, 0.4f,  // Treble rolloff (16-23)
        0.3f, 0.2f, 0.2f, 0.1f, 0.1f, 0.1f, 0.1f, 0.1f   // High freq rolloff (24-31)
    };
};
 
} // namespace audio
} // namespace fl