state_variable_filter.hpp

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#pragma once
 
namespace ts {

enum class FilterTap {
    bypass,
    low_pass,
    band_pass,
    high_pass,
    notch,
};

class StateVariableFilter {
public:
    [[nodiscard]] double low() const noexcept { return low_; }

    [[nodiscard]] double band() const noexcept { return band_; }

    void reset() noexcept
    {
        low_ = 0.0;
        band_ = 0.0;
    }

    [[nodiscard]] double process(double input, double f, double q, FilterTap tap) noexcept
    {
        low_ += f * band_;
        const double high = input - ((q * band_) + low_);
        band_ += f * high;
 
        switch (tap) {
        case FilterTap::low_pass:
            return low_;
        case FilterTap::high_pass:
            return high;
        case FilterTap::band_pass:
            return band_;
        case FilterTap::notch:
            return low_ - high;
        case FilterTap::bypass:
            break;
        }
        return input;
    }
 
private:
    double low_ = 0.0;
    double band_ = 0.0;
};
 
} // namespace ts