spectral_variance_impl.h

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#pragma once
 
#include "fl/stl/span.h"
#include "fl/stl/vector.h"
 
namespace fl {
namespace detail {

template <typename T = float>
class SpectralVarianceImpl {
  public:
    explicit SpectralVarianceImpl(T alpha = T(0.2f), T floor = T(1e-4f))
        : mAlpha(alpha), mFloor(floor), mVariance(T(0)) {}

    T update(fl::span<const T> bins) {
        const int n = static_cast<int>(bins.size());
        if (n == 0) return T(0);
 
        // Initialize on first call or if channel count changes
        if (mSmoothed.size() != static_cast<fl::size>(n)) {
            mSmoothed.resize(n);
            for (int i = 0; i < n; ++i) {
                mSmoothed[i] = bins[i];
            }
            mVariance = T(0);
            return T(0);
        }
 
        T sumRelChange = T(0);
        int activeCount = 0;
 
        for (int i = 0; i < n; ++i) {
            T smoothed = mSmoothed[i];
            T denom = (smoothed > mFloor) ? smoothed : mFloor;
 
            // Only count channels with meaningful energy
            if (smoothed > mFloor) {
                T diff = bins[i] - smoothed;
                T absDiff = (diff >= T(0)) ? diff : -diff;
                sumRelChange += absDiff / denom;
                ++activeCount;
            }
 
            // Update EMA: smoothed = alpha * current + (1-alpha) * smoothed
            mSmoothed[i] = mAlpha * bins[i] + (T(1) - mAlpha) * smoothed;
        }
 
        mVariance = (activeCount >= 2)
                        ? sumRelChange / static_cast<T>(activeCount)
                        : T(0);
        return mVariance;
    }

    T value() const { return mVariance; }

    void reset() {
        mSmoothed.clear();
        mVariance = T(0);
    }

    void setAlpha(T alpha) { mAlpha = alpha; }

    void setFloor(T floor) { mFloor = floor; }

    fl::size size() const { return mSmoothed.size(); }
 
  private:
    T mAlpha;
    T mFloor;
    T mVariance;
    fl::vector<T> mSmoothed;
};
 
} // namespace detail
} // namespace fl