StereoWidth.h

// DSPark - Professional Audio DSP Framework
// Copyright (c) 2026 Cristian Moresi - MIT License
 
#pragma once
 
#include "../Core/DspMath.h"
#include "../Core/AudioSpec.h"
#include "../Core/AudioBuffer.h"
#include "../Core/StateBlob.h"
 
#include <algorithm>
#include <atomic>
#include <cmath>
#include <cstdint>
#include <numbers>
#include <vector>
 
namespace dspark {
 
template <FloatType T>
class StereoWidth
{
public:
    StereoWidth() = default;
    ~StereoWidth() = default; // Removed virtual to prevent vtable overhead
 
    void prepare(double sampleRate) noexcept
    {
        if (!(sampleRate > 0.0) || !std::isfinite(sampleRate)) return;
        sampleRate_ = sampleRate;
        widthMaxStep_ = static_cast<T>(1.0 / (sampleRate * 0.02));
        updateBassMonoCoeff(bassMonoCutoff_.load(std::memory_order_relaxed));
        reset();
    }
 
    void prepare(const AudioSpec& spec) noexcept { prepare(spec.sampleRate); }
 
    void processBlock(AudioBufferView<T> buffer) noexcept
    {
        if (buffer.getNumChannels() >= 2)
            process(buffer.getChannel(0), buffer.getChannel(1), buffer.getNumSamples());
    }
 
    void setWidth(T width) noexcept
    {
        if (!std::isfinite(width)) return;
        width_.store(std::max(T(0), width), std::memory_order_relaxed);
    }
 
    [[nodiscard]] T getWidth() const noexcept { return width_.load(std::memory_order_relaxed); }
 
    void setBassMono(bool enabled, double cutoffHz = 100.0) noexcept
    {
        if (cutoffHz > 0.0 && std::isfinite(cutoffHz))
        {
            bassMonoCutoff_.store(cutoffHz, std::memory_order_relaxed);
            updateBassMonoCoeff(cutoffHz);
        }
        bassMonoEnabled_.store(enabled, std::memory_order_release);
    }
 
    void process(T* left, T* right, int numSamples) noexcept
    {
        if (numSamples <= 0) return;
 
        // Load atomics once per block to allow tight loop vectorization. The
        // width ramp is closed form per sample (moveTowards), so the pure-width
        // loop still vectorizes; settled, it reduces to the constant exactly.
        const T widthTarget = width_.load(std::memory_order_relaxed);
        if (snapWidth_)
        {
            currentWidth_ = widthTarget;
            snapWidth_ = false;
        }
        const T widthStart  = currentWidth_;
        const T widthStep   = widthMaxStep_;
        const bool bassMono = bassMonoEnabled_.load(std::memory_order_acquire);
 
        if (bassMono)
        {
            const T coeff = bassMonoCoeff_.load(std::memory_order_relaxed);
 
            // Bass-mono crossover: the side channel loses its lows through a
            // one-pole high-pass; the mid passes UNTOUCHED. A one-pole split is
            // complementary (LP + HP == 1 exactly), so no phase "compensation"
            // of the mid is needed - the previously used first-order allpass
            // rotated the mid by up to 180 degrees at HF, which swapped and
            // inverted the channels above the transition band.
            for (int i = 0; i < numSamples; ++i)
            {
                T l = left[i];
                T r = right[i];
 
                const T width = moveTowards(widthStart, widthTarget, widthStep * static_cast<T>(i + 1));
                T mid  = (l + r) * T(0.5);
                T side = (l - r) * T(0.5) * width;
 
                // Side processing (1-pole high-pass: side - LP(side))
                T sideLpIn = side;
                sideState_ += coeff * (sideLpIn - sideState_) + antiDenormal_;
                sideState_ -= antiDenormal_; // Denormal flush
                side = sideLpIn - sideState_;
 
                left[i]  = mid + side;
                right[i] = mid - side;
            }
        }
        else
        {
            // Fast-path branch: Pure Width control without filtering overhead
            for (int i = 0; i < numSamples; ++i)
            {
                const T width = moveTowards(widthStart, widthTarget, widthStep * static_cast<T>(i + 1));
                T mid  = (left[i] + right[i]) * T(0.5);
                T side = (left[i] - right[i]) * T(0.5) * width;
                
                left[i]  = mid + side;
                right[i] = mid - side;
            }
        }
        currentWidth_ = moveTowards(widthStart, widthTarget, widthStep * static_cast<T>(numSamples));
    }
 
    void reset() noexcept
    {
        sideState_ = T(0);
        snapWidth_ = true;   // the next block starts settled (no glide on start)
    }
 
 
    [[nodiscard]] std::vector<uint8_t> getState() const
    {
        StateWriter w(stateId("WIDE"), 1);
        w.write("width", width_.load(std::memory_order_relaxed));
        w.write("bassMono", bassMonoEnabled_.load(std::memory_order_relaxed));
        w.write("bassCutoff", static_cast<float>(bassMonoCutoff_.load(std::memory_order_relaxed)));
        return w.blob();
    }
 
    bool setState(const uint8_t* data, size_t size)
    {
        StateReader r(data, size);
        if (!r.isValid() || r.processorId() != stateId("WIDE")) return false;
        setWidth(static_cast<T>(r.read("width", 1.0f)));
        setBassMono(r.read("bassMono", false),
                    static_cast<double>(r.read("bassCutoff", 100.0f)));
        return true;
    }
 
protected:
    void updateBassMonoCoeff(double cutoff) noexcept
    {
        if (sampleRate_ > 0.0)
        {
            // Calculate 1-pole coeff. Stored atomically to prevent data races.
            T coeff = static_cast<T>(1.0 - std::exp(-std::numbers::pi * 2.0 * cutoff / sampleRate_));
            bassMonoCoeff_.store(coeff, std::memory_order_relaxed);
        }
    }
 
private:
    double sampleRate_ = 48000.0;
    
    // Lock-free parameters
    std::atomic<T> width_ { T(1) };
    std::atomic<bool> bassMonoEnabled_ { false };
    std::atomic<double> bassMonoCutoff_ { 100.0 };
    std::atomic<T> bassMonoCoeff_ { T(0) };
 
    // Filter states
    T sideState_ = T(0);
 
    // Width glide (audio thread)
    T currentWidth_ = T(1);
    T widthMaxStep_ = T(1.0 / 960.0);   
    bool snapWidth_ = true;             
 
    // Anti-denormal DC offset (type generic)
    static constexpr T antiDenormal_ = static_cast<T>(1e-15); 
};
 
} // namespace dspark