/*
  ==============================================================================

   This file is part of the JUCE framework.
   Copyright (c) Raw Material Software Limited

   JUCE is an open source framework subject to commercial or open source
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   You may also use this code under the terms of the AGPLv3:
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  ==============================================================================
*/

/**
    Classes for state variable filter processing.
*/
namespace juce::dsp::StateVariableFilter
{
    template <typename NumericType>
    struct Parameters;

    /**
        An IIR filter that can perform low, band and high-pass filtering on an audio
        signal, with 12 dB of attenuation per octave, using a TPT structure, designed
        for fast modulation (see Vadim Zavalishin's documentation about TPT
        structures for more information). Its behaviour is based on the analog
        state variable filter circuit.

        Note: The bandpass here is not the one in the RBJ CookBook as its gain can be
        higher than 0 dB. For the classic 0 dB bandpass, we need to multiply the
        result by R2.

        Note 2: Using this class prevents some loud audio artefacts commonly encountered when
        changing the cutoff frequency using other filter simulation structures and IIR
        filter classes. However, this class may still require additional smoothing for
        cutoff frequency changes.

        see IIRFilter, SmoothedValue

        @tags{DSP}
    */
    template <typename SampleType>
    class Filter
    {
    public:
        //==============================================================================
        /** The NumericType is the underlying primitive type used by the SampleType (which
            could be either a primitive or vector)
        */
        using NumericType = typename SampleTypeHelpers::ElementType<SampleType>::Type;

        /** A typedef for a ref-counted pointer to the coefficients object */
        using ParametersPtr = typename Parameters<NumericType>::Ptr;

        //==============================================================================
       #ifndef DOXYGEN
        /** Creates a filter with default parameters. */
        [[deprecated ("The classes in the StateVariableFilter namespace are deprecated. you should "
                     "use the equivalent functionality in the StateVariableTPTFilter class.")]]
        Filter() : parameters (new Parameters<NumericType>) { reset(); }

        /** Creates a filter using some parameters. */
        [[deprecated ("The classes in the StateVariableFilter namespace are deprecated. you should "
                     "use the equivalent functionality in the StateVariableTPTFilter class.")]]
        Filter (ParametersPtr parametersToUse) : parameters (std::move (parametersToUse)) { reset(); }
       #endif

        /** Creates a copy of another filter. */
        Filter (const Filter&) = default;

        /** Move constructor */
        Filter (Filter&&) = default;

        //==============================================================================
        /** Initialization of the filter */
        void prepare (const ProcessSpec&) noexcept     { reset(); }

        /** Resets the filter's processing pipeline. */
        void reset() noexcept                          { s1 = s2 = SampleType {0}; }

        /** Ensure that the state variables are rounded to zero if the state
            variables are denormals. This is only needed if you are doing
            sample by sample processing.
        */
        void snapToZero() noexcept                     { util::snapToZero (s1); util::snapToZero (s2); }

        //==============================================================================
        /** The parameters of the state variable filter. It's up to the caller to ensure
            that these parameters are modified in a thread-safe way. */
        typename Parameters<NumericType>::Ptr parameters;

        //==============================================================================
        template <typename ProcessContext>
        void process (const ProcessContext& context) noexcept
        {
            static_assert (std::is_same_v<typename ProcessContext::SampleType, SampleType>,
                           "The sample-type of the filter must match the sample-type supplied to this process callback");

            if (context.isBypassed)
                processInternal<true, ProcessContext> (context);
            else
                processInternal<false, ProcessContext> (context);
        }

        /** Processes a single sample, without any locking or checking.
            Use this if you need processing of a single value. */
        SampleType JUCE_VECTOR_CALLTYPE processSample (SampleType sample) noexcept
        {
            switch (parameters->type)
            {
                case Parameters<NumericType>::Type::lowPass:  return processLoop<false, Parameters<NumericType>::Type::lowPass>  (sample, *parameters); break;
                case Parameters<NumericType>::Type::bandPass: return processLoop<false, Parameters<NumericType>::Type::bandPass> (sample, *parameters); break;
                case Parameters<NumericType>::Type::highPass: return processLoop<false, Parameters<NumericType>::Type::highPass> (sample, *parameters); break;
                default: jassertfalse;
            }

            return SampleType{0};
        }

    private:
        //==============================================================================
        template <bool isBypassed, typename Parameters<NumericType>::Type type>
        SampleType JUCE_VECTOR_CALLTYPE processLoop (SampleType sample, Parameters<NumericType>& state) noexcept
        {
            y[2] = (sample - s1 * state.R2 - s1 * state.g - s2) * state.h;

            y[1] = y[2] * state.g + s1;
            s1   = y[2] * state.g + y[1];

            y[0] = y[1] * state.g + s2;
            s2   = y[1] * state.g + y[0];

            return isBypassed ? sample : y[static_cast<size_t> (type)];
        }

        template <bool isBypassed, typename Parameters<NumericType>::Type type>
        void processBlock (const SampleType* input, SampleType* output, size_t n) noexcept
        {
            auto state = *parameters;

            for (size_t i = 0 ; i < n; ++i)
                output[i] = processLoop<isBypassed, type> (input[i], state);

           #if JUCE_DSP_ENABLE_SNAP_TO_ZERO
            snapToZero();
           #endif

            *parameters = state;
        }

        template <bool isBypassed, typename ProcessContext>
        void processInternal (const ProcessContext& context) noexcept
        {
            auto&& inputBlock  = context.getInputBlock();
            auto&& outputBlock = context.getOutputBlock();

            // This class can only process mono signals. Use the ProcessorDuplicator class
            // to apply this filter on a multi-channel audio stream.
            jassert (inputBlock.getNumChannels()  == 1);
            jassert (outputBlock.getNumChannels() == 1);

            auto n = inputBlock.getNumSamples();
            auto* src = inputBlock .getChannelPointer (0);
            auto* dst = outputBlock.getChannelPointer (0);

            switch (parameters->type)
            {
                case Parameters<NumericType>::Type::lowPass:  processBlock<isBypassed, Parameters<NumericType>::Type::lowPass>  (src, dst, n); break;
                case Parameters<NumericType>::Type::bandPass: processBlock<isBypassed, Parameters<NumericType>::Type::bandPass> (src, dst, n); break;
                case Parameters<NumericType>::Type::highPass: processBlock<isBypassed, Parameters<NumericType>::Type::highPass> (src, dst, n); break;
                default: jassertfalse;
            }
        }

        //==============================================================================
        std::array<SampleType, 3> y;
        SampleType s1, s2;

        //==============================================================================
        JUCE_LEAK_DETECTOR (Filter)
    };

    enum class StateVariableFilterType
    {
        lowPass,
        bandPass,
        highPass
    };

    //==============================================================================
    /**
        Structure used for the state variable filter parameters.

        @tags{DSP}
    */
    template <typename NumericType>
    struct Parameters  : public ProcessorState
    {
        //==============================================================================
        using Type = StateVariableFilterType;

        //==============================================================================
        /** The type of the IIR filter */
        Type type = Type::lowPass;

        /** Sets the cutoff frequency and resonance of the IIR filter.

            Note: The bandwidth of the resonance increases with the value of the
            parameter. To have a standard 12 dB/octave filter, the value must be set
            at 1 / sqrt (2).
        */
        void setCutOffFrequency (double sampleRate, NumericType frequency,
                                 NumericType resonance = static_cast<NumericType> (1.0 / MathConstants<double>::sqrt2)) noexcept
        {
            jassert (sampleRate > 0);
            jassert (resonance > NumericType (0));
            jassert (frequency > NumericType (0) && frequency <= NumericType (sampleRate * 0.5));

            g  = static_cast<NumericType> (std::tan (MathConstants<double>::pi * frequency / sampleRate));
            R2 = static_cast<NumericType> (1.0 / resonance);
            h  = static_cast<NumericType> (1.0 / (1.0 + R2 * g + g * g));
        }

        //==============================================================================
        /** The Coefficients structure is ref-counted, so this is a handy type that can be used
            as a pointer to one.
        */
        using Ptr = ReferenceCountedObjectPtr<Parameters>;

        //==============================================================================
        Parameters() = default;
        Parameters (const Parameters& o) : g (o.g), R2 (o.R2), h (o.h) {}
        Parameters& operator= (const Parameters& o) noexcept    { g = o.g; R2 = o.R2; h = o.h; return *this; }

        //==============================================================================
        NumericType g   = static_cast<NumericType> (std::tan (MathConstants<double>::pi * 200.0 / 44100.0));
        NumericType R2  = static_cast<NumericType> (MathConstants<double>::sqrt2);
        NumericType h   = static_cast<NumericType> (1.0 / (1.0 + R2 * g + g * g));
    };

} // namespace juce::dsp::StateVariableFilter
