fftfreq.hpp

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#pragma once
 
#include "NumCpp/Core/Types.hpp"
#include "NumCpp/NdArray.hpp"
 
namespace nc::fft
{
    //===========================================================================
    // Method Description:
    inline NdArray<double> fftfreq(uint32 inN, double inD = 1.)
    {
        if (inN == 0)
        {
            return {};
        }
        else if (inN == 1)
        {
            return { 0 };
        }
 
        if (inD <= 0.)
        {
            return {};
        }
 
        const auto nTimesD = static_cast<double>(inN) * inD;
 
        auto result = NdArray<double>(1, inN);
        result[0]   = 0.;
 
        for (auto i = 1u; i < (inN + 1) / 2; ++i)
        {
            result[i]       = static_cast<double>(i) / nTimesD;
            result[inN - i] = -static_cast<double>(i) / nTimesD;
        }
 
        if (inN % 2 == 0)
        {
            result[inN / 2] = -static_cast<double>(inN / 2) / nTimesD;
        }
 
        return result;
    }
} // namespace nc::fft