FixedFFTLowPass.cpp

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#include "Filters/FixedFFTLowPass.h"
#include "Physics/Physics.h"
#include "Utilities/GSLFFT.h"
 
FixedFFTLowPassFilter::FixedFFTLowPassFilter(const int& N) : number_frequencies_m(N) {}
 
void FixedFFTLowPassFilter::apply(std::vector<double>& LineDensity) {
    const int M = LineDensity.size();
    gsl_fft_halfcomplex_wavetable* hc;
    gsl_fft_real_wavetable* real = gsl_fft_real_wavetable_alloc(M);
    gsl_fft_real_workspace* work = gsl_fft_real_workspace_alloc(M);
    double* LD                   = new double[M];
    for (int i = 0; i < M; ++i) {
        LD[i] = LineDensity[i];
    }
    gsl_fft_real_transform(LD, 1, M, real, work);
 
    for (int i = 2 * number_frequencies_m + 1; i < M; ++i) {
        LD[i] = 0.0;
    }
 
    gsl_fft_real_wavetable_free(real);
    hc = gsl_fft_halfcomplex_wavetable_alloc(M);
 
    gsl_fft_halfcomplex_inverse(LD, 1, M, hc, work);
 
    gsl_fft_halfcomplex_wavetable_free(hc);
    gsl_fft_real_workspace_free(work);
 
    for (int i = 0; i < M; ++i) {
        LineDensity[i] = LD[i];
    }
    delete[] LD;
}
 
void FixedFFTLowPassFilter::calc_derivative(std::vector<double>& LineDensity, const double& h) {
    const int M      = LineDensity.size();
    const double gff = 2. * Physics::pi / (h * (M - 1));
 
    gsl_fft_halfcomplex_wavetable* hc;
    gsl_fft_real_wavetable* real = gsl_fft_real_wavetable_alloc(M);
    gsl_fft_real_workspace* work = gsl_fft_real_workspace_alloc(M);
    double* LD                   = new double[M];
 
    for (int i = 0; i < M; ++i) {
        LD[i] = LineDensity[i];
    }
    gsl_fft_real_transform(LD, 1, M, real, work);
 
    gsl_fft_real_wavetable_free(real);
 
    LD[0] = 0.0;
    for (int i = 1; i < 2 * number_frequencies_m + 1; i += 2) {
        double temp = LD[i];
        LD[i]       = -LD[i + 1] * gff * (i + 1) / 2;
        LD[i + 1]   = temp * gff * (i + 1) / 2;
    }
 
    for (int i = 2 * number_frequencies_m + 1; i < M; ++i) {
        LD[i] = 0.0;
    }
 
    hc = gsl_fft_halfcomplex_wavetable_alloc(M);
 
    gsl_fft_halfcomplex_inverse(LD, 1, M, hc, work);
 
    gsl_fft_halfcomplex_wavetable_free(hc);
    gsl_fft_real_workspace_free(work);
 
    for (int i = 0; i < M; ++i) {
        LineDensity[i] = LD[i];
    }
 
    delete[] LD;
}