test_zero_padding.cxx

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// Example for FFT resampling with zero-padding tricks
#include "WireCellUtil/Waveform.h"

#include <algorithm>

// for FFT
#include <Eigen/Core>
#include <unsupported/Eigen/FFT>

#include <complex>
#include <iostream>

using namespace WireCell;

int main(){
  std::vector<float> a = {1,2,3,2,1};
  // can be sampled to 10 ticks: 1 , 1.35279 , 2 , 2.69443 , 3 , 2.69443 , 2 , 1.35279 , 1 , 0.905573

  auto tran = WireCell::Waveform::dft(a);

  std::cout << " tran = " << std::endl;
  std::cout << tran.size() << std::endl;
  auto mag = WireCell::Waveform::magnitude(tran);
  for(auto x: mag){
    std::cout <<  x << "  ";
  }
  std::cout << std::endl;

  // zero-padding
  int inSmps = tran.size();
  tran.resize(inSmps*2);
  if(inSmps%2==0){
    // even number of samples, eg, inSmps=6, 012345 -(zero-padding)-> 012345,000000 -> 012000,000345
    std::rotate(tran.begin()+inSmps/2, tran.begin()+inSmps, tran.end());
  }
  else{
    // odd number, eg, inSmps=5, 01234,00000 -> 01200,00034
    std::rotate(tran.begin()+(inSmps+1)/2, tran.begin()+inSmps, tran.end());
  }

  //
  std::cout << " tran = " << std::endl;
  std::cout << tran.size() << std::endl;
  for(auto x: tran){
    std::cout << x.real() << "  ";
  }
  std::cout << std::endl;

  // inverse FFT
  auto b = WireCell::Waveform::idft(tran);
  float scale = tran.size() / inSmps;
  //
  std::cout << " b = " << std::endl;
  std::cout << b.size() << std::endl;
  for(auto x: b){
    std::cout << x * scale << "  ";
  }
  std::cout << std::endl;


  return 0;
}