correlations.cpp


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#include <getopt.h>
#include <fstream>
#include <iostream>

#include "eigen/Eigen/Dense"

using Real = float;
using Vector = Eigen::Matrix<Real, Eigen::Dynamic, 1>;

int main(int argc, char* argv[]) {
  unsigned N = 10;
  Real E = 0;
  Real Δt = 1e-4;
  Real Δw = 1e-2;
  Real T0 = 0;
  Real T = 100;

  int opt;

  while ((opt = getopt(argc, argv, "N:E:t:w:0:T:")) != -1) {
    switch (opt) {
    case 'N':
      N = (unsigned)atof(optarg);
      break;
    case 'E':
      E = atof(optarg);
      break;
    case 't':
      Δt = atof(optarg);
      break;
    case 'w':
      Δw = atof(optarg);
      break;
    case '0':
      T0 = atof(optarg);
      break;
    case 'T':
      T = atof(optarg);
      break;
    default:
      exit(1);
    }
  }

  std::string filebase = std::to_string(N) + "_" + std::to_string(E) + "_" + std::to_string(-std::log10(Δt)) + "_" + std::to_string(Δw);

  std::ifstream file(filebase + ".dat", std::ios::binary|std::ios::in|std::ios::ate);

  unsigned size = file.tellg() / (N * sizeof(float)) - 1;
  unsigned i0 = T0 / Δw;

  if (i0 > size) {
    return 0;
  }

  file.seekg((i0 + 1) * sizeof(float) * N);

  Vector x0(N);
  file.read(reinterpret_cast<char*>(x0.data()), N * sizeof(float));

  file.seekg((i0 + 1) * sizeof(float) * N);

  std::ofstream outfile("correlations/" + filebase + "_" + std::to_string(T0) + ".dat");

  for (Real t = 0; t <= T; t += Δw) {
    if (file.peek() == EOF) {
      break;
    }

    Vector x(N);
    file.read(reinterpret_cast<char*>(x.data()), N * sizeof(float));

    outfile << t << "\t" << x0.dot(x) / N << std::endl;
  }


  return 0;
}