abms_and_parallel_computing_teaching/exercise2_lcg.cpp at main · SciCompMod/abms_and_parallel_computing_teaching · GitHub

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#include <cstddef>
#include <iostream>
#include <string>
#include <vector>

typedef struct {
  unsigned int a, c, m, state;
} LCG;

void get_sample_print(LCG &lcg, int n) {
  for (int i = 0; i < n; ++i) {
    lcg.state = (lcg.a * lcg.state + lcg.c) % lcg.m;
    std::cout << (lcg.state)
              << " "; // potentially scale to [0,1] by dividing by m
  }
}

void get_sample_bin_print(LCG &lcg, int n, int shift = 0) {
  for (int i = 0; i < shift; ++i) {
    lcg.state = (lcg.a * lcg.state + lcg.c) % lcg.m;
  }
  double min = 1.0, max = 0.0;
  std::vector<int> bins(10, 0);
  for (int i = 0; i < n; ++i) {
    lcg.state = (lcg.a * lcg.state + lcg.c) % lcg.m;
    bins[(size_t)((lcg.state) / (double)lcg.m * 10)]++;
    // std::cout << ((lcg.state) / (double)lcg.m * 100) << " ";
  }
  for (size_t i = 0; i < bins.size(); ++i) {
    double dummy = bins[i] / 1e4;
    if (dummy > max)
      max = dummy;
    if (dummy < min)
      min = dummy;
    std::cout << i << " " << bins[i] / 1e4 << std::endl;
  }
  std::cout << "min: " << min << ", max: " << max << std::endl;
}

void parse_args(int argc, char *argv[], unsigned int &a, unsigned int &c,
                unsigned int &m, unsigned int &seed) {
  for (int i = 1; i < argc - 1; ++i) {
    std::string arg(argv[i]);
    if (arg == "--a") {
      a = static_cast<unsigned int>(std::stoi(argv[++i]));
    } else if (arg == "--c") {
      c = static_cast<unsigned int>(std::stoi(argv[++i]));
    } else if (arg == "--m") {
      m = static_cast<unsigned int>(std::stoi(argv[++i]));
    } else if (arg == "--s") {
      seed = static_cast<unsigned int>(std::stoi(argv[++i]));
    } else {
      std::cerr << "Unknown or malformed argument. Using standard parameters."
                << std::endl;
      std::cerr << "Usage: " << std::string(argv[0])
                << " [--a A] [--c C] [--m M] [--s S]" << std::endl;
    }
  }
}

int main(int argc, char *argv[]) {
  //     unsigned int seed = 0;
  //     unsigned int a    = 1;
  //     unsigned int c    = 2;
  //     unsigned int m    = 3;

  // unsigned int seed = 1;
  // unsigned int a    = 2;
  // unsigned int c    = 0;
  // unsigned int m    = 9;

  unsigned int seed = 0;
  unsigned int a = 1103515245;
  unsigned int c = 12345;
  unsigned int m = 1U << 31; // 2^31 (glibc (used by GCC), see Wikipedia)

  parse_args(argc, argv, a, c, m, seed);
  std::cout << "Parameters: a=" << a << ", c=" << c << ", m=" << m
            << ", seed: " << seed << std::endl;

  LCG lcg = {a, c, m, static_cast<unsigned int>(seed)};
  // get_sample_print(lcg, 10);

  get_sample_bin_print(lcg, 1e6, 3e5);
}