get every number in [0, n) range only once, in pseudo-random order

pseudo_random_iter.cpp

#include <iostream>
#include <numeric>

//
// `ax + b mod n`, where `[0,n)`
// `a` must be a coprime with `n`,
// `gcd(a, n) = 1`
// `a=1` is bad for obvious reasons, so we gonna pick `a` from `[n/2, n)`.
//

int main() {
  const int b = 7;
  const int n = 120;
  const int a = []() {
    for (int p = n / 2 + b; p < n; p++) {
      if (std::gcd(p, n) == 1) {
        return p;
      }
    }
    return -1;
  }();

  if (a == -1) {
    return -1;
  }

  for (int i = 0; i < n; i++) {
    int v = (a * i + b) % n;
    std::cout << v << '\n';
  }
  return 0;
}