Method boundedRandom

src/pass.cpp:850–871

* @brief Generates a random number bounded by the given value. * @param bound Upper bound (exclusive) * @return Random number in range [0, bound) */

 * @return Random number in range [0, bound)
 */
auto Pass::boundedRandom(quint32 bound) -> quint32 {
  if (bound < 2) {
    return 0;
  }

  quint32 randval;
  // Rejection-sampling threshold to avoid modulo bias.
  // This follows the well-known "arc4random_uniform"-style approach:
  // reject values in the low range [0, min), where
  //   min = 2^32 % bound
  // so that the remaining range size is an exact multiple of `bound`.
  //
  // In quint32 arithmetic, (1 + ~bound) wraps to (2^32 - bound), therefore
  //   (1 + ~bound) % bound == 2^32 % bound.
  const quint32 rejectionThreshold = (1 + ~bound) % bound;

  do {
    randval = QRandomGenerator::system()->generate();
  } while (randval < rejectionThreshold);

  return randval % bound;
}

/**
 * @brief Generates a random password from the given charset.