(Unnecessarily complicated?) fake clock publisher for ROS

fake_clock.cpp

#include <ros/ros.h>
#include <rosgraph_msgs/Clock.h>

template <typename T> auto to_ros(const T &t) {
  auto dur = std::chrono::duration_cast<std::chrono::nanoseconds>(t);
  return std::make_pair(int32_t(std::chrono::duration_cast<std::chrono::seconds>(dur).count()),
                        int32_t(dur.count() % 1000000000UL));
}

int main(int argc, char **argv) {
  ros::init(argc, argv, "fake_clock");
  ros::NodeHandle nh, pnh("~");

  if (!nh.param("/use_sim_time", false)) {
    ROS_FATAL("Param: \"/use_sim_time\" unset or set to false. Exiting");
    return 1;
  }

  auto num = pnh.param("slowdown/num", 1);
  auto den = pnh.param("slowdown/den", 1);
  ROS_ASSERT(num >= 1 && den >= 1);

  auto pub = nh.advertise<rosgraph_msgs::Clock>("/clock", 10);

  using ns = std::chrono::nanoseconds;

  ns t(0), carry(0);
  const ns dt((long long)1e5), num_ns(num);

  ROS_INFO("Starting clock with a slowdown of %i / %i.", num, den);
  rosgraph_msgs::Clock ck;

  const auto [sec, nsec] = to_ros(dt);
  ros::WallRate r(ros::Duration(sec, nsec));

  while (ros::ok()) {
    auto x = dt * den;
    t += x / num;
    carry += x % num;

    if (carry >= num_ns) {
      t += ns{carry / num_ns};
      carry %= num;
    }

    const auto [ts, tns] = to_ros(t);
    ck.clock.sec = ts;
    ck.clock.nsec = tns;

    pub.publish(ck);

    if (!r.sleep()) {
      ROS_ERROR_THROTTLE(1.0, "Clock did not meet required rate.");
    }
  }
  return 0;
}

launch_fake_clock.launch

<?xml version="1.0" ?>
<launch xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="https://gist.githubusercontent.com/nalt/dfa2abc9d2e3ae4feb82ca5608090387/raw/roslaunch.xsd">
    <param name="/use_sim_time" value="true"/>
    <node pkg="fake_clock" type="fake_clock" name="fake_clock">
        <param name="slowdown/num" value="5"/>
        <param name="slowdown/den" value="1"/>
    </node>
</launch>