Being a quick sketch combining <chrono> and <random> functionality, with cryptarithmetic interludes…
At CppCon this year there were several good talks about randomness and time calculations in C++. On randomness: Walter Brown’s What C++ Programmers Need to Know About Header <random> and Cheinan Marks’ I Just Wanted a Random Integer! were both excellent talks. And Howard Hinnant gave several great talks: A <chrono> Tutorial, and Welcome to the Time Zone, a followup to his talk from last year, A C++ Approach to Dates and Times.
CHRONO + RANDOM = HORRID ?
That’s perhaps a little unfair, but recently I ran into the need to compute a random period of time. I think this is a common use case for things like backoff schemes for network retransmission. And it seemed to me that the interaction of <chrono> and <random> was not quite as good as it could be:
system_clock::duration minTime = 0s;
system_clock::duration maxTime = 5s;
uniform_int_distribution<> d(minTime.count(), maxTime.count());
// 'gen' here is a Mersenne twister engine
auto nextTransmissionWindow = system_clock::duration(d(gen));
This code gets more complex when you start computing an exponential backoff. Relatively straightforward, but clumsy, especially if you want a floating-point base for your exponent calculation: system_clock::duration has an integral representation, so in all likelihood you end up having to cast multiple times, using either static_cast or duration_cast. That’s a bit messy.
I remembered some code from another talk: Andy Bond’s AAAARGH!? Adopting Almost Always Auto Reinforces Good Habits!? in which he presented a function to make a uniform distribution by inferring its argument type, useful in generic code. Something like the following:
template ,
typename D = std::uniform_int_distribution>
inline auto make_uniform_distribution(const A& a,
const B& b = std::numeric_limits::max())
-> std::enable_if_t::value, D>
{
return D(a, b);
}
Of course, the standard also provides uniform_real_distribution, so we can provide another template and overload the function for real numbers:
template ,
typename D = std::uniform_real_distribution>
inline auto make_uniform_distribution(const A& a,
const B& b = B{1})
-> std::enable_if_t::value, D>
{
return D(a, b);
}
And with these two in hand, it’s easy to write a uniform_duration_distribution that uses the correct distribution for its underlying representation (using a home-made type trait to constrain it to duration types).
template
struct is_duration : std::false_type {};
template
struct is_duration> : std::true_type {};
template ::value>>
class uniform_duration_distribution
{
public:
using result_type = Duration;
explicit uniform_duration_distribution(
const Duration& a = Duration::zero(),
const Duration& b = Duration::max())
: m_a(a), m_b(b)
{}
void reset() {}
template
result_type operator()(Generator& g)
{
auto d = make_uniform_distribution(m_a.count(), m_b.count());
return result_type(d(g));
}
result_type a() const { return m_a; }
result_type b() const { return m_b; }
result_type min() const { return m_a; }
result_type max() const { return m_b; }
private:
result_type m_a;
result_type m_b;
};
Having written this, we can once again overload make_uniform_distribution to provide for duration types:
template ,
typename D = uniform_duration_distribution>
inline auto make_uniform_distribution(const A& a,
const B& b = B::max()) -> D
{
return D(a, b);
}
And now we can compute a random duration more expressively and tersely, and, I think, in the spirit of the existing functionality that exists in <chrono> for manipulating durations.
auto d = make_uniform_distribution(0s, 5000ms);
auto nextTransmissionWindow = d(gen);
CHRONO + RANDOM = DREAMY
I leave it as an exercise for the reader to solve these cryptarithmetic puzzles. As for the casting problems, for now, I’m living with them.
check out my project: https://github.com/effolkronium/TinyRandom