# CHRONO + RANDOM = ?

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 `duration`s.

```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.

Published
Categorized as C++

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