[#time_generator_v7]
== <boost/uuid/{zwsp}time_generator_v7.hpp>

:idprefix: time_generator_v7_

=== Synopsis

[source,c++]
[subs=+quotes]
----
namespace boost {
namespace uuids {

class time_generator_v7
{
private:

    // exposition only
    _unspecified-csprng-type_ rng_;

public:

    using result_type = uuid;

    time_generator_v7();

    time_generator_v7( time_generator_v7 const& rhs );
    time_generator_v7( time_generator_v7&& rhs ) noexcept;

    time_generator_v7& operator=( time_generator_v7 const& rhs ) noexcept;
    time_generator_v7& operator=( time_generator_v7&& rhs ) noexcept;

    result_type operator()() noexcept;
};

}} // namespace boost::uuids
----

The class `time_generator_v7` generates time-based version 7 UUIDs, as described in https://datatracker.ietf.org/doc/rfc9562/[RFC 9562] section 5.7.

=== Constructors

```
time_generator_v7();
```

Effects: :: Initializes the internal cryptographically strong pseudorandom number generator (CSPRNG) `rng_` using entropy from `std::random_device`.

```
time_generator_v7( time_generator_v7 const& rhs );
```

Effects: :: Copies the state of `rhs` into `*this`, then reseeds `this\->rng_` using entropy from `std::random_device`.

Remarks: :: Reseeding ensures that the two generators don't produce the same random number sequence after the copy.

```
time_generator_v7( time_generator_v7&& rhs ) noexcept;
```

Effects: :: Copies the state of `rhs` into `*this`, then perturbs the state of `rhs.rng_` such that it no longer produces the same random number sequence.

=== Assignment

```
time_generator_v7& operator=( time_generator_v7 const& rhs ) noexcept;
```

Effects: :: Copies the state of `rhs` into `*this`, except for `this\->rng_`, which is left unmodified.

Returns: :: `*this`.

Remarks: :: Not modifying `this\->rng_` ensures that the two generators continue to produce different random numbers.

```
time_generator_v7& operator=( time_generator_v7&& rhs ) noexcept;
```

Effects: :: Copies the state of `rhs` into `*this`, then perturbs the state of `rhs.rng_` such that it no longer produces the same random number sequence.

Returns: :: `*this`.

=== operator()

```
result_type operator()() noexcept;
```

Effects: ::
+
. Obtains a new timestamp as if by getting the system time from `std::chrono::system_clock::now()` and converting it to a number of microseconds from the Unix epoch.
. Creates a new version 7 UUID and initializes its `unix_ts_ms` field with the number of milliseconds in the timestamp.
. Initializes the `rand_a` field with the residual number of microseconds from the timestamp.
. Initializes the 6 bits of the `rand_b` field following the variant to a conflict resolution counter, such that if more than one UUID is generated using the same timestamp, monotonicity is still ensured.
. Initializes the rest of the `rand_b` field to random values obtained from the internal CSPRNG `rng_`.
. Returns the UUID.

Remarks: :: `operator()` generates a monotonically increasing sequence of UUIDs, if the following requirements are met:
+
* The system clock never goes backwards.
* The system clock has at least millisecond resolution.
* No more than 64 UUIDs are generated per microsecond (no more than one every 16 nanoseconds).
