29 Numerics library [numerics]

29.6 Random number generation [rand]

A shuffle_­order_­engine random number engine adaptor produces the same random numbers that are produced by some base engine e, but delivers them in a different sequence. The state xi of a shuffle_­order_­engine engine adaptor object x consists of the state ei of its base engine e, an additional value Y of the type delivered by e, and an additional sequence V of k values also of the type delivered by e. The size of the state is the size of e's state plus k+1.

The transition algorithm permutes the values produced by e. The state transition is performed as follows:

1. a)Calculate an integer j=k(Yemin)emaxemin+1 .

2. b)Set Y to Vj and then set Vj to e().

The generation algorithm yields the last value of Y produced while advancing e's state as described above.

```template<class Engine, size_t k>
class shuffle_order_engine {
public:
// types
using result_type = typename Engine::result_type;

// engine characteristics
static constexpr size_t table_size = k;
static constexpr result_type min() { return Engine::min(); }
static constexpr result_type max() { return Engine::max(); }

// constructors and seeding functions
shuffle_order_engine();
explicit shuffle_order_engine(const Engine& e);
explicit shuffle_order_engine(Engine&& e);
explicit shuffle_order_engine(result_type s);
template<class Sseq> explicit shuffle_order_engine(Sseq& q);
void seed();
void seed(result_type s);
template<class Sseq> void seed(Sseq& q);

// generating functions
result_type operator()();

// property functions
const Engine& base() const noexcept { return e; };

private:
Engine e;           // exposition only
result_type V[k];   // exposition only
result_type Y;      // exposition only
};```

The following relation shall hold: 0 < k.

The textual representation consists of the textual representation of e, followed by the k values of V, followed by the value of Y.

In addition to its behavior pursuant to section [rand.req.adapt], each constructor that is not a copy constructor initializes V[0],,V[k-1] and Y, in that order, with values returned by successive invocations of e().