29 Numerics library [numerics]

29.6 Random number generation [rand]

29.6.4 Random number engine adaptor class templates [rand.adapt] Class template shuffle_­order_­engine [rand.adapt.shuf]

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 {
    // 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
    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()();
    void discard(unsigned long long z);

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

    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().