A discard_block_engine random number engine adaptor produces random numbers selected from those produced by some base engine e. The state xi of a discard_block_engine engine adaptor object x consists of the state ei of its base engine e and an additional integer n. The size of the state is the size of e's state plus 1.
The transition algorithm discards all but r>0 values from each block of p≥r values delivered by e. The state transition is performed as follows: If n≥r, advance the state of e from ei to ei+p−r and set n to 0. In any case, then increment n and advance e's then-current state ej to ej+1.
The generation algorithm yields the value returned by the last invocation of e() while advancing e's state as described above.
template<class Engine, size_t p, size_t r>
class discard_block_engine {
public:
using result_type = typename Engine::result_type;
static constexpr size_t block_size = p;
static constexpr size_t used_block = r;
static constexpr result_type min() { return Engine::min(); }
static constexpr result_type max() { return Engine::max(); }
discard_block_engine();
explicit discard_block_engine(const Engine& e);
explicit discard_block_engine(Engine&& e);
explicit discard_block_engine(result_type s);
template<class Sseq> explicit discard_block_engine(Sseq& q);
void seed();
void seed(result_type s);
template<class Sseq> void seed(Sseq& q);
result_type operator()();
void discard(unsigned long long z);
const Engine& base() const noexcept { return e; };
private:
Engine e; int n; };
The following relations shall hold: 0 < r and r <= p.
The textual representation consists of the textual representation of e followed by the value of n.
In addition to its behavior pursuant to section [rand.req.adapt], each constructor that is not a copy constructor sets n to 0.
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