26 Numerics library [numerics]

26.5 Random number generation [rand]

26.5.7 Utilities [rand.util]

26.5.7.1 Class seed_seq [rand.util.seedseq]

1

No function described in this section [rand.util.seedseq] throws an exception.

class seed_seq{
public:
 // types
 typedef uint_least32_t result_type;

 // constructors
 seed_seq();
 template<class T>
   seed_seq(initializer_list<T> il);
 template<class InputIterator>
   seed_seq(InputIterator begin, InputIterator end);

 // generating functions
 template<class RandomAccessIterator>
   void generate(RandomAccessIterator begin, RandomAccessIterator end);

 // property functions
 size_t size() const;
 template<class OutputIterator>
   void param(OutputIterator dest) const;

 // no copy functions
 seed_seq(const seed_seq& ) = delete;
 void operator=(const seed_seq& ) = delete;

private:
 vector<result_type> v;   // exposition only
};

seed_seq();

2

Effects: Constructs a seed_seq object as if by default-constructing its member v.

template<class T> seed_seq(initializer_list<T> il);

3

Requires: T shall be an integer type.

4

Effects: Same as seed_seq(il.begin(), il.end()).

template<class InputIterator> seed_seq(InputIterator begin, InputIterator end);

5

Requires: InputIterator shall satisfy the requirements of an input iterator (Table [tab:iterator.input.requirements]) type. Moreover, iterator_traits<InputIterator>::value_type shall denote an integer type.

6

Effects: Constructs a seed_seq object by the following algorithm: 

for( InputIterator s = begin; s != end; ++s)
 v.push_back((*s)mod 232);

template<class RandomAccessIterator> void generate(RandomAccessIterator begin, RandomAccessIterator end);

7

Requires: RandomAccessIterator shall meet the requirements of a mutable random access iterator (Table [tab:iterator.random.access.requirements]) type. Moreover, iterator_traits<RandomAccessIterator>::value_type shall denote an unsigned integer type capable of accommodating 32-bit quantities.

8

Effects: Does nothing if begin == end. Otherwise, with s = v.size() and n = end - begin , fills the supplied range [begin,end) according to the following algorithm in which each operation is to be carried out modulo 232, each indexing operator applied to begin is to be taken modulo n, and T(x) is defined as x     xor     (x     rshift     27):

  1. (8.1)

    By way of initialization, set each element of the range to the value 0x8b8b8b8b. Additionally, for use in subsequent steps, let p = (n - t) / 2 and let q = p + t, where t = (n ≥ 623) ? 11 : (n ≥ 68) ? 7 : (n ≥ 39) ? 5 : (n ≥ 7) ? 3 : (n - 1)/2;

  2. (8.2)

    With m as the larger of s + 1 and n, transform the elements of the range: iteratively for k = 0, …, m-1 , calculate values \begin{eqnarray*} r_1 & = & 1664525 \cdot \texttt{T}\left( \texttt{begin[}k\texttt{]} \ensuremath{\, \mathsf{xor} \,} \texttt{begin[}k+p\texttt{]} \ensuremath{\, \mathsf{xor} \,} \texttt{begin[}k-1 \texttt{]} \right) \\ r_2 & = & r_1 + \left\{ \begin{array}{cl} s & \mbox{, } k = 0 \\ k \bmod n + \texttt{v[}k-1\texttt{]} & \mbox{, } 0 < k \le s \\ k \bmod n & \mbox{, } s < k \end{array} \right. \end{eqnarray*} and, in order, increment begin[k+p] by r1, increment begin[k+q] by r2, and set begin[k] to r2.

  3. (8.3)

    Transform the elements of the range again, beginning where the previous step ended: iteratively for k = m, …, m+n-1 , calculate values \begin{eqnarray*} r_3 & = & 1566083941 \cdot \texttt{T}\left( \texttt{begin[}k \texttt{]} + \texttt{begin[}k+p\texttt{]} + \texttt{begin[}k-1\texttt{]} \right) \\ r_4 & = & r_3 - (k \bmod n) \end{eqnarray*} and, in order, update begin[k+p] by xoring it with r3, update begin[k+q] by xoring it with r4, and set begin[k] to r4.

size_t size() const;

9

Returns: The number of 32-bit units that would be returned by a call to param().

10

Complexity: constant time.

template<class OutputIterator> void param(OutputIterator dest) const;

11

Requires: OutputIterator shall satisfy the requirements of an output iterator (Table [tab:iterator.output.requirements]) type. Moreover, the expression *dest = rt shall be valid for a value rt of type result_type.

12

Effects: Copies the sequence of prepared 32-bit units to the given destination, as if by executing the following statement: 

copy(v.begin(), v.end(), dest);