std::seed_seq::generate (3) - Linux Manuals

std::seed_seq::generate: std::seed_seq::generate

NAME

std::seed_seq::generate - std::seed_seq::generate

Synopsis

template< class RandomIt > (since C++11)
void generate( RandomIt begin, RandomIt end );

Fills the range [begin, end) with unsigned integer values i, 0 ≤ i < 232
, based on the data originally provided in the constructor of this seed_seq. The produced values are distributed over the entire 32-bit range even if the initial values were strongly biased.
The following algorithm is used (adapted from the initialization sequence of the Mersenne Twister generator by Makoto_Matsumoto_and_Takuji_Nishimura, incorporating the improvements made by Mutsuo_Saito_in_2007)

* If begin == end, do nothing. Otherwise,

* First, set each element of the output range to the value 0x8b8b8b8b

* Transform the elements of the output range according to the following algorithm:

For k = 0,..., m-1
where m=max(s+1, n)
and n=end-begin
and s=v.size()
and v is the private container holding the values originally provided by the constructor of this seed_seq object,

  1. begin[k+p] += r1
  2. begin[k+q] += r2
  3. begin[k] = r2,

where p=(n-t)/2
and q=p+t
and t=(n >= 623) ? 11 : (n >= 68) ? 7 : (n >= 39) ? 5 : (n >= 7) ? 3 : (n - 1) / 2
and r1=1664525 * T(begin[k]^begin[k+p]^begin[k−1])
and T(x) = x ^ (x >> 27)
and r2=r1+s if k==0, r2=r1 + k%n + v[k-1] if 0<k<=s, r2=r1 + k%n if k>s.
For k = m,..., m+n-1,

  1. begin[k+p] ^= r3
  2. begin[k+q] ^= r4
  3. begin[k]=r4

where r3 = 1566083941 * T(begin[k]+begin[k+p]+begin[k-1])
and r4=r3 - k%n
where all calculations are performed modulo 232
and where the indexing of the output range (begin[x]) is taken modulo n.

Parameters

begin, end - mutable random-access iterators whose std::iterator_traits<>::value_type is an unsigned integer type suitable for storing 32-bit values

Type requirements

-
RandomIt must meet the requirements of LegacyRandomAccessIterator.

Return value

none, the results are written to the [begin, end) range.

Exceptions

Only throws if the operations on begin and end throw.

Example

// Run this code

  #include <random>
  #include <iostream>

  int main()
  {
      std::seed_seq seq({1,2,3,4,5});
      std::vector<std::uint32_t> seeds(10);
      seq.generate(seeds.begin(), seeds.end());
  // Step 1: fill with 0x8b8b8b8b
  // seeds = {2341178251, 2341178251, 2341178251, 2341178251, 2341178251,
  // 2341178251, 2341178251, 2341178251, 2341178251, 2341178251 }
  //
  // Step 2:
  // n = 10, s = 5, t = 3, p = 3, q = 6, m = 10
  //
  // First iteration, k = 0; r1 = 1371501266, r2 = 1371501271
  //
  // seeds = {1371501271, 2341178251, 2341178251, 3712679517, 2341178251,
  // 2341178251, 3712679522, 2341178251, 2341178251, 2341178251 }
  //
  // Iterations from k = 1 to k = 5 (r2 = r1 + k%n + v[k-1])
  //
  // r1 = 2786190137, 3204727651, 4173325571, 1979226628, 401983366
  // r2 = 2786190139, 3204727655, 4173325577, 1979226636, 401983376
  //
  // seeds = {3350727907, 3188173515, 3204727655, 4173325577, 1979226636,
  // 401983376, 3591037797, 2811627722, 1652921976, 2219536532 }
  //
  // Iterations from k = 6 to k = 9 (r2 = r1 + k%n)
  //
  // r1 = 2718637909, 1378394210, 2297813071, 1608643617
  // r2 = 2718637915, 1378394217, 2297813079, 1608643626
  //
  // seeds = { 434154821, 1191019290, 3237041891, 1256752498, 4277039715,
  // 2010627002, 2718637915, 1378394217, 2297813079, 1608643626 }
  //
  // Step 3
  // iterations from k = 10 to k = 19, using ^= to modify the output
  //
  // r1 = 1615303485, 3210438310, 893477041, 2884072672, 1918321961,
  // r2 = 1615303485, 3210438309, 893477039, 2884072669, 1918321957
  //
  // seeds = { 303093272, 3210438309, 893477039, 2884072669, 1918321957,
  // 1117182731, 1772877958, 2669970405, 3182737656, 4094066935 }
  //
  // r1 = 423054846, 46783064, 3904109085, 1534123446, 1495905687
  // r2 = 423054841, 46783058, 3904109078, 1534123438, 1495905678
  //
  // seeds = { 4204997637, 4246533866, 1856049002, 1129615051, 690460811,
  // 1075771511, 46783058, 3904109078, 1534123438, 1495905678 }

      for(std::uint32_t n : seeds)
          std::cout << n << '\n';
  }

Output:

  4204997637
  4246533866
  1856049002
  1129615051
  690460811
  1075771511
  46783058
  3904109078
  1534123438
  1495905678