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std::seed_seq::generate

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< cpp‎ | numeric‎ | random‎ | seed seq
Revision as of 07:32, 4 January 2013 by 80.254.147.84 (Talk)

 
 
 
 
 

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<td >
template< class RandomIt >
void generate( RandomIt begin, RandomIt end );
</td>

<td class="t-dcl-nopad"> </td> <td > (since C++11) </td> </tr> Template:ddcl list 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 RandomAccessIterator.

Return value

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

Example

#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