std::seed_seq::generate
| template< class RandomIt > void generate( RandomIt begin, RandomIt end ); |
(since C++11) | |
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,
- begin[k + p] += r1
- begin[k + q] += r2
- 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,
- begin[k + p] ^= r3
- begin[k + q] ^= r4
- 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
#include <algorithm> #include <cassert> #include <cstdint> #include <iostream> #include <random> // Prototyping the main part of std::seed_seq... struct seed_seq { std::vector<std::uint32_t> v; seed_seq(std::initializer_list<std::uint32_t> const il) : v{il} {} template <typename RandomIt> void generate(RandomIt first, RandomIt last) { if (first == last) return; // // Assuming v = {1,2,3,4,5} and distance(first, last) == 10. // // Step 1: fill with 0x8b8b8b8b // seeds = {2341178251, 2341178251, 2341178251, 2341178251, 2341178251, // 2341178251, 2341178251, 2341178251, 2341178251, 2341178251 } // std::fill(first, last, 0x8b8b8b8b); // // Step 2: // n = 10, s = 5, t = 3, p = 3, q = 6, m = 10 // const std::uint32_t n = last - first; const std::uint32_t s = v.size(); const std::uint32_t t = (n < 7) ? (n - 1) / 2 : (n < 39) ? 3 : (n < 68) ? 5 : (n < 623) ? 7 : 11; const std::uint32_t p = (n - t) / 2; const std::uint32_t q = p + t; const std::uint32_t m = std::max(s + 1, n); // // 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 } // auto begin_mod = [first, n](std::uint32_t u) -> decltype(*first)& { return first[u % n]; // i.e. begin[x] is taken modulo n }; auto T = [](std::uint32_t x) { return x ^ (x >> 27); }; for (std::uint32_t k = 0, r1, r2; k < m; ++k) { r1 = 1664525 * T(begin_mod(k) ^ begin_mod(k + p) ^ begin_mod(k - 1)); r2 = (k == 0) ? r1 + s : (k <= s) ? r1 + k % n + v[k - 1] : r1 + k % n; begin_mod(k + p) += r1; begin_mod(k + q) += r2; begin_mod(k) = r2; } // // 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 k = m, r3, r4; k < m + n; ++k) { r3 = 1566083941 * T(begin_mod(k) + begin_mod(k + p) + begin_mod(k - 1)); r4 = r3 - k % n; begin_mod(k+p) ^= r3; begin_mod(k+q) ^= r4; begin_mod(k) = r4; } } }; int main() { const auto input = std::initializer_list<std::uint32_t>{1,2,3,4,5}; const auto output_size = 10; // using std version of seed_seq std::seed_seq seq(input); std::vector<std::uint32_t> seeds(output_size); seq.generate(seeds.begin(), seeds.end()); for (const std::uint32_t n : seeds) std::cout << n << '\n'; // using custom version of seed_seq seed_seq seq2(input); std::vector<std::uint32_t> seeds2(output_size); seq2.generate(seeds2.begin(), seeds2.end()); assert(seeds == seeds2); }
Output:
4204997637 4246533866 1856049002 1129615051 690460811 1075771511 46783058 3904109078 1534123438 1495905678