squares.h

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//                                OpenRAND                                       *
//   A Performance Portable, Reproducible Random Number Generation Library       *
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#ifndef OPENRAND_SQUARES_H_
#define OPENRAND_SQUARES_H_
 
#include <openrand/base_state.h>
#include <stdint.h>
 
namespace {
 
#define K 0xc58efd154ce32f6d
 
/*
In Squares, since the seed (i.e. key) has to have certain properties to
generate good random numbers, we can't allow user to set arbritray seed.
To avoid the pitfall of weak user supplied seed, we need to hash that seed.
*/
// inline OPENRAND_DEVICE uint64_t hash_seed(uint64_t seed){
//     return (seed + 1) * K;
// }
 
// inline unsigned nthset(uint16_t x, unsigned n) {
//     return _tzcnt_u64(_pdep_u64(1U << n, x));
// }
 
OPENRAND_DEVICE unsigned int get_digit(uint64_t i, char *mask) {
  unsigned int j = 0;
  while (1) {
    while (mask[j] == 0) j++;
    if (i == 0) return j;
    i--;
    j++;
  }
}
 
OPENRAND_DEVICE uint64_t hash_seed(uint32_t seed) {
  constexpr uint64_t factorials[16] = {
      1,         1,          2,           6,
      24,        120,        720,         5040,
      40320,     362880,     3628800,     39916800,
      479001600, 6227020800, 87178291200, 1307674368000};
  uint64_t base = (1ULL << 60);
  uint64_t i = static_cast<uint64_t>(seed);
 
  // NOTE: For CPUs with Bit Manipulation Instruction Set 2 support, we should
  // use the commented out uint16_t mask and nthset function.
 
  // uint16_t mask = 0b1111'1111'1111'1111;
  char mask[16] = {1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1};
 
  uint64_t res = 0;
  int n = 15;
  while (n) {
    uint64_t v = factorials[n];
    unsigned int d = get_digit(i / v, mask);
    // const unsigned int d = nthset(mask, i / v);
    mask[d] = 0;
    // mask = mask & ~(1<<d);
 
    i = i % v;
    res += d * base;
    base = base >> 4;
    n--;
  }
  // unsigned int d = nthset(mask, 0);
  unsigned int d = get_digit(0, mask);
  res += d;
  return res;
}
}  // namespace
 
namespace openrand {
 
class Squares : public BaseRNG<Squares> {
 public:
  OPENRAND_DEVICE Squares(uint32_t seed, uint32_t ctr,
                          uint32_t global_seed = openrand::DEFAULT_GLOBAL_SEED)
      : seed(hash_seed(seed) ^ global_seed), counter(ctr) {
  }
 
  template <typename T = uint32_t>
  OPENRAND_DEVICE T draw() {
    auto round = [](uint64_t x, uint64_t w) {
      x = x * x + w;
      return (x >> 32) | (x << 32);
    };
    _ctr++;
    uint64_t ctr = (static_cast<uint64_t>(counter) << 32) | _ctr;
    uint64_t x = ctr * seed;
    uint64_t y = x;
    uint64_t z = y + seed;
 
    if constexpr (std::is_same_v<T, uint32_t>) {
      x = round(x, y);
      x = round(x, z);
      x = round(x, y);
      return static_cast<uint32_t>((x * x + z) >> 32);
    } else {
      x = round(x, y);
      x = round(x, z);
      x = round(x, y);
      x = round(x, z);
      return x ^ ((x * x + y) >> 32);
    }
  }
 
 private:
  const uint64_t seed;
  const uint32_t counter = 0;
 
  // internal counter
 public:
  uint32_t _ctr = 0;
};  // class Squares
 
}  // namespace openrand
 
#endif  // OPENRAND_SQUARES_H_