ca-rng.H

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/*
                          Aleph_w
 
  Data structures & Algorithms
  version 2.0.0b
  https://github.com/lrleon/Aleph-w
 
  This file is part of Aleph-w library
 
  Copyright (c) 2002-2026 Leandro Rabindranath Leon
 
  Permission is hereby granted, free of charge, to any person obtaining a copy
  of this software and associated documentation files (the "Software"), to deal
  in the Software without restriction, including without limitation the rights
  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  copies of the Software, and to permit persons to whom the Software is
  furnished to do so, subject to the following conditions:
 
  The above copyright notice and this permission notice shall be included in all
  copies or substantial portions of the Software.
 
  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  SOFTWARE.
*/
 
#ifndef CA_RNG_H
#define CA_RNG_H
 
#include <cstddef>
#include <cstdint>
#include <random>
#include <type_traits>
 
#include <ca-traits.H>
 
namespace Aleph {
namespace CA {
 
[[nodiscard]] inline constexpr std::uint64_t splitmix64(std::uint64_t x) noexcept
{
  x += 0x9e3779b97f4a7c15ull;
  x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9ull;
  x = (x ^ (x >> 27)) * 0x94d049bb133111ebull;
  return x ^ (x >> 31);
}
 
[[nodiscard]] inline constexpr std::uint64_t mix_seed(const std::uint64_t a,
                                                      const std::uint64_t b) noexcept
{
  return splitmix64(splitmix64(a + 0x9e3779b97f4a7c15ull) ^ (b + 0xbf58476d1ce4e5b9ull));
}
 
template <std::size_t Rank>
[[nodiscard]] inline constexpr std::uint64_t cell_key_from_coord(const Coord_Vec<Rank> &c) noexcept
{
  std::uint64_t h = 0xcbf29ce484222325ull;  // FNV offset basis.
  for (std::size_t d = 0; d < Rank; ++d)
    {
      const std::uint64_t component = static_cast<std::uint64_t>(static_cast<std::int64_t>(c[d]));
      h = splitmix64(h ^ component);
    }
  return h;
}
 
template <std::size_t Rank>
[[nodiscard]] inline constexpr std::uint64_t cell_seed(const std::uint64_t master_seed,
                                                       const std::size_t step,
                                                       const Coord_Vec<Rank> &coord) noexcept
{
  const std::uint64_t step_hash
    = splitmix64(static_cast<std::uint64_t>(step) + 0xa5a5a5a5a5a5a5a5ull);
  const std::uint64_t coord_hash = cell_key_from_coord<Rank>(coord);
  return splitmix64(master_seed ^ step_hash ^ coord_hash);
}
 
template <std::size_t Rank>
[[nodiscard]] inline constexpr std::uint64_t cell_seed(const std::uint64_t master_seed,
                                                       const Cell_Context<Rank> &ctx) noexcept
{
  return cell_seed<Rank>(master_seed, ctx.step, ctx.coord);
}
 
[[nodiscard]] inline constexpr std::uint64_t thread_step_seed(const std::uint64_t master_seed,
                                                              const std::size_t thread_id,
                                                              const std::size_t step) noexcept
{
  const std::uint64_t tid_hash
    = splitmix64(static_cast<std::uint64_t>(thread_id) + 0xfeedfacecafebee0ull);
  const std::uint64_t step_hash
    = splitmix64((static_cast<std::uint64_t>(step) << 32) ^ 0x0123456789abcdefull);
  return splitmix64(master_seed ^ tid_hash ^ step_hash);
}
 
template <typename Engine>
[[nodiscard]] inline double uniform_unit(Engine &eng)
{
  static_assert(std::is_unsigned_v<typename Engine::result_type>,
                "uniform_unit requires an unsigned result_type");
  const std::uint64_t v = static_cast<std::uint64_t>(eng());
  // 2^53 = 9007199254740992. The shift drops the noisy low 11 bits
  // and leaves a value in [0, 2^53) that maps exactly to [0, 1).
  return static_cast<double>(v >> 11) * (1.0 / 9007199254740992.0);
}
 
template <typename Engine>
[[nodiscard]] inline std::size_t uniform_int(Engine &eng, const std::size_t lo, const std::size_t hi)
{
  if (hi <= lo)
    return lo;
  const double u = uniform_unit(eng);
  const std::size_t span = hi - lo + 1;
  std::size_t pick = lo + static_cast<std::size_t>(u * static_cast<double>(span));
  if (pick > hi)
    pick = hi;
  return pick;
}
 
template <typename Engine = std::mt19937_64>
class Per_Thread_RNG
{
  std::uint64_t master_;
 
public:
  using engine_type = Engine;
  using result_type = typename Engine::result_type;
 
  explicit constexpr Per_Thread_RNG(const std::uint64_t seed = 0) noexcept : master_(seed) {}
 
  [[nodiscard]] constexpr std::uint64_t master_seed() const noexcept
  {
    return master_;
  }
 
  template <std::size_t Rank>
  [[nodiscard]] Engine for_cell(const std::size_t step, const Coord_Vec<Rank> &coord) const
  {
    return Engine{static_cast<result_type>(cell_seed<Rank>(master_, step, coord))};
  }
 
  template <std::size_t Rank>
  [[nodiscard]] Engine for_cell(const Cell_Context<Rank> &ctx) const
  {
    return for_cell<Rank>(ctx.step, ctx.coord);
  }
 
  [[nodiscard]] Engine for_thread_step(const std::size_t thread_id, const std::size_t step) const
  {
    return Engine{static_cast<result_type>(thread_step_seed(master_, thread_id, step))};
  }
};
 
}  // namespace CA
}  // namespace Aleph
 
#endif  // CA_RNG_H