Philox.h

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//============================================================================
//  The contents of this file are covered by the Viskores license. See
//  LICENSE.txt for details.
//
//  By contributing to this file, all contributors agree to the Developer
//  Certificate of Origin Version 1.1 (DCO 1.1) as stated in DCO.txt.
//============================================================================
 
//============================================================================
//  Copyright (c) Kitware, Inc.
//  All rights reserved.
//  See LICENSE.txt for details.
//
//  This software is distributed WITHOUT ANY WARRANTY; without even
//  the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
//  PURPOSE.  See the above copyright notice for more information.
//============================================================================
#ifndef viskores_random_Philox_h
#define viskores_random_Philox_h
 
#include <viskores/Types.h>
 
namespace viskores
{
namespace random
{
namespace detail
{
static inline VISKORES_EXEC_CONT viskores::Vec<viskores::UInt32, 2> mulhilo(viskores::UInt32 a,
                                                                            viskores::UInt32 b)
{
  viskores::UInt64 r = static_cast<viskores::UInt64>(a) * b;
  auto lo = static_cast<viskores::UInt32>(r);
  viskores::UInt32 hi = static_cast<viskores::UInt32>(r >> 32);
  return { lo, hi };
}
 
template <typename UIntType, std::size_t N, UIntType... consts>
struct philox_parameters;
 
template <typename T, T M0, T C0>
struct philox_parameters<T, 2, M0, C0>
{
  static constexpr Vec<T, 1> multipliers = { M0 };
  static constexpr Vec<T, 1> round_consts = { C0 };
};
 
template <typename T, T M0, T C0, T M1, T C1>
struct philox_parameters<T, 4, M0, C0, M1, C1>
{
  static constexpr viskores::Vec<T, 2> multipliers = { M0, M1 };
  static constexpr viskores::Vec<T, 2> round_consts = { C0, C1 };
};
 
template <typename UIntType, std::size_t N, std::size_t R, UIntType... consts>
class philox_functor;
 
template <typename UIntType, std::size_t R, UIntType... consts>
class philox_functor<UIntType, 2, R, consts...>
{
public:
  using counters_type = viskores::Vec<UIntType, 2>;
  using keys_type = viskores::Vec<UIntType, 1>;
 
  VISKORES_EXEC_CONT counters_type operator()(counters_type counters, keys_type keys) const
  {
    for (std::size_t i = 0; i < R; ++i)
    {
      counters = round(counters, keys);
      keys = bump_keys(keys);
    }
    return counters;
  }
 
private:
  static VISKORES_EXEC_CONT counters_type round(counters_type counters, keys_type round_keys)
  {
    auto constexpr multipliers = philox_parameters<UIntType, 2, consts...>::multipliers;
    viskores::Vec<UIntType, 2> r = mulhilo(multipliers[0], counters[0]);
    return { r[1] ^ round_keys[0] ^ counters[1], r[0] };
  }
 
  static VISKORES_EXEC_CONT keys_type bump_keys(keys_type keys)
  {
    auto constexpr round_consts = philox_parameters<UIntType, 2, consts...>::round_consts;
    return { keys[0] + round_consts[0] };
  }
};
 
template <typename UIntType, std::size_t R, UIntType... consts>
class philox_functor<UIntType, 4, R, consts...>
{
  using counters_type = viskores::Vec<UIntType, 4>;
  using keys_type = viskores::Vec<UIntType, 2>;
 
  static VISKORES_EXEC_CONT counters_type round(counters_type counters, keys_type round_keys)
  {
    auto constexpr multipliers = philox_parameters<UIntType, 4, consts...>::multipliers;
    viskores::Vec<UIntType, 2> r0 = mulhilo(multipliers[0], counters[0]);
    viskores::Vec<UIntType, 2> r1 = mulhilo(multipliers[1], counters[2]);
    return {
      r1[1] ^ round_keys[0] ^ counters[1], r1[0], r0[1] ^ round_keys[1] ^ counters[3], r0[0]
    };
  }
 
  static VISKORES_EXEC_CONT keys_type bump_key(keys_type keys)
  {
    auto constexpr round_consts = philox_parameters<UIntType, 4, consts...>::round_consts;
    keys[0] += round_consts[0];
    keys[1] += round_consts[1];
    return keys;
  }
 
public:
  VISKORES_EXEC_CONT counters_type operator()(counters_type counters, keys_type keys) const
  {
    for (std::size_t i = 0; i < R; ++i)
    {
      counters = round(counters, keys);
      keys = bump_key(keys);
    }
    return counters;
  }
};
 
} // namespace detail
 
using PhiloxFunctor2x32x7 = detail::philox_functor<viskores::UInt32, 2, 7, 0xD256D193, 0x9E3779B9>;
using PhiloxFunctor2x32x10 =
  detail::philox_functor<viskores::UInt32, 2, 10, 0xD256D193, 0x9E3779B9>;
 
} // namespace random
} // namespace viskores
#endif //viskores_random_Philox_h