Tpetra_Details_Hash.hpp

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#ifndef TPETRA_DETAILS_HASH_HPP
#define TPETRA_DETAILS_HASH_HPP

#include "Tpetra_ConfigDefs.hpp"
#ifdef TPETRA_USE_MURMUR_HASH
#  include <Kokkos_Functional.hpp> // hash function used by Kokkos::UnorderedMap
#endif // TPETRA_USE_MURMUR_HASH
#include <type_traits> // make_signed

namespace Tpetra {
namespace Details {

namespace Impl {

int getRecommendedSizeInt (const int size);

} // namespace Impl

template<class KeyType,
         class DeviceType,
         class OffsetType = typename std::make_signed<typename Kokkos::View<KeyType*, DeviceType>::size_type>::type,
         class ResultType = int>
struct Hash {
  typedef KeyType argument_type;

  typedef ResultType result_type;

  typedef OffsetType offset_type;

  static KOKKOS_INLINE_FUNCTION result_type
  hashFunc (const argument_type& /*key*/, const offset_type& /*size*/) {
    static_assert (! std::is_same<result_type, int>::value,
                   "Not yet implemented for ResultType != int");
  }

  static result_type getRecommendedSize (const offset_type /*size*/) {
    static_assert (! std::is_same<result_type, int>::value,
                   "Not yet implemented for ResultType != int");
  }
};

template<class KeyType, class DeviceType, class OffsetType>
struct Hash<KeyType, DeviceType, OffsetType, int> {
  typedef KeyType argument_type;

  typedef int result_type;

  typedef OffsetType offset_type;

  static KOKKOS_INLINE_FUNCTION result_type
  hashFunc (const argument_type& key, const offset_type& size)
  {
#ifdef TPETRA_USE_MURMUR_HASH
    Kokkos::pod_hash<argument_type> hash;
    const uint32_t k = hash (key);
    return static_cast<result_type> (k % size);
#else
    // We are using Epetra's hash function by default, as we have
    // observed that it is much faster than the Murmur hash
    // function. However, this is not a good hash function for general
    // sets of keys.  For our typical use case, this is good.  Use
    // Murmur hash if the maps are sparse.
    const unsigned int seed = (2654435761U);
    const int intkey = (int) ((key & 0x000000007fffffffLL) +
                              ((key & 0x7fffffff80000000LL) >> 31));
    return static_cast<result_type> ((seed ^ intkey) % static_cast<int> (size));
#endif
  }

  static result_type getRecommendedSize (const offset_type size)
  {
    return Impl::getRecommendedSizeInt (static_cast<int> (size));
  }
};

} // namespace Details
} // namespace Tpetra

#endif // TPETRA_DETAILS_HASH_HPP