mirror of
https://gitlab.kitware.com/vtk/vtk-m
synced 2024-07-06 07:17:25 +00:00
118 lines
3.8 KiB
C++
118 lines
3.8 KiB
C++
//============================================================================
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// Copyright (c) Kitware, Inc.
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// All rights reserved.
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// See LICENSE.txt for details.
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//
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// This software is distributed WITHOUT ANY WARRANTY; without even
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// the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
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// PURPOSE. See the above copyright notice for more information.
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//============================================================================
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#ifndef vtk_m_Hash_h
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#define vtk_m_Hash_h
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#include <vtkm/TypeTraits.h>
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#include <vtkm/Types.h>
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#include <vtkm/VecTraits.h>
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namespace vtkm
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{
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using HashType = vtkm::UInt32;
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namespace detail
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{
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static constexpr vtkm::HashType FNV1A_OFFSET = 2166136261;
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static constexpr vtkm::HashType FNV1A_PRIME = 16777619;
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/// \brief Performs an FNV-1a hash on 32-bit integers returning a 32-bit hash
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///
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template <typename InVecType>
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VTKM_EXEC_CONT inline vtkm::HashType HashFNV1a32(const InVecType& inVec)
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{
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using Traits = vtkm::VecTraits<InVecType>;
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const vtkm::IdComponent numComponents = Traits::GetNumberOfComponents(inVec);
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vtkm::HashType hash = FNV1A_OFFSET;
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for (vtkm::IdComponent index = 0; index < numComponents; index++)
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{
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vtkm::HashType dataBits = static_cast<vtkm::HashType>(Traits::GetComponent(inVec, index));
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hash = (hash * FNV1A_PRIME) ^ dataBits;
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}
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return hash;
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}
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/// \brief Performs an FNV-1a hash on 64-bit integers returning a 32-bit hash
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///
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template <typename InVecType>
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VTKM_EXEC_CONT inline vtkm::HashType HashFNV1a64(const InVecType& inVec)
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{
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using Traits = vtkm::VecTraits<InVecType>;
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const vtkm::IdComponent numComponents = Traits::GetNumberOfComponents(inVec);
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vtkm::HashType hash = FNV1A_OFFSET;
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for (vtkm::IdComponent index = 0; index < numComponents; index++)
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{
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vtkm::UInt64 allDataBits = static_cast<vtkm::UInt64>(Traits::GetComponent(inVec, index));
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vtkm::HashType upperDataBits =
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static_cast<vtkm::HashType>((allDataBits & 0xFFFFFFFF00000000L) >> 32);
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hash = (hash * FNV1A_PRIME) ^ upperDataBits;
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vtkm::HashType lowerDataBits = static_cast<vtkm::HashType>(allDataBits & 0x00000000FFFFFFFFL);
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hash = (hash * FNV1A_PRIME) ^ lowerDataBits;
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}
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return hash;
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}
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// If you get a compile error saying that there is no implementation of the class HashChooser,
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// then you have tried to make a hash from an invalid type (like a float).
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template <typename NumericTag, std::size_t DataSize>
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struct HashChooser;
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template <>
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struct HashChooser<vtkm::TypeTraitsIntegerTag, 4>
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{
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template <typename InVecType>
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VTKM_EXEC_CONT static vtkm::HashType Hash(const InVecType& inVec)
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{
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return vtkm::detail::HashFNV1a32(inVec);
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}
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};
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template <>
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struct HashChooser<vtkm::TypeTraitsIntegerTag, 8>
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{
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template <typename InVecType>
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VTKM_EXEC_CONT static vtkm::HashType Hash(const InVecType& inVec)
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{
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return vtkm::detail::HashFNV1a64(inVec);
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}
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};
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} // namespace detail
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/// \brief Returns a 32-bit hash on a group of integer-type values.
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///
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/// The input to the hash is expected to be a vtkm::Vec or a Vec-like object. The values can be
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/// either 32-bit integers or 64-bit integers (signed or unsigned). Regardless, the resulting hash
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/// is an unsigned 32-bit integer.
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///
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/// The hash is designed to minimize the probability of collisions, but collisions are always
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/// possible. Thus, when using these hashes there should be a contingency for dealing with
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/// collisions.
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///
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template <typename InVecType>
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VTKM_EXEC_CONT inline vtkm::HashType Hash(const InVecType& inVec)
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{
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using VecTraits = vtkm::VecTraits<InVecType>;
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using ComponentType = typename VecTraits::ComponentType;
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using ComponentTraits = vtkm::TypeTraits<ComponentType>;
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using Chooser = detail::HashChooser<typename ComponentTraits::NumericTag, sizeof(ComponentType)>;
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return Chooser::Hash(inVec);
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}
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} // namespace vtkm
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#endif //vtk_m_Hash_h
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