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meow_hash_cpp

Build Status

This is a rewrite of the Casey's Muratori Meow hash (https://github.com/cmuratori/meow_hash), utilizing modern C++ features to significantly tidy up the codebase and get rid of a large amount of undefined behaviour present in the original implementation.

Compatibility

Compiler Min. Version
MSVC (Visual Studio) 19.1 (VS 2017.3 P2)
clang 3.9
gcc 7
EDG eccp 4.14

Example Usage

// standalone hash 
std::array<int, 4> input {322, 2137, 42069, 65536};
meowh::hash_t<32> hash = meowh::meow_hash<128, 32>(input); 

// extracting parts of the 512 bit long hash result
unsigned int hash_32 = hash[0];
unsigned long long hash_64 = hash.as<64>(0);
__m128i l0 = hash.as<128>(0);
__m128i l1 = hash.as<128>(1);
__m128i l2 = hash.as<128>(2);
__m128i l3 = hash.as<128>(3);

// converting hash result to use a different element type
meowh::hash_t<256> hash256 = hash;
__m256i h0 = hash256[0];
__m256i h1 = hash256[1];

Usage Instructions

The template arguments of the meowh::meow_hash function specify the following:

  • The first argument, N, specifies the version of the hashing algorithm to use. 128 uses MeowHash1, 256 uses MeowHash2, and 512 uses MeowHash4. Nothing else will need to be changed to switch between the different versions when MeowHash2 and MeowHash4 become publically usible.
  • The second argument, Align, specifies whether the input buffer fed to the function is asssumed to be aligned to 16, 32, or 64 bytes, depending on whether MeowHash1, 2, or 4 is being used. If so, an optimization will be enabled in the algorithm. Note that setting Align to true without the input buffer not being actually aligned is undefined behaviour (unaligned read).
  • The second argument, R, specifies the element type the returned 512 bit hash will use. It is set to the same value as N by default.

Due to undefined behavior problems involving switching between unions, the original hash return type, meow_lane, has been replaced by meowh::hash_t. hash_t has a single template argument, which determines the size in bits of the elements of the internal 512 bit long array to hold the hash result. 32, 64, 128, 256 and 512 are valid values. 128, 256 and 512 are only allowed if the target machine supports SSE, AVX, and AVX512F, respectively. Alternatively, if the user wants to manually enable them without relying on feature test macros, macros _MEOWH_128, _MEOWH_256 and _MEOWH_512 should be defined before including meow_hash.cpp. These elements can be accessed by indexing the hash_t object with the array subscript operator ([]). Values of other types than the element type can be obtained with the use of the .as member function. It takes a single template argument, specifying the size in bits of the desired type, and a single function argument, specifying the desired array member of the internal 512 bit long array expressed as an array of the desired type.

meowh::meow_hash provides several convenient overloads, all accepting an optional seed argument:

  • C-style const void* + size_t pair
  • const std::vector<T>&
  • const std::basic_string<T>&
  • A pair of templated iterators
  • const std::array<T, N>&
  • const std::initializer_list<T>&

Build Instructions

The library is provided as a single standalone header, for static linking only. No build instructions are nessessary.