xxHash Clean C Reference Implementation

This is a cleaned up C reference implementation of XXH32, XXH64, XXH3_64bits, and XXH3_128bits, inspired by the HighwayHash C implementation.

Warning: Performance is not the goal of this implementation.

If you want a fast xxHash implementation, see the official version by Cyan4973.

This version focuses on the following:

• Portability: This code is all C90 and C++98 compatible with the additional requirement of stdint.h and long long (XXH32 does not need long long).
• "Port-ability": The single shot code only works with integers and arrays directly, not relying on any library functions like memcpy. The only #includes are for typedefs, so it would be very easy to port to another language. However, the streaming versions do rely on some functions, only because it is impossible or really ugly to do otherwise.
  • memcpy
  • malloc
  • memset
  • free
• Correctness: Everything is (excessively?) const-correct, endian-independent, and standards compliant. Give me -Weverything, I can take it.
• Compactness: The code is very short. Light documentation is added but the code only stands at about 240 lines for XXH64 and 200 lines for XXH32, about 50 of which are from tests. XXH3 squeezes to umder 800 lines.
  • The code also compiles very small.w
• Simplicity: This implementation tries to make the code as clear as possible. Loops are rerolled, pointer arithmetic is removed in favor of counters, etc. Nothing is more complicated than it needs to be.
• Cleanliness: No ugly macros, no #ifdef blocks (except for test data and extern "C"), no ugly SIMD intrinsics, no inline assembly hacks, nothing.

The streaming implementations are in separate files to keep things clean. Only XXH32 and XXH64 are implemented for now.

This does impact performance a lot, for example, Clang will autovectorize XXH32 with a bunch of pshufb instructions when reading the data, resulting in stupid slowdowns.

This also provides xxhsum-example.c which shows how to use xxhash's streaming functions to read and checksum files. It isn't a fully-featured tool, and it shares no code with xxhsum.c from the official repository.

License

xxHash Library
Copyright (c) 2012-2020, Yann Collet
Copyright (c) 2019-2020, Devin Hussey (easyaspi314)
All rights reserved.

Redistribution and use in source and binary forms, with or without modification,
are permitted provided that the following conditions are met:

* Redistributions of source code must retain the above copyright notice, this
  list of conditions and the following disclaimer.

* Redistributions in binary form must reproduce the above copyright notice, this
  list of conditions and the following disclaimer in the documentation and/or
  other materials provided with the distribution.

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