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206 lines
9.6 KiB
Markdown
206 lines
9.6 KiB
Markdown
xxHash - Extremely fast hash algorithm
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======================================
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<!-- TODO: Update. -->
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xxHash is an Extremely fast Hash algorithm, running at RAM speed limits.
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It successfully completes the [SMHasher](https://code.google.com/p/smhasher/wiki/SMHasher) test suite
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which evaluates collision, dispersion and randomness qualities of hash functions.
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Code is highly portable, and hashes are identical on all platforms (little / big endian).
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|Branch |Status |
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|------------|---------|
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|master | [](https://travis-ci.org/Cyan4973/xxHash?branch=master) |
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|dev | [](https://travis-ci.org/Cyan4973/xxHash?branch=dev) |
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Benchmarks
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-------------------------
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The benchmark uses SMHasher speed test, compiled with Visual 2010 on a Windows Seven 32-bit box.
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The reference system uses a Core 2 Duo @3GHz
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| Name | Speed | Quality | Author |
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|---------------|--------------------|:-------:|-------------------|
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| [xxHash] | 5.4 GB/s | 10 | Y.C. |
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| MurmurHash 3a | 2.7 GB/s | 10 | Austin Appleby |
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| SBox | 1.4 GB/s | 9 | Bret Mulvey |
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| Lookup3 | 1.2 GB/s | 9 | Bob Jenkins |
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| CityHash64 | 1.05 GB/s | 10 | Pike & Alakuijala |
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| FNV | 0.55 GB/s | 5 | Fowler, Noll, Vo |
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| CRC32 | 0.43 GB/s † | 9 | |
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| MD5-32 | 0.33 GB/s | 10 | Ronald L.Rivest |
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| SHA1-32 | 0.28 GB/s | 10 | |
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[xxHash]: https://www.xxhash.com
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Note †: SMHasher's CRC32 implementation is known to be slow. Faster implementations exist.
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Q.Score is a measure of quality of the hash function.
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It depends on successfully passing SMHasher test set.
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10 is a perfect score.
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Algorithms with a score < 5 are not listed on this table.
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A more recent version, XXH64, has been created thanks to [Mathias Westerdahl](https://github.com/JCash),
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which offers superior speed and dispersion for 64-bit systems.
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Note however that 32-bit applications will still run faster using the 32-bit version.
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SMHasher speed test, compiled using GCC 4.8.2, on Linux Mint 64-bit.
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The reference system uses a Core i5-3340M @2.7GHz
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| Version | Speed on 64-bit | Speed on 32-bit |
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|------------|------------------|------------------|
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| XXH64 | 13.8 GB/s | 1.9 GB/s |
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| XXH32 | 6.8 GB/s | 6.0 GB/s |
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This project also includes a command line utility, named `xxhsum`, offering similar features to `md5sum`,
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thanks to [Takayuki Matsuoka](https://github.com/t-mat)'s contributions.
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### License
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The library files `xxhash.c` and `xxhash.h` are BSD licensed.
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The utility `xxhsum` is GPL licensed.
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### New hash algorithms
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Starting with `v0.7.0`, the library includes a new algorithm named `XXH3`,
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which is able to generate 64 and 128-bit hashes.
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The new algorithm is much faster than its predecessors for both long and small inputs,
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which can be observed in the following graphs:
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To access these new prototypes, one needs to unlock their declaration, using the build macro `XXH_STATIC_LINKING_ONLY`.
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The algorithm is currently in development, meaning its return values might still change in future versions.
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However, the API is stable, and can be used in production, typically for ephemeral
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data (produced and consumed in same session).
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`XXH3`'s return values will be finalized upon reaching `v0.8.0`.
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### Build modifiers
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The following macros can be set at compilation time to modify libxxhash's behavior. They are all disabled by default.
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- `XXH_INLINE_ALL`: Make all functions `inline`, with implementations being directly included within `xxhash.h`.
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Inlining functions is beneficial for speed on small keys.
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It's _extremely effective_ when key length is expressed as _a compile time constant_,
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with performance improvements being observed in the +200% range .
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See [this article](https://fastcompression.blogspot.com/2018/03/xxhash-for-small-keys-impressive-power.html) for details.
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Note: there is no need to compile an `xxhash.o` object file in this case.
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- `XXH_NO_INLINE_HINTS`: By default, xxHash uses tricks like `__attribute__((always_inline))` and `__forceinline` to try and improve performance at the cost of code size. Defining this to 1 will mark all internal functions as `static`, allowing the compiler to decide whether to inline a function or not. This is very useful when optimizing for the smallest binary size, and it is automatically defined when compiling with `-O0`, `-Os`, `-Oz`, or `-fno-inline` on GCC and Clang. This may also increase performance depending on the compiler and the architecture.
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- `XXH_REROLL`: Reduces the size of the generated code by not unrolling some loops. Impact on performance may vary, depending on the platform and the algorithm.
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- `XXH_ACCEPT_NULL_INPUT_POINTER`: if set to `1`, when input is a `NULL` pointer,
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xxHash'd result is the same as a zero-length input
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(instead of a dereference segfault).
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Adds one branch at the beginning of the hash.
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- `XXH_FORCE_MEMORY_ACCESS`: The default method `0` uses a portable `memcpy()` notation.
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Method `1` uses a gcc-specific `packed` attribute, which can provide better performance for some targets.
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Method `2` forces unaligned reads, which is not standards compliant, but might sometimes be the only way to extract better read performance.
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Method `3` uses a byteshift operation, which is best for old compilers which don't inline `memcpy()` or big-endian systems without a byteswap instruction
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- `XXH_CPU_LITTLE_ENDIAN`: By default, endianess is determined at compile time.
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It's possible to skip auto-detection and force format to little-endian, by setting this macro to 1.
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Setting it to 0 forces big-endian.
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- `XXH_PRIVATE_API`: same impact as `XXH_INLINE_ALL`.
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Name underlines that XXH_* symbols will not be exported.
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- `XXH_NAMESPACE`: Prefixes all symbols with the value of `XXH_NAMESPACE`.
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Useful to evade symbol naming collisions,
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in case of multiple inclusions of xxHash's source code.
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Client applications can still use the regular function name,
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as symbols are automatically translated through `xxhash.h`.
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- `XXH_STATIC_LINKING_ONLY`: gives access to the state declaration for static allocation.
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Incompatible with dynamic linking, due to risks of ABI changes.
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- `XXH_NO_LONG_LONG`: removes support for XXH3 and XXH64 for targets without 64-bit support.
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- `XXH_IMPORT`: MSVC specific: should only be defined for dynamic linking, as it prevents linkage errors.
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### Building xxHash - Using vcpkg
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You can download and install xxHash using the [vcpkg](https://github.com/Microsoft/vcpkg) dependency manager:
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git clone https://github.com/Microsoft/vcpkg.git
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cd vcpkg
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./bootstrap-vcpkg.sh
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./vcpkg integrate install
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./vcpkg install xxhash
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The xxHash port in vcpkg is kept up to date by Microsoft team members and community contributors. If the version is out of date, please [create an issue or pull request](https://github.com/Microsoft/vcpkg) on the vcpkg repository.
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### Example
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Calling xxhash 64-bit variant from a C program:
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```C
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#include "xxhash.h"
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(...)
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XXH64_hash_t hash = XXH64(buffer, size, seed);
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}
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```
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Using streaming variant is more involved, but makes it possible to provide data incrementally:
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```C
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#include "stdlib.h" /* abort() */
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#include "xxhash.h"
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XXH64_hash_t calcul_hash_streaming(FileHandler fh)
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{
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/* create a hash state */
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XXH64_state_t* const state = XXH64_createState();
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if (state==NULL) abort();
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size_t const bufferSize = SOME_SIZE;
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void* const buffer = malloc(bufferSize);
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if (buffer==NULL) abort();
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/* Initialize state with selected seed */
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XXH64_hash_t const seed = 0; /* or any other value */
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if (XXH64_reset(state, seed) == XXH_ERROR) abort();
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/* Feed the state with input data, any size, any number of times */
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(...)
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while ( /* any condition */ ) {
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size_t const length = get_more_data(buffer, bufferSize, fh);
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if (XXH64_update(state, buffer, length) == XXH_ERROR) abort();
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(...)
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}
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(...)
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/* Get the hash */
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XXH64_hash_t const hash = XXH64_digest(state);
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/* State can be re-used; in this example, it is simply freed */
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free(buffer);
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XXH64_freeState(state);
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return hash;
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}
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```
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### Other programming languages
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Aside from the C reference version,
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xxHash is also available in many different programming languages,
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thanks to many great contributors.
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They are [listed here](https://www.xxhash.com/#other-languages).
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### Branch Policy
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> - The "master" branch is considered stable, at all times.
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> - The "dev" branch is the one where all contributions must be merged
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before being promoted to master.
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> + If you plan to propose a patch, please commit into the "dev" branch,
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or its own feature branch.
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Direct commit to "master" are not permitted.
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