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168 lines
7.0 KiB
Markdown
168 lines
7.0 KiB
Markdown
xxHash - Extremely fast hash algorithm
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======================================
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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](http://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]: http://www.xxhash.com
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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 as `md5sum`,
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thanks to [Takayuki Matsuoka](https://github.com/t-mat) 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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### Build modifiers
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The following macros can be set at compilation time,
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they modify xxhash behavior. They are all disabled by default.
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- `XXH_INLINE_ALL` : Make all functions `inline`, with bodies directly included within `xxhash.h`.
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There is no need for an `xxhash.o` module in this case.
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Inlining functions is generally beneficial for speed on small keys.
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It's especially effective when key length is a compile time constant,
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with observed performance improvement 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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- `XXH_ACCEPT_NULL_INPUT_POINTER` : if set to `1`, when input is a null-pointer,
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xxhash result is the same as a zero-length key
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(instead of a dereference segfault).
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- `XXH_FORCE_MEMORY_ACCESS` : 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 standard compliant, but might sometimes be the only way to extract better performance.
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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_FORCE_NATIVE_FORMAT` : on big-endian systems : use native number representation.
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Breaks consistency with little-endian results.
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- `XXH_PRIVATE_API` : same impact as `XXH_INLINE_ALL`.
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Name underlines that symbols will not be published on library public interface.
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- `XXH_NAMESPACE` : prefix 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 source code.
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Client applications can still use regular function name,
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symbols are automatically translated through `xxhash.h`.
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- `XXH_STATIC_LINKING_ONLY` : gives access to 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 XXH64,
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for targets without 64-bit support.
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### Example
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Calling xxhash 64-bit variant from a C program :
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```
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#include "xxhash.h"
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unsigned long long calcul_hash(const void* buffer, size_t length)
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{
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unsigned long long const seed = 0; /* or any other value */
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unsigned long long const hash = XXH64(buffer, length, seed);
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return hash;
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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 in multiple rounds :
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```
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#include "stdlib.h" /* abort() */
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#include "xxhash.h"
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unsigned long long calcul_hash_streaming(someCustomType handler)
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{
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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_VALUE;
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void* const buffer = malloc(bufferSize);
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if (buffer==NULL) abort();
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unsigned long long const seed = 0; /* or any other value */
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XXH_errorcode const resetResult = XXH64_reset(state, seed);
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if (resetResult == XXH_ERROR) abort();
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(...)
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while ( /* any condition */ ) {
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size_t const length = get_more_data(buffer, bufferSize, handler); /* undescribed */
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XXH_errorcode const addResult = XXH64_update(state, buffer, length);
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if (addResult == XXH_ERROR) abort();
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(...)
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}
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(...)
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unsigned long long const hash = XXH64_digest(state);
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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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Beyond the C reference version,
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xxHash is also available on many programming languages,
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thanks to great contributors.
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They are [listed here](http://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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