xxHash/README.md

xxHash - Extremely fast hash algorithm
======================================

xxHash is an Extremely fast Hash algorithm, running at RAM speed limits.
It successfully completes the [SMHasher](http://code.google.com/p/smhasher/wiki/SMHasher) test suite
which evaluates collision, dispersion and randomness qualities of hash functions.
Code is highly portable, and hashes are identical on all platforms (little / big endian).

|Branch      |Status   |
|------------|---------|
|master      | [![Build Status](https://travis-ci.org/Cyan4973/xxHash.svg?branch=master)](https://travis-ci.org/Cyan4973/xxHash?branch=master) |
|dev         | [![Build Status](https://travis-ci.org/Cyan4973/xxHash.svg?branch=dev)](https://travis-ci.org/Cyan4973/xxHash?branch=dev) |



Benchmarks
-------------------------

The benchmark uses SMHasher speed test, compiled with Visual 2010 on a Windows Seven 32-bit box.
The reference system uses a Core 2 Duo @3GHz


| Name          |   Speed     | Quality | Author            |
|---------------|-------------|:-------:|-------------------|
| [xxHash]      | 5.4 GB/s    |   10    | Y.C.              |
| MurmurHash 3a | 2.7 GB/s    |   10    | Austin Appleby    |
| SBox          | 1.4 GB/s    |    9    | Bret Mulvey       |
| Lookup3       | 1.2 GB/s    |    9    | Bob Jenkins       |
| CityHash64    | 1.05 GB/s   |   10    | Pike & Alakuijala |
| FNV           | 0.55 GB/s   |    5    | Fowler, Noll, Vo  |
| CRC32         | 0.43 GB/s † |    9    |                   |
| MD5-32        | 0.33 GB/s   |   10    | Ronald L.Rivest   |
| SHA1-32       | 0.28 GB/s   |   10    |                   |

[xxHash]: http://www.xxhash.com

Note †: SMHasher's CRC32 implementation is known to be slow. Faster implementations exist.

Q.Score is a measure of quality of the hash function.
It depends on successfully passing SMHasher test set.
10 is a perfect score.
Algorithms with a score < 5 are not listed on this table.

A more recent version, XXH64, has been created thanks to [Mathias Westerdahl](https://github.com/JCash),
which offers superior speed and dispersion for 64-bit systems.
Note however that 32-bit applications will still run faster using the 32-bit version.

SMHasher speed test, compiled using GCC 4.8.2, on Linux Mint 64-bit.
The reference system uses a Core i5-3340M @2.7GHz

| Version    | Speed on 64-bit | Speed on 32-bit |
|------------|------------------|------------------|
| XXH64      | 13.8 GB/s        |  1.9 GB/s        |
| XXH32      |  6.8 GB/s        |  6.0 GB/s        |

This project also includes a command line utility, named `xxhsum`, offering similar features as `md5sum`,
thanks to [Takayuki Matsuoka](https://github.com/t-mat) contributions.


### License

The library files `xxhash.c` and `xxhash.h` are BSD licensed.
The utility `xxhsum` is GPL licensed.

### Building xxHash - Using vcpkg

You can download and install xxHash using the [vcpkg](https://github.com/Microsoft/vcpkg) dependency manager:

    git clone https://github.com/Microsoft/vcpkg.git
    cd vcpkg
    ./bootstrap-vcpkg.sh
    ./vcpkg integrate install
    ./vcpkg install xxhash

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.

### Build modifiers

The following macros can be set at compilation time,
they modify libxxhash behavior. They are all disabled by default.

- `XXH_INLINE_ALL` : Make all functions `inline`, with bodies directly included within `xxhash.h`.
                     Inlining functions is beneficial for speed on small keys.
                     It's _extremely effective_ when key length is expressed as _a compile time constant_,
                     with performance improvements observed in the +200% range .
                     See [this article](https://fastcompression.blogspot.com/2018/03/xxhash-for-small-keys-impressive-power.html) for details.
                     Note: there is no need for an `xxhash.o` object file in this case.
- `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.
- `XXH_REROLL` : reduce size of generated code. Impact on performance vary, depending on platform and algorithm.
- `XXH_ACCEPT_NULL_INPUT_POINTER` : if set to `1`, when input is a `NULL` pointer,
                                    xxhash result is the same as a zero-length input
                                    (instead of a dereference segfault).
                                    Adds one branch at the beginning of the hash.
- `XXH_FORCE_MEMORY_ACCESS` : default method `0` uses a portable `memcpy()` notation.
                              Method `1` uses a gcc-specific `packed` attribute, which can provide better performance for some targets.
                              Method `2` forces unaligned reads, which is not standard compliant, but might sometimes be the only way to extract better read performance.
                              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
- `XXH_CPU_LITTLE_ENDIAN` : by default, endianess is determined at compile time.
                            It's possible to skip auto-detection and force format to little-endian, by setting this macro to 1.
                            Setting it to 0 forces big-endian.
- `XXH_PRIVATE_API` : same impact as `XXH_INLINE_ALL`.
                      Name underlines that XXH_* symbols will not be published.
- `XXH_NAMESPACE` : prefix all symbols with the value of `XXH_NAMESPACE`.
                    Useful to evade symbol naming collisions,
                    in case of multiple inclusions of xxHash source code.
                    Client applications can still use regular function name,
                    symbols are automatically translated through `xxhash.h`.
- `XXH_STATIC_LINKING_ONLY` : gives access to state declaration for static allocation.
                              Incompatible with dynamic linking, due to risks of ABI changes.
- `XXH_NO_LONG_LONG` : removes support for XXH64,
                       for targets without 64-bit support.
- `XXH_IMPORT` : MSVC specific : should only be defined for dynamic linking, it prevents linkage errors.


### Example

Calling xxhash 64-bit variant from a C program :

```C
#include "xxhash.h"

    (...)
    XXH64_hash_t hash = XXH64(buffer, size, seed);
}
```

Using streaming variant is more involved, but makes it possible to provide data incrementally :
```C
#include "stdlib.h"   /* abort() */
#include "xxhash.h"


XXH64_hash_t calcul_hash_streaming(FileHandler fh)
{
    /* create a hash state */
    XXH64_state_t* const state = XXH64_createState();
    if (state==NULL) abort();

    size_t const bufferSize = SOME_SIZE;
    void* const buffer = malloc(bufferSize);
    if (buffer==NULL) abort();

    /* Initialize state with selected seed */
    XXH64_hash_t const seed = 0;   /* or any other value */
    if (XXH64_reset(state, seed) == XXH_ERROR) abort();

    /* Feed the state with input data, any size, any number of times */
    (...)
    while ( /* any condition */ ) {
        size_t const length = get_more_data(buffer, bufferSize, fh);   
        if (XXH64_update(state, buffer, length) == XXH_ERROR) abort();
        (...)
    }
    (...)

    /* Get the hash */
    XXH64_hash_t const hash = XXH64_digest(state);

    /* State can be re-used; in this example, it is simply freed  */
    free(buffer);
    XXH64_freeState(state);

    return hash;
}
```

### New experimental hash algorithm

Starting with `v0.7.0`, the library includes a new algorithm, named `XXH3`,
able to generate 64 and 128-bits hashes.

The new algorithm is much faster than its predecessors,
for both long and small inputs,
as can be observed in following graphs :

![XXH3, bargraph](https://user-images.githubusercontent.com/750081/61976096-b3a35f00-af9f-11e9-8229-e0afc506c6ec.png)

![XXH3, latency, random size](https://user-images.githubusercontent.com/750081/61976089-aedeab00-af9f-11e9-9239-e5375d6c080f.png)

The algorithm is currently labeled experimental, its return values can still change in future versions.
It can already be used for ephemeral data, and for tests, but avoid storing long-term hash values yet.

To access experimental prototypes, one need to unlock their declaration using macro `XXH_STATIC_LINKING_ONLY`.
`XXH3` will be stabilized in a future version.
This period is used to collect users' feedback.


### Other programming languages

Beyond the C reference version,
xxHash is also available on many programming languages,
thanks to great contributors.
They are [listed here](http://www.xxhash.com/#other-languages).


### Branch Policy

> - The "master" branch is considered stable, at all times.
> - The "dev" branch is the one where all contributions must be merged
    before being promoted to master.
>   + If you plan to propose a patch, please commit into the "dev" branch,
      or its own feature branch.
      Direct commit to "master" are not permitted.