Duckstation/dep/xxhash/README.md

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

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xxHash is an Extremely fast Hash algorithm, running at RAM speed limits.
It successfully completes the [SMHasher](https://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 &dagger; |    9    |                   |
| MD5-32        | 0.33 GB/s          |   10    | Ronald L.Rivest   |
| SHA1-32       | 0.28 GB/s          |   10    |                   |

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

Note &dagger;: 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 to `md5sum`,
thanks to [Takayuki Matsuoka](https://github.com/t-mat)'s contributions.


### License

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


### New hash algorithms

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

The new algorithm is much faster than its predecessors for both long and small inputs,
which can be observed in the 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)

To access these new prototypes, one needs to unlock their declaration, using the build macro `XXH_STATIC_LINKING_ONLY`.

The algorithm is currently in development, meaning its return values might still change in future versions.
However, the API is stable, and can be used in production, typically for ephemeral
data (produced and consumed in same session).

`XXH3`'s return values will be finalized upon reaching `v0.8.0`.


### Build modifiers

The following macros can be set at compilation time to modify libxxhash's behavior. They are all disabled by default.

- `XXH_INLINE_ALL`: Make all functions `inline`, with implementations being 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 being 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 to compile 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`: Reduces the size of the generated code by not unrolling some loops. Impact on performance may vary, depending on the platform and the algorithm.
- `XXH_ACCEPT_NULL_INPUT_POINTER`: if set to `1`, when input is a `NULL` pointer,
                                   xxHash'd 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`: The 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 standards 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 exported.
- `XXH_NAMESPACE`: Prefixes all symbols with the value of `XXH_NAMESPACE`.
                    Useful to evade symbol naming collisions,
                    in case of multiple inclusions of xxHash's source code.
                    Client applications can still use the regular function name,
                    as symbols are automatically translated through `xxhash.h`.
- `XXH_STATIC_LINKING_ONLY`: gives access to the state declaration for static allocation.
                             Incompatible with dynamic linking, due to risks of ABI changes.
- `XXH_NO_LONG_LONG`: removes support for XXH3 and XXH64 for targets without 64-bit support.
- `XXH_IMPORT`: MSVC specific: should only be defined for dynamic linking, as it prevents linkage errors.


### 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.


### 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;
}
```


### Other programming languages

Aside from the C reference version,
xxHash is also available in many different programming languages,
thanks to many great contributors.
They are [listed here](https://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.