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diff --git a/other-licenses/snappy/src/README.md b/other-licenses/snappy/src/README.md new file mode 100644 index 0000000000..7917d1bf05 --- /dev/null +++ b/other-licenses/snappy/src/README.md @@ -0,0 +1,140 @@ +Snappy, a fast compressor/decompressor. + +[](https://travis-ci.org/google/snappy) +[](https://ci.appveyor.com/project/pwnall/leveldb) + +Introduction +============ + +Snappy is a compression/decompression library. It does not aim for maximum +compression, or compatibility with any other compression library; instead, +it aims for very high speeds and reasonable compression. For instance, +compared to the fastest mode of zlib, Snappy is an order of magnitude faster +for most inputs, but the resulting compressed files are anywhere from 20% to +100% bigger. (For more information, see "Performance", below.) + +Snappy has the following properties: + + * Fast: Compression speeds at 250 MB/sec and beyond, with no assembler code. + See "Performance" below. + * Stable: Over the last few years, Snappy has compressed and decompressed + petabytes of data in Google's production environment. The Snappy bitstream + format is stable and will not change between versions. + * Robust: The Snappy decompressor is designed not to crash in the face of + corrupted or malicious input. + * Free and open source software: Snappy is licensed under a BSD-type license. + For more information, see the included COPYING file. + +Snappy has previously been called "Zippy" in some Google presentations +and the like. + + +Performance +=========== + +Snappy is intended to be fast. On a single core of a Core i7 processor +in 64-bit mode, it compresses at about 250 MB/sec or more and decompresses at +about 500 MB/sec or more. (These numbers are for the slowest inputs in our +benchmark suite; others are much faster.) In our tests, Snappy usually +is faster than algorithms in the same class (e.g. LZO, LZF, QuickLZ, +etc.) while achieving comparable compression ratios. + +Typical compression ratios (based on the benchmark suite) are about 1.5-1.7x +for plain text, about 2-4x for HTML, and of course 1.0x for JPEGs, PNGs and +other already-compressed data. Similar numbers for zlib in its fastest mode +are 2.6-2.8x, 3-7x and 1.0x, respectively. More sophisticated algorithms are +capable of achieving yet higher compression rates, although usually at the +expense of speed. Of course, compression ratio will vary significantly with +the input. + +Although Snappy should be fairly portable, it is primarily optimized +for 64-bit x86-compatible processors, and may run slower in other environments. +In particular: + + - Snappy uses 64-bit operations in several places to process more data at + once than would otherwise be possible. + - Snappy assumes unaligned 32 and 64-bit loads and stores are cheap. + On some platforms, these must be emulated with single-byte loads + and stores, which is much slower. + - Snappy assumes little-endian throughout, and needs to byte-swap data in + several places if running on a big-endian platform. + +Experience has shown that even heavily tuned code can be improved. +Performance optimizations, whether for 64-bit x86 or other platforms, +are of course most welcome; see "Contact", below. + + +Building +======== + +You need the CMake version specified in [CMakeLists.txt](./CMakeLists.txt) +or later to build: + +```bash +git submodule update --init +mkdir build +cd build && cmake ../ && make +``` + +Usage +===== + +Note that Snappy, both the implementation and the main interface, +is written in C++. However, several third-party bindings to other languages +are available; see the [home page](docs/README.md) for more information. +Also, if you want to use Snappy from C code, you can use the included C +bindings in snappy-c.h. + +To use Snappy from your own C++ program, include the file "snappy.h" from +your calling file, and link against the compiled library. + +There are many ways to call Snappy, but the simplest possible is + +```c++ +snappy::Compress(input.data(), input.size(), &output); +``` + +and similarly + +```c++ +snappy::Uncompress(input.data(), input.size(), &output); +``` + +where "input" and "output" are both instances of std::string. + +There are other interfaces that are more flexible in various ways, including +support for custom (non-array) input sources. See the header file for more +information. + + +Tests and benchmarks +==================== + +When you compile Snappy, the following binaries are compiled in addition to the +library itself. You do not need them to use the compressor from your own +library, but they are useful for Snappy development. + +* `snappy_benchmark` contains microbenchmarks used to tune compression and + decompression performance. +* `snappy_unittests` contains unit tests, verifying correctness on your machine + in various scenarios. +* `snappy_test_tool` can benchmark Snappy against a few other compression + libraries (zlib, LZO, LZF, and QuickLZ), if they were detected at configure + time. To benchmark using a given file, give the compression algorithm you want + to test Snappy against (e.g. --zlib) and then a list of one or more file names + on the command line. + +If you want to change or optimize Snappy, please run the tests and benchmarks to +verify you have not broken anything. + +The testdata/ directory contains the files used by the microbenchmarks, which +should provide a reasonably balanced starting point for benchmarking. (Note that +baddata[1-3].snappy are not intended as benchmarks; they are used to verify +correctness in the presence of corrupted data in the unit test.) + + +Contact +======= + +Snappy is distributed through GitHub. For the latest version and other +information, see https://github.com/google/snappy. |