From 5f8de423f190bbb79a62f804151bc24824fa32d8 Mon Sep 17 00:00:00 2001 From: "Matt A. Tobin" Date: Fri, 2 Feb 2018 04:16:08 -0500 Subject: Add m-esr52 at 52.6.0 --- other-licenses/snappy/src/README | 139 +++++++++++++++++++++++++++++++++++++++ 1 file changed, 139 insertions(+) create mode 100644 other-licenses/snappy/src/README (limited to 'other-licenses/snappy/src/README') diff --git a/other-licenses/snappy/src/README b/other-licenses/snappy/src/README new file mode 100644 index 000000000..c60dab9a1 --- /dev/null +++ b/other-licenses/snappy/src/README @@ -0,0 +1,139 @@ +Snappy, a fast compressor/decompressor. + + +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, FastLZ, 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. + + +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 at http://google.github.io/snappy/ +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 + + snappy::Compress(input.data(), input.size(), &output); + +and similarly + + 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, snappy_unittest is compiled in addition to the +library itself. You do not need it to use the compressor from your own library, +but it contains several useful components for Snappy development. + +First of all, it contains unit tests, verifying correctness on your machine in +various scenarios. If you want to change or optimize Snappy, please run the +tests to verify you have not broken anything. Note that if you have the +Google Test library installed, unit test behavior (especially failures) will be +significantly more user-friendly. You can find Google Test at + + http://github.com/google/googletest + +You probably also want the gflags library for handling of command-line flags; +you can find it at + + http://gflags.github.io/gflags/ + +In addition to the unit tests, snappy contains microbenchmarks used to +tune compression and decompression performance. These are automatically run +before the unit tests, but you can disable them using the flag +--run_microbenchmarks=false if you have gflags installed (otherwise you will +need to edit the source). + +Finally, snappy can benchmark Snappy against a few other compression libraries +(zlib, LZO, LZF, FastLZ 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. The testdata/ directory contains the files used by the +microbenchmark, 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, a bug tracker, +and other information, see + + http://google.github.io/snappy/ + +or the repository at + + https://github.com/google/snappy -- cgit v1.2.3