source: bootcd/isolinux/syslinux-6.03/lzo/doc/LZO.TXT

 ============================================================================
 LZO -- a real-time data compression library
 ============================================================================

 Author  : Markus Franz Xaver Johannes Oberhumer
           <markus@oberhumer.com>
           http://www.oberhumer.com/opensource/lzo/
 Version : 2.07
 Date    : 25 Jun 2014


 Abstract
 --------
 LZO is a portable lossless data compression library written in ANSI C.
 It offers pretty fast compression and very fast decompression.
 Decompression requires no memory.

 In addition there are slower compression levels achieving a quite
 competitive compression ratio while still decompressing at
 this very high speed.

 The LZO algorithms and implementations are copyrighted OpenSource
 distributed under the GNU General Public License.


 Introduction
 ------------
 LZO is a data compression library which is suitable for data
 de-/compression in real-time. This means it favours speed
 over compression ratio.

 The acronym LZO is standing for Lempel-Ziv-Oberhumer.

 LZO is written in ANSI C. Both the source code and the compressed
 data format are designed to be portable across platforms.

 LZO implements a number of algorithms with the following features:

 - Decompression is simple and *very* fast.
 - Requires no memory for decompression.
 - Compression is pretty fast.
 - Requires 64 KiB of memory for compression.
 - Allows you to dial up extra compression at a speed cost in the
   compressor. The speed of the decompressor is not reduced.
 - Includes compression levels for generating pre-compressed
   data which achieve a quite competitive compression ratio.
 - There is also a compression level which needs only 8 KiB for compression.
 - Algorithm is thread safe.
 - Algorithm is lossless.

 LZO supports overlapping compression and in-place decompression.


 Design criteria
 ---------------
 LZO was designed with speed in mind. Decompressor speed has been
 favoured over compressor speed. Real-time decompression should be
 possible for virtually any application. The implementation of the
 LZO1X decompressor in optimized i386 assembler code runs about at
 the third of the speed of a memcpy() - and even faster for many files.

 In fact I first wrote the decompressor of each algorithm thereby
 defining the compressed data format, verified it with manually
 created test data and at last added the compressor.


 Performance
 -----------
 To keep you interested, here is an overview of the average results
 when compressing the Calgary Corpus test suite with a blocksize
 of 256 KiB, originally done on an ancient Intel Pentium 133.

 The naming convention of the various algorithms goes LZOxx-N, where N is
 the compression level. Range 1-9 indicates the fast standard levels using
 64 KiB memory for compression. Level 99 offers better compression at the
 cost of more memory (256 KiB), and is still reasonably fast.
 Level 999 achieves nearly optimal compression - but it is slow
 and uses much memory, and is mainly intended for generating
 pre-compressed data.

 The C version of LZO1X-1 is about 4-5 times faster than the fastest
 zlib compression level, and it also outperforms other algorithms
 like LZRW1-A and LZV in both compression ratio and compression speed
 and decompression speed.

 +------------------------------------------------------------------------+
 | Algorithm        Length  CxB   ComLen  %Remn  Bits   Com K/s   Dec K/s |
 | ---------        ------  ---   ------  -----  ----   -------   ------- |
 |                                                                        |
 | memcpy()         224401    1   224401  100.0  8.00  60956.83  59124.58 |
 |                                                                        |
 | LZO1-1           224401    1   117362   53.1  4.25   4665.24  13341.98 |
 | LZO1-99          224401    1   101560   46.7  3.73   1373.29  13823.40 |
 |                                                                        |
 | LZO1A-1          224401    1   115174   51.7  4.14   4937.83  14410.35 |
 | LZO1A-99         224401    1    99958   45.5  3.64   1362.72  14734.17 |
 |                                                                        |
 | LZO1B-1          224401    1   109590   49.6  3.97   4565.53  15438.34 |
 | LZO1B-2          224401    1   106235   48.4  3.88   4297.33  15492.79 |
 | LZO1B-3          224401    1   104395   47.8  3.83   4018.21  15373.52 |
 | LZO1B-4          224401    1   104828   47.4  3.79   3024.48  15100.11 |
 | LZO1B-5          224401    1   102724   46.7  3.73   2827.82  15427.62 |
 | LZO1B-6          224401    1   101210   46.0  3.68   2615.96  15325.68 |
 | LZO1B-7          224401    1   101388   46.0  3.68   2430.89  15361.47 |
 | LZO1B-8          224401    1    99453   45.2  3.62   2183.87  15402.77 |
 | LZO1B-9          224401    1    99118   45.0  3.60   1677.06  15069.60 |
 | LZO1B-99         224401    1    95399   43.6  3.48   1286.87  15656.11 |
 | LZO1B-999        224401    1    83934   39.1  3.13    232.40  16445.05 |
 |                                                                        |
 | LZO1C-1          224401    1   111735   50.4  4.03   4883.08  15570.91 |
 | LZO1C-2          224401    1   108652   49.3  3.94   4424.24  15733.14 |
 | LZO1C-3          224401    1   106810   48.7  3.89   4127.65  15645.69 |
 | LZO1C-4          224401    1   105717   47.7  3.82   3007.92  15346.44 |
 | LZO1C-5          224401    1   103605   47.0  3.76   2829.15  15153.88 |
 | LZO1C-6          224401    1   102585   46.5  3.72   2631.37  15257.58 |
 | LZO1C-7          224401    1   101937   46.2  3.70   2378.57  15492.49 |
 | LZO1C-8          224401    1   100779   45.6  3.65   2171.93  15386.07 |
 | LZO1C-9          224401    1   100255   45.4  3.63   1691.44  15194.68 |
 | LZO1C-99         224401    1    97252   44.1  3.53   1462.88  15341.37 |
 | LZO1C-999        224401    1    87740   40.2  3.21    306.44  16411.94 |
 |                                                                        |
 | LZO1F-1          224401    1   113412   50.8  4.07   4755.97  16074.12 |
 | LZO1F-999        224401    1    89599   40.3  3.23    280.68  16553.90 |
 |                                                                        |
 | LZO1X-1(11)      224401    1   118810   52.6  4.21   4544.42  15879.04 |
 | LZO1X-1(12)      224401    1   113675   50.6  4.05   4411.15  15721.59 |
 | LZO1X-1          224401    1   109323   49.4  3.95   4991.76  15584.89 |
 | LZO1X-1(15)      224401    1   108500   49.1  3.93   5077.50  15744.56 |
 | LZO1X-999        224401    1    82854   38.0  3.04    135.77  16548.48 |
 |                                                                        |
 | LZO1Y-1          224401    1   110820   49.8  3.98   4952.52  15638.82 |
 | LZO1Y-999        224401    1    83614   38.2  3.05    135.07  16385.40 |
 |                                                                        |
 | LZO1Z-999        224401    1    83034   38.0  3.04    133.31  10553.74 |
 |                                                                        |
 | LZO2A-999        224401    1    87880   40.0  3.20    301.21   8115.75 |
 +------------------------------------------------------------------------+

 Notes:
  - CxB is the number of blocks
  - K/s is the speed measured in 1000 uncompressed bytes per second
  - the assembler decompressors are even faster


 Short documentation
 -------------------
 LZO is a block compression algorithm - it compresses and decompresses
 a block of data. Block size must be the same for compression
 and decompression.

 LZO compresses a block of data into matches (a sliding dictionary)
 and runs of non-matching literals. LZO takes care about long matches
 and long literal runs so that it produces good results on highly
 redundant data and deals acceptably with non-compressible data.

 When dealing with incompressible data, LZO expands the input
 block by a maximum of 64 bytes per 1024 bytes input.

 I have verified LZO using such tools as valgrind and other memory checkers.
 And in addition to compressing gigabytes of files when tuning some parameters
 I have also consulted various 'lint' programs to spot potential portability
 problems. LZO is free of any known bugs.


 The algorithms
 --------------
 There are too many algorithms implemented. But I want to support
 unlimited backward compatibility, so I will not reduce the LZO
 distribution in the future.

 As the many object files are mostly independent of each other, the
 size overhead for an executable statically linked with the LZO library
 is usually pretty low (just a few KiB) because the linker will only add
 the modules that you are actually using.

 I first published LZO1 and LZO1A in the Internet newsgroups
 comp.compression and comp.compression.research in March 1996.
 They are mainly included for compatibility reasons. The LZO2A
 decompressor is too slow, and there is no fast compressor anyway.

 My experiments have shown that LZO1B is good with a large blocksize
 or with very redundant data, LZO1F is good with a small blocksize or
 with binary data and that LZO1X is often the best choice of all.
 LZO1Y and LZO1Z are almost identical to LZO1X - they can achieve a
 better compression ratio on some files.
 Beware, your mileage may vary.


 Usage of the library
 --------------------
 Despite of its size, the basic usage of LZO is really very simple.

 Let's assume you want to compress some data with LZO1X-1:
   A) compression
      * include <lzo/lzo1x.h>
        call lzo_init()
        compress your data with lzo1x_1_compress()
      * link your application with the LZO library
   B) decompression
      * include <lzo/lzo1x.h>
        call lzo_init()
        decompress your data with lzo1x_decompress()
      * link your application with the LZO library

 The program examples/simple.c shows a fully working example.
 See also LZO.FAQ for more information.


 Building LZO
 ------------
 As LZO uses Autoconf+Automake+Libtool the building process under
 UNIX systems should be very unproblematic. Shared libraries are
 supported on many architectures as well.
 For detailed instructions see the file INSTALL.

 Please note that due to the design of the ELF executable format
 the performance of a shared library on i386 systems (e.g. Linux)
 is a little bit slower, so you may want to link your applications
 with the static version (liblzo2.a) anyway.

 For building under DOS, Win16, Win32, OS/2 and other systems
 take a look at the file B/00readme.txt.

 In case of troubles (like decompression data errors) try recompiling
 everything without optimizations - LZO may break the optimizer
 of your compiler. See the file BUGS.

 LZO is written in ANSI C. In particular this means:
   - your compiler must understand prototypes
   - your compiler must understand prototypes in function pointers
   - your compiler must correctly promote integrals ("value-preserving")
   - your preprocessor must implement #elif, #error and stringizing
   - you must have a conforming and correct <limits.h> header
   - you must have <stddef.h>, <string.h> and other ANSI C headers
   - you should have size_t and ptrdiff_t


 Portability
 -----------
 I have built and tested LZO successfully on a variety of platforms
 including DOS (16 + 32 bit), Windows 3.x (16-bit), Win32, Win64,
 Linux, *BSD, HP-UX and many more.

 LZO is also reported to work under AIX, ConvexOS, IRIX, MacOS, PalmOS (Pilot),
 PSX (Sony Playstation), Solaris, SunOS, TOS (Atari ST) and VxWorks.
 Furthermore it is said that its performance on a Cray is superior
 to all other machines...

 And I think it would be much fun to translate the decompressors
 to Z-80 or 6502 assembly.


 The future
 ----------
 Here is what I'm planning for the next months. No promises, though...

 - interfaces to .NET and Mono
 - interfaces to Perl, Java, Python, Delphi, Visual Basic, ...
 - improve documentation and API reference


 Some comments about the source code
 -----------------------------------
 Be warned: the main source code in the 'src' directory is a
 real pain to understand as I've experimented with hundreds of slightly
 different versions. It contains many #if and some gotos, and
 is *completely optimized for speed* and not for readability.
 Code sharing of the different algorithms is implemented by stressing
 the preprocessor - this can be really confusing. Lots of marcos and
 assertions don't make things better.

 Nevertheless LZO compiles very quietly on a variety of
 compilers with the highest warning levels turned on, even
 in C++ mode.


 Copyright
 ---------
 LZO is Copyright (C) 1996-2014 Markus Franz Xaver Oberhumer
 All Rights Reserved.

 LZO is distributed under the terms of the GNU General Public License (GPL).
 See the file COPYING.

 Special licenses for commercial and other applications which
 are not willing to accept the GNU General Public License
 are available by contacting the author.