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authorLasse Collin <lasse.collin@tukaani.org>2007-12-09 00:42:33 +0200
committerLasse Collin <lasse.collin@tukaani.org>2007-12-09 00:42:33 +0200
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+
+LZMA Utils history
+------------------
+
+Tukaani distribution
+
+ In 2005, there was a small group working on Tukaani distribution, which
+ was a Slackware fork. One of the project goals was to fit the distro on
+ a single 700 MiB ISO-9660 image. Using LZMA instead of gzip helped a
+ lot. Roughly speaking, one could fit data that took 1000 MiB in gzipped
+ form into 700 MiB with LZMA. Naturally compression ratio varied across
+ packages, but this was what we got on average.
+
+ Slackware packages have traditionally had .tgz as the filename suffix,
+ which is an abbreviation of .tar.gz. A logical naming for LZMA
+ compressed packages was .tlz, being an abbreviation of .tar.lzma.
+
+ At the end of the year 2007, there's no distribution under the Tukaani
+ project anymore. Development of LZMA Utils still continues. Still,
+ there are .tlz packages around, because at least Vector Linux (a
+ Slackware based distribution) uses LZMA for its packages.
+
+ First versions of the modified pkgtools used the LZMA_Alone tool from
+ Igor Pavlov's LZMA SDK as is. It was fine, because users wouldn't need
+ to interact with LZMA_Alone directly. But people soon wanted to use
+ LZMA for other files too, and the interface of LZMA_Alone wasn't
+ comfortable for those used to gzip and bzip2.
+
+
+First steps of LZMA Utils
+
+ The first version of LZMA Utils (4.22.0) included a shell script called
+ lzmash. It was wrapper that had gzip-like command line interface. It
+ used the LZMA_Alone tool from LZMA SDK to do all the real work. zgrep,
+ zdiff, and related scripts from gzip were adapted work with LZMA and
+ were part of the first LZMA Utils release too.
+
+ LZMA Utils 4.22.0 included also lzmadec, which was a small (less than
+ 10 KiB) decoder-only command line tool. It was written on top of the
+ decoder-only C code found from the LZMA SDK. lzmadec was convenient in
+ situations where LZMA_Alone (a few hundred KiB) would be too big.
+
+ lzmash and lzmadec were written by Lasse Collin.
+
+
+Second generation
+
+ The lzmash script was an ugly and not very secure hack. The last
+ version of LZMA Utils to use lzmash was 4.27.1.
+
+ LZMA Utils 4.32.0beta1 introduced a new lzma command line tool written
+ by Ville Koskinen. It was written in C++, and used the encoder and
+ decoder from C++ LZMA SDK with little modifications. This tool replaced
+ both the lzmash script and the LZMA_Alone command line tool in LZMA
+ Utils.
+
+ Introducing this new tool caused some temporary incompatibilities,
+ because LZMA_Alone executable was simply named lzma like the new
+ command line tool, but they had completely different command line
+ interface. The file format was still the same.
+
+ Lasse wrote liblzmadec, which was a small decoder-only library based on
+ the C code found from LZMA SDK. liblzmadec had API similar to zlib,
+ although there were some significant differences, which made it
+ non-trivial to use it in some applications designed for zlib and
+ libbzip2.
+
+ The lzmadec command line tool was converted to use liblzmadec.
+
+ Alexandre Sauvé helped converting build system to use GNU Autotools.
+ This made is easier to test for certain less portable features needed
+ by the new command line tool.
+
+ Since the new command line tool never got completely finished (for
+ example, it didn't support LZMA_OPT environment variable), the intent
+ was to not call 4.32.x stable. Similarly, liblzmadec wasn't polished,
+ but appeared to work well enough, so some people started using it too.
+
+ Because the development of the third generation of LZMA Utils was
+ delayed considerably (roughly two years), the 4.32.x branch had to be
+ kept maintained. It got some bug fixes now and then, and finally it was
+ decided to call it stable, although most of the missing features were
+ never added.
+
+
+File format problems
+
+ The file format used by LZMA_Alone was primitive. It was designed for
+ embedded systems in mind, and thus provided only minimal set of
+ features. The two biggest problems for non-embedded use were lack of
+ magic bytes and integrity check.
+
+ Igor and Lasse started developing a new file format with some help from
+ Ville Koskinen, Mark Adler and Mikko Pouru. Designing the new format
+ took quite a long time. It was mostly because Lasse was quite slow at
+ getting things done due to personal reasons.
+
+ Near the end of the year 2007 the new format was practically finished.
+ Compared to LZMA_Alone format and the .gz format used by gzip, the new
+ .lzma format is quite complex as a whole. This means that tools having
+ *full* support for the new format would be larger and more complex than
+ the tools supporting only the old LZMA_Alone format.
+
+ For the situations where the full support for the .lzma format wouldn't
+ be required (embedded systems, operating system kernels), the new
+ format has a well-defined subset, which is easy to support with small
+ amount of code. It wouldn't be as small as an implementation using the
+ LZMA_Alone format, but the difference shouldn't be significant.
+
+ The new .lzma format allows dividing the data in multiple independent
+ blocks, which can be compressed and uncompressed independenly. This
+ makes multi-threading possible with algorithms that aren't inherently
+ parallel (such as LZMA). There's also a central index of the sizes of
+ the blocks, which makes it possible to do limited random-access reading
+ with granularity of the block size.
+
+ The new .lzma format uses the same filename suffix that was used for
+ LZMA_Alone files. The advantage is that users using the new tools won't
+ notice the change to the new format. The disadvantage is that the old
+ tools won't work with the new files.
+
+
+Third generation
+
+ LZMA Utils 4.42.0alphas drop the rest of the C++ LZMA SDK. The LZMA and
+ other included filters (algorithm implementations) are still directly
+ based on LZMA SDK, but ported to C.
+
+ liblzma is now the core of LZMA Utils. It has zlib-like API, which
+ doesn't suffer from the problems of the API of liblzmadec. liblzma
+ supports not only LZMA, but several other filters, which together
+ can improve compression ratio even further with certain file types.
+
+ The lzma and lzmadec command line tools have been rewritten. They uses
+ liblzma to do the actual compressing or uncompressing.
+
+ The development of LZMA Utils 4.42.x is still in alpha stage. Several
+ features are still missing or don't fully work yet. Documentation is
+ also very minimal.
+