xz/doc/history.txt

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.