1
2History of LZMA Utils and XZ Utils
3==================================
4
5Tukaani distribution
6
7    In 2005, there was a small group working on the Tukaani distribution,
8    which was a Slackware fork. One of the project's goals was to fit the
9    distro on a single 700 MiB ISO-9660 image. Using LZMA instead of gzip
10    helped a lot. Roughly speaking, one could fit data that took 1000 MiB
11    in gzipped form into 700 MiB with LZMA. Naturally, the compression
12    ratio varied across packages, but this was what we got on average.
13
14    Slackware packages have traditionally had .tgz as the filename suffix,
15    which is an abbreviation of .tar.gz. A logical naming for LZMA
16    compressed packages was .tlz, being an abbreviation of .tar.lzma.
17
18    At the end of the year 2007, there was no distribution under the
19    Tukaani project anymore, but development of LZMA Utils was kept going.
20    Still, there were .tlz packages around, because at least Vector Linux
21    (a Slackware based distribution) used LZMA for its packages.
22
23    First versions of the modified pkgtools used the LZMA_Alone tool from
24    Igor Pavlov's LZMA SDK as is. It was fine, because users wouldn't need
25    to interact with LZMA_Alone directly. But people soon wanted to use
26    LZMA for other files too, and the interface of LZMA_Alone wasn't
27    comfortable for those used to gzip and bzip2.
28
29
30First steps of LZMA Utils
31
32    The first version of LZMA Utils (4.22.0) included a shell script called
33    lzmash. It was a wrapper that had a gzip-like command-line interface. It
34    used the LZMA_Alone tool from LZMA SDK to do all the real work. zgrep,
35    zdiff, and related scripts from gzip were adapted to work with LZMA and
36    were part of the first LZMA Utils release too.
37
38    LZMA Utils 4.22.0 included also lzmadec, which was a small (less than
39    10 KiB) decoder-only command-line tool. It was written on top of the
40    decoder-only C code found from the LZMA SDK. lzmadec was convenient in
41    situations where LZMA_Alone (a few hundred KiB) would be too big.
42
43    lzmash and lzmadec were written by Lasse Collin.
44
45
46Second generation
47
48    The lzmash script was an ugly and not very secure hack. The last
49    version of LZMA Utils to use lzmash was 4.27.1.
50
51    LZMA Utils 4.32.0beta1 introduced a new lzma command-line tool written
52    by Ville Koskinen. It was written in C++, and used the encoder and
53    decoder from C++ LZMA SDK with some little modifications. This tool
54    replaced both the lzmash script and the LZMA_Alone command-line tool
55    in LZMA Utils.
56
57    Introducing this new tool caused some temporary incompatibilities,
58    because the LZMA_Alone executable was simply named lzma like the new
59    command-line tool, but they had a completely different command-line
60    interface. The file format was still the same.
61
62    Lasse wrote liblzmadec, which was a small decoder-only library based
63    on the C code found from LZMA SDK. liblzmadec had an API similar to
64    zlib, although there were some significant differences, which made it
65    non-trivial to use it in some applications designed for zlib and
66    libbzip2.
67
68    The lzmadec command-line tool was converted to use liblzmadec.
69
70    Alexandre Sauvé helped converting the build system to use GNU
71    Autotools. This made it easier to test for certain less portable
72    features needed by the new command-line tool.
73
74    Since the new command-line tool never got completely finished (for
75    example, it didn't support the LZMA_OPT environment variable), the
76    intent was to not call 4.32.x stable. Similarly, liblzmadec wasn't
77    polished, but appeared to work well enough, so some people started
78    using it too.
79
80    Because the development of the third generation of LZMA Utils was
81    delayed considerably (3-4 years), the 4.32.x branch had to be kept
82    maintained. It got some bug fixes now and then, and finally it was
83    decided to call it stable, although most of the missing features were
84    never added.
85
86
87File format problems
88
89    The file format used by LZMA_Alone was primitive. It was designed with
90    embedded systems in mind, and thus provided only a minimal set of
91    features. The two biggest problems for non-embedded use were the lack
92    of magic bytes and an integrity check.
93
94    Igor and Lasse started developing a new file format with some help
95    from Ville Koskinen. Also Mark Adler, Mikko Pouru, H. Peter Anvin,
96    and Lars Wirzenius helped with some minor things at some point of the
97    development. Designing the new format took quite a long time (actually,
98    too long a time would be a more appropriate expression). It was mostly
99    because Lasse was quite slow at getting things done due to personal
100    reasons.
101
102    Originally the new format was supposed to use the same .lzma suffix
103    that was already used by the old file format. Switching to the new
104    format wouldn't have caused much trouble when the old format wasn't
105    used by many people. But since the development of the new format took
106    such a long time, the old format got quite popular, and it was decided
107    that the new file format must use a different suffix.
108
109    It was decided to use .xz as the suffix of the new file format. The
110    first stable .xz file format specification was finally released in
111    December 2008. In addition to fixing the most obvious problems of
112    the old .lzma format, the .xz format added some new features like
113    support for multiple filters (compression algorithms), filter chaining
114    (like piping on the command line), and limited random-access reading.
115
116    Currently the primary compression algorithm used in .xz is LZMA2.
117    It is an extension on top of the original LZMA to fix some practical
118    problems: LZMA2 adds support for flushing the encoder, uncompressed
119    chunks, eases stateful decoder implementations, and improves support
120    for multithreading. Since LZMA2 is better than the original LZMA, the
121    original LZMA is not supported in .xz.
122
123
124Transition to XZ Utils
125
126    The early versions of XZ Utils were called LZMA Utils. The first
127    releases were 4.42.0alphas. They dropped the rest of the C++ LZMA SDK.
128    The code was still directly based on LZMA SDK but ported to C and
129    converted from a callback API to a stateful API. Later, Igor Pavlov
130    made a C version of the LZMA encoder too; these ports from C++ to C
131    were independent in LZMA SDK and LZMA Utils.
132
133    The core of the new LZMA Utils was liblzma, a compression library with
134    a zlib-like API. liblzma supported both the old and new file format.
135    The gzip-like lzma command-line tool was rewritten to use liblzma.
136
137    The new LZMA Utils code base was renamed to XZ Utils when the name
138    of the new file format had been decided. The liblzma compression
139    library retained its name though, because changing it would have
140    caused unnecessary breakage in applications already using the early
141    liblzma snapshots.
142
143    The xz command-line tool can emulate the gzip-like lzma tool by
144    creating appropriate symlinks (e.g. lzma -> xz). Thus, practically
145    all scripts using the lzma tool from LZMA Utils will work as is with
146    XZ Utils (and will keep using the old .lzma format). Still, the .lzma
147    format is more or less deprecated. XZ Utils will keep supporting it,
148    but new applications should use the .xz format, and migrating old
149    applications to .xz is often a good idea too.
150
151