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Updated history.txt.

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Lasse Collin 2009-04-13 16:36:41 +03:00
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@ -1,6 +1,6 @@
LZMA Utils history
------------------
History of LZMA Utils and XZ Utils
==================================
Tukaani distribution
@ -15,10 +15,10 @@ Tukaani distribution
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.
At the end of the year 2007, there was no distribution under the
Tukaani project anymore, but development of LZMA Utils was kept going.
Still, there were .tlz packages around, because at least Vector Linux
(a Slackware based distribution) used 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
@ -59,8 +59,8 @@ Second generation
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,
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.
@ -77,8 +77,8 @@ Second generation
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
delayed considerably (3-4 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.
@ -90,51 +90,60 @@ File format problems
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.
Igor and Lasse started developing a new file format with some help
from Ville Koskinen. Also Mark Adler, Mikko Pouru, H. Peter Anvin,
and Lars Wirzenius helped with some minor things at some point of the
development. Designing the new format took quite a long time (actually,
too long time would be more appropriate expression). 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.
Originally the new format was supposed to use the same .lzma suffix
that was already used by the old file format. Switching to the new
format wouldn't have caused much trouble when the old format wasn't
used by many people. But since the development of the new format took
so long time, the old format got quite popular, and it was decided
that the new file format must use a different suffix.
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.
It was decided to use .xz as the suffix of the new file format. The
first stable .xz file format specification was finally released in
December 2008. In addition to fixing the most obvious problems of
the old .lzma format, the .xz format added some new features like
support for multiple filters (compression algorithms), filter chaining
(like piping on the command line), and limited random-access reading.
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.
Currently the primary compression algorithm used in .xz is LZMA2.
It is an extension on top of the original LZMA to fix some practical
problems: LZMA2 adds support for flushing the encoder, uncompressed
chunks, eases stateful decoder implementations, and improves support
for multithreading. Since LZMA2 is better than the original LZMA, the
original LZMA is not supported in .xz.
Third generation
Transition to XZ Utils
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.
The early versions of XZ Utils were called LZMA Utils. The first
releases were 4.42.0alphas. They dropped the rest of the C++ LZMA SDK.
The code was still directly based on LZMA SDK but ported to C and
converted from callback API to stateful API. Later, Igor Pavlov made
C version of the LZMA encoder too; these ports from C++ to C were
independent in LZMA SDK and LZMA Utils.
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 core of the new LZMA Utils was liblzma, a compression library with
zlib-like API. liblzma supported both the old and new file format. The
gzip-like lzma command line tool was rewritten to use liblzma.
The lzma and lzmadec command line tools have been rewritten. They uses
liblzma to do the actual compressing or uncompressing.
The new LZMA Utils code base was renamed to XZ Utils when the name
of the new file format had been decided. The liblzma compression
library retained its name though, because changing it would have
caused unnecessary breakage in applications already using the early
liblzma snapshots.
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.
The xz command line tool can emulate the gzip-like lzma tool by
creating appropriate symlinks (e.g. lzma -> xz). Thus, practically
all scripts using the lzma tool from LZMA Utils will work as is with
XZ Utils (and will keep using the old .lzma format). Still, the .lzma
format is more or less deprecated. XZ Utils will keep supporting it,
but new applications should use the .xz format, and migrating old
applications to .xz is often a good idea too.