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-rw-r--r--doc/history.txt123
1 files changed, 66 insertions, 57 deletions
diff --git a/doc/history.txt b/doc/history.txt
index 55293062..c97492e8 100644
--- a/doc/history.txt
+++ b/doc/history.txt
@@ -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.
-
- 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.
+ 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.
+
+ 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.
+
+ 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.
+
+ 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.
+
+
+Transition to XZ Utils
+
+ 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.
+
+ 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 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 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.