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Merging upstream version 1.6~pre2.

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Signed-off-by: Daniel Baumann <daniel@debian.org>
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Daniel Baumann 2025-02-17 20:33:28 +01:00
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doc/clzip.texi
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@ -6,8 +6,8 @@
@finalout
@c %**end of header
@set UPDATED 30 January 2014
@set VERSION 1.6-pre1
@set UPDATED 6 May 2014
@set VERSION 1.6-pre2
@dircategory Data Compression
@direntry
@ -59,20 +59,36 @@ Clzip is a lossless data compressor with a user interface similar to the
one of gzip or bzip2. Clzip decompresses almost as fast as gzip,
compresses most files more than bzip2, and is better than both from a
data recovery perspective. Clzip is a clean implementation of the LZMA
algorithm.
(Lempel-Ziv-Markov chain-Algorithm) algorithm.
Clzip uses the lzip file format; the files produced by clzip are fully
compatible with lzip-1.4 or newer, and can be rescued with lziprecover.
Clzip is in fact a C language version of lzip, intended for embedded
devices or systems lacking a C++ compiler.
The lzip file format is designed for long-term data archiving and
provides very safe integrity checking. It is as simple as possible (but
not simpler), so that with the only help of the lzip manual it would be
possible for a digital archaeologist to extract the data from a lzip
file long after quantum computers eventually render LZMA obsolete.
The lzip file format is designed for long-term data archiving, taking
into account both data integrity and decoder availability:
@itemize @bullet
@item
The lzip format provides very safe integrity checking and some data
recovery means. The lziprecover program can repair bit-flip errors (one
of the most common forms of data corruption) in lzip files, and provides
data recovery capabilities, including error-checked merging of damaged
copies of a file.
@item
The lzip format is as simple as possible (but not simpler). The lzip
manual provides the code of a simple decompressor along with a detailed
explanation of how it works, so that with the only help of the lzip
manual it would be possible for a digital archaeologist to extract the
data from a lzip file long after quantum computers eventually render
LZMA obsolete.
@item
Additionally lzip is copylefted, which guarantees that it will remain
free forever.
@end itemize
The member trailer stores the 32-bit CRC of the original data, the size
of the original data and the size of the member. These values, together
@ -85,16 +101,21 @@ going undetected are microscopic. Be aware, though, that the check
occurs upon decompression, so it can only tell you that something is
wrong. It can't help you recover the original uncompressed data.
If you ever need to recover data from a damaged lzip file, try the
lziprecover program. Lziprecover makes lzip files resistant to bit-flip
(one of the most common forms of data corruption), and provides data
recovery capabilities, including error-checked merging of damaged copies
of a file.
Clzip uses the same well-defined exit status values used by lzip and
bzip2, which makes it safer than compressors returning ambiguous warning
values (like gzip) when it is used as a back end for tar or zutils.
The amount of memory required for compression is about 1 or 2 times the
dictionary size limit (1 if input file size is less than dictionary size
limit, else 2) plus 9 times the dictionary size really used. The amount
of memory required for decompression is about 46 kB larger than the
dictionary size really used.
Clzip will automatically use the smallest possible dictionary size for
each file without exceeding the given limit. Keep in mind that the
decompression memory requirement is affected at compression time by the
choice of dictionary size limit.
When compressing, clzip replaces every file given in the command line
with a compressed version of itself, with the name "original_name.lz".
When decompressing, clzip attempts to guess the name for the decompressed
@ -135,29 +156,28 @@ Clzip is able to compress and decompress streams of unlimited size by
automatically creating multi-member output. The members so created are
large, about 64 PiB each.
The amount of memory required for compression is about 1 or 2 times the
dictionary size limit (1 if input file size is less than dictionary size
limit, else 2) plus 9 times the dictionary size really used. The amount
of memory required for decompression is about 46 kB larger than the
dictionary size really used.
Clzip will automatically use the smallest possible dictionary size
without exceeding the given limit. Keep in mind that the decompression
memory requirement is affected at compression time by the choice of
dictionary size limit.
@node Algorithm
@chapter Algorithm
@cindex algorithm
Clzip implements a simplified version of the LZMA (Lempel-Ziv-Markov
chain-Algorithm) algorithm. The high compression of LZMA comes from
combining two basic, well-proven compression ideas: sliding dictionaries
(LZ77/78) and markov models (the thing used by every compression
algorithm that uses a range encoder or similar order-0 entropy coder as
its last stage) with segregation of contexts according to what the bits
are used for.
There is no such thing as a "LZMA algorithm"; it is more like a "LZMA
coding scheme". For example, the option '-0' of lzip uses the scheme in
almost the simplest way possible; issuing the longest match it can find,
or a literal byte if it can't find a match. Inversely, a much more
elaborated way of finding coding sequences of minimum price than the one
currently used by lzip could be developed, and the resulting sequence
could also be coded using the LZMA coding scheme.
Lzip currently implements two variants of the LZMA algorithm; fast (used
by option -0) and normal (used by all other compression levels). Clzip
just implements the "normal" variant.
The high compression of LZMA comes from combining two basic, well-proven
compression ideas: sliding dictionaries (LZ77/78) and markov models (the
thing used by every compression algorithm that uses a range encoder or
similar order-0 entropy coder as its last stage) with segregation of
contexts according to what the bits are used for.
Clzip is a two stage compressor. The first stage is a Lempel-Ziv coder,
which reduces redundancy by translating chunks of data to their
@ -165,11 +185,6 @@ corresponding distance-length pairs. The second stage is a range encoder
that uses a different probability model for each type of data;
distances, lengths, literal bytes, etc.
The match finder, part of the LZ coder, is the most important piece of
the LZMA algorithm, as it is in many Lempel-Ziv based algorithms. Most
of clzip's execution time is spent in the match finder, and it has the
greatest influence on the compression ratio.
Here is how it works, step by step:
1) The member header is written to the output stream.
@ -284,7 +299,7 @@ Quiet operation. Suppress all messages.
@itemx --dictionary-size=@var{bytes}
Set the dictionary size limit in bytes. Valid values range from 4 KiB to
512 MiB. Clzip will use the smallest possible dictionary size for each
member without exceeding this limit. Note that dictionary sizes are
file without exceeding this limit. Note that dictionary sizes are
quantized. If the specified size does not match one of the valid sizes,
it will be rounded upwards by adding up to (@var{bytes} / 16) to it.