diff options
Diffstat (limited to 'doc/clzip.info')
| -rw-r--r-- | doc/clzip.info | 102 |
1 files changed, 56 insertions, 46 deletions
diff --git a/doc/clzip.info b/doc/clzip.info index 1819221..a007a92 100644 --- a/doc/clzip.info +++ b/doc/clzip.info @@ -11,7 +11,7 @@ File: clzip.info, Node: Top, Next: Introduction, Up: (dir) Clzip Manual ************ -This manual is for Clzip (version 1.6-pre1, 30 January 2014). +This manual is for Clzip (version 1.6-pre2, 6 May 2014). * Menu: @@ -39,20 +39,31 @@ 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. -Additionally lzip is copylefted, which guarantees that it will remain -free forever. + The lzip file format is designed for long-term data archiving, taking +into account both data integrity and decoder availability: + + * 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. + + * 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. + + * Additionally lzip is copylefted, which guarantees that it will + remain free forever. 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, @@ -66,16 +77,21 @@ 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 @@ -114,30 +130,29 @@ multivolume compressed tar archives. 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. - File: clzip.info, Node: Algorithm, Next: Invoking clzip, Prev: Introduction, Up: Top 2 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 @@ -145,11 +160,6 @@ 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. @@ -261,7 +271,7 @@ The format for running clzip is: '--dictionary-size=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 + size for each 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 (BYTES / 16) to it. @@ -530,13 +540,13 @@ Concept index Tag Table: Node: Top210 -Node: Introduction921 -Node: Algorithm5557 -Node: Invoking clzip8057 -Node: File format13656 -Node: Examples16161 -Node: Problems18130 -Node: Concept index18656 +Node: Introduction916 +Node: Algorithm5823 +Node: Invoking clzip8629 +Node: File format14226 +Node: Examples16731 +Node: Problems18700 +Node: Concept index19226 End Tag Table |