\input texinfo @c -*-texinfo-*- @c %**start of header @setfilename lzip.info @documentencoding ISO-8859-15 @settitle Lzip Manual @finalout @c %**end of header @set UPDATED 11 January 2014 @set VERSION 1.16-pre1 @dircategory Data Compression @direntry * Lzip: (lzip). LZMA lossless data compressor @end direntry @ifnothtml @titlepage @title Lzip @subtitle LZMA lossless data compressor @subtitle for Lzip version @value{VERSION}, @value{UPDATED} @author by Antonio Diaz Diaz @page @vskip 0pt plus 1filll @end titlepage @contents @end ifnothtml @node Top @top This manual is for Lzip (version @value{VERSION}, @value{UPDATED}). @menu * Introduction:: Purpose and features of lzip * Algorithm:: How lzip compresses the data * Invoking lzip:: Command line interface * File format:: Detailed format of the compressed file * Stream format:: Format of the LZMA stream in lzip files * Examples:: A small tutorial with examples * Problems:: Reporting bugs * Reference source code:: Source code illustrating stream format * Concept index:: Index of concepts @end menu @sp 1 Copyright @copyright{} 2008, 2009, 2010, 2011, 2012, 2013, 2014 Antonio Diaz Diaz. This manual is free documentation: you have unlimited permission to copy, distribute and modify it. @node Introduction @chapter Introduction @cindex introduction Lzip is a lossless data compressor with a user interface similar to the one of gzip or bzip2. Lzip is about as fast as gzip, compresses most files more than bzip2, and is better than both from a data recovery perspective. Lzip is a clean implementation of the LZMA algorithm. 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 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 with the value remaining in the range decoder and the end-of-stream marker, provide a 4 factor integrity checking which guarantees that the decompressed version of the data is identical to the original. This guards against corruption of the compressed data, and against undetected bugs in lzip (hopefully very unlikely). The chances of data corruption 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. Lzip uses the same well-defined exit status values used by 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. When compressing, lzip replaces every file given in the command line with a compressed version of itself, with the name "original_name.lz". When decompressing, lzip attempts to guess the name for the decompressed file from that of the compressed file as follows: @multitable {anyothername} {becomes} {anyothername.out} @item filename.lz @tab becomes @tab filename @item filename.tlz @tab becomes @tab filename.tar @item anyothername @tab becomes @tab anyothername.out @end multitable (De)compressing a file is much like copying or moving it; therefore lzip preserves the access and modification dates, permissions, and, when possible, ownership of the file just as "cp -p" does. (If the user ID or the group ID can't be duplicated, the file permission bits S_ISUID and S_ISGID are cleared). Lzip is able to read from some types of non regular files if the @samp{--stdout} option is specified. If no file names are specified, lzip compresses (or decompresses) from standard input to standard output. In this case, lzip will decline to write compressed output to a terminal, as this would be entirely incomprehensible and therefore pointless. Lzip will correctly decompress a file which is the concatenation of two or more compressed files. The result is the concatenation of the corresponding uncompressed files. Integrity testing of concatenated compressed files is also supported. Lzip can produce multi-member files and safely recover, with lziprecover, the undamaged members in case of file damage. Lzip can also split the compressed output in volumes of a given size, even when reading from standard input. This allows the direct creation of multivolume compressed tar archives. Lzip 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 option @samp{-0} is special and only requires about 1.5 MiB at most. The amount of memory required for decompression is about 46 kB larger than the dictionary size really used. Lzip 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 Lzip 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. Lzip is a two stage compressor. The first stage is a Lempel-Ziv coder, which reduces redundancy by translating chunks of data to their 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 lzip'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. 2) The first byte is coded literally, because there are no previous bytes to which the match finder can refer to. 3) The main encoder advances to the next byte in the input data and calls the match finder. 4) The match finder fills an array with the minimum distances before the current byte where a match of a given length can be found. 5) Go back to step 3 until a sequence (formed of pairs, repeated distances and literal bytes) of minimum price has been formed. Where the price represents the number of output bits produced. 6) The range encoder encodes the sequence produced by the main encoder and sends the produced bytes to the output stream. 7) Go back to step 3 until the input data are finished or until the member or volume size limits are reached. 8) The range encoder is flushed. 9) The member trailer is written to the output stream. 10) If there are more data to compress, go back to step 1. @sp 1 @noindent The ideas embodied in lzip are due to (at least) the following people: Abraham Lempel and Jacob Ziv (for the LZ algorithm), Andrey Markov (for the definition of Markov chains), G.N.N. Martin (for the definition of range encoding), Igor Pavlov (for putting all the above together in LZMA), and Julian Seward (for bzip2's CLI). @node Invoking lzip @chapter Invoking lzip @cindex invoking @cindex options @cindex usage @cindex version The format for running lzip is: @example lzip [@var{options}] [@var{files}] @end example Lzip supports the following options: @table @samp @item -h @itemx --help Print an informative help message describing the options and exit. @item -V @itemx --version Print the version number of lzip on the standard output and exit. @item -b @var{bytes} @itemx --member-size=@var{bytes} Set the member size limit to @var{bytes}. A small member size may degrade compression ratio, so use it only when needed. Valid values range from 100 kB to 64 PiB. Defaults to 64 PiB. @item -c @itemx --stdout Compress or decompress to standard output. Needed when reading from a named pipe (fifo) or from a device. Use it to recover as much of the uncompressed data as possible when decompressing a corrupt file. @item -d @itemx --decompress Decompress. @item -f @itemx --force Force overwrite of output files. @item -F @itemx --recompress Force recompression of files whose name already has the @samp{.lz} or @samp{.tlz} suffix. @item -k @itemx --keep Keep (don't delete) input files during compression or decompression. @item -m @var{bytes} @itemx --match-length=@var{bytes} Set the match length limit in bytes. After a match this long is found, the search is finished. Valid values range from 5 to 273. Larger values usually give better compression ratios but longer compression times. @item -o @var{file} @itemx --output=@var{file} When reading from standard input and @samp{--stdout} has not been specified, use @samp{@var{file}} as the virtual name of the uncompressed file. This produces a file named @samp{@var{file}} when decompressing, a file named @samp{@var{file}.lz} when compressing, and several files named @samp{@var{file}00001.lz}, @samp{@var{file}00002.lz}, etc, when compressing and splitting the output in volumes. @item -q @itemx --quiet Quiet operation. Suppress all messages. @item -s @var{bytes} @itemx --dictionary-size=@var{bytes} Set the dictionary size limit in bytes. Valid values range from 4 KiB to 512 MiB. Lzip will use the smallest possible dictionary size for each member 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. For maximum compression you should use a dictionary size limit as large as possible, but keep in mind that the decompression memory requirement is affected at compression time by the choice of dictionary size limit. @item -S @var{bytes} @itemx --volume-size=@var{bytes} Split the compressed output into several volume files with names @samp{original_name00001.lz}, @samp{original_name00002.lz}, etc, and set the volume size limit to @var{bytes}. Each volume is a complete, maybe multi-member, lzip file. A small volume size may degrade compression ratio, so use it only when needed. Valid values range from 100 kB to 4 EiB. @item -t @itemx --test Check integrity of the specified file(s), but don't decompress them. This really performs a trial decompression and throws away the result. Use it together with @samp{-v} to see information about the file. @item -v @itemx --verbose Verbose mode.@* When compressing, show the compression ratio for each file processed. A second @samp{-v} shows the progress of compression.@* When decompressing or testing, further -v's (up to 4) increase the verbosity level, showing status, compression ratio, dictionary size, trailer contents (CRC, data size, member size), and up to 6 bytes of trailing garbage (if any). @item -0 .. -9 Set the compression parameters (dictionary size and match length limit) as shown in the table below. Note that @samp{-9} can be much slower than @samp{-0}. These options have no effect when decompressing. The bidimensional parameter space of LZMA can't be mapped to a linear scale optimal for all files. If your files are large, very repetitive, etc, you may need to use the @samp{--match-length} and @samp{--dictionary-size} options directly to achieve optimal performance. For example, @samp{-9m64} usually compresses executables more (and faster) than @samp{-9}. @multitable {Level} {Dictionary size} {Match length limit} @item Level @tab Dictionary size @tab Match length limit @item -0 @tab 64 KiB @tab 16 bytes @item -1 @tab 1 MiB @tab 5 bytes @item -2 @tab 1.5 MiB @tab 6 bytes @item -3 @tab 2 MiB @tab 8 bytes @item -4 @tab 3 MiB @tab 12 bytes @item -5 @tab 4 MiB @tab 20 bytes @item -6 @tab 8 MiB @tab 36 bytes @item -7 @tab 16 MiB @tab 68 bytes @item -8 @tab 24 MiB @tab 132 bytes @item -9 @tab 32 MiB @tab 273 bytes @end multitable @item --fast @itemx --best Aliases for GNU gzip compatibility. @end table Numbers given as arguments to options may be followed by a multiplier and an optional @samp{B} for "byte". Table of SI and binary prefixes (unit multipliers): @multitable {Prefix} {kilobyte (10^3 = 1000)} {|} {Prefix} {kibibyte (2^10 = 1024)} @item Prefix @tab Value @tab | @tab Prefix @tab Value @item k @tab kilobyte (10^3 = 1000) @tab | @tab Ki @tab kibibyte (2^10 = 1024) @item M @tab megabyte (10^6) @tab | @tab Mi @tab mebibyte (2^20) @item G @tab gigabyte (10^9) @tab | @tab Gi @tab gibibyte (2^30) @item T @tab terabyte (10^12) @tab | @tab Ti @tab tebibyte (2^40) @item P @tab petabyte (10^15) @tab | @tab Pi @tab pebibyte (2^50) @item E @tab exabyte (10^18) @tab | @tab Ei @tab exbibyte (2^60) @item Z @tab zettabyte (10^21) @tab | @tab Zi @tab zebibyte (2^70) @item Y @tab yottabyte (10^24) @tab | @tab Yi @tab yobibyte (2^80) @end multitable @sp 1 Exit status: 0 for a normal exit, 1 for environmental problems (file not found, invalid flags, I/O errors, etc), 2 to indicate a corrupt or invalid input file, 3 for an internal consistency error (eg, bug) which caused lzip to panic. @node File format @chapter File format @cindex file format Perfection is reached, not when there is no longer anything to add, but when there is no longer anything to take away.@* --- Antoine de Saint-Exupery @sp 1 In the diagram below, a box like this: @verbatim +---+ | | <-- the vertical bars might be missing +---+ @end verbatim represents one byte; a box like this: @verbatim +==============+ | | +==============+ @end verbatim represents a variable number of bytes. @sp 1 A lzip file consists of a series of "members" (compressed data sets). The members simply appear one after another in the file, with no additional information before, between, or after them. Each member has the following structure: @verbatim +--+--+--+--+----+----+=============+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ID string | VN | DS | Lzma stream | CRC32 | Data size | Member size | +--+--+--+--+----+----+=============+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ @end verbatim All multibyte values are stored in little endian order. @table @samp @item ID string A four byte string, identifying the lzip format, with the value "LZIP" (0x4C, 0x5A, 0x49, 0x50). @item VN (version number, 1 byte) Just in case something needs to be modified in the future. 1 for now. @item DS (coded dictionary size, 1 byte) Lzip divides the distance between any two powers of 2 into 8 equally spaced intervals, named "wedges". The dictionary size is calculated by taking a power of 2 (the base size) and substracting from it a number of wedges between 0 and 7. The size of a wedge is (base_size / 16).@* Bits 4-0 contain the base 2 logarithm of the base size (12 to 29).@* Bits 7-5 contain the number of wedges (0 to 7) to substract from the base size to obtain the dictionary size.@* Example: 0xD3 = 2^19 - 6 * 2^15 = 512 KiB - 6 * 32 KiB = 320 KiB@* Valid values for dictionary size range from 4 KiB to 512 MiB. @item Lzma stream The lzma stream, finished by an end of stream marker. Uses default values for encoder properties. See the chapter @samp{Stream format} (@pxref{Stream format}) for a complete description. @item CRC32 (4 bytes) CRC of the uncompressed original data. @item Data size (8 bytes) Size of the uncompressed original data. @item Member size (8 bytes) Total size of the member, including header and trailer. This field acts as a distributed index, allows the verification of stream integrity, and facilitates safe recovery of undamaged members from multi-member files. @end table @node Stream format @chapter Format of the LZMA stream in lzip files @cindex format of the LZMA stream The LZMA algorithm has three parameters, called "special LZMA properties", to adjust it for some kinds of binary data. These parameters are; @samp{literal_context_bits} (with a default value of 3), @samp{literal_pos_state_bits} (with a default value of 0), and @samp{pos_state_bits} (with a default value of 2). As a general purpose compressor, lzip only uses the default values for these parameters. Lzip also finishes the LZMA stream with an "End Of Stream" marker (the distance-length pair 0xFFFFFFFFU, 2), which in conjunction with the "member size" field in the member trailer allows the verification of stream integrity. The LZMA stream in lzip files always has these two features (default properties and EOS marker) and is referred to in this document as LZMA-302eos or LZMA-lzip. The second stage of LZMA is a range encoder that uses a different probability model for each type of symbol; distances, lengths, literal bytes, etc. Range encoding conceptually encodes all the symbols of the message into one number. Unlike Huffman coding, which assigns to each symbol a bit-pattern and concatenates all the bit-patterns together, range encoding can compress one symbol to less than one bit. Therefore the compressed data produced by a range encoder can't be split in pieces that could be individually described. It seems that the only way of describing the LZMA-302eos stream is describing the algorithm that decodes it. And given the many details about the range decoder that need to be described accurately, the source code of a real decoder seems the only appropiate reference to use. What follows is a description of the decoding algorithm for LZMA-302eos streams using as reference the source code of "lzd", an educational decompressor for lzip files which can be downloaded from the lzip download directory. The source code of lzd is included in appendix A. @ref{Reference source code} @sp 1 @section What is coded The LZMA stream includes literals, matches and repeated matches (matches reusing a recently used distance). There are 7 different coding sequences: @multitable @columnfractions .35 .14 .51 @headitem Bit sequence @tab Name @tab Description @item 0 + byte @tab literal @tab literal byte @item 1 + 0 + len + dis @tab match @tab distance-length pair @item 1 + 1 + 0 + 0 @tab shortrep @tab 1 byte match at latest used distance @item 1 + 1 + 0 + 1 + len @tab rep0 @tab len bytes match at latest used distance @item 1 + 1 + 1 + 0 + len @tab rep1 @tab len bytes match at second latest used distance @item 1 + 1 + 1 + 1 + 0 + len @tab rep2 @tab len bytes match at third latest used distance @item 1 + 1 + 1 + 1 + 1 + len @tab rep3 @tab len bytes match at fourth latest used distance @end multitable @sp 1 In the following tables, multi-bit sequences are coded in normal order, from MSB to LSB, except where noted otherwise. Lengths (the @samp{len} in the table above) are coded as follows: @multitable @columnfractions .5 .5 @headitem Bit sequence @tab Description @item 0 + 3 bits @tab lengths from 2 to 9 @item 1 + 0 + 3 bits @tab lengths from 10 to 17 @item 1 + 1 + 8 bits @tab lengths from 18 to 273 @end multitable @sp 1 The coding of distances is a little more complicated. LZMA divides the interval between any two powers of 2 into 2 halves, named slots. As possible distances range from 0 to (2^32 - 1), there are 64 slots (0 to 63). The slot number is context-coded in 6 bits. @samp{direct_bits} are the remaining bits (from 0 to 30) needed to form a complete distance, and are calculated as (slot >> 1) - 1. If a distance needs 6 or more direct_bits, the last 4 bits are coded separately. The last piece (direct_bits for distances 4 to 127 or the last 4 bits for distances >= 128) is context-coded in reverse order (from LSB to MSB). For distances >= 128, the @samp{direct_bits - 4} part is coded with fixed 0.5 probability. @multitable @columnfractions .5 .5 @headitem Bit sequence @tab Description @item slot @tab distances from 0 to 3 @item slot + direct_bits @tab distances from 4 to 127 @item slot + (direct_bits - 4) + 4 bits @tab distances from 128 to 2^32 - 1 @end multitable @sp 1 @section The coding contexts These contexts (@samp{Bit_model} in the source), are integers or arrays of integers representing the probability of the corresponding bit being 0. The indices used in these arrays are: @table @samp @item state A state machine (@samp{State} in the source) with 12 states (0 to 11), coding the latest 2 to 4 types of sequences processed. The initial state is 0. @item pos_state Value of the 2 least significant bits of the current position in the decoded data. @item literal_state Value of the 3 most significant bits of the latest byte decoded. @item len_state Coded value of length (length - 2), with a maximum of 3. The resulting value is in the range 0 to 3. @end table In the following table, @samp{!literal} is any sequence except a literal byte. @samp{rep} is any one of @samp{rep0}, @samp{rep1}, @samp{rep2} or @samp{rep3}. The types of previous sequences corresponding to each state are: @multitable {State} {rep or (!literal, shortrep), literal, literal} @headitem State @tab Types of previous sequences @item 0 @tab literal, literal, literal @item 1 @tab match, literal, literal @item 2 @tab rep or (!literal, shortrep), literal, literal @item 3 @tab literal, shortrep, literal, literal @item 4 @tab match, literal @item 5 @tab rep or (!literal, shortrep), literal @item 6 @tab literal, shortrep, literal @item 7 @tab literal, match @item 8 @tab literal, rep @item 9 @tab literal, shortrep @item 10 @tab !literal, match @item 11 @tab !literal, (rep or shortrep) @end multitable @sp 1 The contexts for decoding the type of coding sequence are: @multitable @columnfractions .2 .4 .4 @headitem Name @tab Indices @tab Used when @item bm_match @tab state, pos_state @tab sequence start @item bm_rep @tab state @tab after sequence 1 @item bm_rep0 @tab state @tab after sequence 11 @item bm_rep1 @tab state @tab after sequence 111 @item bm_rep2 @tab state @tab after sequence 1111 @item bm_len @tab state, pos_state @tab after sequence 110 @end multitable @sp 1 The contexts for decoding distances are: @multitable @columnfractions .2 .4 .4 @headitem Name @tab Indices @tab Used when @item bm_dis_slot @tab len_state, bit tree @tab distance start @item bm_dis @tab reverse bit tree @tab after slots 4 to 13 @item bm_align @tab reverse bit tree @tab for distances >= 128, after fixed probability bits @end multitable @sp 1 There are two separate sets of contexts for lengths (@samp{Len_model} in the source). One for normal matches, the other for repeated matches. The contexts in each Len_model are (see @samp{decode_len} in the source): @multitable @columnfractions .2 .4 .4 @headitem Name @tab Indices @tab Used when @item choice1 @tab none @tab length start @item choice2 @tab none @tab after sequence 1 @item bm_low @tab pos_state, bit tree @tab after sequence 0 @item bm_mid @tab pos_state, bit tree @tab after sequence 10 @item bm_high @tab bit tree @tab after sequence 11 @end multitable @sp 1 The context array @samp{bm_literal} is special. In principle it acts as a normal bit tree context, the one selected by @samp{literal_state}. But if the previous decoded byte was not a literal, two other bit tree contexts are used depending on the value of each bit in @samp{match_byte} (the byte at the latest used distance), until a bit is decoded that is different from its corresponding bit in @samp{match_byte}. After the first difference is found, the rest of the byte is decoded using the normal bit tree context. (See @samp{decode_matched} in the source). @sp 1 @section The range decoder The LZMA stream is consumed one byte at a time by the range decoder. (See @samp{normalize} in the source). Every byte consumed produces a variable number of decoded bits, depending on how well these bits agree with their context. (See @samp{decode_bit} in the source). The range decoder state consists of two unsigned 32-bit variables, @code{range} (representing the most significant part of the range size not yet decoded), and @code{code} (representing the current point within @code{range}). @code{range} is initialized to (2^32 - 1), and @code{code} is initialized to 0. The range encoder produces a first 0 byte that must be ignored by the range decoder. This is done by shifting 5 bytes in the initialization of @code{code} instead of 4. (See the @samp{Range_decoder} constructor in the source). @sp 1 @section Decoding the LZMA stream After decoding the member header and obtaining the dictionary size, the range decoder is initialized and then the LZMA decoder enters a loop (See @samp{decode_member} in the source) where it invokes the range decoder with the appropiate contexts to decode the different coding sequences (matches, repeated matches, and literal bytes), until the "End Of Stream" marker is decoded. @node Examples @chapter A small tutorial with examples @cindex examples WARNING! Even if lzip is bug-free, other causes may result in a corrupt compressed file (bugs in the system libraries, memory errors, etc). Therefore, if the data you are going to compress are important, give the @samp{--keep} option to lzip and do not remove the original file until you verify the compressed file with a command like @w{@samp{lzip -cd file.lz | cmp file -}}. @sp 1 @noindent Example 1: Replace a regular file with its compressed version @samp{file.lz} and show the compression ratio. @example lzip -v file @end example @sp 1 @noindent Example 2: Like example 1 but the created @samp{file.lz} is multi-member with a member size of 1 MiB. The compression ratio is not shown. @example lzip -b 1MiB file @end example @sp 1 @noindent Example 3: Restore a regular file from its compressed version @samp{file.lz}. If the operation is successful, @samp{file.lz} is removed. @example lzip -d file.lz @end example @sp 1 @noindent Example 4: Verify the integrity of the compressed file @samp{file.lz} and show status. @example lzip -tv file.lz @end example @sp 1 @noindent Example 5: Compress a whole floppy in /dev/fd0 and send the output to @samp{file.lz}. @example lzip -c /dev/fd0 > file.lz @end example @sp 1 @noindent Example 6: Decompress @samp{file.lz} partially until 10 KiB of decompressed data are produced. @example lzip -cd file.lz | dd bs=1024 count=10 @end example @sp 1 @noindent Example 7: Decompress @samp{file.lz} partially from decompressed byte 10000 to decompressed byte 15000 (5000 bytes are produced). @example lzip -cd file.lz | dd bs=1000 skip=10 count=5 @end example @sp 1 @noindent Example 8: Create a multivolume compressed tar archive with a volume size of 1440 KiB. @example tar -c some_directory | lzip -S 1440KiB -o volume_name @end example @sp 1 @noindent Example 9: Extract a multivolume compressed tar archive. @example lzip -cd volume_name*.lz | tar -xf - @end example @sp 1 @noindent Example 10: Create a multivolume compressed backup of a large database file with a volume size of 650 MB, where each volume is a multi-member file with a member size of 32 MiB. @example lzip -b 32MiB -S 650MB big_db @end example @node Problems @chapter Reporting bugs @cindex bugs @cindex getting help There are probably bugs in lzip. There are certainly errors and omissions in this manual. If you report them, they will get fixed. If you don't, no one will ever know about them and they will remain unfixed for all eternity, if not longer. If you find a bug in lzip, please send electronic mail to @email{lzip-bug@@nongnu.org}. Include the version number, which you can find by running @w{@samp{lzip --version}}. @node Reference source code @appendix Reference source code @cindex reference source code @verbatim /* Lzd - Educational decompressor for lzip files Copyright (C) 2013, 2014 Antonio Diaz Diaz. This program is free software: you have unlimited permission to copy, distribute and modify it. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. */ /* Exit status: 0 for a normal exit, 1 for environmental problems (file not found, invalid flags, I/O errors, etc), 2 to indicate a corrupt or invalid input file. */ #include #include #include #include #include #include #include class State { int st; public: enum { states = 12 }; State() : st( 0 ) {} int operator()() const { return st; } bool is_char() const { return st < 7; } void set_char() { static const int next[states] = { 0, 0, 0, 0, 1, 2, 3, 4, 5, 6, 4, 5 }; st = next[st]; } void set_match() { st = ( st < 7 ) ? 7 : 10; } void set_rep() { st = ( st < 7 ) ? 8 : 11; } void set_short_rep() { st = ( st < 7 ) ? 9 : 11; } }; enum { min_dictionary_size = 1 << 12, max_dictionary_size = 1 << 29, literal_context_bits = 3, pos_state_bits = 2, pos_states = 1 << pos_state_bits, pos_state_mask = pos_states - 1, len_states = 4, dis_slot_bits = 6, start_dis_model = 4, end_dis_model = 14, modeled_distances = 1 << (end_dis_model / 2), // 128 dis_align_bits = 4, dis_align_size = 1 << dis_align_bits, len_low_bits = 3, len_mid_bits = 3, len_high_bits = 8, len_low_symbols = 1 << len_low_bits, len_mid_symbols = 1 << len_mid_bits, len_high_symbols = 1 << len_high_bits, max_len_symbols = len_low_symbols + len_mid_symbols + len_high_symbols, min_match_len = 2, // must be 2 bit_model_move_bits = 5, bit_model_total_bits = 11, bit_model_total = 1 << bit_model_total_bits }; struct Bit_model { int probability; Bit_model() : probability( bit_model_total / 2 ) {} }; struct Len_model { Bit_model choice1; Bit_model choice2; Bit_model bm_low[pos_states][len_low_symbols]; Bit_model bm_mid[pos_states][len_mid_symbols]; Bit_model bm_high[len_high_symbols]; }; class CRC32 { uint32_t data[256]; // Table of CRCs of all 8-bit messages. public: CRC32() { for( unsigned n = 0; n < 256; ++n ) { unsigned c = n; for( int k = 0; k < 8; ++k ) { if( c & 1 ) c = 0xEDB88320U ^ ( c >> 1 ); else c >>= 1; } data[n] = c; } } void update_buf( uint32_t & crc, const uint8_t * const buffer, const int size ) const { for( int i = 0; i < size; ++i ) crc = data[(crc^buffer[i])&0xFF] ^ ( crc >> 8 ); } }; const CRC32 crc32; typedef uint8_t File_header[6]; // 0-3 magic, 4 version, 5 coded_dict_size typedef uint8_t File_trailer[20]; // 0-3 CRC32 of the uncompressed data // 4-11 size of the uncompressed data // 12-19 member size including header and trailer class Range_decoder { uint32_t code; uint32_t range; public: Range_decoder() : code( 0 ), range( 0xFFFFFFFFU ) { for( int i = 0; i < 5; ++i ) code = (code << 8) | get_byte(); } uint8_t get_byte() { return std::getc( stdin ); } int decode( const int num_bits ) { int symbol = 0; for( int i = 0; i < num_bits; ++i ) { range >>= 1; symbol <<= 1; if( code >= range ) { code -= range; symbol |= 1; } if( range <= 0x00FFFFFFU ) // normalize { range <<= 8; code = (code << 8) | get_byte(); } } return symbol; } int decode_bit( Bit_model & bm ) { int symbol; const uint32_t bound = ( range >> bit_model_total_bits ) * bm.probability; if( code < bound ) { range = bound; bm.probability += (bit_model_total - bm.probability) >> bit_model_move_bits; symbol = 0; } else { range -= bound; code -= bound; bm.probability -= bm.probability >> bit_model_move_bits; symbol = 1; } if( range <= 0x00FFFFFFU ) // normalize { range <<= 8; code = (code << 8) | get_byte(); } return symbol; } int decode_tree( Bit_model bm[], const int num_bits ) { int symbol = 1; for( int i = 0; i < num_bits; ++i ) symbol = ( symbol << 1 ) | decode_bit( bm[symbol] ); return symbol - (1 << num_bits); } int decode_tree_reversed( Bit_model bm[], const int num_bits ) { int symbol = decode_tree( bm, num_bits ); int reversed_symbol = 0; for( int i = 0; i < num_bits; ++i ) { reversed_symbol = ( reversed_symbol << 1 ) | ( symbol & 1 ); symbol >>= 1; } return reversed_symbol; } int decode_matched( Bit_model bm[], const int match_byte ) { Bit_model * const bm1 = bm + 0x100; int symbol = 1; for( int i = 7; i >= 0; --i ) { const int match_bit = ( match_byte >> i ) & 1; const int bit = decode_bit( bm1[(match_bit<<8)+symbol] ); symbol = ( symbol << 1 ) | bit; if( match_bit != bit ) { while( symbol < 0x100 ) symbol = ( symbol << 1 ) | decode_bit( bm[symbol] ); break; } } return symbol & 0xFF; } int decode_len( Len_model & lm, const int pos_state ) { if( decode_bit( lm.choice1 ) == 0 ) return decode_tree( lm.bm_low[pos_state], len_low_bits ); if( decode_bit( lm.choice2 ) == 0 ) return len_low_symbols + decode_tree( lm.bm_mid[pos_state], len_mid_bits ); return len_low_symbols + len_mid_symbols + decode_tree( lm.bm_high, len_high_bits ); } }; class LZ_decoder { unsigned long long partial_data_pos; Range_decoder rdec; const unsigned dictionary_size; uint8_t * const buffer; // output buffer unsigned pos; // current pos in buffer unsigned stream_pos; // first byte not yet written to stdout uint32_t crc_; void flush_data(); uint8_t get_byte( const unsigned distance ) const { unsigned i = pos - distance - 1; if( pos <= distance ) i += dictionary_size; return buffer[i]; } void put_byte( const uint8_t b ) { buffer[pos] = b; if( ++pos >= dictionary_size ) flush_data(); } public: LZ_decoder( const unsigned dict_size ) : partial_data_pos( 0 ), dictionary_size( dict_size ), buffer( new uint8_t[dictionary_size] ), pos( 0 ), stream_pos( 0 ), crc_( 0xFFFFFFFFU ) { buffer[dictionary_size-1] = 0; } // prev_byte of first_byte ~LZ_decoder() { delete[] buffer; } unsigned crc() const { return crc_ ^ 0xFFFFFFFFU; } unsigned long long data_position() const { return partial_data_pos + pos; } bool decode_member(); }; void LZ_decoder::flush_data() { if( pos > stream_pos ) { const unsigned size = pos - stream_pos; crc32.update_buf( crc_, buffer + stream_pos, size ); errno = 0; if( std::fwrite( buffer + stream_pos, 1, size, stdout ) != size ) { std::fprintf( stderr, "Write error: %s\n", std::strerror( errno ) ); std::exit( 1 ); } if( pos >= dictionary_size ) { partial_data_pos += pos; pos = 0; } stream_pos = pos; } } bool LZ_decoder::decode_member() // Returns false if error { Bit_model bm_literal[1<> ( 8 - literal_context_bits ); Bit_model * const bm = bm_literal[literal_state]; if( state.is_char() ) put_byte( rdec.decode_tree( bm, 8 ) ); else put_byte( rdec.decode_matched( bm, get_byte( rep0 ) ) ); state.set_char(); } else { int len; if( rdec.decode_bit( bm_rep[state()] ) != 0 ) // 2nd bit { if( rdec.decode_bit( bm_rep0[state()] ) != 0 ) // 3rd bit { unsigned distance; if( rdec.decode_bit( bm_rep1[state()] ) == 0 ) // 4th bit distance = rep1; else { if( rdec.decode_bit( bm_rep2[state()] ) == 0 ) // 5th bit distance = rep2; else { distance = rep3; rep3 = rep2; } rep2 = rep1; } rep1 = rep0; rep0 = distance; } else { if( rdec.decode_bit( bm_len[state()][pos_state] ) == 0 ) // 4th bit { state.set_short_rep(); put_byte( get_byte( rep0 ) ); continue; } } state.set_rep(); len = min_match_len + rdec.decode_len( rep_len_model, pos_state ); } else { rep3 = rep2; rep2 = rep1; rep1 = rep0; len = min_match_len + rdec.decode_len( match_len_model, pos_state ); const int len_state = std::min( len - min_match_len, len_states - 1 ); const int dis_slot = rdec.decode_tree( bm_dis_slot[len_state], dis_slot_bits ); if( dis_slot < start_dis_model ) rep0 = dis_slot; else { const int direct_bits = ( dis_slot >> 1 ) - 1; rep0 = ( 2 | ( dis_slot & 1 ) ) << direct_bits; if( dis_slot < end_dis_model ) rep0 += rdec.decode_tree_reversed( bm_dis + rep0 - dis_slot - 1, direct_bits ); else { rep0 += rdec.decode( direct_bits - dis_align_bits ) << dis_align_bits; rep0 += rdec.decode_tree_reversed( bm_align, dis_align_bits ); if( rep0 == 0xFFFFFFFFU ) // Marker found { flush_data(); return ( len == min_match_len ); // End Of Stream marker } } } state.set_match(); if( rep0 >= dictionary_size || rep0 >= data_position() ) return false; } for( int i = 0; i < len; ++i ) put_byte( get_byte( rep0 ) ); } } return false; } int main( const int argc, const char * const argv[] ) { if( argc > 1 ) { std::printf( "Lzd %s - Educational decompressor for lzip files.\n", PROGVERSION ); std::printf( "Study the source to learn how a lzip decompressor works.\n" "See the lzip manual for an explanation of the code.\n" "It is not safe to use lzd for any real work.\n" "\nUsage: %s < file.lz > file\n", argv[0] ); std::printf( "Lzd decompresses from standard input to standard output.\n" "\nCopyright (C) 2014 Antonio Diaz Diaz.\n" "This is free software: you are free to change and redistribute it.\n" "There is NO WARRANTY, to the extent permitted by law.\n" "Report bugs to lzip-bug@nongnu.org\n" "Lzd home page: http://www.nongnu.org/lzip/lzd.html\n" ); return 0; } for( bool first_member = true; ; first_member = false ) { File_header header; for( int i = 0; i < 6; ++i ) header[i] = std::getc( stdin ); if( std::feof( stdin ) || std::memcmp( header, "LZIP", 4 ) != 0 ) { if( first_member ) { std::fprintf( stderr, "Bad magic number (file not in lzip format)\n" ); return 2; } break; } if( header[4] != 1 ) { std::fprintf( stderr, "Version %d member format not supported.\n", header[4] ); return 2; } unsigned dict_size = 1 << ( header[5] & 0x1F ); dict_size -= ( dict_size / 16 ) * ( ( header[5] >> 5 ) & 7 ); if( dict_size < min_dictionary_size || dict_size > max_dictionary_size ) { std::fprintf( stderr, "Invalid dictionary size in member header\n" ); return 2; } LZ_decoder decoder( dict_size ); if( !decoder.decode_member() ) { std::fprintf( stderr, "Data error\n" ); return 2; } File_trailer trailer; for( int i = 0; i < 20; ++i ) trailer[i] = std::getc( stdin ); unsigned crc = 0; for( int i = 3; i >= 0; --i ) { crc <<= 8; crc += trailer[i]; } unsigned long long data_size = 0; for( int i = 11; i >= 4; --i ) { data_size <<= 8; data_size += trailer[i]; } if( crc != decoder.crc() || data_size != decoder.data_position() ) { std::fprintf( stderr, "CRC error\n" ); return 2; } } if( std::fclose( stdout ) != 0 ) { std::fprintf( stderr, "Can't close stdout: %s\n", std::strerror( errno ) ); return 1; } return 0; } @end verbatim @node Concept index @unnumbered Concept index @printindex cp @bye