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diff --git a/doc/lzip.info b/doc/lzip.info new file mode 100644 index 0000000..7c6d812 --- /dev/null +++ b/doc/lzip.info @@ -0,0 +1,1708 @@ +This is lzip.info, produced by makeinfo version 4.13+ from lzip.texi. + +INFO-DIR-SECTION Compression +START-INFO-DIR-ENTRY +* Lzip: (lzip). LZMA lossless data compressor +END-INFO-DIR-ENTRY + + +File: lzip.info, Node: Top, Next: Introduction, Up: (dir) + +Lzip Manual +*********** + +This manual is for Lzip (version 1.23, 24 January 2022). + +* Menu: + +* Introduction:: Purpose and features of lzip +* Output:: Meaning of lzip's output +* Invoking lzip:: Command line interface +* Quality assurance:: Design, development, and testing of lzip +* Algorithm:: How lzip compresses the data +* File format:: Detailed format of the compressed file +* Stream format:: Format of the LZMA stream in lzip files +* Trailing data:: Extra data appended to the file +* Examples:: A small tutorial with examples +* Problems:: Reporting bugs +* Reference source code:: Source code illustrating stream format +* Concept index:: Index of concepts + + + Copyright (C) 2008-2022 Antonio Diaz Diaz. + + This manual is free documentation: you have unlimited permission to copy, +distribute, and modify it. + + +File: lzip.info, Node: Introduction, Next: Output, Prev: Top, Up: Top + +1 Introduction +************** + +Lzip is a lossless data compressor with a user interface similar to the one +of gzip or bzip2. Lzip uses a simplified form of the 'Lempel-Ziv-Markov +chain-Algorithm' (LZMA) stream format and provides a 3 factor integrity +checking to maximize interoperability and optimize safety. Lzip can compress +about as fast as gzip (lzip -0) or compress most files more than bzip2 +(lzip -9). Decompression speed is intermediate between gzip and bzip2. Lzip +is better than gzip and bzip2 from a data recovery perspective. Lzip has +been designed, written, and tested with great care to replace gzip and +bzip2 as the standard general-purpose compressed format for unix-like +systems. + + For compressing/decompressing large files on multiprocessor machines +plzip can be much faster than lzip at the cost of a slightly reduced +compression ratio. *Note plzip manual: (plzip)Top. + + For creation and manipulation of compressed tar archives tarlz can be +more efficient than using tar and plzip because tarlz is able to keep the +alignment between tar members and lzip members. *Note tarlz manual: +(tarlz)Top. + + The lzip file format is designed for data sharing and long-term +archiving, taking into account both data integrity and decoder availability: + + * The lzip format provides very safe integrity checking and some data + recovery means. The program lziprecover 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. *Note Data safety: (lziprecover)Data + safety. + + * The lzip format is as simple as possible (but not simpler). The lzip + manual provides the source 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 the lzip reference implementation is copylefted, which + guarantees that it will remain free forever. + + A nice feature of the lzip format is that a corrupt byte is easier to +repair the nearer it is from the beginning of the file. Therefore, with the +help of lziprecover, losing an entire archive just because of a corrupt +byte near the beginning is a thing of the past. + + 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 "End Of Stream" marker, provide a 3 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. + + 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 other programs like tar or zutils. + + Lzip will automatically use for each file the largest dictionary size +that does not exceed neither the file size nor the limit given. Keep in +mind that the decompression memory requirement is affected at compression +time by the choice of dictionary size limit. + + 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 +'-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. + + 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: + +filename.lz becomes filename +filename.tlz becomes filename.tar +anyothername becomes anyothername.out + + (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 either the +option '-c' or the option '-o' is specified. + + Lzip will refuse to read compressed data from a terminal or write +compressed data to a terminal, as this would be entirely incomprehensible +and might leave the terminal in an abnormal state. + + Lzip will correctly decompress a file which is the concatenation of two +or more compressed files. The result is the concatenation of the +corresponding decompressed files. Integrity testing of concatenated +compressed files is also supported. + + Lzip can produce multimember files, and lziprecover can safely recover +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 multimember output. The members so created are large, +about 2 PiB each. + + +File: lzip.info, Node: Output, Next: Invoking lzip, Prev: Introduction, Up: Top + +2 Meaning of lzip's output +************************** + +The output of lzip looks like this: + + lzip -v foo + foo: 6.676:1, 14.98% ratio, 85.02% saved, 450560 in, 67493 out. + + lzip -tvvv foo.lz + foo.lz: 6.676:1, 14.98% ratio, 85.02% saved. 450560 out, 67493 in. ok + + The meaning of each field is as follows: + +'N:1' + The compression ratio (uncompressed_size / compressed_size), shown as + N to 1. + +'ratio' + The inverse compression ratio (compressed_size / uncompressed_size), + shown as a percentage. A decimal ratio is easily obtained by moving the + decimal point two places to the left; 14.98% = 0.1498. + +'saved' + The space saved by compression (1 - ratio), shown as a percentage. + +'in' + Size of the input data. This is the uncompressed size when + compressing, or the compressed size when decompressing or testing. + Note that lzip always prints the uncompressed size before the + compressed size when compressing, decompressing, testing, or listing. + +'out' + Size of the output data. This is the compressed size when compressing, + or the decompressed size when decompressing or testing. + + + When decompressing or testing at verbosity level 4 (-vvvv), the +dictionary size used to compress the file and the CRC32 of the uncompressed +data are also shown. + + LANGUAGE NOTE: Uncompressed = not compressed = plain data; it may never +have been compressed. Decompressed is used to refer to data which have +undergone the process of decompression. + + +File: lzip.info, Node: Invoking lzip, Next: Quality assurance, Prev: Output, Up: Top + +3 Invoking lzip +*************** + +The format for running lzip is: + + lzip [OPTIONS] [FILES] + +If no file names are specified, lzip compresses (or decompresses) from +standard input to standard output. A hyphen '-' used as a FILE argument +means standard input. It can be mixed with other FILES and is read just +once, the first time it appears in the command line. + + lzip supports the following options: *Note Argument syntax: +(arg_parser)Argument syntax. + +'-h' +'--help' + Print an informative help message describing the options and exit. + +'-V' +'--version' + Print the version number of lzip on the standard output and exit. This + version number should be included in all bug reports. + +'-a' +'--trailing-error' + Exit with error status 2 if any remaining input is detected after + decompressing the last member. Such remaining input is usually trailing + garbage that can be safely ignored. *Note concat-example::. + +'-b BYTES' +'--member-size=BYTES' + When compressing, set the member size limit to BYTES. It is advisable + to keep members smaller than RAM size so that they can be repaired with + lziprecover in case of corruption. A small member size may degrade + compression ratio, so use it only when needed. Valid values range from + 100 kB to 2 PiB. Defaults to 2 PiB. + +'-c' +'--stdout' + Compress or decompress to standard output; keep input files unchanged. + If compressing several files, each file is compressed independently. + (The output consists of a sequence of independently compressed + members). This option (or '-o') is needed when reading from a named + pipe (fifo) or from a device. Use it also to recover as much of the + decompressed data as possible when decompressing a corrupt file. '-c' + overrides '-o' and '-S'. '-c' has no effect when testing or listing. + +'-d' +'--decompress' + Decompress the files specified. If a file does not exist, can't be + opened, or the destination file already exists and '--force' has not + been specified, lzip continues decompressing the rest of the files and + exits with error status 1. If a file fails to decompress, or is a + terminal, lzip exits immediately with error status 2 without + decompressing the rest of the files. A terminal is considered an + uncompressed file, and therefore invalid. + +'-f' +'--force' + Force overwrite of output files. + +'-F' +'--recompress' + When compressing, force re-compression of files whose name already has + the '.lz' or '.tlz' suffix. + +'-k' +'--keep' + Keep (don't delete) input files during compression or decompression. + +'-l' +'--list' + Print the uncompressed size, compressed size, and percentage saved of + the files specified. Trailing data are ignored. The values produced + are correct even for multimember files. If more than one file is + given, a final line containing the cumulative sizes is printed. With + '-v', the dictionary size, the number of members in the file, and the + amount of trailing data (if any) are also printed. With '-vv', the + positions and sizes of each member in multimember files are also + printed. + + If any file is damaged, does not exist, can't be opened, or is not + regular, the final exit status will be > 0. '-lq' can be used to verify + quickly (without decompressing) the structural integrity of the files + specified. (Use '--test' to verify the data integrity). '-alq' + additionally verifies that none of the files specified contain + trailing data. + +'-m BYTES' +'--match-length=BYTES' + When compressing, 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. + +'-o FILE' +'--output=FILE' + If '-c' has not been also specified, write the (de)compressed output to + FILE; keep input files unchanged. If compressing several files, each + file is compressed independently. (The output consists of a sequence of + independently compressed members). This option (or '-c') is needed when + reading from a named pipe (fifo) or from a device. '-o -' is + equivalent to '-c'. '-o' has no effect when testing or listing. + + In order to keep backward compatibility with lzip versions prior to + 1.22, when compressing from standard input and no other file names are + given, the extension '.lz' is appended to FILE unless it already ends + in '.lz' or '.tlz'. This feature will be removed in a future version + of lzip. Meanwhile, redirection may be used instead of '-o' to write + the compressed output to a file without the extension '.lz' in its + name: 'lzip < file > foo'. + + When compressing and splitting the output in volumes, FILE is used as + a prefix, and several files named 'FILE00001.lz', 'FILE00002.lz', etc, + are created. In this case, only one input file is allowed. + +'-q' +'--quiet' + Quiet operation. Suppress all messages. + +'-s BYTES' +'--dictionary-size=BYTES' + When compressing, set the dictionary size limit in bytes. Lzip will use + for each file the largest dictionary size that does not exceed neither + the file size nor this limit. Valid values range from 4 KiB to + 512 MiB. Values 12 to 29 are interpreted as powers of two, meaning + 2^12 to 2^29 bytes. Dictionary sizes are quantized so that they can be + coded in just one byte (*note coded-dict-size::). If the size specified + does not match one of the valid sizes, it will be rounded upwards by + adding up to (BYTES / 8) 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. + +'-S BYTES' +'--volume-size=BYTES' + When compressing, and '-c' has not been also specified, split the + compressed output into several volume files with names + 'original_name00001.lz', 'original_name00002.lz', etc, and set the + volume size limit to BYTES. Input files are kept unchanged. Each + volume is a complete, maybe multimember, 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. + +'-t' +'--test' + Check integrity of the files specified, but don't decompress them. This + really performs a trial decompression and throws away the result. Use + it together with '-v' to see information about the files. If a file + fails the test, does not exist, can't be opened, or is a terminal, lzip + continues checking the rest of the files. A final diagnostic is shown + at verbosity level 1 or higher if any file fails the test when testing + multiple files. + +'-v' +'--verbose' + Verbose mode. + When compressing, show the compression ratio and size for each file + processed. + 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 data (if any) both in hexadecimal and as a string of printable + ASCII characters. + Two or more '-v' options show the progress of (de)compression. + +'-0 .. -9' + Compression level. Set the compression parameters (dictionary size and + match length limit) as shown in the table below. The default + compression level is '-6', equivalent to '-s8MiB -m36'. Note that '-9' + can be much slower than '-0'. These options have no effect when + decompressing, testing, or listing. + + 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 options '--dictionary-size' and + '--match-length' directly to achieve optimal performance. + + If several compression levels or '-s' or '-m' options are given, the + last setting is used. For example '-9 -s64MiB' is equivalent to + '-s64MiB -m273' + + Level Dictionary size (-s) Match length limit (-m) + -0 64 KiB 16 bytes + -1 1 MiB 5 bytes + -2 1.5 MiB 6 bytes + -3 2 MiB 8 bytes + -4 3 MiB 12 bytes + -5 4 MiB 20 bytes + -6 8 MiB 36 bytes + -7 16 MiB 68 bytes + -8 24 MiB 132 bytes + -9 32 MiB 273 bytes + +'--fast' +'--best' + Aliases for GNU gzip compatibility. + +'--loose-trailing' + When decompressing, testing, or listing, allow trailing data whose + first bytes are so similar to the magic bytes of a lzip header that + they can be confused with a corrupt header. Use this option if a file + triggers a "corrupt header" error and the cause is not indeed a + corrupt header. + + + Numbers given as arguments to options may be followed by a multiplier +and an optional 'B' for "byte". + + Table of SI and binary prefixes (unit multipliers): + +Prefix Value | Prefix Value +k kilobyte (10^3 = 1000) | Ki kibibyte (2^10 = 1024) +M megabyte (10^6) | Mi mebibyte (2^20) +G gigabyte (10^9) | Gi gibibyte (2^30) +T terabyte (10^12) | Ti tebibyte (2^40) +P petabyte (10^15) | Pi pebibyte (2^50) +E exabyte (10^18) | Ei exbibyte (2^60) +Z zettabyte (10^21) | Zi zebibyte (2^70) +Y yottabyte (10^24) | Yi yobibyte (2^80) + + + 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 (e.g., bug) which caused +lzip to panic. + + +File: lzip.info, Node: Quality assurance, Next: Algorithm, Prev: Invoking lzip, Up: Top + +4 Design, development, and testing of lzip +****************************************** + +There are two ways of constructing a software design: One way is to make it +so simple that there are obviously no deficiencies and the other way is to +make it so complicated that there are no obvious deficiencies. The first +method is far more difficult. +-- C.A.R. Hoare + + Lzip is developed by volunteers who lack the resources required for +extensive testing in all circumstances. It is up to you to test lzip before +using it in mission-critical applications. However, a compressor like lzip +is not a toy, and maintaining it is not a hobby. Many people's data depend +on it. Therefore the lzip file format has been reviewed carefully and is +believed to be free from negligent design errors. + + Lzip has been designed, written, and tested with great care to replace +gzip and bzip2 as the standard general-purpose compressed format for +unix-like systems. This chapter describes the lessons learned from these +previous formats, and their application to the design of lzip. + + +4.1 Format design +================= + +When gzip was designed in 1992, computers and operating systems were much +less capable than they are today. The designers of gzip tried to work around +some of those limitations, like 8.3 file names, with additional fields in +the file format. + + Today those limitations have mostly disappeared, and the format of gzip +has proved to be unnecessarily complicated. It includes fields that were +never used, others that have lost their usefulness, and finally others that +have become too limited. + + Bzip2 was designed 5 years later, and its format is simpler than the one +of gzip. + + Probably the worst defect of the gzip format from the point of view of +data safety is the variable size of its header. If the byte at offset 3 +(flags) of a gzip member gets corrupted, it may become difficult to recover +the data, even if the compressed blocks are intact, because it can't be +known with certainty where the compressed blocks begin. + + By contrast, the header of a lzip member has a fixed length of 6. The +LZMA stream in a lzip member always starts at offset 6, making it trivial to +recover the data even if the whole header becomes corrupt. + + Bzip2 also provides a header of fixed length and marks the begin and end +of each compressed block with six magic bytes, making it possible to find +the compressed blocks even in case of file damage. But bzip2 does not store +the size of each compressed block, as lzip does. + + Lziprecover is able to provide unique data recovery capabilities because +the lzip format is extraordinarily safe. The simple and safe design of the +file format complements the embedded error detection provided by the LZMA +data stream. Any distance larger than the dictionary size acts as a +forbidden symbol, allowing the decompressor to detect the approximate +position of errors, and leaving very little work for the check sequence +(CRC and data sizes) in the detection of errors. Lzip is usually able to +detect all possible bit flips in the compressed data without resorting to +the check sequence. It would be difficult to write an automatic recovery +tool like lziprecover for the gzip format. And, as far as I know, it has +never been written. + + Lzip, like gzip and bzip2, uses a CRC32 to check the integrity of the +decompressed data because it provides optimal accuracy in the detection of +errors up to a compressed size of about 16 GiB, a size larger than that of +most files. In the case of lzip, the additional detection capability of the +decompressor reduces the probability of undetected errors several million +times more, resulting in a combined integrity checking optimally accurate +for any member size produced by lzip. Preliminary results suggest that the +lzip format is safe enough to be used in critical safety avionics systems. + + The lzip format is designed for long-term archiving. Therefore it +excludes any unneeded features that may interfere with the future +extraction of the decompressed data. + + +4.1.1 Gzip format (mis)features not present in lzip +--------------------------------------------------- + +'Multiple algorithms' + Gzip provides a CM (Compression Method) field that has never been used + because it is a bad idea to begin with. New compression methods may + require additional fields, making it impossible to implement new + methods and, at the same time, keep the same format. This field does + not solve the problem of format proliferation; it just makes the + problem less obvious. + +'Optional fields in header' + Unless special precautions are taken, optional fields are generally a + bad idea because they produce a header of variable size. The gzip + header has 2 fields that, in addition to being optional, are + zero-terminated. This means that if any byte inside the field gets + zeroed, or if the terminating zero gets altered, gzip won't be able to + find neither the header CRC nor the compressed blocks. + +'Optional CRC for the header' + Using an optional CRC for the header is not only a bad idea, it is an + error; it circumvents the Hamming distance (HD) of the CRC and may + prevent the extraction of perfectly good data. For example, if the CRC + is used and the bit enabling it is reset by a bit flip, the header + will appear to be intact (in spite of being corrupt) while the + compressed blocks will appear to be totally unrecoverable (in spite of + being intact). Very misleading indeed. + +'Metadata' + The gzip format stores some metadata, like the modification time of the + original file or the operating system on which compression took place. + This complicates reproducible compression (obtaining identical + compressed output from identical input). + + +4.1.2 Lzip format improvements over gzip and bzip2 +-------------------------------------------------- + +'64-bit size field' + Probably the most frequently reported shortcoming of the gzip format + is that it only stores the least significant 32 bits of the + uncompressed size. The size of any file larger than 4 GiB gets + truncated. + + Bzip2 does not store the uncompressed size of the file. + + The lzip format provides a 64-bit field for the uncompressed size. + Additionally, lzip produces multimember output automatically when the + size is too large for a single member, allowing for an unlimited + uncompressed size. + +'Distributed index' + The lzip format provides a distributed index that, among other things, + helps plzip to decompress several times faster than pigz and helps + lziprecover do its job. Neither the gzip format nor the bzip2 format + do provide an index. + + A distributed index is safer and more scalable than a monolithic + index. The monolithic index introduces a single point of failure in + the compressed file and may limit the number of members or the total + uncompressed size. + + +4.2 Quality of implementation +============================= + +'Accurate and robust error detection' + The lzip format provides 3 factor integrity checking, and the + decompressors report mismatches in each factor separately. This method + detects most false positives for corruption. If just one byte in one + factor fails but the other two factors match the data, it probably + means that the data are intact and the corruption just affects the + mismatching factor (CRC, data size, or member size) in the member + trailer. + +'Multiple implementations' + Just like the lzip format provides 3 factor protection against + undetected data corruption, the development methodology of the lzip + family of compressors provides 3 factor protection against undetected + programming errors. + + Three related but independent compressor implementations, lzip, clzip, + and minilzip/lzlib, are developed concurrently. Every stable release + of any of them is tested to verify that it produces identical output + to the other two. This guarantees that all three implement the same + algorithm, and makes it unlikely that any of them may contain serious + undiscovered errors. In fact, no errors have been discovered in lzip + since 2009. + + Additionally, the three implementations have been extensively tested + with unzcrash, valgrind, and 'american fuzzy lop' without finding a + single vulnerability or false negative. *Note Unzcrash: + (lziprecover)Unzcrash. + +'Dictionary size' + Lzip automatically adapts the dictionary size to the size of each file. + In addition to reducing the amount of memory required for + decompression, this feature also minimizes the probability of being + affected by RAM errors during compression. + +'Exit status' + Returning a warning status of 2 is a design flaw of compress that + leaked into the design of gzip. Both bzip2 and lzip are free from this + flaw. + + + +File: lzip.info, Node: Algorithm, Next: File format, Prev: Quality assurance, Up: Top + +5 Algorithm +*********** + +In spite of its name (Lempel-Ziv-Markov chain-Algorithm), LZMA is not a +concrete algorithm; it is more like "any algorithm using the LZMA coding +scheme". LZMA compression consists in describing the uncompressed data as a +succession of coding sequences from the set shown in Section 'What is +coded' (*note what-is-coded::), and then encoding them using a range +encoder. 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 size 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). + + 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. + + 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 bytes produced 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. + + + During compression, lzip reads data in large blocks (one dictionary size +at a time). Therefore it may block for up to tens of seconds any process +feeding data to it through a pipe. This is normal. The blocking intervals +get longer with higher compression levels because dictionary size increases +(and compression speed decreases) with compression level. + +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). + + +File: lzip.info, Node: File format, Next: Stream format, Prev: Algorithm, Up: Top + +6 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 + + + In the diagram below, a box like this: + ++---+ +| | <-- the vertical bars might be missing ++---+ + + represents one byte; a box like this: + ++==============+ +| | ++==============+ + + represents a variable number of bytes. + + + A lzip file consists of a series of independent "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 can +encode in compressed form up to 16 EiB - 1 byte of uncompressed data. The +size of a multimember file is unlimited. + + Each member has the following structure: + ++--+--+--+--+----+----+=============+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +| ID string | VN | DS | LZMA stream | CRC32 | Data size | Member size | ++--+--+--+--+----+----+=============+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + + All multibyte values are stored in little endian order. + +'ID string (the "magic" bytes)' + A four byte string, identifying the lzip format, with the value "LZIP" + (0x4C, 0x5A, 0x49, 0x50). + +'VN (version number, 1 byte)' + Just in case something needs to be modified in the future. 1 for now. + +'DS (coded dictionary size, 1 byte)' + The dictionary size is calculated by taking a power of 2 (the base + size) and subtracting from it a fraction between 0/16 and 7/16 of the + base size. + Bits 4-0 contain the base 2 logarithm of the base size (12 to 29). + Bits 7-5 contain the numerator of the fraction (0 to 7) to subtract + 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. + +'LZMA stream' + The LZMA stream, finished by an "End Of Stream" marker. Uses default + values for encoder properties. *Note Stream format::, for a complete + description. + +'CRC32 (4 bytes)' + Cyclic Redundancy Check (CRC) of the original uncompressed data. + +'Data size (8 bytes)' + Size of the original uncompressed data. + +'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 the safe recovery of undamaged members from + multimember files. Member size should be limited to 2 PiB to prevent + the data size field from overflowing. + + + +File: lzip.info, Node: Stream format, Next: Trailing data, Prev: File format, Up: Top + +7 Format of the LZMA stream in lzip files +***************************************** + +The LZMA algorithm has three parameters, called "special LZMA properties", +to adjust it for some kinds of binary data. These parameters are: +'literal_context_bits' (with a default value of 3), +'literal_pos_state_bits' (with a default value of 0), and 'pos_state_bits' +(with a default value of 2). As a general purpose compressor, lzip only +uses the default values for these parameters. In particular +'literal_pos_state_bits' has been optimized away and does not even appear +in the code. + + Lzip finishes the LZMA stream with an "End Of Stream" (EOS) 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 EOS marker is the only marker allowed in lzip files. 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. This +simplified form of the LZMA stream format has been chosen to maximize +interoperability and safety. + + 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 described individually. + + It seems that the only way of describing the LZMA-302eos stream is to +describe 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 decompressor seems the only appropriate 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. Lzd is written in C++11 and its source code is included in +appendix A. *Note Reference source code::. + + +7.1 What is coded +================= + +The LZMA stream includes literals, matches, and repeated matches (matches +reusing a recently used distance). There are 7 different coding sequences: + +Bit sequence Name Description +----------------------------------------------------------------------------- +0 + byte literal literal byte +1 + 0 + len + dis match distance-length pair +1 + 1 + 0 + 0 shortrep 1 byte match at latest used distance +1 + 1 + 0 + 1 + len rep0 len bytes match at latest used distance +1 + 1 + 1 + 0 + len rep1 len bytes match at second latest used + distance +1 + 1 + 1 + 1 + 0 + len rep2 len bytes match at third latest used + distance +1 + 1 + 1 + 1 + 1 + len rep3 len bytes match at fourth latest used + distance + + + In the following tables, multibit sequences are coded in normal order, +from most significant bit (MSB) to least significant bit (LSB), except +where noted otherwise. + + Lengths (the 'len' in the table above) are coded as follows: + +Bit sequence Description +---------------------------------------------------------------------------- +0 + 3 bits lengths from 2 to 9 +1 + 0 + 3 bits lengths from 10 to 17 +1 + 1 + 8 bits lengths from 18 to 273 + + + The coding of distances is a little more complicated, so I'll begin by +explaining a simpler version of the encoding. + + Imagine you need to encode a number from 0 to 2^32 - 1, and you want to +do it in a way that produces shorter codes for the smaller numbers. You may +first encode the position of the most significant bit that is set to 1, +which you may find by making a bit scan from the left (from the MSB). A +position of 0 means that the number is 0 (no bit is set), 1 means the LSB is +the first bit set (the number is 1), and 32 means the MSB is set (i.e., the +number is >= 0x80000000). Then, if the position is >= 2, you encode the +remaining position - 1 bits. Let's call these bits "direct bits" because +they are coded directly by value instead of indirectly by position. + + The inconvenient of this simple method is that it needs 6 bits to encode +the position, but it just uses 33 of the 64 possible values, wasting almost +half of the codes. + + The intelligent trick of LZMA is that it encodes in what it calls a +"slot" the position of the most significant bit set, along with the value +of the next bit, using the same 6 bits that would take to encode the +position alone. This seems to need 66 slots (twice the number of +positions), but for positions 0 and 1 there is no next bit, so the number +of slots needed is 64 (0 to 63). + + The 6 bits representing this "slot number" are then context-coded. If +the distance is >= 4, the remaining bits are encoded as follows. +'direct_bits' is the amount of remaining bits (from 1 to 30) needed to form +a complete distance, and is calculated as (slot >> 1) - 1. If a distance +needs 6 or more direct_bits, the last 4 bits are encoded separately. The +last piece (all the 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 'direct_bits - 4' part is encoded with fixed 0.5 +probability. + +Bit sequence Description +---------------------------------------------------------------------------- +slot distances from 0 to 3 +slot + direct_bits distances from 4 to 127 +slot + (direct_bits - 4) + 4 bits distances from 128 to 2^32 - 1 + + +7.2 The coding contexts +======================= + +These contexts ('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: + +'state' + A state machine ('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. + +'pos_state' + Value of the 2 least significant bits of the current position in the + decoded data. + +'literal_state' + Value of the 3 most significant bits of the latest byte decoded. + +'len_state' + Coded value of the current match length (length - 2), with a maximum + of 3. The resulting value is in the range 0 to 3. + + + The types of previous sequences corresponding to each state are shown in +the following table. '!literal' is any sequence except a literal byte. +'rep' is any one of 'rep0', 'rep1', 'rep2', or 'rep3'. The last type in +each line is the most recent. + +State Types of previous sequences +------------------------------------------------------ +0 literal, literal, literal +1 match, literal, literal +2 rep or (!literal, shortrep), literal, literal +3 literal, shortrep, literal, literal +4 match, literal +5 rep or (!literal, shortrep), literal +6 literal, shortrep, literal +7 literal, match +8 literal, rep +9 literal, shortrep +10 !literal, match +11 !literal, (rep or shortrep) + + + The contexts for decoding the type of coding sequence are: + +Name Indices Used when +---------------------------------------------------------------------------- +bm_match state, pos_state sequence start +bm_rep state after sequence 1 +bm_rep0 state after sequence 11 +bm_rep1 state after sequence 111 +bm_rep2 state after sequence 1111 +bm_len state, pos_state after sequence 110 + + + The contexts for decoding distances are: + +Name Indices Used when +---------------------------------------------------------------------------- +bm_dis_slot len_state, bit tree distance start +bm_dis reverse bit tree after slots 4 to 13 +bm_align reverse bit tree for distances >= 128, after fixed + probability bits + + + There are two separate sets of contexts for lengths ('Len_model' in the +source). One for normal matches, the other for repeated matches. The +contexts in each Len_model are (see 'decode_len' in the source): + +Name Indices Used when +--------------------------------------------------------------------------- +choice1 none length start +choice2 none after sequence 1 +bm_low pos_state, bit tree after sequence 0 +bm_mid pos_state, bit tree after sequence 10 +bm_high bit tree after sequence 11 + + + The context array 'bm_literal' is special. In principle it acts as a +normal bit tree context, the one selected by '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 'match_byte' (the byte at the +latest used distance), until a bit is decoded that is different from its +corresponding bit in 'match_byte'. After the first difference is found, the +rest of the byte is decoded using the normal bit tree context. (See +'decode_matched' in the source). + + +7.3 The range decoder +===================== + +The LZMA stream is consumed one byte at a time by the range decoder. (See +'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 'decode_bit' in the source). + + The range decoder state consists of two unsigned 32-bit variables: +'range' (representing the most significant part of the range size not yet +decoded) and 'code' (representing the current point within 'range'). +'range' is initialized to 2^32 - 1, and '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' instead of 4. (See the 'Range_decoder' constructor in the source). + + +7.4 Decoding and verifying 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 +'decode_member' in the source) where it invokes the range decoder with the +appropriate contexts to decode the different coding sequences (matches, +repeated matches, and literal bytes), until the "End Of Stream" marker is +decoded. + + Once the "End Of Stream" marker has been decoded, the decompressor reads +and decodes the member trailer, and verifies that the three integrity +factors stored there (CRC, data size, and member size) match those computed +from the data. + + +File: lzip.info, Node: Trailing data, Next: Examples, Prev: Stream format, Up: Top + +8 Extra data appended to the file +********************************* + +Sometimes extra data are found appended to a lzip file after the last +member. Such trailing data may be: + + * Padding added to make the file size a multiple of some block size, for + example when writing to a tape. It is safe to append any amount of + padding zero bytes to a lzip file. + + * Useful data added by the user; a cryptographically secure hash, a + description of file contents, etc. It is safe to append any amount of + text to a lzip file as long as none of the first four bytes of the text + match the corresponding byte in the string "LZIP", and the text does + not contain any zero bytes (null characters). Nonzero bytes and zero + bytes can't be safely mixed in trailing data. + + * Garbage added by some not totally successful copy operation. + + * Malicious data added to the file in order to make its total size and + hash value (for a chosen hash) coincide with those of another file. + + * In rare cases, trailing data could be the corrupt header of another + member. In multimember or concatenated files the probability of + corruption happening in the magic bytes is 5 times smaller than the + probability of getting a false positive caused by the corruption of the + integrity information itself. Therefore it can be considered to be + below the noise level. Additionally, the test used by lzip to + discriminate trailing data from a corrupt header has a Hamming + distance (HD) of 3, and the 3 bit flips must happen in different magic + bytes for the test to fail. In any case, the option '--trailing-error' + guarantees that any corrupt header will be detected. + + Trailing data are in no way part of the lzip file format, but tools +reading lzip files are expected to behave as correctly and usefully as +possible in the presence of trailing data. + + Trailing data can be safely ignored in most cases. In some cases, like +that of user-added data, they are expected to be ignored. In those cases +where a file containing trailing data must be rejected, the option +'--trailing-error' can be used. *Note --trailing-error::. + + +File: lzip.info, Node: Examples, Next: Problems, Prev: Trailing data, Up: Top + +9 A small tutorial with 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 +option '--keep' to lzip and don't remove the original file until you verify +the compressed file with a command like 'lzip -cd file.lz | cmp file -'. +Most RAM errors happening during compression can only be detected by +comparing the compressed file with the original because the corruption +happens before lzip compresses the RAM contents, resulting in a valid +compressed file containing wrong data. + + +Example 1: Extract all the files from archive 'foo.tar.lz'. + + tar -xf foo.tar.lz + or + lzip -cd foo.tar.lz | tar -xf - + + +Example 2: Replace a regular file with its compressed version 'file.lz' and +show the compression ratio. + + lzip -v file + + +Example 3: Like example 2 but the created 'file.lz' is multimember with a +member size of 1 MiB. The compression ratio is not shown. + + lzip -b 1MiB file + + +Example 4: Restore a regular file from its compressed version 'file.lz'. If +the operation is successful, 'file.lz' is removed. + + lzip -d file.lz + + +Example 5: Verify the integrity of the compressed file 'file.lz' and show +status. + + lzip -tv file.lz + + +Example 6: The right way of concatenating the decompressed output of two or +more compressed files. *Note Trailing data::. + + Don't do this + cat file1.lz file2.lz file3.lz | lzip -d - + Do this instead + lzip -cd file1.lz file2.lz file3.lz + + +Example 7: Decompress 'file.lz' partially until 10 KiB of decompressed data +are produced. + + lzip -cd file.lz | dd bs=1024 count=10 + + +Example 8: Decompress 'file.lz' partially from decompressed byte at offset +10000 to decompressed byte at offset 14999 (5000 bytes are produced). + + lzip -cd file.lz | dd bs=1000 skip=10 count=5 + + +Example 9: Compress a whole device in /dev/sdc and send the output to +'file.lz'. + + lzip -c /dev/sdc > file.lz + or + lzip /dev/sdc -o file.lz + + +Example 10: Create a multivolume compressed tar archive with a volume size +of 1440 KiB. + + tar -c some_directory | lzip -S 1440KiB -o volume_name - + + +Example 11: Extract a multivolume compressed tar archive. + + lzip -cd volume_name*.lz | tar -xf - + + +Example 12: Create a multivolume compressed backup of a large database file +with a volume size of 650 MB, where each volume is a multimember file with +a member size of 32 MiB. + + lzip -b 32MiB -S 650MB big_db + + +File: lzip.info, Node: Problems, Next: Reference source code, Prev: Examples, Up: Top + +10 Reporting bugs +***************** + +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 +<lzip-bug@nongnu.org>. Include the version number, which you can find by +running 'lzip --version'. + + +File: lzip.info, Node: Reference source code, Next: Concept index, Prev: Problems, Up: Top + +Appendix A Reference source code +******************************** + +/* Lzd - Educational decompressor for the lzip format + Copyright (C) 2013-2022 Antonio Diaz Diaz. + + This program is free software. Redistribution and use in source and + binary forms, with or without modification, are permitted provided + that the following conditions are met: + + 1. Redistributions of source code must retain the above copyright + notice, this list of conditions, and the following disclaimer. + + 2. Redistributions in binary form must reproduce the above copyright + notice, this list of conditions, and the following disclaimer in the + documentation and/or other materials provided with the distribution. + + 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 <algorithm> +#include <cerrno> +#include <cstdio> +#include <cstdlib> +#include <cstring> +#include <stdint.h> +#include <unistd.h> +#if defined __MSVCRT__ || defined __OS2__ || defined __DJGPP__ +#include <fcntl.h> +#include <io.h> +#endif + + +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() + { + 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, + literal_pos_state_bits = 0, // not used + 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 Lzip_header[6]; // 0-3 magic bytes + // 4 version + // 5 coded dictionary size +typedef uint8_t Lzip_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 + { + unsigned long long member_pos; + uint32_t code; + uint32_t range; + +public: + Range_decoder() : member_pos( 6 ), code( 0 ), range( 0xFFFFFFFFU ) + { + for( int i = 0; i < 5; ++i ) code = ( code << 8 ) | get_byte(); + } + + uint8_t get_byte() { ++member_pos; return std::getc( stdin ); } + unsigned long long member_position() const { return member_pos; } + + unsigned decode( const int num_bits ) + { + unsigned symbol = 0; + for( int i = num_bits; i > 0; --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; + } + + unsigned decode_bit( Bit_model & bm ) + { + unsigned 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; + } + + unsigned decode_tree( Bit_model bm[], const int num_bits ) + { + unsigned symbol = 1; + for( int i = 0; i < num_bits; ++i ) + symbol = ( symbol << 1 ) | decode_bit( bm[symbol] ); + return symbol - ( 1 << num_bits ); + } + + unsigned decode_tree_reversed( Bit_model bm[], const int num_bits ) + { + unsigned symbol = decode_tree( bm, num_bits ); + unsigned reversed_symbol = 0; + for( int i = 0; i < num_bits; ++i ) + { + reversed_symbol = ( reversed_symbol << 1 ) | ( symbol & 1 ); + symbol >>= 1; + } + return reversed_symbol; + } + + unsigned decode_matched( Bit_model bm[], const unsigned match_byte ) + { + unsigned symbol = 1; + for( int i = 7; i >= 0; --i ) + { + const unsigned match_bit = ( match_byte >> i ) & 1; + const unsigned bit = decode_bit( bm[symbol+(match_bit<<8)+0x100] ); + symbol = ( symbol << 1 ) | bit; + if( match_bit != bit ) + { + while( symbol < 0x100 ) + symbol = ( symbol << 1 ) | decode_bit( bm[symbol] ); + break; + } + } + return symbol & 0xFF; + } + + unsigned 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_; + bool pos_wrapped; + + void flush_data(); + + uint8_t peek( const unsigned distance ) const + { + if( pos > distance ) return buffer[pos - distance - 1]; + if( pos_wrapped ) return buffer[dictionary_size + pos - distance - 1]; + return 0; // prev_byte of first byte + } + + void put_byte( const uint8_t b ) + { + buffer[pos] = b; + if( ++pos >= dictionary_size ) flush_data(); + } + +public: + explicit 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 ), + pos_wrapped( false ) + {} + + ~LZ_decoder() { delete[] buffer; } + + unsigned crc() const { return crc_ ^ 0xFFFFFFFFU; } + unsigned long long data_position() const + { return partial_data_pos + pos; } + uint8_t get_byte() { return rdec.get_byte(); } + unsigned long long member_position() const + { return rdec.member_position(); } + + 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 ); + 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; pos_wrapped = true; } + stream_pos = pos; + } + } + + +bool LZ_decoder::decode_member() // Returns false if error + { + Bit_model bm_literal[1<<literal_context_bits][0x300]; + Bit_model bm_match[State::states][pos_states]; + Bit_model bm_rep[State::states]; + Bit_model bm_rep0[State::states]; + Bit_model bm_rep1[State::states]; + Bit_model bm_rep2[State::states]; + Bit_model bm_len[State::states][pos_states]; + Bit_model bm_dis_slot[len_states][1<<dis_slot_bits]; + Bit_model bm_dis[modeled_distances-end_dis_model+1]; + Bit_model bm_align[dis_align_size]; + Len_model match_len_model; + Len_model rep_len_model; + unsigned rep0 = 0; // rep[0-3] latest four distances + unsigned rep1 = 0; // used for efficient coding of + unsigned rep2 = 0; // repeated distances + unsigned rep3 = 0; + State state; + + while( !std::feof( stdin ) && !std::ferror( stdin ) ) + { + const int pos_state = data_position() & pos_state_mask; + if( rdec.decode_bit( bm_match[state()][pos_state] ) == 0 ) // 1st bit + { + // literal byte + const uint8_t prev_byte = peek( 0 ); + const int literal_state = prev_byte >> ( 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, peek( rep0 ) ) ); + state.set_char(); + continue; + } + // match or repeated match + int len; + if( rdec.decode_bit( bm_rep[state()] ) != 0 ) // 2nd bit + { + if( rdec.decode_bit( bm_rep0[state()] ) == 0 ) // 3rd bit + { + if( rdec.decode_bit( bm_len[state()][pos_state] ) == 0 ) // 4th bit + { state.set_short_rep(); put_byte( peek( rep0 ) ); continue; } + } + else + { + 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; + } + state.set_rep(); + len = min_match_len + rdec.decode_len( rep_len_model, pos_state ); + } + else // match + { + 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 ); + rep0 = rdec.decode_tree( bm_dis_slot[len_state], dis_slot_bits ); + if( rep0 >= start_dis_model ) + { + const unsigned dis_slot = rep0; + 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 ), + 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 >= pos && !pos_wrapped ) ) + { flush_data(); return false; } + } + for( int i = 0; i < len; ++i ) put_byte( peek( rep0 ) ); + } + flush_data(); + return false; + } + + +int main( const int argc, const char * const argv[] ) + { + if( argc > 2 || ( argc == 2 && std::strcmp( argv[1], "-d" ) != 0 ) ) + { + std::printf( + "Lzd %s - Educational decompressor for the lzip format.\n" + "Study the source to learn how a lzip decompressor works.\n" + "See the lzip manual for an explanation of the code.\n" + "\nUsage: %s [-d] < file.lz > file\n" + "Lzd decompresses from standard input to standard output.\n" + "\nCopyright (C) 2022 Antonio Diaz Diaz.\n" + "License 2-clause BSD.\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", + PROGVERSION, argv[0] ); + return 0; + } + +#if defined __MSVCRT__ || defined __OS2__ || defined __DJGPP__ + setmode( STDIN_FILENO, O_BINARY ); + setmode( STDOUT_FILENO, O_BINARY ); +#endif + + for( bool first_member = true; ; first_member = false ) + { + Lzip_header header; // verify header + for( int i = 0; i < 6; ++i ) header[i] = std::getc( stdin ); + if( std::feof( stdin ) || std::memcmp( header, "LZIP\x01", 5 ) != 0 ) + { + if( first_member ) + { std::fputs( "Bad magic number (file not in lzip format).\n", + stderr ); return 2; } + break; // ignore trailing data + } + 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::fputs( "Invalid dictionary size in member header.\n", stderr ); + return 2; } + + LZ_decoder decoder( dict_size ); // decode LZMA stream + if( !decoder.decode_member() ) + { std::fputs( "Data error\n", stderr ); return 2; } + + Lzip_trailer trailer; // verify trailer + for( int i = 0; i < 20; ++i ) trailer[i] = decoder.get_byte(); + int retval = 0; + unsigned crc = 0; + for( int i = 3; i >= 0; --i ) crc = ( crc << 8 ) + trailer[i]; + if( crc != decoder.crc() ) + { std::fputs( "CRC mismatch\n", stderr ); retval = 2; } + + unsigned long long data_size = 0; + for( int i = 11; i >= 4; --i ) + data_size = ( data_size << 8 ) + trailer[i]; + if( data_size != decoder.data_position() ) + { std::fputs( "Data size mismatch\n", stderr ); retval = 2; } + + unsigned long long member_size = 0; + for( int i = 19; i >= 12; --i ) + member_size = ( member_size << 8 ) + trailer[i]; + if( member_size != decoder.member_position() ) + { std::fputs( "Member size mismatch\n", stderr ); retval = 2; } + if( retval ) return retval; + } + + if( std::fclose( stdout ) != 0 ) + { std::fprintf( stderr, "Error closing stdout: %s\n", + std::strerror( errno ) ); return 1; } + return 0; + } + + +File: lzip.info, Node: Concept index, Prev: Reference source code, Up: Top + +Concept index +************* + + +* Menu: + +* algorithm: Algorithm. (line 6) +* bugs: Problems. (line 6) +* examples: Examples. (line 6) +* file format: File format. (line 6) +* format of the LZMA stream: Stream format. (line 6) +* getting help: Problems. (line 6) +* introduction: Introduction. (line 6) +* invoking: Invoking lzip. (line 6) +* options: Invoking lzip. (line 6) +* output: Output. (line 6) +* quality assurance: Quality assurance. (line 6) +* reference source code: Reference source code. (line 6) +* trailing data: Trailing data. (line 6) +* usage: Invoking lzip. (line 6) +* version: Invoking lzip. (line 6) + + + +Tag Table: +Node: Top203 +Node: Introduction1198 +Node: Output6972 +Node: Invoking lzip8567 +Ref: --trailing-error9356 +Node: Quality assurance18682 +Node: Algorithm27705 +Node: File format31109 +Ref: coded-dict-size32538 +Node: Stream format33773 +Ref: what-is-coded36169 +Node: Trailing data45097 +Node: Examples47358 +Ref: concat-example48800 +Node: Problems50021 +Node: Reference source code50553 +Node: Concept index65411 + +End Tag Table + + +Local Variables: +coding: iso-8859-15 +End: |