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Diffstat (limited to 'README')
| -rw-r--r-- | README | 76 |
1 files changed, 40 insertions, 36 deletions
@@ -11,35 +11,34 @@ compatible with lzip-1.4 or newer, and can be rescued with lziprecover. Clzip is in fact a C language version of lzip, intended for embedded devices or systems lacking a C++ compiler. -The lzip file format is designed for long-term data archiving and -provides very safe integrity checking. It is as simple as possible (but -not simpler), so that with the only help of the lzip manual it would be -possible for a digital archaeologist to extract the data from a lzip -file long after quantum computers eventually render LZMA obsolete. -Additionally lzip is copylefted, which guarantees that it will remain -free forever. - -The 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 clzip (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. +The lzip file format is designed for long-term data archiving, taking +into account both data integrity and decoder availability: + + * The lzip format provides very safe integrity checking and some data + recovery means. The lziprecover program can repair bit-flip errors + (one of the most common forms of data corruption) in lzip files, + and provides data recovery capabilities, including error-checked + merging of damaged copies of a file. + + * The lzip format is as simple as possible (but not simpler). The + lzip manual provides the code of a simple decompressor along with a + detailed explanation of how it works, so that with the only help of + the lzip manual it would be possible for a digital archaeologist to + extract the data from a lzip file long after quantum computers + eventually render LZMA obsolete. + + * Additionally lzip is copylefted, which guarantees that it will + remain free forever. Clzip uses the same well-defined exit status values used by lzip and bzip2, which makes it safer than compressors returning ambiguous warning values (like gzip) when it is used as a back end for tar or zutils. +Clzip will automatically use the smallest possible dictionary size for +each file without exceeding the given limit. Keep in mind that the +decompression memory requirement is affected at compression time by the +choice of dictionary size limit. + When compressing, clzip replaces every file given in the command line with a compressed version of itself, with the name "original_name.lz". When decompressing, clzip attempts to guess the name for the decompressed @@ -78,18 +77,23 @@ Clzip is able to compress and decompress streams of unlimited size by automatically creating multi-member output. The members so created are large, about 64 PiB each. -Clzip will automatically use the smallest possible dictionary size -without exceeding the given limit. Keep in mind that the decompression -memory requirement is affected at compression time by the choice of -dictionary size limit. - -Clzip implements a simplified version of the LZMA (Lempel-Ziv-Markov -chain-Algorithm) algorithm. The high compression of LZMA comes from -combining two basic, well-proven compression ideas: sliding dictionaries -(LZ77/78) and markov models (the thing used by every compression -algorithm that uses a range encoder or similar order-0 entropy coder as -its last stage) with segregation of contexts according to what the bits -are used for. +There is no such thing as a "LZMA algorithm"; it is more like a "LZMA +coding scheme". For example, the option '-0' of lzip uses the scheme in +almost the simplest way possible; issuing the longest match it can find, +or a literal byte if it can't find a match. Inversely, a much more +elaborated way of finding coding sequences of minimum price than the one +currently used by lzip could be developed, and the resulting sequence +could also be coded using the LZMA coding scheme. + +Lzip currently implements two variants of the LZMA algorithm; fast (used +by option -0) and normal (used by all other compression levels). Clzip +just implements the "normal" variant. + +The high compression of LZMA comes from combining two basic, well-proven +compression ideas: sliding dictionaries (LZ77/78) and markov models (the +thing used by every compression algorithm that uses a range encoder or +similar order-0 entropy coder as its last stage) with segregation of +contexts according to what the bits are used for. The ideas embodied in clzip are due to (at least) the following people: Abraham Lempel and Jacob Ziv (for the LZ algorithm), Andrey Markov (for |