mirror of https://git.tukaani.org/xz.git
Minor updates to the file format specification.
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Lasse Collin
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@ -43,10 +43,11 @@ The .lzma File Format
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5.1. Alignment
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5.2. Security
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5.3. Filters
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5.3.1. LZMA2
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5.3.2. Branch/Call/Jump Filters for Executables
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5.3.3. Delta
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5.3.3.1. Format of the Encoded Output
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5.3.1. LZMA
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5.3.2. LZMA2
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5.3.3. Branch/Call/Jump Filters for Executables
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5.3.4. Delta
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5.3.4.1. Format of the Encoded Output
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5.4. Custom Filter IDs
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5.4.1. Reserved Custom Filter ID Ranges
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6. Cyclic Redundancy Checks
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@ -85,7 +86,7 @@ The .lzma File Format
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0.2. Changes
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Last modified: 2008-06-17 14:10+0300
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Last modified: 2008-09-03 14:10+0300
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(A changelog will be kept once the first official version
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is made.)
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@ -530,6 +531,10 @@ The .lzma File Format
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officially defined Filter IDs and the formats of their Filter
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Properties are described in Section 5.3.
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Filter IDs greater than or equal to 0x4000_0000_0000_0000
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(2^62) are reserved for implementation-specific internal use.
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These Filter IDs must never be used in List of Filter Flags.
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3.1.6. Header Padding
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@ -765,20 +770,15 @@ The .lzma File Format
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5.3. Filters
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5.3.1. LZMA2
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5.3.1. LZMA
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LZMA (Lempel-Ziv-Markov chain-Algorithm) is a general-purporse
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compression algorithm with high compression ratio and fast
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decompression. LZMA is based on LZ77 and range coding
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algorithms.
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LZMA2 uses LZMA internally, but adds support for uncompressed
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chunks, eases stateful decoder implementations, and improves
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support for multithreading. Thus, the plain LZMA will not be
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supported in this file format.
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Filter ID: 0x21
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Size of Filter Properties: 1 byte
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Filter ID: 0x40
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Size of Filter Properties: 5 bytes
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Changes size of data: Yes
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Allow as a non-last filter: No
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Allow as the last filter: Yes
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@ -793,6 +793,66 @@ The .lzma File Format
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a separate document, because including the documentation here
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would lengthen this document considerably.
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The format of the Filter Properties field is as follows:
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+-----------------+----+----+----+----+
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| LZMA Properties | Dictionary Size |
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+-----------------+----+----+----+----+
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The LZMA Properties field contains three properties. An
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abbreviation is given in parentheses, followed by the value
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range of the property. The field consists of
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1) the number of literal context bits (lc, [0, 4]);
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2) the number of literal position bits (lp, [0, 4]); and
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3) the number of position bits (pb, [0, 4]).
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In addition to above ranges, the sum of lc and lp must not
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exceed four. Note that this limit didn't exist in the old
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LZMA_Alone format, which allowed lc to be in the range [0, 8].
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The properties are encoded using the following formula:
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LZMA Properties = (pb * 5 + lp) * 9 + lc
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The following C code illustrates a straightforward way to
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decode the properties:
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uint8_t lc, lp, pb;
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uint8_t prop = get_lzma_properties();
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if (prop > (4 * 5 + 4) * 9 + 8)
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return LZMA_PROPERTIES_ERROR;
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pb = prop / (9 * 5);
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prop -= pb * 9 * 5;
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lp = prop / 9;
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lc = prop - lp * 9;
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if (lc + lp > 4)
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return LZMA_PROPERTIES_ERROR;
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Dictionary Size is encoded as unsigned 32-bit little endian
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integer.
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5.3.2. LZMA2
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LZMA2 is an extensions on top of the original LZMA. LZMA2 uses
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LZMA internally, but adds support for flushing the encoder,
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uncompressed chunks, eases stateful decoder implementations,
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and improves support for multithreading. For most uses, it is
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recommended to use LZMA2 instead of LZMA.
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Filter ID: 0x21
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Size of Filter Properties: 1 byte
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Changes size of data: Yes
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Allow as a non-last filter: No
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Allow as the last filter: Yes
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Preferred alignment:
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Input data: Adjustable to 1/2/4/8/16 byte(s)
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Output data: 1 byte
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The format of the one-byte Filter Properties field is as
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follows:
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@ -818,7 +878,7 @@ The .lzma File Format
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37 3 29 1536 MiB
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38 2 30 2048 MiB
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39 3 30 3072 MiB
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40 2 31 4096 MiB
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40 2 31 4096 MiB - 1 B
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Instead of having a table in the decoder, the dictionary size
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can be decoded using the following C code:
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@ -827,11 +887,16 @@ The .lzma File Format
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if (bits > 40)
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return DICTIONARY_TOO_BIG; // Bigger than 4 GiB
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uint32_t dictionary_size = 2 | (bits & 1);
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uint32_t dictionary_size;
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if (bits == 40) {
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dictionary_size = UINT32_MAX;
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} else {
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dictionary_size = 2 | (bits & 1);
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dictionary_size <<= bits / 2 + 11;
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}
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5.3.2. Branch/Call/Jump Filters for Executables
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5.3.3. Branch/Call/Jump Filters for Executables
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These filters convert relative branch, call, and jump
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instructions to their absolute counterparts in executable
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@ -871,7 +936,7 @@ The .lzma File Format
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the Subblock filter.
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5.3.3. Delta
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5.3.4. Delta
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The Delta filter may increase compression ratio when the value
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of the next byte correlates with the value of an earlier byte
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@ -892,7 +957,7 @@ The .lzma File Format
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distance of 1 byte and 0xFF distance of 256 bytes.
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5.3.3.1. Format of the Encoded Output
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5.3.4.1. Format of the Encoded Output
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The code below illustrates both encoding and decoding with
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the Delta filter.
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@ -944,7 +1009,7 @@ The .lzma File Format
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Bits Mask Description
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0-15 0x0000_0000_0000_FFFF Filter ID
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16-55 0x00FF_FFFF_FFFF_0000 Developer ID
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56-62 0x7F00_0000_0000_0000 Static prefix: 0x7F
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56-62 0x3F00_0000_0000_0000 Static prefix: 0x3F
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The resulting 63-bit integer will use 9 bytes of space when
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stored using the encoding described in Section 1.2. To get
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