diff options
Diffstat (limited to 'zlib/doc')
| -rw-r--r-- | zlib/doc/algorithm.txt | 209 | ||||
| -rw-r--r-- | zlib/doc/rfc1950.txt | 619 | ||||
| -rw-r--r-- | zlib/doc/rfc1951.txt | 955 | ||||
| -rw-r--r-- | zlib/doc/rfc1952.txt | 675 | ||||
| -rw-r--r-- | zlib/doc/txtvsbin.txt | 107 |
5 files changed, 0 insertions, 2565 deletions
diff --git a/zlib/doc/algorithm.txt b/zlib/doc/algorithm.txt deleted file mode 100644 index c97f495..0000000 --- a/zlib/doc/algorithm.txt +++ /dev/null @@ -1,209 +0,0 @@ -1. Compression algorithm (deflate) - -The deflation algorithm used by gzip (also zip and zlib) is a variation of -LZ77 (Lempel-Ziv 1977, see reference below). It finds duplicated strings in -the input data. The second occurrence of a string is replaced by a -pointer to the previous string, in the form of a pair (distance, -length). Distances are limited to 32K bytes, and lengths are limited -to 258 bytes. When a string does not occur anywhere in the previous -32K bytes, it is emitted as a sequence of literal bytes. (In this -description, `string' must be taken as an arbitrary sequence of bytes, -and is not restricted to printable characters.) - -Literals or match lengths are compressed with one Huffman tree, and -match distances are compressed with another tree. The trees are stored -in a compact form at the start of each block. The blocks can have any -size (except that the compressed data for one block must fit in -available memory). A block is terminated when deflate() determines that -it would be useful to start another block with fresh trees. (This is -somewhat similar to the behavior of LZW-based _compress_.) - -Duplicated strings are found using a hash table. All input strings of -length 3 are inserted in the hash table. A hash index is computed for -the next 3 bytes. If the hash chain for this index is not empty, all -strings in the chain are compared with the current input string, and -the longest match is selected. - -The hash chains are searched starting with the most recent strings, to -favor small distances and thus take advantage of the Huffman encoding. -The hash chains are singly linked. There are no deletions from the -hash chains, the algorithm simply discards matches that are too old. - -To avoid a worst-case situation, very long hash chains are arbitrarily -truncated at a certain length, determined by a runtime option (level -parameter of deflateInit). So deflate() does not always find the longest -possible match but generally finds a match which is long enough. - -deflate() also defers the selection of matches with a lazy evaluation -mechanism. After a match of length N has been found, deflate() searches for -a longer match at the next input byte. If a longer match is found, the -previous match is truncated to a length of one (thus producing a single -literal byte) and the process of lazy evaluation begins again. Otherwise, -the original match is kept, and the next match search is attempted only N -steps later. - -The lazy match evaluation is also subject to a runtime parameter. If -the current match is long enough, deflate() reduces the search for a longer -match, thus speeding up the whole process. If compression ratio is more -important than speed, deflate() attempts a complete second search even if -the first match is already long enough. - -The lazy match evaluation is not performed for the fastest compression -modes (level parameter 1 to 3). For these fast modes, new strings -are inserted in the hash table only when no match was found, or -when the match is not too long. This degrades the compression ratio -but saves time since there are both fewer insertions and fewer searches. - - -2. Decompression algorithm (inflate) - -2.1 Introduction - -The key question is how to represent a Huffman code (or any prefix code) so -that you can decode fast. The most important characteristic is that shorter -codes are much more common than longer codes, so pay attention to decoding the -short codes fast, and let the long codes take longer to decode. - -inflate() sets up a first level table that covers some number of bits of -input less than the length of longest code. It gets that many bits from the -stream, and looks it up in the table. The table will tell if the next -code is that many bits or less and how many, and if it is, it will tell -the value, else it will point to the next level table for which inflate() -grabs more bits and tries to decode a longer code. - -How many bits to make the first lookup is a tradeoff between the time it -takes to decode and the time it takes to build the table. If building the -table took no time (and if you had infinite memory), then there would only -be a first level table to cover all the way to the longest code. However, -building the table ends up taking a lot longer for more bits since short -codes are replicated many times in such a table. What inflate() does is -simply to make the number of bits in the first table a variable, and then -to set that variable for the maximum speed. - -For inflate, which has 286 possible codes for the literal/length tree, the size -of the first table is nine bits. Also the distance trees have 30 possible -values, and the size of the first table is six bits. Note that for each of -those cases, the table ended up one bit longer than the ``average'' code -length, i.e. the code length of an approximately flat code which would be a -little more than eight bits for 286 symbols and a little less than five bits -for 30 symbols. - - -2.2 More details on the inflate table lookup - -Ok, you want to know what this cleverly obfuscated inflate tree actually -looks like. You are correct that it's not a Huffman tree. It is simply a -lookup table for the first, let's say, nine bits of a Huffman symbol. The -symbol could be as short as one bit or as long as 15 bits. If a particular -symbol is shorter than nine bits, then that symbol's translation is duplicated -in all those entries that start with that symbol's bits. For example, if the -symbol is four bits, then it's duplicated 32 times in a nine-bit table. If a -symbol is nine bits long, it appears in the table once. - -If the symbol is longer than nine bits, then that entry in the table points -to another similar table for the remaining bits. Again, there are duplicated -entries as needed. The idea is that most of the time the symbol will be short -and there will only be one table look up. (That's whole idea behind data -compression in the first place.) For the less frequent long symbols, there -will be two lookups. If you had a compression method with really long -symbols, you could have as many levels of lookups as is efficient. For -inflate, two is enough. - -So a table entry either points to another table (in which case nine bits in -the above example are gobbled), or it contains the translation for the symbol -and the number of bits to gobble. Then you start again with the next -ungobbled bit. - -You may wonder: why not just have one lookup table for how ever many bits the -longest symbol is? The reason is that if you do that, you end up spending -more time filling in duplicate symbol entries than you do actually decoding. -At least for deflate's output that generates new trees every several 10's of -kbytes. You can imagine that filling in a 2^15 entry table for a 15-bit code -would take too long if you're only decoding several thousand symbols. At the -other extreme, you could make a new table for every bit in the code. In fact, -that's essentially a Huffman tree. But then you spend too much time -traversing the tree while decoding, even for short symbols. - -So the number of bits for the first lookup table is a trade of the time to -fill out the table vs. the time spent looking at the second level and above of -the table. - -Here is an example, scaled down: - -The code being decoded, with 10 symbols, from 1 to 6 bits long: - -A: 0 -B: 10 -C: 1100 -D: 11010 -E: 11011 -F: 11100 -G: 11101 -H: 11110 -I: 111110 -J: 111111 - -Let's make the first table three bits long (eight entries): - -000: A,1 -001: A,1 -010: A,1 -011: A,1 -100: B,2 -101: B,2 -110: -> table X (gobble 3 bits) -111: -> table Y (gobble 3 bits) - -Each entry is what the bits decode as and how many bits that is, i.e. how -many bits to gobble. Or the entry points to another table, with the number of -bits to gobble implicit in the size of the table. - -Table X is two bits long since the longest code starting with 110 is five bits -long: - -00: C,1 -01: C,1 -10: D,2 -11: E,2 - -Table Y is three bits long since the longest code starting with 111 is six -bits long: - -000: F,2 -001: F,2 -010: G,2 -011: G,2 -100: H,2 -101: H,2 -110: I,3 -111: J,3 - -So what we have here are three tables with a total of 20 entries that had to -be constructed. That's compared to 64 entries for a single table. Or -compared to 16 entries for a Huffman tree (six two entry tables and one four -entry table). Assuming that the code ideally represents the probability of -the symbols, it takes on the average 1.25 lookups per symbol. That's compared -to one lookup for the single table, or 1.66 lookups per symbol for the -Huffman tree. - -There, I think that gives you a picture of what's going on. For inflate, the -meaning of a particular symbol is often more than just a letter. It can be a -byte (a "literal"), or it can be either a length or a distance which -indicates a base value and a number of bits to fetch after the code that is -added to the base value. Or it might be the special end-of-block code. The -data structures created in inftrees.c try to encode all that information -compactly in the tables. - - -Jean-loup Gailly Mark Adler -jloup@gzip.org madler@alumni.caltech.edu - - -References: - -[LZ77] Ziv J., Lempel A., ``A Universal Algorithm for Sequential Data -Compression,'' IEEE Transactions on Information Theory, Vol. 23, No. 3, -pp. 337-343. - -``DEFLATE Compressed Data Format Specification'' available in -http://tools.ietf.org/html/rfc1951 diff --git a/zlib/doc/rfc1950.txt b/zlib/doc/rfc1950.txt deleted file mode 100644 index ce6428a..0000000 --- a/zlib/doc/rfc1950.txt +++ /dev/null @@ -1,619 +0,0 @@ - - - - - - -Network Working Group P. Deutsch -Request for Comments: 1950 Aladdin Enterprises -Category: Informational J-L. Gailly - Info-ZIP - May 1996 - - - ZLIB Compressed Data Format Specification version 3.3 - -Status of This Memo - - This memo provides information for the Internet community. This memo - does not specify an Internet standard of any kind. Distribution of - this memo is unlimited. - -IESG Note: - - The IESG takes no position on the validity of any Intellectual - Property Rights statements contained in this document. - -Notices - - Copyright (c) 1996 L. Peter Deutsch and Jean-Loup Gailly - - Permission is granted to copy and distribute this document for any - purpose and without charge, including translations into other - languages and incorporation into compilations, provided that the - copyright notice and this notice are preserved, and that any - substantive changes or deletions from the original are clearly - marked. - - A pointer to the latest version of this and related documentation in - HTML format can be found at the URL - <ftp://ftp.uu.net/graphics/png/documents/zlib/zdoc-index.html>. - -Abstract - - This specification defines a lossless compressed data format. The - data can be produced or consumed, even for an arbitrarily long - sequentially presented input data stream, using only an a priori - bounded amount of intermediate storage. The format presently uses - the DEFLATE compression method but can be easily extended to use - other compression methods. It can be implemented readily in a manner - not covered by patents. This specification also defines the ADLER-32 - checksum (an extension and improvement of the Fletcher checksum), - used for detection of data corruption, and provides an algorithm for - computing it. - - - - -Deutsch & Gailly Informational [Page 1] - -RFC 1950 ZLIB Compressed Data Format Specification May 1996 - - -Table of Contents - - 1. Introduction ................................................... 2 - 1.1. Purpose ................................................... 2 - 1.2. Intended audience ......................................... 3 - 1.3. Scope ..................................................... 3 - 1.4. Compliance ................................................ 3 - 1.5. Definitions of terms and conventions used ................ 3 - 1.6. Changes from previous versions ............................ 3 - 2. Detailed specification ......................................... 3 - 2.1. Overall conventions ....................................... 3 - 2.2. Data format ............................................... 4 - 2.3. Compliance ................................................ 7 - 3. References ..................................................... 7 - 4. Source code .................................................... 8 - 5. Security Considerations ........................................ 8 - 6. Acknowledgements ............................................... 8 - 7. Authors' Addresses ............................................. 8 - 8. Appendix: Rationale ............................................ 9 - 9. Appendix: Sample code ..........................................10 - -1. Introduction - - 1.1. Purpose - - The purpose of this specification is to define a lossless - compressed data format that: - - * Is independent of CPU type, operating system, file system, - and character set, and hence can be used for interchange; - - * Can be produced or consumed, even for an arbitrarily long - sequentially presented input data stream, using only an a - priori bounded amount of intermediate storage, and hence can - be used in data communications or similar structures such as - Unix filters; - - * Can use a number of different compression methods; - - * Can be implemented readily in a manner not covered by - patents, and hence can be practiced freely. - - The data format defined by this specification does not attempt to - allow random access to compressed data. - - - - - - - -Deutsch & Gailly Informational [Page 2] - -RFC 1950 ZLIB Compressed Data Format Specification May 1996 - - - 1.2. Intended audience - - This specification is intended for use by implementors of software - to compress data into zlib format and/or decompress data from zlib - format. - - The text of the specification assumes a basic background in - programming at the level of bits and other primitive data - representations. - - 1.3. Scope - - The specification specifies a compressed data format that can be - used for in-memory compression of a sequence of arbitrary bytes. - - 1.4. Compliance - - Unless otherwise indicated below, a compliant decompressor must be - able to accept and decompress any data set that conforms to all - the specifications presented here; a compliant compressor must - produce data sets that conform to all the specifications presented - here. - - 1.5. Definitions of terms and conventions used - - byte: 8 bits stored or transmitted as a unit (same as an octet). - (For this specification, a byte is exactly 8 bits, even on - machines which store a character on a number of bits different - from 8.) See below, for the numbering of bits within a byte. - - 1.6. Changes from previous versions - - Version 3.1 was the first public release of this specification. - In version 3.2, some terminology was changed and the Adler-32 - sample code was rewritten for clarity. In version 3.3, the - support for a preset dictionary was introduced, and the - specification was converted to RFC style. - -2. Detailed specification - - 2.1. Overall conventions - - In the diagrams below, a box like this: - - +---+ - | | <-- the vertical bars might be missing - +---+ - - - - -Deutsch & Gailly Informational [Page 3] - -RFC 1950 ZLIB Compressed Data Format Specification May 1996 - - - represents one byte; a box like this: - - +==============+ - | | - +==============+ - - represents a variable number of bytes. - - Bytes stored within a computer do not have a "bit order", since - they are always treated as a unit. However, a byte considered as - an integer between 0 and 255 does have a most- and least- - significant bit, and since we write numbers with the most- - significant digit on the left, we also write bytes with the most- - significant bit on the left. In the diagrams below, we number the - bits of a byte so that bit 0 is the least-significant bit, i.e., - the bits are numbered: - - +--------+ - |76543210| - +--------+ - - Within a computer, a number may occupy multiple bytes. All - multi-byte numbers in the format described here are stored with - the MOST-significant byte first (at the lower memory address). - For example, the decimal number 520 is stored as: - - 0 1 - +--------+--------+ - |00000010|00001000| - +--------+--------+ - ^ ^ - | | - | + less significant byte = 8 - + more significant byte = 2 x 256 - - 2.2. Data format - - A zlib stream has the following structure: - - 0 1 - +---+---+ - |CMF|FLG| (more-->) - +---+---+ - - - - - - - - -Deutsch & Gailly Informational [Page 4] - -RFC 1950 ZLIB Compressed Data Format Specification May 1996 - - - (if FLG.FDICT set) - - 0 1 2 3 - +---+---+---+---+ - | DICTID | (more-->) - +---+---+---+---+ - - +=====================+---+---+---+---+ - |...compressed data...| ADLER32 | - +=====================+---+---+---+---+ - - Any data which may appear after ADLER32 are not part of the zlib - stream. - - CMF (Compression Method and flags) - This byte is divided into a 4-bit compression method and a 4- - bit information field depending on the compression method. - - bits 0 to 3 CM Compression method - bits 4 to 7 CINFO Compression info - - CM (Compression method) - This identifies the compression method used in the file. CM = 8 - denotes the "deflate" compression method with a window size up - to 32K. This is the method used by gzip and PNG (see - references [1] and [2] in Chapter 3, below, for the reference - documents). CM = 15 is reserved. It might be used in a future - version of this specification to indicate the presence of an - extra field before the compressed data. - - CINFO (Compression info) - For CM = 8, CINFO is the base-2 logarithm of the LZ77 window - size, minus eight (CINFO=7 indicates a 32K window size). Values - of CINFO above 7 are not allowed in this version of the - specification. CINFO is not defined in this specification for - CM not equal to 8. - - FLG (FLaGs) - This flag byte is divided as follows: - - bits 0 to 4 FCHECK (check bits for CMF and FLG) - bit 5 FDICT (preset dictionary) - bits 6 to 7 FLEVEL (compression level) - - The FCHECK value must be such that CMF and FLG, when viewed as - a 16-bit unsigned integer stored in MSB order (CMF*256 + FLG), - is a multiple of 31. - - - - -Deutsch & Gailly Informational [Page 5] - -RFC 1950 ZLIB Compressed Data Format Specification May 1996 - - - FDICT (Preset dictionary) - If FDICT is set, a DICT dictionary identifier is present - immediately after the FLG byte. The dictionary is a sequence of - bytes which are initially fed to the compressor without - producing any compressed output. DICT is the Adler-32 checksum - of this sequence of bytes (see the definition of ADLER32 - below). The decompressor can use this identifier to determine - which dictionary has been used by the compressor. - - FLEVEL (Compression level) - These flags are available for use by specific compression - methods. The "deflate" method (CM = 8) sets these flags as - follows: - - 0 - compressor used fastest algorithm - 1 - compressor used fast algorithm - 2 - compressor used default algorithm - 3 - compressor used maximum compression, slowest algorithm - - The information in FLEVEL is not needed for decompression; it - is there to indicate if recompression might be worthwhile. - - compressed data - For compression method 8, the compressed data is stored in the - deflate compressed data format as described in the document - "DEFLATE Compressed Data Format Specification" by L. Peter - Deutsch. (See reference [3] in Chapter 3, below) - - Other compressed data formats are not specified in this version - of the zlib specification. - - ADLER32 (Adler-32 checksum) - This contains a checksum value of the uncompressed data - (excluding any dictionary data) computed according to Adler-32 - algorithm. This algorithm is a 32-bit extension and improvement - of the Fletcher algorithm, used in the ITU-T X.224 / ISO 8073 - standard. See references [4] and [5] in Chapter 3, below) - - Adler-32 is composed of two sums accumulated per byte: s1 is - the sum of all bytes, s2 is the sum of all s1 values. Both sums - are done modulo 65521. s1 is initialized to 1, s2 to zero. The - Adler-32 checksum is stored as s2*65536 + s1 in most- - significant-byte first (network) order. - - - - - - - - -Deutsch & Gailly Informational [Page 6] - -RFC 1950 ZLIB Compressed Data Format Specification May 1996 - - - 2.3. Compliance - - A compliant compressor must produce streams with correct CMF, FLG - and ADLER32, but need not support preset dictionaries. When the - zlib data format is used as part of another standard data format, - the compressor may use only preset dictionaries that are specified - by this other data format. If this other format does not use the - preset dictionary feature, the compressor must not set the FDICT - flag. - - A compliant decompressor must check CMF, FLG, and ADLER32, and - provide an error indication if any of these have incorrect values. - A compliant decompressor must give an error indication if CM is - not one of the values defined in this specification (only the - value 8 is permitted in this version), since another value could - indicate the presence of new features that would cause subsequent - data to be interpreted incorrectly. A compliant decompressor must - give an error indication if FDICT is set and DICTID is not the - identifier of a known preset dictionary. A decompressor may - ignore FLEVEL and still be compliant. When the zlib data format - is being used as a part of another standard format, a compliant - decompressor must support all the preset dictionaries specified by - the other format. When the other format does not use the preset - dictionary feature, a compliant decompressor must reject any - stream in which the FDICT flag is set. - -3. References - - [1] Deutsch, L.P.,"GZIP Compressed Data Format Specification", - available in ftp://ftp.uu.net/pub/archiving/zip/doc/ - - [2] Thomas Boutell, "PNG (Portable Network Graphics) specification", - available in ftp://ftp.uu.net/graphics/png/documents/ - - [3] Deutsch, L.P.,"DEFLATE Compressed Data Format Specification", - available in ftp://ftp.uu.net/pub/archiving/zip/doc/ - - [4] Fletcher, J. G., "An Arithmetic Checksum for Serial - Transmissions," IEEE Transactions on Communications, Vol. COM-30, - No. 1, January 1982, pp. 247-252. - - [5] ITU-T Recommendation X.224, Annex D, "Checksum Algorithms," - November, 1993, pp. 144, 145. (Available from - gopher://info.itu.ch). ITU-T X.244 is also the same as ISO 8073. - - - - - - - -Deutsch & Gailly Informational [Page 7] - -RFC 1950 ZLIB Compressed Data Format Specification May 1996 - - -4. Source code - - Source code for a C language implementation of a "zlib" compliant - library is available at ftp://ftp.uu.net/pub/archiving/zip/zlib/. - -5. Security Considerations - - A decoder that fails to check the ADLER32 checksum value may be - subject to undetected data corruption. - -6. Acknowledgements - - Trademarks cited in this document are the property of their - respective owners. - - Jean-Loup Gailly and Mark Adler designed the zlib format and wrote - the related software described in this specification. Glenn - Randers-Pehrson converted this document to RFC and HTML format. - -7. Authors' Addresses - - L. Peter Deutsch - Aladdin Enterprises - 203 Santa Margarita Ave. - Menlo Park, CA 94025 - - Phone: (415) 322-0103 (AM only) - FAX: (415) 322-1734 - EMail: <ghost@aladdin.com> - - - Jean-Loup Gailly - - EMail: <gzip@prep.ai.mit.edu> - - Questions about the technical content of this specification can be - sent by email to - - Jean-Loup Gailly <gzip@prep.ai.mit.edu> and - Mark Adler <madler@alumni.caltech.edu> - - Editorial comments on this specification can be sent by email to - - L. Peter Deutsch <ghost@aladdin.com> and - Glenn Randers-Pehrson <randeg@alumni.rpi.edu> - - - - - - -Deutsch & Gailly Informational [Page 8] - -RFC 1950 ZLIB Compressed Data Format Specification May 1996 - - -8. Appendix: Rationale - - 8.1. Preset dictionaries - - A preset dictionary is specially useful to compress short input - sequences. The compressor can take advantage of the dictionary - context to encode the input in a more compact manner. The - decompressor can be initialized with the appropriate context by - virtually decompressing a compressed version of the dictionary - without producing any output. However for certain compression - algorithms such as the deflate algorithm this operation can be - achieved without actually performing any decompression. - - The compressor and the decompressor must use exactly the same - dictionary. The dictionary may be fixed or may be chosen among a - certain number of predefined dictionaries, according to the kind - of input data. The decompressor can determine which dictionary has - been chosen by the compressor by checking the dictionary - identifier. This document does not specify the contents of - predefined dictionaries, since the optimal dictionaries are - application specific. Standard data formats using this feature of - the zlib specification must precisely define the allowed - dictionaries. - - 8.2. The Adler-32 algorithm - - The Adler-32 algorithm is much faster than the CRC32 algorithm yet - still provides an extremely low probability of undetected errors. - - The modulo on unsigned long accumulators can be delayed for 5552 - bytes, so the modulo operation time is negligible. If the bytes - are a, b, c, the second sum is 3a + 2b + c + 3, and so is position - and order sensitive, unlike the first sum, which is just a - checksum. That 65521 is prime is important to avoid a possible - large class of two-byte errors that leave the check unchanged. - (The Fletcher checksum uses 255, which is not prime and which also - makes the Fletcher check insensitive to single byte changes 0 <-> - 255.) - - The sum s1 is initialized to 1 instead of zero to make the length - of the sequence part of s2, so that the length does not have to be - checked separately. (Any sequence of zeroes has a Fletcher - checksum of zero.) - - - - - - - - -Deutsch & Gailly Informational [Page 9] - -RFC 1950 ZLIB Compressed Data Format Specification May 1996 - - -9. Appendix: Sample code - - The following C code computes the Adler-32 checksum of a data buffer. - It is written for clarity, not for speed. The sample code is in the - ANSI C programming language. Non C users may find it easier to read - with these hints: - - & Bitwise AND operator. - >> Bitwise right shift operator. When applied to an - unsigned quantity, as here, right shift inserts zero bit(s) - at the left. - << Bitwise left shift operator. Left shift inserts zero - bit(s) at the right. - ++ "n++" increments the variable n. - % modulo operator: a % b is the remainder of a divided by b. - - #define BASE 65521 /* largest prime smaller than 65536 */ - - /* - Update a running Adler-32 checksum with the bytes buf[0..len-1] - and return the updated checksum. The Adler-32 checksum should be - initialized to 1. - - Usage example: - - unsigned long adler = 1L; - - while (read_buffer(buffer, length) != EOF) { - adler = update_adler32(adler, buffer, length); - } - if (adler != original_adler) error(); - */ - unsigned long update_adler32(unsigned long adler, - unsigned char *buf, int len) - { - unsigned long s1 = adler & 0xffff; - unsigned long s2 = (adler >> 16) & 0xffff; - int n; - - for (n = 0; n < len; n++) { - s1 = (s1 + buf[n]) % BASE; - s2 = (s2 + s1) % BASE; - } - return (s2 << 16) + s1; - } - - /* Return the adler32 of the bytes buf[0..len-1] */ - - - - -Deutsch & Gailly Informational [Page 10] - -RFC 1950 ZLIB Compressed Data Format Specification May 1996 - - - unsigned long adler32(unsigned char *buf, int len) - { - return update_adler32(1L, buf, len); - } - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Deutsch & Gailly Informational [Page 11] - diff --git a/zlib/doc/rfc1951.txt b/zlib/doc/rfc1951.txt deleted file mode 100644 index 403c8c7..0000000 --- a/zlib/doc/rfc1951.txt +++ /dev/null @@ -1,955 +0,0 @@ - - - - - - -Network Working Group P. Deutsch -Request for Comments: 1951 Aladdin Enterprises -Category: Informational May 1996 - - - DEFLATE Compressed Data Format Specification version 1.3 - -Status of This Memo - - This memo provides information for the Internet community. This memo - does not specify an Internet standard of any kind. Distribution of - this memo is unlimited. - -IESG Note: - - The IESG takes no position on the validity of any Intellectual - Property Rights statements contained in this document. - -Notices - - Copyright (c) 1996 L. Peter Deutsch - - Permission is granted to copy and distribute this document for any - purpose and without charge, including translations into other - languages and incorporation into compilations, provided that the - copyright notice and this notice are preserved, and that any - substantive changes or deletions from the original are clearly - marked. - - A pointer to the latest version of this and related documentation in - HTML format can be found at the URL - <ftp://ftp.uu.net/graphics/png/documents/zlib/zdoc-index.html>. - -Abstract - - This specification defines a lossless compressed data format that - compresses data using a combination of the LZ77 algorithm and Huffman - coding, with efficiency comparable to the best currently available - general-purpose compression methods. The data can be produced or - consumed, even for an arbitrarily long sequentially presented input - data stream, using only an a priori bounded amount of intermediate - storage. The format can be implemented readily in a manner not - covered by patents. - - - - - - - - -Deutsch Informational [Page 1] - -RFC 1951 DEFLATE Compressed Data Format Specification May 1996 - - -Table of Contents - - 1. Introduction ................................................... 2 - 1.1. Purpose ................................................... 2 - 1.2. Intended audience ......................................... 3 - 1.3. Scope ..................................................... 3 - 1.4. Compliance ................................................ 3 - 1.5. Definitions of terms and conventions used ................ 3 - 1.6. Changes from previous versions ............................ 4 - 2. Compressed representation overview ............................. 4 - 3. Detailed specification ......................................... 5 - 3.1. Overall conventions ....................................... 5 - 3.1.1. Packing into bytes .................................. 5 - 3.2. Compressed block format ................................... 6 - 3.2.1. Synopsis of prefix and Huffman coding ............... 6 - 3.2.2. Use of Huffman coding in the "deflate" format ....... 7 - 3.2.3. Details of block format ............................. 9 - 3.2.4. Non-compressed blocks (BTYPE=00) ................... 11 - 3.2.5. Compressed blocks (length and distance codes) ...... 11 - 3.2.6. Compression with fixed Huffman codes (BTYPE=01) .... 12 - 3.2.7. Compression with dynamic Huffman codes (BTYPE=10) .. 13 - 3.3. Compliance ............................................... 14 - 4. Compression algorithm details ................................. 14 - 5. References .................................................... 16 - 6. Security Considerations ....................................... 16 - 7. Source code ................................................... 16 - 8. Acknowledgements .............................................. 16 - 9. Author's Address .............................................. 17 - -1. Introduction - - 1.1. Purpose - - The purpose of this specification is to define a lossless - compressed data format that: - * Is independent of CPU type, operating system, file system, - and character set, and hence can be used for interchange; - * Can be produced or consumed, even for an arbitrarily long - sequentially presented input data stream, using only an a - priori bounded amount of intermediate storage, and hence - can be used in data communications or similar structures - such as Unix filters; - * Compresses data with efficiency comparable to the best - currently available general-purpose compression methods, - and in particular considerably better than the "compress" - program; - * Can be implemented readily in a manner not covered by - patents, and hence can be practiced freely; - - - -Deutsch Informational [Page 2] - -RFC 1951 DEFLATE Compressed Data Format Specification May 1996 - - - * Is compatible with the file format produced by the current - widely used gzip utility, in that conforming decompressors - will be able to read data produced by the existing gzip - compressor. - - The data format defined by this specification does not attempt to: - - * Allow random access to compressed data; - * Compress specialized data (e.g., raster graphics) as well - as the best currently available specialized algorithms. - - A simple counting argument shows that no lossless compression - algorithm can compress every possible input data set. For the - format defined here, the worst case expansion is 5 bytes per 32K- - byte block, i.e., a size increase of 0.015% for large data sets. - English text usually compresses by a factor of 2.5 to 3; - executable files usually compress somewhat less; graphical data - such as raster images may compress much more. - - 1.2. Intended audience - - This specification is intended for use by implementors of software - to compress data into "deflate" format and/or decompress data from - "deflate" format. - - The text of the specification assumes a basic background in - programming at the level of bits and other primitive data - representations. Familiarity with the technique of Huffman coding - is helpful but not required. - - 1.3. Scope - - The specification specifies a method for representing a sequence - of bytes as a (usually shorter) sequence of bits, and a method for - packing the latter bit sequence into bytes. - - 1.4. Compliance - - Unless otherwise indicated below, a compliant decompressor must be - able to accept and decompress any data set that conforms to all - the specifications presented here; a compliant compressor must - produce data sets that conform to all the specifications presented - here. - - 1.5. Definitions of terms and conventions used - - Byte: 8 bits stored or transmitted as a unit (same as an octet). - For this specification, a byte is exactly 8 bits, even on machines - - - -Deutsch Informational [Page 3] - -RFC 1951 DEFLATE Compressed Data Format Specification May 1996 - - - which store a character on a number of bits different from eight. - See below, for the numbering of bits within a byte. - - String: a sequence of arbitrary bytes. - - 1.6. Changes from previous versions - - There have been no technical changes to the deflate format since - version 1.1 of this specification. In version 1.2, some - terminology was changed. Version 1.3 is a conversion of the - specification to RFC style. - -2. Compressed representation overview - - A compressed data set consists of a series of blocks, corresponding - to successive blocks of input data. The block sizes are arbitrary, - except that non-compressible blocks are limited to 65,535 bytes. - - Each block is compressed using a combination of the LZ77 algorithm - and Huffman coding. The Huffman trees for each block are independent - of those for previous or subsequent blocks; the LZ77 algorithm may - use a reference to a duplicated string occurring in a previous block, - up to 32K input bytes before. - - Each block consists of two parts: a pair of Huffman code trees that - describe the representation of the compressed data part, and a - compressed data part. (The Huffman trees themselves are compressed - using Huffman encoding.) The compressed data consists of a series of - elements of two types: literal bytes (of strings that have not been - detected as duplicated within the previous 32K input bytes), and - pointers to duplicated strings, where a pointer is represented as a - pair <length, backward distance>. The representation used in the - "deflate" format limits distances to 32K bytes and lengths to 258 - bytes, but does not limit the size of a block, except for - uncompressible blocks, which are limited as noted above. - - Each type of value (literals, distances, and lengths) in the - compressed data is represented using a Huffman code, using one code - tree for literals and lengths and a separate code tree for distances. - The code trees for each block appear in a compact form just before - the compressed data for that block. - - - - - - - - - - -Deutsch Informational [Page 4] - -RFC 1951 DEFLATE Compressed Data Format Specification May 1996 - - -3. Detailed specification - - 3.1. Overall conventions In the diagrams below, a box like this: - - +---+ - | | <-- the vertical bars might be missing - +---+ - - represents one byte; a box like this: - - +==============+ - | | - +==============+ - - represents a variable number of bytes. - - Bytes stored within a computer do not have a "bit order", since - they are always treated as a unit. However, a byte considered as - an integer between 0 and 255 does have a most- and least- - significant bit, and since we write numbers with the most- - significant digit on the left, we also write bytes with the most- - significant bit on the left. In the diagrams below, we number the - bits of a byte so that bit 0 is the least-significant bit, i.e., - the bits are numbered: - - +--------+ - |76543210| - +--------+ - - Within a computer, a number may occupy multiple bytes. All - multi-byte numbers in the format described here are stored with - the least-significant byte first (at the lower memory address). - For example, the decimal number 520 is stored as: - - 0 1 - +--------+--------+ - |00001000|00000010| - +--------+--------+ - ^ ^ - | | - | + more significant byte = 2 x 256 - + less significant byte = 8 - - 3.1.1. Packing into bytes - - This document does not address the issue of the order in which - bits of a byte are transmitted on a bit-sequential medium, - since the final data format described here is byte- rather than - - - -Deutsch Informational [Page 5] - -RFC 1951 DEFLATE Compressed Data Format Specification May 1996 - - - bit-oriented. However, we describe the compressed block format - in below, as a sequence of data elements of various bit - lengths, not a sequence of bytes. We must therefore specify - how to pack these data elements into bytes to form the final - compressed byte sequence: - - * Data elements are packed into bytes in order of - increasing bit number within the byte, i.e., starting - with the least-significant bit of the byte. - * Data elements other than Huffman codes are packed - starting with the least-significant bit of the data - element. - * Huffman codes are packed starting with the most- - significant bit of the code. - - In other words, if one were to print out the compressed data as - a sequence of bytes, starting with the first byte at the - *right* margin and proceeding to the *left*, with the most- - significant bit of each byte on the left as usual, one would be - able to parse the result from right to left, with fixed-width - elements in the correct MSB-to-LSB order and Huffman codes in - bit-reversed order (i.e., with the first bit of the code in the - relative LSB position). - - 3.2. Compressed block format - - 3.2.1. Synopsis of prefix and Huffman coding - - Prefix coding represents symbols from an a priori known - alphabet by bit sequences (codes), one code for each symbol, in - a manner such that different symbols may be represented by bit - sequences of different lengths, but a parser can always parse - an encoded string unambiguously symbol-by-symbol. - - We define a prefix code in terms of a binary tree in which the - two edges descending from each non-leaf node are labeled 0 and - 1 and in which the leaf nodes correspond one-for-one with (are - labeled with) the symbols of the alphabet; then the code for a - symbol is the sequence of 0's and 1's on the edges leading from - the root to the leaf labeled with that symbol. For example: - - - - - - - - - - - -Deutsch Informational [Page 6] - -RFC 1951 DEFLATE Compressed Data Format Specification May 1996 - - - /\ Symbol Code - 0 1 ------ ---- - / \ A 00 - /\ B B 1 - 0 1 C 011 - / \ D 010 - A /\ - 0 1 - / \ - D C - - A parser can decode the next symbol from an encoded input - stream by walking down the tree from the root, at each step - choosing the edge corresponding to the next input bit. - - Given an alphabet with known symbol frequencies, the Huffman - algorithm allows the construction of an optimal prefix code - (one which represents strings with those symbol frequencies - using the fewest bits of any possible prefix codes for that - alphabet). Such a code is called a Huffman code. (See - reference [1] in Chapter 5, references for additional - information on Huffman codes.) - - Note that in the "deflate" format, the Huffman codes for the - various alphabets must not exceed certain maximum code lengths. - This constraint complicates the algorithm for computing code - lengths from symbol frequencies. Again, see Chapter 5, - references for details. - - 3.2.2. Use of Huffman coding in the "deflate" format - - The Huffman codes used for each alphabet in the "deflate" - format have two additional rules: - - * All codes of a given bit length have lexicographically - consecutive values, in the same order as the symbols - they represent; - - * Shorter codes lexicographically precede longer codes. - - - - - - - - - - - - -Deutsch Informational [Page 7] - -RFC 1951 DEFLATE Compressed Data Format Specification May 1996 - - - We could recode the example above to follow this rule as - follows, assuming that the order of the alphabet is ABCD: - - Symbol Code - ------ ---- - A 10 - B 0 - C 110 - D 111 - - I.e., 0 precedes 10 which precedes 11x, and 110 and 111 are - lexicographically consecutive. - - Given this rule, we can define the Huffman code for an alphabet - just by giving the bit lengths of the codes for each symbol of - the alphabet in order; this is sufficient to determine the - actual codes. In our example, the code is completely defined - by the sequence of bit lengths (2, 1, 3, 3). The following - algorithm generates the codes as integers, intended to be read - from most- to least-significant bit. The code lengths are - initially in tree[I].Len; the codes are produced in - tree[I].Code. - - 1) Count the number of codes for each code length. Let - bl_count[N] be the number of codes of length N, N >= 1. - - 2) Find the numerical value of the smallest code for each - code length: - - code = 0; - bl_count[0] = 0; - for (bits = 1; bits <= MAX_BITS; bits++) { - code = (code + bl_count[bits-1]) << 1; - next_code[bits] = code; - } - - 3) Assign numerical values to all codes, using consecutive - values for all codes of the same length with the base - values determined at step 2. Codes that are never used - (which have a bit length of zero) must not be assigned a - value. - - for (n = 0; n <= max_code; n++) { - len = tree[n].Len; - if (len != 0) { - tree[n].Code = next_code[len]; - next_code[len]++; - } - - - -Deutsch Informational [Page 8] - -RFC 1951 DEFLATE Compressed Data Format Specification May 1996 - - - } - - Example: - - Consider the alphabet ABCDEFGH, with bit lengths (3, 3, 3, 3, - 3, 2, 4, 4). After step 1, we have: - - N bl_count[N] - - ----------- - 2 1 - 3 5 - 4 2 - - Step 2 computes the following next_code values: - - N next_code[N] - - ------------ - 1 0 - 2 0 - 3 2 - 4 14 - - Step 3 produces the following code values: - - Symbol Length Code - ------ ------ ---- - A 3 010 - B 3 011 - C 3 100 - D 3 101 - E 3 110 - F 2 00 - G 4 1110 - H 4 1111 - - 3.2.3. Details of block format - - Each block of compressed data begins with 3 header bits - containing the following data: - - first bit BFINAL - next 2 bits BTYPE - - Note that the header bits do not necessarily begin on a byte - boundary, since a block does not necessarily occupy an integral - number of bytes. - - - - - -Deutsch Informational [Page 9] - -RFC 1951 DEFLATE Compressed Data Format Specification May 1996 - - - BFINAL is set if and only if this is the last block of the data - set. - - BTYPE specifies how the data are compressed, as follows: - - 00 - no compression - 01 - compressed with fixed Huffman codes - 10 - compressed with dynamic Huffman codes - 11 - reserved (error) - - The only difference between the two compressed cases is how the - Huffman codes for the literal/length and distance alphabets are - defined. - - In all cases, the decoding algorithm for the actual data is as - follows: - - do - read block header from input stream. - if stored with no compression - skip any remaining bits in current partially - processed byte - read LEN and NLEN (see next section) - copy LEN bytes of data to output - otherwise - if compressed with dynamic Huffman codes - read representation of code trees (see - subsection below) - loop (until end of block code recognized) - decode literal/length value from input stream - if value < 256 - copy value (literal byte) to output stream - otherwise - if value = end of block (256) - break from loop - otherwise (value = 257..285) - decode distance from input stream - - move backwards distance bytes in the output - stream, and copy length bytes from this - position to the output stream. - end loop - while not last block - - Note that a duplicated string reference may refer to a string - in a previous block; i.e., the backward distance may cross one - or more block boundaries. However a distance cannot refer past - the beginning of the output stream. (An application using a - - - -Deutsch Informational [Page 10] - -RFC 1951 DEFLATE Compressed Data Format Specification May 1996 - - - preset dictionary might discard part of the output stream; a - distance can refer to that part of the output stream anyway) - Note also that the referenced string may overlap the current - position; for example, if the last 2 bytes decoded have values - X and Y, a string reference with <length = 5, distance = 2> - adds X,Y,X,Y,X to the output stream. - - We now specify each compression method in turn. - - 3.2.4. Non-compressed blocks (BTYPE=00) - - Any bits of input up to the next byte boundary are ignored. - The rest of the block consists of the following information: - - 0 1 2 3 4... - +---+---+---+---+================================+ - | LEN | NLEN |... LEN bytes of literal data...| - +---+---+---+---+================================+ - - LEN is the number of data bytes in the block. NLEN is the - one's complement of LEN. - - 3.2.5. Compressed blocks (length and distance codes) - - As noted above, encoded data blocks in the "deflate" format - consist of sequences of symbols drawn from three conceptually - distinct alphabets: either literal bytes, from the alphabet of - byte values (0..255), or <length, backward distance> pairs, - where the length is drawn from (3..258) and the distance is - drawn from (1..32,768). In fact, the literal and length - alphabets are merged into a single alphabet (0..285), where - values 0..255 represent literal bytes, the value 256 indicates - end-of-block, and values 257..285 represent length codes - (possibly in conjunction with extra bits following the symbol - code) as follows: - - - - - - - - - - - - - - - - -Deutsch Informational [Page 11] - -RFC 1951 DEFLATE Compressed Data Format Specification May 1996 - - - Extra Extra Extra - Code Bits Length(s) Code Bits Lengths Code Bits Length(s) - ---- ---- ------ ---- ---- ------- ---- ---- ------- - 257 0 3 267 1 15,16 277 4 67-82 - 258 0 4 268 1 17,18 278 4 83-98 - 259 0 5 269 2 19-22 279 4 99-114 - 260 0 6 270 2 23-26 280 4 115-130 - 261 0 7 271 2 27-30 281 5 131-162 - 262 0 8 272 2 31-34 282 5 163-194 - 263 0 9 273 3 35-42 283 5 195-226 - 264 0 10 274 3 43-50 284 5 227-257 - 265 1 11,12 275 3 51-58 285 0 258 - 266 1 13,14 276 3 59-66 - - The extra bits should be interpreted as a machine integer - stored with the most-significant bit first, e.g., bits 1110 - represent the value 14. - - Extra Extra Extra - Code Bits Dist Code Bits Dist Code Bits Distance - ---- ---- ---- ---- ---- ------ ---- ---- -------- - 0 0 1 10 4 33-48 20 9 1025-1536 - 1 0 2 11 4 49-64 21 9 1537-2048 - 2 0 3 12 5 65-96 22 10 2049-3072 - 3 0 4 13 5 97-128 23 10 3073-4096 - 4 1 5,6 14 6 129-192 24 11 4097-6144 - 5 1 7,8 15 6 193-256 25 11 6145-8192 - 6 2 9-12 16 7 257-384 26 12 8193-12288 - 7 2 13-16 17 7 385-512 27 12 12289-16384 - 8 3 17-24 18 8 513-768 28 13 16385-24576 - 9 3 25-32 19 8 769-1024 29 13 24577-32768 - - 3.2.6. Compression with fixed Huffman codes (BTYPE=01) - - The Huffman codes for the two alphabets are fixed, and are not - represented explicitly in the data. The Huffman code lengths - for the literal/length alphabet are: - - Lit Value Bits Codes - --------- ---- ----- - 0 - 143 8 00110000 through - 10111111 - 144 - 255 9 110010000 through - 111111111 - 256 - 279 7 0000000 through - 0010111 - 280 - 287 8 11000000 through - 11000111 - - - -Deutsch Informational [Page 12] - -RFC 1951 DEFLATE Compressed Data Format Specification May 1996 - - - The code lengths are sufficient to generate the actual codes, - as described above; we show the codes in the table for added - clarity. Literal/length values 286-287 will never actually - occur in the compressed data, but participate in the code - construction. - - Distance codes 0-31 are represented by (fixed-length) 5-bit - codes, with possible additional bits as shown in the table - shown in Paragraph 3.2.5, above. Note that distance codes 30- - 31 will never actually occur in the compressed data. - - 3.2.7. Compression with dynamic Huffman codes (BTYPE=10) - - The Huffman codes for the two alphabets appear in the block - immediately after the header bits and before the actual - compressed data, first the literal/length code and then the - distance code. Each code is defined by a sequence of code - lengths, as discussed in Paragraph 3.2.2, above. For even - greater compactness, the code length sequences themselves are - compressed using a Huffman code. The alphabet for code lengths - is as follows: - - 0 - 15: Represent code lengths of 0 - 15 - 16: Copy the previous code length 3 - 6 times. - The next 2 bits indicate repeat length - (0 = 3, ... , 3 = 6) - Example: Codes 8, 16 (+2 bits 11), - 16 (+2 bits 10) will expand to - 12 code lengths of 8 (1 + 6 + 5) - 17: Repeat a code length of 0 for 3 - 10 times. - (3 bits of length) - 18: Repeat a code length of 0 for 11 - 138 times - (7 bits of length) - - A code length of 0 indicates that the corresponding symbol in - the literal/length or distance alphabet will not occur in the - block, and should not participate in the Huffman code - construction algorithm given earlier. If only one distance - code is used, it is encoded using one bit, not zero bits; in - this case there is a single code length of one, with one unused - code. One distance code of zero bits means that there are no - distance codes used at all (the data is all literals). - - We can now define the format of the block: - - 5 Bits: HLIT, # of Literal/Length codes - 257 (257 - 286) - 5 Bits: HDIST, # of Distance codes - 1 (1 - 32) - 4 Bits: HCLEN, # of Code Length codes - 4 (4 - 19) - - - -Deutsch Informational [Page 13] - -RFC 1951 DEFLATE Compressed Data Format Specification May 1996 - - - (HCLEN + 4) x 3 bits: code lengths for the code length - alphabet given just above, in the order: 16, 17, 18, - 0, 8, 7, 9, 6, 10, 5, 11, 4, 12, 3, 13, 2, 14, 1, 15 - - These code lengths are interpreted as 3-bit integers - (0-7); as above, a code length of 0 means the - corresponding symbol (literal/length or distance code - length) is not used. - - HLIT + 257 code lengths for the literal/length alphabet, - encoded using the code length Huffman code - - HDIST + 1 code lengths for the distance alphabet, - encoded using the code length Huffman code - - The actual compressed data of the block, - encoded using the literal/length and distance Huffman - codes - - The literal/length symbol 256 (end of data), - encoded using the literal/length Huffman code - - The code length repeat codes can cross from HLIT + 257 to the - HDIST + 1 code lengths. In other words, all code lengths form - a single sequence of HLIT + HDIST + 258 values. - - 3.3. Compliance - - A compressor may limit further the ranges of values specified in - the previous section and still be compliant; for example, it may - limit the range of backward pointers to some value smaller than - 32K. Similarly, a compressor may limit the size of blocks so that - a compressible block fits in memory. - - A compliant decompressor must accept the full range of possible - values defined in the previous section, and must accept blocks of - arbitrary size. - -4. Compression algorithm details - - While it is the intent of this document to define the "deflate" - compressed data format without reference to any particular - compression algorithm, the format is related to the compressed - formats produced by LZ77 (Lempel-Ziv 1977, see reference [2] below); - since many variations of LZ77 are patented, it is strongly - recommended that the implementor of a compressor follow the general - algorithm presented here, which is known not to be patented per se. - The material in this section is not part of the definition of the - - - -Deutsch Informational [Page 14] - -RFC 1951 DEFLATE Compressed Data Format Specification May 1996 - - - specification per se, and a compressor need not follow it in order to - be compliant. - - The compressor terminates a block when it determines that starting a - new block with fresh trees would be useful, or when the block size - fills up the compressor's block buffer. - - The compressor uses a chained hash table to find duplicated strings, - using a hash function that operates on 3-byte sequences. At any - given point during compression, let XYZ be the next 3 input bytes to - be examined (not necessarily all different, of course). First, the - compressor examines the hash chain for XYZ. If the chain is empty, - the compressor simply writes out X as a literal byte and advances one - byte in the input. If the hash chain is not empty, indicating that - the sequence XYZ (or, if we are unlucky, some other 3 bytes with the - same hash function value) has occurred recently, the compressor - compares all strings on the XYZ hash chain with the actual input data - sequence starting at the current point, and selects the longest - match. - - The compressor searches the hash chains starting with the most recent - strings, to favor small distances and thus take advantage of the - Huffman encoding. The hash chains are singly linked. There are no - deletions from the hash chains; the algorithm simply discards matches - that are too old. To avoid a worst-case situation, very long hash - chains are arbitrarily truncated at a certain length, determined by a - run-time parameter. - - To improve overall compression, the compressor optionally defers the - selection of matches ("lazy matching"): after a match of length N has - been found, the compressor searches for a longer match starting at - the next input byte. If it finds a longer match, it truncates the - previous match to a length of one (thus producing a single literal - byte) and then emits the longer match. Otherwise, it emits the - original match, and, as described above, advances N bytes before - continuing. - - Run-time parameters also control this "lazy match" procedure. If - compression ratio is most important, the compressor attempts a - complete second search regardless of the length of the first match. - In the normal case, if the current match is "long enough", the - compressor reduces the search for a longer match, thus speeding up - the process. If speed is most important, the compressor inserts new - strings in the hash table only when no match was found, or when the - match is not "too long". This degrades the compression ratio but - saves time since there are both fewer insertions and fewer searches. - - - - - -Deutsch Informational [Page 15] - -RFC 1951 DEFLATE Compressed Data Format Specification May 1996 - - -5. References - - [1] Huffman, D. A., "A Method for the Construction of Minimum - Redundancy Codes", Proceedings of the Institute of Radio - Engineers, September 1952, Volume 40, Number 9, pp. 1098-1101. - - [2] Ziv J., Lempel A., "A Universal Algorithm for Sequential Data - Compression", IEEE Transactions on Information Theory, Vol. 23, - No. 3, pp. 337-343. - - [3] Gailly, J.-L., and Adler, M., ZLIB documentation and sources, - available in ftp://ftp.uu.net/pub/archiving/zip/doc/ - - [4] Gailly, J.-L., and Adler, M., GZIP documentation and sources, - available as gzip-*.tar in ftp://prep.ai.mit.edu/pub/gnu/ - - [5] Schwartz, E. S., and Kallick, B. "Generating a canonical prefix - encoding." Comm. ACM, 7,3 (Mar. 1964), pp. 166-169. - - [6] Hirschberg and Lelewer, "Efficient decoding of prefix codes," - Comm. ACM, 33,4, April 1990, pp. 449-459. - -6. Security Considerations - - Any data compression method involves the reduction of redundancy in - the data. Consequently, any corruption of the data is likely to have - severe effects and be difficult to correct. Uncompressed text, on - the other hand, will probably still be readable despite the presence - of some corrupted bytes. - - It is recommended that systems using this data format provide some - means of validating the integrity of the compressed data. See - reference [3], for example. - -7. Source code - - Source code for a C language implementation of a "deflate" compliant - compressor and decompressor is available within the zlib package at - ftp://ftp.uu.net/pub/archiving/zip/zlib/. - -8. Acknowledgements - - Trademarks cited in this document are the property of their - respective owners. - - Phil Katz designed the deflate format. Jean-Loup Gailly and Mark - Adler wrote the related software described in this specification. - Glenn Randers-Pehrson converted this document to RFC and HTML format. - - - -Deutsch Informational [Page 16] - -RFC 1951 DEFLATE Compressed Data Format Specification May 1996 - - -9. Author's Address - - L. Peter Deutsch - Aladdin Enterprises - 203 Santa Margarita Ave. - Menlo Park, CA 94025 - - Phone: (415) 322-0103 (AM only) - FAX: (415) 322-1734 - EMail: <ghost@aladdin.com> - - Questions about the technical content of this specification can be - sent by email to: - - Jean-Loup Gailly <gzip@prep.ai.mit.edu> and - Mark Adler <madler@alumni.caltech.edu> - - Editorial comments on this specification can be sent by email to: - - L. Peter Deutsch <ghost@aladdin.com> and - Glenn Randers-Pehrson <randeg@alumni.rpi.edu> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -Deutsch Informational [Page 17] - diff --git a/zlib/doc/rfc1952.txt b/zlib/doc/rfc1952.txt deleted file mode 100644 index a8e51b4..0000000 --- a/zlib/doc/rfc1952.txt +++ /dev/null @@ -1,675 +0,0 @@ - - - - - - -Network Working Group P. Deutsch -Request for Comments: 1952 Aladdin Enterprises -Category: Informational May 1996 - - - GZIP file format specification version 4.3 - -Status of This Memo - - This memo provides information for the Internet community. This memo - does not specify an Internet standard of any kind. Distribution of - this memo is unlimited. - -IESG Note: - - The IESG takes no position on the validity of any Intellectual - Property Rights statements contained in this document. - -Notices - - Copyright (c) 1996 L. Peter Deutsch - - Permission is granted to copy and distribute this document for any - purpose and without charge, including translations into other - languages and incorporation into compilations, provided that the - copyright notice and this notice are preserved, and that any - substantive changes or deletions from the original are clearly - marked. - - A pointer to the latest version of this and related documentation in - HTML format can be found at the URL - <ftp://ftp.uu.net/graphics/png/documents/zlib/zdoc-index.html>. - -Abstract - - This specification defines a lossless compressed data format that is - compatible with the widely used GZIP utility. The format includes a - cyclic redundancy check value for detecting data corruption. The - format presently uses the DEFLATE method of compression but can be - easily extended to use other compression methods. The format can be - implemented readily in a manner not covered by patents. - - - - - - - - - - -Deutsch Informational [Page 1] - -RFC 1952 GZIP File Format Specification May 1996 - - -Table of Contents - - 1. Introduction ................................................... 2 - 1.1. Purpose ................................................... 2 - 1.2. Intended audience ......................................... 3 - 1.3. Scope ..................................................... 3 - 1.4. Compliance ................................................ 3 - 1.5. Definitions of terms and conventions used ................. 3 - 1.6. Changes from previous versions ............................ 3 - 2. Detailed specification ......................................... 4 - 2.1. Overall conventions ....................................... 4 - 2.2. File format ............................................... 5 - 2.3. Member format ............................................. 5 - 2.3.1. Member header and trailer ........................... 6 - 2.3.1.1. Extra field ................................... 8 - 2.3.1.2. Compliance .................................... 9 - 3. References .................................................. 9 - 4. Security Considerations .................................... 10 - 5. Acknowledgements ........................................... 10 - 6. Author's Address ........................................... 10 - 7. Appendix: Jean-Loup Gailly's gzip utility .................. 11 - 8. Appendix: Sample CRC Code .................................. 11 - -1. Introduction - - 1.1. Purpose - - The purpose of this specification is to define a lossless - compressed data format that: - - * Is independent of CPU type, operating system, file system, - and character set, and hence can be used for interchange; - * Can compress or decompress a data stream (as opposed to a - randomly accessible file) to produce another data stream, - using only an a priori bounded amount of intermediate - storage, and hence can be used in data communications or - similar structures such as Unix filters; - * Compresses data with efficiency comparable to the best - currently available general-purpose compression methods, - and in particular considerably better than the "compress" - program; - * Can be implemented readily in a manner not covered by - patents, and hence can be practiced freely; - * Is compatible with the file format produced by the current - widely used gzip utility, in that conforming decompressors - will be able to read data produced by the existing gzip - compressor. - - - - -Deutsch Informational [Page 2] - -RFC 1952 GZIP File Format Specification May 1996 - - - The data format defined by this specification does not attempt to: - - * Provide random access to compressed data; - * Compress specialized data (e.g., raster graphics) as well as - the best currently available specialized algorithms. - - 1.2. Intended audience - - This specification is intended for use by implementors of software - to compress data into gzip format and/or decompress data from gzip - format. - - The text of the specification assumes a basic background in - programming at the level of bits and other primitive data - representations. - - 1.3. Scope - - The specification specifies a compression method and a file format - (the latter assuming only that a file can store a sequence of - arbitrary bytes). It does not specify any particular interface to - a file system or anything about character sets or encodings - (except for file names and comments, which are optional). - - 1.4. Compliance - - Unless otherwise indicated below, a compliant decompressor must be - able to accept and decompress any file that conforms to all the - specifications presented here; a compliant compressor must produce - files that conform to all the specifications presented here. The - material in the appendices is not part of the specification per se - and is not relevant to compliance. - - 1.5. Definitions of terms and conventions used - - byte: 8 bits stored or transmitted as a unit (same as an octet). - (For this specification, a byte is exactly 8 bits, even on - machines which store a character on a number of bits different - from 8.) See below for the numbering of bits within a byte. - - 1.6. Changes from previous versions - - There have been no technical changes to the gzip format since - version 4.1 of this specification. In version 4.2, some - terminology was changed, and the sample CRC code was rewritten for - clarity and to eliminate the requirement for the caller to do pre- - and post-conditioning. Version 4.3 is a conversion of the - specification to RFC style. - - - -Deutsch Informational [Page 3] - -RFC 1952 GZIP File Format Specification May 1996 - - -2. Detailed specification - - 2.1. Overall conventions - - In the diagrams below, a box like this: - - +---+ - | | <-- the vertical bars might be missing - +---+ - - represents one byte; a box like this: - - +==============+ - | | - +==============+ - - represents a variable number of bytes. - - Bytes stored within a computer do not have a "bit order", since - they are always treated as a unit. However, a byte considered as - an integer between 0 and 255 does have a most- and least- - significant bit, and since we write numbers with the most- - significant digit on the left, we also write bytes with the most- - significant bit on the left. In the diagrams below, we number the - bits of a byte so that bit 0 is the least-significant bit, i.e., - the bits are numbered: - - +--------+ - |76543210| - +--------+ - - This document does not address the issue of the order in which - bits of a byte are transmitted on a bit-sequential medium, since - the data format described here is byte- rather than bit-oriented. - - Within a computer, a number may occupy multiple bytes. All - multi-byte numbers in the format described here are stored with - the least-significant byte first (at the lower memory address). - For example, the decimal number 520 is stored as: - - 0 1 - +--------+--------+ - |00001000|00000010| - +--------+--------+ - ^ ^ - | | - | + more significant byte = 2 x 256 - + less significant byte = 8 - - - -Deutsch Informational [Page 4] - -RFC 1952 GZIP File Format Specification May 1996 - - - 2.2. File format - - A gzip file consists of a series of "members" (compressed data - sets). The format of each member is specified in the following - section. The members simply appear one after another in the file, - with no additional information before, between, or after them. - - 2.3. Member format - - Each member has the following structure: - - +---+---+---+---+---+---+---+---+---+---+ - |ID1|ID2|CM |FLG| MTIME |XFL|OS | (more-->) - +---+---+---+---+---+---+---+---+---+---+ - - (if FLG.FEXTRA set) - - +---+---+=================================+ - | XLEN |...XLEN bytes of "extra field"...| (more-->) - +---+---+=================================+ - - (if FLG.FNAME set) - - +=========================================+ - |...original file name, zero-terminated...| (more-->) - +=========================================+ - - (if FLG.FCOMMENT set) - - +===================================+ - |...file comment, zero-terminated...| (more-->) - +===================================+ - - (if FLG.FHCRC set) - - +---+---+ - | CRC16 | - +---+---+ - - +=======================+ - |...compressed blocks...| (more-->) - +=======================+ - - 0 1 2 3 4 5 6 7 - +---+---+---+---+---+---+---+---+ - | CRC32 | ISIZE | - +---+---+---+---+---+---+---+---+ - - - - -Deutsch Informational [Page 5] - -RFC 1952 GZIP File Format Specification May 1996 - - - 2.3.1. Member header and trailer - - ID1 (IDentification 1) - ID2 (IDentification 2) - These have the fixed values ID1 = 31 (0x1f, \037), ID2 = 139 - (0x8b, \213), to identify the file as being in gzip format. - - CM (Compression Method) - This identifies the compression method used in the file. CM - = 0-7 are reserved. CM = 8 denotes the "deflate" - compression method, which is the one customarily used by - gzip and which is documented elsewhere. - - FLG (FLaGs) - This flag byte is divided into individual bits as follows: - - bit 0 FTEXT - bit 1 FHCRC - bit 2 FEXTRA - bit 3 FNAME - bit 4 FCOMMENT - bit 5 reserved - bit 6 reserved - bit 7 reserved - - If FTEXT is set, the file is probably ASCII text. This is - an optional indication, which the compressor may set by - checking a small amount of the input data to see whether any - non-ASCII characters are present. In case of doubt, FTEXT - is cleared, indicating binary data. For systems which have - different file formats for ascii text and binary data, the - decompressor can use FTEXT to choose the appropriate format. - We deliberately do not specify the algorithm used to set - this bit, since a compressor always has the option of - leaving it cleared and a decompressor always has the option - of ignoring it and letting some other program handle issues - of data conversion. - - If FHCRC is set, a CRC16 for the gzip header is present, - immediately before the compressed data. The CRC16 consists - of the two least significant bytes of the CRC32 for all - bytes of the gzip header up to and not including the CRC16. - [The FHCRC bit was never set by versions of gzip up to - 1.2.4, even though it was documented with a different - meaning in gzip 1.2.4.] - - If FEXTRA is set, optional extra fields are present, as - described in a following section. - - - -Deutsch Informational [Page 6] - -RFC 1952 GZIP File Format Specification May 1996 - - - If FNAME is set, an original file name is present, - terminated by a zero byte. The name must consist of ISO - 8859-1 (LATIN-1) characters; on operating systems using - EBCDIC or any other character set for file names, the name - must be translated to the ISO LATIN-1 character set. This - is the original name of the file being compressed, with any - directory components removed, and, if the file being - compressed is on a file system with case insensitive names, - forced to lower case. There is no original file name if the - data was compressed from a source other than a named file; - for example, if the source was stdin on a Unix system, there - is no file name. - - If FCOMMENT is set, a zero-terminated file comment is - present. This comment is not interpreted; it is only - intended for human consumption. The comment must consist of - ISO 8859-1 (LATIN-1) characters. Line breaks should be - denoted by a single line feed character (10 decimal). - - Reserved FLG bits must be zero. - - MTIME (Modification TIME) - This gives the most recent modification time of the original - file being compressed. The time is in Unix format, i.e., - seconds since 00:00:00 GMT, Jan. 1, 1970. (Note that this - may cause problems for MS-DOS and other systems that use - local rather than Universal time.) If the compressed data - did not come from a file, MTIME is set to the time at which - compression started. MTIME = 0 means no time stamp is - available. - - XFL (eXtra FLags) - These flags are available for use by specific compression - methods. The "deflate" method (CM = 8) sets these flags as - follows: - - XFL = 2 - compressor used maximum compression, - slowest algorithm - XFL = 4 - compressor used fastest algorithm - - OS (Operating System) - This identifies the type of file system on which compression - took place. This may be useful in determining end-of-line - convention for text files. The currently defined values are - as follows: - - - - - - -Deutsch Informational [Page 7] - -RFC 1952 GZIP File Format Specification May 1996 - - - 0 - FAT filesystem (MS-DOS, OS/2, NT/Win32) - 1 - Amiga - 2 - VMS (or OpenVMS) - 3 - Unix - 4 - VM/CMS - 5 - Atari TOS - 6 - HPFS filesystem (OS/2, NT) - 7 - Macintosh - 8 - Z-System - 9 - CP/M - 10 - TOPS-20 - 11 - NTFS filesystem (NT) - 12 - QDOS - 13 - Acorn RISCOS - 255 - unknown - - XLEN (eXtra LENgth) - If FLG.FEXTRA is set, this gives the length of the optional - extra field. See below for details. - - CRC32 (CRC-32) - This contains a Cyclic Redundancy Check value of the - uncompressed data computed according to CRC-32 algorithm - used in the ISO 3309 standard and in section 8.1.1.6.2 of - ITU-T recommendation V.42. (See http://www.iso.ch for - ordering ISO documents. See gopher://info.itu.ch for an - online version of ITU-T V.42.) - - ISIZE (Input SIZE) - This contains the size of the original (uncompressed) input - data modulo 2^32. - - 2.3.1.1. Extra field - - If the FLG.FEXTRA bit is set, an "extra field" is present in - the header, with total length XLEN bytes. It consists of a - series of subfields, each of the form: - - +---+---+---+---+==================================+ - |SI1|SI2| LEN |... LEN bytes of subfield data ...| - +---+---+---+---+==================================+ - - SI1 and SI2 provide a subfield ID, typically two ASCII letters - with some mnemonic value. Jean-Loup Gailly - <gzip@prep.ai.mit.edu> is maintaining a registry of subfield - IDs; please send him any subfield ID you wish to use. Subfield - IDs with SI2 = 0 are reserved for future use. The following - IDs are currently defined: - - - -Deutsch Informational [Page 8] - -RFC 1952 GZIP File Format Specification May 1996 - - - SI1 SI2 Data - ---------- ---------- ---- - 0x41 ('A') 0x70 ('P') Apollo file type information - - LEN gives the length of the subfield data, excluding the 4 - initial bytes. - - 2.3.1.2. Compliance - - A compliant compressor must produce files with correct ID1, - ID2, CM, CRC32, and ISIZE, but may set all the other fields in - the fixed-length part of the header to default values (255 for - OS, 0 for all others). The compressor must set all reserved - bits to zero. - - A compliant decompressor must check ID1, ID2, and CM, and - provide an error indication if any of these have incorrect - values. It must examine FEXTRA/XLEN, FNAME, FCOMMENT and FHCRC - at least so it can skip over the optional fields if they are - present. It need not examine any other part of the header or - trailer; in particular, a decompressor may ignore FTEXT and OS - and always produce binary output, and still be compliant. A - compliant decompressor must give an error indication if any - reserved bit is non-zero, since such a bit could indicate the - presence of a new field that would cause subsequent data to be - interpreted incorrectly. - -3. References - - [1] "Information Processing - 8-bit single-byte coded graphic - character sets - Part 1: Latin alphabet No.1" (ISO 8859-1:1987). - The ISO 8859-1 (Latin-1) character set is a superset of 7-bit - ASCII. Files defining this character set are available as - iso_8859-1.* in ftp://ftp.uu.net/graphics/png/documents/ - - [2] ISO 3309 - - [3] ITU-T recommendation V.42 - - [4] Deutsch, L.P.,"DEFLATE Compressed Data Format Specification", - available in ftp://ftp.uu.net/pub/archiving/zip/doc/ - - [5] Gailly, J.-L., GZIP documentation, available as gzip-*.tar in - ftp://prep.ai.mit.edu/pub/gnu/ - - [6] Sarwate, D.V., "Computation of Cyclic Redundancy Checks via Table - Look-Up", Communications of the ACM, 31(8), pp.1008-1013. - - - - -Deutsch Informational [Page 9] - -RFC 1952 GZIP File Format Specification May 1996 - - - [7] Schwaderer, W.D., "CRC Calculation", April 85 PC Tech Journal, - pp.118-133. - - [8] ftp://ftp.adelaide.edu.au/pub/rocksoft/papers/crc_v3.txt, - describing the CRC concept. - -4. Security Considerations - - Any data compression method involves the reduction of redundancy in - the data. Consequently, any corruption of the data is likely to have - severe effects and be difficult to correct. Uncompressed text, on - the other hand, will probably still be readable despite the presence - of some corrupted bytes. - - It is recommended that systems using this data format provide some - means of validating the integrity of the compressed data, such as by - setting and checking the CRC-32 check value. - -5. Acknowledgements - - Trademarks cited in this document are the property of their - respective owners. - - Jean-Loup Gailly designed the gzip format and wrote, with Mark Adler, - the related software described in this specification. Glenn - Randers-Pehrson converted this document to RFC and HTML format. - -6. Author's Address - - L. Peter Deutsch - Aladdin Enterprises - 203 Santa Margarita Ave. - Menlo Park, CA 94025 - - Phone: (415) 322-0103 (AM only) - FAX: (415) 322-1734 - EMail: <ghost@aladdin.com> - - Questions about the technical content of this specification can be - sent by email to: - - Jean-Loup Gailly <gzip@prep.ai.mit.edu> and - Mark Adler <madler@alumni.caltech.edu> - - Editorial comments on this specification can be sent by email to: - - L. Peter Deutsch <ghost@aladdin.com> and - Glenn Randers-Pehrson <randeg@alumni.rpi.edu> - - - -Deutsch Informational [Page 10] - -RFC 1952 GZIP File Format Specification May 1996 - - -7. Appendix: Jean-Loup Gailly's gzip utility - - The most widely used implementation of gzip compression, and the - original documentation on which this specification is based, were - created by Jean-Loup Gailly <gzip@prep.ai.mit.edu>. Since this - implementation is a de facto standard, we mention some more of its - features here. Again, the material in this section is not part of - the specification per se, and implementations need not follow it to - be compliant. - - When compressing or decompressing a file, gzip preserves the - protection, ownership, and modification time attributes on the local - file system, since there is no provision for representing protection - attributes in the gzip file format itself. Since the file format - includes a modification time, the gzip decompressor provides a - command line switch that assigns the modification time from the file, - rather than the local modification time of the compressed input, to - the decompressed output. - -8. Appendix: Sample CRC Code - - The following sample code represents a practical implementation of - the CRC (Cyclic Redundancy Check). (See also ISO 3309 and ITU-T V.42 - for a formal specification.) - - The sample code is in the ANSI C programming language. Non C users - may find it easier to read with these hints: - - & Bitwise AND operator. - ^ Bitwise exclusive-OR operator. - >> Bitwise right shift operator. When applied to an - unsigned quantity, as here, right shift inserts zero - bit(s) at the left. - ! Logical NOT operator. - ++ "n++" increments the variable n. - 0xNNN 0x introduces a hexadecimal (base 16) constant. - Suffix L indicates a long value (at least 32 bits). - - /* Table of CRCs of all 8-bit messages. */ - unsigned long crc_table[256]; - - /* Flag: has the table been computed? Initially false. */ - int crc_table_computed = 0; - - /* Make the table for a fast CRC. */ - void make_crc_table(void) - { - unsigned long c; - - - -Deutsch Informational [Page 11] - -RFC 1952 GZIP File Format Specification May 1996 - - - int n, k; - for (n = 0; n < 256; n++) { - c = (unsigned long) n; - for (k = 0; k < 8; k++) { - if (c & 1) { - c = 0xedb88320L ^ (c >> 1); - } else { - c = c >> 1; - } - } - crc_table[n] = c; - } - crc_table_computed = 1; - } - - /* - Update a running crc with the bytes buf[0..len-1] and return - the updated crc. The crc should be initialized to zero. Pre- and - post-conditioning (one's complement) is performed within this - function so it shouldn't be done by the caller. Usage example: - - unsigned long crc = 0L; - - while (read_buffer(buffer, length) != EOF) { - crc = update_crc(crc, buffer, length); - } - if (crc != original_crc) error(); - */ - unsigned long update_crc(unsigned long crc, - unsigned char *buf, int len) - { - unsigned long c = crc ^ 0xffffffffL; - int n; - - if (!crc_table_computed) - make_crc_table(); - for (n = 0; n < len; n++) { - c = crc_table[(c ^ buf[n]) & 0xff] ^ (c >> 8); - } - return c ^ 0xffffffffL; - } - - /* Return the CRC of the bytes buf[0..len-1]. */ - unsigned long crc(unsigned char *buf, int len) - { - return update_crc(0L, buf, len); - } - - - - -Deutsch Informational [Page 12] - diff --git a/zlib/doc/txtvsbin.txt b/zlib/doc/txtvsbin.txt deleted file mode 100644 index 3d0f063..0000000 --- a/zlib/doc/txtvsbin.txt +++ /dev/null @@ -1,107 +0,0 @@ -A Fast Method for Identifying Plain Text Files -============================================== - - -Introduction ------------- - -Given a file coming from an unknown source, it is sometimes desirable -to find out whether the format of that file is plain text. Although -this may appear like a simple task, a fully accurate detection of the -file type requires heavy-duty semantic analysis on the file contents. -It is, however, possible to obtain satisfactory results by employing -various heuristics. - -Previous versions of PKZip and other zip-compatible compression tools -were using a crude detection scheme: if more than 80% (4/5) of the bytes -found in a certain buffer are within the range [7..127], the file is -labeled as plain text, otherwise it is labeled as binary. A prominent -limitation of this scheme is the restriction to Latin-based alphabets. -Other alphabets, like Greek, Cyrillic or Asian, make extensive use of -the bytes within the range [128..255], and texts using these alphabets -are most often misidentified by this scheme; in other words, the rate -of false negatives is sometimes too high, which means that the recall -is low. Another weakness of this scheme is a reduced precision, due to -the false positives that may occur when binary files containing large -amounts of textual characters are misidentified as plain text. - -In this article we propose a new, simple detection scheme that features -a much increased precision and a near-100% recall. This scheme is -designed to work on ASCII, Unicode and other ASCII-derived alphabets, -and it handles single-byte encodings (ISO-8859, MacRoman, KOI8, etc.) -and variable-sized encodings (ISO-2022, UTF-8, etc.). Wider encodings -(UCS-2/UTF-16 and UCS-4/UTF-32) are not handled, however. - - -The Algorithm -------------- - -The algorithm works by dividing the set of bytecodes [0..255] into three -categories: -- The white list of textual bytecodes: - 9 (TAB), 10 (LF), 13 (CR), 32 (SPACE) to 255. -- The gray list of tolerated bytecodes: - 7 (BEL), 8 (BS), 11 (VT), 12 (FF), 26 (SUB), 27 (ESC). -- The black list of undesired, non-textual bytecodes: - 0 (NUL) to 6, 14 to 31. - -If a file contains at least one byte that belongs to the white list and -no byte that belongs to the black list, then the file is categorized as -plain text; otherwise, it is categorized as binary. (The boundary case, -when the file is empty, automatically falls into the latter category.) - - -Rationale ---------- - -The idea behind this algorithm relies on two observations. - -The first observation is that, although the full range of 7-bit codes -[0..127] is properly specified by the ASCII standard, most control -characters in the range [0..31] are not used in practice. The only -widely-used, almost universally-portable control codes are 9 (TAB), -10 (LF) and 13 (CR). There are a few more control codes that are -recognized on a reduced range of platforms and text viewers/editors: -7 (BEL), 8 (BS), 11 (VT), 12 (FF), 26 (SUB) and 27 (ESC); but these -codes are rarely (if ever) used alone, without being accompanied by -some printable text. Even the newer, portable text formats such as -XML avoid using control characters outside the list mentioned here. - -The second observation is that most of the binary files tend to contain -control characters, especially 0 (NUL). Even though the older text -detection schemes observe the presence of non-ASCII codes from the range -[128..255], the precision rarely has to suffer if this upper range is -labeled as textual, because the files that are genuinely binary tend to -contain both control characters and codes from the upper range. On the -other hand, the upper range needs to be labeled as textual, because it -is used by virtually all ASCII extensions. In particular, this range is -used for encoding non-Latin scripts. - -Since there is no counting involved, other than simply observing the -presence or the absence of some byte values, the algorithm produces -consistent results, regardless what alphabet encoding is being used. -(If counting were involved, it could be possible to obtain different -results on a text encoded, say, using ISO-8859-16 versus UTF-8.) - -There is an extra category of plain text files that are "polluted" with -one or more black-listed codes, either by mistake or by peculiar design -considerations. In such cases, a scheme that tolerates a small fraction -of black-listed codes would provide an increased recall (i.e. more true -positives). This, however, incurs a reduced precision overall, since -false positives are more likely to appear in binary files that contain -large chunks of textual data. Furthermore, "polluted" plain text should -be regarded as binary by general-purpose text detection schemes, because -general-purpose text processing algorithms might not be applicable. -Under this premise, it is safe to say that our detection method provides -a near-100% recall. - -Experiments have been run on many files coming from various platforms -and applications. We tried plain text files, system logs, source code, -formatted office documents, compiled object code, etc. The results -confirm the optimistic assumptions about the capabilities of this -algorithm. - - --- -Cosmin Truta -Last updated: 2006-May-28 |