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diff --git a/zlib/doc/algorithm.txt b/zlib/doc/algorithm.txt new file mode 100644 index 000000000..c97f49502 --- /dev/null +++ b/zlib/doc/algorithm.txt @@ -0,0 +1,209 @@ +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 new file mode 100644 index 000000000..ce6428a0f --- /dev/null +++ b/zlib/doc/rfc1950.txt @@ -0,0 +1,619 @@ + + + + + + +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 new file mode 100644 index 000000000..403c8c722 --- /dev/null +++ b/zlib/doc/rfc1951.txt @@ -0,0 +1,955 @@ + + + + + + +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 new file mode 100644 index 000000000..a8e51b456 --- /dev/null +++ b/zlib/doc/rfc1952.txt @@ -0,0 +1,675 @@ + + + + + + +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 new file mode 100644 index 000000000..3d0f0634f --- /dev/null +++ b/zlib/doc/txtvsbin.txt @@ -0,0 +1,107 @@ +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 |