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[13/55] [abbrv] incubator-corinthia git commit: removed zlib
http://git-wip-us.apache.org/repos/asf/incubator-corinthia/blob/1a48f7c3/DocFormats/platform/3rdparty/zlib-1.2.8/doc/algorithm.txt
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-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
http://git-wip-us.apache.org/repos/asf/incubator-corinthia/blob/1a48f7c3/DocFormats/platform/3rdparty/zlib-1.2.8/doc/rfc1950.txt
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diff --git a/DocFormats/platform/3rdparty/zlib-1.2.8/doc/rfc1950.txt b/DocFormats/platform/3rdparty/zlib-1.2.8/doc/rfc1950.txt
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-
-
-
-
-
-
-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: <gh...@aladdin.com>
-
-
- Jean-Loup Gailly
-
- EMail: <gz...@prep.ai.mit.edu>
-
- Questions about the technical content of this specification can be
- sent by email to
-
- Jean-Loup Gailly <gz...@prep.ai.mit.edu> and
- Mark Adler <ma...@alumni.caltech.edu>
-
- Editorial comments on this specification can be sent by email to
-
- L. Peter Deutsch <gh...@aladdin.com> and
- Glenn Randers-Pehrson <ra...@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);
- }
-
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-
-
-
-
-
-
-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.
-
-
-
-
-
-
-
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-Deutsch Informational [Page 7]
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-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]
-
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-
-
- 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:
-
-
-
-
-
-
-
-
-
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-
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-
-
- 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
-
-
-
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-
-
- 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)
-
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- (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
-
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- 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.
-
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-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.
-
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-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: <gh...@aladdin.com>
-
- Questions about the technical content of this specification can be
- sent by email to:
-
- Jean-Loup Gailly <gz...@prep.ai.mit.edu> and
- Mark Adler <ma...@alumni.caltech.edu>
-
- Editorial comments on this specification can be sent by email to:
-
- L. Peter Deutsch <gh...@aladdin.com> and
- Glenn Randers-Pehrson <ra...@alumni.rpi.edu>
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