United States Patent [191
`
`5,379,036
`[11] Patent Number:
`Storer
`[45] Date of Patent: Jan. 3, 1995
`
`
`
`||||||||I|l|||||||||||||||||I|l|l||l||||||||||l||||||||||Illlllllllllllllll
`US005379036A
`
`[57]
`
`ABSTRACI
`
`A data compression system utilizes multiple en-
`coding/decoding processing units arranged in a mes-
`sively parallcl systolic pipe array for compressing a
`stream of input characters received at an input end of
`the pipe. The systolic pipe executes data compression
`by textual substitution, converting the uncompressed
`stream of input characters into a compressed set of
`pointers representative of the input. The processors
`store a dictionary of strings of previously-read input
`characters in association with corresponding pointers.
`The processors match portions of the input stream with
`the stored strings, and transmit the corresponding point-
`ers in place of the strings. In order to execute decoding
`or decompression of the stream of pointers, the proces-
`sors maintain a dictionary identical to that generated
`during encoding. The processors match received point-
`ers with stored strings of data and transmit the corre-
`sponding data strings in place of the pointers. Novel
`methods for matching, update, and deletion in a mas-
`sively parallel systolic pipe structure enhance process-
`ing speed.
`
`[S4] MEI‘!-[OD AND APPARATUS FOR DATA
`COMPRESSION
`
`[76]
`
`Inventor:
`
`James A. Stnrer, 89 8. Great Rd..
`Lincoln, Mass. 01773
`
`[21] Appl. No.: 861,572
`
`[22] Filed:
`
`Apr. 1, I992
`
`H03M 7/30
`Int. Cl.‘
`[51]
`......... .. 341/51
`[52] US. Cl.
`[58] Field of Search ........................................ .. 341/51
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`34!/51
`34l/95
`341/SI
`341/51
`.'.':.'... 341/31
`341/s1
`
`
`
`4,553,302 I2/1985 Welch
`4,814,746 3/1989 Miller ct al.
`4,347,619 7/I989 Kato et al.
`4,376,541 10/1989 Storer
`4,881,075 I I/1939
`Wong
`5,175,543 12/1992
`
`OTI-[ER PUBLICATIONS
`
`Gonmlez—Smith, et 21]., "Parallel Algorithms for Data
`Compression"; Journal of the Association for Comput-
`ing Machinery; vol. 32 No. 2, Apr. 1935 pp. 344-373.
`
`Primary Exarmirer-—I-Icrward L. Williams
`
`23C.'la.iIns,6DrawingShee1s
`
`1-BIT REGISTERS
`
`IFULLB
`IFULLA
`OMARKA
`
`ORACLE 1017
`
`

`
`U.S. Patent
`
`Jan. 3, 1995
`
`Sheet 1 of 6
`
`5,379,036
`
`(101)
`
`(102)
`
`INIHIALIZE THE LOCAL DICTIONARY D TO HAVE ONE ENTRY
`FOR EACH CI-[ARACTER OF THE INPUT ALPHABET
`
`
`REPEAT FQEEVER:
`
`
`(GET THE CURRENT MATCH STRING S:)
`
`USE A MATCH OPERATION MX TO READ 8
`FROM 'I'I-IE INPUT.
`
`
`
`TRANSMIT (LOCQ IDI) BITS FOR THE INDEX
`OF S.
`
`
`
`
`
`(IFDIS FULL,'I'I-IENFIRSTI-EMPLOYA
`II ELETION OPERATION DX TO MAKE SPACE) .
`
`ADD EACH OF THE STRINGS SPECIFIED
`BY AN UPDATE OPERATION UX TO D.
`
`FIG. 1
`
`

`
`US. Patent
`
`Jan. 3, 1995
`
`Sheet 2 of 6
`
`5,379,036
`
`(201)
`
`
`
`INITIALIZE THE LOCAL DICTIONARY D
`BY PERFORMING STEP I OF THE ENCODING OPERATION.
`
`
`
`(202)
`
`
`(GET THE CURRENT MATCH STRING Sz)
`
`RECEIVE“-0(5[DDB]'I'S PDRTI-IE INDEX
`OF S.
`
`RETREIVE S FROM DANDOU'I'PUTTI-[E
`CHARACTERS OFS.
`
`
`
`
`
` (UPDATE D1)
`
`PERFORM STEP 2!) OF THE ENCODING
`ALGORITHM.
`
`FIG. 2
`
`

`
`U.S. Patent
`
`Jan. 3, 1995
`
`Sheet 3 of 6
`
`5,379,036
`
`I I I I I I *""°
`
`10.1
`
`10.
`
`10.
`
`10.
`
`10.
`
`10.
`
`I I I I I I
`
`10.9
`
`10.8
`
`10.
`
`T
`
`I ‘ I I I
`
`FIG.3
`
`

`
`U.S. Patent
`
`Jan. 3, 1995
`
`Sheet 4 of 6
`
`V D
`
`RESET
`
`1-BIT REGISTERS
`
`0 FULL B
`I RESET
`0 FULL. A
`. MODE
`C MARK A
`QFLUSI-I
`QLEADER 0MARK B
`C ACTIVE C DULIMY
`0 STOP
`
`MARK
`
`I E 5 D
`ER
`P
`
`STO
`
`1 2-BI'T
`DATA
`
`N11.
`
`GND
`
`FIG. 4
`
`VDD
`
`RESET
`
`MARK
`
`LEADER
`
`STOP
`
`12-BIT
`DATA
`
`NIL.
`
`GND
`
`

`
`U.S. Patent
`
`Jan. 3, 1995
`
`Sheet 5 of 6
`
`5,379,036
`
`
`
`PROCESSOR ARRAY
`
`PROCESSOR ARRAY
`
`FIG. 5
`
`

`
`US. Patent
`
`Jan. 3, 1995
`
`Sheet 6 of 5
`
`5,379,036
`
`INDEXING SCI-IEMCE
`
`- This is one possible indexing scheme. The indentations reflect the
`symmetry of the numbering sequence in the upper four bits.
`
`chipB1[831. . .8?e] --> chip02[881. . .8'Fe] —->
`chip@3[901. . .9?e] --> chip64[981. . .9fe] -->
`chipB5[a01. . .a?e] --> chip@6[o81. . .afe -->
`chip87[b01. . .b?e] ——> chip@8 [b81. . .bfe] -—>
`chip09[c61. . . c?e] --> chi p10[c81. . . cfe] -->
`chip11[d@1. . .d7e] —-3» chip12[d81. . .dfe] -->
`chip13[e01. . .e?e] --> chip14[e81. . .efe] —->
`chip1S[fB1. . .F?e] --> chip16[‘F81. . .1-Te] -->
`ch'ip1?[161. . .17e'_'| --> chip18[181. . .1fe] -—>
`chi.p19[201. . .27e] --> chip20[281. . .2fe] -->
`chip21[3@1. . .3?e] --> chip22[381. . .3'Fe]
`--:~
`chip23[4@1. . .47e] --> chip24[481. . .4fe] -->
`chip2S[SB1. . . 579] --> chip26[581. . . Sfej
`-—>
`chip27[661. . .6712] --> chip28 [681. . .6fe] -->
`chip29[701. . .7713] --> chip3[781. . .?fe]
`
`0 During encode chips 15 and 16 need f(hex) as their upper four hits; but
`during decode they need 0(hex) as their upper four bits. This is to be
`handled by the external control logic.
`
`FIG. 6
`
`

`
`1
`
`5,379,036
`
`2
`
`METHOD AND APPARATUS FOR DATA
`COLIPRESSION
`
`BACKGROUND OF THE INVENTION
`
`This invention relates generally to systems for digital
`data processing, and, more particularly,
`relates to
`highly parallel methods and apparatus for dynamic,
`high speed. on-line compression and decompression of
`digital data.
`Data compression methods. most of which exploit the
`redundancy that is characteristic of data streams, are
`becoming an essential element ofmany high speed data
`communications and storage systems. In ccmmuuim-
`tious systems, a transmitter can compress data before
`transmitting it, and a receiver can decompress the data
`after receiving it. thus increasing the effective data rate
`of the communications channel. In storage applications,
`data can be compressed before storage and decom-
`pressed after retrieval, thereby increasing the efiective
`capacity of the storage device.
`Partictilar applications of data compression include
`magnetic and optical disk interfaces, satellite communi-
`cations, computer network communications, intercon-
`nections between computers and attached devices, tape
`drive interface, and write-once media, where storage
`space reduction is critical.
`Compression of data streams can be lossless or lossy.
`Lossless data compression transforms a body of data
`into a smaller body of data from which it is possible to
`exactly and uniquely recover the original data. Lossy
`data comprwsicu transforms a body of data into a
`smaller one from which an acceptable approximation of
`the original—as defined by a selected fidelity criter-
`ion—can be constructed.
`Lossless compression is appropriate for applications
`in which it is unacceptable to lose even a single bit of
`information. These include transmission or storage of
`textual data, such as primed human language, program-
`ming language source code or object code. database
`information, numerical data, and electronic mail. Loss-
`less compression is also used for devices such as disk
`controllers that must provide exact preservation and
`retrieval of nncorrupted data.
`Lossy compression is useful in specialized applica-
`tions such as the transmission and storage of digitally
`sampled analog data, including speech, music, images,
`and video. Lossy compression ratios are typically much
`higher than those attainable by purely lossless compres-
`sion, depending upon on the nature of the data and the
`degree of fidelity required. For example, digitized
`speech that has been sampled 8,000 times per second,
`with 8 bits per sample. typically compresses by less than
`a factor of 2 with any lossless algorithm. However, by
`first quantizing each sample, which preserves accept-
`able quality for many applications, compression ratios
`exceeding 20 to 1 can be achieved. In contrast, typical
`lossless compression ratios are 3 to 1 for English text, 5
`to I for programming language source code, and 10 to
`I for spreadsheets or other easily compressible data.
`The difference is even greater for highly compressible
`sources such as video. In such cases. quantization and
`dithering techniques may be applied in combination
`with otherwise lcssless compression to achieve high
`ratios of lossy compression.
`Among the most powerful approaches to lossless data
`compression are textual substitution methods, in which
`frequently-appearing dam strings are replaced by
`
`5
`
`IO
`
`15
`
`20
`
`25
`
`35
`
`40
`
`shorter indexes or pointers stored in correspondence
`with the data strings in a dictionary. Typically, an en-
`coder module and a decoder module maintain identical
`copies of a dictionary containing data strings that have
`appeared in the input stream. The encoder finds
`matches between portions of the input stream and the
`previous]y—encountered strings stored in the dictionary.
`The encoder then transmits, in place of the matched
`string, the dictionary index or pointer corresponding to
`the string.
`The encoder can also update the dictionary with an
`entry based on the current match and the current con-
`tents of the dictionary. If insufficient space is available
`in the dictionary, space is created by deleting strings
`from the dictionary.
`The decoder, operating in a converse manner, re-
`ceives at its input a set of indexes, retrieves each corre-
`sponding dictionary entry as a “current ma
`", and
`updates its dictionary. Because the encoder and decoder
`work in a "lock-step" fashion to maintain identical dic-
`tionaries, no additional communicaticn is necessary
`between the encoder and decoder.
`Thus, the input to the coder of an on-line textual
`substitution data compressor is a stream of bytes or
`characters, and the output is a sequence of pointers.
`Conversely, the input to the decoder is a stream of
`pointers and the output is a stream of bytes or charac-
`l.e.1's.
`
`A common example of a textual substitution method
`is the publicly available COMPRESS command of the
`UNIX system, which implements the method devel-
`oped by Lempel and Ziv. See J. Ziv, A. Lempel, "A
`Universal Algorithm for Sequential Data Commu-
`sicu,” IEEE Transactions on Infiarrnurion Theory, Vol.
`1'1‘-23, No. 5, pp. 337-343, 1977; and J. Ziv. A. Lempel,
`"Compression of Individual Sequences Via Variable
`Rate Coding," IEEE Trcnsocttbns on Infortrtcrian The-
`ory, Vol. IT-24, No. 5. pp. 530-536, 1978.
`Textual substitution methods are generally superior
`to conventional methods such as Huffman coding. For
`example, the COMPRESS command of the LTNDC sys-
`tem easily out-performs the COMPACT command of
`UNIX, whichisbased onl-luifmancoding.
`Further examples of data compression methods and
`apparatus are disclosed in the following:
`1.5.5. Pat. No. 4.875.541.
`U.S. Pat. No. 4,814-.746.
`S. Henriques, N. Rsnganathan. "A Parallel Architec-
`ture for Data Compression.” P:-oceedings of the IEEE
`Symposium on Parallel and Distributed Pracemfrrg, pp.
`2tS0—266, December 1990.
`R. Zito-Wolf, "A Broadcast/Reduce Architecture
`for I-Iigh-Speed Data Compression," Proceedings cfrhe
`IEEE Symposiurn on Parallel and Distributed Program-
`ing, pp. 174-131. December 1990.
`S. Bunion, G. Borriello, “Practical Dictionary Man-
`agement for Hardware Data Compression." Apr. 2,
`1990.
`
`65
`
`R. Zito-Wolf, “A Systolic Architecture for Sliding-
`Window Data Compression," Proceeding of the IEEE
`Workshop on VLSI Stgml .Pl'BC£S.§'Il'lg, pp. 339-351, 1990.
`C. Thomborson, B. Wei, “Systolic Implementations
`of a Move-to-Front Text Compressor.“ Journal of the
`Asrociotr'on fitr Comparing Machinery, pp. 283-290. 1989.
`J. Storer. Data Comp:-essr'on. Computer Science Press,
`pp. 163466, 1988.
`
`

`
`3
`J. Storer, "Textual Substitution Techniques for Data
`Compression," Combinatorial Algorithm: on Words.
`Springer-Verlag (Apostolico and Galil,
`ed.). pp.
`lll-130, 1985.
`V. Miller. M. Wegman, “Variations on a Theme by
`Ziv and Lempel,” Combinatorial Algorithxns on Words,
`Springer-Verlag (Apostolico and Galil, ed.), pp.
`131-140, 1935.
`M. Gonzalez Smith. 3. Storm’, "Parallel Algorithms
`for Data Compression," Journal of the Association fivr
`Computing Machinery. Vol. 32, No. 2. PP. 344-373,
`April 1935.
`U.S. Pat. No. 4,876,541 discloses a data compression
`system including an encoder for compressing an input
`stream and a decoder for decompressing the com-
`pressed data. The encoder and decoder maintain dictio-
`naries for storing strings of frequently appearing char-
`acters. Each string is stored in association with a corre-
`sponding pointer. The encoder matches portions of the
`input stream with strings stored in the encoder dictio-
`nary, and transmits the corresponding pointers in place
`ofthc strings, thacby providing data compression. The
`decoder decompresses the compressed dam in a con-
`verse manner. The system utilizes selected matching,
`dictionary update. and deletion methods to maintain
`processing efficiency.
`U.S. Pat. No. 4,814,746 discloses a data compression
`method including the steps of establishing a dictionary
`of strings of frequently appearing characters, determin-
`ing a longt string of the input stream that matches a
`string in the dictionary. transmitting the pointer associ-
`ated with that string in place of the string. and adding a
`new string to the dictionary. The new string is a concat-
`enation or linking of a previous matched string and the
`current matched string. The method also includes the
`step of deleting a least recently used string from the
`dictionary if the dictionary is full.
`Henriques et al. discloses a systolic array of proces-
`sors for executing sliding window data compression in
`accordance with the Ziv and Lempel method. Data
`compression is divided into parsing and coding. During
`parsing. the input string of symbols is split into sub-
`strings. During coding, each substring is sequentially
`coded into a fixed length code.
`Zito-Wolf ("A Broadcast/Reduce Architecture for
`High-Speed Data Compression") discusses a data com-
`pression system utilizing a sliding window dictionary
`and a combination of a systolic array and pipelined trees
`for broadcast of input characters and reduction of re-
`sults. in this system, for every character position of the
`input stream, the processor computes a pair (length,
`ofiket) identifying the longest matched string ending at
`that character.
`
`IO
`
`20
`
`30
`
`35
`
`45
`
`5,379,036
`
`4
`Thomborson et al. discloses systolic implementations
`of data compression utilizing move-to-front encoding.
`A move-to-front encoder finds the current ordinal posi-
`tion of a symbol in the input stream, transmits that ordi-
`nal position, and moves the symbol to the front of the
`list. A characteristic of this encoder is that more-fre-
`quently occurring input symbols will be at the front of
`the list.
`Storer (1988) discusses parallel processing implemen-
`tations utilizing the dynamic dictionary model of data
`compression. An ID heuristic is implemented for updat-
`ing the dictionary, and a SWAP heuristic is executed
`for deleting entries from the dictionary to create space
`for new entries The SWAP heuristic is implemented by
`doubling the storage elements and adding a controller
`to each end of the pipe, for switching input/output lines
`appropriately as the dictionaries are switched. A sys-
`tolic pipe irnplemenlation utilizes a "bottom up" match-
`i.ttgtechnique.1nparticular.asastreamot'charactersor
`pointers
`flows
`through the systolic pipe,
`longer
`matched strings are constructed from pairs of snmller
`matched strings. Each processor supports a FLAG bit
`that can be set during data compression to designate ll
`“ earning" processor in the pipe. The learning proces-
`sor is the tirst processor along the pipe that contains a
`dictionary try.
`Storer (1935) discusses ofi‘-line and on-line data com-
`pression methods
`textual substitution in which
`pointers are transmitted in place of character strings.
`An on-line implementation described therein utilizes an
`encoder and decoder, each having a fixed amount of
`local memory forming a local dictionary. Another im-
`plementation utilizes a systolic array of processing ele-
`ments utilizing a sliding dictionary to match strings in
`the input stream. The sliding dictionary stores the last N
`characters procmsed in the systolic pipeline. one char-
`acter per processing element The dictionary is updated
`by sliding old characters to the left in order to bring in
`new characters.
`Miller et al. (1935) discusses enhancements to the data
`compression methods of Ziv and Lempel. including a
`dictionary replacement strategy that enables the use of
`a fixed sine dictionary, methods for using a larger class
`ofstringsthatcanbeaddedtothedictionary,a.nd1ech-
`niques that avoid tmcomprmed output.
`Gonzalez—Smith er al. discloses systolic arrays for
`parallel implementation of data comprusion by textural
`substitution. The systems described therein implement a
`sliding dictionary method. in which characters being
`read in are compared with each of the elements of a
`dictionary that spans the N characters preceding the
`current character.
`
`Bunton et al. discusses data compression methods
`utilizing a dictionary tagging technique for deleting
`selected entries from the data compression dictionary.
`The TAG dictionary management scheme disclosed
`therein employs a structure known as a trio data struc-
`ture with tagged nodes.
`R. Zito-Wolf (“A Systolic Architecture for Sliding-
`Window Data tbmpression") discusses a systolic pipe
`implementation of textual substitution data compression
`utilizing a sliding window. The systolic-array architec-
`ture codes substrings of the input by reference to identi-
`cal sequences occurring in a bounded window of pre-
`ceding characters of the input, wherein the contents of
`the window form a dictionary of referenceable strings.
`
`65
`
`These publications accordingly disclose various sys-
`tems for data compression. However, one deficiency
`shared by conventional data compression methods and
`systems, such as those described above, is the tradeofi
`between the benefits of data compression and the com-
`putational costs of the encoding and subsequent decod-
`ing. There exists a need for data compression systems
`that provide high compression ratios and high speed
`operation, without necemitating complex encoding and
`decoding modules.
`It is thus an object of the invention to provide lossless
`data compression systems utilizing massively parallel
`architectures to compress and decompress data streams
`at high rates, and with high compression ratios.
`
`

`
`5
`It is a further object of the invention to provide such
`methods and apparatus that can be bodied in rela-
`tively simple, low-cost digital hardware.
`A further object of the invention is to provide data
`compression methods and apparatus that dynamically
`adapt to changing data streams.
`Other general and specific objects of the invention
`will in part be obvious and will in part appear hereinaf-
`ter.
`
`5
`
`SUMMARY OF T'I-IE INVENTION
`
`25
`
`30
`
`The foregoing objects are attained by the invention,
`one aspect of which provides a data compression sys-
`tem utilizing multiple encoding/decoding processing
`units arranged in a systolic pipe array for compressing a
`stream of input data received at an input end ofthe pipe.
`The systolic pipe executes data compression by tex-
`tual substitution, convening the uncompressed stream
`of input data into a oomprmed set of pointers represen-
`tative of‘ the input. The processors store a dictionary of 20
`previously-read strings of input data in association with
`corresponding pointers The processors match portions
`of the input stream with the stored strings, and transmit
`the corresponding pointers in place ofthe strings.
`In order to execute decoding or decompression of the
`stream of pointers, the processors maintain a decoding
`dictionary identical to that generated during encoding.
`The processors match the pointers with stored strings of
`data. and transmit the corresponding data strings in
`place of the pointers.
`A further aspect of the invention utilizes selected
`matching, update, and deletion methods to increase
`pnocwsing eflicieucy. The matching methods can in-
`clude a modified "greedy" heuristic; the updating fea-
`ture can include an implementation of a modified "iden-
`tity" ("ID") heuristic: and thedeletion methods include
`a “swap'’ heuristic.
`One practice of the invention utilizes a "bottom up”
`approach to finding the longest possible match between
`the input stream and a stored string, in which succes-
`sively longer match strings are constructed from
`shorter matches.
`The invention will next be described in connection
`with certain illustrated embodiments; however. it will
`be clear to those skilled in the art that various modifica-
`tions, additions and subtractions can be made without
`departing front the spirit or scope of the claims.
`DESCRIPTION OF THE DRAWHNIGS
`
`45
`
`50
`
`For a fuller understanding of the nature and objects
`of the invention, reference should be made to the fol-
`lowing detailed description and the accompanying
`drawings, in which:
`FIG. 1 is a flow chart of the overall data compres-
`sion/encoding method of the invention;
`FIG. 2 is a flow chart of the overall data decompres-
`sion/decoding method of the invention,-
`FIG. 3 is a schematic diagram of a systolic pipe for
`data compression in accord with the invention;
`FIG.4isase.hematicdiagramofaprocessorinthe
`systolic pipe of FIG. 3;
`FIG. 5 is a schematic diagram of a complete data
`compression system constructed in accordance with the
`invention, including two processor arrays and control-
`ler logic for executing a SWAP operation; and
`FIG. Gisaschemaficdiagramofanindexingsc
`quence utilized in one practice of the invention.
`
`55
`
`60
`
`65
`
`5,379,036
`
`6
`
`DESCRIPTION OF ILLUSTRATED
`EMBODl.'MIENTS
`
`Overview: FIGS. 1-3 depict the overall structure and
`function of a data compression system constructed in
`accord with the invention. In particular. FIGS. 1 and 2
`are flow charts of encoding and decoding operations
`utilizing on-line dynamic textual substitution. The en-
`coding operation depicted in FIG. 1 reads a stream of
`characters and writes a compressed stream of corre-
`sponding hits, while the decoding operation depicted in
`FIG. 2 receives the compressed stream of bits and gen-
`erates a stream of corresponding characters.
`FIG. 3 shows the systemic pipe structure of a data
`compression system constructed in accord with the
`invention, for executing the method steps depicted in
`FIGS. 1 and 2. A systolic pipe is a linear array of pro-
`cessors in which each processor communicates only
`with left and right adjacent processors. As indicated in
`FIGS. 1-3, the invention provides a massively parallel,
`systolic pipe multiprocessor system that compresses
`data by means of textual substitution
`Parallel implementations of dynamic textual substitu-
`tion were previously thought to require data hroadcmt
`or other complex global communication. The invention,
`however, exploits the massively parallel nature of the
`systolic pipe to execute dynamic textual substitution at
`high speed, without complex interprocessor connec-
`nous.
`
`Encoding/Decoding: These advantages are provided
`by implentatiou of the steps depicted in FIGS. 1 and
`2. As indicated therein, the encoder and decoder each
`have a tired, finite quantity of local memory forming a
`local dictionary D. The two local dictionaries are ini-
`tializedatatin1e'Il0tobeempty.Dur-ingencodingand
`decoding operations, the dictionaries change dynami-
`cally, and are maintained by a novel variant of an iden-
`tity ("ID") update technique, a SWAP deletion tech-
`nique. and a "bottom up" parallel version of the
`"greedy" match technique. These methods are dis-
`cussed in detail hereinafter.
`
`The illustrated methods and apparatus constitute a
`real-time. on-line data compression system. In an on-line
`systern, neither the sender nor the receiver can “see” all
`of the data at once. Data are constantly passed from the
`sender through the encoder, then through the decoder,
`and on to the receiver. The encoding and decoding
`operations shown in FIGS. 1 and 3 satisfy the require-
`ments of the following definition: an on-line compres-
`sion/decompression method is real-time if there exists a
`constant It (which is independent of the data being pro-
`cessed or the size of the dictionary) such that for every
`I: units of time, exactly one new character is read by the
`encoder and exactly one character is written by the
`decoder.
`
`Theonly exceptiontothis ruleis theallowancecfa
`small “ g” between the encoder and decoder. Thus,
`there exists another constant I. (that is independent of
`thedatabeingprocessed)suchthat (i) duriugthefirstl.
`units of time the decoder produces no characters. and
`(ii) given an input of finite length, L time units may
`elapse between the time that the encoder has finished
`reading characters, and the time that the decoder is
`finished transmitting characters.
`Further explanation of the invention is simplified by
`assuming that the input date stream is a sequence of 8-bit
`characters. The set of all possible characters. ie.. the
`integers 0 through 255, is referred to herein as the input
`
`

`
`7
`alphabet S. In addition, data handling in compressed
`form is assumed to be noiseless, such that no bits are
`added, lost, or changed. While this assumption is made
`for convenience, those skilled in the art will recognize
`that known error detection and correction techniques
`can be utilized to ensure that no hits are corrupted or
`lost.
`The input data stream is then encoded by executing
`the following steps, as depicted in FIG. 1:
`(101)
`the local dictionary D to have one
`entry for each character of the input alphabet. (102)
`repeat forever:
`(at) (Get the current match string Use a match opera-
`tion MX to read s from the input. Transmit [log2|D|]
`bits for the index of s.
`
`(b) (Update D:) Add each of the strings specified by
`anupdateoperateUXtoD (il'D isfull, then fi.rstem-
`ploy a deletion operation DX to make space).
`The compressed stream can then be decoded by exe-
`cuting the following steps, as depicted in FIG. 2:
`(201) Initialize the local dictionary D by performing
`Step 1 of the encoding operation.
`(202) repeat forever:
`(a)
`(Get
`the current match string 3:) Receive
`[logzlD [] bits for the index ofs. Retrieve s from D and
`output the characters of s.
`(b) (Update D:) Perform step 2!: of the encoding
`algorithm.
`The encoding and decoding operations of FIGS. 1
`and 2 are executed in lock-step to maintain identical
`copies of the constantly changing dictionary D. The
`encoder repeatedly finds a match between the incoming
`characters of the input stream and the dictionary, de-
`leta these characters from the input stream, transmits
`the index of the corresponding dictionary entry. and
`updates the dictionary. The dictionary update opera-
`tion, described hereinafter, generates an update string
`that is a function of the current contents of the diode-
`nary and the most recent match. If i.nsu.fficienl space is
`available in the dictionary for updating, a deletion oper-
`ation must be performed, as discussed in below.
`Similarly, the decoder repeatedly receives an index,
`retrieves. the corresponding dictionary entry as the
`“current match", and then executes the same operation
`as the encoder to update its dictionary.
`the number of
`In accordance with the invention,
`entries in the dictionary D can be varied. The term
`D”, is used herein to denote the maximum number of
`entries that the dictionary D may contain. A typical
`value is D,,m=4,(l96. Experimental simulations indi-
`cate that this value permits the system to achieve most
`of the compression attainable by the underlying en-
`coding/decoding method. while providing a reasonable
`limit on the number of entries in the dictionary. In one
`practice of the invention, the local dictionary D utilizes
`a data structure in which each dictionary entry occupies
`a constant amount of data space, independent of the
`actual length of the corresponding string.
`MX UX, DX: In executing the steps depicted in
`FIGS. 1 and 2., the invention performs the following
`operations:
`The match operation, MX: This operation removes
`from the input stream a string ‘'5'’ that is contained
`within the dictionary D. Since the charam of S are
`implicitly in D and can never be deleted, there is always
`at least one such 3.
`
`The update operation, UK: This operation that takes
`the local dictionary D and returns a set of strings that
`
`5
`
`20
`
`25
`
`35
`
`-15
`
`50
`
`55
`
`60
`
`65
`
`5,379,036
`
`8
`should be added to the dictionary if space can be found
`for them.
`The deletion operation DX: This operation takes the
`local dictionary D and returns a string of D that may be
`deleted.
`
`the following
`In accordance with the invention,
`methods are utilized for the matching, updating, and
`deleting tasks:
`The match operation MX provides an approximate
`determination of the longest possible match string, uti-
`lining a modified "greedy" match heuristic (which takes
`the longest possible match between the input stream and
`a string in D) in eonjunctionwilh a “bottom up" tech-
`nique, in which longer match strings are constructed
`from shorter matches.
`The update operation UK employs a modified ver-
`sion of an identity (ID) technique. A typical ID opera-
`tion adds the previous match concatenated with the
`current match. In conventional data compression sys-
`tems, the ID technique required complex data struc-
`tures that were characteriaed by undesirable worst-case
`properties. However, by employing a modified ID tech-
`nique in conjunction with the massively parallel archi-
`tecture depicted in FIG. 3, the invention avoids the
`need for complex data structures. The modified ID
`method is described hereinafter.
`In one practice of the invention, the deletion opera-
`tion DX plays a SWAP technique that utilizes pri-
`mary and auxiliary
`When the primary dic-
`tionary first fills to capacity, an auxiliary dictionary is
`started. but comprmion based on the primary dictio-
`nary is continued. From this point on, each time the
`auxiliary dictionary becomes full, the roles of the pri-
`mary and auxiliary dictionaries are reversed, and the
`auxiliary dictionary is reset to be empty. A structure for
`providing this function is depicted in FIG. 5, using two
`identical copies of the systolic pipe architecture (10,
`10'),
`in conjunction with conventional control logic
`elements (34, 36, 38) for switching the roles of the pri-
`mary and auxiliary dictionaries.
`Those skilled in the art will appreciate that the inven-
`tion can be implemented using other DX methods. Al-
`ternatives for D):
`include the known FREEZE
`method, in which the dictionary remains unchanged
`after it fills to capacity, and the conventional LRU
`(least recently used) method, in which the processors
`delete the least recently matched dictionary string. The
`FREEZE heuristic, however, can be unstable if the
`data characteristics change after the dictionary fills.
`Moreover. the SWAP method described above in con-
`nection with FIG. 5 is better adapted for parallel imple-
`mentations than is an LRU technique.
`Systolic Pipe: As indicated in FIG. 3, a preferred
`embodiment of the invention executes the above-
`described method steps using a systolic pipe 10. The
`pipe 10 includes a linear array of processors 10.1, 10.2.
`10.3, . .
`. , l|J.N, each connected only to left and right
`adjacent processors. The "snake" arrangement depicted
`in FIG. 3 can implement textual substitution methods at
`much higher data rates than are achievable in serial
`structures.
`
`A simple example of a systolic pipe is an automobile
`assembly line, which may produce a new automobile
`every twenty minutes even though each car is in the
`assembly line for a day. Although each station in the
`automobile assembly line performs a different task, the
`stations are at least conceptually identical, if we view
`then: all as taking as input a partially built car, perform-
`
`

`
`9
`ing an elementary operation (suchas welding), and then
`transmitting a partially completed car to the next sta-
`tion.
`
`10
`numbered so that ii'i<j, then processor i is to the left of
`processor j and occurs earlier in the pipe than processor
`_1.
`
`5,379,036
`
`As indicated in FIG. 4, a one-bit LEADER register is
`5 provided in register module 22 of each proomsor in the
`pipe. The LEADER BIT is initially ONE for the left-
`most processor and ZERO for all others. It is always
`the case that at most one processor is the LEADER
`PROCESSOR, that the processors to its left contain
`dictionary entries, and that processors to its right are
`empty.
`As the data str passes through the processors to
`the left of the LEADER PROCESSOR, it is encoded.
`In particular. whenever a pair of pointers enter a pro-
`cessor and matches the pair of pointers stored in the
`storage registers of that processor (see FIG. 4). that pair
`of pointers is removed from the data stream and re-
`placed by a single pointer—the index of that processor.
`Specifically, upon detection of a match. the index stored
`at index register 34 is pushed down into the Built regis-
`ter 28. Data passes unchanged through the processors to
`the right of the LEADER PROCESSOR. When a pair
`of pointers ters the LEADER PROCESSOR, they
`represent adjacent substrings of the original data re-
`ferred to as the "previous" -match and the “current"
`match. The LEADER PROCESSOR can simply adopt
`thispair ofpointersasits
`,setits LE.ADERBITto
`Z