ANSI INCITS 241-1994 Information Systems - Data Compression Method - Adaptive Coding with Sliding Window for Information Interchange.pdf
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1、ANSI INCITS 241-1994 (R1999)(formerly ANSI X3.241-1994 (R1999)for Information Systems Data Compression Method Adaptive Coding with Sliding Windowfor Information InterchangeAmericanNationalStandardApproval of an American National Standard requires review by ANSI that therequirements for due process,
2、consensus, and other criteria for approval havebeen met by the standards developer.Consensus is established when, in the judgment of the ANSI Board of StandardsReview, substantial agreement has been reached by directly and materiallyaffected interests. Substantial agreement means much more than a si
3、mplemajority, but not necessarily unanimity. Consensus requires that all views andobjections be considered, and that a concerted effort be made toward theirresolution.The use of American National Standards is completely voluntary; their existencedoes not in any respect preclude anyone, whether he ha
4、s approved the standardsor not, from manufacturing, marketing, purchasing, or using products, processes,or procedures not conforming to the standards.The American National Standards Institute does not develop standards and will inno circumstances give an interpretation of any American National Stand
5、ard.Moreover, no person shall have the right or authority to issue an interpretation ofan American National Standard in the name of the American National StandardsInstitute. Requests for interpretations should be addressed to the secretariat orsponsor whose name appears on the title page of this sta
6、ndard.CAUTION NOTICE: This American National Standard may be revised orwithdrawn at any time. The procedures of the American National StandardsInstitute require that action be taken periodically to reaffirm, revise, or withdrawthis standard. Purchasers of American National Standards may receive curr
7、entinformation on all standards by calling or writing the American National StandardsInstitute.CAUTION: The developers of this standard have requested that holders of patents that may be required for theimplementation of the standard disclose such patents to the publisher. However, neither the devel
8、opers nor the publisherhave undertaken a patent search in order to identify which, if any, patents may apply to this standard. As of the date ofpublication of this standard, following calls for the identification of patents that may be required for the implementation ofthe standard, notice of one or
9、 more such claims has been received. By publication of this standard, no position is takenwith respect to the validity of this claim or of any rights in connection therewith. The known patent holder(s) has (have),however, filed a statement of willingness to grant a license under these rights on reas
10、onable and nondiscriminatory termsand conditions to applicants desiring to obtain such a license. Details may be obtained from the publisher. No furtherpatent search is conducted by the developer or publisher in respect to any standard it processes. No representation ismade or implied that this is t
11、he only license that may be required to avoid infringement in the use of this standard.Published byAmerican National Standards Institute11 West 42nd Street, New York, New York 10036Copyright 1994 by Information Technology Industry Council (ITI)All rights reserved.No part of this publication may be r
12、eproduced in anyform, in an electronic retrieval system or otherwise,without prior written permission of ITI, 1250 Eye Street NW,Washington, DC 20005.Printed in the United States of AmericaANSIX3.241-1994American National Standardfor Information Systems Data Compression Method Adaptive Coding with S
13、liding Windowfor Information InterchangeSecretariatComputer and Business Equipment Manufacturers AssociationApproved August 30, 1994American National Standards Institute, Inc.iiContentsPageForewordii1 Scope and conformance 12 Normative references.13 Definitions.14 Algorithm identifier.15 Data format
14、 .2AnnexesA Encoding overview.5B Example compression encoding method.8iiiForeword (This foreword is not part of American National Standard X3.241-1994.)This standard specifies a lossless data compression method that is intendedfor general purposes. It contains features that make it particularly appl
15、icableto systems for recording information on interchangeable media.This standard was developed by Technical Committee X3B5, by X3 project882. The first draft was produced in November 1991. The second draft wasproduced in March 1992.There are two annexes in this standard. Both are informative and ar
16、e notconsidered part of this standard.Requests for interpretation, suggestions for improvement or addenda, ordefect reports are welcome. They should be sent to the X3 Secretariat,Computer and Business Equipment Manufacturers Association, 1250 EyeStreet NW, Suite 200, Washington, DC 20005.This standa
17、rd was processed and approved for submittal to ANSI by theAccredited Standards Committee on Information Processing Systems, X3.Committee approval of this standard does not necessarily imply that allcommittee members voted for its approval. At the time it approved thisstandard, the X3 Committee had t
18、he following members:James D. Converse, ChairDonald C. Loughry, Vice-ChairJoanne Flanagan, SecretaryOrganization Represented Name of RepresentativeAmerican Nuclear Society Geraldine C. MainSally Hartzell (Alt.)AMP, IncEdward KellyCharles Brill (Alt.)Apple Computer, Inc. .Karen HigginbottomDavid K. M
19、ichael (Alt.)AT an offset field (described in 5.5); a length field (described in 5.6).5.5 Offset fieldThe offset field of a string token is a variable-length bit pattern that represents the distance5 Data format5.1 OverviewThe data compression encoding method isdesigned to support an adaptive string
20、 com-pression algorithm that can find redundantmultiple byte patterns in the input data streamand replace them with shorter tokens in thecompressed output data stream. The outputdata stream alone may be used to reconstructthe original input stream completely andexactly.5.2 Principle of operationThe
21、input data stream and the output datastream shall consist of a stream of bytes.Within a byte, the bits shall be arranged withbit b8 as the most significant, and bit b1 asthe least significant.The output data bytes are composed of astream of fields with a variable number of bitsin each field. The mos
22、t significant bit of a fieldshall be placed into the most significantunused bit location of an output byte. Theother bits of a field shall be placed in bit loca-tions in an output byte by proceeding insequence towards the least significant end ofthe output byte. The end marker (see 5.7)may be used t
23、o force the output data streamto a byte-boundary.The output data stream consists of the follow-ing three field types: a raw byte token; a string token; an end marker.The structure of these fields is describedlater. Each field may be present multiple timesand may appear in any order. Each field shall
24、be completed before a new field may begin.The encoding algorithm shall maintain a 2048-byte history buffer. Each byte that enters theencoding algorithm from the input data streamshall be placed into the history buffer. Whenthe history buffer becomes full, the newestbytes shall replace the oldest byt
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