US20030102989A1 - Coding apparatus and decoding apparatus - Google Patents

Abstract

A symbol string detection unit detects the second symbol string matching the first symbol string having a predetermined length n from input character strings. A matching length detection unit detects a matching length k between the third symbol string following the first symbol string and the fourth symbol string following the second symbol string. A coding unit codes an input symbol string based on the symbol string detected by the symbol string detection unit and the matching length k detected by the matching length detection unit.

Description

BACKGROUND OF THE INVENTION
• 1. Field of the Invention[0001]
• The present invention relates to a coding apparatus and a decoding apparatus which can be optimally applied in compressing and reconstructing various data such as CAD data, document data, etc.[0002]
• 2. Description of the Related Art[0003]
• Recently, an increasing volume of various types of data such as character codes, image data, etc. have been processed in a computer. When such large volume of data is stored and transmitted to a distant destination, it is common to compress the data with the redundant portion of the data removed to reduce the storage capacity and improve the transmission speed.[0004]
• There are two common data compressing systems. They are a dictionary type coding system based on the similarity in data sequences; and a probability statistic type coding system based on the frequency of occurrences of data strings.[0005]
• A typical example of the dictionary type coding system is an LZ77 system and an LZ78 system.[0006]
• In the LZ77 system, a predetermined buffer is provided, the position of the previous data matching in longest length is retrieved from the previously input data in the buffer, and the matching position and the matching length are used as codes.[0007]
• FIG. 1 shows the coding method in the conventional LZ77 system.[0008]
• In FIG. 1, assume that ‘a b a b c d e f a b c d e f g h . . . ’ is input as data to be compressed, and each character of the data to be compressed is assigned an input number indicating an occurrence position.[0009]
• First, if ‘a’ having the[0010]input number 1 is input, then the character ‘a’ is coded as is because it has no preceding characters. Then, when a character ‘b’ having theinput number 2 is input, it is compared with the previously input characters. However, there are no characters matching the character ‘b’, the character ‘b’ is coded as is. Furthermore, when a character string ‘a b’ having theinput numbers 3 and 4 is input, it is compared with the previously input character strings. As a result, since the character string matches a character string ‘a b’ having theinput numbers 1 and 2, the character string ‘a b’ having theinput numbers 3 and 4 is coded using the matching position and matching length. In this example, since the matching position is the position of the character ‘a’ having theinput number 1, and the matching length is 2, ‘(1, 2)’ is coded as the code of the character string ‘a b’ having theinput numbers 3 and 4.
• Next, when a character ‘c’ having the[0011]input number 5 is input, it does not match any of the previously input characters. Therefore, the character ‘c’ is coded as is. When a character ‘d’ having theinput number 6 is input, it does not match any of the previously input characters. Therefore, the character ‘d’ is coded as is. When a character ‘e’ having theinput number 7 is input, it does not match any of the previously input characters. Therefore, the character ‘e’ is coded as is. When a character ‘f ’ having theinput number 8 is input, it does not match any of the previously input characters. Therefore, the character ‘f’ is coded as is.
• Then, when a character string ‘a b c d e f’ having the[0012]input numbers 9 through 14 is input, it matches a character string ‘a b c d e f’ having theinput numbers 3 through 8. Therefore, the character string ‘a b c d e f’ having theinput numbers 9 through 14 is coded using the matching position and the matching length. In this example, since the matching position is position of the character ‘a’ having theinput number 3, and the matching length is 6, ‘(3, 6)’ is coded as the code of the character string ‘a b c d e f’ having theinput numbers 9 through 14.
• When a character ‘g’ having the[0013]input number 15 is input, it does not match any of the previously input characters. Therefore, the character ‘g’ is coded as is. When a character ‘h’ having theinput number 16 is input, it does not match any of the previously input characters. Therefore, the character ‘h’ is coded as is. On the other hand, in the LZ78 system, a previously input character string is entered in a dictionary, and an entered input number is coded.
• The LZ77 system has higher compression performance than the LZ78 system for data containing a repetition of a long character string. On the other hand, the LZ78 system has higher compression performance than the LZ77 system for data containing a repetition of a comparatively short character string. The LZ77 system and the LZ78 system are described in, for example, the document “The Introduction to the Document Data Compression Algorithm” by Tomohiko Uematsu published by CQ Publishing Company.[0014]
• A typical system of the probability statistic type coding system can be the arithmetic coding system and the Huffman coding system. Both arithmetic coding system and Huffman coding system obtain a compression effect by allotting a short code length to a character having a high occurrence probability according to the statistic occurrence frequency of each character[0015]
• The arithmetic coding system is described in, for example, the document “Arithmetic coding revisited” by Alister Moffat et al., 1995, IEEE Data Compression Conference, p202-211. The Huffman coding system is described in, for example, the document “The Introduction to the Document Data Compression Algorithm” by Tomohiko Uematsu published by CQ Publishing Company.[0016]
• To obtain a higher compression effect, a variable length coding method has been suggested based on the conditional occurrence probability (P[Xt|Xt−1]) in which not the occurrence probability (P(Xt)) of a single character but the dependence (hereinafter referred to as a context) between an input character and its previous is taken into account. This method is described in, for example, the document “Unbounded Length Contexts for PPM” by John G. Cleary et al., 1995, IEEE Data Compression Conference, p52-61.[0017]
• The probability statistic type coding system as well as the LZ78 system has higher compression performance for data containing a repetition of a comparatively short character string. Normally, the LZ78 system has a higher processing speed than the probability statistic type coding system. On the other hand, the probability statistic type coding system has a higher compression rate than the LZ78 system.[0018]
• However, the LZ78 system and the probability statistic type coding system have high compression rate for data containing a repetition of a comparatively short character string, but cannot have sufficient compression rate for data containing a repetition of a long character string.[0019]
• On the other hand, the LZ77 system has high compression rate for data containing a repetition of a long character string, but cannot have sufficient compression rate for data containing a repetition of a comparatively short string.[0020]
• Therefore, the conventional compression systems have difficulty in obtaining high compression rate for data containing a repetition of long character strings and comparatively short character strings.[0021]
• The present invention aims at providing a data coding apparatus capable of efficiently compressing both long and short character strings.[0022]
SUMMARY OF THE INVENTION
• To solve the above described problem, the present invention includes a symbol string detection unit for detecting a second symbol string matching a first symbol string having a predetermined length from an input symbol string; a matching length detection unit for detecting a matching length between a third symbol string following the first symbol string and a fourth symbol string following the second symbol string; and a coding unit for coding the input symbol string based on the symbol string detected by the symbol string detection unit and the matching length detected by the matching length detection unit.[0023]
• Thus, for input data having a repetition of long symbol strings, a part of matching symbol string can be coded based on the matching length. Accordingly, the input data having a repetition of long symbol strings can be efficiently compressed. In addition, since a remaining portion of a matching symbol string is used as a code for use in detecting a matching position, the matching position can be detected without newly inserting a code for use in detecting the matching position. As a result, even when input data having a repetition of short symbol strings is coded using a matching length, the deterioration of a compression rate can be prevented from being caused by a large number of new codes inserted for detection of a matching position.[0024]
• Furthermore, according to an aspect of the present invention, when a first symbol string matching a second symbol string having a predetermined length occurs, a third symbol string following the first symbol string is coded based on the matching length between the third symbol string and a fourth symbol string following the second symbol string. The portion not coded based on the matching length is coded using the code of a symbol immediately succeeding a symbol string which is a context.[0025]
• Thus, for input data having a repetition of long symbol strings, a matching symbol string can be coded based on a matching string. Input data having a repetition of short symbol strings can be coded by allotting a shorter code length to a symbol string having a higher occurrence probability. As a result, a high compression rate can be attained for both data having a repetition of long symbol strings and data having a repetition of short symbol strings.[0026]
• Furthermore, according to another aspect of the present invention, when a first symbol string matching a second symbol string having a predetermined length occurs, a third symbol string following the first symbol string is coded based on the matching length between the third symbol string and a fourth symbol string following the second symbol string. The portion not coded based on the matching length is coded by retrieving a coded word corresponding to the current symbol string from the dictionary in which the symbol strings occurred previously are entered in association with coded words.[0027]
• Thus, for input data having a repetition of long symbol strings, a matching symbol string can be coded based on a matching string, and input data having a repetition of short symbol strings can be coded by the LZ 78 system. As a result, a high compression rate can be attained for both data having a repetition of long symbol strings and data having a repetition of short symbol strings.[0028]
• According to a further aspect of the present invention, when a first symbol string matching a second symbol string having a predetermined length occurs, a third symbol string following the first symbol string is coded based on the matching length between the third symbol string and a fourth symbol string following the second symbol string. The data coded based on the matching length is further coded using the code of a symbol immediately succeeding a symbol string which is a context.[0029]
• Thus, for input data having a repetition of long symbol strings, a matching symbol string can be coded based on the matching length. Accordingly, the input data having a repetition of long symbol strings can be efficiently compressed. In addition, when a short symbol string repeatedly occurs in compressed data coded based on a matching length, the compressed data coded based on the matching length can be furthermore compressed by allotting a short code length to a symbol string having a high occurrence probability, thereby attaining a high compression rate.[0030]
• According to a further aspect of the present invention, when a first symbol string matching a second symbol string having a predetermined length occurs, a third symbol string following the first symbol string is coded based on the matching length between the third symbol string and a fourth symbol string following the second symbol string. The data coded based on the matching length is further coded by retrieving a coded word corresponding to the current symbol string from the dictionary in which the symbol strings occurred previously are entered in association with coded words.[0031]
• Thus, for input data having a repetition of long symbol strings, a matching symbol string can be coded based on the matching length. Accordingly, the input data having a repetition of long symbol strings can be efficiently compressed. In addition, the compressed data coded based on the matching length can be further compressed by the LZ78 system. Therefore, a high compression rate can be attained for both data having a repetition of long symbol strings and data having a repetition of short symbol strings.[0032]
• According to a further aspect of the present invention, the occurrence position of a symbol string which previously occurred is stored in association with a predetermined code, and it is checked whether or not a code corresponding to a symbol string immediately before a symbol string coded based on a matching length is stored, thereby detecting the occurrence position of a previous symbol string to be compared when the symbol string is coded based on the matching length.[0033]
• Thus, when the occurrence position of the previous symbol string to be compared based on the matching length is checked, it is not necessary to check back one by one the previous symbol strings until a symbol string matching in a previous symbol string can be detected, thereby performing a process at a higher speed.[0034]
• Furthermore, according to a further aspect of the present invention, when a matching length is shorter than a predetermined value, the symbol string is not coded based on the matching length.[0035]
• Thus, when a matching length is short, the deterioration of a compression rate caused by adding a code indicating a matching length can be successfully avoided, thereby improving the compression rate in a coding process.[0036]
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 shows the encoding method using the conventional LZ77 code;[0037]
• FIG. 2 is a block diagram of the configuration of the coding apparatus according to the first embodiment of the present invention;[0038]
• FIG. 3 shows the coding method and the decoding method according to the first embodiment of the present invention;[0039]
• FIG. 4 shows the data compressing method according to the second embodiment of the present invention;[0040]
• FIG. 5 is a block diagram of the configuration of the data compression apparatus according to the second embodiment of the present invention;[0041]
• FIG. 6 is a block diagram of the configuration of the data reconstruction apparatus according to the second embodiment of the present invention;[0042]
• FIG. 7 shows the data compressing method according to the third embodiment of the present invention;[0043]
• FIG. 8 is a block diagram of the configuration of the data compression apparatus according to the third embodiment of the present invention;[0044]
• FIG. 9 is a block diagram of the configuration of the data reconstruction apparatus according to the third embodiment of the present invention;[0045]
• FIG. 10 shows the data compressing method according to the fourth embodiment of the present invention;[0046]
• FIG. 11 is a block diagram of the configuration of the data compression apparatus according to the fourth embodiment of the present invention;[0047]
• FIG. 12 is a block diagram of the configuration of the data reconstruction apparatus according to the fourth embodiment of the present invention;[0048]
• FIG. 13 shows the coding method and decoding method according to the fifth embodiment of the present invention;[0049]
• FIG. 14 is a block diagram of the configuration of the data compression apparatus according to the sixth embodiment of the present invention;[0050]
• FIG. 15 is a block diagram of the configuration of the data reconstruction apparatus according to the sixth embodiment of the present invention;[0051]
• FIG. 16 is a block diagram of the configuration of the data compression apparatus according to the seventh embodiment of the present invention;[0052]
• FIG. 17 is a block diagram of the configuration of the data reconstruction apparatus according to the seventh embodiment of the present invention;[0053]
• FIG. 18 is a block diagram of the configuration of the coding apparatus according to the eighth embodiment of the present invention;[0054]
• FIG. 19 shows the encoding method according to the eighth embodiment of the present invention;[0055]
• FIG. 20 shows the data compressing method according to the ninth embodiment of the present invention;[0056]
• FIG. 21 is a flowchart of the data compressing method according to the tenth embodiment of the present invention;[0057]
• FIG. 22 is a flowchart of the data compressing method according to the eleventh embodiment of the present invention;[0058]
• FIG. 23 is a flowchart of the data compressing method according to the twelfth embodiment of the present invention;[0059]
• FIG. 24 is a flowchart of the data compressing method according to the thirteenth embodiment of the present invention;[0060]
• FIG. 25 is a block diagram of the configuration of the data compression apparatus according to the fourteenth embodiment of the present invention; and[0061]
• FIG. 26 is a block diagram of the system configuration of the coding apparatus and the decoding apparatus according to an embodiment of the present invention.[0062]
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
• The coding apparatus according to an embodiment of the present invention is described below by referring to the attached drawings.[0063]
• FIG. 2 is a block diagram of the configuration of the coding apparatus according to the first embodiment of the present invention.[0064]
• In FIG. 2, a symbol[0065]string detection unit1 detects a second symbol string matching a first symbol string having a predetermined length of n from input symbol strings. A matchinglength detection unit2 detects a matching length k between a third symbol string following the first symbol string and a fourth symbol string following the second symbol string. Acoding unit3 codes an input symbol string based on the symbol string detected by the symbolstring detection unit1 and the matching length k detected by the matchinglength detection unit2.
• As a result, when the third symbol string is coded based on the matching length k, the occurrence position of the fourth symbol string corresponding to the third symbol string can be detected based on the first symbol string, and it is not necessary to insert an excess code indicating the occurrence position of the fourth symbol string. Therefore, a high compression rate can be maintained for a repetition of long symbol strings while the reduction of the compression rate for a repetition of short symbol strings can be avoided.[0066]
• FIG. 3 shows the coding method and the decoding method according to the first embodiment of the present invention.[0067]
• In FIG. 3, a second symbol string having the length of n is input as input data. Assuming that, after the second symbol string is input, a first symbol string having the length of n is input. Assuming that the first symbol string matches the second symbol string, the first symbol string is sequentially compared with the symbol strings input previously to detect the second symbol string matching the first symbol string, thereby specifying the occurrence position of the second symbol string.[0068]
• When the occurrence position of the second symbol string can be specified, the matching length k between the third symbol string following the first symbol string and the fourth symbol string following the second symbol string is checked, and the third symbol string is coded base on the matching length k.[0069]
• As a result, a code corresponding to the first symbol string of the input data, a code corresponding to the third symbol string of the input data, a code of the second symbol string of the input data, and a code of the matching length k can be generated as coded data.[0070]
• When the coded data are decoded, the code corresponding to the second symbol string, the code corresponding to the fourth symbol string, and the code corresponding to the first symbol string are decoded. Then, a symbol string matching the first symbol string is retrieved from the decoding result. If the second symbol string is detected as a symbol string matching the first symbol string, then the code of the matching length k following the code of the first symbol string is decoded to obtain the matching length k. If the matching length k is obtained, the fourth symbol string having the matching length k and following the second symbol string is output as a decoding result of the third symbol string.[0071]
• Thus, when the first and second symbol strings match each other, the third symbol string is coded based on the matching length k. As a result, when long symbol strings repeatedly occur, the repetition of the long symbol strings can be coded based on the matching length k, thereby efficiently compressing data. In addition, since the first symbol string is coded based on the symbol string, the fourth symbol string required when the third symbol string coded based on the matching length k is decoded can be detected using the first symbol string. As a result, it is not necessary to insert an identification code for use in detecting the fourth symbol string into coded data. Accordingly, data can also be efficiently compressed when short symbol strings repeatedly occurs.[0072]
• FIG. 4 shows the data compressing method according to the second embodiment of the present invention. According to the second embodiment, an n-character string is retrieved from the previously input data, and the currently input data is coded based on the matching length k of the matching portion from the (n+1)th character. According to the embodiment described below, a character is input. However, the present invention is not limited to the compression of character codes, but can be applied to various data. That is, based on an information logic, one work unit of data is referred to as a character (alphabet), and a string of any number of words is referred to as a character string.[0073]
• In FIG. 4, assume that a character string ‘a b a b c d e f a b c d e f g h . . . ’ is input as data to be compressed, and an input number indicating an occurrence position is assigned to each character of the data to be compressed. In addition, assume that n=3, a 3-character string is retrieved from the previously input data, and the currently input data is coded based on the matching length k of the matching portion from the fourth character.[0074]
• First if ‘a’ having the[0075]input number 1 is input, then the character ‘a’ is coded as is because it has no preceding characters. Then, when a character ‘b’ having theinput number 2 is input, it is compared, with the previously input characters. However, there are no characters matching the character ‘b’, the character ‘b’ is coded as is. Furthermore, when a character string ‘a b’ having theinput numbers 3 and 4 is input, it is compared with the previously input character strings. As a result, the character string matches a character string ‘a b’ having theinput numbers 1 and 2. However, since the length of the character string ‘a b’ is 2, and is smaller thann 3, the character string ‘a b’ having theinput numbers 3 and 4 are coded as is.
• Next, when a character ‘c’ having the[0076]input number 5 is input, it does not match any of the previously input characters. Therefore, the character ‘c’ is coded as is. When a character ‘d’ having theinput number 6 is input, it does not match any of the previously input characters. Therefore, the character ‘d’ is coded as is. When a character ‘e’ having theinput number 7 is input, it does not match any of the previously input characters. Therefore, the character ‘e’ is coded as is. When a character ‘f’ having theinput number 8 is input, it does not match any of the previously input characters. Therefore, the character ‘f’ is coded as is.
• When a character string ‘a b c’ having the[0077]input numbers 9 through 11 is input, it matches the character string ‘a b c’ having theinput numbers 3 through 5, and n=3. Therefore, the character string ‘a b c’ having theinput numbers 9 through 11 is coded as is, and the character string having the input numbers of and after 12 is coded based on the matching length k of the character string having the input numbers of and after 6. Since the character string ‘d e f’ having theinput numbers 6 through 8 matches the character string ‘d e f’ having theinput numbers 12 through 14, and the matching length k=3, the character string ‘d e f’ having theinput numbers 12 through 14 is coded based on the matching length of 3.
• Next, when a character ‘g’ having the[0078]input number 15 is input, it does not match any of the previously input characters. Therefore, the character ‘g’ is coded as is. When a character ‘h’ having theinput number 16 is input, it does not match any of the previously input characters. Therefore, the character ‘h’ is coded as is.
• As a result, coded data ‘[0079]a b a b c d e f a b c 3 g h . . . ’ is obtained for the data ‘a b a b c d e f a b c d e f g h . . . ’ to be compressed.
• Thus, the character string ‘a b c’ having the[0080]input numbers 9 through 11 is not coded based on a matching length even if it matches the character string ‘abc’ having theinput numbers 3 through 5. In this case, the character string ‘a b c’ having theinput numbers 9 through 11 is coded, and the character string ‘d e f’ having theinput numbers 12 through 14 following the character string ‘a b c’ having theinput numbers 9 through 11 is coded based on a matching length.
• Next, when the coded data ‘[0081]a b a b c d e f a b c 3 g h . . . ’ is decoded, the character string ‘a b a b c d e f’ is first decoded. Then, after the character string ‘a b a b c d e f’ is decoded, the character string ‘a b c’ immediately before the code indicating the matching length of 3 is decoded. When the code indicating the matching length of 3 following the character string ‘a b c’ is decoded, the previous character string matching the character string ‘a b c’ is retrieved from the previously decoded character string ‘a b a b c d e f’. When the previous character string ‘a b c’ is detected from the character string ‘a b a b c d e f’, the next code indicates a matching length. Thus, the 3-character string ‘d e f’ following the previous character string ‘a b c’ can be obtained, and the character string ‘d e f’ is output as a decoding result of a code indicating the matching length of 3 following the character string ‘a b c’.
• As described above, unlike the LZ77 system, only a matching length is used as a code according to the fourth embodiment of the present invention. Therefore, a code indicating a matching position is not required, thereby realizing efficient data compression even when short character strings repeatedly occur.[0082]
• FIG. 5 is a block diagram of the configuration of the data compression apparatus according to the second embodiment of the present invention.[0083]
• In FIG. 5A, an[0084]input buffer11 is provided with aLempel buffer18 for storing an already compressed portion of the data to be compressed shown in FIG. 5B; and aZiv buffer19 for storing a portion to be compressed of the data to be compressed. An occurrenceposition holding unit13 stores a part or all of the occurrence position of the character string having the length of n (hereinafter referred to as an n-character string) in theLempel buffer18.
• An occurrence/[0085]non-occurrence check unit14 checks whether or not the occurrence position of the previous n-character string which is the same as the n-character string immediately before theZiv buffer19 is stored in the occurrenceposition holding unit13, and notifies aswitch unit12 of the check result. When theswitch unit12 receives a notification that the occurrence position of the previous n-character string which is the same as the n-character string immediately before theZiv buffer19 is stored, theswitch unit12 instructs an occurrenceposition obtaining unit15 to obtain the occurrence position of the previous n-character string which is the same as the n-character string immediately before theZiv buffer19.
• When the occurrence[0086]position obtaining unit15 obtains the occurrence position of the previous n-character string which is the same as the n-character string immediately before theZiv buffer19, a matchinglength obtaining unit16 obtains the matching length k between the character string after the occurrence position of the previous n-character string in theLempel buffer18 and the character string at and after the start of theZiv buffer19. When the matching length k is obtained, a matchinglength coding unit17 codes based on the matching length k the portion, in the character string at and after the start of theZiv buffer19, matching the character string at and after the occurrence position of the previous n-character string in theLempel buffer18.
• On the other hand, when the[0087]switch unit12 receives a notification that the occurrence position of the previous n-character string which is the same as the n-character string immediately before theZiv buffer19 is not stored, the character string at and after the start of theZiv buffer19 is coded as is, and output to the output buffer.
• The occurrence[0088]position holding unit13 can be designed to hold only the latest occurrence position in the same n-character string in theLempel buffer18.
• FIG. 6 is a block diagram of the configuration of the data reconstruction apparatus according to the second embodiment of the present invention.[0089]
• In FIG. 6A, an[0090]output buffer27 is provided with aLempel buffer28 for storing an already reconstructed portion of the compressed data shown in FIG. 6B; and aZiv buffer29 for storing a portion to be reconstructed in the compressed data. An occurrenceposition holding unit23 stores a part or all of the occurrence position of the n-character string in theLempel buffer28.
• An occurrence/[0091]non-occurrence check unit22 checks whether or not the occurrence position of the previous n-character string which is the same as the n-character string immediately before theZiv buffer29 is stored in the occurrenceposition holding unit23, and notifies aswitch unit21 of the check result. When theswitch unit21 receives a notification that the occurrence position of the previous n-character string which is the same as the n-character string immediately before theZiv buffer29 is stored, theswitch unit21 instructs an occurrenceposition obtaining unit24 to obtain the occurrence position of the previous n-character string which is the same as the n-character string immediately before theZiv buffer29.
• When an occurrence[0092]position obtaining unit24 obtains the occurrence position of the previous n-character string which is the same as the n-character string immediately before theZiv buffer29, a matchinglength reconstruction unit25 reconstructs the matching length k from the code after the n-character string immediately before theZiv buffer29. When the matching length k is reconstructed, acopy unit26 copies a character string having the matching length k following the previous n-character string in theLempel buffer28, and outputs the copy result as a result of reconstructing the code after the n-character string immediately before theZiv buffer29.
• On the other hand, when the[0093]switch unit21 receives a notification that the occurrence position of the previous n-character string which is the same as the n-character string immediately before theZiv buffer29 is not stored, the input coded data is output as is to the output buffer.
• FIG. 7 shows the data compressing method according to the third embodiment of the present invention. According to the third embodiment, data to be compressed is coded by the method shown in FIG. 4, and the portion not replaced with a matching length is coded by the probability statistic type coding system using context.[0094]
• In FIG. 7, assume that a character string ‘a b a b c d e f a b c d e f g h . . . ’ is input as data to be compressed, and an input number indicating an occurrence position is assigned to each character of the data to be compressed. In addition, assume that n=3, a 3-character string is retrieved from the previously input data, and the currently input data is coded based on the matching length k of the matching portion from the fourth character.[0095]
• Furthermore, assume that there are a code table[0096]31 of characters; a code table32 of characters occurring after a context a; a code table33 of characters occurring after a context b; a code table34 of characters occurring after a context c; and a code table35 of characters occurring after a context ‘a b’.
• Also assume that, in the code table[0097]31, acode 111 is assigned to a character ‘a’, a code ‘110’ is assigned to a character ‘b’, a code ‘101’ is assigned to a character ‘c’, a code ‘100’ is assigned to a character ‘d’, a code ‘011’ is assigned to a character ‘e’, a code ‘010’ is assigned to a character ‘f’, a code ‘001’ is assigned to a character ‘g’, and a code ‘000’ is assigned to a character ‘h’,
• In addition, assume that, in the code table[0098]32, a code ‘1’ is assigned when the character ‘b’ occurs after the context ‘a’, and a code ‘0’ is assigned when a character other than the character ‘b’ occurs after the context ‘a’; in the code table33, a code ‘11’ is assigned when the character ‘a’ occurs after the context ‘b’, a code ‘10’ is assigned when the character ‘c’ occurs after the context ‘b’, and a code ‘01’ is assigned when a character other than the characters ‘a’ and ‘c’ occurs after the context ‘b’; in the code table 34, a code ‘1’ is assigned when the character ‘d’ occurs after the context ‘c’, a code ‘0’ is assigned when a character other than the character ‘d’ occurs after the context ‘c’; and in the code table35, a code ‘11’ is assigned when the character ‘a’ occurs after the context ‘a b’, a code ‘10’ is assigned when the character ‘c’ occurs after the context ‘a b’, and a code ‘01’ is assigned when a character other than the characters ‘a’ and ‘c’ occurs after the context ‘a b’.
• First, when the character ‘a’ having the[0099]input number 1 is input, ‘111’ is output as a code corresponding to the character ‘a’ by referring to the code table31. Next, when the character ‘b’ having theinput number 2 is input, the code table32 is referred to using as a context the character ‘a’ immediately before the character ‘b’ having theinput number 2, and ‘1’ is output as a code corresponding to the character ‘b’ having theinput number 2. Then, when the character ‘a’ having theinput number 3 is input, the code table33 is referred to using as a context the character ‘b’ immediately before the character ‘a’ having theinput number 3, and ‘11’ is output as a code corresponding to the character ‘a’ having theinput number 3. Next, when the character ‘b’ having theinput number 4 is input, the code table32 is referred to using as a context the character ‘a’ immediately before the character ‘b’ having theinput number 4, and ‘1’ is output as a code corresponding to the character ‘b’ having theinput number 4. Then, when the character ‘c’ having theinput number 5 is input, the code table35 is referred to using as a context the character string ‘a b’ immediately before the character ‘c’ having theinput number 5, and ‘10’ is output as a code corresponding to the character ‘c’ having theinput number 5. Next, when the character ‘d’ having theinput number 6 is input, the code table 34 is referred to using as a context the character ‘c’ immediately before the character ‘d’ having theinput number 6, and ‘l’ is output as a code corresponding to the character ‘d’ having theinput number 6.
• Next, when the character ‘e’ having the[0100]input number 7 is input, ‘011’ is output as a code corresponding to the character ‘e’ by referring to the code table31. Then, when the character ‘f’ having theinput number 8 is input, ‘010’ is output as a code corresponding to the character ‘f’ by referring to the code table31. Next, when the character ‘a’ having theinput number 9 is input, ‘111’ is output as a code corresponding to the character ‘a’ by referring to the code table31. When the character ‘b’ having theinput number 10 is input, the code table32 is referred to using as a context the character ‘a’ immediately before the character ‘b’ having theinput number 10, and ‘1’ is output as a code corresponding to the character ‘b’ having theinput number 10. Next, when the character ‘c’ having theinput number 11 is input, the code table35 is referred to using as a context the character string ‘a b’ immediately before the character ‘c’ having theinput number 11, and ‘10’ is output as a code corresponding to the character ‘c’ having theinput number 11.
• Since the character string ‘a b c’ having the[0101]input numbers 9 through 11 matches the character string ‘abc’ having theinput numbers 3 through 5, and n=3, the character strings having the input numbers equal to and larger than 12 are coded based on the matching length with the character strings having the input numbers equal to and larger than 6. Since the character string ‘d e f’ having theinput numbers 6 through 8 matches the character string ‘d e f’ having theinput numbers 12 through 14, and the matching length is 3, the character string ‘d e f’ having theinput numbers 12 through 14 is coded based on the matching length of 3, and ‘0011’ is output as a code corresponding to the matching length of 3.
• Next, when the character ‘g’ having the[0102]input number 15 is input, ‘001’ is output as a code corresponding to the character ‘g’ by referring to the code table31. Then, when the character ‘h’ having theinput number 16 is input, ‘000’ is output as a code corresponding to the character ‘h’ by referring to the code table31.
• According to the above described embodiment, a matching length code is expressed by 4 bits, but can be expressed by any number of bits. In addition, a code table can be prepared for a matching length, and a matching length can be coded based on the code table.[0103]
• FIG. 8 is a block diagram of the configuration of the data compression apparatus according to the third embodiment of the present invention[0104]
• In FIG. 8, an[0105]input buffer41 comprises a Lempel buffer for storing an already compressed portion of the data to be compressed; and a Ziv buffer for storing a portion to be compressed of the data to be compressed. An occurrenceposition holding unit43 stores a part or all of the occurrence position of the n-character string in the Lempel buffer.
• An occurrence/non-occurrence check unit[0106]44 checks whether or not the occurrence position of the same character string as the n-character string immediately before the Ziv buffer is stored in the occurrenceposition holding unit43, and notifies aswitch unit42 of the check result. When theswitch unit42 receives a notification that the occurrence position of the same character string as the n-character string immediately before the Ziv buffer is stored, it instructs an occurrenceposition obtaining unit45 to obtain the occurrence position of the same character string as the n-character string immediately before the Ziv buffer.
• When the occurrence[0107]position obtaining unit45 obtains the occurrence position of the same character string as the n-character string immediately before the Ziv buffer, a matchinglength obtaining unit46 obtains the matching length k between the character string at and after the occurrence position in the Lempel buffer and the character string at and after the start of the Ziv buffer. When the matching length k is obtained, a matchinglength coding unit47 codes a portion matching the character strings at and after the occurrence position in the character strings at and after the start of the Ziv buffer.
• On the other hand, when the[0108]switch unit42 receives a notification that the occurrence position of the same character string as the n-character string immediately before the Ziv buffer is not stored, it instructs a character-with-context retrieval unit48 to receive the next character from the data to be compressed using the immediatelyprevious character string 0 through (m−1) as a context, and retrieve it from a character-with-contextcode holding unit49. The character-with-contextcode holding unit49 holds a code corresponding to each type of next occurring character using the immediately previous 0 through (m−1) (m≧1) character string as a context, When the character-with-context retrieval unit48 retrieves a character with a context, a character-with-context coding unit50 obtains a code corresponding to the character with a context from the character-with-contextcode holding unit49.
• If an n-character string obtained by combining a context having the length of (n−1) with the following character is linked to the occurrence position of the n-character string in the occurrence[0109]position holding unit43, and the length of the context checked by the character-with-context retrieval unit48 is (n−1), then the notification that the occurrence position of the same character string as the n-character string immediately before the Ziv buffer is stored can be provided for the occurrence/non-occurrence check unit44.
• FIG. 9 is a block diagram of the configuration of the data reconstruction apparatus according to the third embodiment of the present invention.[0110]
• In FIG. 9, an[0111]output buffer70 comprises a Lempel buffer for storing an already reconstructed portion of compressed data and a Ziv buffer for storing a portion to be reconstructed of the compressed data. An occurrenceposition holding unit63 holds a part or all of the occurrence position of an n-character string in the Lempel buffer.
• An occurrence/non-occurrence check unit[0112]62 checks whether or not the occurrence position of the same character string as the n-character string immediately before the Ziv buffer is held by the occurrenceposition holding unit63, and notifies aswitch unit61 of the check result. When theswitch unit61 receives a notification that the occurrence position of the same character string as the n-character string immediately before the Ziv buffer is stored, it instructs an occurrenceposition obtaining unit64 to obtain the occurrence position of the same character string as the n-character string immediately before the Ziv buffer.
• When the occurrence[0113]position obtaining unit64 obtains the occurrence position of the same character string as the n-character string immediately before the Ziv buffer, a matchinglength reconstruction unit65 reconstructs the matching length k from an input code. When the matching length k is reconstructed, acopy unit66 copies a character string of the matching length k following the n-character string in the Lempel buffer, and outputs the copy result to the output buffer as the reconstruction result of the code after the n-character string immediately before the Ziv buffer.
• On the other hand, when the[0114]switch unit61 receives a notification that the occurrence position of the same character string as the n-character string immediately before the Ziv buffer is not stored, it instructs acontext retrieval unit67 to retrieve a code with the immediately previous 0 through (m−1) character string as a context from a character-with-contextcode holding unit68. The character-with-contextcode holding unit68 holds a code of the next character using the immediately previous 0 through (m−1) character string as a context. A character-with-context reconstruction unit69 reconstructs a character from the code retrieved by thecontext retrieval unit67.
• FIG. 10 shows the data compressing method according to the fourth embodiment of the present invention. In the fourth embodiment, data to be compressed is coded in the method shown in FIG. 3, and a portion not replaced with a matching length is coded by the LZ 78 system.[0115]
• In FIG. 10, assume that a character string ‘a b a b c d e f a b c d e f g h . . . ’ is input as data to be compressed, and an input number indicating an occurrence position is assigned to each character of the data to be compressed. In addition, assume that n=3, a 3-character string is retrieved from the previously input data, and the currently input data is coded based on the matching length k of the matching portion from the fourth character.[0116]
• In addition, assume that a[0117]dictionary81 in which characters ‘a’ through ‘h’ are entered is provided, and the characters ‘a’ through ‘h’ are respectively assigned thereference numbers 1 through 8.
• First, when a character ‘a’ having the[0118]input number 1 is entered, areference number 1 is output as a code corresponding to the character ‘a’ having theinput number 1 by referring to thedictionary81. Next, when a character ‘b’ having theinput number 2 is entered, areference number 2 is output as a code corresponding to the character ‘b’ having theinput number 2 by referring to thedictionary81. A character string ‘a b’ having theinput numbers 1 and 2 is entered in thedictionary81, and thedictionary81 is updated into adictionary82. Then, 9 is entered as a reference number of the character string ‘a b’.
• Next, when a character string ‘a b’ having the[0119]input numbers 3 and 4 is entered, thedictionary82 is referred to, and thereference number 9 is output as a code corresponding to the character string ‘a b’ having theinput numbers 3 and 4.
• Next, when a character ‘c’ having the[0120]input number 5 is entered, areference number 3 is output as a code corresponding to the character ‘c’ having theinput number 5 by referring to thedictionary 82. A character string ‘a b c’ having theinput numbers 3 through 5 is entered in thedictionary83, and 10 is entered as a reference number of the character string ‘a b c’.
• Next, when a character ‘d’ having the[0121]input number 6 is entered, areference number 4 is output as a code corresponding to the character ‘d’ having theinput number 6 by referring to thedictionary82. Then, when a character ‘e’ having theinput number 7 is entered, areference number 5 is output as a code corresponding to the character ‘e’ having theinput number 7 by referring to thedictionary82. A character string ‘d e’ is entered in thedictionary83, and 11 is entered as a reference number. When a character ‘f’ having theinput number 8 is entered, thedictionary82 is referred to, and thereference number 6 is output as a code corresponding to the character ‘f’ having theinput number 8. Then, a character string ‘e f’ is entered in thedictionary83, and 12 is entered as a reference number.
• Next, when a character string ‘a b c’ having the[0122]input numbers 9 through 11 is input, the character string ‘a b c’ having theinput numbers 9 through 11 is coded using thereference number 10 entered in thedictionary 83. The character string ‘a b c’ having theinput numbers 9 through 11 matches the previous character string ‘a b c’ having theinput numbers 3 through 5, and n=3, the character strings having the input numbers equal to and larger than 12 are coded based on the matching length with the character strings having the input numbers equal to and larger than 6. As a result, the character string ‘d e f’ having theinput numbers 6 through 8 matches the character string ‘d e f’ having theinput numbers 12 through 14, and the matching length k is equal to 3. Therefore, the character string ‘d e f’ having theinput numbers 12 through 14 is coded based on the matching length of 3.
• When a character ‘g’ having the[0123]input number 15 is input, it does not match any of the previously input characters. Therefore, thedictionary83 is referred to, and thereference number 7 is output as a code corresponding to the character ‘g’ having theinput number 15. Next, when a character ‘h’ having theinput number 16 is input, it does not match any of the previously input characters. Therefore, thedictionary83 is referred to, and thereference number 8 is output as a code corresponding to the character ‘h’ having theinput number 16.
• Thus, an initial dictionary has all characters that possibly occur and are assigned respective codes, and the characters are sequentially coded and entered in the dictionary to output a code which is assigned to characters/character strings entered in the dictionary and match each other based on the longest matching length (LZ78 code). In addition, when the same character string as the previous n-character string occurs (point 3), the matching length at and after the (n+1) character string is coded and output, and then back to the LZ78 code. By processing the matching starting portion as the character string as is, and coding it by the LZ 78 system or the probability statistic type coding system, a code indicating whether or not the subsequent code is a matching length code is not required. As a result, a high compression rate can be attained using a matching length code for a repetition of long character strings while a high compression rate by the LZ78 system and the probability statistic type coding system can be utilized as is for a repetition of short character strings.[0124]
• FIG. 11 is a block diagram of the configuration of the data compression apparatus according to the fourth embodiment of the present invention[0125]
• In FIG. 11, an[0126]input buffer91 comprises a Lempel buffer for storing an already compressed portion of the data to be compressed; and a Ziv buffer for storing a portion to be compressed of the data to be compressed. An occurrenceposition holding unit93 stores a part or all of the occurrence position of the n-character string in the Lempel buffer.
• An occurrence/non-occurrence check unit[0127]94 checks whether or not the occurrence position of the same character string as the n-character string immediately before the Ziv buffer is stored in the occurrenceposition holding unit93, and notifies aswitch unit92 of the check result. When theswitch unit92 receives a notification that the occurrence position of the same character string as the n-character string immediately before the Ziv buffer is stored, it instructs an occurrenceposition obtaining unit95 to obtain the occurrence position of the same character string as the n-character string immediately before the Ziv buffer.
• When the occurrence[0128]position obtaining unit95 obtains the occurrence position of the same character string as the n-character string immediately before the Ziv buffer, a matchinglength obtaining unit96 obtains the matching length k between the character string at and after the occurrence position in the Lempel buffer and the character string at and after the start of the Ziv buffer. When the matching length k is obtained, a matchinglength coding unit97 codes a portion matching the character strings at and after the occurrence position in the character strings at and after the start of the Ziv buffer.
• When a character/character[0129]string holding unit99 holds a code corresponding to a character or a character string having the length of 1 through m, and theswitch unit92 receives a notification that the occurrence position of the same character as the n-character string-immediately before the Ziv buffer is not held, it instructs a character/characterstring retrieval unit98 to retrieve a character or a character string matching the data to be compressed based on the longest matching length from the character or the character string hold in the character/characterstring holding unit99. A character/characterstring coding unit100 then codes the character or the character string retrieved by the character/characterstring retrieval unit98 using the code held by the character/characterstring holding unit99.
• If the character string having the length of n held in the character/character[0130]string holding unit99 is linked to the occurrence position of the n-character string in the occurrenceposition holding unit93, and the length of the character or the character string coded by the character/characterstring coding unit100 is n, then the occurrence/non-occurrence check unit94 can be informed that the occurrence position of the same character string as the n-character string immediately before the Ziv buffer is held.
• FIG. 12 is a block diagram of the configuration of the data reconstruction apparatus according to the fourth embodiment of the present invention.[0131]
• In FIG. 12, an[0132]output buffer119 comprises a Lempel buffer for storing an already reconstructed portion of compressed data and a Ziv buffer for storing a portion to be reconstructed of the compressed data. An occurrenceposition holding unit113 holds a part or all of the occurrence position of an n-character string in the Lempel buffer.
• An occurrence/non-occurrence check unit[0133]112 checks whether or not the occurrence position of the same character string as the n-character string immediately before the Ziv buffer is held by the occurrenceposition holding unit113, and notifies aswitch unit111 of the check result. When theswitch unit 111 receives a notification that the occurrence position of the same character string as the n-character string immediately before the Ziv buffer is stored, it instructs an occurrenceposition obtaining unit114 to obtain the occurrence position of the same character string as the n-character string immediately before the Ziv buffer.
• When the occurrence[0134]position obtaining unit114 obtains the occurrence position of the same character string as the n-character string immediately before the Ziv buffer, a matchinglength reconstruction unit115 reconstructs the matching length k from an input code. When the matching length k is reconstructed, acopy unit116 copies a character string of the matching length k following the n-character string in the Lempel buffer, and outputs the copy result to theoutput buffer119 as the reconstruction result of the code after the n-character string immediately before the Ziv buffer.
• When a character/character[0135]string holding unit117 holds a code corresponding to a character or a character string having the length of 1 through m, and theswitch unit111 receives a notification that the occurrence position of the same character as the n-character string immediately before the Ziv buffer is not held, it instructs a character/characterstring reconstruction unit118 to reconstruct a character or a character string based on the code held by the character/character stringcode holding unit117.
• Thus, if the occurrence[0136]position holding unit113 does not hold the immediately previous n-character string, it is not output as is, but is coded by the probability statistic type coding system or the LZ78 system. As a result, a high compression rate of the LZ78 system or the probability statistic type coding system can be obtained for a repetition of short character strings in a data string to be compressed, and a high compression rate can be obtained based on a matching length code for a repetition of long character strings.
• FIG. 13A shows the coding method according to the fifth embodiment of the present invention.[0137]
• In FIG. 13A, a[0138]first coding process121 is performed on input data, and then asecond coding process122 is performed on the input data, thereby performing a 2-step coding process on the input data.
• In this example, the[0139]first coding process121 codes a part of a matching symbol string based on a matching length, and thesecond coding process122 furthermore codes by the probability statistic type coding system the symbol string coded in thefirst coding process121. In addition, as a probability statistic type code, a Shannon Fanno code, a Huffman code, an arithmetic code, a Jones code, an adaptive Huffman code, an adaptive arithmetic code, a code using a context model, etc. can be used.
• Furthermore, the[0140]first coding process121 codes a part of a matching symbol string based on a matching length, and thesecond coding process122 furthermore codes by the dictionary type coding system the symbol string coded in thefirst coding process121. In addition, as a dictionary type code, an LZ78 code, an LZW code, an LZC code, an LZY code, an LZT code, an LZFG code, etc. can be used.
• FIG. 13B shows the decoding method according to the fifth embodiment of the present invention.[0141]
• In FIG. 13B, a[0142]first decoding process123 is performed on coded data, and then asecond decoding process124 is performed on the coded data, thereby performing a 2-step decoding process on coded data.
• FIG. 14 is a block diagram of the configuration of the data compression apparatus according to the sixth embodiment of the present invention. In this sixth embodiment, data to be compressed is coded by the method shown in FIG. 4, and the data coded by the method shown in FIG. 4 is furthermore coded by the probability statistic type coding system using a context.[0143]
• In FIG. 14, an[0144]input buffer131 comprises a Lempel buffer for storing an already compressed portion of the data to be compressed; and a Ziv buffer for storing a portion to be compressed of the data to be compressed. An occurrenceposition holding unit133 stores a part or all of the occurrence position of the n-character string in the Lempel buffer.
• An occurrence/[0145]non-occurrence check unit134 checks whether or not the occurrence position of the same character string as the n-character string immediately before the Ziv buffer is stored in the occurrenceposition holding unit133, and notifies aswitch unit132 of the check result. When theswitch unit132 receives a notification that the occurrence position of the same character string as the n-character string immediately before the Ziv buffer is stored, it instructs an occurrenceposition obtaining unit135 to obtain the occurrence position of the same character string as the n-character string immediately before the Ziv buffer.
• When the occurrence[0146]position obtaining unit135 obtains the occurrence position of the same character string as the n-character string immediately before the Ziv buffer, a matchinglength obtaining unit136 obtains the matching length k between the character string at and after the occurrence position in the Lempel buffer and the character string at and after the start of the Ziv buffer. When the matching length k is obtained, a matchinglength coding unit137 codes a portion matching the character strings at and after the occurrence position in the character strings at and after the start of the Ziv buffer, and outputs the result to anintermediate buffer138.
• When the[0147]switch unit132 receives a notification that the occurrence position of the same character as the n-character string immediately before the Ziv buffer is not held, it outputs the character string at and after the start of the Ziv buffer as is to theintermediate buffer138.
• The[0148]intermediate buffer138 holds a code indicating the matching length transmitted from the matchinglength coding unit137, and holds the data to be compressed and transmitted from theswitch unit132.
• A character-with-context[0149]code holding unit139 stores a code of a character occurring next using the immediatelyprevious character string 0 through (m−1) as a context. A character-with-context retrieval unit140 retrieves the subsequent character from the character-with-contextcode holding unit139 using the immediatelyprevious character string 0 through (m−1). When the character-with-context retrieval unit140 retrieves a character with a context, a character-with-context coding unit141 codes the character with a context based on the code held in the character-with-contextcode holding unit139.
• FIG. 15 is a block diagram of the configuration of the data reconstruction apparatus according to the sixth embodiment of the present invention.[0150]
• In FIG. 15, a character-with-context[0151]code holding unit151 stores a code of a character occurring next using the immediatelyprevious character string 0 through (m−1) as a context. Acontext retrieval unit152 retrieves a code having the immediatelyprevious character string 0 through (m−1) as a context from the character-with-contextcode holding unit151 using the immediatelyprevious character string 0 through (m−1). A character-with-context reconstruction unit153 reconstructs a character from the code retrieved by the character-with-contextcode holding unit151.
• An[0152]intermediate buffer154 holds the data reconstructed by the character-with-context reconstruction unit153. Anoutput buffer161 comprises a Lempel buffer storing an already reconstructed portion of input data held in theintermediate buffer154; and a Ziv buffer storing a portion to be reconstructed of input data held in theintermediate buffer154. An occurrenceposition holding unit157 holds a part or all of the occurrence position of the n-character string in the Lempel buffer.
• An occurrence/[0153]non-occurrence check unit156 checks whether or not the occurrence position of the same character string as the n-character string immediately before the Ziv buffer is stored in the occurrenceposition holding unit157, and notifies aswitch unit155 of the check result. When theswitch unit155 receives a notification that the occurrence position of the same character string as the n-character string immediately before the Ziv buffer is stored, it instructs an occurrence position obtaining unit158 to obtain the occurrence position of the same character string as the n-character string immediately before the Ziv buffer.
• When the occurrence position obtaining unit[0154]158 obtains the occurrence position of the same character string as the n-character string immediately before the Ziv buffer, a matchinglength reconstruction unit159 reconstructs the matching length k from the code held in theintermediate buffer154. When the matching length k is reconstructed, acopy unit160 copies a character string of the matching length k following the n-character string in the Lempel buffer, and outputs the copy result to theoutput buffer161 as a result of reconstructing the code after the n-character string immediately before the Ziv buffer.
• On the other hand, when the[0155]switch unit155 receives a notification that the occurrence position of the same character string as the n-character string immediately before the Ziv buffer is not held, it outputs the character string in theintermediate buffer154 as is.
• FIG. 16 is a block diagram of the configuration of the data compression apparatus according to the seventh embodiment of the present invention. According to the seventh embodiment, data to be compressed is coded in the method shown in FIG. 4, and the data coded in the method shown in FIG. 4 is furthermore coded by the LZ78 system.[0156]
• In FIG. 16, an[0157]input buffer171 comprises a Lempel buffer for storing an already compressed portion of the data to be compressed; and a Ziv buffer for storing a portion to be compressed of the data to be compressed. An occurrenceposition holding unit173 stores a part or all of the occurrence position of the n-character string in the Lempel buffer.
• An occurrence/[0158]non-occurrence check unit174 checks whether or not the occurrence position of the same character string as the n-character string immediately before the Ziv buffer is stored in the occurrenceposition holding unit173, and notifies aswitch unit172 of the check result. When theswitch unit172 receives a notification that the occurrence position of the same character string as the n-character string immediately before the Ziv buffer is stored, it instructs an occurrenceposition obtaining unit175 to obtain the occurrence position of the same character string as the n-character string immediately before the Ziv buffer.
• When the occurrence[0159]position obtaining unit175 obtains the occurrence position of the same character string as the n-character string immediately before the Ziv buffer, a matchinglength obtaining unit176 obtains the matching length k between the character string at and after the occurrence position in the Lempel buffer and the character string at and after the start of the Ziv buffer. When the matching length k is obtained, a matchinglength coding unit177 codes the portion matching with the character strings at and after the occurrence position in the character strings at and after the start of the Ziv buffer, and outputs the result to anintermediate buffer178.
• When the[0160]switch unit172 receives a notification that the occurrence position of the same character as the n-character string immediately before the Ziv buffer is not held, it outputs the character string at and after the start of the Ziv buffer as is to theintermediate buffer178.
• The[0161]intermediate buffer178 holds a code indicating the matching length transmitted from the matchinglength coding unit177, and holds the data to be compressed and transmitted from theswitch unit172.
• A character/character[0162]string holding unit179 holds a code corresponding to a character or a character string having the length of 1 through m, and a character/character string retrieval unit180 retrieves a character or a character string matching in longest length with the data stored in theintermediate buffer178 from the characters or character strings stored in the character/character stringcode holding unit179.
• A character/character[0163]string coding unit181 codes the character or character string retrieved by the character/character string retrieval unit180 according to the code held in the character/character stringcode holding unit179.
• FIG. 17 is a block diagram of the configuration of the data reconstruction apparatus according to the seventh embodiment of the present invention.[0164]
• In FIG. 17, a character/character string[0165]code holding unit191 holds a code corresponding to a character or a character string having the length of 1 through m, and a character/character string reconstruction unit192 reconstructs a character or a character string according to a code held in the character/character stringcode holding unit191. Anintermediate buffer193 holds the data reconstructed by the character/character string reconstruction unit192.
• An[0166]output buffer200 comprises a Lempel buffer storing an already reconstructed portion of input data held in theintermediate buffer193; and a Ziv buffer storing a portion to be reconstructed of input data held in theintermediate buffer193. An occurrenceposition holding unit196 holds a part or all of the occurrence position of the n-character string in the Lempel buffer.
• An occurrence/[0167]non-occurrence check unit195 checks whether or not the occurrence position of the same character string as the n-character string immediately before the Ziv buffer is stored in the occurrenceposition holding unit196, and notifies aswitch unit194 of the check result. When theswitch unit194 receives a notification that the occurrence position of the same character string as the n-character string immediately before the Ziv buffer is stored, it instructs an occurrenceposition obtaining unit197 to obtain the occurrence position of the same character string as the n-character string immediately before the Ziv buffer.
• When the occurrence[0168]position obtaining unit197 obtains the occurrence position of the same character string as the n-character string immediately before the Ziv buffer, a matchinglength reconstruction unit198 reconstructs the matching length k from the code held in theintermediate buffer193. When the matching length k is reconstructed, acopy unit199 copies a character string of the matching length k following the n-character string in the Lempel buffer, and outputs the copy result to theoutput buffer200 as a result of reconstructing the code after the n-character string immediately before the Ziv buffer.
• On the other hand, when the[0169]switch unit194 receives a notification that the occurrence position of the same character string as the n-character string immediately before the Ziv buffer is not held, it outputs the character string in theintermediate buffer193 as is.
• Thus, when an immediately previous n-character string is held by an occurrence[0170]position holding unit196, the data coded based on a matching length is temporarily held in theintermediate buffer193, and is further coded by the probability statistic type coding system or the LZ78 system, thereby effectively assigning a code to a long character string using the existing compression application as is.
• A code used in the probability statistic type coding system or the LZ78 system can be obtained in a static coding process in which a predetermined code is used from start to end, or in a dynamic coding process in which codes are sequentially updated based on coded data.[0171]
• As described above, according to the above described embodiment, a code indicating whether or not the following code is a matching length code is not required. Furthermore, based on the LZ78 system and the probability statistic type coding system, a matching length between the current and previous character strings can be output as a code as in the LZ77 system only when long character strings are repeated. Therefore, a high compression rate can be attained using a matching length code when long character strings are repeated while a high compression rate can also be attained by the LZ78 system or the probability statistic type coding system for a repetition of short character strings, thereby successfully obtaining a high compression rate for both short and long character strings.[0172]
• FIG. 18 is a block diagram of the configuration of the coding apparatus according to the eighth embodiment of the present invention. According to the embodiment shown in FIG. 3, when the same symbol string occurred previously, the following symbol string is coded by a matching length code. In this case, the time taken in the coding process is mainly occupied by the retrieval of the occurrence position of the previous symbol string having the length of n. Therefore, in the eighth embodiment of the present invention, the occurrence position of a previous symbol string having the length of n is coded and stored, and then obtained based on the storage result. Thus, the time taken to retrieve the previous occurrence position of the symbol string having the length of n can be shortened.[0173]
• In FIG. 18, an occurrence[0174]position storage unit211 codes and stores the occurrence position of an input symbol string. An occurrenceposition retrieval unit212 checks whether or not the code corresponding to the first symbol string having a predetermined length of n is stored in the occurrenceposition storage unit211. If the code is stored, the occurrence position of the second symbol string associated with the code is obtained. It is not necessary that the first symbol string always matches the second symbol string. That is, the first symbol string can be different from the second symbol string if the occurrence position of the second symbol string can be specified from the first symbol string. However, a higher compression rate can be obtained when the first symbol string matches the second symbol string. That is, when the first symbol string matches the second symbol string, there is a higher probability that a longer matching length k can be obtained between the third symbol string following the first symbol string and the fourth symbol string following the second symbol string.
• When the occurrence position of the second symbol string is detected, a matching[0175]length detection unit213 detects the matching length k between the fourth symbol string and the third symbol string by comparing the fourth symbol string following the second symbol string with the third symbol string following the first symbol string. When the matching length k is detected, a matchinglength coding unit214 codes the third symbol string based on the matching length k.
• FIG. 19 shows the coding method according to the eighth embodiment of the present invention.[0176]
• In FIG. 19, when input data is entered, a hash code is assigned to the symbol string having the length n, and the occurrence position of the symbol string is stored in a[0177]storage unit215. In this example, assume that a hash code B is assigned to the previous second symbol string having the length n, and the occurrence position A of the second symbol string is stored in thestorage unit215 corresponding to the hash code B.
• Next, when the first symbol string having the length n is input, a hash code corresponding to the first symbol string is generated, and it is checked whether or not a hash code corresponding to the first symbol string is stored in the[0178]storage unit215. Assume that a hash code B is assigned to the first symbol string, the occurrence position A stored corresponding to the hash code B is read from thestorage unit215. When the occurrence position A is obtained, a matching length k between the third symbol string following the first symbol string and the fourth symbol string at and after the occurrence position A is obtained, and the third symbol string is coded based on the matching length k.
• The first and second symbol strings are used to specify the position of the fourth symbol string for use in coding the third symbol string based on the matching length k. Thus, the position of the fourth symbol string for use in coding the third symbol string based on the matching length k should be specified using the first and second symbol strings. Therefore, the first and second symbol strings do not always have to match each other.[0179]
• FIG. 20 shows the data compressing method according to the ninth embodiment of the present invention. In the ninth embodiment, a hash code is generated from the three characters immediately before the character string to be coded, and the occurrence position of the previous character string for use in comparison is obtained from the generated hash code.[0180]
• Assume that a character string ‘a b c d e f a b q a a b c d a a b d e a b c d a a a q . . . ’ is input as data to be compressed, and the portion ‘a b c d e f a b q a a b c d a a b d e a b c’ has already been coded as shown in FIG. 20. In this case, a hash code is generated from the three characters in a[0181]window221 in the already coded character string, and the occurrence position of the three-character strings is stored in astorage unit226. For example, assume that, corresponding to a character string222 ‘a b c’, a hash code B is generated by a hashcode generation unit224, and an occurrence position A is stored in thestorage unit226 corresponding to the hash code B.
• Next, to code the character string ‘d a a a q . . . ’ after the already coded character string, a 3-character string[0182]223 ‘a b c’ immediately before the character string ‘d a a a q . . . ’ to be coded is observed, and a hash code of the 3-character string223 ‘a b c’ is obtained. In this example, since the 3-character string ‘a b c’ is assigned the hash code B, the hash code B is generated as a hash code of the 3-character string223 ‘a b c’.
• Next, it is checked whether or not the hash code B is stored in the[0183]storage unit226. If the hash code B is stored in thestorage unit226, then the occurrence position corresponding to the hash code B is obtained. In this example, since the hash code B has already been stored in thestorage unit226, the occurrence position A is obtained as the occurrence position of the previous 3-character string222 corresponding to the 3-character string223 ‘a b c’. When the occurrence position A is obtained, the character string ‘d a a a q . . . ’ to be coded is compared with the character string ‘d a a b d e a b c’ after the occurrence position A to obtain the matching length between these character strings. Since the 3-character strings ‘d a a’ match, the matching length is 3. When the matching length of 3 is obtained, the portion ‘d a a’ which is a character string to be coded is coded based on the matching length of 3.
• Thus, the time required to retrieve the previous occurrence position of the[0184]character string222 can be shortened by storing the previous occurrence position of thecharacter string222 in association with the hash code B, and obtaining the previous occurrence position of thecharacter string222 corresponding to thecharacter string223 through the hash retrieval.
• In the hash retrieval, there are a plurality of character strings for which the same hash codes are allocated. That is, the character string at the previous occurrence position associated with a hash code can be different from the character string for which the hash code is originally generated (generating the same hash codes from different character strings is referred to as a conflict). Therefore, when the hash retrieval is performed, the original key is normally checked for matching. However, according to the embodiment, the hash retrieval is performed as a key for use in determining whether or not the subsequent character string is to be coded using a matching length code. Since the character string used in the hash retrieval is not contained in the matching length code, it is not necessary that the character string matches the original key. Therefore, the present invention improves the processing speed not only by the hash retrieval but also by omitting a matching check.[0185]
• However, the compression rate is higher when the character string matches the original key. Therefore, a matching check can be made for the previous p characters. For example, the length n of the previous character string for use in generating a hash code can be 3 characters, and the length p of the previous character string for use in a matching check for a conflict in the hash can be 2 characters.[0186]
• In addition, different hash codes can be assigned to different character strings to prevent a conflict.[0187]
• FIG. 21 is a flowchart of the data compressing method according to the tenth embodiment of the present invention. According to the tenth embodiment of the present invention, the occurrence position of the previous character string corresponding to the character string to be coded based on a matching length is obtained by the hash retrieval. If the hash retrieval fails, the character string is coded by a statistic type code or an LZ78 code.[0188]
• In FIG. 21, an array address at which the occurrence position of a previous n-character string is hash-coded and stored is initialized (step S[0189]1). Next, a hash code of the n-character string immediately before the character string to be coded is generated (step S2), and it is checked whether or not the occurrence position corresponding to the hash code is stored at the array address (step S3). If the occurrence position corresponding to the hash code is stored at the array address, then the occurrence position of the previous n-character string corresponding to the n-character string immediately before the character string to be coded is obtained from the array address. Then, the matching length of the character string after the n-character string is checked, and the character string to be coded is coded based on the matching length (step S4).
• On the other hand, if the occurrence position corresponding to the hash code is not stored at an array address, then the character or the character string to be coded is coded by a statistic type code or an LZ78 code (step S[0190]5).
• Then, corresponding to the hash code of the n-character string immediately before the coded character string, the occurrence position of the n-character string is stored at an array address (step S[0191]6). Then, it is determined whether or not the coding process has been completed to the end of the input data (step S7). If the coding process has not been completed, then control is returned to step S2, and the above described processes are repeated.
• Thus, the time required to retrieve a character string can be shortened by hash-retrieving the occurrence position of the previous character string corresponding to the character string to be coded based on a matching length, thereby realizing a processing speed at a practical level.[0192]
• FIG. 22 is a flowchart of the data compressing method according to the eleventh embodiment of the present invention. According to the embodiment shown in FIG. 21, a coding process is performed based on a matching length without checking the matching between the n-character string immediately before the character string to be coded and the previous n-character string corresponding to the n-character string. However, according to the embodiment shown in FIG. 22, a matching check is made for the p characters immediately before the character string to be coded. Since the processes in steps S[0193]1 through S7 according to the embodiment shown in FIG. 22 are matching the processes according to the embodiment shown in FIG. 21, the descriptions are omitted here, and only the portions different from those in the embodiment shown in FIG. 21 are described below.
• In FIG. 22, when the hash code of the n-character string immediately before the character string to be coded is stored at an array address (step S[0194]3), it is checked whether or not the p-character string immediately before the character string to be coded matches the previous p-character string at the occurrence position retrieved by the hash retrieval (step S11). Then, if the p-character string immediately before the character string to be coded matches the previous p-character string at the occurrence position retrieved by the hash retrieval, then the character string to be coded is coded based on a matching length (step S4). On the other hand, if the p-character string immediately before the character string to be coded does not match the previous p-character string at the occurrence position retrieved by the hash retrieval, then the character or the character string to be coded is coded by a statistic type code or an LZ78 code (step S5).
• Thus, a previous character having a longer matching length can be retrieved by performing a matching check between a character string for retrieval for use in performing a coding process based on a matching length and a previous character string. As a result, a compression rate can be successfully improved.[0195]
• It is desired that n is set to 3, and p is set to 2. By setting these values, the retrieval time can be prevented from being prolonged with the compression rate improved.[0196]
• FIG. 23 is a flowchart of the data compressing method according to the twelfth embodiment of the present invention. In the above described embodiment, a previous character string is searched for using n-character string immediately before the character string to be coded, and the portion matching the previous character string is coded based on a matching length. However, when the length of a large number of repeated character strings is n, a code indicating the matching length of 0 is frequently added, thereby lowering the compression rate. Accordingly, in the embodiment shown in FIG. 23, when the character string following the n-character string used in retrieving the previous character string does not match the previous character string (in this case, the matching length=0), the next occurrence of the character string does not imply the coding based on the matching length. Therefore, a code indicating the matching length of 0 is prevented from being added, thereby preventing the compression rate from being lowered.[0197]
• In FIG. 23, the array address at which the occurrence position of the previous n-character string is stored after being hash-coded (step S[0198]21), and the hash code of the n-character string immediately before the character string to be coded is generated (step S22). Then, it is checked whether or not the contents of the array address specified by the hash code are ‘NOT_LEN’. If the contents of the array address is ‘NOT_LEN’, then the character or the character string to be coded is coded by a statistic type code or an LZ78 code (step S24). On the other hand, when the contents of the array address are not ‘NOT_LEN’, it is checked whether or not the occurrence position corresponding to the hash code is stored at the array address (step S25). When the occurrence position corresponding to the hash code is stored at the array address, the occurrence position of the previous n-character string corresponding to the n-character string immediately before the character string to be coded is obtained from the array address. Then, the matching length of the character string after the n-character strings is checked, and the character string to be coded is coded based on the matching length (step S26).
• On the other hand, the occurrence position corresponding to the hash code is not stored at the array address, the character or the character string to be coded is coded by a statistic type code or an LZ78 code (step S[0199]27).
• Next, it is checked whether or not the matching length computed in step S[0200]26 is 0 (step S28). If the matching length is 0, then ‘NOT_LEN’ is stored at the array address specified by the hash code (step S29). On the other hand, if the matching length is not 0, then the occurrence position of the n-character string is stored at the array address specified by the hash code (step S30). Next, it is determined whether or not the coding process has been completed to the end of the input data (step S31). If the coding process has not been completed, control is returned to step S22, and the above described process is repeated.
• In the above described embodiment, the coding process is not performed only when the matching length is 0, but the matching length can be any other values than 0.[0201]
• In addition, when it is determined whether or not the matching length between the current character string and the previous character string indicates a value larger than a predetermined value. If the matching length between the current character string and the previous character string indicates a value larger than a predetermined value, the coding process is performed using an LZ77 code. If it indicates a value smaller than a predetermined value, the coding process is performed using an LZ78 code or a probability statistic type code.[0202]
• FIG. 24 is a flowchart of the data compressing method according to the thirteenth embodiment of the present invention. According to the embodiment shown in FIG. 23, when the matching length after the n-character string is 0, the subsequent character string is not coded based on a matching length even if the n-character string occurs next. However, it is recommendable that the coding process based on a matching length is not limited for data which possibly contains a large number of repetitions of the same characters (for example, ‘0000’, ‘FFFF’, etc.). Therefore, according to the embodiment shown in FIG. 24, when an immediately previous character string contains a repetition of the same characters, switching into a matching length code is not limited. Thus, the compression rate can be improved for data which possibly contains a large number of repetitions of the same characters. Since the processes in steps S[0203]21 through S31 shown in FIG. 24 are the same as those according to the embodiment shown in FIG. 23, the detailed descriptions are omitted here, and only the portions different from those according to the embodiment shown in FIG. 23 are described below.
• In FIG. 24, when it is determined that a matching length is 0 in step S[0204]28, it is checked whether or not the character immediately before the character string to be coded matches the character before it (step S41). If they do not match each other, ‘NOT_LEN’ is stored at the array address specified by the hash code of the character string (step S29). On the other hand, if they match each other, the occurrence position of the n-character string is stored at the array address specified by the hash code of the character string (step S30).
• According to the embodiment shown in FIG. 24, when an immediately previous character string having the length of 2 contains a repetition of the same characters, no switching into a matching length code is performed. The character string immediately before can have the length of 2 or larger.[0205]
• FIG. 25 is a block diagram of the configuration of the data compressing apparatus according to the fourteenth embodiment of the present invention. Normally, when data is compressed by the probability statistic type coding system, the highest compression rate can be obtained using a character string having the length of 5 as the maximum context (referred to as a 5th context) (refer to, for example, the above described document “Unbounded Length Contexts for PPM”). However, the 5th context makes a slow process. Therefore, a coding process is performed with a matching length code combined with the probability statistic type coding system of the fixed primary context according to the embodiment shown in FIG. 25. By combining a matching length code with the probability statistic type coding system of the fixed primary context, a high compression rate can be obtained even if the process is simplified by limiting a compression target in the probability statistic type coding system to a short character string of two- or three-character string. In addition, as compared with the coding process using only a 5th context, the combination can furthermore improve the process speed.[0206]
• A statistic[0207]type coding unit231 and a matchinglength coding unit237 are provided as shown in FIG. 25. The statistictype coding unit231 performs a coding process using a fixed primary context, and256 code tables233 through235 corresponding to an immediately previous character are held in a codetable holding unit232.
• The original data is input to the statistic[0208]type coding unit231 and the matchinglength coding unit237. When the original data is input, the statistictype coding unit231 selects the code tables233 through235 corresponding to the character immediately before the input character string. Acoding unit236 codes the character string using the code entered in the selected code tables233 through235, and outputs the coding result. When the original data is entered, the matchinglength coding unit237 codes the input character string based on a matching length, and outputs the coding result.
• In the example shown in FIG. 25, the code tables[0209]233 through235 are provided corresponding to all 1-byte characters of 256 types. However, the number of code tables can be reduced by providing code tables corresponding to the features extracted from the immediately previous character. For example, a code table can be provided corresponding to, excluding the highest order bit of the character data, the remaining 7 bits of the character data.
• FIG. 26 is a block diagram of the system configuration of the coding apparatus and decoding apparatus (or the data compression apparatus and the data reconstruction apparatus) according to an embodiment of the present invention.[0210]
• In FIG. 26, a central processing unit (CPU)[0211]241 performs a general process,242 is read-only memory (ROM),243 is random access memory (RAM),244 is a communications interface,245 is a communications network,246 is an input/output interface, adisplay247 displays document data, etc., aprinter248 prints document data, etc.,memory249 temporarily stores document data, etc. read by ascanner250 for reading an input image, etc.,251 is a keyboard,252 is a pointing device such as a mouse, adriver253 drives a storage medium,254 is a hard disk,255 is an IC memory card,256 is a magnetic tape,257 is a floppy disk,258 is an optical disk such as CD-ROM, DVD-ROM, etc., and259 is a bus.
• A program for compressing data, a program for reconstructing data, data to be compresses, compressed data, etc. are stored in storage media such as the[0212]hard disk254, theIC memory card255, themagnetic tape256, thefloppy disk257, theoptical disk258, etc. Data can be compressed by reading a program for compressing data, and data to be compressed from these storage media to theRAM243. In addition, data can be reconstructed by reading a program for reconstructing data, and data to be compressed from these storage media to theRAM243. Furthermore, a program for compressing data and a program for reconstructing data can be stored in theROM242.
• Furthermore, a program for compressing data, a program for reconstructing data, data to be compressed, compressed data, etc. can be retrieved from the[0213]communications network245 through thecommunications interface244. Thecommunications network245 connected to thecommunications interface244 can be, for example, a LAN (local area network), a WAN (wide area network), Internet, an analog telephone network, a digital telephone network (ISDN: integral service digital network), a PHS (personal handy system), wireless communications networks such as satellite communications, etc.
• When a program for compressing data is activated, the[0214]central processing unit241 obtains data to be compressed from a storage medium such as thehard disk254 or thecommunications network245, etc. The obtained data to be compressed is compressed by the method shown in FIGS. 4, 7,10,20 through24, etc. The compressed data is stored in a storage medium such as thehard disk254, etc., and transmitted through thecommunications network245.
• By compressing data, the storage capacity can be reduced when data is stored in a storage medium such as the[0215]hard disk254, or the transmission time can be shortened when data is transmitted through thecommunications network245.
• In addition, a compressing time can be shortened by hash-coding and storing a symbol string which previously occurred in the[0216]RAM243, and by performing hash retrieval to search for the previous symbol string to be referred to when a coding process is performed based on a matching length.
• Furthermore, when a program for reconstructing data is activated, the[0217]central processing unit241 obtains compressed data from a storage medium such as thehard disk254, etc. or thecommunications network245, etc. Then, the obtained compressed data is reconstructed, and the reconstructed data is stored in a storage medium such as thehard disk254, etc., transmitted through thecommunications network245, displayed on thedisplay247, and printed on theprinter248.
• Thus, according to the present invention, a part of matching symbol string can be coded based on the matching length. Accordingly, the input data having a repetition of long symbol strings can be efficiently compressed. In addition, since a remaining portion of a matching symbol string is used as a code for use in detecting a matching position, the matching position can be detected without newly inserting a code for use in detecting the matching position. As a result, even when input data having a repetition of short symbol strings is coded using a matching length, the deterioration of a compression rate can be prevented from being caused by a large number of new codes inserted for detection of a matching position.[0218]
• According to another aspect of the present invention, a part of a matching symbol string is coded based on a matching length while a portion not coded based on the matching length is coded using the code of a symbol occurring next when a previous symbol string is a context. As a result, for input data having a repetition of long symbol strings, a matching symbol string can be coded based on a matching string. Input data having a repetition of short symbol strings can be coded by allotting a shorter code length to a symbol string having a higher occurrence probability. As a result, a high compression rate can be attained for both data having a repetition of long symbol strings and data having a repetition of short symbol strings.[0219]
• According to a further aspect of the present invention, a part of a matching symbol string is coded based on a matching length, and a portion not coded based on the matching length is coded by retrieving a coded word corresponding to the current symbol string from the dictionary in which the symbol strings occurred previously are entered in association with coded words. Thus, for input data having a repetition of long symbol strings, a matching symbol string can be coded based on a matching string, and input data having a repetition of short symbol strings can be coded by the LZ78 system. As a result, a high compression rate can be attained for both data having a repetition of long symbol strings and data having a repetition of short symbol strings.[0220]
• According to a further aspect of the present invention, a part of a matching symbol string is coded based on a matching length, and the data coded based on the matching length is further coded using the code of a symbol occurring next when a previous symbol string is a context. Thus, for input data having a repetition of long symbol strings, a matching symbol string can be coded based on the matching length. Accordingly, the input data having a repetition of long symbol strings can be efficiently compressed. In addition, when a short symbol string repeatedly occurs in the compressed data, the compressed data can be furthermore compressed by allotting a short code length to a symbol string having a high occurrence probability, thereby attaining a high compression rate.[0221]
• According to a further aspect of the present invention, a part of a matching symbol string is coded based on a matching length, and the coded data is further coded by retrieving a coded word corresponding to the current symbol string from the dictionary in which the symbol strings occurred previously are entered in association with coded words. Thus, for input data having a repetition of long symbol strings, a matching symbol string can be coded based on the matching length. Accordingly, the input data having a repetition of long symbol strings can be efficiently compressed. In addition, the compressed data can be further compressed by the LZ78 system. Therefore, a high compression rate can be attained for both data having a repetition of long symbol strings and data having a repetition of short symbol strings.[0222]
• According to a further aspect of the present invention, by storing the occurrence position of a symbol string to be used in retrieval for checking the occurrence position of a previous symbol string to be compared based on a matching length, it is not necessary to check back one by one the previous symbol strings until a matching symbol string can be detected, thereby performing a coding process and a decoding process based on a matching length at a higher speed.[0223]
• According to a further aspect of the present invention, when a matching length is shorter than a predetermined value, the symbol string is not coded based on the matching length. Thus, when a matching length is short, the deterioration of a compression rate caused by performing a coding process can be successfully avoided, thereby improving the compression rate in a coding process.[0224]

Claims (36)

What is claimed is:

1. A coding apparatus comprising:

symbol string detection means for detecting a second symbol string matching a first symbol string having a predetermined length from symbol strings;

matching length detection means for detecting a matching length between a third symbol string following the first symbol string and a fourth symbol string following the second symbol string; and

coding means for coding input symbol strings based on the symbol string detected by said symbol string detection means and the matching length detected by said matching length detection means.

2. A coding apparatus comprising:

occurrence position storage means for storing an occurrence position of a symbol string;

occurrence position retrieval means for retrieving an occurrence position of a second symbol string associated with a code corresponding to a first symbol string from the occurrence position storage means;

matching length detection means for detecting a matching length between a third symbol string following the first symbol string and a fourth symbol string following the second symbol string; and

a matching length coding means for coding the third symbol string by coding the matching length detected by said matching length detection means.

3. The apparatus according toclaim 1, wherein

when said matching length is smaller than a predetermined value, a coding process is not performed on a corresponding symbol string based on a matching length.

4. A decoding apparatus comprising:

coded data obtaining means for obtaining data coded based on a matching length of a third symbol string following a first symbol string having a predetermined length;

symbol string detection means for detecting a second symbol string matching the first symbol string; and

decoding means for obtaining a matching length from a code following a code indicating the first symbol string, and decoding a fourth symbol string for the matching length following the second symbol string as a third symbol string.

5. A decoding apparatus comprising:

coded data obtaining means for obtaining data coded based on a matching length of a third symbol string following a first symbol string having a predetermined length;

occurrence position storage means for storing an occurrence position of a decoded symbol string;

occurrence position retrieval means for retrieving an occurrence position of a second symbol string associated with a code corresponding to the first symbol string from the occurrence position storage means; and

decoding means for obtaining a matching length from a code following a code indicating the first symbol string, and decoding a fourth symbol string for the matching length following the second symbol string as a third symbol string.

6. A data transmission apparatus comprising:

input means for inputting a symbol string;

first symbol string detection means for detecting a second symbol string matching a first symbol string having a predetermined length from input symbol strings;

matching length detection means for detecting a matching length between a third symbol string following the first symbol string and a fourth symbol string following the second symbol string;

coding means for coding input symbol strings based on the symbol string detected by said symbol string detection means and the matching length detected by said matching length detection means;

coded data transmission means for transmitting data coded by said coding means;

coded data reception means for receiving data coded based on a matching length between the first symbol string and the third symbol string;

second symbol string detection means for detecting the second symbol string matching the first symbol string from a result of decoding the coded data; and

decoding means for decoding the fourth symbol string following the second symbol string as the third symbol string.

7. A data compressing apparatus having an input buffer for holding a string of data to be compressed, said input buffer comprising a Lempel buffer for storing an already compressed portion and a Ziv buffer for storing a portion to be compressed, comprising:

occurrence position holding means for holding a part or all of an occurrence position of an n-character string (n≧1) in the Lempel buffer;

occurrence/non-occurrence check means for checking whether or not said occurrence position holding means holds an occurrence position of a character string matching the n-character string immediately before the Ziv buffer;

occurrence position obtaining means for obtaining the occurrence position of the character string matching the n-character string immediately before the Ziv buffer when the occurrence position is held by said occurrence position holding means;

matching length detection means for detecting a matching length between a character string after the occurrence position and a character string after a start of the Ziv buffer; and

matching length coding means for coding a portion matching the character string after the occurrence position based on the matching length when said occurrence/non-occurrence check means confirms that the occurrence position of the n-character string immediately before the Ziv buffer is held.

8. The apparatus according toclaim 7, further comprising:

character-with-context code holding means for holding a code of a character immediately succeeding 0 through (m−1) character string (m≧1) as a context;

character-with-context retrieval means for retrieving a character immediately succeeding 0 through (m−1) character string as a context from said character-with-context code holding means;

character-with-context coding means for coding a character retrieved by said character-with-context retrieval means based on a code held by said character-with-context code holding means; and

switch means for switching a coding process between said matching length coding means and said character-with-context coding means.

9. The apparatus according toclaim 8, further comprising:

link means for linking an n-character string of a context having the length of n−1 and a subsequent character to an occurrence position of an n-character string in said occurrence position holding means;

context length check means for checking whether or not a length of a character coded by said character-with-context coding means is n−1; and

notification means for notifying said occurrence/non-occurrence check means that said occurrence position holding means holds the occurrence position when the length of the context is n−1.

10. The apparatus according toclaim 7, further comprising:

character/character string code holding means for holding a code corresponding to a character or a character string having a length of 1 through m;

character/character string retrieval means for retrieving a character or a character string matching for a longest length the data to be compressed from a character or a character string held by said character/character string code holding means;

character/character string coding means for coding a character or a character string retrieved by said character/character string retrieval means according to a code held by said character/character string code holding means; and

switch means for switching a coding process between said matching length coding means and said character/character string coding means.

11. The apparatus according toclaim 10, further comprising:

link means for linking an n-character string held by said character/character string code holding means to an occurrence position of an n-character string in said occurrence position holding means;

context length check means for checking whether or not a length of a character or a character string coded by said character/character string coding means is n; and

notification means for notifying said occurrence/non-occurrence check means that when a length of the character or the character string is n, said occurrence position holding means holds the occurrence position.

12. The apparatus according toclaim 7, further comprising:

an intermediate buffer for holding a matching portion after an occurrence position of a character string matching the n-character string in the lempel buffer, with the matching portion replaced with a code indicating the matching length, and with the other portions held as data to be compressed as is;

character-with-context code holding means for holding a code of a next occurring character using an immediately previous 0 through (m−1) character string as a context;

character-with-context retrieval means for retrieving a subsequent character from said character-with-context code holding means using an immediately previous 0 through (m−1) character string as a context; and

character-with-context coding means for coding a character-with-context retrieved by said character-with-context retrieval means according to a code held by said character-with-context code holding means.

13. The apparatus according toclaim 7, further comprising:

an intermediate buffer for holding a matching portion after an occurrence position of a character string matching the n-character string in the lempel buffer, with the matching portion replaced with a code indicating the matching length, and with the other portions held as data to be compressed as is;

character/character string code holding means for holding a code corresponding to a character or a character string having a length of 1 through m;

character/character string retrieval means for retrieving a character or a character string matching for a longest length the data held in said intermediate buffer from a character or a character string held by said character/character string code holding means; and

character/character string coding means for coding a character or a character string retrieved by said character/character string retrieval means according to a code held by said character/character string code holding means.

14. A data compressing apparatus having an input buffer for holding a string of data to be compressed, said input buffer comprising a Lempel buffer for storing an already compressed portion and a Ziv buffer for storing a portion to be compressed, comprising:

hash code generation means for generating a hash code from a character string having the length n;

occurrence position holding means for holding an occurrence position of the n-character string in the Lempel buffer in association with the hash code generated from the n-character string in the Lempel buffer;

occurrence position obtaining means for obtaining an occurrence position associated with the hash code from said occurrence position holding means by retrieving the hash code generated from the character string having the length n immediately before the Ziv buffer from said occurrence position holding means;

matching length obtaining means for comparing a character string after the occurrence position with a character string after a start of the Ziv buffer, and obtaining a matching length; and

matching length coding means for coding the character string after the start of the Ziv buffer based on the matching length.

15. The apparatus according toclaim 14, further comprising:

hash conflict confirmation means for comparing a character string in the Lempel buffer specified by said occurrence position obtaining means with a character string having the length p immediately before the Ziv buffer, wherein

said matching length coding means codes a character string after a start of the Ziv buffer based on the matching length only when all of p characters match the character string.

16. The apparatus according toclaim 15, wherein

n=3 and p=0 or 2.

17. A data compressing apparatus having an input buffer for holding a string of data to be compressed, said input buffer comprising a Lempel buffer for storing an already compressed portion and a Ziv buffer for storing a portion to be compressed, comprising:

occurrence position holding means for holding a part or all of an occurrence position of a character string having a length n in the Lempel buffer;

occurrence/non-occurrence check means for checking whether or not an occurrence position in the Lempel buffer of a character string matching a character string having a length n immediately before the Ziv buffer is held;

occurrence position obtaining means for obtaining an occurrence position in the Lempel buffer of a character string matching an n-character string immediately before the Ziv buffer;

matching length obtaining means for comparing for matching a character string after the occurrence position with a character string after a start of the Ziv buffer, and obtaining a matching length;

non-matching information holding means for holding non-matching information in association with the n-character string when a matching length obtained by said matching length obtaining means is 0; and

matching length coding means for coding the character string after the start of the Ziv buffer based on the matching length when the non-matching information is not associated with the character string having the length n immediately before the Ziv buffer.

18. The apparatus according toclaim 17, further comprising:

same character sequence confirmation means for confirming whether or not all of characters in a character string having a length q immediately before the Ziv buffer are same characters, wherein

when the all characters in the immediately previous character string having the length q are identical, said non-matching information holding means does not set the non-matching information for the n-character string even when the matching length is 0.

19. The apparatus according toclaim 18, wherein

q equals 2.

20. A data compressing apparatus having an input buffer for holding a string of data to be compressed, said input buffer comprising a Lempel buffer for storing an already compressed portion and a Ziv buffer for storing a portion to be compressed, comprising:

occurrence position holding means for holding a part or all of an occurrence position of a character string having a length n in the Lempel buffer;

occurrence/non-occurrence check means for checking whether or not an occurrence position in the Lempel buffer of a character string matching a character string having a length n immediately before the Ziv buffer is held;

occurrence position obtaining means for obtaining an occurrence position in the Lempel buffer of a character string matching an n-character string immediately before the Ziv buffer;

matching length obtaining means for comparing for matching a character string after the occurrence position with a character string after a start of the Ziv buffer, and obtaining a matching length;

matching length coding means for coding the character string after the start of the Ziv buffer based on the matching length when said occurrence position obtaining means holds the occurrence position in the Lempel buffer of the character string matching the n-character string immediately before the Ziv buffer;

character-with-context code holding means for holding a code of a next occurring character using an immediately previous character or a feature extracted from the immediately previous character as a context; and

character-with-context coding means for coding a character based on the code when said occurrence position obtaining means does not hold the occurrence position in the Lempel buffer of the character string matching the n-character string immediately before the Ziv buffer.

21. A data reconstruction apparatus having an output buffer for holding original data obtained by reconstructing compressed data comprising an Lempel buffer for storing already reconstructed data and a Ziv buffer for storing data to be reconstructed, comprising:

occurrence position holding means for holding a part or all of an occurrence position of an n-character string in the Lempel buffer;

occurrence/non-occurrence check means for checking whether or not said occurrence position holding means holds an occurrence position of a character string matching the n-character string immediately before the Ziv buffer;

occurrence position obtaining means for obtaining the occurrence position of the character string matching the n-character string immediately before the Ziv buffer;

matching length reconstruction means for reconstructing a matching length from compressed data;

character string reconstruction means for obtaining a character string for the matching length following the n-character string in the Lempel buffer as a next reconstruction result of the n-character string immediately before the Ziv buffer.

22. The apparatus according toclaim 21, further comprising:

character-with-context code holding means for holding a code of a character next occurring with an immediately previous 0 through (m−1) character string as a context;

character-with-context retrieval means for retrieving a code from said character-with-context code holding means with the immediately previous 0 through (m−1) character string as a context;

character-with-context reconstruction means for reconstructing a character from the code retrieved by said character-with-context retrieval means; and

switch means for instructing said character-with-context reconstruction means to reconstruct a character when said occurrence position holding means does not hold the occurrence position of the character string matching the n-character string immediately before the Ziv buffer.

23. The apparatus according toclaim 21, further comprising:

character/character string code holding means for holding a code corresponding to a character or a character string having a length of 1 through m;

character/character string reconstruction means for reconstructing a character or a character string according to a code held by said character/character string code holding means; and

switch means for instructing said character/character string reconstruction means to reconstruct a character or a character string when said occurrence position holding means does not hold the occurrence position of the character string matching the n-character string immediately before the Ziv buffer.

24. A data reconstruction apparatus, comprising:

character-with-context code holding means for holding a code of a character next occurring with an immediately previous 0 through (m−1) character string as a context;

character-with-context retrieval means for retrieving a code with the immediately previous 0 through (m−1) character string as a context;

character-with-context reconstruction means for reconstructing a character from compressed data according to the code held by said character-with-context code holding means;

an intermediate code buffer for holding data reconstrucred by said character-with-context reconstruction means;

an output buffer for storing already reconstructed data when data held in said intermediate code buffer defined as input data, said buffer comprising a Lempel buffer for storing already reconstructed data and a Ziv buffer for storing data to be reconstructed;

occurrence position holding means for holding a part or all of an occurrence position of an n-character string in the Lempel buffer;

occurrence/non-occurrence check means for checking whether or not said occurrence position holding means holds an occurrence position of a character string matching the n-character string immediately before the Ziv buffer;

occurrence position obtaining means for obtaining the occurrence position of the character string matching the n-character string immediately before the Ziv buffer when the occurrence position is held by said occurrence position holding means;

matching length reconstruction means for reconstructing a matching length from an intermediate code;

character string reconstruction means for obtaining a character string for the matching length following the n-character string in the Lempel buffer as a next reconstruction result of the n-character string immediately before the Ziv buffer; and

output means for outputting the intermediate code as a reconstruction result as is when said occurrence position holding means does not hold the occurrence position.

25. A data reconstruction apparatus, comprising:

character/character string code holding means for holding a code corresponding to a character or a character string having a length of 1 through m;

character/character string reconstruction means for reconstructing a character or a character string according to a code held by said character/character string code holding means;

an intermediate code buffer for holding data reconstructed by said character/character string reconstruction means;

an output buffer for storing already reconstructed data when data held in said intermediate code buffer defined as input data, said buffer comprising a Lempel buffer for storing already reconstructed data and a Ziv buffer for storing data to be reconstructed;

occurrence position holding means for holding a part or all of an occurrence position of an n-character string in the Lempel buffer;

occurrence/non-occurrence check means for checking whether or not said occurrence position holding means holds an occurrence position of a character string matching the n-character string immediately before the Ziv buffer;

occurrence position obtaining means for obtaining the occurrence position of the character string matching the n-character string immediately before the Ziv buffer when the occurrence position is held by said occurrence position holding means;

matching length reconstruction means for reconstructing a matching length from an intermediate code;

character string reconstruction means for obtaining a character string for the matching length following the n-character string in the Lempel buffer as a next reconstruction result of the n-character string immediately before the Ziv buffer; and

output means for outputting the intermediate code as a reconstruction result as is when said occurrence position holding means does not hold the occurrence position.

26. A data reconstruction apparatus having an output buffer for holding original data reconstructed, said buffer comprising a Lempel buffer for storing an already reconstructed portion, and a Ziv buffer for storing a portion to be reconstructed, comprising:

hash code generation means for generating a hash code from a character string having the length n;

occurrence position holding means for holding an occurrence position of the n-character string in the Lempel buffer in association with the hash code generated from the n-character string in the Lempel buffer;

occurrence position obtaining means for obtaining an occurrence position associated with the hash code generated from the character string having the length n immediately before the Ziv buffer;

matching length reconstruction means for reconstructing a matching length from a leading code in the Ziv buffer; and

character reconstruction means for defining a character string for the matching length after the occurrence position in the Lempel buffer as a reconstruction result of the code at the start in the Ziv buffer.

27. The apparatus according toclaim 26, further comprising:

hash conflict confirmation means for comparing a character string in the Lempel buffer specified by said occurrence position obtaining means with a character string having a length p immediately before the Ziv buffer, wherein

said character string reconstruction means defines as a reconstruction result of a leading code in the Ziv buffer a character string having the matching length after the occurrence position in the Lempel buffer only when all of the p characters match the character string.

28. A data reconstruction apparatus having an output buffer for holding original data reconstructed, said buffer comprising a Lempel buffer for storing an already reconstructed portion, and a Ziv buffer for storing a portion to be reconstructed, comprising:

occurrence position holding means for holding a part or all of an occurrence position of an n-character string in the Lempel buffer;

occurrence/non-occurrence check means for checking whether or not said occurrence position holding means holds an occurrence position of a character string matching the n-character string immediately before the Ziv buffer;

occurrence position obtaining means for obtaining the occurrence position of the character string matching the n-character string immediately before the Ziv buffer;

matching length reconstruction means for reconstructing a matching length from a leading code in the Ziv buffer;

non-matching information holding means for holding non-matching information in association with the n-character string when a matching length obtained by said matching length obtaining means is 0; and

character reconstruction means for defining as a reconstruction result of a leading code in the Ziv buffer a character string having the matching length after the occurrence position in the Lempel buffer when the non-matching information is not associated with the character string having the length n immediately before the Ziv buffer.

29. A data reconstruction apparatus having an output buffer for holding original data reconstructed, said buffer comprising a Lempel buffer for storing an already reconstructed portion, and a Ziv buffer for storing a portion to be reconstructed, comprising:

occurrence position holding means for holding a part or all of an occurrence position of an n-character string in the Lempel buffer;

occurrence/non-occurrence check means for checking whether or not said occurrence position holding means holds an occurrence position of a character string matching the n-character string immediately before the Ziv buffer;

occurrence position obtaining means for obtaining the occurrence position of the character string matching the n-character string immediately before the Ziv buffer;

matching length reconstruction means for reconstructing a matching length from a leading code in the Ziv buffer;

character reconstruction means for defining a character string having the matching length after the occurrence position in the Lempel buffer as a reconstruction result of a leading code in the Ziv buffer when said occurrence position obtaining means holds the occurrence position in the Lempel buffer of the character string matching the n-character string immediately before the Ziv buffer;

character-with-context code holding means for holding a code of a next occurring character using an immediately previous character or a feature extracted from the immediately previous character as a context; and

character-with-context code reconstruction means for reconstructing a character based on the code when said occurrence position obtaining means does not hold the occurrence position in the Lempel buffer of the character string matching the n-character string immediately before the Ziv buffer.

30. A data compressing method comprising the steps of:

coding parts of matching symbol strings based on a matching length; and

coding a symbol string not coded based on the matching length by a probability statistic type coding system or a dictionary type coding system.

31. A data compressing method comprising the steps of:

coding parts of matching symbol strings based on a matching length; and

coding the symbol strings coded based on the matching length furthermore by a probability statistic type coding system or a dictionary type coding system.

32. A data compressing method for coding a current character string based on a matching length with a previous character string, comprising the step of

using a character string immediately before the current character string for searching an occurrence position of the previous character string.

33. A data compressing method comprising the steps of:

storing a occurrence position of a previous second character string;

detecting a first character string immediately before a third character string;

obtaining a occurrence position of the second character string matching the first character string for each character;

detecting a matching length between a fourth character string following the second character string and the third character string; and

coding the third character string based on the matching length.

34. A data compressing method comprising the steps of:

determining whether or not a matching length between a current character string and a previous character string is equal to or larger than a predetermined value;

coding the current character string by a coding method of a high compression rate on a repetition of short character strings when the matching length is equal to or smaller than the predetermined value; and

coding the current character string by a coding method of a high compression rate on a repetition of long character strings when the matching length exceeds the predetermined value.

35. A computer-readable storage medium comprising:

a data structure in which a first symbol string matching a second symbol string having a predetermined length is coded; and

a data structure in which a third symbol string following the first symbol string is coded based on a matching length with a fourth symbol string following the second symbol string.

36. A computer-readable storage medium storing a program used to direct a computer to perform the steps of:

detecting a second symbol string matching a first symbol string having a predetermined length from symbol strings;

detecting a matching length between a third symbol string following the first symbol string and a fourth symbol string following the second symbol string; and

coding the third symbol string based on a matching length between the third and fourth symbol strings.

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