JP2012065097A - Compression device, compression method, compression program, and restoration device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To compress a data string efficiently.SOLUTION: A return distance threshold table associates and stores data and a threshold of a shift distance by which the data can be shifted according to a frequency of appearance of the data determining whether the data is to be exchanged or not. A character string conversion unit goes on reading from an attention position of an object data string to be compressed; and determines the shift distance by which different data can be shifted based on the return distance threshold table, when the data differing from the data of the attention position appears. The character string conversion unit shifts the attention position to the position where the different data appears, when there is no data being identical with the different data within a range not exceeding a shift distance of the different data. When there is data being identical with the different data; the character string conversion unit exchanges the next data of the identical data for the different data, stores the distance from an original point of the beginning of the object data string to be compressed to the exchanged data and the difference between the exchanged data in an exchange history table, and shifts the original point and the attention position to the position of the exchanged data.

Description

  The present invention relates to a compression device, a compression method, a compression program, and a decompression device.

Conventionally, a data compression / decompression technique has been developed that compresses the amount of data without losing the contents of the original data string, or restores the compressed data string to the original data string. As one of the techniques, there is R _LE (Run Length Encoding). This R _LE is a technique for encoding a data string in which the same data continues with a pair of a data type and a length in which the data continues.

For example, when the data string S represented by the following formula (1) is encoded by R _LE , the continuous sequence “bbbb” of the data string S is compressed to “b, 4”. When the other sequences of the data string S are similarly encoded, the data string S is compressed into a data string R _LE (S) represented by the following equation (2). In this case, since the data amount of the data string S is 19 bytes and the data amount of the data string R _LE (S) is 12 bytes, the data amount of the data string S is compressed by 7 bytes.

S = bbbbababbbbbcccbbbbdd ... (1)
R _LE (S) = (b, 4) (a, 2) (b, 4) (c, 3) (b, 3) (d, 3) (2)

However, if _RLE is executed on a data string in which the same data is rarely continuous, the data amount may be increased. For example, when the data string S represented by the following expression (3) is encoded by R _LE , the data string S becomes a data string R _LE (S) represented by the following expression (4). In this case, the amount of data increases from 9 bytes to 14 bytes, and increases by 5 bytes. For this reason, there is an improved technique in which each data is moved so that the same data is continuous, the data string is converted so that the data can be efficiently compressed, and then compressed by _RLE .

S = abccabaab (3)
R _LE (S) = (a, 1) (b, 1) (c, 2) (a, 1) (b, 1) (a, 2) (b, 1) (4)

For example, in this improved technique, when the head is 0th, the first data “b” of the data string S shown in Expression (3) is moved to the fourth position so that the data “b” is continuous. To. Other data in the data string S is also moved so that the same data is continuous, and the data string S is converted into a data string T represented by the following equation (5). When this data string T is encoded by R _LE , the data string T becomes a data string R _LE (T) represented by the following equation (6). In this case, the data amount is changed from 9 bytes to 6 bytes and compressed by 3 bytes.

T = aaaabbbcc (5)
R _LE (T) = (a, 4) (b, 3) (c, 2) (6)

By the way, in the process of restoring the data string S from the data string R _LE (T) compressed by this improved technique, it is necessary to reversely convert the data string T into the data string S. In this improved technique, when the data string S is converted into the data string T, a conversion function π indicating the correspondence between the position before movement of each data and the position after movement is generated, and this conversion function π Perform inverse transformation using. FIG. 25 is a diagram illustrating an example of a conventional conversion function. As shown in FIG. 25, the conversion function π associates the n-th data string S with the π (n) -th data string T. That is, assuming that the nth data in the data string S is S [n] and the π (n) th data in the data string T is T [π (n)], S [n] = T [π (n) ] Holds. For example, when the first data in the data string S is restored, S [1] = T [4], so the fourth “b” in the data string T is moved to the first in the data string S. . In this way, the data string T is inversely converted into the data string S by sequentially moving each data of the data string T using the conversion function π. Note that n is a non-negative integer (0, 1, 2,... N).

By Masatatsu K'z, “Compression Algorithm”, Softbank Publishing

  However, the conventional technique has a problem that the data string cannot be efficiently compressed.

  For example, in the above prior art, it is necessary to try a number of patterns for moving each data in order to make the sequence of the same data continuous longer. For this reason, when the original data string becomes long, the number of patterns increases exponentially, and a huge processing load is applied.

Further, for example, in the above conventional technique, it is necessary to store a conversion function in order to inversely convert the converted data string. For this reason, even if the data string is converted so that it can be efficiently compressed and then compressed by _RLE , the entire data amount including the conversion function is hardly compressed, and the entire data amount increases. There was also.

  The disclosed technology has been made in view of the above, and an object thereof is to provide a compression device, a compression method, a compression program, and a decompression device that can efficiently compress a data string.

  In one aspect, the technology disclosed in the present application includes a movement distance table, a movement distance determination unit, and a replacement processing unit. The movement distance table stores the data and the threshold of the movement distance that can move the data according to the appearance frequency of the data that determines whether or not the data is replaced. The movement distance determination unit reads data from the target position of the data string to be compressed, and when data different from the data of the target position appears, based on the different data and the movement distance table, The movement distance that can move different data is determined. When the same data as the different data does not exist within a range that does not exceed the movement distance determined by the movement distance determination unit from the position where the different data appears, the replacement processing unit sets the attention position as the different data. Move to the position where it appears. In addition, when the same data as the different data exists in a range not exceeding the movement distance determined by the movement distance determination unit from the position where the different data appears, the replacement processing unit Swap data and the different data. Then, after replacing the data, the replacement processing unit stores the distance from the starting origin of the data string to be compressed to the replaced data and the distance between the replaced data in the history table, and at the position of the replaced data. The origin and the target position are moved.

  According to one aspect of the technology disclosed in the present application, there is an effect that a data string can be efficiently compressed.

FIG. 1 is a diagram illustrating the configuration of the data compression / decompression apparatus according to the present embodiment. FIG. 2 is a diagram illustrating an example of a data structure of the return distance threshold table. FIG. 3 is a diagram (1) illustrating an example of the data structure of the replacement history table. FIG. 4 is a diagram for explaining various terms. FIG. 5 is a diagram (1) for explaining the processing of the character string conversion unit in detail. FIG. 6 is a diagram (2) for explaining the processing of the character string conversion unit in detail. FIG. 7 is a diagram (3) for explaining the processing of the character string conversion unit in detail. FIG. 8 is a diagram (4) for explaining the processing of the character string conversion unit in detail. FIG. 9 is a diagram (5) for explaining the processing of the character string conversion unit in detail. FIG. 10 is a diagram (6) for explaining the processing of the character string conversion unit in detail. FIG. 11 is a diagram (7) for explaining the processing of the character string conversion unit in detail. FIG. 12 is a diagram (1) illustrating an example of a data structure of a replacement history table temporarily held by the character string conversion unit. FIG. 13 is a diagram (2) illustrating an example of the data structure of the replacement history table temporarily held by the character string conversion unit. FIG. 14 is a diagram (3) illustrating an example of the data structure of the replacement history table temporarily held by the character string conversion unit. FIG. 15 is a diagram (2) illustrating an example of the data structure of the replacement history table. FIG. 16 is a diagram for explaining the process of restoring the origin information. FIG. 17 is a diagram (1) for explaining the processing of the character string reverse conversion unit in detail. FIG. 18 is a diagram (2) for explaining the processing of the character string reverse conversion unit in detail. FIG. 19 is a flowchart illustrating the processing procedure of the compression unit. FIG. 20 is a flowchart illustrating a processing procedure of threshold value table generation processing. FIG. 21 is a flowchart showing the processing procedure of the character string conversion processing. FIG. 22 is a flowchart illustrating the processing procedure of the character string reverse conversion unit. FIG. 23 is a diagram (3) illustrating an example of the data structure of the replacement history table. FIG. 24 is a diagram illustrating an example of a computer that executes a compression / decompression program. FIG. 25 is a diagram illustrating an example of a conventional conversion function.

  Hereinafter, embodiments of a compression device, a compression method, a compression program, and a decompression device disclosed in the present application will be described in detail with reference to the drawings. Note that the present invention is not limited to the embodiments. Each embodiment can be appropriately combined within a range in which processing contents do not contradict each other.

  An example of the configuration of the data compression / decompression apparatus according to the present embodiment will be described. FIG. 1 is a diagram illustrating the configuration of the data compression / decompression apparatus according to the present embodiment. As illustrated in FIG. 1, the data compression / decompression apparatus 100 includes an input unit 110, an output unit 120, an input / output control unit 130, a storage unit 140, a compression unit 150, and a decompression unit 160.

  The input unit 110 is an input device that receives input of various types of information. For example, the input unit 110 corresponds to a keyboard, a mouse, or the like. The output unit 120 is an output device that outputs various types of information. For example, the output unit 120 corresponds to a display, a monitor, or the like. The input / output control unit 130 is a processing unit that controls input / output of various types of information among the input unit 110, the output unit 120, the storage unit 140, the compression unit 150, and the restoration unit 160. For example, the input / output control unit 130 corresponds to an ASIC (Application Specific Integrated Circuit) that controls input / output of various types of information.

  The storage unit 140 includes an input file 141, a return distance threshold table 142, a replacement history table 143, and an output file 144. The storage unit 140 corresponds to, for example, a semiconductor memory device such as a random access memory (RAM), a read only memory (ROM), or a flash memory, and a storage device such as a hard disk or an optical disk.

  The input file 141 is a file including a plurality of input character strings. For example, the input character string S is a character string represented by the following equation (7).

    S = aacabaaabcaaaaababaaa (7)

  The return distance threshold table 142 is a table that holds a return distance threshold for each character. This return distance threshold is a threshold set according to the appearance frequency of each character appearing in the input character string S. As the return distance threshold, a smaller value is set as the appearance frequency of the corresponding character is higher, and a larger value is set as the appearance frequency is lower. For example, the return distance threshold table 142 holds the number of appearances and the return distance threshold in association with each character included in the input character string S.

  FIG. 2 is a diagram illustrating an example of a data structure of the return distance threshold table. As illustrated in FIG. 2, the return distance threshold table 142 holds the number of appearances “14” and the return distance threshold “0” in association with the character “a”. Further, the return distance threshold table 142 stores the number of appearances “4” and the return distance threshold “4” in association with the character “b”. In addition, the return distance threshold table 142 stores the number of appearances “2” and the return distance threshold “9” in association with the character “c”.

The replacement history table 143 is a table that holds data used when the character string T converted to a character string convenient for _RLE compression is returned to the input character string S before conversion. For example, the replacement history table 143 holds an offset and a return distance in association with each other. Details regarding the replacement history table 143 will be described later.

  FIG. 3 is a diagram illustrating an example of the data structure of the replacement history table. As illustrated in FIG. 3, the replacement history table 143 holds an offset “5” and a return distance “3” in association with each other. Also, the replacement history table 143 holds the offset “4” and the return distance “7” in association with each other. Also, the replacement history table 143 holds the offset “8” and the return distance “3” in association with each other.

The output file 144 is a file including a character string compressed by the compression unit 150. For example, the character string compressed by the compression unit 150 is an output character string R _LE (T) represented by the following equation (8).

R _LE (T) = (a, 2) (c, 2) (b, 2) (a, 8) (b, 2) (a, 4) (8)

Returning to the description of FIG. The compression unit 150 is a processing unit that compresses an input character string included in the input file 141. The compression unit 150 includes a threshold table generation unit 151, a character string conversion unit 152, and an R _LE encoding unit 153.

  The threshold value table generating unit 151 is a processing unit that calculates a return distance threshold value for each character included in the input character string and generates the return distance threshold value table 142 shown in FIG. Hereinafter, the processing of the threshold table generation unit 151 will be specifically described.

  For example, the threshold value table generation unit 151 acquires an input character string from the input file 141, and reads the acquired input character string character by character from the beginning to the end. The threshold value table generation unit 151 counts the number of appearances for each character, and records the counted number of appearances for each character in the return distance threshold value table 142. The threshold table generation unit 151 sorts the return distance threshold table 142 in descending order using the number of appearances as a sort key. Then, the threshold value table generation unit 151 divides, for each character, the sum of the number of appearances of characters having a greater number of appearances than the corresponding character by the number of appearances of the corresponding character, and rounded off the first decimal place of the divided value. The return distance threshold value 142 of each character is recorded in the return distance threshold value table 142.

  For example, when the threshold value table generating unit 151 reads the input character string S shown in Expression (7), the threshold value table generating unit 151 returns “14” as the appearance number of the character “a” and returns the distance threshold value table 142. To record. Similarly, the threshold value table generation unit 151 records “4” as the number of appearances of the character “b” and records “2” as the number of appearances of the character “c”. Here, since there is no character having a higher number of appearances than the character “a”, the sum of the number of appearances of characters having a higher number of appearances than the character “a” is “0”. Therefore, the threshold value table generation unit 151 divides this “0” by the appearance number “14” of the character “a” and calculates “0” as the return distance threshold value of the character “a”. Further, since the character having a higher appearance number than the character “b” is the character “a”, the sum of the appearance numbers of characters having a higher appearance number than the character “b” is “14”. Therefore, the threshold value table generation unit 151 divides this “14” by the appearance number “4” of the character “b” and calculates “4” as the return distance threshold value of the character “b”. Further, since the characters having more appearances than the character “c” are the characters “a” and “b”, the sum of the appearances of the characters having more appearances than the character “c” is “18”. Therefore, the threshold value table generation unit 151 divides this “18” by the appearance number “2” of the character “c”, and calculates “9” as the return distance threshold value of the character “c”.

String conversion unit 152, so that a good sorted convenient for compression scheme R _LE, a processing unit for converting the order of the characters in the input file 141. That is, the character string conversion unit 152 converts each character string by moving each character so that the same character continues. Note that the character moving distance is limited by a return distance threshold.

  Here, before describing the processing of the character string conversion unit 152, terms used in describing this processing will be described. FIG. 4 is a diagram for explaining various terms. The slide buffer is a buffer for storing a part of the input character string S. Each time the conversion of the input character string S in the slide buffer is completed, the character string conversion unit 152 sequentially stores the unconverted input character string S in the slide buffer.

  The origin o indicates the position of the reference character. The attention position p indicates a position serving as a reference when detecting the replacement source character, and moves from the origin o toward the end. The offset m is a relative distance from the origin o to the replacement source character. The return distance n corresponds to the moving distance from the replacement source character to the replacement destination character when the character is replaced. For example, when the bold characters “a” and “b” shown in FIG. 4 are replaced, the offset m is “6” and the return distance n is “1”.

  Processing in which the character string conversion unit 152 converts a character string will be described. The character string conversion unit 152 reads the input character string S character by character toward the end until it detects a character y different from the character x indicated by the target position p. When the character string conversion unit 152 detects the character y, the character string conversion unit 152 moves the input character string S toward the head one by one until it detects the character y ′ that is present at the head of the character y and is identical to the character y. Continue reading.

  When y ′ is detected within the return distance threshold of the character y, the character string conversion unit 152 replaces the character z with the character z next to the character y ′. The character string conversion unit 152 records the origin o, the offset m, and the return distance n in the replacement history table 143 in association with each other. Then, the character string conversion unit 152 sets the origin o and the target position p to the position of the offset m, and repeatedly executes the same processing. On the other hand, if y ′ is not detected within the return distance threshold of the character y, the character string conversion unit 152 sets the attention position p to the character y and repeatedly executes the same processing.

  Next, the processing of the character string conversion unit 152 will be described in detail. FIGS. 5-11 is a figure for demonstrating in detail the process of a character string conversion part. 12 to 14 are diagrams illustrating an example of a data structure of a replacement history table temporarily held by the character string conversion unit. Here, for convenience of explanation, it is assumed that the input character string S can be stored in all slide buffers. Further, the input character string S is S = aacabaaabcaaaaabaaaa.

  FIG. 5 will be described. The character string conversion unit 152 stores the input character string S in the slide buffer. In addition, the character string conversion unit 152 sets the origin o and the target position p to the first character “a” of the input character string S. In this case, the origin o = 0 (step S10). Further, the character string conversion unit 152 reads one character at a time until it detects a character different from the character “a” indicated by the target position p, and reads the character “c” at a position where the offset m = 2. It detects (step S11). The character string conversion unit 152 reads one character at a time from the character “c” toward the head, but does not detect the character “c” within the return distance threshold “9” of the character “c”. Set to the position of offset m = 2.

  Shifting to the description of FIG. The character string conversion unit 152 reads one character at a time toward the end until a character different from the character “c” indicated by the target position p is detected, and detects the character “a” at a position where the offset m = 3. (Step S12). The character string conversion unit 152 reads one character at a time from the character “a” toward the top, but does not detect the character “a” within the return distance threshold “0” of the character “a”. Set to the position of offset m = 3.

  The character string conversion unit 152 reads one character toward the end until a character different from the character “a” indicated by the target position p is detected, and detects the character “b” at a position where the offset m = 4. (Step S13). The character string conversion unit 152 reads one character at a time from the character “b” toward the head, but does not detect the character “c” within the return distance threshold “4” of the character “b”. Set to the position of offset m = 4.

  The character string conversion unit 152 reads one character at a time toward the end until a character different from the character “b” indicated by the target position p is detected, and detects the character “a” at a position where the offset m = 5. (Step S14). The character string conversion unit 152 reads one character at a time from the character “a” toward the top, but does not detect the character “a” within the return distance threshold “0” of the character “a”. Set to the position of offset m = 5.

  Shifting to the description of FIG. The character string conversion unit 152 reads one character at a time toward the end until a character different from the character “a” indicated by the target position p is detected, and detects the character “b” at a position where the offset m = 8. (Step S15). The character string conversion unit 152 reads one character at a time from the character “b” toward the beginning, and sets the character “b” at a return distance n = 4 within the return distance threshold “4” of the character “b”. To detect. The character string conversion unit 152 replaces the character “a” next to the character “b” and the character “b” present at the position of the offset m = 8 (step S16). The character string conversion unit 152 sets the origin o and the target position p to the position of the offset m = 8.

  The character string conversion unit 152 stores the origin o “0”, the offset m “8”, and the return distance n “4” in association with each other in the replacement history table 143 when the process of step S16 ends. FIG. 12 shows the data contents of the replacement history table at the time when step S16 is completed.

  Shifting to the description of FIG. The character string conversion unit 152 reads one character at a time toward the end until a character different from the character “a” indicated by the target position p is detected, and detects the character “c” at a position where the offset m = 1. (Step S17). The character string conversion unit 152 reads the character “c” from the character “c” one by one toward the head, and sets the character “c” at a return distance n = 7 that is within the return distance threshold “7” of the character “c”. To detect. The character string conversion unit 152 replaces the character “a” next to the character “c” and the character “c” present at the position of the offset m = 1 (step S18). The character string conversion unit 152 sets the origin o and the target position p to the position where the offset m = 1.

  The character string conversion unit 152 stores the origin o “8”, the offset m “1”, and the return distance n “7” in association with each other in the replacement history table 143 when the process of step S18 ends. FIG. 13 shows the data contents of the replacement history table at the time when step S18 is completed.

  The description shifts to the description of FIG. The character string conversion unit 152 reads one character at a time toward the end until a character different from the character “a” indicated by the target position p is detected, and detects the character “b” at a position where the offset m = 5. (Step S19). The character string conversion unit 152 reads one character at a time from the character “b” toward the head, but does not detect the character “b” within the return distance threshold “4” of the character “b”. Set to the position of offset m = 5.

  The character string conversion unit 152 reads one character at a time toward the end until a character different from the character “b” indicated by the target position p is detected, and detects the character “a” at a position where the offset m = 6. (Step S20). The character string conversion unit 152 reads one character at a time from the character “a” toward the top, but does not detect the character “a” within the return distance threshold “0” of the character “a”. Set to the position of offset m = 6.

  The description shifts to the description of FIG. The character string conversion unit 152 reads the characters one by one toward the end until a character different from the character “a” indicated by the target position p is detected, and detects the character “b” at a position where the offset m = 7. (Step S21). The character string conversion unit 152 reads the character “b” from the character “b” one by one toward the head, and sets the character “b” at the return distance n = 2 within the return distance threshold “4” of the character “b”. To detect. The character string conversion unit 152 replaces the character “a” next to the character “b” and the character “b” present at the position of the offset m = 7 (step S22). The character string conversion unit 152 sets the origin o and the target position p to the position of the offset m = 7.

  The character string converting unit 152 stores the origin o “9”, the offset m “7”, and the return distance n “2” in association with each other in the replacement history table 143 when the process of step S22 ends. FIG. 14 shows the data contents of the replacement history table at the time when step S22 is completed.

  The description shifts to the description of FIG. The character string conversion unit 152 reads the characters one by one toward the end until a character different from the character “a” indicated by the target position p is detected. However, the end of the slide buffer is reached before the corresponding character is detected (step S23). The character string conversion unit 152 sets the character string stored in the slide buffer as the character string T. Further, the character string conversion unit 152 stores the information obtained by removing the information on the origin o of the replacement history table illustrated in FIG. 14 in the replacement history table 143 (step S24).

As described above, the input character string S is converted into the character string T by the character string conversion unit 152 executing the processes of steps S10 to S24. The character string conversion unit 152 outputs the character string T = aaccbbaaaaaaaabbaaaa to the R _LE encoding unit 153.

  Further, as shown in step S24, the character string conversion unit 152 does not store the replacement history table in the storage unit 140 as it is. The information on the origin of the replacement history table can be uniquely derived from the relationship between the offset and the return distance. For this reason, the character string conversion unit 152 stores the replacement history table 143 from which the origin information is removed in the storage unit 140, thereby reducing the amount of data to be stored in the storage unit 140.

  Further, the character string conversion unit 152 stores the pair of the offset and the return distance in the replacement history table 143 with a data amount of 1 byte. FIG. 15 is a diagram (2) illustrating an example of the data structure of the replacement history table. In the example illustrated in FIG. 15, the character string conversion unit 152 stores the offset “8” in the first row of the replacement history table 143 in 4 bits and stores the return distance “4” in 4 bits. , 4) is stored in 1 byte. Similarly, the character string conversion unit 152 stores (1, 7) in 1 byte by storing the offset “1” of the second row in 4 bits and storing the return distance “7” in 4 bits. . The character string conversion unit 152 stores (7, 2) in 1 byte by storing the offset “7” in the third row in 4 bits and the return distance “2” in 4 bits. That is, the character string conversion unit 152 stores the replacement history table 143 shown in FIG. 15 in the storage unit 140 with a data amount of 3 bytes.

Returning to the description of FIG. The R _LE encoding unit 153 is a processing unit that compresses a character string based on the R _LE compression method. Compression schemes _{R LE} which R _LE coding unit 153 performs is the same as the conventional. The R _LE encoding unit 153 stores the compressed character string in the output file 144 as an output character string.

For example, the R _LE encoding unit 153 converts the character string T = aaccbbaaaaaaaabbaaaaa input by the character string conversion unit 152 into the output character string R _LE (T) = (a, 2) (c, 2) (b, 2). It encodes to (a, 8) (b, 2) (a, 4).

The restoration unit 160 is a processing unit that restores the input file 141 from the output file 144. The restoration unit 160 includes an R _LE decoding unit 161 and a character string reverse conversion unit 162.

The R _LE decoding unit 161 is a processing unit that decodes the output character string based on the R _LE decoding method. Decoding scheme _{R LE} which R _LE decoding unit 161 performs is the same as the conventional. For example, the R _LE decoding unit 161 traces the output character string from the first character, and decodes the output character string to a character string before encoding based on the type of consecutive characters and the length of consecutive characters. The R _LE decoding unit 161 outputs the decoded character string to the character string reverse conversion unit 162.

For example, the R _LE decoding unit 161 outputs the output character string R _LE (T) = (a, 2) (c, 2) (b, 2) (a, 8) (b, 2) stored in the output file 144. ) (A, 4) is decoded into the character string T = aaccbbaaaaaaaabbaaaaa.

String inverse conversion unit 162 is a processing unit for inversely converting the converted character string to be a good sorted convenient for compression method R _LE to the original string. Hereinafter, the process of the character string reverse conversion unit 162 will be described in detail. The character string reverse conversion unit 162 reads the replacement history table 143 from the storage unit 140, restores the origin information of the replacement history table 143, and then reversely converts the character string. Here, the character string T to be reversely converted is assumed to be T = aaccbbaaaaaaaabbaaaaa. The data structure of the replacement history table 143 is shown in FIG.

  A process in which the character string reverse conversion unit 162 restores the origin information will be described. FIG. 16 is a diagram for explaining the process of restoring the origin information. Here, a case will be described in which the origin of the replacement history table shown in FIG. 15 is restored. The character string inverse conversion unit 162 obtains the value of the origin of the n-th row by adding the offset value of the n-1 row to the origin of the n-1 row. However, the value of the origin of the first line is 0. In the example shown in FIG. 16, the value of the origin of the first row is 0. The value of the origin of the second line is 8. The value of the origin of the first line is 9.

  Processing in which the character string reverse conversion unit 162 reversely converts the character string will be described. The character string reverse conversion unit 162 reads the replacement history table whose origin has been restored line by line from the last line, and determines two characters to be replaced. One character to be replaced is a character corresponding to the position of “origin o + offset m” from the beginning of the character string. The other character to be replaced is a character corresponding to the position “origin o + offset m−return distance n + 1” from the beginning of the character string. The character string reverse conversion unit 162 replaces each character after specifying two characters to be replaced. The character string reverse conversion unit 162 performs reverse conversion of the character string by repeatedly executing the above processing. The character string reverse conversion unit 162 may output the reversely converted character string to the output unit 120 or may store the character string in the storage unit 140.

  Next, the processing of the character string reverse conversion unit 162 will be described in detail. 17 and 18 are diagrams for explaining the processing of the character string reverse conversion unit in detail. The replacement history table with the origin restored is shown on the right side of FIG.

  FIG. 17 will be described. The character string reverse conversion unit 162 reads the character string T = aaccbbaaaaaaaabbaaaa to be converted into the buffer (step S25). The character string reverse conversion unit 162 reads the data in the third row of the replacement history table and determines two characters to be replaced. The data in the third row of the replacement history table has an origin o = 9, an offset m = 7, and a return distance n = 2. Therefore, the characters to be replaced are the 16th character “a” and the 15th character “b” from the top. The character string reverse conversion unit 162 replaces the 16th character “a” from the top with the 15th character “b” (step S26).

  The description shifts to the description of FIG. The character string reverse conversion unit 162 reads the data in the second row of the replacement history table and determines two characters to be replaced. The data in the second row of the replacement history table has an origin o = 8, an offset m = 1, and a return distance n = 7. Therefore, the characters to be replaced are the ninth character “c” and the third character “a” from the top. The character string reverse conversion unit 162 replaces the ninth character “c” from the top with the third character “a” (step S27).

  The character string reverse conversion unit 162 reads the data in the first row of the replacement history table and determines two characters to be replaced. The data in the first row of the replacement history table has an origin o = 0, an offset m = 8, and a return distance n = 4. Therefore, the characters to be replaced are the eighth character “b” and the fifth character “a” from the top. The character string reverse conversion unit 162 replaces the eighth character “b” from the top with the fifth character “a” (step S28). When step S28 ends, all the replacements corresponding to the replacement history table are completed.

As described above, the character string reverse conversion unit 162 executes the processing of steps S25 to S28, so that the character string T = aaccbbaaaaaaaaaabaaaa is inversely converted to the character string T = aaabaaaaaaaaaaaaa. The reversely converted character string matches the character string before being converted in accordance with the _RLE compression method.

  Meanwhile, the compression unit 150 and the restoration unit 160 illustrated in FIG. 1 correspond to an integrated device such as an application specific integrated circuit (ASIC) or a field programmable gate array (FPGA), for example. Or the compression part 150 and the decompression | restoration part 160 respond | correspond to electronic circuits, such as CPU (Central Processing Unit) and MPU (Micro Processing Unit), for example.

  Next, a processing procedure of the data compression / decompression apparatus 100 according to the present embodiment will be described. FIG. 19 is a flowchart illustrating the processing procedure of the compression unit. The process illustrated in FIG. 19 is executed when the input file 141 is stored in the storage unit 140, for example.

As illustrated in FIG. 19, the compression unit 150 executes a threshold value table generation process (step S101) and executes a character string conversion process (step S102). Then, the compression unit 150 compresses the character string based on the R _LE compression method (step S103).

  Next, the threshold value table generation process shown in step S101 of FIG. 19 will be described. FIG. 20 is a flowchart illustrating a processing procedure of threshold value table generation processing. As illustrated in FIG. 20, the threshold value table generation unit 151 acquires an input character string from the input file 141, and reads the acquired input character string character by character from the beginning to the end (step S201). The threshold value table generating unit 151 counts the number of appearances for each character, and records the counted number of appearances for each character in the return distance threshold value table 142 (step S202).

  The threshold value table generating unit 151 sorts the return distance threshold value table 142 in descending order using the number of appearances as a sort key (step S203). For each character, the threshold value table generating unit 151 divides the sum of the number of appearances of characters having a larger number of appearances than the corresponding character by the number of appearances of the corresponding character, and rounds off the first decimal place of the divided value to each character. Is recorded in the return distance threshold table 142 (step S204).

  Next, the character string conversion process shown in step S102 of FIG. 19 will be described. FIG. 21 is a flowchart showing the processing procedure of the character string conversion processing. As shown in FIG. 21, the character string converter 152 reads an input character string from the input file 141 into the slide buffer (step S301), and performs an initialization process (step S302). In the initialization process in step S302, the character string conversion unit 152 sets the origin o and the target position p at the head of the slide buffer.

  The character string converter 152 reads the input character string S one character at a time until it detects a character y different from the character x indicated by the target position p (step S303). When the character string conversion unit 152 detects the character y before reaching the end of the slide buffer (No in step S304), the character string conversion unit 152 exists on the front side of the character y and is the same character y as the character y. Until 'is detected, one character is read at the head side (step S305).

  When the character string conversion unit 152 detects y ′ within the return distance threshold of the character y (Yes in Step S306), the character string conversion unit 152 replaces the character z with the character z next to the character y ′ (Step S307). . The character string conversion unit 152 records the origin o, the offset m, and the return distance n in association with each other in the replacement history table (step S308). The character string conversion unit 152 sets the origin o and the position of interest p to the position of the offset m (step S309), and proceeds to step S303.

  On the other hand, when the character string conversion unit 152 does not detect y ′ within the return distance threshold of the character y (No at Step S306), the character string conversion unit 152 sets the target position p to the position of the offset m (Step S310). The process proceeds to step S303.

  Incidentally, in step S304, when the character string conversion unit 152 reaches the end of the slide buffer before detecting the character y (step S304, Yes), the character string conversion unit 152 updates the character string in the slide buffer (step S311). . That is, the character string conversion unit 152 reads a character string from the input file 141 and stores the read character string in the slide buffer.

If the character string conversion unit 152 has not reached the end of the input file (No at Step S312), the character string conversion unit 152 proceeds to Step S303. On the other hand, when the character string conversion unit 152 reaches the end of the input file (Yes in step S312), the character string conversion unit 152 outputs the character string in the slide buffer to the _RLE encoding unit 153 (step S313), and ends the process. To do.

  Next, the processing procedure of the character string reverse conversion unit 162 shown in FIG. 1 will be described. FIG. 22 is a flowchart illustrating the processing procedure of the character string reverse conversion unit according to the present embodiment. The process illustrated in FIG. 22 is executed, for example, when the replacement history table 143 and the output file 144 are stored in the storage unit 140.

  As shown in FIG. 22, the character string reverse conversion unit 162 reads the replacement history table 143 (step S401), and restores the origin of the replacement history table 143 (step S402). The character string reverse conversion unit 162 reads the output character string T into the buffer (step S403), and selects an unselected line from the end of the replacement history table (step S404).

  When all the rows of the replacement history table have been selected (step S405, Yes), the character string reverse conversion unit 162 outputs the character string T (step S406) and ends the process. On the other hand, when all the rows of the replacement history table are not selected (step S405, No), the character string reverse conversion unit 162 replaces T [o + m] and T [o + m−n + 1] in the output character string T. (Step S407), the process proceeds to Step S404. Here, o is the origin, m is the offset, and n is the return distance.

Next, a comparison result between the number of bytes when the input character string S is directly compressed by the _RLE compression method and the number of bytes when the compression unit 150 compresses the input character string S after converting it to the character string T. Indicates. The number of bytes when the input character string S is compressed after being converted to the character string T includes the number of bytes of the replacement history table that is necessary when the character string T is converted back to the input character string S. One character is 1 byte, and each numerical value in the replacement history table is 1 byte.

Let the input character string S be S = aacabaaabcaaaaabaaaa. When this input character string S is compressed with R _LE as in the conventional case, R _LE (S) = (a, 2) (c, 1) (a, 1) (b, 1) (a, 3) (b , 1) (c, 1) (a, 4) (b, 1) (a, 1) (b, 1) (a, 3). Therefore, the data amount of R _LE (S) is “24” bytes.

A character string obtained by converting the input character string S into an arrangement order convenient for the R _LE compression method is defined as a character string T = aaccbbaaaaaaaabbaaaa. FIG. 23 shows a replacement history table for reversely converting the character string T into the input character string S. FIG. 23 is a diagram (3) illustrating an example of the data structure of the replacement history table. When the string T is compressed by _{R LE,} the R LE (T) = (a , 2) (c, 2) (b, 2) (a, 8) (b, 2) (a, 4). Therefore, the data amount of R _LE (T) is “12” bytes. The data amount of the replacement history table shown in FIG. 23 is 3 bytes when the origin information is omitted and the pair of offset and return distance is stored in 1 byte. Therefore, the sum of the data amount of R _LE (T) and the data amount of the replacement history table is “15” bytes.

  Therefore, the compression unit 150 can reduce the data amount by comparing the data amount of the character string compressed by the conventional compression method even when the data amount of the replacement history table is combined. . In the example shown above, the compression unit 150 can reduce 9 bytes compared to the conventional technique.

  Next, effects of the data compression / decompression apparatus 100 according to the present embodiment will be described. In the prior art, as shown in FIG. 25, the positional relationship before and after movement is stored for all characters included in the character string. On the other hand, the data compression / decompression apparatus 100 does not store the positional relationship for characters that are not replaced among the characters included in the character string, but stores the positional relationship only for the replaced characters. For this reason, the data compression / decompression apparatus 100 can reduce the amount of data that the storage unit 140 should store.

  Further, in the prior art, when storing the positional relationship before and after the movement, the number indicating the position of the character is stored in the form as it is, so the longer the character string to be compressed, the larger the integer, that is, I remembered it with a long bit length. For example, in the prior art, when the 1516177th character and the 1516179th character are replaced, (1516177, 1516179) is stored. On the other hand, the data compression / decompression apparatus 100 stores the offset and the return distance. Since both the offset and the return distance represent the positional relationship of characters as a difference, the data compression / decompression apparatus 100 stores a small integer, that is, a short bit length even if the character string to be compressed is long. be able to. For this reason, the data compression / decompression apparatus 100 can reduce the amount of data that the storage unit 140 should store.

  In the prior art, when the length of the character string is n, the memory cost is O (n). On the other hand, since the data compression / decompression apparatus 100 uses a slide buffer, the memory cost is O (1), and the memory cost can be reduced as compared with the prior art.

  In addition, when converting the input character string S, the data compression / decompression device 100 sets the character after the origin as a replacement target and restricts the area of the character that is the replacement source. For this reason, since the origin is reset to the position of the character that is the replacement source every time replacement is performed, it is possible to prevent the character that has been replaced once from being replaced again. Further, when detecting a replacement destination character, the data compression / decompression apparatus 100 sets a return distance threshold value that becomes smaller as the replacement source character appears more frequently. For this reason, characters with higher appearance frequencies are less likely to be replaced, so that the replacement frequency can be suppressed. For this reason, the data compression / decompression apparatus 100 can reduce the processing load for compression. Specifically, when the length of the character string is n, the calculation cost of the prior art is O (nlogn). On the other hand, the calculation cost of the present invention is O (n), and the calculation cost can be reduced as compared with the prior art.

  Further, when restoring the compressed character string, the data compression / decompression apparatus 100 restores the origin of the replacement history table, and based on the restored origin, the offset, and the return distance, the compressed character string is restored. Decrypt and reverse transform. For this reason, even when only the offset and the return distance are stored in the replacement history table, the character string can be accurately restored.

  Incidentally, each component of the data compression / decompression apparatus 100 shown in FIG. 1 is functionally conceptual, and does not necessarily need to be physically configured as illustrated. That is, the specific form of distribution / integration of the data compression / decompression apparatus 100 is not limited to the one shown in the drawing, and all or a part of the data compression / decompression apparatus 100 can be functionally or physically processed in an arbitrary unit according to various loads or usage conditions. Can be distributed and integrated. For example, it is not necessary that the same apparatus has the compression unit 150 and the restoration unit 160 illustrated in FIG. Different devices may include the compression unit 150 and the restoration unit 160, respectively.

  In addition, among the processes described in the present embodiment, all or part of the processes described as being automatically performed can be performed manually, or the processes described as being performed manually can be performed. All or a part can be automatically performed by a known method. For example, the processing of the compression unit 150 has been described as being automatically executed when the input file 141 is stored in the storage unit 140, but is not limited thereto. The processing of the compression unit 150 may be started manually after the input file 141 is stored in the storage unit 140. In addition, the processing procedures, control procedures, specific names, and information including various data and parameters shown in the above-described document and drawings can be arbitrarily changed unless otherwise specified. For example, the return distance threshold table 142 has been described as holding the number of appearances and the return distance threshold in association with each character included in the input character string S, but is not limited to this. The return distance threshold table 142 may hold only the return distance threshold in association with each character included in the input character string S.

  In the present embodiment, the case where the data compression / decompression apparatus 100 processes character data has been described. However, the present invention is not limited to this. For example, the data compression / decompression apparatus may process image data as a processing target.

  In the present embodiment, the case has been described in which the character string conversion unit 152 stores the replacement history table 143 in the storage unit 140 by storing the offset / return distance pair of the replacement history table 143 in one byte. However, the present invention is not limited to this. For example, the character string conversion unit 152 may store the replacement history table 143 in the storage unit 140 using a variable length code.

  For example, the character string conversion unit 152 stores the replacement history table 143 in the storage unit 140 using a Weyl code that is a variable length code. The Weyl code is a variable-length code that represents an arbitrary integer with a bit length corresponding to the magnitude of the numerical value. For example, the integers “1 to 4” are expressed by 3 bytes as “0xx” when a Weyl code is used. Further, the integer “5 to 8” is expressed by 5 bytes as “10xxx” when a Weyl code is used. In addition, the integer “9 to 16” is expressed by “110xxxx” and 7 bytes when the Weyl code is used. Further, the integer “17 to 32” is expressed by 9 bytes as “1110xxxx” when the Weyl code is used. “X” is 0 or 1.

  In the example illustrated in FIG. 15, the character string conversion unit 152 stores the offset “8” of the first row of the replacement history table 143 in 5 bits and stores the return distance “4” in 3 bits. Further, the character string conversion unit 152 stores the offset “1” of the second row in 3 bits and stores the return distance “7” in 5 bits. Further, the character string conversion unit 152 stores the offset “7” of the third row in 5 bits and stores the return distance “2” in 3 bits. That is, the character string conversion unit 152 stores the replacement history table 143 shown in FIG. 15 with a data amount of 24 bits (3 bytes).

  When a pair of offset and return distance is stored in 1 byte, the numerical value of the offset and the return distance that can be stored in the storage unit 140 by the character string conversion unit 152 are restricted by themselves. For example, when the offset is stored in 4 bits, the numerical value of the offset that can be stored by the character string conversion unit 152 is limited to “0 to 15”. On the other hand, when a variable length code is used, it is stored in the storage unit 140 with a bit length corresponding to the numerical value. For this reason, the data compression / decompression apparatus 100 can store the replacement history table 143 in the storage unit 140 regardless of the numerical values of the offset and the return distance.

  Further, the processing of the data compression / decompression apparatus 100 and the like described in the above-described embodiments can be realized by executing a program prepared in advance on various computers. Here, an example of a computer that executes a compression / decompression program that realizes the same function as the processing performed by the data compression / decompression apparatus 100 described in the above embodiment will be described with reference to FIG. FIG. 24 is a diagram illustrating an example of a computer that executes a compression / decompression program.

  As shown in FIG. 24, a computer 200 functioning as the data compression / decompression apparatus 100 includes a CPU (Central Processing Unit) 201 that executes various arithmetic processes, an input device 202 that receives input of data from a user, and a monitor 203. Have. The computer 200 also includes a medium reading device 204 that reads programs and the like from a storage medium, and a network interface device 205 that exchanges data with other computers via a network. The computer 200 also includes a RAM (Random Access Memory) 206 that temporarily stores various information and a hard disk device 207. Each device 201 to 207 is connected to a bus 208.

  The hard disk device 207 stores a compression program 207a that exhibits the same functions as those of the data compression / decompression device 100 described above, a restoration program 207b, and various data 207c. The various data 207c corresponds to the input file 141, the return distance threshold table 142, the replacement history table 143, the output file 144, and the like shown in FIG. Note that the compression program 207a, the restoration program 207b, and the various data 207c may be appropriately distributed and stored in a storage unit of another computer that is communicably connected via a network.

  Then, the CPU 201 reads the compression program 207a from the hard disk device 207 and expands it in the RAM 206, whereby the compression program 207a functions as the compression process 206a. This compression process 206a corresponds to the compression unit 150 shown in FIG.

  When the CPU 201 reads the restoration program 207b from the hard disk device 207 and expands it in the RAM 206, the restoration program 207b functions as the restoration process 206b. The restoration process 206b corresponds to the restoration unit 160 illustrated in FIG. Further, the CPU 201 reads various data 207 c from the hard disk device 207 and stores it in the RAM 206.

  The compression process 206a performs compression processing on the input file included in the various data 206c. The restoration process 206b restores the compressed character string included in the various data 206c based on the replacement history table.

  Note that the compression program 207a and the restoration program 207b are not necessarily stored in the hard disk device 207 from the beginning. For example, each program is stored in a “portable storage medium” such as a flexible disk (FD), a CD-ROM, a DVD disk, a magneto-optical disk, and an IC card inserted into the computer 200. Then, the computer 200 may read and execute each program from these.

  The following supplementary notes are further disclosed with respect to the embodiments including the above examples.

(Supplementary Note 1) A movement distance table that stores data and a threshold of a movement distance that can move the data according to the appearance frequency of the data that determines whether or not to replace the data;
The data is read from the target position of the data string to be compressed, and when different data from the data at the target position appears, the different data can be moved based on the different data and the moving distance table. A movement distance determination unit for determining a movement distance;
If the same data as the different data does not exist within the range that does not exceed the movement distance determined by the movement distance determination unit from the position where the different data appears, the target position is moved to the position where the different data appears. Let
When the same data as the different data exists within a range not exceeding the movement distance determined by the movement distance determination unit from the position where the different data appears, the next data of the same data and the different data are The distance from the first origin of the data sequence to be replaced and compressed to the replaced data and the distance between the replaced data are stored in the history table, and the origin and the target position are moved to the position of the replaced data. A compression apparatus comprising: a replacement processing unit.

(Additional remark 2) The compression apparatus which hold | maintains the movement distance table which matches and memorize | stores data and the threshold value of the movement distance which can move this data according to the appearance frequency of this data which determines whether this data is replaced | exchanged But,
The data is read from the target position of the data string to be compressed, and when different data from the data at the target position appears, the different data can be moved based on the different data and the moving distance table. A moving distance determining step for determining a moving distance;
If the same data as the different data does not exist within the range that does not exceed the movement distance determined in the movement distance determination step from the position where the different data appears, the target position is moved to the position where the different data appears. Let
When the same data as the different data exists within a range not exceeding the movement distance determined by the movement distance determination step from the position where the different data appears, the next data of the same data and the different data are The distance from the first origin of the data sequence to be replaced and compressed to the replaced data and the distance between the replaced data are stored in the history table, and the origin and the target position are moved to the position of the replaced data. A compression method comprising: performing a replacement processing step.

(Supplementary Note 3) In a computer that holds a movement distance table that stores data and a threshold of a movement distance that can move the data according to the appearance frequency of the data that determines whether or not to replace the data. ,
The data is read from the target position of the data string to be compressed, and when different data from the data at the target position appears, the different data can be moved based on the different data and the moving distance table. A moving distance determination procedure for determining a moving distance;
If the same data as the different data does not exist within the range that does not exceed the moving distance determined by the moving distance determination procedure from the position where the different data appears, the target position is moved to the position where the different data appears. Let
When the same data as the different data exists within a range not exceeding the movement distance determined by the movement distance determination procedure from the position where the different data appears, the next data of the same data and the different data are The distance from the first origin of the data sequence to be replaced and compressed to the replaced data and the distance between the replaced data are stored in the history table, and the origin and the target position are moved to the position of the replaced data. A compression program characterized by causing a replacement processing procedure to be executed.

(Supplementary Note 4) An origin calculation unit that calculates the origin based on the distance from the origin to the replaced data included in the history table described in Supplementary Note 1,
A data determination unit that determines a set of replaced data based on the origin, a distance from the origin included in the history table to the replaced data, and a distance between the replaced data;
A restoration apparatus comprising: a restoration unit that restores a data string by replacing the same set of data determined by the data determination unit.

(Appendix 5) The restoration device is
An origin calculation step for calculating the origin based on a distance from the origin to the replaced data included in the history table according to appendix 1,
A data determination step of determining a set of replaced data based on the origin, a distance from the origin included in the history table to the replaced data, and a distance between the replaced data;
A restoration method comprising: performing a restoration step of restoring a data string by replacing the same set of data determined by the data determination step.

(Appendix 6)
An origin calculation procedure for calculating the origin based on the distance from the origin to the replaced data included in the history table described in Supplementary Note 1,
A data determination procedure for determining a set of replaced data based on the origin, a distance from the origin included in the history table to the replaced data, and a distance between the replaced data;
And a restoration procedure for restoring a data string by exchanging the same set of data determined by the data determination procedure.

100 Data compression / decompression apparatus 110 Input unit 120 Output unit 130 Input / output control unit 140 Storage unit 141 Input file 142 Return distance threshold table 143 Replacement history table 144 Output file 150 Compression unit 151 Threshold table generation unit 152 Character string conversion unit 153 R _LE Encoding unit 160 Restoration unit 161 R _LE decoding unit 162 Character string reverse conversion unit 200 Computer 201 CPU
202 Input Device 203 Monitor 204 Medium Reading Device 205 Network Interface Device 206a Compression Process 206b Restoration Process 206c Various Data 207 Hard Disk Device 207a Compression Program 207b Restoration Program 207c Various Data 208 Bus

Claims (4)

A movement distance table that associates and stores data and a threshold of a movement distance that can move the data according to the appearance frequency of the data that determines whether or not to replace the data;
The data is read from the target position of the data string to be compressed, and when different data from the data at the target position appears, the different data can be moved based on the different data and the moving distance table. A movement distance determination unit for determining a movement distance;
If the same data as the different data does not exist within the range that does not exceed the movement distance determined by the movement distance determination unit from the position where the different data appears, the target position is moved to the position where the different data appears. Let
When the same data as the different data exists within a range not exceeding the movement distance determined by the movement distance determination unit from the position where the different data appears, the next data of the same data and the different data are The distance from the first origin of the data sequence to be replaced and compressed to the replaced data and the distance between the replaced data are stored in the history table, and the origin and the target position are moved to the position of the replaced data. A compression apparatus comprising: a replacement processing unit. A compression apparatus that holds a movement distance table that stores data and a threshold of a movement distance that can move the data according to the appearance frequency of the data that determines whether or not to replace the data.
The data is read from the target position of the data string to be compressed, and when different data from the data at the target position appears, the different data can be moved based on the different data and the moving distance table. A moving distance determining step for determining a moving distance;
If the same data as the different data does not exist within the range that does not exceed the movement distance determined in the movement distance determination step from the position where the different data appears, the target position is moved to the position where the different data appears. Let
When the same data as the different data exists within a range not exceeding the movement distance determined by the movement distance determination step from the position where the different data appears, the next data of the same data and the different data are The distance from the first origin of the data sequence to be replaced and compressed to the replaced data and the distance between the replaced data are stored in the history table, and the origin and the target position are moved to the position of the replaced data. A compression method comprising: performing a replacement processing step. A computer that holds a movement distance table that associates and stores data and a threshold of a movement distance that can move the data according to the appearance frequency of the data that determines whether to replace the data,
The data is read from the target position of the data string to be compressed, and when different data from the data at the target position appears, the different data can be moved based on the different data and the moving distance table. A moving distance determination procedure for determining a moving distance;
If the same data as the different data does not exist within the range that does not exceed the moving distance determined by the moving distance determination procedure from the position where the different data appears, the target position is moved to the position where the different data appears. Let
When the same data as the different data exists within a range not exceeding the movement distance determined by the movement distance determination procedure from the position where the different data appears, the next data of the same data and the different data are The distance from the first origin of the data sequence to be replaced and compressed to the replaced data and the distance between the replaced data are stored in the history table, and the origin and the target position are moved to the position of the replaced data. A compression program characterized by causing a replacement processing procedure to be executed. An origin calculation unit that calculates the origin based on a distance from the origin to the replaced data included in the history table according to claim 1;
A data determination unit that determines a set of replaced data based on the origin, a distance from the origin included in the history table to the replaced data, and a distance between the replaced data;
A restoration apparatus comprising: a restoration unit that restores a data string by replacing the same set of data determined by the data determination unit.

Images