JP3100206B2 - Data compression method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a data compression method, and more particularly, to a method of obtaining a character string that matches an input character string from encoded character strings that have already been input, and using the number of the matched character string to determine an input character string. And a data compression method for encoding.

In recent years, office automation (OA)
Has been developed, and images are increasingly handled by computers as black-and-white gradation image information. The data amount of these pieces of image information is as large as several Mbytes per sheet. Therefore, in order to efficiently handle image information in storage, transmission, and the like, it is essential to reduce the data amount by adding efficient data compression.

[0003]

2. Description of the Related Art As a method of efficiently compressing data,
A compression method using a universal code has been put to practical use. This universal code is an information preserving type data compression method, and does not presuppose a statistical property of an information source at the time of data compression, so that it can be applied to data of various types (character codes, object codes, etc.). .
Document images have similarities in character outlines and character intervals, and halftone images have similar dot periodicity and halftone dot shape. Redundancy having this similarity can be reduced by the universal code, and effective compression can be performed.
In the following, following the name used in information theory, one word unit of data will be called a character, and data connected with an arbitrary word will be called a character string.

[0004] As a typical method of the universal code,
There is a Ziv-Lempel code. For example, Munakata "Ziv-Lempel Data Compression Method", Information Processing, Vol. 26, No.
See 1,1985. In this Ziv-Lempel code, two algorithms of a universal type and an incremental decomposition type (Incremental parsing) have been proposed.As a practical method using the universal type algorithm, an LZSS code (T.
C. Bell, "Better OMP / LText Compression", IEEE Tran
s. on Commun., Vol. COM-34, No. 12, Dec. 1986), and a practical method using an incremental decomposition type algorithm is an LZW (Lempel-Ziv-Welch) code ( TA
Welch, "A Technique for High-Performance Data Co
mpression ", Computer, June 1984).

LZSS encoding In LZSS encoding as a practical method using a universal algorithm, input data that has already appeared and has been encoded is stored in a storage unit (P buffer), and encoded data is encoded. Is searched for a partial data string that matches the maximum length of the input data string from the partial data string starting from an arbitrary position of the above. The information indicating the address and the match length of the first character of the matching partial data string in the storage unit (P buffer) , Encode the input data sequence. In this LZSS encoding, a large amount of calculation is performed, but a high compression rate can be obtained.

FIG. 7 is an explanatory diagram of such LZSS encoding, wherein 1 is a Q buffer and 2 is a P buffer. The Q buffer 1 has, for example, 4-bit index information (address).
The Have, which stores now 16 (= 2 ⁴⁾ for coding pieces of string, P buffer 2 has, for example, 12 bits of index information (address), 4096 encoded the latest (= 2 ¹²⁾ Is stored.

A universal encoding unit (not shown) collates a character string from the head of the Q buffer 1 with a character string starting from an arbitrary position in the P buffer 2 to obtain a maximum length matching partial character string 3. Using the matching start position p ₁ of the character string in the P buffer and the “matching length q ₁ of the partial character string”,
The partial character string 3 'of the buffer is encoded and stored. Thereafter, the universal encoding unit moves the encoded character string 3 'in the Q buffer 1 to the P buffer 2 and discards the oldest encoded character string corresponding to the number of the character strings from the P buffer 2 and performs encoding. A new character string corresponding to the number of characters of the character string 3 'is input into the Q buffer 1, and thereafter, the above-described encoding processing is continued. If the maximum match length is 1, no encoding is performed, and the first character data of the Q buffer 1 (called raw data)
Is stored as it is. This is because encoded data requires 2 bytes, but raw data only requires 1 byte.

When eight encoded data or raw data are stored, as shown in FIG. 7 (b), it is composed of eight flag bits for indicating the discrimination between the encoded data and the raw data. The identification data is added to the head ("0" is code data, "1" is raw data), and this set of data is sequentially output.

In LZW encoding as a practical method using an LZW encoding incremental decomposition type algorithm, a rewritable dictionary is provided,
Divides the input character string into different character strings, assigns numbers to the character strings in the order in which they appear, and registers them in the dictionary, and stores only the dictionary number of the longest matching character string registered in the dictionary with the currently input character string. And encode it. According to the LZW coding, the compression ratio is inferior to that of the LZSS coding, but since it is simple, easy to calculate, and can perform high-speed processing, it is used for file compression of storage devices and data transmission.

FIG. 8 is an explanatory diagram of LZW encoding, FIG. 9 is an explanatory diagram of a dictionary configuration, and FIG. 10 is a flowchart of LZW encoding processing. For the sake of simplicity, it is assumed that a character string composed of three characters a, b, and c is subjected to LZW encoding and data compression. In advance, a character string (a,
b, c) is given a registration number and is initially registered in the dictionary, and the number N of registrations in the dictionary is set to the number M of character types (M → N).・
・ Step 101

In this state, the first character K is input, the registration number of the character is set as a reference number ω, and this is referred to as an initial character string (p
refix string) (step 102). Next, the next character K of the input data is read (step 103),
Step 103 is applied to the word string ω obtained in step 102.
A search is performed to determine whether or not the character string (ωK) to which the character K read in (1) is added in the current dictionary (step 104).

If the character string (ωK) exists in the dictionary, the character string (ωK) is replaced with ω (step 105), and thereafter, it is determined whether the input data has been completed (step 10).
6) If the data is not completed, the process returns to step 103 and the subsequent processing is repeated, and the search for the maximum matching length character string is continued until the character string (ωK) cannot be searched from the dictionary. On the other hand, if the input data is completed in step 106,
The reference number ω is output as the code word code (ω) (step 107), and the encoding process ends. In this case, the codeword code (ω) (= ω) is represented by “log ₂ N” (the smallest integer greater than log ₂ N) bits.

If the search for the longest matching character string continues and the character string (ωK) does not exist in the dictionary in step 104, the reference number ω is set to the code word code (ω) and “log ₂
[N] bits and outputs the result, and adds a new registration number N to the character string (ωK), registers it in the dictionary, and replaces the registration number of the character K read in step 103 with the reference number ω. , The dictionary address N is incremented by one (step 108). Next, it is determined in step 106 whether the input data has been completed, and the subsequent processing is repeated according to the determination result.

The LZW encoding will be specifically described with reference to FIGS. 8 and 9. That is, FIG.
Is read one character at a time from left to right.
When the first character a is read, there is no matching character string other than a in the dictionary, so the registration number “1” of a (reference number ω
= 1) as a codeword (code (ω)). Then, the extended character string ab is assigned a registration number 4 and registered in the dictionary. The actual registration is in the form of a character string "1b". Subsequently, the second character b becomes the head of the input character string. Since there is no matching character string other than b in the dictionary, the registration number (reference number) 2 of b is output as a code word, and the expanded character string ba is actually assigned a registration number 5 in the form of 2a. Register in the dictionary.

As described above, the third character a becomes the head of the input character string. Since the first character a exists in the dictionary, a character string “1b” obtained by adding the next character b to the registration number 1 of the character
Check if exists. Since the character string "1b" exists,
A character string “4” in which the following character c is added to the registration number 4 of the character string
c "is present. Since the character string "4c" does not exist, the registration number "4" of the longest matching character string "1b" is output as a code word, and the registration number 6 is added to the expanded character string "4c".
Is added to the dictionary, and the encoding and dictionary registration are repeated in the same manner to execute the LZW encoding process for all input characters.

FIG. 11 is a flowchart of the LZW decoding process. In the decoding process, the reverse operation of the encoding is performed. That is, at the time of decoding, as in the case of encoding, a character string (a, b, c) consisting of one character for every character is assigned a registration number and initially registered in the dictionary, and the number N of registered dictionary is changed to the number of character types. Let M be (M → N).・ ・ Step 201 Then, read the first code CODE and replace the code OLDcode
And Also, since the first code corresponds to one of the registration numbers of one character already registered in the dictionary, the input code CO
The character K indicated by DE (= registration number) is output. The output character K is set as char for later exception processing.・
・ Step 202

Thereafter, the next code CODE is read and NEWc is read.
ode (step 203) and code CO
It is checked whether DE (= registration number) is defined (registered) in the dictionary (step 204). Normally, input code C
Since the ODE (= registration number) has been registered in the dictionary in the processing up to the previous time, "NO" is obtained in step 204. Next, the registered character string of the dictionary indicated by the code CODE (= registration number) is ( ωK). That is, it is determined whether the registered character string of the dictionary indicated by the code CODE is a combined character string of the reference number ω and the character K, such as (ωK) (step 2).
05).

If the character string is a combination of the reference number ω and the character K, the character K is temporarily stacked and the code word of the reference number ω is
code (ω) (actually code (ω) = ω) is set as a new CODE, and the number of characters C is incremented by 1 (step 20).
6) Return to step 205. Thereafter, step 205,
Step 206 is recursively repeated until the registered character string indicated by CODE reaches one character.

In step 205, if the character string indicated by the CODE is one character, that is, if the registered character string in the dictionary indicated by the code CODE is (K), K is output, and then the stack is executed. LIFO (Last i
n Pop up and output in Fast Out) format. A character string (OLDcode, OLDcode, OLDcode) obtained by adding the first character K of the character string decoded this time to the code OLDcode used in the previous decoding.
K) is registered in the dictionary with a registration number N, and N is incremented by 1 (N + 1 → N). Further, the first character K of the decoded character string is set to char, and NEWcode is set to OLDcode.・ ・
Step 207

Thereafter, it is determined whether code input has been completed (step 208). If not completed, the process returns to step 203 to read the next code and repeat the decoding process. by the way,
In the encoding process, since the encoding of a certain character string and the dictionary registration of the character string in which the next leading character is added to the character string are performed at the same time, in the next encoding process, Codewords can be used. However, in the decoding process, since the character string obtained by adding the head character string of the currently decoded character string to the character string decoded immediately before is registered in the dictionary, dictionary registration is delayed by one time as compared with the encoding process. For this reason, if the code word of the character string coded immediately before is used in the encoding process, the code word may not be registered (defined) in the decoding process. This is step 2
In 04, no code is defined, and "Y
ES ”.

For example, at the time of encoding, as shown in FIG. 12, an OLD code is output for a character string "a... Z", and the character string "a. Is output by NEWcode, and the character string "a ... zab" is registered in the dictionary. Now,
When the decoding side reads the code word NEWcode, the decoding cannot be performed because the code word is not registered in the dictionary on the decoding side. However, comparing NEWcode and OLDcode, the following relationship NEWc
The character string of ode = the character string of OLDcode + the first character (char) of the character string of OLDcode. Therefore, if "NO" in step 204, the set char is stacked, the OLDcode is regarded as a CODE, and the chord is set to the OLDcode.
The character string to which ar is added is set as NEWcode (step 209),
Thereafter, the processing from step 205 is performed using the CODE.

The decoding process will be described below in detail with reference to FIG. The first input code is “1”, and one character a, b, c is already registered with the registration number 1,
Since the dictionary is registered as 2, 3 (similar to FIG. 9),
By referring to the dictionary, it is replaced with the character string a of the registration number that matches the code “1” and output. Next, the code "2" is similarly replaced with the character b and output. At this time, a new registration number 4 is added to “1b”, which is a combination of the code “1” processed last time and the first character b decoded this time, and registered in the dictionary.

The third code "4" is obtained by searching the dictionary.
The character string “ab” is output by replacing “1b” with “ab”. At the same time, the character string “2a (= a) is obtained by combining the code“ 2 ”processed last time and the first character a decoded this time.
b) is added to the new registration number 5 and registered in the dictionary.
Hereinafter, similarly, the decoding process is repeated. Note that the exception processing in step 209 in FIG. 11 occurs when the sixth input code “8” is decoded. The code “8” is not defined in the dictionary at the time of decoding and cannot be decoded. In this case, a character string “5b” is obtained by adding the first character b of the character string “ba” decoded last time to the code “5” processed last time, and further obtains “2ab”,
The output is replaced with “bab”. Then, a registration number "8" is added to the character string "5b" obtained by adding the character b of the character string decoded this time to the previous code word "5", and the dictionary is registered in the dictionary.

[0024]

As described above, the universal code is a compression method in which, even if the nature of the object to be encoded is unknown, the encoding is performed while learning it. When the same data sequence appears, the registration position or registration number of the dictionary is transmitted as coded data (codeword). That is, in the conventional universal encoding, a character string that matches the longest from the beginning of the input character string (the longest matching character string) is searched from the dictionary, and the input character string is encoded by its registration position or registration number. Things.

Therefore, in the case where various character strings are registered in the dictionary as shown in the right column of FIG. 14, when encoding the input character string "ABCDEFGHIJK" shown in the left column, a conventional universal encoding method is used. Then, "AB", "CDE", "F", "GHI", "JK" and the longest matching character string are searched, and these are sequentially encoded.
However, if the first character "A" is separated, the longest matching character string "BCDEFGHIJK" can be searched next, and encoding is completed with two codewords, so that the compression efficiency can be improved as compared with the former case. In other words, the conventional compression method based on universal coding may not perform compression sufficiently, and there is room for improvement.

Therefore, the longest matching character string of the next character string is obtained when the search is terminated with the current matching character string in the middle of the matching search and when the matching search is continued as it is. In consideration of the total encoding efficiency of the first and next two character strings, it is possible to determine where the match search can be terminated. However, in such a method, the above-described processing must be performed during every match search, and there is a problem that the amount of calculation increases and high-speed encoding cannot be performed.

Accordingly, an object of the present invention is to provide a data compression method capable of improving a data compression ratio by minimizing a decrease in processing speed by increasing a small amount of calculation. Another object of the present invention is to determine the longest matching character string of the next character string in each of the case where the first character is separated and the case where the search processing of the longest matching character string is continued without separating, In consideration of the total encoding efficiency of the following two character strings, a data compression method is provided to improve the data compression efficiency by judging whether to encode with the first character or to encode without dividing. It is to be.

It is still another object of the present invention to provide a data compression method capable of accurately calculating a compression ratio and dividing and encoding an input character string in a manner having a high compression ratio. Another object of the present invention is to provide a data compression method capable of further improving the compression ratio by expressing the first character with a code word shorter than the code word of the longest matching character string when encoding by separating with the first character. It is to be. It is still another object of the present invention to provide a data compression method capable of improving the compression ratio by dividing data into blocks by a predetermined amount and coding the blocks based on an optimum coding method.

[0029]

FIG. 1 is a diagram illustrating the principle of the present invention. CTR is input string, CTR ₁₁ is the first character string consisting of only the first character of the input string, CTR ₁₂ is a dictionary that are registered encoded character string and second character string after the first character matching the longest, CDW ₁₁ is the code word of the first character string,
CDW ₁₂ is the code word of the second character string, CTR ₂₁ is the first character string that matches the encoded character string registered in the dictionary from the first character of the input character string and the longest, and CTR ₂₂ is the code registered in the dictionary. CDW ₂₁ is the code word of the first character string, CDW ₂₁ is the second character string after the first character string that matches the longest
₂₂ is a code word of the second character string, CPR ₁ and CPR ₂ are compression ratio calculation units, COMP is a comparison unit that compares the compression ratio, CD
OT is a codeword output unit.

[0030]

In the universal encoding, the input character string CT
If separated R in the first character C _1, with the tip initials first string CTR _11, the encoded character string C ₂ C ₃ · · matching the input character string and the longest of the first character string and later・ C _n
, And obtains the outermost length matching character string as the second string CTR _12. Further, an encoded character string C ₁ C ₂ C ₃ ... C _i that is the longest match from the _first character C ₁ of the input character string CTR is obtained, and the longest match character string is set as the first character string CTR _21. obtains an encoded character string C _{i + 1} ··· C _m matching the first string CTR ₂₁ after the string and the longest in the string, the outermost length matching character string and the second character string CTR _22. Then
Each longest matching character string CTR ₁₂ , CTR ₂₁ , CTR ₂₂ is converted into a code word CDW ₁₂ , CDW ₁₂ using its dictionary number (registration number).
₂₁ and CDW ₂₂ and the first character
The character string CTR ₁₁ is represented by a code word CDW ₁₁ shorter than the code word of the longest matching character string, and the first and second character strings CTR ₁₁ , CT
Compression rate when the R ₁₂ is coded, first, second string CT
The compression ratios at the time of encoding R ₂₁ and CTR ₂₂ are obtained by the compression ratio calculation units CPR ₁ and CPR ₂ , respectively, and the respective compression ratios are compared by the comparison unit COMP. The code output unit CDOT has the higher compression ratio first outputs a string code word CTR ₂₁ or CTR ₁₁ of, thereafter, continue to encode the next character in the first character string in the input string CTR as the first character.

As described above, the case where the character is separated by the first character,
In each case where the search processing of the longest matching character string is continued without separating, the longest matching character string of the next character string is obtained, and the total encoding efficiency of the first and next two character strings in each case is considered. Then, since data is compressed by judging whether encoding is performed by dividing by the first character or encoding without dividing, efficient data compression can be performed with a small increase in the amount of calculation.

In the case of encoding by separating with the first character,
Since the first character is represented by a code word shorter than the code word of the longest matching character string, the compression ratio can be further improved. Further, the compression ratio is obtained by dividing the sum of the characters of the first and second character strings by the sum of the bit numbers of the code words of the first and second character strings. In this way, the compression ratio can be calculated accurately, the input character string can be segmented and coded in a manner with a high compression ratio, and the compression ratio can be improved.

The number of times the code word of the first character string CTR ₁₁ is output is counted by temporarily executing the encoding for a certain number of input character strings. Formally perform processing and output codewords, and if it is less than a certain value, perform the same encoding as before, so that it is possible to adopt an encoding method according to the character string characteristics and improve the data compression rate Can be.

[0034]

【Example】

(a) First Embodiment Configuration FIG. 2 is a block diagram of an encoder for realizing a data compression method according to the present invention and a schematic explanatory diagram of data compression. Reference numeral 11 denotes a universal encoding and output of an input character string CTR. The encoding unit 12 is a dictionary unit that sequentially adds numbers (referred to as registration numbers or reference numbers) to character strings that have already been input and encoded, and stores them.

The data compression schematic universal coding unit 11, first, as shown in FIG. 2 (b), the case where the separated input string CTR in the first character C _1, the distal acronym first string CTR ₁₁ with a, a coded string C ₂ C ₃ ··· C _n that matches the input character string and the longest of the first character string and later searches the dictionary part 12, a second string outermost length matching string and CTR _12, the first, second string CTR _11, CTR ₁₂ encodes, the respective code word CDW _11, CDW _12.

First character string CTR consisting of one character₁₁Sign of
Word CDW₁₁Is one character with m bits (for example, 8 bits)
If it is composed, the character itself is expressed.
M-bit raw data and a file
Expressed as a total of 9 bits with lag bits. Also the longest
Second character string CTR that is a matching character string₁₂Code word CDW ₁₂
Is N, the number of character strings registered in the dictionary unit 12
And the second character string CTR ₁₂"Log representing the registration number of_Two
N] (log_TwoThe smallest integer greater than N) bits and the longest one
Total with flag bit indicating that it is a match character string
("Log_TwoN] +1) bits. ...
Above, 1st compression method

Next, the first and second character strings CTR ₁₁ , CT
By dividing the sum of the character of R ₁₂ in the total number of bits of the code word CDW _11, CDW ₁₂ calculates the compression ratio r _1. When the calculation of the compression ratio r ₁ of the first compression method is completed, the universal encoding unit 11, as shown in FIG. 2C, encodes the encoded character that matches the longest character than the _first character C ₁ of the input character string CTR. Row C ₁ C
₂ C ₃ ... C _i are obtained from the dictionary unit 12, and the longest matching character string is set as a first character string CTR ₂₁ . Also, calculated from the first character dictionary unit 12 an encoded character string C _{i + 1} ··· C _m matching the column after the string and the longest in the input string, outermost length matching character string and the second character string CTR ₂₂ . Thereafter, the first and second character strings CTR ₂₁ and CTR ₂₂ are encoded, and are respectively referred to as code words CDW ₂₁ and CDW ₂₂ .

The first and second character strings C which are the longest matching character strings
Assuming that the number of character strings registered in the dictionary unit 12 is N, the codewords CDW ₂₁ and CDW ₂₂ of TR ₂₁ and CTR ₂₂ are represented by “log ₂ N” representing the registration numbers of the first and second character strings. (Log ₂
The total of bits (the smallest integer greater than N) and a flag bit indicating the longest matching character string (“log ₂ N”)
+1) bits. ...
The second compression method

Next, the first and second character strings CTR_{twenty one}, CT
R_{twenty two}To the code word CDW_{twenty one}, CDW_{twenty two}Compression when encoding with
Rate r_TwoIs calculated. The compression ratio is required for both compression methods
For example, the universal encoding unit 11 calculates the compression rate r ₁, R_TwoBig and small
And the first character string CTR with the higher compression ratio₁₁Or C
TR_{twenty one}Of the first character string CTR₁₁or
Is CTR_{twenty one}The next letter C_TwoOr C_{i + 1}As the first character
Continue encryption.

As described above, the case where the character is delimited by the first character,
In each case where the longest matching character string search processing is continued without separating, the longest matching character string of the next character string is obtained, and the total compression ratio of the first and next two character strings in each case is taken into consideration. Therefore, efficient data compression can be achieved depending on whether encoding is performed by dividing by the first character or by encoding without dividing.

Details of Data Compression FIGS. 3 and 4 are flowcharts of the data compression processing of the present invention. A variable LEN for checking the character string matching length is set to 0 in advance.
(Step 301) is initialized, the initial registration in the dictionary are given the respective registration numbers of all characters (= 256 = 2 ⁸⁾ of 8 bits, and the character type number registration number N of dictionary M (= 256) (M → N). In this state, the first character K is input, the registration number of the character is set as a reference number ω, and this is set as a prefix string (step 302).

Next, the next character K of the input character string is read, and it is checked whether the character exists (step 30).
3, 304). If the input data is completed, since the character K does not exist, the reference number ω is expressed by “log ₂ N” bits, and a flag bit indicating the longest matching character string is added (“log ₂ N”). _A 2N] +1) -bit code word code (ω) is output (step 305), and the coding process is terminated.

On the other hand, if the next character K exists,
Is searched for in the current dictionary (step 305). String (ω
If K) exists in the dictionary, check if LEN = 0, in other words, check if character K is the second character (step 306). If LEN = 0 and the character K is the second character, the second character string search routine of FIG.
Second character string CTR ₁₂ in the first compression method (see FIG. 2)
(Step 307). That is, ω is held, and the registration number of the second character K is ω ′ (step 401). Next, the next character (third character) K 'of the input character string is read, and it is determined whether the character K' exists, in other words, whether the input character string has been completed (steps 402 and 403).

If the character K 'exists, the dictionary is searched for a character string (ω'K') obtained by adding the character K 'to the reference number ω' (step 404), and the character string (ω'K If ') exists in the dictionary, the character string (ω'K') is replaced with ω '(step 405). Thereafter, the flow returns to step 402 and the subsequent processing is repeated until the character string (ω′K ′) cannot be searched from the dictionary or until the next character does not exist, and the maximum matching length character string (second character string CTR ₁₂ ) Continue searching for.

Then, in step 403, the character K '
If no longer exists, or if the character string (ω′K ′) cannot be found in the dictionary in step 404, the second
The character string search process ends. Note that the number of characters of the second character string CTR ₁₂ obtained by the second character string search routine is n ₁₂ , and the number of bits required to represent the reference number ω ′ of the second character string CTR ₁₂ is log ₂ ( ω ′).

Next, the universal encoding unit 11 calculates the compression ratio r _{1 in the} “first compression scheme” by the following equation: r ₁ = (1 + n ₁₂ ) / {9 + log ₂ (ω ′) + 1} (1) (Step 308). The numerical values 9 in the denominator is the number of bits for representing the first character string CTR ₁₁ consisting of the first _{character, (log 2 (ω ')} + 1) and the second string is the longest matching character string the number of bits for representing the CTR _12.

As described above, when the calculation processing of the compression ratio r _{1 in} the “first compression method” is completed, the first character, that is, the reference number (registration number) ω of the first character string CTR ₁₁ is held in the buffer, and ω → BUFω), and the matching length LEN is set to 1 (steps 309 and 310). The reason why the reference number (registration number) ω of the first character string CTR ₁₁ is stored in the buffer is that the “first compression method” has a higher compression ratio than the “second compression method”. This is because the code word code (ω) of the reference number (registration number) ω of the first character string CTR ₁₁ needs to be output in a later process.

When the above processing is completed, the first and second character strings CTR ₂₁ and CTR ₂₂ in the “second compression method” will be described hereinafter.
Perform search processing. That is, first, the character string (ωK) from the head to the second character in the input character string is replaced with ω (step 311).

Next, returning to step 303, the next character K is read, and it is checked whether the character exists (steps 303 and 304). If the input data is finished,
Since the letter K no longer exists, the reference number ω
₂ N] bits, and a flag bit indicating the longest matching character string is added to the total (“log ₂ N]
A code word code (ω) of +1) bits is output (step 305), and the encoding process ends.

On the other hand, if the next character K exists,
A search is performed to determine whether or not a character string (ωK) obtained by adding to the reference number ω is present in the dictionary (step 305), and the character string (ωK)
If exists in the dictionary, it is checked whether LEN = 0 (step 306). LEN is already 1 in step 310
Is "NO", and the character string (ωK) is changed to ω
In addition, LEN is incremented by 1 (steps 312 and 313), and thereafter, the process returns to step 303, and the search for the first character string CTR ₂₁ in the "second compression method" is continued.

The above search processing is repeated to obtain a character string (ω
If K) cannot be found from the dictionary, "NO" is determined in the step 305, and the search processing of the first character string CTR ₂₁ in the "second compression method" is completed. Note that the number of characters of the first character string CTR ₂₁ obtained in the first character string search processing is n ₂₁ , and the number of bits required to represent the reference number ω of the first character string CTR ₂₁ is log ₂ (ω ).

The first character string CT in the "second compression method"
If search processing for R ₂₁ is complete, then executes the second character string search routine shown in FIG. 4 performs a search process of the second string CTR ₂₂ in the "second format" (Step 3
14).

That is, the current ω (first character string CTR ₂₁
Is stored, and the registration number of the character K (the character read in step 304) next to the first character string CTR ₂₁ is set to ω ′ (step 401). Next, the next character K 'in the input character string is read, and it is determined whether or not the character K' exists, in other words, whether the input character string has been completed (steps 402 and 403).

If the character K 'exists, the dictionary is searched for a character string (ω'K') obtained by adding the character K 'to the reference number ω' (step 404), and the character string (ω'K If ') exists in the dictionary, the character string (ω'K') is replaced with ω '(step 405). Thereafter, the process returns to step 402 and the subsequent processing is repeated until the character string (ω′K ′) cannot be searched from the dictionary or until the next character does not exist, and the maximum matching length character string (second character string CTR ₂₂ ) Continue searching for.

Then, at step 403, the character K '
If no longer exists, or if the character string (ω′K ′) cannot be found in the dictionary in step 404, the second
The character string search process ends. The number of characters of the second character string CTR ₂₂ obtained by the above-described second character string search routine is n ₂₂ , and the number of bits required to express the reference number ω ′ of the second character string CTR ₂₂ is log ₂ ( ω ′).

Next, the universal encoding unit 11
Equation r_Two= (N_{twenty one}+ N_{twenty two}) / [｛Log_Two(Ω) +1｝ + ｛log_Two(Ω ′) + 1}], the compression ratio r in the “second compression method”_TwoCalculate
(Step 315). Note that ｛log in the denominator_Two(Ω)
+1} is the first character string CTR that is the longest matching character string _{twenty one}The table
Number of bits to represent, ｛log_Two(Ω ') + 1｝ is the longest one
Second character string CTR that is a match character string_{twenty two}To express
Number of units.

As described above, when the calculation of the compression ratio r _{2 in} the “second compression method” is completed, the matching length LEN is returned to 0 (step 316), and the compression ratios r in both compression methods are determined.
The magnitudes of ₁ and r ₂ are compared (step 317). r ₁ ≧ r ₂
, The reference number ω of the first character in the input character string stored in the buffer, in other words, the “first compression method”
, The reference number of the first character string CTR ₁₁ is returned as ω (BUFω → ω, step 318), and the codeword code () expressing the reference number ω with a total of 9 bits of 8-bit raw data and 1-bit flag bit ω) (Step 3)
19).

After that, the next character of the first character string is set as the first character, and the registration number is set as a new reference number ω, and the processing after step 303 is repeated to encode the input character string. On the other hand, if r ₁ <r ₂ in step 317,
The reference number ω of the first character string CTR ₂₁ in the “second compression method” held in step 401 is changed to “log ₂ N] (log
₂ (the smallest integer greater than N) and a flag bit indicating the longest match string ("log
₂ N] +1) and outputs the code word code (omega) representing a bit string (omega obtained by adding the next character K to the first character string
K) is added to the registration number N and registered in the dictionary.
Is replaced with the reference number ω, and the dictionary address N is incremented by one (step 320). Thereafter, the processing of step 303 and subsequent steps is repeated to encode the input character string.

(B) Second Embodiment According to the encoding processing according to FIG. 3 described above, it is possible to encode with 9 bits when divided by one character.
(“Log ₂ N] +1) bits can be shortened compared to the case of encoding. However, when the output frequency of the first character string (maximum matching length character string) in the second compression method increases, only the flag amount is used. A conventional coding method that is accumulated and does not use a flag (FIG. 1)
In some cases, the coding efficiency may be lower than that in the case of (0). Therefore, the encoding process of FIG. 3 is temporarily performed for a file in a certain section (a fixed amount of input character strings), and the number of times the code word of the first character string CRT ₁₁ is output by the “first compression method” is monitored. If the value is smaller than the set value, compression is performed by the conventional encoding method. If the value is larger than the set value, the encoding process of the present invention according to FIG. Compression can be performed.

FIG. 5 is a block diagram showing an embodiment of an encoder for realizing such a compression method. Reference numeral 11 denotes a universal encoding unit for encoding an input character string CTR according to FIG. 3 or FIG. A dictionary unit 21 stores character strings that have been encoded in advance by sequentially assigning numbers (registration numbers or reference numbers), and 21 is a temporary compression unit.
3) is temporarily executed on the input character string CTR of (byte), and the number of times the code word of the first character string CRT ₁₁ according to the “first compression method” is output is monitored. The provisional compression unit 22 determines whether to perform the encoding process of the present invention in accordance with FIG. 3 formally or to perform the encoding process without using the flag as in the related art. It is a storage unit for storing. The storage unit 22 may not be provided, and the input character string may be input to the universal encoding unit 11 after a predetermined amount of the input character string is temporarily compressed. Further, the temporary compression unit 21 and the universal encoding unit 11 may be configured in common so that both the temporary compression and the main compression are used.

FIG. 6 is a flowchart of the data compression processing of the present invention. A certain amount of the input character string CTR is sequentially input to the temporary compression unit 21 and the storage unit 22. The temporary compression unit 21 temporarily executes the encoding process according to FIG. 3, and the storage unit 22 stores a fixed amount of the input character string CTR (steps 501 and 502). The temporary compression unit 21 performs an encoding process on the input character string CTR, and performs the first character string CTR using the “first compression method”.
Number of times ₁₁ code words are output (raw data output number) P
(i) is monitored (step 503).

When the fixed amount of temporary compression is completed, the raw data output frequency P (i) is compared with a predetermined fixed number C (step 504). Raw data output count P
If (i) is equal to or less than a certain number C, MODE = 0 (conventional encoding processing), and if it is more than a certain number, MODE = 1 (the encoding processing of the present invention according to FIG. 3). ..Step 505,
506

Next, it is checked whether or not the mode has been changed, and if so, mode change data is output (step 5).
07, 508). The universal encoding unit 11 determines whether MODE = 1 has been instructed by the mode change data (step 509). If MODE = 0, the universal encoding unit 11 performs an encoding process that does not use a conventional flag (step 510). = 1
If so, the encoding process according to the present invention according to FIG. 3 is executed (step 511), and compressed data is output (step 51).
2) Thereafter, step 5 is performed for the next fixed amount of input character strings.
The processing after 01 is repeated.

As the mode change data, an 8-bit code not used as a character is determined, and the mode change is instructed by the code. Alternatively, the mode is designated by outputting a 1-bit mode designation flag for designating the mode for each fixed input character string (one block). As described above, the present invention has been described with reference to the embodiments. However, the present invention can be variously modified in accordance with the gist of the present invention described in the claims, and the present invention does not exclude these.

[0065]

As described above, according to the present invention, the longest matching character string of the next character string is obtained in each of the case where the first character is separated and the case where the search processing of the longest matching character string is continued without separating. In consideration of the total encoding efficiency of the first and next two character strings in each case, data compression is performed by judging whether encoding is performed by delimiting with the first character or encoding without delimiting. Therefore, efficient data compression can be achieved with a small increase in the amount of calculation, in other words, a decrease in the calculation speed is minimized.

Further, according to the present invention, when encoding by dividing by the first character, the first character is represented by a code word shorter than the code word of the longest matching character string, so that the compression ratio is further improved. it can.

Further, according to the present invention, the compression ratio is set to the first,
Since the sum of the characters of the second character string is obtained by dividing the sum of the number of bits of the code words of the first and second character strings, the compression ratio can be accurately calculated and the compression ratio can be accurately increased. Can be used to separate and encode the input character string and improve the compression ratio.

Further, according to the present invention, the number of times that raw data is output by temporarily executing encoding for a fixed number of input character strings is counted. The code word is output by performing the formalization process, and if the value is less than a certain value, the encoding is performed as before.Therefore, the encoding method according to the character string characteristics can be adopted, and the data compression ratio can be reduced. Can be improved.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the principle of the present invention.

FIG. 2 is a configuration diagram of an encoder for realizing a data compression method according to the present invention and a schematic diagram of data compression.

FIG. 3 is a first flowchart of a data compression process of the present invention.

FIG. 4 is a second flowchart of the data compression processing of the present invention.

FIG. 5 is a configuration diagram of another embodiment of the present invention.

FIG. 6 is a flowchart of another compression process of the present invention.

FIG. 7 is an explanatory diagram of LZSS encoding.

FIG. 8 is an explanatory diagram of LZW encoding.

FIG. 9 is an explanatory diagram of a dictionary configuration.

FIG. 10 is a flowchart of LZW encoding.

FIG. 11 is a flowchart of LZW decoding.

FIG. 12 is an explanatory diagram at the time of exception of LZW decoding.

FIG. 13 is an explanatory diagram of LZW decoding.

FIG. 14 is an explanatory diagram of a problem of conventional data compression.

[Explanation of symbols]

CTR ·· input string CTR ₁₁ the second string of the first character string CTR ₁₂ ·· first character after which consists of only the first character of ... the input string CDW ₁₁ ·· first character string code word CDW ₁₂ ·· the second string CDW ₂₁ · · first character that matches the first character string CTR ₂₂ · · encoded strings and longest matching codeword CTR ₂₁ · · encoded string and the longest second string string of the code word CDW ₂₂ ... second string of code words CPR _1, comparing section cdot ... codeword output unit 11 ... universal code for comparing the magnitude of CPR ₂ · compression ratio calculation unit COMP · compression ratio Transformation unit 12 Dictionary unit 21 Temporary compression unit

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hirotaka Chiba 1015 Kamiodanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture Inside Fujitsu Limited (56) References JP-A-2-34038 (JP, A) JP-A-3-78322 (JP, A) JP-A-4-86126 (JP, A) JP-A-5-11973 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G06F 5/00 H03M 7 / 30-7/46

Claims (5)

(57) [Claims] 1. A data compression method for obtaining a character string that matches an input character string from an already input encoded character string and encoding the input character string by using the number of the matched character string. The encoded character string that matches the longest character from the first character of (CTR) is defined as the first character string (CTR ₂₁ ), and the encoded character string that matches the longest character string after the first character string in the input character string is The first step to obtain the second character string (CTR ₂₂ ). When the input character string (CTR) is separated by the first character, the first character is used as the first character string (CTR ₁₁ ), and the input after the first character string A second step of obtaining a coded character string that matches the longest character string as a second character string (CTR ₁₂ ); a first character string (CT) consisting of one character obtained in the second step
R ₁₁ ) is represented by a code word shorter than the code word of the longest matching character string. The compression ratio when the first and second character strings (CTR ₂₁ , CTR ₂₂ ) in the first step are encoded , The first and second character strings of the second step
A fourth step of comparing the magnitude of the compression rate when encoding (CTR ₁₁ , CTR ₁₂ ); a fifth step of outputting the code word of the first string having the higher compression rate; a first step in the input string A sixth step of continuing encoding with the next character of the character string as the leading character. 2. The compression ratio in the fourth step is obtained by dividing the sum of the characters of the first and second character strings by the sum of the bit numbers of the code words of the first and second character strings. 2. The data compression method according to claim 1, wherein 3. When one character is composed of m bits,
First character string (CTR ₁₁ ) consisting of one character in the second step
With m-bit raw data representing the character itself;
(M +
2. The data compression method according to claim 1, wherein the encoding is performed by 1) bits. 4. Assuming that the number of encoded character strings is n, “log ₂ N] (l
(“log ₂ N] + the smallest integer greater than og ₂ N) and a flag bit indicating the longest matching string
2. The data compression method according to claim 1, wherein each longest matching character string is encoded by 1) bits. 5. The method according to claim 5, wherein said encoding is temporarily performed on a fixed number of input character strings, and the number of times the code word of the first character string is output in the second step is counted.
2. The data compression method according to claim 1, further comprising the step of formally performing the encoding process and outputting a codeword.