JPH0683573A - Data compression system - Google Patents

Abstract

PURPOSE:To attain the coding properly corresponding to the requirement of high speed processing and high compression by generating a Hash address resulting from a reference number of a partial string in addition to the information of an element of an input character. CONSTITUTION:A coding means 2 divides a coded character string into different partial strings, adds a different reference number from each section and registers the result in a dictionary 1, and compresses data by the coding designated by a reference number of a partial string whose maximum length is coincident with that among partial strings in the dictionary 1 from the input retrieval character string. A dictionary retrieval means 3 uses the external Hash mesh method for the retrieval of the partial string, a Hash address resulting from adding information Km extracted from the element of an input character K to a reference number (i) of the partial string registered in the dictionary 1 is generated to generate a contact list with a division number in response to the bit number of additional information Km for the retrieval of the dictionary 1. Thus, the dynamic data compression processing suitable for high compression rate processing or high speed processing is properly implemented.

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 by LZW coding as an improvement of an incremental decomposition type which is a kind of universal coding. In recent years, various types of data such as character codes, vector information, and images have been handled by computers, and the amount of handled data has been increasing rapidly. When handling a large amount of data, the redundant portion of the data is omitted and the amount of data is compressed so that the storage capacity can be reduced or the data can be transmitted at high speed.

Universal coding has been proposed as a method of compressing various kinds of data by one method. Here, the field of the present invention is not limited to compression of character codes, but can be applied to various data, but in the following, one word of data will be used, following the name used in information theory. A unit is called a character, and a unit of data consisting of a plurality of words is called a character string.

As a typical method of the universal code,
There is a Ziv-Lempel code (for details,
For example, see Munakata “Ziv-Lempel Data Compression Method”, Information Processing, Vol. 26, No. 1, 1985). Two algorithms of universal type and incremental decomposition type (Incremental parsing) have been proposed for the Differenpel code.

Further, as an improvement of the universal type algorithm, there is LZSS code (TCBell, "Better OPM").
/ L Text Compression ”, IEEE Trans. On Commun., Vol.
See COM-34, No. 12, DEC. 1986). As an improvement of the incremental decomposition type algorithm, LZW (Lempel-Ziv-Welch)
Signed (TAWelch, “A Technique for High-Perfo
rmance Data Compression ”, Computer, June 1984).

Among these codes, the LZW code has come to be used for file compression of a storage device because of its high-speed processing and the simplicity of the algorithm.

[0006]

2. Description of the Related Art FIG. 9 shows an encoding process flow using a conventional LZW code, and FIG. 10 shows a decoding process flow. First, the LZW encoding process has a rewritable dictionary, divides the input character string into different character strings (substrings), adds reference numbers in the order in which they appear, and registers them in the dictionary. The character string currently input is represented by the reference number of the longest matching character string registered in the dictionary and is encoded.

FIG. 11 shows an explanatory diagram of LZW encoding, FIG. 13 shows an explanatory diagram of LZW decoding, and FIG. 12 shows an example of a dictionary structure created at the time of encoding and decoding.
Note that, in FIGS. 11, 12, and 13, for simplification of description, an example of compressing and restoring data consisting of a combination of three letters abc is taken. In the LZW encoding process of FIG. 9, first, in step S1, a character string consisting of one character for every character is registered in the dictionary in advance as an initial value, and then encoding is started.

In the encoding in step S1, a reference number ω is obtained by searching the dictionary with the input first character K, and this is used as the initial character string. Next, in step S2, the next character K of the input data is read, and in step S3 it is checked whether or not the character input is completed. Then, the process proceeds to step S4 and step S4.
It is searched whether or not the extended character string (ωK) obtained by adding the character K read in step S2 to the initial character string ω obtained in 1 is in the dictionary.

If the character string (ωK) is not in the dictionary in step S4, the process proceeds to step S6, the reference number ω of the character K obtained in step S1 is output as the code word code (ω), and the character string (ωK) is also output. A new reference number is added to and registered in the dictionary, the input character K in step S2 is replaced with the reference number ω, the dictionary address n is incremented, and the process returns to step S2 to read the next character K.

On the other hand, if the character string (ωK) is found in the dictionary at step S4, the character string (ωK) is referred to as reference number ω at step S5.
, And the process returns to step S2 and the search for the maximum matching length is continued until the character string (ωK) cannot be searched from the dictionary in step S4.

The LZW coding will be described in detail with reference to FIGS. First, the input data input in FIG. 11 is read from left to right. When the first character a is input, since there is no matching character string other than the character a in the dictionary, OUTPUT CODE 1 (reference number ω) is coded and output. Then, the letter a is set to the initial letter string ω.

Next, if the second character b is input,
Extended character string ωK = which adds this input character to the initial character string ω
Since ab is not in the dictionary, OUTPUT CODE 2 of the character b is output as a code word. Then, the extended character string ωK = ab
Is registered in the dictionary with reference numeral 4. The actual dictionary registration is registered as a character string 1b as shown on the right side of FIG. Then, the letter b becomes the initial letter string ω.

Next, if the third character a is input, an extended character string ωK = b obtained by adding the initial character string ω to the character b.
Since a = 2a is not in the dictionary, the OUTPUT CODE of the character a
After outputting 1 as a code word, the extended character string ωK = ba is represented by 2a, and the reference number 5 is attached to register it in the dictionary. Then, the character a becomes a new initial character string ω. As for the fourth input character b, the extended character string ωK = ab is already registered in the dictionary as the code word 4 of 1b, so the character string ωK is set as a new initial character string ω and the fifth character c is input. Then, the extended character string ωK = 4c = abc is created. This extended string ω
Since K = abc is not registered in the dictionary, OUTPUT CODE4 of the character string ab = 1b is output as a code word, and the extended character string ωK = abc is registered in the dictionary as a code word 6 in the form of 4c. This process is continued in the same manner thereafter.

The decoding process of FIG. 10 performs the reverse operation of the encoding of FIG. In the LZW decoding of FIG. 10, similarly to the case of encoding, a character string consisting of one character for all characters is registered in the dictionary in advance as an initial value and then decoding is started. First, in step S1, the first code (reference number) is read and the current CODE is
Input code CO because the first code corresponds to any one-character reference number already registered in the dictionary.
Find the character code (K) that matches DE and output the character K.

The output character K is for exception processing later.
Set to FINchar. Next, in step S2, the next code is read and set as CODE in INcode. In step S3, it is checked whether or not there is a new code, that is, whether or not the code input has been completed, and the process proceeds to step S4. In step S3, it is checked whether the code CODE input in step S3 is defined (registered) in the dictionary. . Normally, the input codeword is registered in the dictionary by the processing up to the previous time, so step S
5, the character string code (ωK) corresponding to the code CODE is read from the dictionary, the character K is temporarily stacked in step S6, the reference number CODE (ω) is set as a new code CODE, and the process returns to step S5. The procedure of steps S5 and S6 is recursively repeated until the reference number ω reaches one character K, and finally, the process proceeds to step S7, and the characters stacked in step S6 are popped up in the LIFO (Last In Fast Out) format and output. To do. At the same time, in step S7,
A new reference number is added to the character string in which the code ω used last time and the first character K of the character string restored this time are represented as a set (ωK) and registered in the dictionary.

The LZW decoding process will be described in detail with reference to FIG. First, in FIG. 13, the first input code word (INPUT CODE) is 1, and the characters a, b, and c are already registered in the dictionary as reference numbers 1, 2, and 3 as shown in FIG. Is output by replacing with the character string a of the reference number that matches the code word 1.

Similarly for the next code word 2, the character b
Replace with and output. Codeword 1 processed last time
A new reference number 4 is added to the character string ωK = 1b, which is a combination of the first character b of the character string decoded this time, and registered in the dictionary. The third code word 4 replaces the character string 1b obtained by searching the dictionary with the character string ab and outputs the character string ab. At the same time, a character string ωK = 2a obtained by combining the previously processed codeword 2 and the first character a of the character string decoded this time
A new reference number 5 is added to (= ba) and registered in the dictionary.

Similarly, this process is repeated.

The LZW decoding of FIG. 13 has the following exception processing. This exception processing occurs in the decoding of the sixth input codeword 8. Codeword 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 previously decoded character string ba to the code word 5 processed last time, and further replaced with 5b = 2ab = bab to perform exceptional processing. Then, after the character string is output, the reference number 8 is added to the character string 5b obtained by adding the first character b of the character string decoded this time to the previous code word 5 and registered in the dictionary.

This exception processing is performed through the processing of steps S4 and S8 of the decoding processing flow of FIG. 10, and finally, in step S7, the output of the character string and the addition of a reference number to the new character string are made to the dictionary. Is registered in step S7. Note that the LZW decoding of FIGS. 10 and 13 has been described as a case where a dictionary is created in real time while decoding the code on the decoding side, but the dictionary created at the time of encoding is used as it is as a copy on the decoding side. It may be encoded by that. In this case, the exception processing on the decoding side becomes unnecessary.

However, if LZW encoding is performed according to the procedure shown in the flowchart of FIG. 9, the worst case is that the entire dictionary must be searched every time one character string is searched for in the dictionary. There was a problem that takes. Therefore, in the conventional dictionary search method, the external hash method (open has
The processing speed is increased by using hing or chaining).

First, in a dictionary search by a general hash method, when considering a set S consisting of a plurality of character strings,
The storage position of the character string x of the set S can directly calculate the address indicating the storage position from the character string x itself, and high-speed dictionary search can be performed. String storage location,
That is, if addresses from 0 to m-1 are given to the hash table, in the hash method, one function h: S → [0,1, ..., m-1] is defined and the set S The address of the character string x of h (x)
Ask as. This function h is a hash function, and the value h (x)
Is called the hash address of the character string x.

The hash method is usually used when the size of the set S is much larger than the number of addresses m.
However, no matter how the hash function h is selected, it is possible that h (x1) = h (x2) hash addresses match with different character strings x1 and x2 of the set S.
This is called a collision, and an external hashing method (open hashing, or chaining) is used as one of the countermeasures against the collision.

In the external hash method, as shown in FIG. 14, a linked list is prepared for each hash address i indicated by an index (directory), and a character string x having a collision hash address h (x) = i is Store them in order from the beginning of the linked list. Each linked list with the same hash address h (x) is called a bucket.

FIG. 15 shows a processing flow of LZW encoding using a list structure of the external hash method for dictionary search. 16 shows an example of the structure of a conventional dictionary, and FIG. 17 shows the arrangement on the dictionary memory corresponding to this dictionary structure. Figure 1
7, the dictionary memory is a first memory (fir).
st) 100, next memory (next) 200, and extended memory (extension; abbreviated as ext) 30
It consists of 0. Here, the first memory 100 is shown in FIG.
14 corresponds to the index (directory) of the external hash method shown in FIG. 4, the next memory 200 corresponds to “next” in the linked list of FIG. 14, and the extended memory 300 corresponds to “name” of FIG.

In the dictionary structure of FIG. 16, the following information is shown in one node as shown in the lower right part. (1) In the node; Extended memory registration symbol (2) Node upper left; Address (3) Node lower left; Next first memory address (4) Node lower right; Next memory address Note that the value O is empty. Is shown.

The LZW encoding process of FIG. 15 will be described below by taking as an example the case where three characters A, B, and C are used for the sake of simplicity. First step S
At 1, the following initialization processing is performed. (1) Initialize the dictionary to include the first character.
Since the three letters of the alphabets A, B, and C are targeted here, the character codes of A, B, and C are used as the hash address as they are, and the address 1 of the dictionary memory in FIG.
Register in a few steps.

(2) The current character registration number n in the dictionary is set to the number of characters registered in (2) above. In the case of three letters of the alphabet, n = 3. (3) The first input character K is the initial character string i. In this case, since the first input character is "A", the initial character string i = 1. (4) The dictionary search array is initialized to 0. That is, the search array for the first, next, and extended memories is first
Since it is represented by [1, Nmax], next [1, Nmax], EXT [1, Nmax], it is initialized to 0.

When the initialization process of step S1 is completed, the process proceeds to step S2, and as a result, it is assumed that the dictionaries shown in FIGS. 16 and 17 are currently created. A process for inputting and encoding the character string "AAAA" in this state will be described.

Since the initialization of step S1 has been completed,
The first input character “A” is the initial character string ω = 1, the first input character “A” is the initial character string ω = 1 in step S1, and the second input character “A” is read in step S2. . Then, in step S3, it is determined that there is an unprocessed character, and the process proceeds to the dictionary search step shown in steps S5 to S9.

In the dictionary search step, first, step S5
The initial character string ω = 1 is set in the counter i with i = 1, and the j counter is set with j = 0. Here, the counter i is the address value of the dictionary memory specified by the stored value of the first memory, and the counter j is the address value of the dictionary memory specified by the stored value of the next memory.

Next, in step S6, the contents of address 1 of the dictionary memory of FIG. 17 designated by the i counter are read, and "A" is read from the extension memory 300 as a symbol (smbol).
Is read out, the next first address “4” is read out from the first memory 100, and the i counter is set to i = 4. Subsequently, the process proceeds to step S7, and it is checked whether i = 0 to determine whether to proceed to the dictionary registration step. Since i = 4 at this time, the process proceeds to step S8, and expansion of address 1 in step S6. The symbol “A” obtained by referring to the memory 300 and the first input character “A”
Determine the match with. In this case, since the two match, the process returns to step S2 and the third input character "A" is read.

Then, step S5 is executed through step S3.
Then, the value i of the counter i at that time i = 4 is set to the address ω of the dictionary memory, and the address 4 of the dictionary memory is referred to. Next, in step S6, the contents of address 4 of the dictionary memory are read, and the symbol (s
“B” is read out as the MMBol), the next first address “6” is read out from the first memory 100, and the i counter is set to i = 6.

Then, in step S7, it is checked whether i = 0. Since i = 6 at this time, step S7 is executed.
8, the expansion memory 30 of address 4 in step S6
It is determined whether the symbol “B” obtained from 0 matches the input character “A” obtained in step S2. In this case, since the two do not match, the process proceeds to step S9. Step S9
Then, first, the value of j = 10 obtained from the next memory 200 by referring to the address 4 of the dictionary memory is set in the i counter to set i = 10. The replacement of the i counter with the j counter is to make it possible for the j counter because the determination in step S7 is made only for the i counter.

Next, referring to the address 10 of the dictionary memory designated by the i counter which has been replaced, the address 1
The symbol “A” stored in the extended memory 300 of 0 is read, and the address value 11 of the next first memory stored in the first memory 100 of address 10 is set to i.
Set in the counter. Next, the process returns to step S7. Since i = 11 at this time, the symbol "A" of the address 10 obtained in step S9 is compared with the input character "A". If they match, the process proceeds to step S2. Proceed to processing the third character.

The same processing as that of the first input character is performed for the third and fourth input characters "A", and the addresses 10 to 11 and the addresses 11 to 12 of the dictionary memory are further processed.
When the processing of the address 12 is completed, there is no character to be processed in step S3, and therefore the processing proceeds to step S16, where the final address ω = 12 is output as the code word code (ω), and the series of processing ends.

Next, the processing of the dictionary registration step of steps S11 to S15 will be described. The dictionary registration is performed when i = 0 in the search of the first memory 100 or the next memory 200 in the dictionary search step. That is, when i = 0 is determined in step S7, the dictionary search can no longer be performed, and the dictionary address ω at that time is output as the code word code (ω) in step S10 to enter the dictionary registration step.

In the dictionary registration step, first, step S1
At 1, the number n of characters currently registered in the dictionary memory at that time is set in the i counter, and n is further incremented by 1.
Then, in step S12, it is checked whether or not j = 0, and j =
If it is not 0, i = 0. Therefore, the process proceeds to step S13 and the registration process of the first memory 100 is performed. If j = 0, the process proceeds to step S14 to perform the next memory registration process.

First memory 100 in step S13
The registration process of (1) indicates the next registration destination in the first memory 100 at the memory address n designated by the i counter (n +
1) The value is stored, and (2) the expansion memory 10 of the next memory address (n + 1) is stored.
The input character K is registered in 0 as a symbol. Specifically, in the case where the input character “A” is registered following the address 11 in FIGS. 16 and 17, the next registration destination is stored in the first memory 100 at the memory address 11 designated by the i counter. Is stored, and the input character “A” is registered as a symbol in the extension memory 100 at the next memory address 12.

On the other hand, the next memory 2 in step S14
The registration process of 00 is as follows: (1) The value of (n + 1) indicating the next registration destination is stored in the next memory 200 of the memory address specified by the i counter, and (2) the next memory address (n + 1) Extended memory 10
The input character K is registered in 0 as a symbol.

Specifically, in the case where the input character "A" is registered following the address 11 in FIGS.
The address value 10 indicating the next registration destination is stored in the next memory 200 of the memory address 11 designated by the counter, and the input character “A” is registered as a symbol in the expansion memory 100 of the next memory address 10. When the above registration process is completed, the registered character K is set in the i counter and the process returns to the dictionary search step from step S2.

[0042]

[Problems to be Solved by the Invention] Such a conventional LZ
Since the W code requires a lot of time for the dictionary search process when encoded by software, the W code is speeded up by using the external hash method for the dictionary search. However, in the book search by the external hash method, since the input character and the candidate character are collated sequentially, the dictionary search time is about 80% of the total time.
%, Which is a drawback that it is difficult to speed up.

On the other hand, the inventors of the present application can speed up the processing by dividing the linked list into a plurality of pieces and searching by using the information of the input characters that have already been encoded when performing the dictionary search. A coding scheme is proposed. However, in the actual encoding, the usable memory capacity is predetermined, and depending on the size of the input data, the encoding may end without using all the dictionary memory. In addition, depending on the application, it may be desired to prioritize the processing time over the compression rate for encoding.

However, the conventional encoding method has a problem that it is difficult to properly combine the request for high speed and the request for high compression for encoding. The present invention has been made in view of the above conventional problems, and an object of the present invention is to provide a data compression method capable of encoding appropriately corresponding to demands for high speed and high compression.

[0045]

FIG. 1 is a diagram for explaining the principle of the present invention. First, according to the present invention, an encoded character string is divided into different partial strings, a different reference number is added to each partial string and registered in the dictionary 1, and the input character string is stored in the dictionary 1. Among them, the encoding means 2 that compresses the data by the encoding specified by the reference number of the one having the maximum length match, and the external hash method is used to search for the substring, and the input character is input to the reference number i of the substring registered in the dictionary 1. A dictionary search means 3 for searching the dictionary 1 by generating a hash address to which the information Km extracted from the element of K is added to generate a linked list of the number of divisions according to the number of bits of the additional information Km. Data compression methods are targeted.

In the present invention regarding such a data compression method, the number of bits of the information Km extracted from the element of the input character K added to the reference number i of the subsequence is appropriately changed and the number of divisions of the linked list is changed. It is characterized in that a division number specifying means 4 for specifying is provided. Here, the division number designating unit 4 sets the number of bits of the information Km extracted from the element of the input character K added to the reference number i of the partial sequence to the number of bits suitable for high-speed processing (see FIG. 3B) or high. The number of bits suitable for the compression processing (see FIG. 3A) is designated.

Specifically, the number of bits is designated based on the number of divisions determined from the information added to the beginning of the input character string data by the retrieval division number determination means 5. Further, the division number designating means 4 uses the information Km extracted from the elements of the input character K.
The number of bits may be specified as the number of bits corresponding to the previously designated division.

[0048]

According to the data compression method of the present invention having such a configuration, the following effects can be obtained. As a processing condition for data compression, there are a case where a high compression rate is required even if it takes a long time, and a case where the processing time is prioritized over the compression rate and high-speed processing is desired.

Although such a high compression rate and high speed are contradictory processing conditions, in the present invention, as the hash address used in the external hasher method, the reference number of the subsequence,
That is, by dividing the address i by the information Km of the element of the input character K, that is, the number Km of bits of the input character code, the hash address is divided according to the number of bits Km of the additional information, and the linked list is divided. By arbitrarily designating the effective bit number Km of the character code used as the additional information for determining the number, it is possible to appropriately perform the dynamic data compression processing suitable for each of the high compression rate and the high speed processing.

[0050]

FIG. 2 is a block diagram of an embodiment showing an embodiment of a data compression system having a dictionary search function of the present invention. Figure 2
In the above, the original data (character data or code word data) 10 to be processed is DMA (Direct Memory Access).
It is input via the control circuit 12. M as control means
The PU 14 uses the input original data 10 as one character K and a plurality of hash addresses obtained by adding the element bit Km of the character code of the one character K to the reference number i of the character string so far in the dictionary search circuit 16. After setting in the character reading circuit 18, the dictionary search circuit 16 is activated. The element bit of the character code of this 1 character K determines the number of search divisions of the linked list in the external hash method. When the character code is 8 bits, the effective bit number of the element bit Km is selected from, for example, the higher order. The following nine types of linked list search division numbers are obtained. When the number of divisions of such a linked list is 256, which corresponds to all 256 character types to be processed, a perfect hash is obtained, and the dictionary can be searched only once.

In the present invention, encoding is performed by designating the number of divisions that matches the designated processing condition from among the nine division numbers. The specific number of divisions can be specified by setting the number of bits of the element bit Km of the next one character K added to the reference number (dictionary address) i of the immediately preceding character string already encoded. The function as the division number designating means 4 shown in the principle explanatory diagram of FIG. 1 for this purpose is realized by the program control of the MPU 14. FIG. 3 shows the number of search divisions of the linked list with respect to the size (size) of the input data. FIG. 3 (a) shows characteristics suitable for high compression processing conditions, and FIG. 3 (b) shows high speed. It shows the characteristics suitable for the processing conditions.

That is, when it is desired to obtain a high compression rate even if it takes time, the number of divisions that is inversely proportional to the input data size in FIG. 3A is designated. In this case, the larger the input data size, the smaller the number of divisions of the linked list, and the longer the maximum length of the matching subsequence in the dictionary, so that a high compression rate can be obtained. However, the length of the matching substring increases the number of times the dictionary is searched, and the processing time increases. Further, when the memory capacity is constant, unused memory can be effectively used.

On the other hand, if it is desired to shorten the processing time without obtaining the compression rate, the division number proportional to the input data size shown in FIG. 3B is designated. In this case, the number of divisions increases according to the input data size, and the maximum number of divisions 256
Since it is a perfect hash, it can be encoded by one dictionary search. Further, when the memory capacity is constant, it means that all can be processed in a constant time at the expense of the compression rate regardless of the input data size.

To specify the number of divisions based on the input data size that meets any of the conditions shown in FIGS. 3A and 3B, the operator knows the data size to be processed and the MPU
The number of divisions may be directly specified for 14. Further, a value indicating the size of the data at the head of the input data is set in advance, the size of this data is read by the MPU 14, and the value shown in FIG.
It is also possible to automatically convert into the optimum number of divisions corresponding to the size of the input data according to the division characteristics shown in FIG. The function of determining the size of the input data by the MPU 14 is the function as the search division number determining means 5 shown in the principle explanatory diagram of FIG.

As shown in FIG. 4 (a), the input data format for this purpose has a value indicating the size of the input data at the beginning, followed by the original data series. Further, as shown in FIG. 4B, the encoded data has the size of the dictionary used for encoding, that is, the used dictionary size (the number of divisions and the used size of each divided dictionary) at the beginning, and the encoded data The converted data follows.

Therefore, at the time of restoration, it is possible to decide the dictionary having the maximum size to be used for restoration from the size of the leading dictionary used for restoration. Referring again to FIG. 2, the dictionary search path 16 thereafter reads the candidate character of the character string extended by one character from the dictionary memory 20, and the match checking circuit 22 checks the input character and the candidate character for matching (matching). The connection detection circuit 24 detects the presence / absence of candidate characters.

The pipeline control circuit 26 reads the next candidate character into the dictionary memory 20 in parallel with the matching check circuit 22 collating the input character with the candidate character and the concatenation detection circuit 24 detecting the presence or absence of the candidate character. Call. By carrying out the pipeline processing in the pipeline control circuit 26 in this way, it is possible to execute the search and collation processing for each of a plurality of candidate characters in the cycle time of the dictionary memory 20.

Further, the dictionary search circuit 16 is provided with a continuous address circuit 28, which generates continuous addresses and causes the plural character reading circuit 18 to have a dictionary memory 20.
The hash address and the candidate character registered in the continuous address of are read. In encoding the LZW code, a character string having the maximum length matching in the dictionary memory 20 is obtained. Therefore, it is understood that the input character is added and the character string is sequentially extended character by character, and when there are no candidate characters, the character string has the maximum matching length. At this time, up to the maximum matching length character string is represented by a reference number using the address ω, and the reference number ω is output from the input / output port 30 to the outside as a compressed code word code (ω). . FIG. 5 is a block diagram of an embodiment showing details of the dictionary search circuit 16 of FIG.

In FIG. 5, the MPU 1 first sets, for example, the most significant bit Km (Km, i) of the 8-bit character code of the first character reference number i and the second character K of the character string.
Is set in the address register 18-1 and the input second character K is set in the register 22-1. Next, the dictionary control circuit 16 is added to the pipeline control circuit 26.
Command to start.

The pipeline control circuit 26 first detects the FF2.
After 8-1 is reset to Km = 0, the dictionary memory 20 is read. The FF 24-2 is the most significant bit (MSB) of the address of the dictionary memory 20, and the address register 1
The content of 8-1 becomes the lower address and the area corresponding to the array First of the dictionary memory 20 is read. Here, an example of the configuration of the dictionary memory 20 is shown in FIG. 6, and the arrangement of the dictionary memory 20 corresponding to FIG. 6 is shown in FIG. 6 and 7
For simplification of description, the three-character encoding of a, b, and c is taken as an example.

In this memory array, first0,
1 is determined by the most significant bit Km of the next one character added to the original hash address i. Km = 0 first1 Km = 1 specifies first1.

Therefore, in FIG. 5, the address (reference number) i of the first character and the most significant bit K of the second character are shown.
When the dictionary memory 20 is accessed with m as an address,
At this time, since Km = 0 due to the reset of the FF 28-1, the array first0 in the dictionary memory of FIG.
The areas corresponding to (first first memory) and extension (extended memory) are read. Dictionary memory 20
A portion (first0) corresponding to the linked list address in the read one word content is set in the address register 18-1 and a portion (e) corresponding to the candidate character K ′ is set.
xtention) is set in the register 18-2.

At the same time, the address register 18-1
The part i of the contents already stored in the register except the most significant bit Km of the character K is transferred to the register 18-3. Also, F
The content Km of F28-1 is transferred to FF24-2. In parallel with the reading of the dictionary memory 20, the input character in the register 22-1 and the candidate character in the register 18-2 are compared and collated by the coincidence comparison circuit 22-2.

By comparing and collating, the input character K and the candidate character K '
, The pipeline control circuit 26 sets the next input character in the register 22-1. At this time, the FF 28
Since -1 remains as it is after reset, Km = 0
Becomes the array first0 and extent of the dictionary memory 20 of the address specified by the address register 18-1.
region corresponding to the linked list address (first 0) of the contents of 1 word read from the dictionary memory 20 is read from the address register 18
-1 is set, and the part corresponding to the candidate character K '(ext
(ention) is set in the register 18-2, comparison and collation are performed, and the same is repeated thereafter.

At the time of such comparison and comparison, at the same time as the comparison and comparison, it is determined whether or not the contents of the address register 18-1 read from the dictionary memory 20 and stored by the NOR circuit 24-1 are all 0. If all 0s, it is detected that there are no candidate characters. The output of the NOR circuit 24-1 when there are no candidate characters is MPU14.
And the pipeline control circuit 26,
Outputs the value of the memory address where the comparison and collation finally match as a code word, and moves to the search for the next input character. Similarly for the search for the next character, the set of the content (registered immediately before the reference number) i of the register 18-3 and the most significant bit Km of the input character K is set in the address register 18-1. , The input character is set in the register 22-1, and the search process for the second and subsequent characters is performed.

On the other hand, if the result of comparison and collation is a mismatch, the area of the array next of the same address is read and set in the address register 18-1, and the next dictionary memory 20 is read to find a matching candidate. The reading of the array next is repeated until a character is obtained. The dictionary search of the present invention will be described in detail with reference to FIGS. 6 and 7.

Assume that the character string "aaaa" is encoded with the dictionary constructed in FIGS. 6 and 7. Here, the most significant bit of the character “a, c” is Km = 0, and the most significant bit of the character “b” is Km = 1. First, since the reference number of the first character "a" is i = 1 and the most significant bit MSB of the character code of the second character "a" is Km = 0, the address of the dictionary memory 1 in FIG. Km in 1
Content of first0 specified by = 0 and the extent
The candidate character "a" of ntion is read. in this case,
Since the input character “a” and the candidate character “a” match, the next dictionary memory is accessed by the address 10 obtained from first0 and the candidate character “a” is read out.
Further, the content 11 of the array first0 designated by the most significant bit Km = 0 of the third character "a" is read. The third character "a" is also matched with the candidate character "a", the fourth and fifth characters are processed in the same manner, and the content of the array fifth0 of the last fifth character "a" is Since it is 0, it is determined that there are no candidate characters, and the final address 12 is output as the code word of the input character string “aaaa”.

On the other hand, for the character string "abc", since the most significant bit of the second character is Km = 1, the content 4 of the array first1 of the address 1 is read out, and the match with the candidate character is used as a condition. Read the candidate character at address 5,
Finally, a match with the candidate character at address 6 is obtained, and the code word 6 of the input character "abc" is output. Note that the next search when the match with the candidate character is not obtained is the same as the conventional one.

FIG. 8 shows a flowchart of the encoding algorithm according to the present invention, which is basically the same as the conventional system of FIG. The difference is that (1) the memory address 1 is created by a combination of the reference number i of the character string encoded immediately before as the memory address in step S5 and the element bit of the next one character K, for example, the most significant bit Km. Points,
(2) In step S6, the divided first in the next memory address i is obtained by reading the dictionary memory first [l] determined by the set of l = (Km, i), (3)
Furthermore, at the time of registering the candidate character to the address i in step S13, which of the first 0 and the first 1 of the previous address i-1 to register the address i is distinguished according to the most significant bit Km of the candidate character. ,.

In the above embodiment, the method of storing the candidate characters as they are in the dictionary memory and comparing them has been described. However, in order to reduce the memory capacity, the candidate characters are only bits excluding the bit Km added to the hash address. You may give it. Also, as another embodiment of the present invention, instead of adding a specific bit Km of the input character to the hash address,
It is obvious that the same can be realized by adding bits of information created by processing the input character.

[0071]

As described above, according to the present invention, the characteristic of the number of divisions with respect to the input data size of the dictionary memory is selected in accordance with the processing condition of high speed or high compression, and the size of the input data is selected. By automatically or artificially specifying according to the above, it is decided dynamically for each encoding so that the encoding can be performed. Data compression by can be realized.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of the principle of the compression method of the present invention.

FIG. 2 is a block diagram of an embodiment of the present invention.

FIG. 3 is a characteristic diagram showing the number of divisions with respect to the input data size of the present invention divided into processing conditions.

FIG. 4 is an explanatory diagram showing an input data format and an encoded data format of the present invention.

5 is a block diagram of an embodiment showing details of the dictionary search circuit of FIG.

6 is an explanatory diagram showing a configuration of a dictionary memory used for encoding in FIG.

FIG. 7 is an explanatory diagram of an arrangement of a dictionary memory corresponding to FIG.

FIG. 8 is a flowchart showing an encoding algorithm of the present invention.

FIG. 9 is a flowchart of a conventional LZW encoding algorithm.

FIG. 10 is a flowchart of a conventional LZW decoding algorithm.

FIG. 11 is an explanatory diagram of a specific example of conventional LZW encoding.

FIG. 12 is an explanatory diagram of a dictionary configuration example.

FIG. 13 is an explanatory diagram of a specific example of conventional LZW decoding.

FIG. 14 is an explanatory diagram of a list structure of the external hash method.

FIG. 15 is a flowchart showing a conventional LZW code encoding algorithm using the external hash method.

16 is an explanatory diagram showing the configuration of a dictionary memory used for encoding in FIG.

FIG. 17 is an explanatory diagram of an arrangement of a dictionary memory corresponding to FIG.

[Explanation of symbols]

 1: Dictionary 2: Dictionary search means 3: Data addition means 4: Judgment means 10: Original data 12: DMA control circuit 14: MPU 16: Dictionary search means (dictionary search circuit) 18: Multiple character reading circuit 18-1: Address Registers 18-2, 18-3: Register 20: Dictionary memory 22: Matching check circuit 22-1: Register 22-2: Comparator 24: Connection detection circuit 24-1: NOR circuit 24-2: FF 26: Pipeline Control circuit 28-1: FF

Continued Front Page (72) Inventor Yasuhiko Nakano 1015 Kamiodanaka, Nakahara-ku, Kawasaki-shi, Kanagawa Fujitsu Limited

Claims (5)

[Claims] 1. An encoded character string is divided into different partial strings, different reference numbers are added to the respective partial strings, and registered in the dictionary (1), and the input character string is stored in the dictionary (1). An encoding means (2) for compressing data by designating by encoding with a reference number of a substring having the maximum length match among the substrings, and an external hash method is used for searching the substring, and the dictionary ( By generating the hash address by adding the information (Km) extracted from the element of the input character (K) to the reference number (i) of the subsequence registered in 1), the additional information (K) is generated.
In the data compression method, which comprises a dictionary search means (3) for generating and searching a linked list having the number of divisions corresponding to the number of bits of m), an input character ( A data compression method characterized in that a division number designating means (4) for designating the number of divisions of the linked list by appropriately changing the number of bits of the information (Km) extracted from the element K) is provided. 2. The data compression method according to claim 1, wherein the division number designating means (4) is information extracted from an element of an input character (K) added to a reference number (i) of the subsequence. A data compression method characterized in that the number of bits of (Km) is designated as the number of bits suitable for high-speed processing or the number of bits suitable for high-compression processing. 3. The data compression method according to claim 1, wherein the division number designating means (4) is information extracted from an element of an input character (K) added to a reference number (i) of the subsequence. A data compression method, wherein the number of bits of (Km) is designated based on information indicating the size of character string data to be encoded. 4. The data compression method according to claim 1, further comprising search division number determination means (5) for determining the division number from the information added to the beginning of the character string. method. 5. The data compression method according to claim 1, wherein the division number designating means (4) is information extracted from an element of an input character (K) added to a reference number (i) of the subsequence. A data compression method, wherein the number of bits of (Km) is set to the number of bits corresponding to a predetermined number of divisions.