Statistical data compression/decompression method
First Claim
1. A statistical data compression method for storing a character string (context) as well as a probability of occurrence of a character which appears following the context, and encoding a character to a variable-length code using the probability, comprising the steps of:
- extending a number of characters of the context (degree of context) based upon an entered character, and storing the context as well as the probability of occurrence of said context;
selecting a context for encoding one of the entered characters from among the stored contexts satisfying a predetermined condition; and
encoding the entered character using the probability of occurrence of said entered character following the selected context,wherein when a frequency of occurrence of said stored context is greater than a predetermined value, it is judged that said predetermined condition is satisfied.
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Abstract
Provided is a probabilistic statistical data compression/restoration method for variable-length encoding a source character and restoring a variable-length code to a character using the probability of occurrence of the source character which appears following a character string (context) of n characters which immediately precede the source character. When variable-length encoding is performed, a context registration unit successively registers context based upon an entered character without fixing the length (degree) of context, and a compressing encoder selects registered context satisfying a predetermined condition, e.g., registered context for which the frequency of occurrence is greater than a fixed value, as context (encoding context) used in encoding, and variable-length encodes a source character by using this encoding context. A restoration unit restores a code to a character by executing processing which is the reverse of the processing executed by the compressing encoder.
62 Citations
15 Claims
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1. A statistical data compression method for storing a character string (context) as well as a probability of occurrence of a character which appears following the context, and encoding a character to a variable-length code using the probability, comprising the steps of:
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extending a number of characters of the context (degree of context) based upon an entered character, and storing the context as well as the probability of occurrence of said context; selecting a context for encoding one of the entered characters from among the stored contexts satisfying a predetermined condition; and encoding the entered character using the probability of occurrence of said entered character following the selected context, wherein when a frequency of occurrence of said stored context is greater than a predetermined value, it is judged that said predetermined condition is satisfied.
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2. A statistical data decompression method for storing a character string (context) as well as a probability of occurrence of a character which appears following the context, and decoding a variable-length code to a character using the probability, comprising the steps of:
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extending a number of characters of the context (degree of context) based upon a decoded character, and storing the context as well as the probability of occurrence of said context; selecting a context for decoding the variable-length code from among the stored contexts satisfying a predetermined condition; and decoding the variable-length code to a character using the probability of occurrence of said character following the selected context, wherein when a frequency of occurrence of an arbitrary character following to the stored context is greater than a predetermined value, it is judged that said stored context satisfies said predetermined condition.
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3. A statistical data compression method for storing a character string (context) as well as a probability of occurrence of a character which appears following the context, and encoding a character to a variable-length code using the probability, comprising the steps of:
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comparing a number n of characters of the context that is used in encoding of an entered character and a number Pn of characters of the context that was used in encoding of a previous character; deciding the context to be stored, based upon the context that was used in encoding of the previous character when the number n is smaller than the number Pn; storing the decided context as well as a probability of occurrence of the entered character which appears following the context; and selecting a prescribed stored context as the context used in encoding of a succeeding entered character and encoding the entered character using the probability of occurrence of said entered character following the selected context. - View Dependent Claims (4)
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5. A statistical data compression method for storing a character string (context) as well as a probability of occurrence of a character which appears following the context, and encoding a character to a variable-length code using the probability, comprising the steps of:
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calculating the number of times the degrees of encoding contexts used in encoding previous characters are successively equal to or greater than degree 1; obtaining Pn by adding 1 to said number; comparing a number n of characters of the context that is used in encoding of an entered character and the value of Pn; deciding the context to be stored, based upon the context that was used in encoding of the previous character when the number n is smaller than the value Pn; storing the decided context as well as the probability of occurrence of the entered character which appears following the context; and selecting a prescribed stored context as the context used in encoding of a succeeding entered character and encoding the character using the probability of occurrence of said entered character following the selected context, wherein in a case where the number of characters of each encoding context used for encoding each previous character is continuously equal to or greater than 1, said context to be stored includes a context comprising each of the contexts and the previous character. - View Dependent Claims (6, 7, 8)
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9. A statistical data decompression method for storing a character string (context) as well as a probability of occurrence of a character which appears following the context, and decoding a variable-length code to the character using the probability, comprising the steps of:
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comparing a number n of characters of the context that is used in decoding of the variable-length code and a number Pn of characters of the context that was used in decoding of a previous variable-length code which precedes said variable-length code; deciding the context to be stored, based upon the context that was used in decoding of the previous variable-length code to a previous character when the number n is smaller than the number Pn; storing the decided context as well as the probability of occurrence of the character which appears following the context; and selecting a prescribed stored context as the context used in decoding of the variable-length code and decoding the variable-length code to the character using the probability of occurrence of said character following the selected context. - View Dependent Claims (10)
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11. A statistical data decompression method for storing a character string (context) as well as a probability of occurrence of a character which appears following the context, and decoding a variable-length code to a character using the probability, comprising the steps of:
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calculating the number of times the degrees of decoding contexts used in decoding previous variable-length codes are successively equal to or greater than degree 1; obtaining Pn by adding 1 to said number; comparing a number n of characters of the context that is used in decoding an entered variable-length code to a character and the value of Pn; deciding the context to be stored, based upon a context that was used in decoding the previous variable-length code to a previous character when the number n is smaller than the value Pn; storing the decided context as well as the probability of occurrence of a decoded character which appears following the context; and selecting a prescribed stored context as the context used in decoding of a succeeding entered variable-length code and decoding the variable-length code using the probability of occurrence of the character following the selected context, wherein when the number of characters of each decoding context used for decoding each variable-length code is continuously equal to or greater than 1, said context to be stored includes a context comprising each of the contexts and the previous decoded character. - View Dependent Claims (12, 13, 14)
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15. A statistical data compression apparatus storing a character string (context) as well as a probability of occurrence of a character which appears following the context, and encoding a character to a variable-length code using the probability, comprising:
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a first unit extending a number of characters of the context (degree of context) based upon an entered character, and storing the context as well as the probability of occurrence of said context; a second unit selecting a context for encoding one of the entered characters from among the stored contexts satisfying a predetermined condition; and a third unit encoding the entered character using the probability of occurrence of said entered character following the selected context; wherein when a frequency of occurrence of said stored context is greater than a predetermined value, it is judged that said predetermined condition is satisfied.
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Specification