WO1995019662B1

WO1995019662B1 - Data compression apparatus and method

Info

Publication number: WO1995019662B1
Application number: PCT/US1994/014823
Authority: WO
Filing date: 1994-12-21
Publication date: 1995-08-10

Abstract

Apparatus and method for compression of a digital sequence. The compressed sequence may be buffered and a compression procedure altered depending on buffer usage. This altering may include changing the depth of searching for matches performed in linked lists depending on the amount of the sequence being buffered. An adjacent element can be found in a linked list using a pointer, and a match can also be found in a history buffer using the same pointer. Frequency values may be updated for symbols based on a sampled subset of the symbols, and a token may be derived, such as by Huffman encoding, for each symbol from the updated frequency values. A subsequence may be encoded using an offset referenced to an earlier match. An insert pointer and a check pointer into a history buffer may be maintained, and a determination can be made as to whether accessing the history buffer is valid based on these check pointers. The maximum amount of the history buffer in which accessing is valid may be set. Decompression of sequences is also presented.

Claims

AMENDED CLAIMS

[received by the International Bureau on 13 July 1995 (13.07.95); original claims 1-26 cancelled; remaining claims unchanged (2 pages)]

27. Apparatus for compressing a digital sequence, comprising: a history buffer for receiving and buffering the digital sequence, match detection logic responsive to the history buffer, for detecting matches between a present subsequence in a history buffer and a previous subsequence in a history buffer, and coding logic responsive to the match detection logic to selectively encode a matched present subsequence as either an absolute offset referenced to the previous subsequence or a relative offset referenced to an earlier match.

28. The apparatus of claim 27 wherein the coding logic further includes logic to generate an indicator for indicating whether the matched present subsequence is encoded as an absolute offset or a relative offset.

29. A method of compressing a digital sequence, comprising: detecting a first match between a first and a second subsequence in the digital sequence, detecting a second match between a third and a fourth subsequence in the digital sequence, the first and second subsequences being different from the third and fourth subsequences, and encoding the fourth subsequence using an offset referenced to the first match.

30. The method of claim 29 wherein the step of encoding is performed in response to a further step of determining, the step of determining including determining whether to encode the fourth subsequence using an offset referenced to the first match instead of an offset from the beginning of a history buffer.

31. The method of claim 29 further including the step of receiving packet-switched data as the digital sequence.

32. A method of decompressing a digital sequence, comprising: receiving a first match token, after receiving the first match token, receiving a second match token including an offset, determining the position of a subsequence in a history buffer based on the offset and the first match token, and substituting the subsequence for the second match token in the digital sequence.

33. The method of claim 32 further including the step of reading an offset mode indication from the second match token to determine that the step of determining is to be based on the second match token and the offset instead of being based on the offset alone.

34. Apparatus for compressing a digital sequence, comprising: a history buffer for receiving and buffering the digital sequence, match detection logic responsive to the history buffer, for detecting matches between a present subsequence in a history buffer and a previous subsequence in a history buffer, and control logic for identifying a portion of the history buffer as a valid portion, the control logic further being responsive to the match detection logic to determine whether the matches detected by the match detection logic are within the valid portion of the history buffer.

35. The apparatus of claim 34 wherein the control logic is further operations to control the maximum size of the valid portion of the history buffer.

36. A method of compressing a digital sequence, comprising: maintaining an insert pointer and a check pointer into a history buffer, inserting characters at the insert pointer, accessing the history buffer to search for matches in the history buffer, and determining whether the step of accessing the history buffer is valid based on the insert and check pointers.

37. The method of claim 36 wherein the step of maintaining includes maintaining the check pointer to set the maximum amount of the history buffer in which the step of determining will determine that the step of accessing is valid.

38. The method of claim 37 further including the step of initializing the buffer by setting the insert and check pointers to be equal.