Compression the process of coding that will effectively reduce the total number of bits needed to represent certain information.
Lossless Compression • data compression technique that reduces the size of a file without sacrificing any original data information from the file is still there, nothing is deleted lets you recreate the original file exactly • suitable for: text and computer code • example: ZIP archiving technology (WinZip & PKZIP)
Lossless Compression pro. exact duplicate of the original file con. compression ratio is not all that high Compression ratio is the ratio of the size or rate of the original data to the size or rate of the compressed data.
Lossless Compression Principle of Lossless Compression Algorithms any non-random file will contain duplicated information that can be condensed using statistical modeling techniques that determine the probability of a character or phrase appearing
Run-Length Encoding compression technique that replaces runs of two or more of the same character with a number which represents the length of the run, followed by the original character; single characters are coded as runs of 1.
Burrows-Wheeler Transform o technique invented in 1994 that aims to reversibly transform a block of input data such that the amount of runs of identical characters is maximized. o does not perform any compression operations, it simply transforms the input such that it can be more efficiently coded by a Run-Length Encoder or other secondary compression technique.
Burrows-Wheeler Transform (cont’d) Algorithm I. Create a string array. II. Generate all possible rotations of the input string, storing each in the array. III. Sort the array alphabetically. IV.Return the last column of the array