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When using LZ77 why is the compressed message longer than the decompressed version?
Dictionary coders (such as LZ77) store a segment of the message in a dictionary, and then replace each occurrence with the index into the dictionary. This works very well for repeated values, or messages that use a small portion of the possible values (such as 26 letters out of the 256 possible values in a byte). However, if the data does not repeat, or uses almost all of the possible values randomly, there will be the space used for the dictionary PLUS the space used for indexes of single occurrences. This can make the message longer. In general terms, if the message entropy is too high, any lossless compression scheme will fail to compress the data.
What is a series of instructions telling a computer how to read a compressed file?
A series of instructions telling a computer how to read a compressed file is typically referred to as a decompression algorithm. This algorithm processes the compressed data, interpreting its format and reconstructing the original file by reversing the compression process. Common decompression algorithms include ZIP, GZIP, and LZ77, each designed for specific compression techniques. The computer executes these instructions to restore the file to its usable form.
