Data compression dictionary techniques

A dictionary coder, also sometimes known as a substitution coder, is a class of lossless data compression algorithms which operate by searching for matches between the text to be compressed and a set of strings contained in a data structure (called the 'dictionary') maintained by the encoder.These techniques—both static and adaptive (or dynamic)—build a list of commonly occurring patterns and encode these patterns by transmitting their index in the list. They are most useful with sources that generate a relatively small number of patterns quite frequently, such as text sources and computer commands.The compression techniques we have seen so far replace individual symbolswith a variable length codewords. In dictionary compression, variable lengthsubstrings are replaced by short, possibly even xed length codewords.Compression is achieved by replacing long strings with shorter codewords.Data compression can be performed by using smaller strings of bits (0s and 1s) in place of the original string and using a ‘dictionary’ to decompress the data if required. Other techniques include the introduction of pointers (references) to a string of bits that the compression program has become familiar with or removing redundant characters.