Dictionary Methods

Statistical compression methods use a statistical model of the data, and the quality of compression they achieve depends on how good that model is. Dictionary-based compression methods do not use a statistical model, nor do they use variable-size codes. Instead they select strings of symbols and encode each string as a token using a dictionary. The dictionary holds strings of symbols and it may be static or dynamic (adaptive). The former is permanent, sometimes allowing the addition of strings but no deletions, whereas the latter holds strings previously found in the input stream, allowing for additions and deletions of strings as new input is being read.