Off-line dictionary-based compression

Dictionary-based modeling is a mechanism used in many practical compression schemes. In most implementations of dictionary-based compression the encoder operates on-line, incrementally inferring its dictionary of available phrases from previous parts of the message. An alternative appproach is to use the full message to infer a complete dictionary in advance, and include an explicit representation of the dictionary as part of the compressed message. In this investigation, we develop a compression scheme that is a combination of a simple but powerful phrase derivation method and a compact dictionary encoding. The scheme is highly efficient, particularly in decompression, and has characteristics that make it a favourable choice when compressed data is to be searched directly. We describe data structures and algorithms that allow our mechanism to operate in linear time and space.