ABSTRACT
The methods to be described in this chapter are, as in Chapters 5 and 6, methods for lossless encoding of source text over a fixed source alphabet S = {s1, . . . ,sm}. The new feature is that no statistical study of the source text need be done beforehand. The encoder starts right in encoding the source text, initially according to some convention, but with the intention of modifying the encoding procedure as more and more source text is scanned. In the cases of adaptive Huffman (Section 8.1) and adaptive arithmetic (Section 8.3) encoding, the encoder keeps statistics, in the form of counts of source letters, as the encoding proceeds, and modifies the encoding according to those statistics. In interval and in recency rank encoding (Section 8.4), encoding of a source letter also depends on the statistical nature of the source text preceding the letter, but the statistics gathering is not boundless; the encoding rules do not change, but rather cleverly take into account the recent statistical history of the source text.
