Data Compression Lec#12 PowerPoint

Dictionary Methods
LZ78
The LZ78 method (which is sometimes referred to as LZ2) [Ziv and Lempel 78] does not use any search buffer, look-ahead buffer, or sliding window. Instead, there is a dictionary of previously encountered strings. This dictionary starts empty (or almost empty).
The encoder outputs two-field tokens. The first field is a pointer to the dictionary; the second is the code of a symbol. Tokens do not contain the length of a string, since this is implied in the dictionary. Each token corresponds to a string of input symbols, and that string is added to the dictionary after the token is written on the compressed stream.
The dictionary starts with the null string at position zero. As symbols are input and encoded, strings are added to the dictionary at positions 1, 2, and so on. When the next symbol x is read from the input stream, the dictionary is searched for an entry with the one-symbol string x. If none are found, x is added to the next available position in the dictionary, and the token (0, x) is output. This token indicates the string “null x”(a concatenation of the null string and x). If an entry with x is found (at, say, position 37), the next symbol y is read, and the dictionary is searched for an entry containing the two-symbol string xy. If none are found, then string xy is added to the next available position in the dictionary, and the token (37, y) is output. This token indicates the string xy, since 37 is the dictionary position of string x. The process continues until the end of the input stream is reached.