On Minimum-Redundancy Fix-Free Codes

Fix-free codes are variable length codes in which no codeword is the prefix or suffix of another codeword. They are used in video compression standards because their property of efficient decoding in both the forward and backward directions assists with error resilience. This property also potentially halves the average search time for a string in a compressed file relative to unidirectional variable length codes. Relatively little is known about minimum-redundancy fix-free codes, and we describe some characteristics of and observations about such codes. We introduce a new heuristic to produce fix-free codes which is influenced by these ideas. The design of minimum-redundancy fix-free codes is an example of a constraint processing problem, and we offer the first approach to constructing them and a variation with an additional symmetry requirement.