On the Relationships Among Optimal Symmetric Fix-Free Codes

Symmetric fix-free codes are prefix condition codes in which each codeword is required to be a palindrome. Their study is motivated by the topic of joint source-channel coding and by some information retrieval problems. Although they have been considered by a few communities they are not well understood. In earlier work, we used a collection of instances of Boolean satisfiability problems as a tool in the generation of all optimal binary symmetric fix-free codes with n codewords and observed that the number of different optimal codelength sequences grows slowly compared with the corresponding number for prefix condition codes. We demonstrate that all optimal symmetric fixfree codes can alternatively be obtained by sequences of codes generated by simple manipulations starting from one particular code. We also discuss simplifications in the process of searching for this set of codes as well as a conjecture, which if correct, together with the other results leads to a relatively fast algorithm which has been implemented in MATLAB to construct all optimal binary symmetric fix-free codes.