Recompression: Technique for Word Equations and Compressed Data

Abstract

In this talk I will present the recompression technique on the running example of word equations. In word equation problem we are given an equation u = v, where both u and v are words of letters and variables, and ask for a substitution of variables by words that equalizes the sides of the equation. The recompression technique is based on employing simple compression rules (replacement of two letters ab by a new letter c, replacement of maximal repetitions of a by a new letter), and modifying the equations (replacing a variable X by bX or Xa) so that those operations are sound and complete. The simple analysis focuses on the size of the instance and not on the combinatorial properties of words that are used. The recompression-based algorithm for word equations runs in nondeterministic linear space.The approach turned out to be quite robust and can be applied to various generalized, simplified and related problems, in particular, to problems in the area of grammar compressed words. I will comment on some of those applications.