Finding submasses in weighted strings with Fast Fourier Transform

We study the Submass Finding Problem: given a string s   over a weighted alphabet, i.e., an alphabet ΣΣ with a weight function μ:Σ→Nμ:Σ→N, we refer to a mass M∈NM∈N as a submass of s if s has a substring whose weights sum up to M  . Now, for a set of input masses {M1,…,Mk}{M1,…,Mk}, we want to find those MiMi which are submasses of s  , and return one or all occurrences of substrings with mass MiMi. We present efficient algorithms for both the decision and the search problem. Furthermore, our approach allows us to compute efficiently the number of different submasses of s.The main idea of our algorithms is to define appropriate polynomials such that we can determine the solution for the Submass Finding Problem from the coefficients of the product of these polynomials. We obtain very efficient running times by using Fast Fourier Transform to compute this product. Our main algorithm for the decision problem runs in time O(μslogμs)O(μslogμs), where μsμs is the total mass of string s  . Employing methods for compressing sparse polynomials, this runtime can be viewed as O(σ(s)log2σ(s)), where σ(s)σ(s) denotes the number of different submasses of s. In this case, the runtime is independent of the size of the individual masses of characters.