Fast and flexible packed string matching

Searching for all occurrences of a pattern in a text is a fundamental problem in computer science with applications in many other fields, like natural language processing, information retrieval and computational biology. In the last two decades a general trend has appeared trying to exploit the power of the word RAM model to speed-up the performances of classical string matching algorithms. In this model an algorithm operates on words of lengthw, grouping blocks of characters, and arithmetic and logic operations on the words take one unit of time. In this paper we use specialized word-size packed string matching instructions, based on the Intel streaming SIMD extensions (SSE) technology, to design a very fast string matching algorithm. We evaluate our solution in terms of efficiency, stability and flexibility, where we propose to use the deviation in running time of an algorithm on distinct equal length patterns as a measure of stability. From our experimental results it turns out that, despite their quadratic worst case time complexity, the new presented algorithm becomes the clear winner on the average in many cases, when compared against the most recent and effective algorithms known in literature.