Optimal packed string matching

In the packed string matching problem, each machine word accommodates α characters, thus an n-character text occupies n/α memory words. We extend the Crochemore-Perrin constant-space O(n)-time string matching algorithm to run in optimal O(n/α) time and even in real-time, achieving a factor α speedup over traditional algorithms that examine each character individually. Our solution can be efficiently implemented, unlike prior theoretical packed string matching work. We adapt the standard RAM model and only use its AC⁰instructions (i.e., no multiplication) plus two specialized AC⁰packed string instructions. The main string-matching instruction is available in commodity processors (i.e., Intel's SSE4.2 and AVX Advanced String Operations); the other maximal-suffix instruction is only required during pattern preprocessing. In the absence of these two specialized instructions, we propose theoretically-efficient emulation using integer multiplication (not AC⁰) and table lookup.