Online Grammar Transformation Based on Re-Pair Algorithm

The Re-Pair algorithm (Re-Pair), proposed by Larsson and Moat, is a simple grammar-based compression method that achieves a good compression ratio. Although Re-Pair runs in O(n) time and space for an input of length n, it cannot be used with a large input, because it runs oine and consumes substantial memory space. In this paper, we propose an online grammar transformation algorithm based on a modied Re-Pair along with a compression method using the algorithm. The proposed algorithm runs in O(n log^ h) time using O(g) space, where g and ^h are the number of production rules in a grammar and the maximum height of syntax trees generated by the rules, respectively. We implemented our method and demonstrated that it signicantly reduces memory usage with little sacrice of compression ratio in comparison with the original Re-Pair.