A substring search algorithm in extendible hashing

Abstract

An extendible hashing scheme resolves bucket overflows by reorganizing the hash function and file structure locally, so it is very suitable for fast key retrievals of dynamic key sets. However, it cannot search keys that contain a given string as substrings efficiently. In this paper, in order to design this substring search in extendible hashing, signature vectors are introduced as hash values, and a trie structure as an extendible hash table, where each vector is composed by a bit stream. Pseudo signature vectors are defined to identify the buckets, and a constrained depth-first search is presented to traverse the arcs of the trie structure. To construct a compact trie despite an increase in the number of keys, uniform signature vectors are introduced, and the method for an incremental expansion of the hash table is proposed. This approach can restrict the size of the bit stream for each key, making constrained depth-first search efficient. From simulation results, by applying the presented schemes to Japanese and English key sets, it was shown that the number of accessed buckets decreased from 40% to 10% in comparison with traditional extendible hashing for which only descriptors were used. In addition, the search time cost of the presented approach is 2–10 times faster.