A new compression method for double-array structures by a hierarchical representation

Abstract

To store and retrieve keyword sets, a trie that is a tree structure is utilised in many applications for processing strings. The double-array and level-order unary degree sequence LOUDS efficiently represent the trie. The double-array provides fast retrieval for the trie, but its space usage is not so compact. On the other hand, LOUDS represents the trie compactly, but its retrieval speed is not so fast. This paper presents a new compression method for the double-array. Our new method represents the double-array by a hierarchical structure and changes allocations of the double-array. Theoretical observations show that the new method reduces the space usage of the double-array to ∼60%. Moreover, experimental results for English keywords show that the new method reduces the space usage of the double-array to ∼60-62% without impairing the high-speed performance. The retrieval speed of the new method is ∼17-24 times faster than that of LOUDS.