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Abstract-Trie is one of the data structures for keyword matching. The trie is used in natural language processing, IP address routing, and so on. It is represented by the matrix form, the link form, the double array, and LOUDS. The double array combines retrieval speed of the matrix form with compactness of the list form. LOUDS is a succinct data structure using bit-string. Retrieval speed of LOUDS is not faster than that of double array, but the dictionary size is smaller. This paper proposes a compression data structure of the double array by dividing trie into each depth and removing the BASE array from that double array. From experimental results, the retrieval speed was almost the same as double array, and the size of the presented method was the most compact in other methods including LOUDS for a large set of keywords with fixed length.

A trie is one of the data structures for keyword search algorithms and is utilized in natural language processing, reserved words search for compilers and so on. The doublearray and LOUDS are efficient representation methods for the trie. The double-array provides fast traversal at time complexity of O(1), but the space usage of the double-array is larger than that of LOUDS. LOUDS is a succinct data structure with bit-string, and its space usage is extremely compact. However, its traversal speed is not so fast. This paper presents a new compression method of the double-array with keeping the retrieval speed. Our new method compresses the double-array by dividing the double-array into blocks and by using linear functions. Experimental results for varied keywords show that our new method reduced space usage of the double-array up to about 44 %, and the retrieval speed of the new method was 9-14 times faster than that of LOUDS. Moreover, the results show that the construction speed of the new method was faster than that of the conventional method for a large keyword set.

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