Suffix Arrays on Words

Surprisingly enough, it is not yet known how to build directly a suffix array that indexes just the k positions at word-boundaries of a text T [1, n], taking O(n) time and O(k) space in addition to T . We propose a class-note solution to this problem that achieves such optimal time and space bounds. Word-based versions of indexes achieving the same time/space bounds were already known for suffix trees [1, 2] and (compact) DAWGs [3,4]. Our solution inherits the simplicity and efficiency of suffix arrays, with respect to such other word-indexes, and thus it foresees applications in word-based approaches to data compression [5] and computational linguistics [6]. To support this, we have run a large set of experiments showing that word-based suffix arrays may be constructed twice as fast as their full-text counterparts, and with a working space as low as 20%. The space reduction of the final word-based suffix array impacts also in their query time (i.e. less random access binary-search steps!), being faster by a factor of up to 3.