Abstract

Abstract. A compressed full-text self-index is a data structure that replaces a text and in addition gives indexed access to it, while taking space proportional to the compressed text size. The LZ-index, in particular, requires 4uHk(1 + o(1)) bits of space, where u is the text length in characters and Hk is its k-th order empirical entropy. Although in practice the LZ-index needs 1.0-1.5 times the text size, its construction requires much more main memory (around 5 times the text size), which limits its applicability to large texts. In this paper we present a practical space-efficient algorithm to construct LZ-index, requiring (4+ǫ)uHk+o(u) bits of space, for any constant 0 < ǫ < 1, and O(σu) time, being σ the alphabet size. Our experimental results show that our method is efficient in practice, needing an amount of memory close to that of the final index. 1 Introduction and Previous Work A full-text database is a system providing fast access to a large mass of textual data. The simplest (yet realistic and rather common) scenario is as follows. The text collection is regarded as a unique sequence of characters T1...u over an alphabet Σ of size σ,

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