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Improved Grammar-Based Compressed Indexes

We introduce the first grammar-compressed representation of a sequence that supports searches in time that depends only logarithmically on the size of the grammar. Given a text $T[1..u]$ that is represented by a (context-free) grammar of $n$ (terminal and nonterminal) symbols and size $N$ (measured as the sum of the lengths of the right hands of the rules), a basic grammar-based representation of $T$ takes $N\lg n$ bits of space. Our representation requires $2N\lg n + N\lg u + ε\, n\lg n + o(N\lg n)$ bits of space, for any $0<ε\le 1$. It can find the positions of the $occ$ occurrences of a pattern of length $m$ in $T$ in $O((m^2/ε)\lg (\frac{\lg u}{\lg n}) +occ\lg n)$ time, and extract any substring of length $\ell$ of $T$ in time $O(\ell+h\lg(N/h))$, where $h$ is the height of the grammar tree.