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arXiv · 2607.00612

Online computation of maximal closed substrings

Hiroki Shibata·Haruki Umezaki·Takuya Mieno·Yuto Nakashima·Shunsuke Inenaga
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Abstract

A non-empty string is closed if it has length one or its longest border appears exactly twice in the string. An occurrence of a closed substring is a maximal closed substring (MCS) if it cannot be extended to the left or to the right while preserving closedness. MCSs can be regarded as a general class of maximal repetitive structures including runs. In this paper, we study the computation of MCSs of a string given in an online manner, where one character is appended to the string at a time. Our algorithm detects newly formed MCSs after each append operation by using the rightmost previous occurrence of each suffix. To support this efficiently, we introduce the link-cut suffix tree (LCST), a novel data structure combining an online suffix tree with a link-cut tree. The LCST maintains rightmost occurrence information for substrings represented in the suffix tree in $O(n \log n)$ total time and $O(n)$ space, where $n$ is the length of the input string. Using the LCST, we obtain an $O(n \log n)$-time online algorithm for computing all MCSs, which is worst-case optimal. As further direct applications of the LCST, we obtain online algorithms for rightmost LZ77 factorizations and most recent match queries.

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Hiroki Shibata, Haruki Umezaki, Takuya Mieno, Yuto Nakashima, Shunsuke Inenaga. 2026-08-20. Online computation of maximal closed substrings. https://arxiv.org/abs/2607.00612

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