Compact Enumeration of Maximal Closed Substrings in Run-Length Encoded Strings

Abstract

A string $w$ is closed if $|w|=1$, or if $w$ has a non-empty border occurring only as its prefix and suffix. A maximal closed substring (MCS) is a maximal occurrence of a closed string; equivalently, it is a maximal closed repeat (MCR). We study MCS enumeration directly from the run-length encoding (RLE) of a string. For a string $T$ of length $n$ with RLE size $m$, we introduce a compact family representation for all MCS occurrences. We prove that $O(m^2)$ families are always sufficient and sometimes necessary. The representation relies on consecutive occurrence pairs of longest borders, classified by the RLE length of the border. The non-unary non-periodic cases are handled uniformly by a sparse suffix tree on run-start suffixes and height-based three-sided range reporting over RLE-boundary point sets; periodic cases are treated separately. Using McCreight's balanced priority search trees, the compact representation $\mathcal F$ of all MCSs can be listed in $O(m\log^2 m + |\mathcal F|\log m)$ time with $O(m)$ working space.