Faster Compact Top-k Document Retrieval

An optimal index solving top-k document retrieval [Navarro and Nekrich, SODA'12] takes O(m+k) time for a pattern of length m, but its space is at least 80n bytes for a collection of n symbols. We reduce it to 1.5n-3n bytes, with O(m + (k+log log n)log log n) time, on typical texts. The index is up to 25 times faster than the best previous compressed solutions, and requires at most 5% more space in practice (and in some cases as little as one half). Apart from replacing classical by compressed data structures, our main idea is to replace suffix tree sampling by frequency thresholding to achieve compression.