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arXiv 2602.17201cs.DS
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适应性编码用于小而快的压缩后缀数组

Adaptive encodings for small and fast compressed suffix arrays

Department of Computer Science, University of Helsinki(赫尔辛基大学计算机系)

Diego Díaz-Domínguez, Veli Mäkinen

AI总结:

本文提出可变长度阻塞编码技术,用于优化压缩后缀数组的空间与查询速度,通过动态调整索引信息以提升性能。

摘要:

压缩后缀数组(CSAs)用于索引大规模重复性集合,在许多文本应用中起关键作用。r-index及其衍生方法结合了运行长度Burrows-Wheeler变换(BWT)与后缀数组采样,以实现空间大小与BWT中的等符号运行数成比例。尽管在近似相同字符串中效果显著,但随着变化增加,其大小增长迅速,因为BWT运行数对编辑敏感。现有方法通常在空间和查询速度之间进行取舍,限制了其在大规模应用中的实用性。我们引入了可变长度阻塞(VLB),一种针对基于BWT的CSA的编码技术,通过适应本地压缩性来调整索引信息的量。BWT被递归地划分为最多w个运行的块(一个参数),并组织成树状结构。可压缩区域出现在根附近,存储少量辅助数据;不可压缩区域位于更深处,保留额外信息以加速访问。查询遍历一条短的根到叶路径,然后进行小的运行扫描。这种策略通过将压缩区域节省的位转移到不可压缩区域的访问加速中,平衡了空间和查询速度。回溯搜索依赖于BWT上的排名和后继查询。我们引入了一种采样技术,仅在有效的回溯搜索状态上保证正确性,从而在不影响查询性能的情况下减少空间。我们扩展VLB以编码子采样的r-index(sr-index)。实验表明,基于VLB的技术在查询时间上优于r-index和sr-index,同时保持空间接近sr-index。与移动数据结构相比,VLB提供了更有利的空间-时间权衡。

英文摘要:

Compressed suffix arrays (CSAs) index large repetitive collections and are key in many text applications. The r-index and its derivatives combine the run-length Burrows-Wheeler Transform (BWT) with suffix array sampling to achieve space proportional to the number of equal-symbol runs in the BWT. While effective for near-identical strings, their size grows quickly as variation increases, since the number of BWT runs is sensitive to edits. Existing approaches typically trade space for query speed, or vice versa, limiting their practicality at large scale. We introduce variable-length blocking (VLB), an encoding technique for BWT-based CSAs that adapts the amount of indexing information to local compressibility. The BWT is recursively divided into blocks of at most w runs (a parameter) and organized into a tree. Compressible regions appear near the root and store little auxiliary data, while incompressible regions lie deeper and retain additional information to speed up access. Queries traverse a short root-to-leaf path followed by a small run scan. This strategy balances space and query speed by transferring bits saved in compressible areas to accelerate access in incompressible ones. Backward search relies on rank and successor queries over the BWT. We introduce a sampling technique that guarantees correctness only along valid backward-search states, reducing space without affecting query performance. We extend VLB to encode the subsampled r-index (sr-index). Experiments show that VLB-based techniques outperform the r-index and sr-index in query time, while retaining space close to that of the sr-index. Compared to the move data structure, VLB offers a more favorable space-time tradeoff.

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