Count-Min Tree Sketch: Approximate counting for NLP

The Count-Min Sketch is a widely adopted structure for approximate event counting in large scale processing. In a previous work we improved the original version of the Count-Min-Sketch (CMS) with conservative update using approximate counters instead of linear counters. These structures are computationaly efficient and improve the average relative error (ARE) of a CMS at constant memory footprint. These improvements are well suited for NLP tasks, in which one is interested by the low-frequency items. However, if Log counters allow to improve ARE, they produce a residual error due to the approximation. In this paper, we propose the Count-Min Tree Sketch (Copyright 2016 eXenSa. All rights reserved) variant with pyramidal counters, which are focused toward taking advantage of the Zipfian distribution of text data.