Practical Implementation of Encoding Range Top-2 Queries.

We design a practical variant of an encoding for range Top-2 queries (RT2Q), and evaluate its performance. Given an array A[1,n] of n elements from a total order, the range Top-2 encoding problem is to construct a data structure that can answer RT2Q queries, which return the positions of the first and the second largest elements within a given query range of A, without accessing the array A at query time. Davoodi et al. [Phil. Trans. Royal Soc. A, 2016] proposed a (3.272n + o(n))-bit encoding, which answers RT2Q queries in O(1) time, while Gawrychowski and Nicholson [ICALP, 2015] gave an optimal (2.755n + (n))-bit encoding which doesn't support efficient queries. In this paper, we propose the first practical implementation of the encoding data structure for answering RT2Q. Our implementation is based on an alternative representation of Davoodi et al.’s data structure. The experimental results show that our implementation is efficient in practice, and gives improved time-space trade-offs compared to the indexing data structures (which keep the original array A as part of the data structure) for range maximum queries.