Partial fillup and search time in LC tries

TL;DR

This paper provides a theoretical analysis of partial fillup in LC tries, showing that it slightly improves search times compared to original LC tries, with search depth typically proportional to loglog n.

Contribution

It offers a rigorous justification for experimental results on partial fillup in LC tries, quantifying the typical search depth and its dependence on parameters.

Findings

Partial fillup levels are concentrated on two values with high probability.

Typical search depth in alpha-LC tries is C loglog n, with C depending on p.

Search time in alpha-LC tries is smaller but of the same order as in original LC tries.

Abstract

Andersson and Nilsson introduced in 1993 a level-compressed trie (in short: LC trie) in which a full subtree of a node is compressed to a single node of degree being the size of the subtree. Recent experimental results indicated a 'dramatic improvement' when full subtrees are replaced by partially filled subtrees. In this paper, we provide a theoretical justification of these experimental results showing, among others, a rather moderate improvement of the search time over the original LC tries. For such an analysis, we assume that n strings are generated independently by a binary memoryless source with p denoting the probability of emitting a 1. We first prove that the so called alpha-fillup level (i.e., the largest level in a trie with alpha fraction of nodes present at this level) is concentrated on two values with high probability. We give these values explicitly up to O(1), and observe that the value of alpha (strictly between 0 and 1) does not affect the leading term. This result directly yields the typical depth (search time) in the alpha-LC tries with p not equal to 1/2, which turns out to be C loglog n for an explicitly given constant C (depending on p but not on alpha). This should be compared with recently found typical depth in the original LC tries which is C' loglog n for a larger constant C'. The search time in alpha-LC tries is thus smaller but of the same order as in the original LC tries.