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Balancing weight-balanced trees

Part of: JFP Research Articles

Published online by Cambridge University Press:  27 May 2011

YOICHI HIRAI  and
KAZUHIKO YAMAMOTO
YOICHI HIRAI
Affiliation:
The University of Tokyo, JSPS Research Fellow (e-mail: yh@lyon.is.s.u-tokyo.ac.jp)
KAZUHIKO YAMAMOTO
Affiliation:
IIJ Innovation Institute Inc. (e-mail: kazu@iij.ad.jp)
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Abstract

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A weight-balanced tree (WBT) is a binary search tree, whose balance is based on the sizes of the subtrees in each node. Although purely functional implementations on a variant WBT algorithm are widely used in functional programming languages, many existing implementations do not maintain balance after deletion in some cases. The difficulty lies in choosing a valid pair of rotation parameters: one for standard balance and the other for choosing single or double rotation. This paper identifies the exact valid range of the rotation parameters for insertion and deletion in the original WBT algorithm where one and only one integer solution exists. Soundness of the range is proved using a proof assistant Coq. Completeness is proved using effective algorithms generating counterexample trees. For two specific parameter pairs, we also proved in Coq that set operations also maintain balance. Since the difference between the original WBT and the variant WBT is small, it is easy to change the existing buggy implementations based on the variant WBT to the certified original WBT with a rational solution.

Type
Articles
Information
Journal of Functional Programming , Volume 21 , Issue 3 , May 2011 , pp. 287 - 307
Copyright
Copyright © Cambridge University Press 2011

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