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arXiv · 2309.11056

A Verified Cost Analysis of Joinable Red-Black Trees

Runming Li·Harrison Grodin·Robert Harper
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Abstract

Ordered sequences of data, specified with a join operation to combine sequences, serve as a foundation for the implementation of parallel functional algorithms. This abstract data type can be elegantly and efficiently implemented using balanced binary trees, where a join operation is provided to combine two trees and rebalance as necessary. In this work, we present a verified implementation and cost analysis of joinable red-black trees in $\textbf{calf}$, a dependent type theory for cost analysis. We implement red-black trees and auxiliary intermediate data structures in such a way that all correctness invariants are intrinsically maintained. Then, we describe and verify precise cost bounds on the operations, making use of the red-black tree invariants. Finally, we implement standard algorithms on sequences using the simple join-based signature and bound their cost in the case that red-black trees are used as the underlying implementation. All proofs are formally mechanized using the embedding of $\textbf{calf}$ in the Agda theorem prover.

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Runming Li, Harrison Grodin, Robert Harper. 2023-09-20. A Verified Cost Analysis of Joinable Red-Black Trees. https://arxiv.org/abs/2309.11056

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