Fenwick trees demystified en

By Soultaker on Thursday 23 January 2014 06:00 - Comments (5)
Category: Algorithms & Data Structures, Views: 17.004

A Fenwick tree is a clever way to represent a list of numbers in an array, which allows arbitrary-length prefix sums to be calculated efficiently. (For example, the list [1,2,3,4,5] has a length-3 prefix [1,2,3] with sum 1+2+3 = 6.) This is useful in various scenarios, most notably to implement arithmetic coding, which requires dynamic tracking of cumulative frequencies of previously encountered symbols.

The data structure was introduced by Peter Fenwick in 1994, in a report titled “A new data structure for cumulative frequency tables”. Fenwick called it a Binary Indexed Tree, after the observation that the binary representation of indices determines the implicit tree structure, but the term Fenwick tree seems to be more popular today. Many articles are already available online that explain how a Fenwick tree may be implemented. Unfortunately, these articles invariably fail to explain how it was derived.

Fenwick himself shares the responsibility for the confusion, since he did not bother to discuss the history of the data structure in his publication. This has lead readers to believe there is something magical about the particular layout that is used, and caused programmers to blindly copy the source code from Fenwick's report or another source, instead of trying to understand the underlying principle first and then deriving the necessary code themselves. That's a pity, since proper understanding of a solution is necessary to extend, adapt or reconstruct it.

In this article I will try to fill this gap in public knowledge by explaining how the Fenwick tree structure and the algorithms that operate on it can be derived from scratch.

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