Core Concepts
A simple and efficient algorithm for sorting a set of items with an unknown total order by performing binary comparisons, given the outcomes of some pre-existing comparisons.
Abstract
The paper presents a new algorithm called "topological heapsort" for sorting a set of items with an unknown total order, given the outcomes of some pre-existing comparisons. The algorithm combines three classic algorithms - topological sort, heapsort, and efficient insertion into a sorted list - in a novel way.
The key highlights of the algorithm are:
- It runs in O(m + n + log T) time and performs O(log T) comparisons, where n is the number of items, m is the number of pre-existing comparisons, and T is the number of total orders consistent with the pre-existing comparisons.
- This time and comparison bound is best possible up to constant factors, resolving a problem that has been studied intensely since 1976.
- The algorithm uses a heap with the "working-set" bound, which allows it to achieve the optimal bounds.
- The authors also present a variant called "topological heapsort with insertion" that eliminates the additive n term in the comparison bound, making it purely logarithmic in T.
- The approach is similar to a recent result on the distance-ordering problem, showing the connection between these two seemingly different problems.
- The authors also discuss how to efficiently estimate the value of log T, which is a key parameter in the algorithm's performance.
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