Optimal strategies for the list update problem under the MRM alternative cost model

Abstract

We give an explicit representation for the offline optimum strategy for list update under the MRM model of Martínez and Roura [C. Martínez, S. Roura, On the competitiveness of the move-to-front rule, Theoret. Comput. Sci. 242 (1–2) (2000) 3130–325] and Munro [J.I. Munro, On the competitiveness of linear search, in: Proc. 8th Annual European Symposium on Algorithms (ESA 2000), in: Lecture Notes in Comput. Sci., vol. 1879, 2000, pp. 338–345] and give an O ( n 3 ) algorithm to compute it. This is in contrast to the standard model of Sleator and Tarjan [D.D. Sleator, R.E. Tarjan, Amortized efficiency of list update and paging rules, Commun. ACM 28 (2) (1985) 202–208] under which computing the offline optimum was shown to be NP-hard [C. Ambühl, Offline list update is NP-hard, in: Proc. 8th Annual European Symposium on Algorithms (ESA 2000), in: Lecture Notes in Comput. Sci., vol. 1879, 2000, pp. 42–51]. This algorithm follows from a new characterization theorem for realizable visiting sequences in the MRM model.