Summary: A Combined BIT and TIMESTAMP Algorithm for
the List Update Problem
Susanne Albers, Bernhard von Stengel, Ralph Werchner
International Computer Science Institute, 1947 Center Street, Berkeley, CA 94704, USA
Email: falbers, stengel, firstname.lastname@example.org
Abstract. A simple randomized on-line algorithm for the list update problem
is presented that achieves a competitive factor of 1.6, the best known so far.
The algorithm makes an initial random choice between two known algorithms
that have di erent worst-case request sequences. The rst is the BIT algorithm
that, for each item in the list, alternates between moving it to the front of the
list and leaving it at its place after it has been requested. The second is a
TIMESTAMP algorithm that moves an item in front of less often requested
items within the list.
Keywords. On-line algorithms, analysis of algorithms, competitive analysis,
linear lists, list-update.
1. Description of the algorithm
The list update problem is one of the rst on-line problems that have been studied
with respect to competitiveness (see 5] and references). The problem is to maintain
an unsorted list of items so that access costs are kept small. An initial list of items
is given. A sequence of requests must be served in that order. A request speci es an