from [Czyzowicz et al., ALGOSENSORS 2014, LNCS 8847, pp. 3-21 (2014], we prove that the 1-source Beachcombers' Problem on the cycle is NP-hard, and we derive approximation algorithms for the problem. For the t-source variant of the Beachcombers' Problem on the cycle and on the finite segment, we also derive efficient approximation algorithms.One important contribution of our work is that, in all variants of the offline Beachcombers' Problem that we discuss, we allow the robots to change direction of movement and search points of the domain on both sides of their respective starting positions. This represents a significant generalization compared to the model considered in [Czyzowicz et al., ALGOSENSORS 2014, LNCS 8847, pp. 3-21 (2014], in which each robot had a fixed direction of movement that was specified as part of the solution to the problem. We manage to prove that changes of direction do not help the robots achieve optimality.
IntroductionA group of n mobile robots have to explore collectively a given one-dimensional domain. The robots may be initially collocated or dispersed in the domain. At every moment of time, a robot can be either in walking mode or in searching mode. A robot in walking mode traverses the domain with a speed not exceeding its maximal walking speed. A robot in searching mode can travel using at most its maximal searching speed, which is strictly smaller than its walking speed, reflecting the fact that a searching activity is more time-consuming. Different robots may have distinct maximal walking and searching speeds. A robot can change mode, speed, and direction of movement instantaneously. There is no communication between the robots during the execution of the algorithm. In the Beachcombers' Problem, the goal is to design a schedule for the movement of all robots so that the domain is searched as fast as possible. A domain is said to be searched under a given schedule, if every point of the domain is visited by at least one robot in searching mode.As pointed out in [12], where the Beachcombers' Problem was introduced, there are numerous examples in quite diverse domains in which exploration using two-speed robots arises as a natural model for the underlying processes. For example, foraging or harvesting a field may take longer than inadvertent walking. In computer science, web page indexing or code inspection require a more involved investigation. A common feature of these examples is that the activity of searching, or other action to be performed on the territory, takes more time than casual territory traversal. The analogy to beachcombers has been introduced in [12] to bring out that, e.g., a beachcomber looking for things of value performs a meticulous search of the beach, which takes significantly more time than simply walking from one point of the beach to another. Further motivation for the two-speed model can be found in [12,13].Preliminaries and notation. We consider searching schedules using two-speed robots in the following one-dimensional geometric domains: the cycle of ...