Time Optimal Algorithms for Black Hole Search in Rings

In a network environment supporting mobile entities (called robots or agents), a black hole is a harmful site that destroys any incoming entity without leaving any visible trace. The black-hole search problem is the task of a team of k > 1 mobile entities, starting from the same safe location and executing the same algorithm, to determine within finite time the location of the black hole. In this paper, we consider the black hole search problem in asynchronous ring networks of n nodes, and focus on time complexity. It is known that any algorithm for black-hole search in a ring requires at least 2(n− 2) time in the worst case. The best known algorithm achieves this bound with a team of n − 1 agents with an average time cost of 2(n − 2), equal to the worst case. In this paper, we first show how the same number of agents using 2 extra time units in the worst case, can solve the problem in only 7 4 n−O(1) time on the average. We then prove that the optimal average case complexity of 3 2 n − O(1) can be achieved without increasing the worst case using 2(n − 1) agents. Finally, we design an algorithm that achieves asymptotically optimal both worst and average case time complexities employing an optimal team of k = 2 agents, thus improving on the earlier results that required O(n) agents.