Worst-Case External-Memory Priority Queues

A priority queue Q is a data structure that maintains a collection of elements, each element having an associated priority drawn from a totally ordered universe, under the operations Insert, which inserts an element into Q, and DeleteMin, which deletes an element with the minimum priority from Q. In this paper a priority-queue implementation is given which is efficient with respect to the number of block transfers or I/Os performed between the internal and external memories of a computer. Let B and M denote the respective capacity of a block and the internal memory measured in elements. The developed data structure handles any intermixed sequence of Insert and DeleteMin operations such that in every disjoint interval of B consecutive priorityqueue operations at most clogM/B N/M I/Os are performed, for some positive constant c. These I/Os are divided evenly among the operations: if B ≥ clogM/B N/M, one I/O is necessary for every B/(clogM/B N/M)th operation and if B < clogM/B N/M, c/B log M/B N/M I/Os are performed per every operation. Moreover, every operation requires O(log2 N) comparisons in the worst case. The best earlier solutions can only handle a sequence of S operations with O(σ i=1 S 1/B log M/B Ni/M) I/Os, where N i denotes the number of elements stored in the data structure prior to the ith operation, without giving any guarantee for the performance of the individual operations.