Datensatz

Zusammenfassung

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 {\sc Insert}, which inserts an element into $Q$, and {\sc 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 {\sc Insert} and {\sc DeleteMin} operations such that in every disjoint interval of $B$ consecutive priority-queue operations at most $c \log_{M/B} \frac{N}{M}$ I/Os are performed, for some positive constant $c$. These I/Os are divided evenly among the operations: if $B \geq c \log_{M/B} \frac{N}{M}$, one I/O is necessary for every $B/(c\log_{M/B} \frac{N}{M})$th operation and if $B < c \log_{M/B} \frac{N}{M}$, $\frac{c}{B}\log_{M/B} \frac{N}{M}$ I/Os are performed per every operation. Moreover, every operation requires $O(\log_2 N)$ comparisons in the worst case. The best earlier solutions can only handle a sequence of $S$ operations with $O(\sum_{i=1}^{S}\frac{1}{B}\log_{M/B}\frac{N_{i}}{M})$ I/Os, where $N_{i}$ denotes the number of elements stored in the data structure prior to the $i$th operation, without giving any guarantee for the performance of the individual operations.