Unidirectional Input/Output Streaming Complexity of Reversal and Sorting

We consider unidirectional data streams with restricted access, such as read-only and write-only streams. For read-write streams, we also introduce a new complexity measure called expansion, the ratio between the space used on the stream and the input size. We give tight bounds for the complexity of reversing a stream of length n in several of the possible models. In the read-only and write-only model, we show that p-pass algorithms need memory space Θ(n/p). But if either the output stream or the input stream is read-write, then the complexity falls to Θ(n/p). It becomes polylog(n) if p = O(logn) and both streams are read-write. We also study the complexity of sorting a stream and give two algorithms with small expansion. Our main sorting algorithm is randomized and has O(1) expansion, O(logn) passes and O(logn) memory.