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HOME > PAST ISSUE > July-August 2008 > Article Detail

COMPUTING SCIENCE

The Britney Spears Problem

Tracking who's hot and who's not presents an algorithmic challenge

Brian Hayes


Logjam

Not every property of a stream can be computed in constant space and time. Consider a device that reports the number of distinct elements in a stream of integers. (For example, although the stream 3, 1, 8, 3, 1, 4, 3 has a total of seven elements, it has only four distinct elements: 3, 1, 8 and 4.)

There's a straightforward method for counting distinct elements: Start a counter at 0 and increment the counter whenever the stream delivers a number you haven't seen before. But how do you know that a given number is making its first appearance? The obvious answer is to keep a list of all the integers you've encountered so far, and compare each incoming value n with the numbers in the stored set. If n is already present in the list, ignore the new copy. If n is not found, append it to the stored set and increment the counter.

The trouble is, this is not a constant-space algorithm. The set of stored values can grow without limit. In the worst case—where every element of the stream is unique—the size of the stored set is equal to the length of the stream. No fixed amount of memory can satisfy this appetite.

A%20stream%20algorithmClick to Enlarge Image The problem of counting distinct elements is closely related to the problem of identifying the most-frequent elements in a stream. Having a solution to one problem gives us a head start on the other. The distinct-elements algorithm needs to keep a list indicating which stream elements have been observed so far. The frequent-elements algorithm needs the same list, but instead of simply noting whether or not an element is present, the program must maintain a counter for each element, giving its frequency. Since the list of elements and their counters can grow without limit, demands for space are again potentially unbounded.

Both of the algorithms could run out of time as well as space, depending on details of implementation. If it's necessary to search through all the stored numbers one by one for every newly received stream element, then the time needed grows in proportion to the size of the stored set. Eventually, the delay must exceed t , the interval between stream elements, at which point the algorithm can no longer keep up with the incoming data. Other data structures, such as trees and indexed arrays, allow for quicker access, although other time constraints may remain.



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