Algorithmica

, Volume 8, Issue 1–6, pp 285–319 | Cite as

Optimal parallel detection of squares in strings

  • Alberto Apostolico
Article

Abstract

A stringw isprimitive if it is not a power of another string (i.e., writingw =v k impliesk = 1. Conversely,w is asquare ifw =vv, withv a primitive string. A stringx issquare-free if it has no nonempty substring of the formww. It is shown that the square-freedom of a string ofn symbols over an arbitrary alphabet can be tested by a CRCW PRAM withn processors inO(logn) time and linear auxiliary space. If the cardinality of the input alphabet is bounded by a constant independent of the input size, then the number of processors can be reduced ton/logn without affecting the time complexity of this strategy. The fastest sequential algorithms solve this problemO(n logn) orO(n) time, depending on whether the cardinality of the input alphabet is unbounded or bounded, and either performance is known to be optimal within its class. More elaborate constructions lead to a CRCW PRAM algorithm for detecting, within the samen-processors bounds, all positioned squares inx in timeO(logn) and using linear auxiliary space. The fastest sequential algorithms solve this problem inO(n logn) time, and such a performance is known to be optimal.

Key words

Parallel computation Combinatorial algorithms on words String matching Avoidable regularities Squares and repetitions in a string 

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Copyright information

© Springer-Verlag New York Inc. 1992

Authors and Affiliations

  • Alberto Apostolico
    • 1
    • 2
  1. 1.Department of Computer SciencePurdue UniversityWest LafayetteUSA
  2. 2.Dipartimento di Matematica Pura e ApplicataUniversità de L'Aquila L'AquilaItaly

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